# Thread: Einstein's Theory - Which One is Moving?

1. This speed of light thingamey … and time slowing down the higher the speed and all that business … what has always puzzled me about this mind boggling stuff is how you decide something is actually moving in the first place, never mind moving near the speed of light. I mean, how do you judge speed? Speed is not a definite fixed measurement (like light), it's a relative measurement. You can't decide that something is moving without relating it to something, and even then it might be that other something that is actually doing the moving (i.e the old 'train in the station' illusion). On the Earth it's easy peasy, we always know a speed measurement is relative to the Earth, but out in the depths of space, what on earth … .. I mean what in the universe do you relate it to? The nearest star? And if that star is also moving?

I think, repeat think, I actually understand the idea that the speed of light is a constant (well I thought I did before someone went and spoilt it by stating that it displays a doppla effect - red shift - maybe I'm up a gum tree with that one, anyway I digress). A commonly repeated phrase in this field of science is "Suppose you are travelling close to the speed of light" ….. well, I fall down at that first hurdle. How do you judge such aspeed? Relative to what? And then when the theory goes further and suggests one object is moving faster than another and experiences a slowing down of time relative to the other I get even more confused. I mean, how do you decide which one has moved the faster? It seems to depend on which one has experienced a 'change' in speed rather than just a speed.

For example:- Suppose the Earth is travelling in space at 24,000 miles/hr in its orbit around the Sun (speed judged relative to the Sun of course). A rocket is fired into space from the Earth at 24,000 miles/hr (relative to the Earth) in the exact opposite direction to which the Earth is travelling. Both the Earth and the rocket have atomic clocks set to exactly the same time at take off. Which one will have slowed down when the two eventually meet again in a year's time?

From the point of view of the Earth it is the rocket that has moved. It is the rocket that has shot off into space and come back in a year's time. Therefore it's the rocket's clock that should be slower.

From the point of view of the Sun it's the Earth that hasmoved. The Sun would have seen a rocket sudden stop dead in space and the Earthcarry on travelling around at 24,000 miles/hr and eventually meet up with thisstationary rocket in a year's time. Therefore the Sun would conclude that it'sthe Earth's clock that should have slowed down.

It seems to me that it's the one that experiences acceleration and deceleration that is the deciding factor, but I cannot see how that fits in with Einstein's theory. In the above example I suppose it would be the rocket's clock that would have slowed down, but yet …. for a whole year that clock was 'stationary'.

PS. Why have a I just had to go through my whole text and separate out words that have been joined together? My original was alright. Is Einstein having a go at me?

2.

3. It is all relative. (hence the "Theory of Relativity"). It starts from the fact that if you are in a steady state of motion (same speed, no change of direction, no acceleration) in a sealed vehicle with no windows, then there is no way of telling if you are moving or not. You can only judge movement relative to something else.

If you have two people, A and B, then it may be natural for A to consider themselves stationary and B as moving. At the same time it is natural for B, from their point of view, to consider themselves stationary and A moving.

A will look at B and see B's clocks running slower than A's.
B will look at A and see A's clocks running slower than B's.

If the situation involves acceleration of one person, e.g. the Twins "Paradox", then the situation is no longer symmetrical. You can feel acceleration so you know that you have accelerated (and the other person hasn't).

4. Originally Posted by RickArcher
Speed is not a definite fixed measurement (like light), it's a relative measurement.
That is absolutely correct.

well I thought I did before someone went and spoilt it by stating that it displays a doppla effect - red shift -
Doppler effect and red shift are distinct phenomena, but in both cases all that changes are the wavelengths of the light - the speed of light remains constant nonetheless, so your understanding need not be affected.

How do you judge such aspeed? Relative to what?
Relative to another observer or frame of reference.

I mean, how do you decide which one has moved the faster? It seems to depend on which one has experienced a 'change' in speed rather than just a speed.
If you have two frames ( without acceleration ) you can decide only their relative speed with respect to one another. The question as to who goes faster is meaningless. You would need to have at least one other frame of reference to compare the first ones to, in order to judge who goes "faster".

