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This is one where it doesn't matter how many books I've read about it, they all seem to evade the elephant in the room - from examples like the mirror on the boat reflecting a single photon up and down to a rocket approaching the speed of light.

The question is: the books talk about no absolute speed or movement ie - if two bodies are floating in space then from both perspectives neither of them can say who is "moving" therefore who has a "speed". A train leaving a platform can be perceptibly seen to be moving away from he platform - but from the passenger's point of view - is not the platform also moving away from the train?

On these bases - in the case of the astronaut "approaching the speed of light" - my question is "compared to what"?

And if there is an answer to this - e.g "the earth" - then why do we insist there is no absolute position in space? One thing must be moving and the other must be still in order for this to be logical - otherwise they are both moving, relative to each other and the effect of speed should logically cancel out? or maybe I am missing the effect of "mass".

I am not classically trained in Physics but an keenly interested so for me the popular books seem to very frustratingly gloss over this nuance and make the whole Relativity thing difficult to accept ...

sammy gerbil
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BluePowder
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  • It's hard to understand exactly what you're confused about. " is not the platform also moving away from the train?" YES! Are you asking how the statement "Time goes slower in the moving system" doesn't contradict this symmetry between train and platform? – OON Dec 23 '16 at 09:04
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    In any case, I think it would be useful for you to read the first two answers to this question http://physics.stackexchange.com/questions/155015/a-sees-bs-clock-running-slow-and-b-sees-as-clock-running-slow?noredirect=1&lq=1 – OON Dec 23 '16 at 09:51
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    I think so yes. In the case of train/platform there I do not see how there can be a difference in speed and therefore how we can derive a conclusion based on the difference in speed. in the above case, one cannot say that one is moving faster (or slower) than teh other therefore the time dilation point seems to have no basis. – BluePowder Dec 23 '16 at 11:10
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    See: What is time dilation really? – John Rennie Dec 23 '16 at 11:18
  • In the train's frame, the train is still, the platform is moving, and the clock on the platform is running slow. In the platform's train, the platform is still, the train is moving, and the clock on the train is running slow. Both descriptions are equally valid. By analogy: In my frame, I'm facing forward, my wife is facing backward and the living room is to the left; in my wife's frame, she's facing forward, I'm facing backward, and the living room is to the right (she happens to be facing me). Do you feel the need to ask "facing forward compared to what?" – WillO Aug 03 '18 at 02:47

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I guess the essence of your question is in here:

On these bases - in the case of the astronaut "approaching the speed of light" - my question is "compared to what"?

And if there is an answer to this - e.g "the earth"...

No, not "the earth".

When someone says "astronaut is approaching speed of light" it is assumed "in my frame of reference". Or at least it should be obvious what frame of reference he is talking about.

The question is: the books talk about no absolute speed or movement ie - if two bodies are floating in space then from both perspectives neither of them can say who is "moving" therefore who has a "speed". A train leaving a platform can be perceptibly seen to be moving away from he platform - but from the passenger's point of view - is not the platform also moving away from the train?

I think you are correct here.

We have some process (let it better be "train is moving along the platform"). And we can look at this process from different frames of reference. In the frame of reference of a person who is staying on platform, the platform if at rest, the train is moving. In the frame of reference of a person who is sitting on the train, the train is at rest, the platform is moving.

There is no way to tell, which frame of reference is better, which one is "truly staying still". How would possibly these two persons argue whose frame of reference is better? "Almost everything around me is not moving, so my velocity is really zero!". "Not at all, it's only the earth which is not moving in your frame of reference, there are other, bigger and possibly better planets which are moving, so what?".

Better argument would be something like "Look, when this toy canon fires, it does not matter which way it is directed, the maximum fire distance is always 1 meter! That means I am not moving. Would I be moving, the fire distance would be longer (or shorter?) when I point the canon in the direction of my own velocity!". You see, it's some experiment that one can make and find out if it is moving or not without looking around. But the results of this experiment would be the same on the moving train. It will not help to find out if the train is moving or staying.

And no other experiment would do. This is because physics lows are the same in all the systems of reference. (well, in "inertial" systems of reference. Physics lows are quite different in f.e. rotating systems of reference - that's why it's possible to find out if the room you are in is rotating or not without looking out of the window)

You can choose any inertial frame of reference to describe the world around. Some of them may be more or less convenient for some particular problem, but there are no major differences.

And this is the main point of relativity theory! You can choose any inertial frame of reference, and neither of them is better than another.

This was quite obvious for physicists for couple of hundred years until they found that speed of light does not depend on speed of the source of light. Here it is, the speed of light is that much only in some truly staying frame of reference! One can measure the speed of light and find out if he is moving or not.

And all the special relativity theory is the explanation how can it be that speed of light does not depend on speed of it's source, but still there is no "truly not moving" frame of reference.

lesnik
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The question is the valid thing to ask in this subject. Exactly!

You have to choose a reference frame, and then start your physics, you would get consistent answers in any of them and later you using the special relativity would be able to translate your solution to other reference frames. But you have to stick with the one you chose and never change it during the experiment.

I think you have to note that in these kinds of questions there's an initial problem, which if you answer, you ultimately have the problem solved. If one sees something and the other sees it differently, how are they going to communicate about it?

Kid A
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All speeds are relative, as you say. When you see a sentence such as 'a spaceship moving close to the speed of light', it is just a lazy conventional way of describing a situation in which you have one frame which is taken to be stationary, and another which is not. Generally, when you have two frames, A and B, which are moving relative to each other, you can take either of them to be stationary. In practice, therefore, people often only specify one of the frames (eg the spaceship), and assume the reader will understand that the unspecified other frame is taken to be stationary.

Sadly a lot of textbooks and other written explanations of special relativity are riddled with the kind of ambiguity you have identified, and other forms of sloppy terminology, which introduces scope for various common mis-understandings.

Most of the key effects of special relativity are symmetric, in the sense that it doesn't matter which frame you take to be stationary. For example, you might read that a muon speeding through the atmosphere will be time dilated to some extent in the frame of the Earth. Well, you can consider the muon to be stationary, and the Earth to be time dilated to exactly the same extent in the frame of the muon.

The symmetry can break down when you have acceleration, since acceleration is not relative in the sense that speed is. If two people are coasting inertially relative to each other, and one of them accelerates, that will lead to asymmetrical effects. The most famous example is the so-called twin paradox, in which one twin stays on Earth and the other travels way before returning to find the stay-at-home twin has aged more in the meantime. The fact that the two twins have not aged by the same amount is related to the fact that only the travelling twin underwent a change of reference frame when they reached the turn-around point.

Marco Ocram
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