Thread: moving faster than light violates causality?

I was wondering why moving faster than light violates causality. I understand Relativity of Simultaneity(or think I do).

Example why it makes my brain hurt to think it does:

Person A sends message to Person B as some distance d at the speed of light at time t0. Person B receives the message at time t0 + cd. He then sends message back at t0 +cd. Person A receives message at time t0 + cd + cd.

Person A sends message to Person B at same distance d at speed faster than light x at time t0. B receives message at time t0 + xd and so on.

This is with messages though. I know that if you move faster than light, then the record of your existence can cause someone to see you in two places at once as they will see the record of your location at A and receive the record of your new location B at the same time, but I'm just confused why that would change causality. Does moving faster then light make you move back in time or something or am I missing something completely?

Thank you very much.

Does moving faster then light make you move back in time or something or am I missing something completely?

Since nothing can move faster than light, your question is moot.

Originally Posted by AlexG

Does moving faster then light make you move back in time or something or am I missing something completely?

Since nothing can move faster than light, your question is moot.

Well, what about hypothetical Tachyons. While only hypothetical, they don't violate SR in principle.

Originally Posted by AlexG

Does moving faster then light make you move back in time or something or am I missing something completely?

Since nothing can move faster than light, your question is moot.

Unhelpful. Rather than dismissing a question off hand because it conflicts with your interpretation of the facts, evaluate it on its own merits. It is called applying the skills of critical thinking. I have some extra reading for you, Alex G. Critical Thinking

Originally Posted by KALSTER

Originally Posted by AlexG

Does moving faster then light make you move back in time or something or am I missing something completely?

Since nothing can move faster than light, your question is moot.

Well, what about hypothetical Tachyons. While only hypothetical, they don't violate SR in principle.

Why do tachyons not violate special relativity?

Originally Posted by Physics_

Originally Posted by KALSTER

Originally Posted by AlexG

Does moving faster then light make you move back in time or something or am I missing something completely?

Since nothing can move faster than light, your question is moot.

Well, what about hypothetical Tachyons. While only hypothetical, they don't violate SR in principle.

Why do tachyons not violate special relativity?

The best I can do is provide a link to Wiki: Tachyon - Wikipedia, the free encyclopedia

Sorry to answer your question with a link.

You are basically asking, 'if we could break the laws of physics, what would happen'. If the universe did not operate the way it operates, anything could happen. It's not MY interpretation of the facts, it's the laws of physics.

Originally Posted by Physics_

This is with messages though. I know that if you move faster than light, then the record of your existence can cause someone to see you in two places at once as they will see the record of your location at A and receive the record of your new location B at the same time, but I'm just confused why that would change causality. Does moving faster then light make you move back in time or something or am I missing something completely?

Causality:
Imagine things like this: you look up to the dark night sky and you see a star blink (lets say it as point A), and then a moment later you see another star blink at a point on the right (lets say point B), assume this 2 point are saperated by a thumb size region of space. Would point A caused point B to blink, like a cascade? -If light took more time to travel from point A to point B (in space) than the time required for the point to blink consequtively: then point A couldn't possible caused point B to blink, that's all there is (because we know nothing can travel faster than light).

When something can travel faster than light, then that is when this principle is violated.
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Simultaneity:
However, the sequence of those blink doesn't necessarily true in all world. In one world you might see them as simultaneously, and in another you might see them in reverse, and yet in another you see them like ours (but maybe quicker or slower). What this prove is that: simultaneity is not true for all point of view, one might see them as faster/slower or in reverse sequence. (This is the principle of simultaneity in relativity).

But how can this become soo bizzare, at least for simultaneity? This is because of lenght contraction and time dilation that happen in different frame of reference. Frame that moves in different velocity relative to others has contracted sense of lenght or time *so that they view the same speed of light*, as a result the physics are same in different point of view caused by this contraction and skewing (if without this time&lenght inconsistency: physic will breakdown. For example: important constant in electromagnetism will change to compensate for different speed of light, and different electromagnetism mean different chemical properties and possibly different universe altogether.)
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P/S: one might use principle of causality to test if one thing could be caused by another thing since principle of simultaneity clearly state that nothing of in sequence or simultaneous can be true in all point of view.

Let me try to answer the question in the OP as I understand it (or as it was explained to me, in other words!)