Both the Earth and the rocket have atomic clocks set to exactly the same time at take off. Which one will have slowed down when the two eventually meet again in a year's time?
This isn't easy to answer since time dilation is influenced not just by speed, but also by acceleration and the presence of gravitational fields. In this particular example one would have to actually do the maths to see whose clock shows what, it depends on the exact motion of the rocket.
In general terms, it is stationary observers which experience the most proper time; if the rocket goes fast enough and far enough, then returns to earth, the atomic clock in the rocket will show less elapsed time than the clock left behind on earth.

It seems to me that it's the one that experiences acceleration and deceleration that is the deciding factor, but I cannot see how that fits in with Einstein's theory.
To correctly and comprehensively treat acceleration one needs to use General Relativity as opposed to Special Relativity. Under general relativity acceleration is equivalent to the presence of gravitational fields ( this is called the equivalence principle ), and the appropriate mathematical tools are available to handle just such problems. So it fits in just fine !

5. Cripes Strange, that was a quick reply. Can you travel close to the speed of light and not told anyone?

Thanks Strange and Markus for your replies but you'll have to give me some 'time'on this one. Gosh, this is confusing stuff. I keep on looking up a word (normally directed to Wikipedia), then find more words I don't understand, have to look them up …. then more words … look them up … yet more words to look up …. in the end I've forgotten why I'm looking up all these bloomin' words in the first place.

I'll be back.

6. I don't seem to be getting very far by reading about it, I'll have to ask more questions.

Originally Posted by Strange
A will look at B and see B's clocks running slower than A's.
B will look at A and see A's clocks running slower than B's.

If the situation involves acceleration of one person, e.g. the Twins "Paradox", then the situation is no longer symmetrical. You can feel acceleration so you know that you have accelerated (and the other person hasn't).
If B accelerated would B see A's clock speeding up?

Markus - Ah I see about the red shift thing now. Of course, the doppla effect doesn't alter the speed of sound, just the wave length.

Originally Posted by Markus Hanke
Originally Posted by RickArcher
How do you judge such a speed? Relative to what?
Relative to another observer or frame of reference.
But if that other observer is also moving relative to something else? I mean, suppose that relative to object A you accelerate away to close the speed of light, but if A is already traveling at close to the speed of light relative to B, in the same direction? What happens in that scenario? What speed would you be doing relative to B?

It seems to me that Einstein's theory is all to do with acceleration, not speed i.e. nothing can accelerate faster than the speed of light, but something can actually travel faster than the speed of light, depends what you relate it to. On the other hand, when do you decide something can't accelerate faster than the speed of light? Does that have to be all in one go, from zero to light speed plus? I mean, suppose you accelerated up to 75% the speed of light and then switched your engines off. Can you then take it that you are starting from zero again and then accelerate up to 75% light speed again? How do you define a starting point to this acceleration?

Originally Posted by MarkusHanke
In general terms, it is stationary observers which experience the most proper time; if the rocket goes fast enough and far enough, then returns to earth, the atomic clock in the rocket will show less elapsed time than the clock left behind on earth.
But how do you decide who is stationary? The rocket in my example doesn't go anywhere as far as an outside observer is concerned, such as the Sun. It only goes as fast in one direction as the Earth is going in the opposite direction, therefore stopping it, in other words you could say the rocket did not accelerate, it deccelerated. It deccelerated to a stop.

Also, this time thing. Is there a difference between matter slowing down and time slowing down? I mean, when the atomic clock and your body clock slows down due to this theory, is that perhaps ALL that's happening? Nothing to do with actual time istelf, it's just that the activity of matter is slowing down, like when things get cold … the atomic clock is ACTUALLY slowing down, your body clock is ACTUALLY slowing down, not time slowing down …. or is that one and the same?

This gets extremely confusing. I think I need to stick my head in a bucket of cold water now … I'm about to blow!!!!

7. Originally Posted by RickArcher
If B accelerated would B see A's clock speeding up?
Generally no, but the question is not actually as straightforward as that, since acceleration is equivalent to local gravitational forces. The final answer to this would thus depend on the actual amount of acceleration.

[QUOTE]But if that other observer is also moving relative to something else?[/QUOTE]

That's fine, then we have three distinct frames to consider.

What happens in that scenario? What speed would you be doing relative to B?
The total speed would be one very close to the speed of light, but never exceeding it. The calculation itself is done using the so-called Lorentz velocity addition formula.

nothing can accelerate faster than the speed of light,
That does not make sense, because light doesn't accelerate.

