# On the idea of time in physics-relativity

Discussion in 'Physics & Math' started by ash64449, Mar 15, 2013.

1. ### ash64449Registered Senior Member

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Length Contraction and Time dilation are not on beams.It is on the train. and train is shorter and time dilates does not make light fast or slower. it makes Light speed constant. As a result,Galilean Invariant remains Invariant.

3. ### ash64449Registered Senior Member

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Einstein's Thought Experiment is Galilean Invariant!! You mean this one right: Lighting strokes and simultaneous events and all?? that is Galilean Invariant...

5. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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I didn't say it was on the beams, it is on the train.

Say for example the train contracted, the beam would be measured to be faster since it had to travel less distance. The ongoing beam and the oncoming beam couldn't both be measured to be faster because of length contraction and still be measured to travel at the same speed. The same amount of length contraction alone would NOT compensate for beams traveling in both directions.

7. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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It actually isn't. If it was Galilean Invariant then the observer on the train could assume that he is at rest and that the flashes of light reach him at the same time. The mind experiment doesn't say the observer on the train receives the flashes at the same time, but that he observes them to reach him at different times. If it was Galilean Invariant then the observer would measure the beams to reach him at the same time in the same way they found in the M&M experiment. He didn't know what the actual experiment said about this situation.

8. ### SyneSine qua nonValued Senior Member

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It does not, and the video you linked shows this.

If the speed of light were not constant then the velocity of the train would have no impact on the observed timing between the two events. The only way both observers could see both events as simultaneous would be for light to have a variable speed or for them to be at rest relative to each other. Since we know that neither is the case, both cannot observe these events as simultaneous.

9. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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If the observer on the train assumed he was at rest, then the observer on the station would not observe the beams to reach them at the same time. So then it would create a paradox. Both observers would say that the other didn't receive the flashes at the same time. But, since the speed of light is constant in all frames and they can all assume that they are at rest while in constant motion, then they can all say that the beams would reach them at the same time, so then there is no such paradox. The M&M experiment was said to be accelerating with the motions of the Earth and the Sun, the beams of light reached the detector at the same time!

I think this mind experiment has been swept under the rug in mainsteam physics, but then somehow has exploded out of porportions on the internet. I think it was as easy as saying that it comes from one of Einsteins books, but I don't think SR really has keeped the same form as when Einstein wrote this. I had a friend a long time ago that got this book and asked me what I thought about it, and it didn't seem to go along with any other book I have read about SR. It may have just been a dumbed down version of a more complex theory. I have never heard of any example as being totally accurate in every way.

10. ### SyneSine qua nonValued Senior Member

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Actually, it is the fact that the train observer can assume himself at rest that allows the platform observer to see his non-simultaneous strikes as simultaneous. Just like your linked video pointed out, the platform observer will correctly predict the train passenger's observation. The train passenger can also correctly predict that the platform observer will see simultaneous strikes. No paradox at all. MMX is not analogous to this thought experiment, as MMX does not compare relatively moving observers, nor spatially separated events. Spacetime events (those that happen at the same place and time, i.e. coincident events) are coincident in all inertial frames. If the platform observer sees both strikes as simultaneous then all other inertial observers will see the light from both strike events reach that observer at the same time. And the same goes for the non-simultaneous events for the train observer. All observers agree on what the other sees, just not on the timing of the spatially separated events. The result of the MMX detector is a coincident event all observers would agree on, even if in relative motion.

The only thing seemingly "swept under the rug" here is the fact that you do not have the requisite understanding of SR to be offering anyone your opinion on it. I do not know what "other book [you] have read on SR", but if it advocates a variable speed of light then...garbage in-garbage out.

11. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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This is complete garbage. I don't care that they dug up Einsteins old garbage and proclaimed it as the undying truth about relativity on the internet, so then you will make up whatever garbage possible in order to back it.

Why would I then consider any of your opinions, if for one you cannot even correctly state my own opinions, and two you are nieve enough to think that there are books written about variable speed of light. That is just garbage that you read on the internet. I haven't ever read such garbage in any book, or claimed it myself.

12. ### ash64449Registered Senior Member

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795

You interpreted in a wrong way. it is like this:

The train is moving uniformly in straight direction. Since train is moving uniformly,observer in train won't "feel" the motion of train. But that doesn't mean that he is not moving. He is moving.As a result,speed of light measured would be less. But This "violates" the Special Theory of Relativity is Restricted sense. All general Laws of nature hold good exactly to all Galilean system of co-ordinates. Very Important law here is that SPEED OF LIGHT IS CONSTANT. IT CANNOT CHANGE AND SHOULD NOT CHANGE. Both,SR and Law of light are elegant. So Both of them cannot be wrong. So to show how Time dilates,he introduced simultaneous. In order to test simultaneity,he provided an instrument to an observer in train to see both the lightning at the same time. So if the observer sees both the light at the same time,then both are simultaneous. So in this way he developed the idea of time in physics. If two clocks are of identical settings,then their pointers will move in such a way that their pointers are simultaneously the same. Now we are able to describe the 'time' of event the reading (position of the hands) of that one of these clocks which is in the immediate vicinity (in space) of event. And Also if they are of identical construction,then these clocks go at the SAME RATE.... So then we try this: Are Simultaneous events to the observer in rest simultaneous to the observer in the train??

