Thread: Time travel- Serious business.

the premise: Time travel is possible. We have the technology!

The question: Rules?


I personally subscribe to a 'back the future' and 'Futurama' methodology here. Back AND forth, assuming the machine goes back or forth with you and it's not just some random time portal/rift/elevator/whatever that you step onto and then its gone. I particularily like the idea of a cascading effect of what you do causes stuff later. Futurama nailed this pretty well in that award winning episode where Fy becomes his own grandfather, but it did create a bit of a paradox. I'd also say that they abused time travel better than anyone ever in the first movie they made!

So aside from a debate about how timetravel might work, I'd like to ask how time paradoxes would be dealt with. For example- you go back in time, kill your own self as a baby. So, you wouldn't ever have made it long enough to go back in time to kill yourself. However, if you couldn't go back to kill yourself, you wouldn't die as a baby and therefore you COULD go back and do it. Would it be an auto-correct process, where some mitigating factor would prevent you from killing yourself or something?

Some people have tried to take care of the paradoxes by asserting that every single moment results in an alternate universe with just a single minute difference. So in a setup such as this, going back would put you in another universe altogether so you could kill your grandfather, which would eliminate the you in that universe, but leave the you from your original universe free to do what you please in that universe's future where the other you never existed. :? Conservation of energy might be a barrier to this idea though.

How about if the act of time travel creates a duplicate traveler "you"? Then a 20-year-old might travel ahead 60 years and surprise her 80-year-old self in a nursing home... no showstopper paradox there. The opposite direction, I guess we do need some mitigating factor. Could it be elastic? As in some changes would be slack and easy, while increasingly paradoxical changes would be harder to pull off? Remove free will, then time-travelers "just happen to" keep within bounds.

I don't know, I think that futurama kinda nailed it with their 'paradox fixing thinger' (don't remember the specific title the gave it). This is where the time duplicate invariably dies at some point by some slight change in the timeline that really affects nothing else.

My own theory as to how these paradoxes might be solved is a little different.

When you travel back in time, the future ceases to exist. This is because travelling back in time causes slight quantum changes, which change the future. What was the past for you is now the present, and you have perfect autonomy, because you've already changed the future by materialising into the past. Remember that in the present, the future has not yet been defined. Likewise, for you in the past, the future has not yet been defined and ceases to exist.

You have complete freedom in the past. If you were born in the future, and go back to the past, the universe behaves like you were born in the past instead of the future. It doesn't matter where you come from. You can kill your grandfather and still be alive, and watch the new future dawn. All actions you do became intrinsic to the new future.

if the future is erased, then so would the event where you travel back in time, right? So how could you have gone back? Wouldn't that create its own paradox?

It won't actually. The universe, I believe, behaves as if you were born in the past than in the future once you arrive back in the past. Never mind the fact that you arrived out of nowhere, bringing in extra energy (which is against the law of conservation of energy, by the way), and you were born in the future, the universe would still allow you to exist. This gives you complete freedom to do whatever you wish.

You could kill your grandparents, your mother, father, or your whole family, safe in the knowledge that whatever you do won't affect your chances of existence.

Of course, as long as the law of conservation of energy barrier (you can't go back in time, as that would bring in more energy into the universe) holds, time travel must be considered impossible.

the premise of this thread is that time travel is possible.

JEEZ


Okay, anyways, I think you're greatly flawed. How can you reconcile going back in time, killing every single specific person that lead to you being alive biologically, and then still existing anyways because of 'extra energy'? Your claim of being born in the future stands no ground if you go back to when you yourself were born and kill yourself, because you can't be born in both the future and the present if they are the same moment.

You don't continue to exist because of extra energy. You continue to exist because your existence here implies a causal event. This causal event may not be in the future, as the future has not yet been decided. Therefore, the causal event must be in the past, and you are free to exist and do whatever you choose.

Now the problem remains as to what this causal event in the past may be. Perhaps spontaneous creation by a random mixture of particles in such a way that you emerge? Just joking,lol, although that may be considered possible. Extremely unlikely, but possible.

If you lived in the future and occur as a series of discrete events in said future, then going back in time surely must imply something in the 'future' has happened. If you mitigate the events leading up to said events, ie your birth, then the future will change and you will cease to exist.

You're thinking in terms of the future affecting the past, which is in itself illogical. The time-travel paradox is a bit like the twin paradox in this sense, where the paradox ceases to be a paradox once we alter our view of things.

Think of this not as if you're looking from a series of events going from the future to the past, but from another viewpoint. You can mitigate the events; your mere existence has already mitigated the events, in fact. However, the fact that you do exist, as evidenced by the changes you have made in the quantum world, implies that there must be something that has caused you to exist. That is where my chain of reasoning emerges.

You must exist in the past, for otherwise the future could not have been erased. Obviously, since the future has been changed, your causal event is no longer in the future. However, you cannot not exist, because you've already changed things. So, there must be a causal event in the past which allows you to exist now, in the present.

The paradox ceases to be a paradox once you alter your view of things.

Originally Posted by Liongold

You must exist in the past, for otherwise the future could not have been erased. Obviously, since the future has been changed, your causal event is no longer in the future. However, you cannot not exist, because you've already changed things. So, there must be a causal event in the past which allows you to exist now, in the present.

Okay so what part of this doesn't equate to the future changing the past? Remember the present IS the future to the past.

and here the issue I have with your post comes full circle:

You're thinking in terms of the future affecting the past, which is in itself illogical.

But the present is not affecting the past at all. I am considering this from the viewpoint of someone who meets the time-traveller but does not know that he comes from the future. He will logically conclude that the time-traveller was born in the past. Likewise, let us assume the universe can think and place him in the shoes of this person. The universe will also conclude that the time-traveller was born in the past. If it investigates and finds that no proof of this exists, he could also conclude that the time-traveller was a result of a random configuration of atoms.

The time-traveller in the present has not affected the past, but has affected the future. The fact that he has changed the future implies that he must exist. this in turn implies that something must have caused him to exist. The universe, in the shoes of the person, must conclude that he was born in the past. Nothing he does now on, then, will keep him from existing.

For clarity, let's assume two pasts from now on: Past1, which is the past to the future, and Past2 which is the past of the past in which the time-traveller appears in.

Past2 is before Past1, obviously. Past1 is the present from the view of Past2.

Exactly where am I assuming that the future affects the past?

okay so the causal event that occurs in the past to allow you to exist in the present that didn't previous occur except that you came from the future isn't a direct alteration of the past? In the VERY LEAST, the future is changing at least one single discrete event to allow for you to exist in the present, since you changed the past (or the present at least) as an entity that exists only in the future- a future that relies on its past, which is now the present which you altered.

So, if you alter all the above things, the discrete event where you physically travel back in time would be changed. Therefore, you couldn't have gone back in time to begin with.

Basically, it's still a paradox.

okay so the casual event that occurs in the past to allow you to exist in the present that didn't previous occur except that you came from the future isn't a direct alteration of the past?

Forgive me if I misunderstood parts of this statement; time travel is highly confusing world. :wink:

Basically, you mean to say that my idea of the causal event that allows the time-traveller to exist is caused by the future, thereby invalidating the idea? Am I right?

But now you're assuming that the causal event must exist. But it doesn't; the universe thinks so on the basis of its logic. In fact, the causal event need not even be caused by the future.

EDIT: I am now assuming that a causal event must exist. It does not have to, but if we really want to investiagte, then if the causal event occured at the time the traveller appeared in the past, then I will proceed to show, assuming that a random configuration of particles created the travel, that it doesn't have to be created by the future at all.

Remember that in the present, the future does not exist. However, particles in the present are uncertain, to say the least. It is possible for them to do anything; they might suddenly combine to form a television set, or leap away from each other. These are all possible, but simply unlikely.

In the present then, the particles might suddenly choose to create a real live grown man, with memories of a non-existent future. There is no need to assume that the future caused this; it could be the choice of the particles themselves. The time-traveler might suddenly vanish in the future; particles in the past (which is now the present) might suddenly choose to create a person, thereby changing the future. Does this imply that the two events are related? Correlation, as I once remember reading, does not imply causation.

Basically, what I am saying is that the future does not have to cause a causal event in the past. The causal event is simply the choice of the particles in the present; it does not have any links to the future. The future is not changing anything at all; the present, with its randomness, affected the future in such a way that we can conclude that the person has travelled in time, yet allow him freedom to do as he wishes.

You've now entered an abstract realm into which even the greatest minds would answer with this:




seriously though you are getting more and more abstract. seriously:

In the present then, the particles might suddenly choose to create a real live grown man, with memories of a non-existent future

The causal event is simply the choice of the particles in the present

Care to explain this recurring topic more?

I am quite willing to explain.

The fact that a time-traveler exists changes the future. Yet he cannot not exist, as I explained, for otherwise the changes would not have been created. However, the fact that he exists implies a causal event. The causal event, if one were to think about it, could not occur in the future, because the future has already changed. So, the causal event must take place in the past, according to logic.

However, now our problem is with the causal event. What is this causal event? Now, I'm assuming that the time in which the traveller drops into behaves like the present, instead of the past. Since the time into which he drops into is the present, it has the capacity to change.

Now, since the causal event cannot take place in the past of the time traveller (as that would mean the future has affected the past), it must take place at the exact time at which the traveller appeared. That implies that the present has changed.

I then reverted to quantum mechanical arguments to propose a possible way in which the time-traveler was caused to exist, and showed that that this does not mean that the future creates the event at all.

Quantum mechanics allows particles to "choose" their path, or future. In the present, the particles have the choice to suddenly form a time-traveller out of thin air. It would be an extremely unlikely event, but it is possible.

This still implies that the future caused the past. This does not have to be so, as there is no link between the choice of the particles and the future. The time traveller might disappear in the future; in the past (which behaves like the present), particles might suddenly choose to recreate the time-traveller, thereby changing the future. These two events do not have to be connected; correlation does not imply causation.

