Thread: the doppler effect energy problem

1. Originally Posted by Ghrasp
If I want to call my cat foton at least one foton is a cat.
You might call your cat a physicist, and then go consult with him. It could not possibly hurt.

2. Originally Posted by Ghrasp
In the sense of the smallest part where you can still speak of wavelength, color, frecquency, energy etc. Like an atom is for a specific substance. If you go further you can,t speak of a molecule any more. If you go further with light you could still speak of a quantum but not of light and thus not of a lightquantum. That seems a simple logic that lies in the meaning of the words. Discussing there would come easily to discussing the meaning of words. That can be done but it is rather stupid when you think it is a discussion about content (as words can have different meaning in different contexts or used by different people you can get endless discussions and misunderstandings).

The word foton is a good example for this. Sometimes you find it used as another word for a "quantum of light" thus having a frecquency and wavelength which makes it impossible to be "a point".

Or it is used as referring to only the particle character. Only the particle character the photon can't be a quantum off light off course that would do no justice to the wavecharacter of light. So to become light the wavecharacter must be added first.

Then you come to the problem of the drunken particle that has to follow a wavy pattern because light is supposed to have a wavecharacter also.
That's why it's important that you define things when confusion arises. And that's not what the word quantum means to everyone else. Quantum is not the smallest something, it's a discrete something. Electrons can only exists at discrete energy levels, so moving between those levels takes a quantum of energy. (This is dervied from the word quantized.)

Anyway, if I asked you to plot the function (sin t, 0, t), would you agree that it is a wave and that all that is waving is a single point?

3. In general a kwantum is just a latin word meaning the smallest distinguishable part of something where you van speak of that something. If you like it or not but the meaning of the word is rather generic. What the use is at universities is not that important I (or anyone) can use the word for a as long as the context makes it clear how it is used. You can,t posses a word. Off course you can discuss how words are used but not end a discussion by defining a word.

Off course I understand that if you put something on a springfeather and give it a frecquency that it will have energy according to the frecquency. But that idea is strongly bounded to the idea that the objekt has weight. Without that I can,t see how a point would get energy related to a frecquency. So it can,t have a meaning for me because I can;t relate to it. Just while for instance Feynmann would define it like that and say so doesn,t change that. But sadly it does for many.

If you interpret the wave-particle duality like that you actually deny it. The wave particle duality (or whatever one would refer to it ) counts for everything including a foton or a point. It is inherent to reality and something always has both characters at the same time.
How you experience it or how you think it will "at one time" doesn,t mean that it is only that (a particle or a wave) at that time. It,s just saying something about the way you detect and measure.

4. (What is with you ant the weird spellings?)

So just because you can't relate to it means that it's invalid? The universe has to mean something to you for it to exist? The universe is under no obligation to make sense, to you or to anyone else.

5. English is not my usual language, is it that bad ? Kwantum I did on purpose to accentuate the greek - I think it was - origin.

So just because you can't relate to it means that it's invalid?
No, in general it means there is
no way to make out if it is valid or not
if you can,t relate to it somehow. if the relation is completely abstract it is not yet real.

So you have an abstraction (I mean a point how much more abstract can you get ?) on which a sentence builds and you have something we now from daily life : light (and shadow). How can I connect the abstract to the real world in a sensible way but with meaning.

For instance Dr Rocket mentions that a foton is a point. So you could change foton into a point ? Then the sentence says "a point is a point". That has no real meaning.

Or are there types of points to distinguish ? Then what,s the difference between a point being a foton and a point that is not a foton ?

What is a point anyway ? Put a pencil on paper and you certainly don,t have a point, that,s a dot or a spot. Make a movement and you certainly don,t have a line that,s a stripe.

Paint half of the paper black and where the white can be distinguished from black allready looks more like it. That determines a line on the white paper to where the black covers the white paper. It may not be a straight line if you look at it with a microscope but we see it as straight to a certain extend (the stripe is also not straigth as it,s lines aren,t).

So maybe that,s what mr rocket means with a point in reality ? How he images a point conscious or subconscious as a dot on paper in a fysics drawing making it real for him ? That would at least explain somehow why the problem is not clear that such sentences have no sense of reality because of complete abstractness. And if that is what is meant by a point (something I would refer to as a dot in drawing ) then what is the difference between a dot and a marble ? except for the thickness ? I could make a dot of glass on paper by melting the glass and use it as paint. Then you have a very flat marble only the shape being different.

So no I,m not saying anything is invalid, just asking what am I supposed to imagine with it. Where is the sensible aspect as light and shadow is sensible.

The universe has to mean something to you for it to exist?
Well I,m a part of it aint I. So that makes it a very complex word. It,s hard to imagine the universe without me imagining it and also with me imagining it as I can,t step out of me.

But if the word universe has no meaning in this sentence :

The universe is under no obligation to make sense, to you or to anyone else.
It can not be distinguished from any other word that has no meaning. The whole sentence can,t mean a thing.

6. Originally Posted by Ghrasp
English is not my usual language, is it that bad ? Kwantum I did on purpose to accentuate the greek - I think it was - origin.
That, and you keep changing ph to f. They may sound the same, but typed it looks weird (and wrong).

Originally Posted by Ghrasp
So just because you can't relate to it means that it's invalid?
No, in general it means there is
no way to make out if it is valid or not
if you can,t relate to it somehow. if the relation is completely abstract it is not yet real.
Sorry, but that's not true. Just because you can't relate to it doesn't mean no one can, or that no one has found a way of testing these things.

Originally Posted by Ghrasp
So you have an abstraction (I mean a point how much more abstract can you get ?) on which a sentence builds and you have something we now from daily life : light (and shadow). How can I connect the abstract to the real world in a sensible way but with meaning.

For instance Dr Rocket mentions that a foton is a point. So you could change foton into a point ? Then the sentence says "a point is a point". That has no real meaning.
That's just inane. Saying a photon is a point and saying a point is a point are two completely unrelated statements, and trying to say they mean the same thing is just dishonest. There's no way someone could actually draw that conclusion.

Originally Posted by Ghrasp
Or are there types of points to distinguish ? Then what,s the difference between a point being a foton and a point that is not a foton ?
Yes. The word point describes any number of things, including photons. There are other types of point particles and there are other things that are points.

Originally Posted by Ghrasp
What is a point anyway ? Put a pencil on paper and you certainly don,t have a point, that,s a dot or a spot. Make a movement and you certainly don,t have a line that,s a stripe.

Paint half of the paper black and where the white can be distinguished from black allready looks more like it. That determines a line on the white paper to where the black covers the white paper. It may not be a straight line if you look at it with a microscope but we see it as straight to a certain extend (the stripe is also not straigth as it,s lines aren,t).

So maybe that,s what mr rocket means with a point in reality ? How he images a point conscious or subconscious as a dot on paper in a fysics drawing making it real for him ? That would at least explain somehow why the problem is not clear that such sentences have no sense of reality because of complete abstractness. And if that is what is meant by a point (something I would refer to as a dot in drawing ) then what is the difference between a dot and a marble ? except for the thickness ? I could make a dot of glass on paper by melting the glass and use it as paint. Then you have a very flat marble only the shape being different.

So no I,m not saying anything is invalid, just asking what am I supposed to imagine with it. Where is the sensible aspect as light and shadow is sensible.
Imagining all the details all the way up from a single photon to a shadow is very difficult, and it's not at all surprising you can't. There are probably very few people that can. I know I can't. But that doesn't mean that those details are wrong, just complicated.