Answer is negative. That is what we have been discussing here. Now this is the part that you don't understand right?? So let me try to make you understand...
One of the reason why you aren't able to understand is because you are confusing with uniform motion and it's "feeling" and rest. Look,when object moves in uniform motion,he doesn't understand the motion and he "feels" he is in rest. But He is "Actually" in motion. So You see,Relative velocity effects does happen to observer. Relative velocity doesn't depend upon "feeling" Of rest. So He would measure speed of light to be less even though he "feels" he is in rest. But this is in violation of relativity: Speed of light should be CONSTANT in relative velocity also.(Conclusion from SR). But he is "moving". So relative velocity should be there. So how to accommodate these possibilities????

Easy. When Lighting strikes both ends simultaneously( simultaneously in reference to observer who is in rest), Light travels. So in order to accommodate SR And Law of Constant of light: Let us think that light is moving with its own speed. And relative velocity too be seen light travelling at the speed of light. But observer is moving.. he is not in rest. SO he is hastening towards the light.But train is hastening away from the lighting at the back. So Two lighting are not simultaneous. SO Those two observers have different time. SO THERE IS NO TIME OF EVENT. THESE EVENTS TOOK PLACE IN DIFFERENT TIME IN DIFFERENT REFERENCE FRAMES.(Observer in train is in middle)

13. ### SyneSine qua nonValued Senior Member

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So what part do you not understand? Or are you satisfied making crank proclamations that well-known science is "garbage"? Nothing about SR has appreciably changed since this thought experiment to alter its implications in any way. So it seems you are just determined to remain ignorant.

14. ### Write4UValued Senior Member

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As I understand it, Doppler effect is a result of relative motion.

Two persons standing along a train track, some distance apart.
Between them a train is moving at constant speed. While moving the train blows it's whistle. Later that day the two persons meet and one says, "I like that new train whistle, it is tuned to the note of D sharp". The other person replies, "Yes it is very loud, but it is tuned to note D flat".
Who is lying and who is correct?
They are both telling truth, but they are both wrong. To a third person on the train the whistle would have sounded the correct tuned note of D

15. ### SyneSine qua nonValued Senior Member

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Another non sequitur, off-topic post.

16. ### Write4UValued Senior Member

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What topic?? The topic title reads; "On the idea of time in physics-relativity"

By your strict standards of scientific correctness this topic should not have been allowed at all. The topic has no topic. If you know what this topic is all about, please explain it to me, so that I can learn. If you cannot, afford me the same courtesy I afford you, ok?

17. ### SyneSine qua nonValued Senior Member

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The topic is usually defined in the OP, which in this case is simultaneity. You are familiar with forums in general, right?

18. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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That has nothing to do with why I said the mind experiment is not Galilean Invariant. My exact words was that it is not Galilean Invariant in the consistancy of the constant speed of light. This means that the observer on the train assumes that he measures light to travel at the same speed as the observer on the station, this is wrong. The laws of physics would have only changed in the sense that the observer on the train will not measure a flash of light as reaching him at the same time that is the same distance away from him, that is all. I am saying that alll observers in constant motion can assume that they are at rest and then would observe two flashes of light the same distance away from them to reach them at the same time. How the passenger on the train feels is irrelevent to this issue. The mind experiment just doesn't assume Galilian Invarience of the speed of light.

19. ### Neddy BateValued Senior Member

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That is only true if the two flashes were emitted at the same in the first place. And the test to determine if they were emitted at the same time is the very same test that you propose. Therefor, if the two flashes from the same distance away do not reach the observer at the same time, he or she must interpret this to mean the flashes were not emitted at the same time. And he or she would be 100% correct to do so, because the speed of light is already defined, and not up for re-definition every time someone sees two things at two different times.

20. ### SyneSine qua nonValued Senior Member

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This TE does demonstrate the invariance of the laws of physics. You simply have not figured out that this invariance requires giving up Newton's absolute simultaneity. You seriously need to get with the physics of 100 years ago.

21. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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This is why I brought up the M&M experiment, it assumes that the Earth is in motion, so then by sending beams of light in different directions then you could measure the speed of light against that motion. The beams of light in the experiment reach the detector at the same time. It doesn't matter that a beam is sent with the motion, against the motion, or perpendicular to the direction of motion, the beams of light will reach a location the same distance away at the same time!

22. ### Prof.Laymantotally internally reflectedRegistered Senior Member

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The only reason you say this is because the only physics you have gotten with is 100 years or older. The problem is that it doesn't match up with the properties of light. In modern physics, a light beam the same distance away from an observer as another light beam will reach the observer at the same time no matter what degree of motion that observer is in, period.

23. ### Neddy BateValued Senior Member

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Yes, but only if the beams were emitted at the same time to begin with. In Einstein's thought experiment with two different strokes of lightning, as far as the train is concerned, there is no requirement that the beams were emitted at the same time to begin with.