That is what I meant. Do you get it now?

This still implies that the future caused the past

and then you say they don't have to be connected etc.

Really?

Also explaining a time paradox resolution in this way through quantum mechanics to a palaeontologist is like smacking a child on christmas and explaining to them that it's for their own good because masochism exists in some places.

Wow even I went abstract there.

and then you say they don't have to be connected etc.

Really?

Well, it was only a possible way. It makes perfect sense to me.

Also explaining a time paradox resolution in this way through quantum mechanics to a palaeontologist is like smacking a child on christmas and explaining to them that it's for their own good because masochism exists in some places

You're a palaeontologist? I apologise then, if you didn't understand what I was rambling on about quantum mechanics. Would you appreciate a simplification or did you get it?

Wow even I went abstract there.

No, you didn't. That was a perfectly legitimate example, albeit inappropriate for some age groups, lol.

Originally Posted by Liongold

No, you didn't. That was a perfectly legitimate example, albeit inappropriate for some age groups, lol.

Story of my life...


Anyways, quantum mechanics is something I'm not really ready to embrace yet. Maybe that's because nobody understands it at all. I've heard all kinds of crackpot ideas about quantum mechanics and to be honest it sounds alot like handwaving, special effects and some magic.

explain more please.

Well, quantum mechanics has been around for a century now, in fact. It explains the behaviour of subatmoic particles extremely well, and changed our view of the world.

Quantum mechanics holds that what we see as particles are also waves, and vice-versa. That is to say, a particle is a mixture of both a wave and a particle. It also says one thing of extreme importance: the uncertainty principle i.e. the more you know of the position of a particle, the less you know of its speed and vice-versa. All light waves are emitted in "packets", or certain amounts of energy.

The uncertainty principle means that we can never know exactly everything about a particle. There is a little "cloud" of uncertainty around it, as we can never define its position exactly, so we have to guess, approximately where it is.

This uncertainty means that we can never predict what happens next to a particle. Feynman's idea of a sum over histories, in which a particle takes all possible paths and then the paths cancel each other out, is an example of this. A particle's path might suddenly become this or perhaps that. A partticle, in effect, has the right to "choose" it's path.

A mass of particles also have the right to choose their path. As a result, they might suddenly leap together to form an object. The only obstacle is unlikeliness. It is extremely unlikely that particles would leap together to form something as small as a grain of sand. It is even more unlikely that they might form a time-traveller, but it is possible.

If you want to know more, I suggest you read Hawking's A Brief History of Time. It explains everything far more clearly than I can.

uncertainty principle maintains that you cannot know anything about quantum physics? That is rather...copoutish.

Actually, it states that you may now only parts of a quantum system, while consigning to uncertainty other parts. If you know its position, you cannot know its velocity. Likewise, you cannot know the position if you know its velocity.

Again, that just sounds copoutish to me. How can you measure its velocity unless you know its exact place in any given instant?

Originally Posted by Liongold

I am quite willing to explain.

Quantum mechanics allows particles to "choose" their path, or future. In the present, the particles have the choice to suddenly form a time-traveller out of thin air. It would be an extremely unlikely event, but it is possible.

This still implies that the future caused the past. This does not have to be so, as there is no link between the choice of the particles and the future. The time traveller might disappear in the future; in the past (which behaves like the present), particles might suddenly choose to recreate the time-traveller, thereby changing the future. These two events do not have to be connected; correlation does not imply causation.

That is what I meant. Do you get it now?

Quantum mechanics probably does allow for time travel, it's just that in order to use it, you have to place yourself in a state where virtually all of the existing physical laws are changed in equal and opposite ways.

You can't just change one aspect of your experience of time. You'd have to change all your aspects, or none.

In other words: Marty can't just walk around freely in the 1950's. (In Back to the Future) His entire experience of time and space has to change, not just the one part.

To be more exact:


It's a Schrödinger's cat problem.

http://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat

Schrödinger's cat is a thought experiment, often described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics being applied to everyday objects. The thought experiment presents a cat that might be alive or dead, depending on an earlier random event. In the course of developing this experiment, he coined the term Verschränkung (entanglement).


...

Broadly stated, a quantum superposition is the combination of all the possible states of a system (for example, the possible positions of a subatomic particle). The Copenhagen interpretation implies that the superposition only undergoes collapse into a definite state at the exact moment of quantum measurement.

Until you open the box, and see whether the cat died or not, some part of the universe honestly hasn't made a decision yet.

More accurately: The cat's space-time line, and yours haven't connected yet.

My theory is derived from this. Your first interaction with another space-time line need not occur at any particular date. If it's your first interaction, you can make it as early as you want, or as late as you want in time, even to times that are technically in the past.

However, if your space-time line is already tangled with the other. (Or the other space-time line is part of your space-time-line), then the current moment, and the future is all you have available to you. You can't remake your own past decisions, including the causes of those past decisions.

This would allow time travel in a limited sense. You could glimpse tomorrow's lottery numbers, buy a ticket, and win. You couldn't do anything to change the outcome of tomorrow's lottery number drawing, however. Once you've seen it (or interacted with it in any way), it's beyond your power to change.

In a sense, time travel leaves portions of the past unchangeable, and it can make portions of the future unchangeable, so it's a double edged sword. (Because if you never time travel at all, at least the future is always changeable.)

I think the reason this stuff is most commonly observed in sub-atomic particles is because it's the easiest to isolate their time lines from each other. They can be made to fully not interact with each other for a moment, whereas macro-scale objects almost always interact too much to isolate.

Once you've seen it (or interacted with it in any way), it's beyond your power to change.

so you're saying if I saw myself as a 500lbs man who ate himself to death, I couldn't say "damn I better get one a treadmill or I'm screwed in 15 years", thereby changing the future that I saw?

I call BS on that, flat out.

Yes.

Either that, or the event of that happening you would be absolutely un-glimpse-able. More precisely: You'd have to make up your mind, in an absolutely irreversible way, to never change what you saw regardless of how bad it was, in order to see it in the first place.

You're part of the quantum mechanical system you're observing.

Here's a little more from the same Wiki Article:

In the Copenhagen interpretation of quantum mechanics, a system stops being a superposition of states and becomes either one or the other when an observation takes place. This experiment makes apparent the fact that the nature of measurement, or observation, is not well defined in this interpretation. Some interpret the experiment to mean that while the box is closed, the system simultaneously exists in a superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat", and that only when the box is opened and an observation performed does the wave function collapse into one of the two states. More intuitively, some feel that the "observation" is taken when a particle from the nucleus hits the detector. This line of thinking can be developed into Objective collapse theories. In contrast, the many worlds approach denies that collapse ever occurs.

I'm suggesting that the wave function collapses if you decide to act on what you see. Or rather, it collapses when you decide to act on what you see.

If you saw yourself as a 500 lb man soon to die from it (but hadn't seen the actual death yet), you could write a letter to yourself, or arrange for a personal trainer to contact you a second after the moment in time you're looking at.

You'd have to then erase your own memory, and allow the event to happen exactly as you had seen it.

But no... you can't change the events you see when you use time travel to look.

What I mean is: You can only see the moment of collapse. If you want to see an event 15 years in the future, you have to also delay the collapse until 15 years in the future. You can also arrange for some collapses to happen in the past.

Originally Posted by mormoopid

Again, that just sounds copoutish to me. How can you measure its velocity unless you know its exact place in any given instant?

You are succumbing to the Argument from Incredulity logical fallacy.

The Difficulty with Time Travel, is how do we physical manpulate the 4th Dimension, Time, which is not physical.

As well, Time is like a River, Only moving one direction so going back in time would have very great difficulties, while going into the future might be a easier way.

In General, to manipulate time one would have to move faster then Time it's self, something which Light can already do, therefore meaning Time must literally catch up with light. One interesting thing is that Astronauts are actually 2 Microseconds into the future from us, the speed for overcoming gravity moves them ahead in time.

In General, to manipulate time one would have to move faster then Time it's self, something which Light can already do, therefore meaning Time must literally catch up with light. One interesting thing is that Astronauts are actually 2 Microseconds into the future from us, the speed for overcoming gravity moves them ahead in time.

We don't know the speed of time, Thargana. Relatively speaking, according to special relativity, in order for us to observe a constant value for the speed of light, space and time distort. Time slows down for you the faster you go, and the smaller you become in length. We wouldn't be able to go faster than time as nothing may go faster than light.

If you travelled faster than light, you could go back into the past, of course...

Again, that just sounds copoutish to me. How can you measure its velocity unless you know its exact place in any given instant?

This is a thought experiment to describe how.

According to Max Planck, light is emitted in the form of a quanta, or packet of energy. More easily, imagine light to be emitted as a particle.

Now, in order to locate the position of a particle, you will obviously shine a beam of light on it. You cannot use any energy less than a single quantum; the least you can use is one quantum. To find the position even more accurately, you will use light waves with a very short wavelength, as these limit the number of possible areas where the particle exists. However, short wavelengths imply a higher amount of energy. When the quantum hits the particle, it must give all its energy to the particle, increasing its speed; changing the speed, in fact, making any measurement of it's speed useless.

So, you'll know its position, but won't be able to find its speed accurately. This also works vice-versa, where you use waves with a long wavelength. You'll give the particle less energy, but you'll have to sacrifice knowledge of its position.

still not convinced, even in theory. What else you got?

Originally Posted by Tharghana

The Difficulty with Time Travel, is how do we physical manpulate the 4th Dimension, Time, which is not physical.

As well, Time is like a River, Only moving one direction so going back in time would have very great difficulties, while going into the future might be a easier way.

In General, to manipulate time one would have to move faster then Time it's self, something which Light can already do, therefore meaning Time must literally catch up with light. One interesting thing is that Astronauts are actually 2 Microseconds into the future from us, the speed for overcoming gravity moves them ahead in time.