To show how FTL violates causality requires FTL communication combined with good old relative motion, because the key to this is in the relativity of simultaneity. What is also required is the understanding that this needs to be considered from the view of each of the frames of reference involved, rather than from a "god's eye view" as it were.

Let me start by stating that we have to relax the rules (forgetting things like having to create tachyons, or even better, finding some way to exploit quantum entanglement hitherto thought impossible for instant communications) in order to consider this, but we also have to apply the principles of SR to both the frames involved.

This requires either very large distances and moderate relative speeds, or if the distances involved are small it requires very large relative speeds.

It is easiest to explain using "instantaneous" communication (anything FTL will do, but instantaneous is easiest), with the large distances, so consider a star 10 light-years away, that is moving away from us at around 30 km/s. If you work out the simultaneity shift involved, from our frame of reference, "now" is shifting backwards in that distant Solar System, relative to us. If it were moving towards us, "now" would be shifting forwards. You should understand this if you understand the relativity of simultaneity and how it works from a given frame of reference.

So, now we concoct a message that contains the instructions on how to build an instantaneous communicator in only a few minutes! We use our own communicator to send the message to that distant star, and it reaches that star "now", from our frame of reference. The message contains the stock prices as they are right now, with a request to send them straight back as soon as the communicator is built at the other end.

Our alien friends receive a message telling them how to build an instantaneous communicator in a few minutes, and asking them to send the stock prices back to Earth. They build it and are amazed at the new technology and are more than happy to send that message to our Solar System, which is 10 light-years away and receding at 30 km/s relative to them.

But in the few minutes it has taken them to build the communicator, the Earth has been receding from them, and "now" on Earth is moving backwards, relative to them. They send the message back, instantaneously.

On Earth, we receive a message with the stock prices, four hours before we sent them.

Originally Posted by SpeedFreek

So, now we concoct a message that contains the instructions on how to build an instantaneous communicator in only a few minutes! We use our own communicator to send the message to that distant star, and it reaches that star "now", from our frame of reference. The message contains the stock prices as they are right now, with a request to send them straight back as soon as the communicator is built at the other end.

Our alien friends receive a message telling them how to build an instantaneous communicator in a few minutes, and asking them to send the stock prices back to Earth. They build it and are amazed at the new technology and are more than happy to send that message to our Solar System, which is 10 light-years away and receding at 30 km/s relative to them.

But in the few minutes it has taken them to build the communicator, the Earth has been receding from them, and "now" on Earth is moving backwards, relative to them. They send the message back, instantaneously.

On Earth, we receive a message with the stock prices, four hours before we sent them.

I don't get it. We sent a message to an alien planet that recede away from us at great speed, but then the message reached them and we got back our message 4 hour before we sent them??? (like in the past???) -If they could send an instantenous message won't their message arrive to us as soon as our message was read by them?

Originally Posted by msafwan

I don't get it. We sent a message to an alien planet that recede away from us *at great speed*, but then the message reached them and we got back our message 4 hour before we sent them??? (like in the past???) -If they could send an instantenous message won't their message arrive to us as soon as our message was read by them?

You are only thinking of the relativity of simultaneity in one direction here, but it is a relationship that is symmetrical between the frames involved, which is why FTL communications can violate causality and thus makes no sense.

Once the information is at the "other end", you need to consider it from the frame of reference at the other end. Any time lag between their receiving and sending the message back will mean they are sending the message back to our past, from their frame of reference, if we are receding from them.

Originally Posted by SpeedFreek

Once the information is at the "other end", you need to consider it from the frame of reference at the other end. Any time lag between their receiving and sending the message back will mean they are sending the message back to our past, from their frame of reference, if we are receding from them.

Aren't both planet are moving away from each other? So Earth POV sees a reply as soon as it reached the other planet, while other planets POV has to wait for 4 hours before they sees Earth had received their reply. -We don't receive reply 4 hour in the past.

According to prof Brian Greene in his famous book "The Fabric of the Cosmos", the relativity of simultaneity tells us that as you run backwards and forwards in your living room, in a very distant galaxy in your direction of motion, time is jumping backwards and forwards by centuries! The principle of relativity also tells us that the same would be true, the other way round.

Think about the implications of this.

Originally Posted by SpeedFreek

According to prof Brian Greene in his famous book "The Fabric of the Cosmos", the relativity of simultaneity tells us that as you run backwards and forwards in your living room, in a very distant galaxy in your direction of motion, time is jumping backwards and forwards by centuries! The principle of relativity also tells us that the same would be true, the other way round.