Originally Posted by Ghrasp
The universe has to mean something to you for it to exist?
Well I,m a part of it aint I. So that makes it a very complex word. It,s hard to imagine the universe without me imagining it and also with me imagining it as I can,t step out of me.

But if the word universe has no meaning in this sentence :

The universe is under no obligation to make sense, to you or to anyone else.
It can not be distinguished from any other word that has no meaning. The whole sentence can,t mean a thing.
... What? "Universe" means this universe we are living in. It is indeed complex. More complex than anyone can currently imagine. And there's no reason to think it should be otherwise. Complex and wrong are two different things.

Seriously. I understand that you don't understand all of these details. Neither do I. But just because you can't understand it doesn't mean its wrong.

7. Originally Posted by MagiMaster
Originally Posted by Ghrasp
English is not my usual language, is it that bad ? Kwantum I did on purpose to accentuate the greek - I think it was - origin.
That, and you keep changing ph to f. They may sound the same, but typed it looks weird (and wrong).
I think the problem is deeper than simply language.

I am reminded of an incident from graduate school. There were two new Chnese graduate students. One of them spoke English quite well and was easily understood. The second one was totally incomprehensible. Finally one of the U.S. students asked the understandable Chinese student, why it was that he was so easily understood while the second was was essentially impossible to understand. The reply --- "Oh, he doesn't make sense in Chinese either."

8. Foton is dutch Pronounciation, I will see to it to use ph.

That's just insane. Saying a photon is a point and saying a point is a point are two completely unrelated statements, and trying to say they mean the same thing is just dishonest. There's no way someone could actually draw that conclusion.
But do you realize what is the difference ? The difference is that in the first meaning a foton is described as a point for instance in the math. A certain description is always open for discussion, the description is no more then that it,s not what it describes. Saying a foton is a point makes that impossible.
Like a landscape can be painted (descriptive also) in many ways. There is not necessarily a right or wrong way to do it and even painting without being descriptive is possible.

.. What? "Universe" means this universe we are living in.
You are also part of the universe while using the word not just living in it, it is not a room.

But just because you can't understand it doesn't mean its wrong.
But does it mean it,s right then ? Nescio.

9. Originally Posted by Ghrasp
But does it mean it,s right then ?
No, but based on the available it does greatly increase the probability that it is right.

10. Originally Posted by Ghrasp
Foton is dutch Pronounciation, I will see to it to use ph.
If it's written that way in another language, whatever, but this is an English forum and foton and fysics looks weird.

Originally Posted by Ghrasp
That's just insane. Saying a photon is a point and saying a point is a point are two completely unrelated statements, and trying to say they mean the same thing is just dishonest. There's no way someone could actually draw that conclusion.
Inane is a word, and distinct from insane.

Originally Posted by Ghrasp
But do you realize what is the difference ? The difference is that in the first meaning a foton is described as a point for instance in the math. A certain description is always open for discussion, the description is no more then that it,s not what it describes. Saying a foton is a point makes that impossible.
Like a landscape can be painted (descriptive also) in many ways. There is not necessarily a right or wrong way to do it and even painting without being descriptive is possible.
It's fine to make the case that the math says it's a point particle, but that doesn't necessarily mean it's physically a point. On the other hand, we have no better description for what it is, so we have no good reason to assume it's anything else, and intuition is not a good reason.

Also that doesn't excuse purposefully mangling someone else's sentences. "A photon is a point" and "a point is a point" are still completely unrelated sentences.

Originally Posted by Ghrasp
.. What? "Universe" means this universe we are living in.
You are also part of the universe while using the word not just living in it, it is not a room.
:? What's your point. I never said I wasn't part of the universe. I said that the universe is not required to make sense to anyone, especially not anyone in particular. It certainly doesn't make perfect sense to me. I have no clue how my own brain works beyond the rudimentary idea of signals passed between neurons. I can't imagine the interactions between two atoms from the quarks up. But that still doesn't mean anything at all. Nothing in science is contingent on my (or your) understanding.

Originally Posted by Ghrasp
But just because you can't understand it doesn't mean its wrong.
But does it mean it,s right then ? Nescio.
No, but the weight of observational evidence does.

11. Originally Posted by MagiMaster

It's fine to make the case that the math says it's a point particle, but that doesn't necessarily mean it's physically a point. On the other hand, we have no better description for what it is, so we have no good reason to assume it's anything else, and intuition is not a good reason.
Elementary particles are treated as points in current theory (QED), and for good reason.

First, the theory formulated in that manner gives excellent predictions. Not just extremely accurate, but accurate to as many decimal places as experiment has been able to determine.

Second, other models have been tried and found to produce problems. In particular models of the electron as a small charged sphere run into serious trouble. In fact, so does the point model, with apparently infinite self-energy. But if one ignores that issue, the theory works quite well.

This is not to say that QED is perfect, or that it is the final theory. It is not. If one applies QED in the usual manner one gets estimates for the cosmological constant, based on the zero point energy of the vacuum that are absurd. That is an important open issue. It may take a unified theory of all 4 forces to resolve it, but nobody really knows.

In the meantime, QED gives extraordinarily accurate predictions of all everyday phenomena associated with light and and non-nuclear phenomena involving matter. In particular QED, in principle, explains all of chemistry and material science. In practice the equations are too hard to solve.

One thing that modern physical theory has shown conclusively is that ordinary intuition is not a good guide. The intuition of experts like Feynman, Weinberg, and Wilczek is useful in guiding the development of sophisticated theories, but only in formulating mathematical descriptions that can be used to make testable predictions. It is the predictions of the theory and their confirmation by experiment that determine the validity of the formulation. Naive intuition, or worse yet, misguided intuition is worthless or worse than useless. Ghrasp's intuition cannot only be safely ignored, it must be completely ignored if one hopes to gain any understanding of real physics.

12. intuition is not a good reason
All the fysics we know of comes from the intuition of fysicists who where not afraid to think. Intuition is part of thinking and you cant put just that away without getting a disformed way of thinking.

If you are told to believe that nature can,t be intuitively understood you accept it maybe while I and at least some others activ on this forum don,t. That could be the main reason of conflicts like this.

If someone tells me that nature is counterintuitive I would understand that the same as when someone tells me intuition is against nature.

Thinking to me is a produkt of intuition and instinct and how they combine in a person moe or less balanced. Let go of intuition believing that it lies to you simply means animalistic instincts will rule.

Proof for this can be found in abundancy in the uncivil way some here react to posts of not just me. fomrer topics and encounters are rememberd as personal encounters bringing that back in new ones etc. All typical primitiv behaviour from an overly instinctiv way of reacting. A psychology student could graduate from making a study of how people react to each other here.

But at least we agree that a foton allthough it may be described as a point in current fysics it doesn,t mean it has to be seen as "being a point". As we agree that the descrition is not what is described. "ceci n'est pas une pipe" n'est pas ?

13. No we don't agree. I said it was alright to make that point, not that I agree with it. I also said that no one has a better description. Specifically, small balls don't work. That was tried many times and points work better.

And no, human intuition sucks. The physicists your talking about didn't look at the universe and suddenly understand it. They studied it carefully and at length and finally had a breakthrough, which they then had to hammer out into mathematical detail. Intuition played a very small part in any of that, maybe showing them which direction to look next, but never showing them anything solid. No scientist would back anything by intuition alone.

Consider medical sciences. Would you trust a doctor that told you he hadn't studied medicine, but he had a great intuition for surgery?