It's all perspective. The 4 dimensions are kind of interchangeable. Backwards relative to North, is forwards relative to South. If you go slower North, then you're going faster in the time direction. (more time is passing with less activity) If you go faster North, then you're going slower in the time dimension.

Light is the slowest you can go, in the time dimension.

If you cryogenicly froze yourself, you'd perceive a huge amount of time to have passed instantly. So zero spatial motion is fast time.

Originally Posted by mormoopid

still not convinced, even in theory. What else you got?

Well, what kind of explanation do you want? Could you tell me what still prevents you from accepting this and perhaps we can iron it out?

You do not need to know the position of something in order to figure out its velocity. You treat them both as unknowns. Then, in actually trying to figure out, what they are, you do a version of the same thought experiment I outlined in my last post. That effectively means you can only know either velocity or position of a particle.

You're saying in order to act on something you see in the future you have to erase your memory, the whole basis for your action in other words, before you can do it? Otherwise it will create a wave function collapse? Why would I want to even prevent a wave function collapse? It doesn't really sound that bad, I mean when was that last time a wave function did anything for me anyways?

I don't care much for your lack of free will in the given example of the fat man.


Liongold,

okay well lets say you have an equation:

Velcoity = distance/Time
V=S/D, pretty basic stuff I know

Are you saying though that we have two unknowns, Distance (or relative position) and velocity? I think we can use time as a constant here or can measure it. in this case you cannot solve this equation at all.

Or are you alternatively saying that we have two/three of the necessary variable, assuming we have time as a constant or can measure it? In that case, having 2/3 of the variable, finding number 3 really isn't hard at all...

My mind really doesn't agree with abstractness, even if it has logic behind it.

You're saying in order to act on something you see in the future you have to erase your memory, the whole basis for your action in other words, before you can do it? Otherwise it will create a wave function collapse? Why would I want to even prevent a wave function collapse? It doesn't really sound that bad, I mean when was that last time a wave function did anything for me anyways?

I don't care much for your lack of free will in the given example of the fat man

Well, the wave function determines what state you are in. The wave function can state whether you are alive, dead, happy, hungry, a gelatinous blob on the beach or a monstrous predator at the foot of the mountains. If you collapse a wave function, then what it reveals is what you become. If you cause a wave function collapse, well, you have just changed your future.

Velcoity = distance/Time
V=S/D, pretty basic stuff I know

Are you saying though that we have two unknowns, Distance (or relative position) and velocity? I think we can use time as a constant here or can measure it. in this case you cannot solve this equation at all.

Or are you alternatively saying that we have two/three of the necessary variable, assuming we have time as a constant or can measure it? In that case, having 2/3 of the variable, finding number 3 really isn't hard at all...

My mind really doesn't agree with abstractness, even if it has logic behind it

No. You are confusing position with distance, which is not the same thing at all. Distance is the amount you've covered when you are travelling; position is where you are right now. You can't tell where a particle is, in the uncertainty principle, although its possible to record the distance covered.

By trying to find the exact location of the particle, you must give up your idea of how fast its moving. V=S/d doesn't fit over here.

I'm pretty sure that your spiel of wave function stuff makes no sense to me, a biologist, who is pretty firm in his understanding that biological processes govern life/death etc, rather than some wave function stuff.

Okay so wave function collapse = changing the future...so that would mean that allowing for a wave function collapse would be counter intuitive in an idea where you cannot change your future?

Okay so I'll admit to almost failing grade 10 physics and never touching it again.

So there is no way in which your ideas allow for velocity and position to reconcile into a single observation? I just don't understand why if you know the distance travelled, the time it took to do that distance and the velocity travelled you can't just use some simple math and say "yeah okay it was at this point at this time and knowing the direction it is moving, we can say it will be here at this time".

I guess I'm assuming you know every other variable in all this though.

I'm pretty sure that your spiel of wave function stuff makes no sense to me, a biologist, who is pretty firm in his understanding that biological processes govern life/death etc, rather than some wave function stuff.

Of course biological processes govern life and death. Changing the wave function changes those biological processes.

Okay so wave function collapse = changing the future...so that would mean that allowing for a wave function collapse would be counter intuitive in an idea where you cannot change your future?

Yep. Got it in one.

Okay so I'll admit to almost failing grade 10 physics and never touching it again.

Well, you did start a thread on time travel. Just joking. Physics isn't really that hard to understand. I'm 14, by the way, so you can understand how easy it is. All you need is a little imagination.

I just don't understand why if you know the distance travelled, the time it took to do that distance and the velocity travelled you can't just use some simple math and say "yeah okay it was at this point at this time and knowing the direction it is moving, we can say it will be here at this time".

That is correct. But you misunderstand me. By trying to measure the position you automatically change the speed. You will no longer be able to tell where the particle will be. You will know its position while giving up any idea of what the velocity is, because you just changed it. By how much? You don't know.

Besides, time slows down at fast speeds. Using V=D/T then really wouldn't work, then.

I know this thread assumes time travel is possible but it isn't, plain and simple, it's impossible, time is not a dimension, time has no real way of being measured, no way of being properly defined, it just is. There are only moments and experiences, there is only reality and the perceptions that come with it, there is only here and now. If it were possible, every time I moved forward in time and left the the past behind me, a replica of myself would be left in that past, it sort of implies that there are static moments in the past that don't move forward, they are left there for me to go visit via time travel. Time is not a river in which I can swim backward, it is not something that splits into into every possible outcome, for every universe, think of it like a hamster ball, you are stuck in it. There is no point racking your brain to solve the paradoxes involved, they are paradoxes for a reason, the situations are impossible.

Originally Posted by mormoopid

...so that would mean that allowing for a wave function collapse would be counter intuitive in an idea where you cannot change your future?

Here is where the biology comes in. We evolved as a species that travels at a tiny fraction of the velocity of light. Our neurological system can cope readily with complex Newtonian mechanics, involving swinging in trees, or throwing rocks at hyenas. We can do these things 'instinctively', though doubtless practice helps (except for my golf swing). But the universe is different at 90% of the speed of light, or on the scale of a proton, not the scale of a rock. We cannot 'intuitively' understand that universe. Not very surprising really.

Originally Posted by GHOST

I know this thread assumes time travel is possible but it isn't, plain and simple, it's impossible, time is not a dimension, time has no real way of being measured, no way of being properly defined, it just is. There are only moments and experiences, there is only reality and the perceptions that come with it, there is only here and now. If it were possible, every time I moved forward in time and left the the past behind me, a replica of myself would be left in that past, it sort of implies that there are static moments in the past that don't move forward, they are left there for me to go visit via time travel. Time is not a river in which I can swim backward, it is not something that splits into into every possible outcome, for every universe, think of it like a hamster ball, you are stuck in it. There is no point racking your brain to solve the paradoxes involved, they are paradoxes for a reason, the situations are impossible.

Ghost, let us just assume we can, for one moment. all right? I myself can show that time travel is impossible for any reason whatsoever, but what if it was possible? All we're doing is trying to think of a way to avoid those darned paradoxes. It's a bit like trying to find the proof of a long and exceedingly difficult statement, but at least it's fun.

Yeah, I don't mean to spoil your fun, it is a cool topic. I used to think about it all the time. Who knows, it could be possible but wouldn't we see people popping into our time? I think the main paradox is essentially the existence of a past that can be revisited. It would mean that just behind me in time is another me, and behind that, another me, and a me for every single moment in time. So if a go back 10 mins in time to the past and my old me is still moving forward in time then time must loop around, there mustn't be a start to my existence, nor an end, the are infinite amounts of me existing in different times, all moving forward, and a new me being born every moment in time, my older instances would also die at the same rate. This would eliminate all paradoxes (I think) because if I kill myself, I'm still going to get born, there will be a missing me in the flow of time but I would still exist. There would also be a missing me in the time socket I left. So now there is an empty me slot in time and I'm fraudulently stealing my other self's socket in time or if I don't kill myself, there will be two of me existing at the same time which shouldn't create problems because there's an infinite amount of me.

I would wonder how a time machine could re-create the entire universe, with everything back to where it was, moment to moment, over billions of years, at a time one wished to travel.

Just a somewhat tiny flaw.

and then q ruins the thread with his consistent rightness



Yeah my mind doesn't like imagination on certain topics, liongold

By changing the universe, Q. Obviously.

Originally Posted by Liongold

By changing the universe, Q. Obviously.

Of course, how silly of me. :wink:

So, how many DeLoreans were in BTTF II in the 1955 time period?

So, how many DeLoreans were in BTTF II in the 1955 time period?

Well, the universe hasn't changed yet, so I can't give you a straight answer. Well, I can, but not with a straight face.

Anyway, for a serious answer to your original question, by changing just one moment, you automatically change the next moment, which in turn affects the next and so on, creating a cascading effect that rewrites the universe's timeline.

If you travel back in time, from your viewpoint, you'd have changed just one moment. Because now you exist in the past, your past behaves like the present for you, so you're able to see the effects of the changes you make. If you now choose to travel in the future, it'll be like you've just jumped several key moments which you could have observed, and simply come to the future, where the sum total of the changes you've wrought comes into fore.

It's not that hard a question.

Originally Posted by Liongold

Anyway, for a serious answer to your original question, by changing just one moment, you automatically change the next moment, which in turn affects the next and so on, creating a cascading effect that rewrites the universe's timeline.

What happens to the universe you left behind? How much energy is required to change the universe? If a star goes supernova and turns into a black hole, do you have enough energy to put it back together when you come back to this time?

Btw, the answer to my question is (3) DeLoreans.

Originally Posted by (Q)

Originally Posted by Liongold

Anyway, for a serious answer to your original question, by changing just one moment, you automatically change the next moment, which in turn affects the next and so on, creating a cascading effect that rewrites the universe's timeline.

What happens to the universe you left behind? How much energy is required to change the universe? If a star goes supernova and turns into a black hole, do you have enough energy to put it back together when you come back to this time?

Btw, the answer to my question is (3) DeLoreans.

Actually, I believe it is 4.