How can it be so?

I just went over the information that you all have given me and it clicked for me. The reason moving faster than light violates causality is because an observer in a different reference frame can see an effect before a cause relative to another reference frame. This would allow you to react before the cause happened, causing a paradoxical effect.

....its hard to put in words this without making your brain call you crazy. Reading this: Understanding Special Relativity combined with earlier explanations made it for me. Thank you all!

I strongly disagree that faster-than-light (FTL) communication can create causality-violation or somekind of a sci-fi "time-travel" in a sense of "going-back-in-time". For one: we are discussing on an object that is light-year (or light-minute) apart such as stars and planet which are causally saperated from each other by the such amount of time (years or minute) in which case FTL only function is to decrease such distant to seconds or to make it instant. -It can never give you information on something that hasn't yet happen like proposed in the article in the link...

The train experiment did show us that when you move torward point A (eg: the right) you see point A happen first, but when you move toward point B (eg: left) you see point B happen first, and if you stay stationary with the train: you see both point A & B happen simultaneously. When a person on right send an FTL containing info on A to person on left, then person on left basically see A&B simultaneously like the stationary person does. -The significance of A&B happening simultaneusly is such that A is never caused by B and B is never caused by A such that they can happen in any order.

This is not "time-travel".

The definition of "time-travel" is that to see an event B before it was triggered by A. Such thing cannot be fixed with FTL. -Even if you move torward to point B (eg: to the left) in the train: you will still see A first before seeing B IF B is triggered by point A, (even stationary viewer will see A then B in such case). (the trigger has a maximum speed at speed-of-light)

When something hit you the speed of light, it means that he knew your relative speed before he came here, and that in itself has been transfer of information faster then light

Originally Posted by AlexG

You are basically asking, 'if we could break the laws of physics, what would happen'. If the universe did not operate the way it operates, anything could happen. It's not MY interpretation of the facts, it's the laws of physics.

None of the laws of physics can be broken, but sometimes they can be circumvented. Gravity is a fine example. I just flew on a plane last Sunday, but I still believe gravity is a law.

I just flew on a plane last Sunday, but I still believe gravity is a law.

Sure, and if I pick up a pencil from my desk, I'm 'circumventing' gravity? Perhaps there are simply other forces at play?

Originally Posted by Physics_

I just went over the information that you all have given me and it clicked for me. The reason moving faster than light violates causality is because an observe in a different reference frame an see an effect before a cause relative to another reference frame. This would allow you to react before the cause happened, causing a paradoxical effect.

Yes, that's right.

Originally Posted by msafwan

Originally Posted by SpeedFreek

According to prof Brian Greene in his famous book "The Fabric of the Cosmos", the relativity of simultaneity tells us that as you run backwards and forwards in your living room, in a very distant galaxy in your direction of motion, time is jumping backwards and forwards by centuries! The principle of relativity also tells us that the same would be true, the other way round.

How can it be so?

Because simultaneity is not absolute, it is relative.

You can extend your notion of "now" to a distant frame of reference, using Einstein synchronisation and the Lorentz contraction, and you wil get the correct result, relative to your frame, but it might not be the same result as is found from the other frame of reference, when there is motion between the frames involved. This is shown in the famous train and embankment thought experiment.

And the same is true the other way around!

When relative motion is involved, whilst one frame might calculate the distance between themselves and two events, and calculate the light travel time to those events, and calculate that those events were simultaneous relative to that frame, another frame who also knows their distance to the events and the light travel time will calculate those events to have occurred at different times, rather than simultaneously.

Note, this is just not about what you see when the light from an event reaches you, it is about when you calculate those events to have occurred relative to yourself, after calculating the distance to those events and time the light from them took to reach you.

So, one oberver can find two events to have been simultaneous, whilst another can find the same two events to have happened at different times.

Now, when it comes to causality, if one event causes another, different observers may find a different interval betwen those events, but nobody in the universe will find the events to have occurred in the wrong order, as that would violate causality.

But where FTL is concerned, we can have observers who DO find events to have occurred in the wrong order, which is why FTL movement or communication is thought to be impossible.

Originally Posted by kojax

Originally Posted by AlexG

You are basically asking, 'if we could break the laws of physics, what would happen'. If the universe did not operate the way it operates, anything could happen. It's not MY interpretation of the facts, it's the laws of physics.