How do you define a starting point to this acceleration?
I don't understand this question. Acceleration is simply a change in speed.

But how do you decide who is stationary?
Stationary is a relative term, you can be stationary only in relation to some other point of reference.

Also, this time thing. Is there a difference between matter slowing down and time slowing down?
Time never 'slows down'. All that changes is the causal relationships between observers - you need at least two distinct observers to define the term 'time dilation'. Locally, a clock always ticks at exactly one second per second, so to speak. If observers are in relative motion to each other, then the geometric relationship between them in space-time changes. Time dilation then goes hand-in-hand with length contraction; where one observer sees time dilation, the other one will see length contraction, and vice versa.

8. Originally Posted by RickArcher
nothing can accelerate faster than the speed of light,

Originally Posted by Markus Hanke
That does not make sense, because light doesn't accelerate.

Okey, how about 'nothing can accelerate to a speed greater than the speed of light'?

Originally Posted by RickArcher
How do you define a starting point to this acceleration?

Originally Posted by Markus Hanke
I don't understand this question. Acceleration is simply a change in speed.

Yes but if that change in speed is to be measured, if the increasing speed is to be measured relative to the speed of light, you have to have a starting point to know when it is approaching the speed of light. In the example I gave, which starting point do you take as zero? Quote:-

" … suppose you accelerated up to 75% the speed of light and then switched your engines off. Can you then take it that you are starting from zero again and then accelerate up to 75% light speed again?"

When I refer to "switching your engines off" I am referring to being in space, therefore your speed will stay at 75% light speed relative to your starting point, unlike in Star Trek when Scotty says"They canna keep up this speed Capt'n". Yes they can, just switch them off.

Am I right that after the first increase to 75% light speed your mass has increased i.e. you've become denser? Does that increase in density remain even though you have now stopped accelerating? I will presume it does, therefore is this a method of deciding whether something has already experienced acceleration and how much it has experienced?

Originally Posted by Markus Hanke
Stationary is a relative term, you can be stationary only in relation to some other point of reference.

Exactly, so how can you ever decide that an object has started accelerating from zero in order to be able to measure its speed relative to light?

Originally Posted by RickArcher
Also, this time thing. Is there a difference between matter slowing down and time slowing down?

Originally Posted by Markus Hanke
Time never 'slows down'. All that changes is the causal relationships between observers ........ Time dilation then goes hand-in-hand with length contraction; where one observer sees time dilation, the other will see length contraction, and vice versa.

Crumbs, that's a paragraph that threatens to grab hold of my brain and strangle it!! That last sentence, when I earlier asked the question"If B accelerated would B see A's clock speeding up?" your answer was (Post #6) "Generally no, but the question is not actually as straight forward as that, since acceleration is equivalent to local gravitational forces. The final answer to this would thus depend on the actual amount of acceleration."

I can't seem to marry these two statements together.

Also, in that rocket example I gave, how do you decide if something is accelerating or decelerating? It's like what happens if you jump off a train, you don't accelerate, you decelerate. From Earth the rocket appears to be accelerating out into space but actually it's decelerating to a stop (relative to the Sun). It depends which way you are facing inside the rocket. Is it only the Earth's gravity that decides it is accelerating and therefore it's the rocket's time that will slow down? If you take the Earth out of the equation and replace it with another rocket, in other words you have two rockets orbiting the Sun at 24,000mph in one direction and then one of them turns to face the opposite direction and accelerates up to 24,000mph (or decellerates to a stop). In that case big gravitational forces wouldn't apply because the Earth's gone, therefore the latter rocket's clock will be faster than the other rocket's.

I think this is not really a subject one can discuss by text. It's just too complicated, we could be forever back and forth. It really needs to be discussed face to face. Thanks for your help in this matter Markus.

9. Originally Posted by RickArcher
Okey, how about 'nothing can accelerate to a speed greater than the speed of light'?
Replace "greater than" with "equal to" and it sounds OK.

Yes but if that change in speed is to be measured, if the increasing speed is to be measured relative to the speed of light, you have to have a starting point to know when it is approaching the speed of light.
You can't measure your own speed relative to the speed of light. You will always see light travelling at the same speed (the speed of light, c). You can only measure the speed of something else relative to you as a proportion of the speed of light. So in that case, you are the "starting point" (rest frame).