Continue to trust your intuition if you want, but it won't get you very far in science.

Oh yeah, and your description of thought as just intuition and instict is about the stupidest idea ever. I for one can actually think and that has nothing to do with either.

14. Originally Posted by Ghrasp

Thinking to me is a produkt of intuition and instinct and how they combine in a person moe or less balanced. Let go of intuition believing that it lies to you simply means animalistic instincts will rule.
I am glad to hear that you are considering doing some of that "thinking" stuff. I hope it works out for you. Start slowly and work your way into it.

15. Originally Posted by MagiMaster

Oh yeah, and your description of thought as just intuition and instict is about the stupidest idea ever. I for one can actually think and that has nothing to do with either.
I am not sure I agree.

It is in the top three.

I am pretty sure that Ghrasp owns everything higher up the totem pole also.

Maybe he will change his description after he has actually tried thinking for a while. Practice might help. Or not.

16. Oh yeah, and your description of thought as just intuition and instict is about the stupidest idea ever.
I don,t know if you read a lot but in this case you've done a lousy job.

I referred to thinking as the produkt of intuition and instinct, imagination and dreams. Coming from both not being one of these as you try to put on to me.

Actually I started to like the idea of a foton being viewd at as a point because as you know you can't detect a foton by looking at a beam between a source and something it shines light on (or there must be air, moist dust etc in the room. You can only see and detect a photon when it it is coming to you (or a machine, photosensitive material etc). You have to catch it to observe it and then it is always coming towards you even if it would be reflected from a mirror.
So we can,t speak of foton,s as we would speak of cars while standing at the side of a highway and see cars pass by.

But looking in a direktion and observing a point the point might just as well be a snare. A mathematical line in perspective is observed as a point and line and point can,t be distinguished from each other. Because the direction of sight exactly falls together with that line.

So a photon could be a line or snare just as well if you look further then you,re own eye or a fotograph and take the specific perspective in consideration.

17. Instead of telling each other how wrong we are, let's just discuss what has experimentally confirmed. The double slit experiment, for example, has been conducted using light sources that emit exactly one photon at a time. The detection screen registers that a photon of light hit it at one, and exactly one place, per photon. The location where the photon will strike it is completely random, other than that it falls within the range of locations where 2 photons would not have destructively interfered with each other. (which is wierd)

The same experiments have been done by emitting electrons one at a time, and carbon atoms one at a time, and with basically the same result as when they used photons one at a time.

It would be really hard to think of a situation where a whole wave would hit the detection screen, and that screen only detects it at one location, and detects no activity at all at any other location. There's lots of ways we could interpret this, but then we have to start getting deeper into QM theory, and then things will just keep getting stranger and stranger.

18. Originally Posted by DrRocket
Originally Posted by himnextdoor
The thing is that the photon isn't red-shifted; only the portion of the light-wave that is travelling towards you is. The part of the wave that is moving away from you (which you can never see) is blue-shifted.

I think it is better to think of a photon as an event that causes an electo-magnetic wave in the same way that a stone dropped into a pool of water causes a ripple. A photon can be any event from an electron changing its energy state to nuclear collisions in the same way that a stone can be anything from a grain of sand to a boulder of rock.
No. It is indeed the photon that is red-shifted. Electromagnetic waves are just a lot of photons. You cannot spit a photon into two parts.

You are thinking of photons in the wrong way.
This should address the conservation issue, though, because for every red shifted photon, there is probably and equally blue shifted one in the wave somewhere.

What would be really interesting to examine is what happens to the energy when a photon is reflected off of a mirror that is moving away from us? Where does that energy go?

19. Originally Posted by kojax

What would be really interesting to examine is what happens to the energy when a photon is reflected off of a mirror that is moving away from us? Where does that energy go?
The mirror.

Look, in terms of energy conservation, it really isn't any different than throwing a ball at the back of a truck receding from you. If you throw the ball at 60 mph and the truck is moving at 20 mph, then the ball will hit and rebound from the truck at 40 mph relative to the truck, meaning it will return at 20 mph relative to you. Where did that energy go? The answer is that in rebounding from the truck, there was a transfer of momentum. The truck was moving just slightly faster than 20 mph afterwards.

The same happens with the mirror, After the photon reflects of of it, it has a slightly different velocity. The difference with light is that it doesn't show its change of energy by a change of speed, but by a change of wavelength.

20. Originally Posted by Ghrasp
Oh yeah, and your description of thought as just intuition and instict is about the stupidest idea ever.
I don,t know if you read a lot but in this case you've done a lousy job.

I referred to thinking as the produkt of intuition and instinct, imagination and dreams. Coming from both not being one of these as you try to put on to me.

Actually I started to like the idea of a foton being viewd at as a point because as you know you can't detect a foton by looking at a beam between a source and something it shines light on (or there must be air, moist dust etc in the room. You can only see and detect a photon when it it is coming to you (or a machine, photosensitive material etc). You have to catch it to observe it and then it is always coming towards you even if it would be reflected from a mirror.
So we can,t speak of foton,s as we would speak of cars while standing at the side of a highway and see cars pass by.

But looking in a direktion and observing a point the point might just as well be a snare. A mathematical line in perspective is observed as a point and line and point can,t be distinguished from each other. Because the direction of sight exactly falls together with that line.

So a photon could be a line or snare just as well if you look further then you,re own eye or a fotograph and take the specific perspective in consideration.
Your first paragraph made no sense. I can't tell what it is you're trying to say, but it seems you're still trying to discount higher though or something. Anyway, this isn't a discussion for the physics subforum.

For the rest of your description, it sounds like you're talking about the spacetime view, where photons would indeed be a line. (More precisely, the world line of photon, or any point particle, would be a mathematical line.)

21. Originally Posted by Janus
Originally Posted by kojax

What would be really interesting to examine is what happens to the energy when a photon is reflected off of a mirror that is moving away from us? Where does that energy go?
The mirror.

Look, in terms of energy conservation, it really isn't any different than throwing a ball at the back of a truck receding from you. If you throw the ball at 60 mph and the truck is moving at 20 mph, then the ball will hit and rebound from the truck at 40 mph relative to the truck, meaning it will return at 20 mph relative to you. Where did that energy go? The answer is that in rebounding from the truck, there was a transfer of momentum. The truck was moving just slightly faster than 20 mph afterwards.

The same happens with the mirror, After the photon reflects of of it, it has a slightly different velocity. The difference with light is that it doesn't show its change of energy by a change of speed, but by a change of wavelength.
In theory, does this mean that a laser could be used as a means of propulsion? If we launched a satellite in a direction moving away from Earth, and pointed a laser at it long enough, would that actually accelerate it over time? I'm guessing not very much, but suppose we kept the laser on it for a few hundred years or something?

Now, the other side of that question. Would a stationary object (stationary relative to the laser) receive any momentum? In theory, the beam of light is not getting red shifted, so there shouldn't be any transfer of energy, right? But somehow that seems counter intuitive.

22. Originally Posted by kojax

In theory, does this mean that a laser could be used as a means of propulsion? If we launched a satellite in a direction moving away from Earth, and pointed a laser at it long enough, would that actually accelerate it over time? I'm guessing not very much, but suppose we kept the laser on it for a few hundred years or something?
Yes, it is the principle upon which the light-sail is based.

Now, the other side of that question. Would a stationary object (stationary relative to the laser) receive any momentum? In theory, the beam of light is not getting red shifted, so there shouldn't be any transfer of energy, right? But somehow that seems counter intuitive.
What do you mean by stationary to the laser? Do you mean attached to the laser and using the laser as a reaction motor? In that case, yes. Light is created with a momentum in one direction, which must be balanced by momentum in the other. In this case it is a recoil by the laser.