1.The DeLorean Marty originally traveled to 1955 in.
2.The DeLorean Old Biff used to return to 1955.
3.The DeLorean Doc and Marty returned to 1955 in.
and
4.The DeLorean Doc sealed in the abandoned mine in 1885.

Originally Posted by Janus

4.The DeLorean Doc sealed in the abandoned mine in 1885.

Damn! I missed that one. Good call, Janus.

Q thanks for saving the thread

What happens to the universe you left behind? How much energy is required to change the universe? If a star goes supernova and turns into a black hole, do you have enough energy to put it back together when you come back to this time?

Why, the universe you just left behind behaves as the past now. In fact, simply existing in the past will cause small quantum changes. This change will cause a cascading effect; depending upon the amount you've changed and the time between the event and another point in the future, some changes could come faster than others.

For example, waving your hand will add up over a long, long period of time, while killing the President of the United States could change history in a small period of time.

Here's a small formula I've devised for it:



Where m is the magnitude of the event, and t is the time since. To change the universe, there is no limit on the energy. You could change the universe just by breathing, but the effects will be very small.

Now, to your last question. You have to go back a long, long time in order to change the future of the black hole, since the magnitude of your chaging event will be very small. For best results, go back to the Big Bang. :wink:


Now, back to your last question.

Originally Posted by Liongold

Why, the universe you just left behind behaves as the past now.

But, then you now have two universes, the one you left behind and the one you've apparently created. One has a star while the other has the same star that went supernova and is now a black hole. To have created another universe would have required a lot of mass/energy, yes?

Maybe gods are just time-travelers?

Here's a small formula I've devised for it:



Where m is the magnitude of the event, and t is the time since.

I'm ready to plug in elements if you are.

For best results, go back to the Big Bang.

hehe Sounds like a recipe.

Originally Posted by Janus

4.The DeLorean Doc sealed in the abandoned mine in 1885.

I got a question, Janus, or anyone. Would this DeLorean have been there in the abandoned mine before the lightning bolt that hit it to send it back to 1885?

But, then you now have two universes, the one you left behind and the one you've apparently created. One has a star while the other has the same star that went supernova and is now a black hole. To have created another universe would have required a lot of mass/energy, yes?

Ah, I see the problem now. You're talking about the parallel worlds solution to the paradoxes, while I'm not. I was talking about the classical viewpoint, where you travel back and forth in just one universe.

I have to agree with you, that does seem to be a problem. Although ideas about the parallel worlds suppose that they are created at every moment, so I imagine there must be a solution to your problem.

Maybe gods are just time-travelers?

Interesting idea...

I'm ready to plug in elements if you are.

Absolutely. Shall we do it on the count of three? :wink:

hehe Sounds like a recipe.

Serve for four, then, I'm hungry.

Originally Posted by (Q)

Originally Posted by Janus

4.The DeLorean Doc sealed in the abandoned mine in 1885.

I got a question, Janus, or anyone. Would this DeLorean have been there in the abandoned mine before the lightning bolt that hit it to send it back to 1885?

I pondered that, and here's what I came up with.

It depends on what you mean by "before".

Each incursion into the past creates a new time line.

Thus in the time line that exists just prior to our seeing the DeLorean being struck by lightning, there were 3 Deloreans existing at the same time at some moment in 1955. Upon the Delorean being struck by Lightning and being sent back to 1885, a New timeline is created, one in which the 4th DeLorean was sitting in the mine. So in this new time line, the 4th Delorean was sitting in the mine before the lightning sent it back to 1885.

That being said, the movies are a little inconsistent in dealing with these time line changes. For instance, the messenger delivering the message appears just moments after the Delorean is struck by lightning, indicating an almost instantaneous shift of time lines. However, when Biff steals the DeLorean and takes it back to 1955, the 2015 timeline doesn't apparently change until after Marty and Doc leave to return to 1985. (taking into account the vast changes Biff made by 1985 in the alternate timeline, these changes should have have even grown larger by 2015.

Exactly what is a DeLorean?

Originally Posted by Liongold

Exactly what is a DeLorean?

See here: http://en.wikipedia.org/wiki/DeLorean_DMC-12
and watch the classic timetravel trilogy
Back to the Future

If there is a way to timetravel that doesn't involve a DeLorean I don't want to hear about it

Originally Posted by Janus

Thus in the time line that exists just prior to our seeing the DeLorean being struck by lightning, there were 3 Deloreans existing at the same time at some moment in 1955. Upon the Delorean being struck by Lightning and being sent back to 1885, a New timeline is created, one in which the 4th DeLorean was sitting in the mine. So in this new time line, the 4th Delorean was sitting in the mine before the lightning sent it back to 1885.

But, there should only be 3 DeLoreans before and after, in that the DeLorean sitting in the mine could not have been there until it was struck by lightning. At that point in time in 1955, that DeLorean is instantly transported to the abandoned mine, hence there were only 3 DeLoreans, not 4. I'm getting a headache.

That being said, the movies are a little inconsistent in dealing with these time line changes. For instance, the messenger delivering the message appears just moments after the Delorean is struck by lightning, indicating an almost instantaneous shift of time lines. However, when Biff steals the DeLorean and takes it back to 1955, the 2015 timeline doesn't apparently change until after Marty and Doc leave to return to 1985. (taking into account the vast changes Biff made by 1985 in the alternate timeline, these changes should have have even grown larger by 2015.

True. I think there's a term for that, but I can't recall it just now. Something about writers prerogative.

Originally Posted by Liongold

Ah, I see the problem now. You're talking about the parallel worlds solution to the paradoxes, while I'm not. I was talking about the classical viewpoint, where you travel back and forth in just one universe.

I still can't quite see that. What happens to this universe if you travel back through it to another time? Where does everything go?

Originally Posted by (Q)

Originally Posted by Janus

Thus in the time line that exists just prior to our seeing the DeLorean being struck by lightning, there were 3 Deloreans existing at the same time at some moment in 1955. Upon the Delorean being struck by Lightning and being sent back to 1885, a New timeline is created, one in which the 4th DeLorean was sitting in the mine. So in this new time line, the 4th Delorean was sitting in the mine before the lightning sent it back to 1885.

But, there should only be 3 DeLoreans before and after, in that the DeLorean sitting in the mine could not have been there until it was struck by lightning. At that point in time in 1955, that DeLorean is instantly transported to the abandoned mine, hence there were only 3 DeLoreans, not 4. I'm getting a headache.

By that logic, when Marty originally goes back to 1955, and destroys the pine tree, The "Twin Pines" mall sign shouldn't change to "Lone Pine", until after the DeLorean leaves 1985. But when Marty returns to 1985 and prior to watching the Events leading up to his original trip back into time, the sign reads "Lone Pine".

Besides, the Delorean was in 1885. Doc put in the mine in 1885. So why wouldn't it be there between the time he puts it in the mine and the time Marty and Doc take it out of the Mine.

The logical explanation is that once the DeLorean is struck by lightning and is sent back to 1885, a whole new time line is created with its own history, which includes the DeLorean being put in the mine and being in the mine during the period that the other three DeLoreans were in 1955.

So "before" the lightning strike, Marty is in a time line where there were only Three Deloreans. "After" the strike he is in a time line with its own history,which includes the Delorean being in the mine the whole time the other earlier events took place.

Originally Posted by Janus

By that logic, when Marty originally goes back to 1955, and destroys the pine tree, The "Twin Pines" mall sign shouldn't change to "Lone Pine", until after the DeLorean leaves 1985. But when Marty returns to 1985 and prior to watching the Events leading up to his original trip back into time, the sign reads "Lone Pine".

Besides, the Delorean was in 1885. Doc put in the mine in 1885. So why wouldn't it be there between the time he puts it in the mine and the time Marty and Doc take it out of the Mine.

How about this? The Doc wouldn't have needed to put the DeLorean in the abandoned mine because Marty came back with a DeLorean in which they both attempted to use to go back to 1985. Doc probably used that Delorean to help build his steam driven locomotive time machine after Marty left 1885.

The time circuits were blown on the one in the mine, yet it still had gasoline, which was the problem they had trying to get back from 1885 in the first place, a punctured gas tank from the arrow shot by the Indians.

As well, didn't the sign for Clayton Ravine turn to Eastwood Ravine only after Marty returned from 1885?

Originally Posted by mormoopid

You're saying in order to act on something you see in the future you have to erase your memory, the whole basis for your action in other words, before you can do it? Otherwise it will create a wave function collapse? Why would I want to even prevent a wave function collapse? It doesn't really sound that bad, I mean when was that last time a wave function did anything for me anyways?

I don't care much for your lack of free will in the given example of the fat man.

The wave function collapse is the moment at which something is decided. If you aren't going to erase your memory, then most likely it collapses before you even see it. (The decision is that you don't get to see it in the first place)

You're right, though, about free will. Just as you have no free will about the past, after you've seen it. (You can't go back in time and erase your mistakes), you also have no free will about the future, after you've seen it. Abandonment of your free will about a future event is a necessary prerequisite to seeing that future event.

I'm not talking about the probable future, here. I'm talking about the literal future. You can change the probability-determined future (I mean "guessed at" future) all you want, but you can't change a time-machine-glimpsed future.

Originally Posted by mormoopid

I'm pretty sure that your spiel of wave function stuff makes no sense to me, a biologist, who is pretty firm in his understanding that biological processes govern life/death etc, rather than some wave function stuff.

Okay so wave function collapse = changing the future...so that would mean that allowing for a wave function collapse would be counter intuitive in an idea where you cannot change your future?

No. wave function collapse = determining the future. It's never a change.

It can also mean determining the past, but that too is also never a change.

All paradoxes are resolved in nature. You can't go back in time and kill your grandfather.

You can't see yourself as a fat man at 50, and then not get fat... because then you would never have seen yourself as a fat man at 50..... which means you wouldn't adjust your diet....... which means you will get fat..... which means you see yourself as a fat man at 50...... which means you change your diet...... and we've created an eternal cycle.