Am I right that after the first increase to 75% light speed your mass has increased i.e. you've become denser? Does that increase in density remain even though you have now stopped accelerating? I will presume it does, therefore is this a method of deciding whether something has already experienced acceleration and how much it has experienced?
Relativistic mass (which a lot of people think shouldn't be used as a concept at all because it causes exactly this confusion) is relative. So, as you accelerate away from the Earth, you will not find yourself getting heavier or denser. But the people on Earth could make observations of you that they could interpret as increased mass. (And you could make exactly the same observations about them).

So, you accelerate from space for a while and then shut off the engines and coast. You and everything in you ship will have exactly the same mass (as measured by you) as it did on the ground. If your ship now releases a tank containing all the garbage and waste that has accumulated. It will coast alongside you at zero relative velocity. You can use that as a reference point to accelerate to 75% of c, relative to that container. You can keep doing that forever. You will approach closer and closer to c, relative to your original rest frame (Earth) but never reach c. To understand why not, you need to look at the formula for relativistic addition of velocity.

Also, in that rocket example I gave, how do you decide if something is accelerating or decelerating?
Good question. You can't. You can be accelerating relative to one object but decelerating relative to another. All you can tell is that there is a change in velocity (or direction).

Remember: changes in measured time, distance, mass, energy, etc are relative. They are what one observer sees happen to another due to their relative velocity. And, if you take acceleration out of it, the situation is perfectly symmetrical. A seems B's clock run slower, length decrease, mass increase. And B sees exactly the same in A.

Think about perspective. As someone walks away from you, they appear to get smaller. When they look back at you, they see you as smaller as well.

10. Also, in that rocket example I gave, how do you decide if something is accelerating or decelerating? It's like what happens if you jump off a train, you don't accelerate, you decelerate. From Earth the rocket appears to be accelerating out into space but actually it's decelerating to a stop (relative to the Sun). It depends which way you are facing inside the rocket.
The answer to this one is the equivalence principle. What this says is basically that locally acceleration is indistinguishable from the presence of gravitational fields. In other words, an observer inside the rocket would measure a force during acceleration, and this force is quite independent on which way the observer faces, and also quite independent on any other points of reference. Motion is relative, but acceleration is not due to the presence of forces which can be locally measured.
Btw, the only difference between acceleration and deceleration is the direction of the resulting force. This direction is indeed relative, but not the magnitude of the force, i.e. acceleration.

11. Originally Posted by RickArcher
Quote:-

" … suppose you accelerated up to 75% the speed of light and then switched your engines off. Can you then take it that you are starting from zero again and then accelerate up to 75% light speed again?"
What you run up against here is how velocities really add up. The correct equation to use is

You might notice that if v1 and v2 are small when compared to c, the speed of light, the answer comes out to be very close to v1+v2.

However, 0.75c plus 0.75c results in an answer of only 0.96c

Or to put it the terms of your example, let's say, that after reaching 0.75c with respect to your starting point, you shut down our engines and deploy a buoy. The buoy shares your 0.75c velocity with respect to your starting point. You now accelerate to 0.75c with respect to this buoy, then shut your engines down and measure your relative speed with respect to both the buoy and your starting point. Youy will measure your speed wrt to the buoy as being 0.75c and your speed with respect your staring point as being 0.96c.

Someone at the buoy would measure the relative speed between him and both you and starting point as being 0.75c

Someone at the starting point would measure the buoy moving at 0.75c and you moving at 0.96c.

This all is a result of the fact that observers in different inertial frames measure time and space differently.

When I refer to "switching your engines off" I am referring to being in space, therefore your speed will stay at 75% light speed relative to your starting point, unlike in Star Trek when Scotty says"They canna keep up this speed Capt'n". Yes they can, just switch them off.

Remember, this is a TV show. Besides, what you are dealing with is the ship at "warp speed". In essence, the ship's engines are warping space in order to allow them to travel at greater than light speeds. We have to assume that it takes energy to do this and to keep the warp from reverting to it normal state. Thus if they were to shut down the engines, the warp would "flatten out" and they would revert to traveling at less than light speed.

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