23. Originally Posted by Janus
"

Yes, it is the principle upon which the light-sail is based.
Yes, but light sails are usually based on using light from the sun. It would take an enormous laser, based on Earth, to be effective.

You could also take the laser with you and simply point it aft. That would also impart momentum to the spacecraft, with enormous Isp and very poor energy efficiency.
This scheme would also take a gigantic laser to work.

There are much more efficient methods of deep space propulsion, ion engines for instance.

24. So all the stars from different directions are pushing the earth (as a sail) all in different directions as a counter force for gravity ? What if the sun would have more of this "pushing anergy" then it has gravitation ?

25. So all the stars from different directions are pushing the earth (as a sail) all in different directions as a counter force for gravity ? What if the sun would have more of this "pushing anergy" (that it obviously hasn,t) then it has gravitation ?

26. Originally Posted by Ghrasp
So all the stars from different directions are pushing the earth (as a sail) all in different directions as a counter force for gravity ?
Yes the light from stars is pushing on the earth from all directions. It is a rather small effect to say the least.

Originally Posted by Ghrasp
What if the sun would have more of this "pushing anergy" then it has gravitation ?
What if frogs had wings ?

Ans. They wouldn't bump their ass.

27. Yes the light from stars is pushing on the earth from all directions. It is a rather small effect to say the least.
Gravitational force is assumed to be very weak also and the sun is much closer to earth then any other star.

If the sun acts with a pushing force towards earth as it is the most nearby star it would be very coincidental if the forces from all other directions would compensate for it so the total of pushing energy was evened out. Therefor the orbitation of sun and earth would need a bigger gravitational force to compensate for this pushing energy of the sun.

The sun being so nearby would result in a force that would push the earth away.
if you push against something with 10 n and attract it at the same time with 15 you keep only 5 N as a resulting attractive force. So the gravitational force would have to be 15 newton to give a resulting attractive force of 5 n.

There is a difference though, the earth is not as reflective as a reflective sail. The effect will have to do with the reflection. Earth has reflective properties and absorbing as well. How would a sail act a laser if it had both of these properties also, parts being reflective material parts absorbing material likeblack cloth ?

Maybe the absorbtion would result in an attracting force ?

Then if both reflection and absorbtion are in balance thedistance would stay constant.

28. Originally Posted by Ghrasp
Yes the light from stars is pushing on the earth from all directions. It is a rather small effect to say the least.
Gravitational force is assumed to be very weak also and the sun is much closer to earth then any other star.

If the sun acts with a pushing force towards earth as it is the most nearby star it would be very coincidal if the forces of all other stars would compensate for it so the total of pushing force is evened out. Therefor theories for the orbitation of sun and earth would need a bigger gravitational force to compensate for this pushing energy of the sun.

The sun being so nearby would result in a force that would push the earth away.
if you push against something with 10 n and attract it at the same time with 15 you keep only 5 N as a resulting attractive force. So the gravitational force would have to be 15 newton to give a resulting attractive force of 5 n.

How much it is relevant is also not that important.
The radiation pressure the Sun exerts on the Earth equals 1.27 x10^9 Newtons. While this might seem like a lot, keep in mind that the mass of the Earth is 6 x10^24 kg. The resulting acceleration from light pressure alone would be only 2.1 x10-16 m/s^2.
Now compare this to the force due to gravity on the Earth due to the Sun. This works out to be 3.6 x 10^22 Newtons, over 10^13 time larger than the radiation pressure. In other words, the radiation pressure is insignificant compared to gravity.

Another way to look at it is this: If you were to take that acceleration of 2.1 x 10^-16 m/s^2 and apply it to the Earth in the direction of its orbital motion, in about 150 billion years, the Earth will have increased its orbital speed by 1 km/sec and only receded an additional 13 million km from the Sun. Now, applying the acceleration in this direction is the most efficient way to raise the Earth's orbit. Radiation pressure, however, pushes outward, which makes it even less effective in altering the Earth's distance.

There is a difference though, the earth is not as reflective as a reflective sail. The effect will have to do with the reflection. Earth has reflective properties and absorbing as well. How would a sail act a laser if it had both of these properties also, parts being reflective material parts absorbing material likeblack cloth ?

Maybe the absorbtion would result in an attracting force ?

Then if both reflection and absorbtion are in balance thedistance would stay constant.
No, the result would still be a push. The Earth would take on the momentum of the light. The result would just be 1/2 of that of reflection.

29. Originally Posted by DrRocket
Originally Posted by Janus
"

Yes, it is the principle upon which the light-sail is based.
Yes, but light sails are usually based on using light from the sun. It would take an enormous laser, based on Earth, to be effective.

You could also take the laser with you and simply point it aft. That would also impart momentum to the spacecraft, with enormous Isp and very poor energy efficiency.
This scheme would also take a gigantic laser to work.

There are much more efficient methods of deep space propulsion, ion engines for instance.
True. Besides, I always thought that solar sails operated by catching the solar wind, like protons and such that are being expelled by the Sun, rather than the light itself.

I was mostly asking about light's propulsion properties just because of what it might tell us about light and momentum. Also, because pushing on a space ship using a laser would mean the space ship doesn't need to carry any of its own propellant (at least while it is accelerating away.) I guess you'd need a very very strong laser to get any real effect going, though, and the light would need to be able to travel really far without diminishing in strength to be of use. So.... probably not a likely candidate for a deep space mission.....

30. A space based laser would be able to travel a very long way without too much dispersion for a sail.

31. The radiation pressure the Sun exerts on the Earth equals 1.27 x10^9 Newtons. While this might seem like a lot, keep in mind that the mass of the Earth is 6 x10^24 kg. The resulting acceleration from light pressure alone would be only 2.1 x10-16 m/s^2.
On a longer time scale such a small accelleration still would have huge consecquences off course because of the s^2. Also what do you take as the surface ? the diameter of the earth or the diameter of the atmosphere ? Ionosphere ? Clouds. which all have more or less strong reflective properties ?

Even if the effect is quantatively small qualitative it means a lot. If some one would ask "is there beside gravitation also an opposing force between stars/planets according to fysics" One could not say "no there isn,t, just gravitation". Would have to put some nuance to it or be incomplete.

And how would it work when you have two lasers that are opposed in direktion. For instance the spaceship would have a laser on it,s own also and directs it towards the other laser ? Could it increase the acceleration (backward from the other one)?

32. Originally Posted by Ghrasp

On a longer time scale such a small accelleration still would have huge consecquences off course because of the s^2.
I already showed where you would need a time scale many times longer than the age of the universe for there to be any significant consequences.
Also what do you take as the surface ? the diameter of the earth or the diameter of the atmosphere ? Ionosphere ? Clouds. which all have more or less strong reflective properties ?
I considered a perfectly reflective and flat disc the diameter of the The Earth. Adding the thickness of the atmosphere would result in a change many times smaller than the rounding error. Besides, my estimate is already overly generous. The Earth is not perfectly reflective, nor is it flat. Both f these facts greatly reduce the actual acceleration value.

Even if the effect is quantatively small qualitative it means a lot. If some one would ask "is there beside gravitation also an opposing force between stars/planets according to fysics" One could not say "no there isn,t, just gravitation". Would have to put some nuance to it or be incomplete.
You act as if this is some new profound revelation. Radiation pressure has been known about for a long time. In those instances where it is a consequence, it is taken into account.