None of the laws of physics can be broken, but sometimes they can be circumvented. Gravity is a fine example. I just flew on a plane last Sunday, but I still believe gravity is a law.

I don't realy enjoy calling people out on bad examples, but this is one you should probably rethink. Planes don't fly via anti-gravity just yet. Jumping, for instance, is not a loophole in gravity theory.

Originally Posted by SpeedFreek

When relative motion is involved, whilst one frame might calculate the distance between themselves and two events, and calculate the light travel time to those events, and calculate that those events were simultaneous relative to that frame, another frame who also knows their distance to the events and the light travel time will calculate those events to have occurred at different times, rather than simultaneously.

Note, this is just not about what you see when the light from an event reaches you, it is about when you calculate those events to have occurred relative to yourself, after calculating the distance to those events and time the light from them took to reach you.

So, one oberver can find two events to have been simultaneous, whilst another can find the same two events to have happened at different times.

Now, when it comes to causality, if one event causes another, different observers may find a different interval betwen those events, but nobody in the universe will find the events to have occurred in the wrong order, as that would violate causality.

But where FTL is concerned, we can have observers who DO find events to have occurred in the wrong order, which is why FTL movement or communication is thought to be impossible.

Okay, so when we see a supernova before it expected to happen then it doesn't mean we have looked into the future, its just mean we seen an FTL information. This didn't violate causality at all. It just mean that the supernova had traveled in space but not in time, thus giving us instant information.

If this is what we call "time-travel" then it is pointless... it is just a thought game on an abstract concept of space-time.

Originally Posted by msafwan

Okay, so when we see a supernova before it expected to happen then it doesn't mean we have looked into the future, its just mean we seen an FTL information. This didn't violate causality at all. It just mean that the supernova had traveled in space but not in time, thus giving us instant information.

If this is what we call "time-travel" then it is pointless... it is just a thought game on an abstract concept of space-time.

I am afraid I have absolutely no idea what you are talking about here. What do you mean by "seeing a supernova before it was expected to happen"? We only know a supernova has occurred once we have seen it. The light from a supernova cannot reach us faster than light, of course.

I think my wording might have confused you. What I was trying to say is that this is not about light-delay, it is about what you calculate to have happened once you subtract out the light-delay. So, when you see the light from a distant event, and measure the distance to that event, and know the speed that light travels, you can calculate when that event actually happened, relative to you. But someone else in motion relative to you may calculate it to have happened at a different time, relative to them.

What Special Relativity tells us is that there is no absolute space, no absolute time and no absolute way that "now" can be synchronised between frames of reference in relative motion. There are no problems here as long as nothing travels through space faster than light. We have no evidence that anything ever travels through space faster than light, so there is no problem.

If something could travel faster than light, it would cause problems with causality, as I have explained above, but it seems you didn't understand me.

Do you actually understand the implications of the train and embankment thought experiment? Just ask if you want me to take you through it.

Okay, what do you think about being stationary with respect to train, and being stationary with respect to the ground mean to causality? Does event A (left) is causally related to event B (on right) in any frame-of-reference? (what does causality mean anyway?). -And, what happen to causality when both observer communicate with FTL?

Causality gets different meanings when faster communication then speed of light, you have to check things like strength and sometimes gets things transparent

Originally Posted by msafwan

Okay, what do you think about being stationary with respect to train, and being stationary with respect to the ground mean to causality? Does event A (left) is causally related to event B (on right) in any frame-of-reference? (what does causality mean anyway?). -And, what happen to causality when both observer communicate with FTL?

Let's stick with the train example:

We start with an observer M standing along the tracks and an observer m sitting in the train the train is moving to the right relative to the tracks. We start when M and m are next to each other: Also, in the frame of the tracks a & A and B&b are next to each other:

a............m............b
A______M______B

Now assume that lightning strikes Aa and Bb

A sends a instantaneous signal to M, telling him of the lightning strike. ( and incidentally that a passenger at 'a' was killed.)
M shares this info with M (as they are right next to each other). and A sends and instantaneous message back to 'a' to confirm.

However, this is where it gets tricky. In the last diagram, we showed how everything lines up according to the railway. However, remember, the train is moving relative to the railway and thus is length contracted. Thus is it the "length contracted" distance between 'a' and 'b' that just fits between A and B in the railway frame.

