# On the idea of time in physics-relativity

#### ash64449

Registered Senior Member
Hello friends,

i am now reading Theory of relativity written by Albert Einstein.It described on the idea of time in physics by the use of simultaneity.Lightning struck on two extreme parts of the train.In order to test whether the two lightning is simultaneous,a person is made to sit in the middle and two mirrors to let him see two lightning at the same time,if it does,it is simultaneous.. but why is the observer kept at the middle of the train?? Won't those events to him be simultaneous even if he was not sitting in the middle??

Hello friends,
but why is the observer kept at the middle of the train?? Won't those events to him be simultaneous even if he was not sitting in the middle??

Because the speed of light is not infinite. Let's say you are half way between the earth and the moon. A light is flashed on the moon and the earth at the same time - you would see them as simultaneous. If the same flashes of light were done again and you were on earth 10 feet away from the 'earth' light they would not appear simultaneous to you, would they? The flash on the moon would be more than a second after the earth flash.

Light travels at a finite speed, so by staying at the center of the train, the physical observer assures that he will see the light from each flash reach him at the same time only if they were originally simultaneous -- at least from his perspective. Since he saw both flashes at the same time, that's a fact which cannot be changed by a change of perspective.

Now for someone who doesn't have the train's state of motion, the observer still sees the flashes arrive to him at the same time, but that may be because the flashes started at different times, and the relative motion of the train moved the observer towards the direction of one of the flashes.

So what is simultaneous and far-apart from one person's perspective may not be simultaneous to another, and the key factor that makes the difference is the relative motion between what those people consider "at rest."

Because the speed of light is not infinite. Let's say you are half way between the earth and the moon. A light is flashed on the moon and the earth at the same time - you would see them as simultaneous. If the same flashes of light were done again and you were on earth 10 feet away from the 'earth' light they would not appear simultaneous to you, would they? The flash on the moon would be more than a second after the earth flash.

That means those events are simultaneous but because of the finite velocity of light,it is "appeared" that the events are not simultaneous??

That means those events are simultaneous but because of the finite velocity of light,it is "appeared" that the events are not simultaneous??

Yes.

Light travels at a finite speed, so by staying at the center of the train, the physical observer assures that he will see the light from each flash reach him at the same time only if they were originally simultaneous -- at least from his perspective. Since he saw both flashes at the same time, that's a fact which cannot be changed by a change of perspective.

Now for someone who doesn't have the train's state of motion, the observer still sees the flashes arrive to him at the same time, but that may be because the flashes started at different times, and the relative motion of the train moved the observer towards the direction of one of the flashes.

So what is simultaneous and far-apart from one person's perspective may not be simultaneous to another, and the key factor that makes the difference is the relative motion between what those people consider "at rest."

If observer moves in opposite direction to that of the train in the same velocity. for an observer outside he won't be moving.. But for that man in the train,he is moving. because he moves in direction opposite to the train,won't he perceive the simultaneous events to outside observer simultaneous to that man too?

then why not the case of what i said?? Man was moving,so it "appeared" to him that events are not simultaneous...

Physical observers in the same state of inertial motion conclude that events that to one are simultaneous are also simultaneous to the other.

Physical observers in the same state of inertial motion conclude that events that to one are simultaneous are also simultaneous to the other.

What?? I didn't understand.. Can you please tell in simple manner than just statements? I really want to understand...

That means those events are simultaneous but because of the finite velocity of light,it is "appeared" that the events are not simultaneous??
then why not the case of what i said?? Man was moving,so it "appeared" to him that events are not simultaneous...

Origin was talking about the example of a situation of geometry when the observer is not at the midpoint. You put "appeared" in scare quotes because you knew that a reasonable observer would factor in the light-travel time and so by asking a leading question you shifted the topic of discussion to perception of light flashes, not simultaneity.

When you changed the topic of discussion again, Origin learned a lesson about your intellectual honesty and the folly of answering leading questions as if they were simple questions.

What?? I didn't understand.. Can you please tell in simple manner than just statements? I really want to understand...

Oh!! I understood what you said.. Thank you!

I have heard people on the net act like there is some big connection with things being seen as simultaneous and time dilation. What is this big connection really? All observers measure the speed of light to be the same speed, but then each observer then recieves a flash of light at different times. I have read about this example before, but I never read about it really being used as an explanation for time dilation itself.

If the train went 10% the speed of light, then he would measure it to travel 10% slower than an observer on the station. If he measured the beam of light to travel 10% slower then wouldn't they still observe the flash to reach them at the same time? Then wouldn't he measure a flash of light behind him to be 10% faster?

To me, it seems like it would only be something true for lightning caused by electrons, that are not always measured to travel at the same speed.

I have heard people on the net act like there is some big connection with things being seen as simultaneous and time dilation. What is this big connection really? All observers measure the speed of light to be the same speed, but then each observer then recieves a flash of light at different times. I have read about this example before, but I never read about it really being used as an explanation for time dilation itself.