And how would it work when you have two lasers that are opposed in direktion. For instance the spaceship would have a laser on it,s own also and directs it towards the other laser ? Could it increase the acceleration (backward from the other one)?
If the spaceship reflects/absorbs light from the other laser this will result in an added transfer of momentum.

33. Originally Posted by Janus
Originally Posted by Ghrasp

On a longer time scale such a small accelleration still would have huge consecquences off course because of the s^2.
I already showed where you would need a time scale many times longer than the age of the universe for there to be any significant consequences.
In fact time scales are not important. Classically both the attractive gravitational forcde and the miniscule repulsive force of light pressure follow inverse square laws. The only effect would be a tiny correction to the gravitational force, effectively an insignificant reduction in the gravitational constant (so far to the right of the decimal point to be well within the error limits for that constant or even GM where G is the universal constant and M is the mass of the sun anyway.)

34. If the spaceship reflects/absorbs light from the other laser this will result in an added transfer of momentum.
No sail, two identic lasers pointed against each other with identic diameter of the beam. So instead of using a sail.

I don,t think one laser would absorb the light of the other just like it wouldn,t absorb it,s own light coming back from the mirror. As reflected light from one laser is not assumed to be different then light coming from a different but identic laser coming from opposite direktion. So I would think this should also work.
and twice as strong as with one laser of same energy on a sail offcourse.

35. A large part of the point of sails is so the ship won't have to carry around things like rockets or lasers.

36. I,m not planning to take a sailing trip Just wondered if this effect would occur between two stars.
What happens in this respect if two lasers are pointed at each other. You sure won,t see a lightspot on both lasers of the opposite laser. So where does the energy go if there is (almost) nothing absorbent or reflective in between. Some interference ?
But why would there be more interference then compared with a reflected beam with same wavelength and frecquency ?

The only effect would be a tiny correction to the gravitational force
Newtons formula is only about an attractive force.
On theoretical level 10 + (-3) is different then 7. Only the result can be the same.

Besides putting it away in the gravitational constant the force would be con rsidered as elated to the mass of the sun and the earth while it was assumed to be related to the energy of the sun and the reflective surface.

37. Originally Posted by Ghrasp
I,m not planning to take a sailing trip Just wondered if this effect would occur between two stars.
What happens in this respect if two lasers are pointed at each other. You sure won,t see a lightspot on both lasers of the opposite laser. So where does the energy go if there is (almost) nothing absorbent or reflective in between. Some interference ?
But why would there be more interference then compared with a reflected beam with same wavelength and frecquency ?

The only effect would be a tiny correction to the gravitational force
Newtons formula is only about an attractive force.
On theoretical level 10 + (-3) is different then 7. Only the result can be the same.

Besides putting it away in the gravitational constant the force would be con rsidered as elated to the mass of the sun and the earth while it was assumed to be related to the energy of the sun and the reflective surface.
You would indeed see a light spot on each laser from the opposite laser. (You would not actually see it since lasers are pretty bright, but the photons from each laser would hit the other one.)

38. You would indeed see a light spot on each laser from the opposite laser. (You would not actually see it since lasers are pretty bright,
So you would not see a lightspot then.

And how is this in the case of shing a laser in the mirror then. You would see a lightspot on the laser of it,s own light reflected from the mirror ?

If not why the difference ?

Also if you would see the lightspot on the laser. Do you assume one laser reflectant to the other or absorbent ? If it would be absorbent the light would disappear I assume, it would get dark if both lasers are indentical. So there must be a reflectancy then.

In that case it would also account for stars. and as the energy is supposed to be the same from the start of the universe and distances much shorter the effect becomes significant there.

With decrease of distance the energy is much more concentrated. Different then by a laser but with same result.

With two stars the relation of energy density would also be to the fourth of the distance I suppose.

39. Originally Posted by Ghrasp
You would indeed see a light spot on each laser from the opposite laser. (You would not actually see it since lasers are pretty bright,
So you would not see a lightspot then.

And how is this in the case of shing a laser in the mirror then. You would see a lightspot on the laser of it,s own light reflected from the mirror ?

If not why the difference ?

Also if you would see the lightspot on the laser. Do you assume one laser reflectant to the other or absorbent ? If it would be absorbent the light would disappear I assume, it would get dark if both lasers are indentical. So there must be a reflectancy then.

In that case it would also account for stars. and as the energy is supposed to be the same from the start of the universe and distances much shorter the effect becomes significant there.

With decrease of distance the energy is much more concentrated. Different then by a laser but with same result.

With two stars the relation of energy density would also be to the fourth of the distance I suppose.
PLEASE go take a physics course somewhere. Anywhere.

"That's not right. It's not even wrong." -- Wolfgang Pauli

40. Well well,

You would indeed see a light spot on each laser from the opposite laser. (You would not actually see it since lasers are pretty bright,
You come up with a lightspot one would indeed see but actually would not see.
And then you come up with this pompous reply ?

41. Originally Posted by Ghrasp
Well well,

You would indeed see a light spot on each laser from the opposite laser. (You would not actually see it since lasers are pretty bright,
You come up with a lightspot I would indeed see but actually would not see.
And now you,re insulted or something ?
Or are you part of some religious "seeing the light sekt" making people read the good books to enlight themselves.
OK, if you are really that stupid, go ahead and stare directly into a laser. You won't see the spot for long. You won't see much of anything for long.

WARNING: For the more intelligent readers -- don't look directly into a laser. It may permanently damage your eyes.

42. This baffles me, I was referring to two lasers all the time, in you,re quote it is also about two lasers so why would I look into a laserlight (in which case I wouldn,t even see a lightspot but more of a dark spot.)

You can direkt two lasers to each other just like towards anything. My question remains what would they do push each other away or maybe sideways due to an opposit momentum ?
(because the direction is opposed the momentum would oppose also)

43. Originally Posted by Ghrasp
This baffles me, I was referring to two lasers all the time, in you,re quote it is also about two lasers so why would I look into a laserlight (in which case I wouldn,t even see a lightspot but more of a dark spot.)

You can direkt two lasers to each other just like towards anything. My question remains what would they do push each other away or maybe sideways due to an opposit momentum ?
(because the direction is opposed the momentum would oppose also)
Of course they would. Light carries momentum. Light impinging on a rock tends to push it away. It doesn't matter if the light impinges on a rock or another laser. Momentum is still conserved.

It also doesn't matter if there are two lasers. Each laser emits light and that light impinges on another body, momentum will be transferred. A laser is just a body that emits light.

This also has nothing to do with lasing. You would get the same effect with two flashlights.

And BTW when you look into a laser you see a VERY bright spot, as it burns your retna.

44. Originally Posted by DrRocket
WARNING: For the more intelligent readers -- don't look directly into a laser. It may permanently damage your eyes.
Let's be more realistic and replace that "may" with a "will"...

Ghrasp, stare directly into a laser and tell us what you see!

45. Originally Posted by Arcane_Mathematician
Originally Posted by DrRocket
WARNING: For the more intelligent readers -- don't look directly into a laser. It may permanently damage your eyes.
Let's be more realistic and replace that "may" with a "will"...

Ghrasp, stare directly into a laser and tell us what you see!
No, it is a may. It depends highly on the specific laser. There are some lasers that are sufficiently low in power that you don't get permanent damage. But don't count on it.

Even so-called "eye safe" lasers can be pretty painful if you get one directly in the eye. Many available lasers, particularly stand-along laser pointers, are sufficiently high in power to be a concern.