There are no such cycles in nature. They all figure themselves out at the moment of wave function collapse. The only way to see the future now is to make sure the wave function collapses in such a manner as to allow you to see it now, and that's a very complicated task that no machine can just up and do for you.

This can be a very useful skill/tool. You know, you can still win the lottery.

You can't see yourself as a fat man at 50, and then not get fat... because then you would never have seen yourself as a fat man at 50..... which means you wouldn't adjust your diet....... which means you will get fat..... which means you see yourself as a fat man at 50...... which means you change your diet...... and we've created an eternal cycle.

That's basically my opinion right there

Originally Posted by (Q)

How about this? The Doc wouldn't have needed to put the DeLorean in the abandoned mine because Marty came back with a DeLorean in which they both attempted to use to go back to 1985. Doc probably used that Delorean to help build his steam driven locomotive time machine after Marty left 1885.

Now your trying to create on of those time-paradoxes that could destroy the universe (or at least our own galaxy). Doc would have had to put the DeLorean in the mine in order for it to be there for Marty to use in 1955. Now, following the logic used in the rest of the films, After Marty returns to 1885, Doc could then decide not to put the DeLorean in the mine, but then the DeLorean Marty returned in would likely vanish.

The time circuits were blown on the one in the mine, yet it still had gasoline, which was the problem they had trying to get back from 1885 in the first place, a punctured gas tank from the arrow shot by the Indians.

Doc likely drained the gas before storing the car. It would have gone bad and fouled the fuel system if left. Doc probably used the gas for something else before Marty came back. (with the storage facilities in 1885 it wouldn't have had a long shelf life.)

As well, didn't the sign for Clayton Ravine turn to Eastwood Ravine only after Marty returned from 1885?

Did it? All we know is that the sign read Eastwood Ravine when Marty returned (we didn't actually see it change), so we don't know how long prevously to that it read such.

I still can't quite see that. What happens to this universe if you travel back through it to another time? Where does everything go?

Well, to put it simply, it disappears. This may not be as big a violation as you think; the universe can be considered a system, yes? Then, it stands to reason that the total energy of the universe is constant at all points of time. You do not change the energy when you go back in time; you merely change the form of energy.

And now it seems I don't understand anything on this thread, thanks to the sudden influx of DeLorean stuff...

Originally Posted by Janus

Now your trying to create on of those time-paradoxes that could destroy the universe (or at least our own galaxy). Doc would have had to put the DeLorean in the mine in order for it to be there for Marty to use in 1955. Now, following the logic used in the rest of the films, After Marty returns to 1885, Doc could then decide not to put the DeLorean in the mine, but then the DeLorean Marty returned in would likely vanish.

It's funny there was no mention of the DeLorean in the mine when Marty went back to 1885. I believe Doc went to 1885 on January 1 and the letter he sent to Marty was dated September 1. But, Marty also went back to 1885 on September 1 in the morning, but didn't see Doc until the next day due to his continued head bumping incidents. He really should have worn a helmet. Curious. As you say, probably one of those galaxy-class time-paradoxes.

Doc likely drained the gas before storing the car. It would have gone bad and fouled the fuel system if left. Doc probably used the gas for something else before Marty came back. (with the storage facilities in 1885 it wouldn't have had a long shelf life.)

Very good assumption. I remember Doc talking about the gasoline issue, but never mentioned the gas from the DeLorean he stored in the mine and what he may have done with it.

Did it? All we know is that the sign read Eastwood Ravine when Marty returned (we didn't actually see it change), so we don't know how long prevously to that it read such.

True, there was only mention of the name of the ravine from Marty when he explained how the ravine got it's name, we didn't actually get to see the original Clayton Ravine sign, or seen it change.

I think I'm going to need to draw out all the timelines created to see where, if any, the flaws present themselves. At this point, though, I'd agree with you that there were probably four DeLoreans in 1955.

Originally Posted by Liongold

Well, to put it simply, it disappears.

Ok, but if that is the case, then we would all simply disappear if a time traveler went into the future and did not return. Or, if he did return, then would the star that went supernova and became a black hole be returned to it's original state?

And now it seems I don't understand anything on this thread, thanks to the sudden influx of DeLorean stuff...

hehe, well it's kinda interesting to watch the BTTF trilogy and see how writers attempt to make time travel somewhat consistent, relevant and credible. If nothing else, it's good brain food.

Originally Posted by (Q)

Originally Posted by Liongold

Well, to put it simply, it disappears.

Ok, but if that is the case, then we would all simply disappear if a time traveler went into the future and did not return. Or, if he did return, then would the star that went supernova and became a black hole be returned to it's original state?

At least it provides an energy/matter source for the new universe.

This is an interesting discussion for quantum physics. Maybe the meaning of the whole wave function notion is that the system is undergoing every possible time travel scenario in order to finally revise itself into the one most..... I don't know what basis it's decided on..... final state.

Nobody knows why the photon in the double slit experiment chooses one place on the screen to land, as opposed to another, only that sending a large number of them one at a time always fills out a diffraction pattern indicative of a wave interfering with itself. And the big controversy is how it's possible that a single photon is doing this.

I'm curiously watching to see if some aspects of the discussion on this thread, even the BTTF series stuff, might shed some light. (The horrible pun aside)

Ok, but if that is the case, then we would all simply disappear if a time traveler went into the future and did not return. Or, if he did return, then would the star that went supernova and became a black hole be returned to it's original state?

We die. It's a tragic ending, but at least science can claim to know what happens after you die. :wink:

Quite seriously, your premise is indeed correct. We will all vanish; however, only from the viewpoint of the time-traveller. We, however, will simply keep living, only in a changed universe.

One of the things you must understand about time travel is that travelling in time prevents your ever going to the future. Why? Because when you go the past, the past becames your present, and the future is never defined in the present (obviously). You have an infinite number of possible futures, and since you can't exist in all of them at once (this is only from a realist's view; from quantum mechanics, you most certainly can) you can't ever go to the future.

However, quantum mechanics is a very real part of the universe. Let's look at Feynamn's sum-over-histories.

Basically, that says a particle takes all possible paths to a point and the probabililties of it going off on a non-Newtonian path i.e. in straight lines, or gentle curves, cancel out, leaving only a Newtonian path. I have no idea if this is true for a particle going through time, and not space, but assuming that it does so, then you will simply land in a changed universe.

So, if you've changed the universe enough, then certainly the black hole might be prevented from becoming a black hole, at least for a little while longer.

This is an interesting discussion for quantum physics. Maybe the meaning of the whole wave function notion is that the system is undergoing every possible time travel scenario in order to finally revise itself into the one most..... I don't know what basis it's decided on..... final state

Interesting idea. However, I don't think it changes anything we think we know about wave functions, mainly because wave functions are travelling through time: they are going forward in time! So, yes, the wave function must revise itself continually at every instant of time to account for the next moment in time.

Interesting. It wouldn't have occured to me that wave functions really might be doing this all the time (not stealing the credit here; it''s your idea, mate) if you hadn't raised it up.

Nobody knows why the photon in the double slit experiment chooses one place on the screen to land, as opposed to another, only that sending a large number of them one at a time always fills out a diffraction pattern indicative of a wave interfering with itself. And the big controversy is how it's possible that a single photon is doing this

You know, I had a very odd idea about that recently. It's quite frankly absurd, but hear me out.

We know from the uncertainty principle that we may never know the exact measure of either velocity or position without giving up knowledge of either.

So, we can't tell where a proton is if we know its speed, and we can't tell how fast its going if we know where it is.

However, what about the photon? We know its speed; heck, we must always measure its speed to be 300 million metres per second, thanks to special relativity. So the uncertainty principle says that since we know the speed of the photon, then we can never know where it is.

This is ridiculous, because it implies that a photon, after just being emitted, is already at the edge of the universe, which contradicts all known knowledge. I've been trying to see if there is some fault in my reasoning ever since; so far, it has stood intact.

Your thoughts?

My thinking is that, from the photon's perspective, every location it will ever occupy is where it is now. I'm still banging my head against the wall trying to understand relativity, but every description I've heard so far seems to support that.

I mean, say we emit a photon and allow it to travel 300 meters before we measure its location. From our perspective, a microsecond has passed. From the photon's perspective, no time has passed at all.

So... what confuses me is wondering, which relativistic perspective we're supposed to use: ours or the object we're observing?


I think sometimes that photon causality will end up being the key to unlocking time travel, and it worries me because it would mean that, when a device is finally devised, it could be a machine a terrorist can build in his/her garage.

Originally Posted by (Q)

Originally Posted by Janus

Doc likely drained the gas before storing the car. It would have gone bad and fouled the fuel system if left. Doc probably used the gas for something else before Marty came back. (with the storage facilities in 1885 it wouldn't have had a long shelf life.)

Very good assumption. I remember Doc talking about the gasoline issue, but never mentioned the gas from the DeLorean he stored in the mine and what he may have done with it.

You'd think that Doc Brown, with all his technical saavy, would have known how to refine gasoline out of lamp oil, or whiskey, or how to shale coal.

My thinking is that, from the photon's perspective, every location it will ever occupy is where it is now. I'm still banging my head against the wall trying to understand relativity, but every description I've heard so far seems to support that.

Oh, relativity is easy to understand. Basically, the faster you go, the slower time will seem for you, the greater your mass will appear, and the shorter you will be to an observer.

Actually, the photon will think that it's already where it is supposed to be sent. If you emit a photon now, it will think it is both at the end of the universe and just a meter away from your head.

In fact, once you consider it, position is also relative. You define your position with respect to other objects. For example, I can say I am sitting five metres from my TV. I can never specify my position without an outside inference. Likewise, perhaps the photon cannot specify its own position; it doesn't know where it is, because it always moves at the same speed, and the uncertainty principle prevents it from acknowledging its position at the same time.