In the train frame, the train is its proper length and it is the railway that is length contracted. Ergo, from the train frame, things are like this when m and M are next to each other.

a___________m___________b
................A......M......B...............

And from the train's frame A, M and B are moving to the left.

But this means that a and A have yet to align, and since the Lightning strike occurs when A and 'a' are aligned, the lightning has not struck 'a' yet. If m sends an instantaneous message to 'a', 'a' will receive it before 'a' is hit by lightning. The person at 'a' can now move before the strike and not be killed. Which means that A will no longer send that part of the message when the lightning strike does occur... You can see how fast things can become higgidly-piggidly .

Even though I accept the standard model, I still think that things can move faster than light, but it is practical in many ways to use light as we will not ever be able to see anything move faster without equipment, besides the fact it will take an age for our brains to process it.

There are strong mathematics in favour of it all, solve that and you will be a happy person...maybe then causality will be moved up a notch, or removed from thought altogether...who knows ey...

Originally Posted by Janus

We start with an observer M standing along the tracks and an observer m sitting in the train the train is moving to the right relative to the tracks. We start when M and m are next to each other: Also, in the frame of the tracks a & A and B&b are next to each other:

a............m............b
A______M______B

Now assume that lightning strikes Aa and Bb

A sends a instantaneous signal to M, telling him of the lightning strike. ( and incidentally that a passenger at 'a' was killed.)
M shares this info with M (as they are right next to each other). and A sends and instantaneous message back to 'a' to confirm.

However, this is where it gets tricky. In the last diagram, we showed how everything lines up according to the railway. However, remember, the train is moving relative to the railway and thus is length contracted. Thus is it the "length contracted" distance between 'a' and 'b' that just fits between A and B in the railway frame.

In the train frame, the train is its proper length and it is the railway that is length contracted. Ergo, from the train frame, things are like this when m and M are next to each other.

a___________m___________b
................A......M......B...............

And from the train's frame A, M and B are moving to the left.

But this means that a and A have yet to align, and since the Lightning strike occurs when A and 'a' are aligned, the lightning has not struck 'a' yet. If m sends an instantaneous message to 'a', 'a' will receive it before 'a' is hit by lightning. The person at 'a' can now move before the strike and not be killed. Which means that A will no longer send that part of the message when the lightning strike does occur... You can see how fast things can become higgidly-piggidly .

I think that is not accurate... I disagree.
-This is what actually happen with finite speed of information: you *in the train* saw the lightning hit the front seat before another one hit the back seat, but a person stationary on the ground saw both seat struck by lightning at the same time.
-This is what happen when person on the ground has FTL communication: you saw the person on the ground says lightning hit the front seat, then you sent message to front seat at a finite speed, and information arrive instantly after the lightning strike and seat is burning, then you saw lightning strike the front seats.

There is no time-travel. Its just a sense of "premonition" or fortune telling but not an actual time-travel. -Time dilation & lenght contraction did not cause time-travel; it exist so that you saw the same speed of light (c) even though light appears to approach you slower & faster with respect to the person on the ground.

Forget "time-travel", this is not really about "time-travel", it is about how the concept of simultaneity is relative, rather than absolute.

Janus illustrated this using the train and embankment, where lightning strikes are simultaneous in one frame, but are not simultaneous in another frame due to the constancy of the speed of light to both frames, which means one frame will find time-dilation and length contraction for the other.

The upshot of all this is that "now" is a local concept, rather than being absolute across the universe. This has no bearing on cause and effect, because no observer will ever calculate the effect to have occurred before the cause, whatever their motion relative to those events.

But, if it were possible for information to travel faster than light (which is impossible), then it would mean that there are observers who would calculate the effect to have occurred before the cause, which is obviously not a description of reality.

The problem is that if you take for example the twin paradox , one time going fast and one slow and if you look from the other direction it reversed Which brings the contradiction that probably got from parallel universes and was created from negative movement of time . Thanks
( you just cant add )

Originally Posted by Water Nosfim

The problem is that if you take for example the twin paradox , one time going fast and one slow and if you look from the other direction it reversed

The "paradox" comes from not taking into account which twin remained in an inertial frame throughout, and which twin changed frames. If both twins remain in inertial frames, the situation is as you described, but the twins will never meet again and will not be able to compare their ages. In order for them to meet up, one twin has to change reference frame, which causes the asymmetry between them.

U can and i can

it's seems to be violating