There is sufficient variability among lasers that looking directly into one, except in exceptional circumstances, is pretty stupid. There is no point to it and the downside is severe.

46. You use the term impinge I suppose to avoid you have to choose between reflective and absorbent. Impinge is not satisfactory though because I suppose they choose reflective material for such sails because the reflective properties have a part of it.

So a laser would have to reflect the light of another laser.

In that case if a laser is pointed at a spot on a screen and another laser is pointed at the first opposing direktion but with slight angle, beside the spot of the first laser there would be a second spot for the second laser. You understand that doesn,t work. So impinge would mean absorbent then ?

But if two lasers would absorb the light of the other when pointed against each other they both would emit what they absorb and absorb what the other emits while both are identical and emit the same ? There would have to be some interference of there light in between them. That would mean loss of energy but it is not lost if it acts as a force.

47. Originally Posted by Ghrasp
You use the term impinge I suppose to avoid you have to choose between reflective and absorbent. Impinge is not satisfactory though because I suppose they choose reflective material for such sails because the reflective properties have a part of it.

So a laser would have to reflect the light of another laser.

In that case if someone points a laser at a spot on a screen and another laser is pointed at the first beside the spot of the first laser there would be a second spot for the second laser. You understand that doesn,t work. So impinge would mean absorbent then ?

But if two lasers are pointed exactly to each other and each would absorb the light of the other they both would emit what they absorb and absorb what the other emits ? Makes no sense to me.
No, I used "impinge" because I meant impinge. The difference between absorption and reflection, in this particular application, is a factor of 2 in the momentum that is gained by the object on which the beam impinges.

Whether a laser would absorb or reflect or partially reflect the incident beam from a second laser depends on the details of the construction of the laser.

PLEASE go learn some physics and stop spouting nonsense.

48. So you think it is possible to construkt a laser that (when active offcourse) reflects the light of another one on a screen ? When its off yes but on no way. It doesn,t work with a flashlight and it wont work with a laser either

49. What do you mean by "reflects something on a screen"?

50. Originally Posted by Ghrasp
So you think it is possible to construkt a laser that (when active offcourse) reflects the light of another one on a screen ? When its off yes but on no way. It doesn,t work with a flashlight and it wont work with a laser either
Flashlight reflectors will reflect the light of an incoming beam, and in the ususal design would concentrate that light on the filament of the bulb.

A solid rod laser would also reflect the light of an incoming laser, partially, as the design is normally a partiallly reflective mirror on the rod end.

As always, you have no idea what you are talking about.

The mirrors are the same whether the device is on or off. That is what determines reflection. It has nothing whatever to do with the emissions that may or may be involved in the operation of the device.

Are you faking this or is your knowledge of basic physics really that poor ?

51. Reflects on the screen I mean for instance a toy laser pointed to a filmscreen in the dark, you get a lightspot on the screen as you would with a filmcamera. The filmcamera shows a divergent beam but the laser has no or litle divergency.

Another identic laser is pointed from a position close to the screen directly beside the lightspot of the first and aiming at the first laser. If that laser would reflect the light of the second laser a reflected lightspot could be seen on the secreen nearby the first lightspot. That is if one laser would reflect the other.

That a laser reflector can reflect the light of another laser Is obvious when the reflecting laser is out of funktion. Then you could use the reflector as any mirror and get a reflected lightspot as a result. But when the reflecting laser is put on I would espect that lightspot to disappear or at least change, decrease. And at no angle directly opposed it would disappear completely.
With a reflective material there is reflected light from opposite direction. So also light coming from two directions.

So what i,m questioning is does the light of the laser really push against the screen mechanical like if you would throw pingpong balls at it or does the magnetic field of one do something to the electrical field of the other change it and with that change, deform the beam and vice versa the reflecting beam also. In that case you could say that there is not a force acting on the screen but the fields are deformed changed and the objekts react accordinglly, somewhat similar as you can look at gravity as from a power force acting from a distance on something or as a deformation of timespace.

These sails do not really work with a pushing force what I read about it. More of a force that reminds me to lorentzforce the laser pushes the sail sideways. That gives an increase in distance also.
Therefor the magnetic aspect of the light will play a role as well.

52. What do you mean by "reflects something on a screen"?

Like in a cinema when the film is broken a lightspot is reflected on the screen.

For instance a toy laser pointed to a filmscreen in the dark, you get a lightspot on the screen as you would with a filmcamera. The filmcamera shows a divergent beam but the laser has no or litle divergency.

Another identic laser is pointed from a position close to the screen directly beside the lightspot of the first and aiming at the first laser. If that laser would reflect the light of the second laser a reflected lightspot could be seen on the secreen nearby the first lightspot. That is if one laser would reflect the other.

That a laser reflector can reflect the light of another laser when it is of is obvious when the reflecting laser is out of funktion (it,s not a laser then) . Then you could use it as any mirror and get a reflected lightspot as a result. But when the reflecting laser is put on I would expect that lightspot to disappear or at least change, decrease. And at no angle directly opposed it would disappear completely.
With a reflective material there is reflected light from opposite direction. So also light coming from two directions.

So what i,m questioning is does the light of the laser really push against the screen mechanical like if you would throw pingpong balls at it (no, photons are supposed to have no mass) or does the magnetic field of one do something to the electrical field of the other change it and with that change, deform the beam and vice versa the reflecting beam also. In that case you could say that there is not a force acting on the screen but the fields are deformed changed in a lorentz type of way (with a perpendicularity) and the objekts react accordinglly.

Because these sails do not really work with a pushing force what I read about it. More of a force that reminds me to lorentzforce the laser pushes the sail sideways. That gives an increase in distance also.
Therefor the magnetic aspect of the light will play a role as well.

53. It is easy to understand how a perpendicular force - in general - can seem to be a pushing force with this example.

Suppose from a satellite a projektile is fired exactly horizontal (determinded locally inside the satellite) in the orbitational direktion and perpendicular to the direction of gravitation.

The energy to do this still "pushes" the projektile to a higher orbit. Fired in opposite direktion it will reach a lower orbit then the satellite.

54. There is an edit button you know.

Anyway, no. In your set up with the two lasers and the screen, turning the reflecting laser on should not significantly effect the reflected spot.

And yes, photons push the same way ping pong balls do. It's simply conservation of momentum.

And no, you can easily tell the difference between a push in one direction and a push in another, even in orbit.

55. So if someone would be at the carnival in a merry go round and wants to throw a ball to someone standing at a distance of the merry go round. Increasing the distance to the rotational axis of the merry go round it would not be possible to do that and at the same time throw exactly in the rotalional direction (of that moment, lokally) and perpendicular to the imaginary towards the rotational axis ?

A simple math of a cirkle and a tangent, the line of the circle is curved the line of throw is straight so they run apart directly after the ball comes loose from the hand.

Something similar happens with the satellites as the line of orbitation is also curved just as with the maerry go round and therefor the projektile reaches a higher orbit even if thrown perpendicular to the vertical direktion (thus horizontal).

Basic geometry and confirmed with what is within daily experience.

With the laser pointed towards a sail the sail would come in a different angle to the laser and therefor it has to be rotated towards the laser constantly. That,s part of the construktion. Then the laser seems to push the sail further and further. If it was a real push the construktion would not need this and could be more similar to a sail on a boat sailing with wind from the back.