So the photon's position is always relative to the observer. Space and time may distort to prevent you from reading a speed different from the one it has, but what if the same thing happens to prevent you from ever knowing its position? Could there possibly any way to measure how the universe is affected to prevent us from knowing its position?

What do you think?

This is an interesting possibility.

I'm thinking that knowledge of an object's position, in the Uncertainty Principle, has to be knowledge both of its position and at what time. I mean, knowing when is just as important as knowing where. If the beam of light has no "when", then I guess a "where" is meaningless without a "when".

Suppose we look at an object that's just traveling really close to C, but not at C. Say it travels from low Earth orbit to Proxima Centauri. 4 years pass in our time frame. A few seconds pass in its time frame.

How accurate would its clock have to be in order to say what time it was when it passed Jupiter?

Suppose we look at an object that's just traveling really close to C, but not at C. Say it travels from low Earth orbit to Proxima Centauri. 4 years pass in our time frame. A few seconds pass in its time frame.

How accurate would its clock have to be in order to say what time it was when it passed Jupiter?

It's a brilliant idea. However, I can't really understand by what you mean by "accurate". Do you mean what time it will measure as it passes by Jupiter?

Well, assuming this is so, it will measure this at a slower time because of the object's fast speed. In such a case, the object will think his position is changing more slowly.

However, an observer will think the object is changing position more quickly, because of his faster time.

So a photon thinks its position never changes, while observers at rest will think its everywhere in the universe. Speedy observers will be able to define the photon's position more accurately.

I mean the clock would have to keep time down to something less than nano-seconds in order to avoid giving really ambiguous information.

The passing of Jupiter and Saturn could both appear to have occurred in the same nanosecond, given the speed the object observes itself to be traveling. So, unless it can read trillionths of a second, or smaller, it won't know when, exactly, it passed Jupiter.

If temporal ambiguity equates to spatial ambiguity, this means the object doesn't know, at any given time, whether it's passing Jupiter or Saturn (unless it can keep time in trillionths of a second)

I mean the clock would have to keep time down to something less than nano-seconds in order to avoid giving really ambiguous information

In that case, close to the speed of light, it would have to be extremely accurate to measure the position.

I guess I'm confusing a couple of things. Am I understanding relativity right in this example:

A star ship departs from Earth, on the way to Proxima Centauri traveling close to C, and travels all the way to Proxima Centauri.

On Earth, we perceive the trip to have taken 8 years (4 to arrive, and 4 more for the light from the ship's arrival to reach our telescopes). The crew of the ship perceives the trip to have taken say... 8 minutes.

Does this mean, from their perspective, Jupiter just flew by them in a flash? Or am I wrong about all of this? Did Jupiter appear to be passing them slowly instead?

Does this mean, from their perspective, Jupiter just flew by them in a flash? Or am I wrong about all of this? Did Jupiter appear to be passing them slowly instead?

Your second version is correct. Jupiter does indeed seem, from their perspective, to be moving slowly, thanks to time dilation. At the speed of light, howebver, this effect is more pronounced than anything; for example, a light beam will think Jupiter hasn't moved at all.

This would seem to imply that time is moving fast for them.

I mean, are we talking about what happens when a Meth addict takes Meth, and all of a sudden it seems like the whole world just slowed down around them? (Which means the addict's metabolism just sped up)


Or are we talking about what would happen if you took a massive depressant and everyone around you seemed to be moving really really fast? (Which means your metabolism just slowed down)

Correct. I can say no more.

Basically, if you're going fast, it will appear as if weverything else has slowed down- and vice-versa.

However, after some thought, I've realised that position doers not require a specific time. Time arises only when you consider the concept of speed; for a static, unchanging position, no time is necessary to define it.

Time is a process, after all i.e. it emerges once you consider something it affects. Obviously, time does not affect a motionless particle; it will remain where it is. I do not mean that the object will not feel time pass, tohugh.

To make it easier, think of a three-dimensional Cartesian system. Defining a point i easy, requiring only the three dimensions of space; if the point is moving i.e. its coordinates are changing, then it traces out a line. To define a single point in that line, you require time. Only where velocity is concerned should we use time to define position.

By the way, I apologise for my late reply.

Only where velocity is concerned should we use time to define position.

In your example you still have the Cartesian grid to measure the particle's position relative to. So for movement to occur the particle would have to move relative to where it was a moment ago, but in the real state of things relative movement must always exist. The movement of the particle relative to an observer is then defined on the Cartesian grid with the point of reference (observer) being at the origin of the grid. So time can never stop, unless all points and matter are in one spot (before the big bang).

So, if the past can affect the present, and the present can affect the future, then who is to say that foing into the past would not affect our present looking into our future? Sorry if that didnt make much sense, this topic is a bit...complicated.
I would think that if one could actually travel in space, it would be easier to travel by tesseract. If a fold in time and space could be computed, which it has been, then why is it so hard to imagine a collection of particles diffusing and then rearranging themselves to form molecules-organs-you. The rearrangement would be difficult, but theoretically it seems a lot easier than some time machine warping you back into a new time.
The only problem that I can see with this tesseract idea, is accidently crossing into a different dimension because of the quantum qualifications of space and time.
The future would be affected by the past, which would really be the present if we were in the past, and so if our future was the farther future, than if, theoretically of course, we were able to travel to the past, we would be able to predict the future in whihc we lived. But how would we get back?

sorry if im a bit behind in the conversation, im still mixed up with the earlier conversation

Originally Posted by FringeGirl

But how would we get back?

wait for time to keep on truckin', I guess.

In your example you still have the Cartesian grid to measure the particle's position relative to. So for movement to occur the particle would have to move relative to where it was a moment ago, but in the real state of things relative movement must always exist. The movement of the particle relative to an observer is then defined on the Cartesian grid with the point of reference (observer) being at the origin of the grid. So time can never stop, unless all points and matter are in one spot (before the big bang).

I'm sorry, I don't quite understand what you mean, Kalster. I'm saying that in order to define a single position when an object is moving using a Cartesian grid, you should require a fourth dimension or axis i.e. time.

I'm taking the origin of the grid as the location of the observer, and using that to define the direction of motion.

Sorry if you might be confused; kojax and I began to go off-topic.

So, if the past can affect the present, and the present can affect the future, then who is to say that foing into the past would not affect our present looking into our future? Sorry if that didnt make much sense, this topic is a bit...complicated.

Ah, FringeGirl. Firstly I must say I read some of your earlier posts; you strike me as being very intelligent. That may seem like flattery, but I assure you I am impressed.

Secondly, as to what you say, assuming you can travel back in time, you are quite correct. We would instantly change the future, sometimes in such a way that the very device enabling us to travel is not invented in the future. This is the time-traveller's paradox: to go back into the past might change the future in such a way that you would prevent yourself in ever going back in time. A brilliant paradox.

I would think that if one could actually travel in space,

By space, I assume you mean dimensions? Certainly, we already possess that ability; simply moving your arm in a circle means you have just moved through three dimensions. Actually moving from dimension to dimension is, correctly, impossible. What would happen to our mass, for example? It would vanish in a lower dimension. I am not above believing in a four dimensions; however, I can show that we will never be able to visualise or even see another dimension.

If a fold in time and space could be computed, which it has been, then why is it so hard to imagine a collection of particles diffusing and then rearranging themselves to form molecules-organs-you.

Mainloy because then you're talking about wormholes, which is hard to create. A simply curvature is possible, and is the standard explanation for gravity; the difficulty is in managing to get the space to attach to another segment of space. Most attempts are extremely difficult and require impossible things; take Kip Thorne's attempt, which requires negative energy.

The future would be affected by the past, which would really be the present if we were in the past, and so if our future was the farther future, than if, theoretically of course, we were able to travel to the past, we would be able to predict the future in whihc we lived. But how would we get back?

Actually, no. The very act of going back in time changes the future. After a certain time has elapsed, noticable diferences will start to crop up. You will then no longer be able to acurately predict the future.

As to getting back, I'd say the using the same method we used to go back in time.

Sorry if you might be confused; kojax and I began to go off-topic.

No, I am sorry. I was pretty tired that day and was thinking even less clearly than normal. We are in agreement. Don't mind me! :?

Originally Posted by Liongold

So, if the past can affect the present, and the present can affect the future, then who is to say that foing into the past would not affect our present looking into our future? Sorry if that didnt make much sense, this topic is a bit...complicated.

Ah, FringeGirl. Firstly I must say I read some of your earlier posts; you strike me as being very intelligent. That may seem like flattery, but I assure you I am impressed.

Secondly, as to what you say, assuming you can travel back in time, you are quite correct. We would instantly change the future, sometimes in such a way that the very device enabling us to travel is not invented in the future. This is the time-traveller's paradox: to go back into the past might change the future in such a way that you would prevent yourself in ever going back in time. A brilliant paradox.

I would think that if one could actually travel in space,

By space, I assume you mean dimensions? Certainly, we already possess that ability; simply moving your arm in a circle means you have just moved through three dimensions. Actually moving from dimension to dimension is, correctly, impossible. What would happen to our mass, for example? It would vanish in a lower dimension. I am not above believing in a four dimensions; however, I can show that we will never be able to visualise or even see another dimension.

....

The future would be affected by the past, which would really be the present if we were in the past, and so if our future was the farther future, than if, theoretically of course, we were able to travel to the past, we would be able to predict the future in whihc we lived. But how would we get back?

Actually, no. The very act of going back in time changes the future. After a certain time has elapsed, noticable diferences will start to crop up. You will then no longer be able to acurately predict the future.

As to getting back, I'd say the using the same method we used to go back in time.

Supposing a time machine were built. I don't think it would be like in classic science fiction. What the machine would do is to make the 4th dimension (time) traverse-able in the same way the other 3 dimensions are traverse-able.

I think you'd have to give up one of the other three to get the fourth.

I think the universe has a rule that you can't ever occupy the same 4 dimensional coordinates twice. Indeed, I think the rule is that not even your actions, or even their consequences, are allowed to have conflicting effects on an area of 4 dimensional space-time.