56. Originally Posted by Ghrasp

But if two lasers would absorb the light of the other when pointed against each other they both would emit what they absorb and absorb what the other emits while both are identical and emit the same ? There would have to be some interference of there light in between them. That would mean loss of energy but it is not lost if it acts as a force.
Actually, there's no interference in the sense of what you're talking about. Light has a property called "superposition", which means that two photons can pass through each other like ghosts, without affecting each other in any way whatsoever.

Interference only happens at the point where the photon is detected. If two photons in opposite phase strike the detector at exactly the same time, they will cancel, and nothing will be detected. But if they meet each other away from the detector, they just pass through each other and nothing happens.

Originally Posted by Ghrasp
This baffles me, I was referring to two lasers all the time, in you,re quote it is also about two lasers so why would I look into a laserlight (in which case I wouldn,t even see a lightspot but more of a dark spot.)

You can direkt two lasers to each other just like towards anything. My question remains what would they do push each other away or maybe sideways due to an opposit momentum ?
(because the direction is opposed the momentum would oppose also)
The effect of pointing two lasers directly at each other is no different than the effect of pointing two lasers at two walls. There's nothing special about using a laser as the surface the light strikes.

Originally Posted by DrRocket
Originally Posted by Janus
Originally Posted by Ghrasp

On a longer time scale such a small accelleration still would have huge consecquences off course because of the s^2.
I already showed where you would need a time scale many times longer than the age of the universe for there to be any significant consequences.
In fact time scales are not important. Classically both the attractive gravitational forcde and the miniscule repulsive force of light pressure follow inverse square laws. The only effect would be a tiny correction to the gravitational force, effectively an insignificant reduction in the gravitational constant (so far to the right of the decimal point to be well within the error limits for that constant or even GM where G is the universal constant and M is the mass of the sun anyway.)
Would this effect become strong enough to matter near an incredibly bright light source, like a quasar?

Would the repulsive effect of light trapped in the middle of a black hole be of any importance, if a lot of it accrued over time?

57. Actually, there's no interference in the sense of what you're talking about. Light has a property called "superposition", which means that two photons can pass through each other like ghosts, without affecting each other in any way whatsoever.
I can understand that if a foton is considered a mathematical point
(except maybe that I can't imagine anything going through something that has no width or surface)

but light is not just fotons. There is (as you know) more to it then a foton defined that way.

So the space properties (magnetic field electric field) between the lasers can be influenced.
Hence the "particle" ( naming a point a particle is still strange to me) character of one laser could be influenced by the other character or side coming from the other laser and offcourse vice versa. As light is electro-magnetic radiation, The electric side from one laser could be influenced by the magnetic component coming from the other.

58. Originally Posted by Ghrasp
Actually, there's no interference in the sense of what you're talking about. Light has a property called "superposition", which means that two photons can pass through each other like ghosts, without affecting each other in any way whatsoever.
I can understand that if a foton is considered a mathematical point
(except maybe that I can't imagine anything going through something that has no width or surface)

but light is not just fotons. There is (as you know) more to it then a foton defined that way.

So the space properties (magnetic field electric field) between the lasers can be influenced.
Hence the "particle" ( naming a point a particle is still strange to me) character of one laser could be influenced by the other character or side coming from the other laser and offcourse vice versa. As light is electro-magnetic radiation, The electric side from one laser could be influenced by the magnetic component coming from the other.
Totally, completely and utterly wrong.

You have no idea what you are talking about.

59. At wikipedia (it,s not always right...I know. It,s not always wrong either)

There is a description that it is not a real pushing force.

60. It's still ping pong balls, just thrown at an angle to the wall. Because the ping pong ball leaves at an angle, the transfer of momentum is in a different direction, but it's the same math.

And it's still easy to tell a push in one direction from a push in another. There's nothing perpendicular to the direction of the light.

61. Originally Posted by Ghrasp
At wikipedia (it,s not always right...I know. It,s not always wrong either)

There is a description that it is not a real pushing force.
Wiki says nothing of the sort.:

"Escaping planetary orbit
Sails orbit, and therefore do not need to hover or move directly toward or away from the sun. Almost all missions would use the sail to change orbit, rather than thrusting directly away from a planet or the sun. The sail is rotated slowly as the sail orbits around a planet so the thrust is in the direction of the orbital movement to move to a higher orbit or against it to move to a lower orbit. When an orbit is far enough away from a planet, the sail then begins similar maneuvers in orbit around the sun.[3]"

http://en.wikipedia.org/wiki/Solar_sail

If they had said what you are saying, which Wiki did not, then it would be wrong.

What they are describing are sailing maneuvers that are exactly the same as those executed by an ordinary sailboat in taking a course that is not directly downwind, with added dimension of orbital mechanics.

62. Try to sail someday in a boat without a keel or a rudder and try steering just from the sail. At best use a boat that is shaped perfectly round.

Comparing with a boat at sea is misleading because the sail and manipulating it is not only steering the boat. The water and the construktion of the boat (the keel, rudder, the flanks, it,s shape) can,t be missed otherwise any boat could only sail in winddirection. You can,t use that in space.

If you sail away from an island you can do it downwind but also under ninety degrees from the wind, Gain speed and you get more and more distance from the island keeping the winddirektion halfwind. Sailing is a game of wind, water and the boat.

You can't put a game in a simple theory, then it stops "being a game".
That counts for me as well.

I mentioned earlier there will be a pusheffect also (so both and then you have two forces to steer and play the game as when sailing)

But I don,t see it as a real force, as pushing like with fysical contact of a pingpongball or marble when it hits something. A particle considered a geometrical point (or line ?) is imaginary by definition (not meaning not real, imagination is real ain,t it ?).

More or less the same as you can see gravity as not a real force.

63. You're going to have to define "not a real force" because for all purposes both examples act like any "real force."

64. In the way Einstein mentioned gravity not being a real force.

If you put a magnet on a table vertical and roll a charged (relative to the magnet) sphere of iron, with the diameter the same as the magnet is long, past it, when it passes a force from the magnet seems to push it away or attract it. But it,s never a pushing force or attracting force between both. The force is constantly perpendicular to the direction the sphere rolls and the vertical orientation.

In this case you use the field idea of the magnet to explain it and get insight. It,s also called a force but in a different way, it,s not a pulling or an attracting force (or at least depends which side the sphere passes and what it,s direction of motion is.)

65. Originally Posted by Ghrasp
Try to sail someday in a boat without a keel or a rudder and try steering just from the sail. At best use a boat that is shaped perfectly round.

Comparing with a boat at sea is misleading because the sail and manipulating it is not only steering the boat. The water and the construktion of the boat (the keel, rudder, the flanks, it,s shape) can,t be missed otherwise any boat could only sail in winddirection. You can,t use that in space.

If you sail away from an island you can do it downwind but also under ninety degrees from the wind, Gain speed and you get more and more distance from the island keeping the winddirektion halfwind. Sailing is a game of wind, water and the boat.

You can't put a game in a simple theory, then it stops "being a game".
That counts for me as well.

I mentioned earlier there will be a pusheffect also (so both and then you have two forces to steer and play the game as when sailing)

But I don,t see it as a real force, as pushing like with fysical contact of a pingpongball or marble when it hits something. A particle considered a geometrical point (or line ?) is imaginary by definition (not meaning not real, imagination is real ain,t it ?).

More or less the same as you can see gravity as not a real force.
Wrong.

Go read up on momentum and conservation of momentum.