So, there can never be a "time machine" that doesn't include the user as one of its components. The user has to participate in the process of sending themself back in time by obeying the paradox rule, and being sure not to affect any part of time they, themselves, have experienced.

I think that's consistent with the Quantum Mechanics theory of the observer affecting what is observed.

Basically, wave function collapse (which is considered to happen at the moment of observation), is only relative to the observer's perspective. If you've observed something (caused the wave function to collapse for you), that doesn't necessarily mean that the wave function has collapsed for me (or any other observer).

I can't go back in time into my own past, because my own past consists exclusively of wave functions that have collapsed (from my perspective), but I could back in time to someone else's past (so long as their past and my past don't coincide).

Suppose, for example, that it turns out there are Martians living on Mars, and, without knowing anything about them, I go back a thousand years into the past, and visit Mars. I could freely interact with their history, because it consists almost exclusively of wave functions that (from my perspective) have yet to collapse.

I think you'd have to give up one of the other three to get the fourth.

I'm sorry, kojax, but that is ridiculous. If what you sday is correct, in order to go back in time, you'd have to give up height, length or breadth. That doesn't even make sense.

I think the universe has a rule that you can't ever occupy the same 4 dimensional coordinates twice. Indeed, I think the rule is that not even your actions, or even their consequences, are allowed to have conflicting effects on an area of 4 dimensional space-time.

The Time Traveller Protection Hypothesis. Yes, Stephen Hawking thinks the same way.

Suppose, for example, that it turns out there are Martians living on Mars, and, without knowing anything about them, I go back a thousand years into the past, and visit Mars. I could freely interact with their history, because it consists almost exclusively of wave functions that (from my perspective) have yet to collapse.

Good point. But what about back in the future? From their perspective, the past is already determined. The Martians of the future will remember who you are. What about your own wave function? By collapsing the wave function of the martians and interfering, you collapse your own wave function, thereby setting in motion the ridulcous notion of two copies of you being in two places simultaneously.

Originally Posted by Liongold

I think you'd have to give up one of the other three to get the fourth.

I'm sorry, kojax, but that is ridiculous. If what you sday is correct, in order to go back in time, you'd have to give up height, length or breadth. That doesn't even make sense.

I mean that you sacrifice a degree of control over one or more of the other three in order to gain control over the fourth (which you normally don't have any control over).

What I'm trying to get at is that, to a time traveler moving around in a previous time to their own, certain areas of 3D space are totally inaccessible to them. They just can't go there. They can't even interact indirectly through a proxy. It's plain off limits.

I think the universe has a rule that you can't ever occupy the same 4 dimensional coordinates twice. Indeed, I think the rule is that not even your actions, or even their consequences, are allowed to have conflicting effects on an area of 4 dimensional space-time.

The Time Traveller Protection Hypothesis. Yes, Stephen Hawking thinks the same way.

Suppose, for example, that it turns out there are Martians living on Mars, and, without knowing anything about them, I go back a thousand years into the past, and visit Mars. I could freely interact with their history, because it consists almost exclusively of wave functions that (from my perspective) have yet to collapse.

Good point. But what about back in the future? From their perspective, the past is already determined. The Martians of the future will remember who you are. What about your own wave function? By collapsing the wave function of the martians and interfering, you collapse your own wave function, thereby setting in motion the ridulcous notion of two copies of you being in two places simultaneously.

If the Martians were going to have technology that might enable them to find me on Earth during my earlier life and disrupt my own history, then you're right. That wouldn't be a good example.

The fact of them not having bothered me during my early life is a collapsed wave function from my own perspective, and therefore lies outside the limits of what I can change.

Basically, I can change anything that won't impact my own past.

Let's suppose instead that I find the ruins of a lost Martian civilization, one that never made it into space, and died off a few thousand years ago. I could go back in time and interact with them all I wanted, so long as I didn't share any technology with them, nor prevent their extinction.

What I'm trying to get at is that, to a time traveler moving around in a previous time to their own, certain areas of 3D space are totally inaccessible to them. They just can't go there. They can't even interact indirectly through a proxy. It's plain off limits

Ah, now I understand. It's a bit like being confined to wherever you have not walked before, am I right?

Now, that is something. However, consider this: you may not go there, absolutely. What about looking at it? If you were to look at an electron, that by definition is interacting with the electron, because in order to see it, you would exchange a photon with it, thereby disrupting it. You have changed the future, and since, by exchanging a photon, making a single electron in your way recoil in a way that previously did not exist, you have interfered with your own past.

Now let's take what you say, that you can't interfere with your own past. All right, so all I can do is go to a place that I have never seen before, but heard of. Here's the rub: simultaneously, you will be in two places at once, one going through his own life, or the events in the past, and the time traveller you, who's gone to a place he's never been before. You have a memory of the past, which exists as collapsed wave functions. However, by going back into the past, every memory of yours about the future instantly vanishes (you can't remember a non-existant wave function).

Further, what about the chaos effect? The mere act of going back into the past will eventually change the world so much that your past will be affected. Say you were to go to a war-stricken region. You no longer remember the future, confused and disoriented, you are taken hostage. Your country finds out and invades. As a result a new war begins that did not exist before. Naturally, your own past will be affected, leading back to the paradox.

Basically, as the act of going into the past changes the past, you will, no matter how hard you try, eventually interfere with your own past, thereby coming back again to the time-traveller paradox.

Well, according to my theory, whatever you do while in the past, is what happened the first time. I don't think the universe has multiple versions of history.

It will just so happen, unbelievably, that those electrons you disrupt actually had been disrupted when you saw that moment the first time. You didn't know this (which is why those specific electrons were not a part of the collapsed wave function).

It's possible for parts of a moment in your history to collapse, whilst other parts don't collapse.

There's a non-zero chance that nobody in the history of the world has ever died. There was always an invisible time traveler standing nearby ready to harvest data, or switch the brain in their skull for a fake. That possibility would not be inconsistent with our known version of history. Or... another way to say that would be to suggest that that possibility is not yet a collapsed wave function.

Well, according to my theory, whatever you do while in the past, is what happened the first time. I don't think the universe has multiple versions of history.

Quantum indeterminancy, my dear friend; by going back into the past, you do not guarantee that the past will be exactly as it was.

What you are now proposing is an alternate version of the parallel universes theory.

The trick with my theory is that you are *already* guaranteed, absolutely to infinity that *nothing* about the past will ever change. However, the guarantee is only that *your* past will never change.

Going back to the ancient martian civilization. Until you interact with them at some level of detail, from your perspective, they are something that hasn't happened yet.

I'm suggesting something somewhat similar to relativity. There is no absolute standard of time. It's all relative to the observer. There's no absolute clock in the universe that would agree with you (nor disagree) that the martian civilization was genuinely 1000 years ago instead of today or tomorrow.

You, the observer, are the only thing that creates a continuum. If you see the ruins of the martian civilization, then your options on how to interact with them are limited to those things which will not change the ruins you've seen. You will fail in any attempt to interact with them in a way that would change it.


Your experience of the universe is like a book, where you can determine any part of the story you haven't already read. It doesn't matter what page it's on, only whether you've read it.

However, the guarantee is only that *your* past will never change.

I think I already answered as to why this has a flaw in it. Nevertheless, let me explain again.

Imagine that you go back in time. Now, taking your idea to be true, you cannot interfere with your past; everything else, however, is changeable. Yet the butterfly effect guarantees that even if you went back into the past you would change your past, even by some almost negligible amount, contradicting your own idea.

However, let's take your idea that, to quote:

It will just so happen, unbelievably, that those electrons you disrupt actually had been disrupted when you saw that moment the first time.

But if they had been disrupted, then you would have appeared back in your past, leading again to the same sequence of events you followed that brought you back to the past. In effect, you form a cycle of events that lack the potential to change.

The reason this is inconceivable is that that would mean that you can define a particle at any given moment when you go back in time. You could tell us accurately, where it was going to be or what speed it was going, after time-travelling back to the past. This violates the uncertainty principle; even in time-travel, the laws of the universe must be upheld.

Originally Posted by Liongold

However, the guarantee is only that *your* past will never change.

I think I already answered as to why this has a flaw in it. Nevertheless, let me explain again.

Imagine that you go back in time. Now, taking your idea to be true, you cannot interfere with your past; everything else, however, is changeable. Yet the butterfly effect guarantees that even if you went back into the past you would change your past, even by some almost negligible amount, contradicting your own idea.

However, let's take your idea that, to quote:

It will just so happen, unbelievably, that those electrons you disrupt actually had been disrupted when you saw that moment the first time.

But if they had been disrupted, then you would have appeared back in your past, leading again to the same sequence of events you followed that brought you back to the past. In effect, you form a cycle of events that lack the potential to change.

Here you're using this word "disrupted". "Disrupted" relative to what? The universe never had an alternative version of history without the time traveler in it. There is no original state from which you are changing things.

What you're doing is conforming, perfectly, to that original state that was always there.

And, yes, it is an unchangeable loop. That's actually what time travel requires. Every loop must be unchangeable. If the loop had the potential to lead to a change, that would be a contradiction.

You can't go back in time and kill your grandfather, then have that lead to a set of consequences where you're a different person (different grandfather, most likely). You could maybe go back in time and cause your grandfather and grandmother to meet, however. (There's no contradiction in creating a set of events that lead to the present world you know).

The thing of it is... it's not impossible that a time traveling version of yourself actually *did* cause them to meet.

The reason this is inconceivable is that that would mean that you can define a particle at any given moment when you go back in time. You could tell us accurately, where it was going to be or what speed it was going, after time-travelling back to the past. This violates the uncertainty principle; even in time-travel, the laws of the universe must be upheld.

As far as the uncertainty principle: There's no guarantee that the uncertainty principle holds. Even if it does (and it probably does), I don't see how your time travel concern violates it anyway.