66. Originally Posted by Ghrasp
In the way Einstein mentioned gravity not being a real force.

If you put a magnet on a table vertical and roll a charged (relative to the magnet) sphere of iron, with the diameter the same as the magnet is long, past it, when it passes a force from the magnet seems to push it away or attract it. But it,s never a pushing force or attracting force between both. The force is constantly perpendicular to the direction the sphere rolls and the vertical orientation.

In this case you use the field idea of the magnet to explain it and get insight. It,s also called a force but in a different way, it,s not a pulling or an attracting force (or at least depends which side the sphere passes and what it,s direction of motion is.)
Wrong.

There are two forces in your scenario, related forces.

Since your sphere is iron, a magnetic material, there is the magnetic force attracting the sphere towards the magnet.

Second, since the sphere is hypothesized to be electrically charged, there is the Lorentz force exerted on a moving charge in a magnetic field, which is perpendicular to both the magnetic field and the velocity of the charge.

Both are forces in every sense of the word.

67. Not only that, but it effects both objects equally. I can't imagine how you've never seen a magnet and a roughly equally sized piece of metal jump towards each other.

68. Since your sphere is iron, a magnetic material, there is the magnetic force attracting the sphere towards the magnet.
The difference in distance of a pole of the sphere to the two poles of the magnet is minimimal but you,re right it would still be there. But as the ball rolls it will tend to toll the magnetization with it. The ball will tend to keep rolling relative to the magnetic orientation. There is a tension between that (magnetisation has a resistance changing it there will be a resistance so the spin of the rolling ball and it,s magnetic orientation will struggle against each other. The ball tends to roll the magnetisation with it because of this and the magnet try,s keeping it straight up. What happens ? Induction like effects maybe so that there will build up a charge difference inside the sphere and therefor an electric field perpendicular to the rolling direction and the magnetic fieldlines (in the way as with lorentzforce).

Anyway no matter what there will be a lorentzforce in this case if there is a charge difference. I used the example because I wanted to explain that a lorentzforce can't be put as a force coming directly in a straight line from the magnet. It acts as a fieldforce. At times it can seem a pushing or attracting force but it is misleading to look at it that way.

That is a general thing with lorentzforce and thus I used the example to illustrate what I meant with "not a real (pushing or attracting) force".

69. So a real force has to be in a straight line between two objects? What's the point of such a distinction?

70. No In relation to this the lorentzforce is an example of a force that is not a pushing or attracting force allthough it can seem to act that way. The force is related to the orbitational direction and the "field" it moves in.

So it is never a force that influences the motion of a particle, it doesn,t push or pull. If you want to name it a force as well it,s fine offcourse the distinction is with pulling or attracting forces. You could make that distinction by using different words for it but not necessarily, different type of force or whatever is fine also as long as the distinction is clear it suffices for understanding the distinction.

I prefer looking at lorentzforce related to relativity (Lorentz and Einstein where acquinted and influenced each other in there time) and in relativity it,s about "deforming spacetime" distinct from a pulling force. Therefor I sometimes prefer to think of lorentzforce in similar terms instead of a force to see how it can be related to Einsteins ideas because the word force automatically tends to distinct it from the ideas of deforming spacetime as there word force is not used there.
But it is about the distinction more then the words used to make the distinction and make it clear.

71. Originally Posted by Waveman28
Originally Posted by DrRocket
NO. An electromagnetic wave consists of a great many photons.
I strongly beg to differ. Single photons cannot have a frequency, wavelength, a phase or even an amplitude, which are all well known properties of light, it just makes no sense. The wave model of light is much more logical and correct. The photon concept arose during the irrational era of quantum mechanics, and should be avoided. Lets face it, light is a wave: How can photons explain interference patterns? How can they explain refraction? How can they explain standing waves? How can they explain phase change upon reflection? How can they explain polarization? This list goes on and on. Light MUST be a wave. We made an error by changing our model because we misunderstood one thing: the photoelectric effect. We call light a particle to suit one experiment, which then contradicts HUNDREDS of earlier experiments. Even worse, some people think that light is both a particle and a wave, and this is just delirious.
There is a QED primer posted in this very forum that addresses every one of your questions. The primer links to a series of videos of lectures given by Richard Feynman (to a lay audience). I have watched these lectures several times. They are absolutely oozing with good information.

72. Originally Posted by salsaonline
There is a QED primer posted in this very forum that addresses every one of your questions. The primer links to a series of videos of lectures given by Richard Feynman (to a lay audience). I have watched these lectures several times. They are absolutely oozing with good information.
They are indeed wonderful.

Just as note to interested readers. These lectures are the basis for Feynman's book QED which is an explanation of quantum electrodynamics directed at a general audience. The book is also a masterpiece of exposition by a genuine master of the subject.

73. Originally Posted by Ghrasp

So it is never a force that influences the motion of a particle, it doesn,t push or pull.
Totally wrong.

The Lorentz force is precisely the force that determines the path of an electron from the electron gun to the phosphorent screen in a cathode ray tube. If it did not influence the motion of a particle, neither oscilloscopes nor televisions with picture tubes would work.

74. Originally Posted by Ghrasp

Comparing with a boat at sea is misleading because the sail and manipulating it is not only steering the boat. The water and the construktion of the boat (the keel, rudder, the flanks, it,s shape) can,t be missed otherwise any boat could only sail in winddirection. You can,t use that in space.
Space would be like frictionless water. The geometry of the boat only matters in you scenario because of the friction effects of the water acting on the hull.

Originally Posted by Ghrasp
Actually, there's no interference in the sense of what you're talking about. Light has a property called "superposition", which means that two photons can pass through each other like ghosts, without affecting each other in any way whatsoever.
I can understand that if a foton is considered a mathematical point
(except maybe that I can't imagine anything going through something that has no width or surface)

but light is not just fotons. There is (as you know) more to it then a foton defined that way.

So the space properties (magnetic field electric field) between the lasers can be influenced.
Hence the "particle" ( naming a point a particle is still strange to me) character of one laser could be influenced by the other character or side coming from the other laser and offcourse vice versa. As light is electro-magnetic radiation, The electric side from one laser could be influenced by the magnetic component coming from the other.
If you take photons out of it, superposition becomes more true, not less true. Light waves do nothing to each other. No matter what. There is no way to make a light wave push another light wave, or move another light wave, or steer another light wave.

As I said, they pass through each other like ghosts.

The only effect they might exert on each other is from their gravity, and.... I am loathe to point that one out because it's such a small effect it would never be measurable. The mass of a photon is abysmally small. Your computer keyboard exerts a gravitational force on your head many million times stronger than one light beam would ever exert on another light beam.

75. Space would be like frictionless water.
That was what I meant, because of that the sail might as well be a loose sail attaching it to anything won,t help to stabilize or steer it. The tension between water and wind, boat and sail, typical for sailing would all be missing. Steering it with different laser to different points of the sail won,t work also, it would only start the sail to rotate.

76. Bouncing ping pong balls off the wall at an angle will result in the wall gaining momentum at an angle compared to the direction the ping pong ball was coming from. Plus, most light sail designs include other forms of propulsion for attitude control and other low-delta-v manuevers.

77. Originally Posted by Ghrasp
Space would be like frictionless water.
That was what I meant, because of that the sail might as well be a loose sail attaching it to anything won,t help to stabilize or steer it. The tension between water and wind, boat and sail, typical for sailing would all be missing. Steering it with different laser to different points of the sail won,t work also, it would only start the sail to rotate.
The way they make up for that is using gyroscopes. With gyroscopes you can change the vessel's orientation arbitrarily to face any way you want, and keep that orientation stable.

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