If I measure the electron's position 10 minutes in the future, that won't tell me, with any certainty, where it was 10 minutes before I measured it (and therefore won't tell me its speed).

Or maybe you're talking about another aspect? The randomness of trying to predict where any given photon will land in the double slit experiment?

What you're doing is conforming, perfectly, to that original state that was always there.

And, yes, it is an unchangeable loop. That's actually what time travel requires. Every loop must be unchangeable. If the loop had the potential to lead to a change, that would be a contradiction.

So, if we were to go back in time, we can easily anticipate what will happen in the future, without any fear of changing it. A good idea, but it still doesn't make sense. For one thing, it doesn't take into account what we may choose to do. To be more specific, it refuses to acknowledge free will.

I might have witnessed, for example, the horrific death of my friend. By going back in time, I attempt to save him; an event which I should have seen, let us suppose, which I did not, thereby changing the past. Yet according to you, such an event can never happen. However, assuming we still have the illusion of free will, how do you then propose to fight this one?

(There's no contradiction in creating a set of events that lead to the present world you know).

That depends. Your grandmother and grandfather might remember an alternate set of events than the one they would have remembered if you'd never gone back. Even microscopic changes cannot be accepted in the future you travelled back from.

As far as the uncertainty principle: There's no guarantee that the uncertainty principle holds. Even if it does (and it probably does), I don't see how your time travel concern violates it anyway.

Alright, let us assume it does. Basically, what happens then is:

Suppose you measure the position of a particle two minutes ago. You then go back into the past, and again measure the prticle, but this time you measure its velocity. Hence, you know now both its position and velocity, shattering the principle into pieces.

However, I will admit this version has flaws. By going back in time, you will meet your past self, and the situation will dissolve into the "two physicists measuring both velocity and position of a particle" argument of Einstein against the uncertainty principle. I do not remember how, but I believe it was eventually overcome.

So let's try something else. You measure the velocity of the particle and then go back in time. You tell your earlier self, its velocity and instruct him to measure its position, which he then tells you. He then goes back in time to repeat the same, and you are now the sole possessor of the information of botha particle's position and velocity.

See it now?

If I measure the electron's position 10 minutes in the future, that won't tell me, with any certainty, where it was 10 minutes before I measured it (and therefore won't tell me its speed).

We're assuming here that everything that happened in the past stays that way in the future. If we don't, however, that would mean that your idea of an unchanging loop in time is flawed: it allows changes to happen.

Originally Posted by Liongold

What you're doing is conforming, perfectly, to that original state that was always there.

And, yes, it is an unchangeable loop. That's actually what time travel requires. Every loop must be unchangeable. If the loop had the potential to lead to a change, that would be a contradiction.

So, if we were to go back in time, we can easily anticipate what will happen in the future, without any fear of changing it. A good idea, but it still doesn't make sense. For one thing, it doesn't take into account what we may choose to do. To be more specific, it refuses to acknowledge free will.

I might have witnessed, for example, the horrific death of my friend. By going back in time, I attempt to save him; an event which I should have seen, let us suppose, which I did not, thereby changing the past. Yet according to you, such an event can never happen. However, assuming we still have the illusion of free will, how do you then propose to fight this one?

I don't intend to fight this one. I'm sorry that it may be insulting to the human ego, but a time traveler cannot simultaneously hold on to their free will (at least not all of it) and travel backwards in time.

As far as this issue is concerned, there is no free will, and you'd quickly lose the illusion if you started using a time machine.

(There's no contradiction in creating a set of events that lead to the present world you know).

That depends. Your grandmother and grandfather might remember an alternate set of events than the one they would have remembered if you'd never gone back. Even microscopic changes cannot be accepted in the future you travelled back from.

If they remember a different set of events, and have made you aware of it somehow, then that area of space/time is off limits to you.

If they remember a different set of events, and have not made you aware of it, then it will randomly turn out that you just so happen not to contradict what they remember.

As far as the uncertainty principle: There's no guarantee that the uncertainty principle holds. Even if it does (and it probably does), I don't see how your time travel concern violates it anyway.

Alright, let us assume it does. Basically, what happens then is:

Suppose you measure the position of a particle two minutes ago. You then go back into the past, and again measure the prticle, but this time you measure its velocity. Hence, you know now both its position and velocity, shattering the principle into pieces.

However, I will admit this version has flaws. By going back in time, you will meet your past self, and the situation will dissolve into the "two physicists measuring both velocity and position of a particle" argument of Einstein against the uncertainty principle. I do not remember how, but I believe it was eventually overcome.

So let's try something else. You measure the velocity of the particle and then go back in time. You tell your earlier self, its velocity and instruct him to measure its position, which he then tells you. He then goes back in time to repeat the same, and you are now the sole possessor of the information of botha particle's position and velocity.

See it now?

If I measure the electron's position 10 minutes in the future, that won't tell me, with any certainty, where it was 10 minutes before I measured it (and therefore won't tell me its speed).

We're assuming here that everything that happened in the past stays that way in the future. If we don't, however, that would mean that your idea of an unchanging loop in time is flawed: it allows changes to happen.

Well of course you can't change the past under my theory, so you just plain couldn't send the information backwards if they were going to do something different in measuring the electron because of you.

If I am aware that my previous self had not measured the position of the electron, and try to send a message back in time telling myself to measure it, .... that's exactly the kind of contradiction you can't possibly make.

If you mean I measure the velocity now, and arrange for my previous self to measure the position 10 minutes ago, well, that won't tell me what the particle's velocity was prior to having its position measured.

I could do that without time travel. Measure a particle's position right now, wait 10 minutes, then measure its velocity and compile the data together.

I don't intend to fight this one. I'm sorry that it may be insulting to the human ego, but a time traveler cannot simultaneously hold on to their free will (at least not all of it) and travel backwards in time.

As far as this issue is concerned, there is no free will, and you'd quickly lose the illusion if you started using a time machine.

In which case determinancy rules. Einstein celebrates in his grave.

Exactly how do you propose free will is controlled? I'd like to hold on to that illusion, you see. Exactly what prevents an electron from choosing another part than the one it chose long ago?

From a wave function viewpoint, we must remember that the wave-function we collapse is still alive and operational in the past. A paradox here arises: we know what the particle will do, even though the wave function has not yet been collapsed.

How do you propose we escape this paradox?

If they remember a different set of events, and have made you aware of it somehow, then that area of space/time is off limits to you.

Question: how would that happen? What would actually make a region of space-time inaccessible for you? Impenetrable walls? Invisible barriers?

That is a hole in your theory, kojax.

If you mean I measure the velocity now, and arrange for my previous self to measure the position 10 minutes ago, well, that won't tell me what the particle's velocity was prior to having its position measured.

I could do that without time travel. Measure a particle's position right now, wait 10 minutes, then measure its velocity and compile the data together.

No, no. I'm saying that you can measure the position now, and convey that information to your past self, who then chooses to measure the velocity, violating the uncertainty principle.

Well of course you can't change the past under my theory, so you just plain couldn't send the information backwards if they were going to do something different in measuring the electron because of you.

All right. What if you chose to actually go back in the past, in effect destroying your future, and stayed there? Consequently, all actions on terra firma will no longer affect you. I'm assuming here, of course, that now your future consists only of possibilities; as, according to you, the future will still somehow guarantee that you exist, then even if you went and violated the uncertainty principle, you would still, somehow, find a way to survive.

Another interesting point: if, by going back into the past, you destroy your future, exactly how can you exist without your parents ever having conceived you? It would be a relentless paradox.

K, that last post of mine was really long and droning, so I deleted it. The short version:


The trick is that time machines don't work in a time-forward way. Even their controls don't work like that. I can decide right now that I will push the button tomorrow, to tell the machine to send me somewhere today, and I get sent there right now, even though I haven't pushed the button yet.

Now, if I decided after getting sent back in time that I don't want to push the button tomorrow (or at any time in the future when I could push it to accommodate having been sent back in time today), we have a paradox.

However, the machine is a prophet. It forsees the paradox. It not only foresees the contingency, but responds, and I don't get sent back in time today.

The time machine is basically a computer that is able to see a large amount of time as being just one second. That means that everything I will do over that period of time is data it has in its mind before it acts. It can't create paradoxes, because it only acts after it has confirmed (with absolute certainty) that there won't be any.

And, I'm being metaphorical in a sense. It's not that the machine is intelligent, per se, but the system of phenomena it manipulates approximates to being a very intelligent machine. You can't fool that machine. Nothing will fool it.

The trick is that time machines don't work in a time-forward way. Even their controls don't work like that. I can decide right now that I will push the button tomorrow, to tell the machine to send me somewhere today, and I get sent there right now, even though I haven't pushed the button yet.

Wow. That's quite a machine; chock full of features that haven't been invented yet. :wink:

Still, let's work with this.

Now, if I decided after getting sent back in time that I don't want to push the button tomorrow (or at any time in the future when I could push it to accommodate having been sent back in time today), we have a paradox.

However, the machine is a prophet. It forsees the paradox. It not only foresees the contingency, but responds, and I don't get sent back in time today.

Now, of course, we're assuming that it is possible to see the future from the past.

However, I highly doubt that is feasible. For one thing, by trying to look into the future you automatically change it, again thanks to the uncertainty principle. That means that everytime your machine looks at something in the future, it gets a different scenario each time.

Say it looks at the future a second ago. Then it looks at the future now. Each time, it will encounter different futures than the one it saw a mere second ago, and that's with not taking your actions in account.

See the problem here? The machine won't be able to tell the future that will really happen; it'll start responding to situations tht won't even exist.

And, I'm being metaphorical in a sense. It's not that the machine is intelligent, per se, but the system of phenomena it manipulates approximates to being a very intelligent machine. You can't fool that machine. Nothing will fool it.

Considering that the most intelligent species (homo sapiens) in the world can be fooled time and time again, I find it hard to believe such a machine would not be intelligent. Certainly, to prevent being fooled it would have to be vastly more intelligent than, say, Gary Kasparov.