If the train went 10% the speed of light, then he would measure it to travel 10% slower than an observer on the station. If he measured the beam of light to travel 10% slower then wouldn't they still observe the flash to reach them at the same time? Then wouldn't he measure a flash of light behind him to be 10% faster?

To me, it seems like it would only be something true for lightning caused by electrons, that are not always measured to travel at the same speed.

how it is explains time dialation is like this: imagine 3 clocks in the train. Two on extreme parts and one in the middle. It is set in such a way that their pointers are simultaneouly the same. So now we are able to explain the time of the event when it happened with the help of simultanity.. As a result we have arrivied on the idea of time in physics..

I have heard people on the net act like there is some big connection with things being seen as simultaneous and time dilation. What is this big connection really? All observers measure the speed of light to be the same speed, but then each observer then recieves a flash of light at different times. I have read about this example before, but I never read about it really being used as an explanation for time dilation itself.

If the train went 10% the speed of light, then he would measure it to travel 10% slower than an observer on the station. If he measured the beam of light to travel 10% slower then wouldn't they still observe the flash to reach them at the same time? Then wouldn't he measure a flash of light behind him to be 10% faster?

To me, it seems like it would only be something true for lightning caused by electrons, that are not always measured to travel at the same speed.

no.. U didn't understand relativity.. Speed of light is the same for all observers irrespective of their motion(uniform or rest). When we move in uniform motion along a straight line and light too move in same direction,we would have measured less. But this contradicts principle of relativity in restricted sense.so either principle of relativity or law of light is wrong. But relativity shows that both are correct.. If both are correct,the something should change.. That change is distance,time as a result simultaneous events.

no.. U didn't understand relativity..
That is what the guy said that I talked to, go figure. Small world? He seemed to believe that time doesn't actually slow down and it is just the flashes of light that just reaches us at different times. In every book I have read on relativity says that time actually slows down. Then goes on about things actually slowing down uniformly so that is how we could theoretically not notice time actually slowing down. So for me it was a hard thing to chew on. I only read this example once, and it only claimed that was one of Einsteins thought experiments that he used to develop his theory, not that it is the actual cause of time dilation itself. So it has been kind of gnawing at me for a while now, since I never cared to ask why it is viewed as only being a matter of simultaneity.

I have heard people on the net act like there is some big connection with things being seen as simultaneous and time dilation. What is this big connection really? All observers measure the speed of light to be the same speed, but then each observer then recieves a flash of light at different times. I have read about this example before, but I never read about it really being used as an explanation for time dilation itself.

If the train went 10% the speed of light, then he would measure it to travel 10% slower than an observer on the station. If he measured the beam of light to travel 10% slower then wouldn't they still observe the flash to reach them at the same time? Then wouldn't he measure a flash of light behind him to be 10% faster?

To me, it seems like it would only be something true for lightning caused by electrons, that are not always measured to travel at the same speed.

No one here was talking about relativistic speeds (significant fraction of the speed of light), so this is an off-topic non sequitur. At non-relativistic speeds, time dilation does not have an appreciable effect on the time it takes a finite speed light signal to reach observers at different distances.

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No one here was talking about relativistic speeds (significant fraction of the speed of light), so this is an off-topic non sequitur. At non-relativistic speeds, time dilation does not have an appreciable effect on the time it takes a finite speed light signal to reach observers at different distances.
The M&M experiment was used to measure light traveling at the same speed in different directions, with no velocity relative to the Earth. You cannot assume that all observers will measure light to travel at the same speed, but that if it is a slow speed then that is okay. I was just wondering how people have taken one of Einsteins mind experiments, and then have proclaimed it to be the Holy Grail of physics. I wonder if Einstein new any better about the difference of the speed of photons and electrons at the time he came up with this, or if he did and that is why he used lightning in his example instead of flashes of light. It is the odd ball of theoretical physics writings, and then we have held it higher than any other one as though it has more significance. I wonder if it was even valid. Did Einstein even assume that the speed of light was constant at this time? Or was he dodging the question by saying that it was not light in the problem?

The M&M experiment was used to measure light traveling at the same speed in different directions, with no velocity relative to the Earth. You cannot assume that all observers will measure light to travel at the same speed, but that if it is a slow speed then that is okay. I was just wondering how people have taken one of Einsteins mind experiments, and then have proclaimed it to be the Holy Grail of physics. I wonder if Einstein new any better about the difference of the speed of photons and electrons at the time he came up with this, or if he did and that is why he used lightning in his example instead of flashes of light. It is the odd ball of theoretical physics writings, and then we have held it higher than any other one as though it has more significance. I wonder if it was even valid. Did Einstein even assume that the speed of light was constant at this time? Or was he dodging the question by saying that it was not light in the problem?

Incomprehensible post. How does any of that justify your non sequitur introduction of a relativistic observer?

Incomprehensible post. How does any of that justify your non sequitur introduction of a relativistic observer?
I have heard that relativity of simultaneity proves that time dilation doesn't actually slow down time, that is in contrary to what I have read about the subject, so then I question its validity. It doesn't assume the constant speed of light, if it doesn't assume the constant speed of light then I would agree that the relativity of simultaneity does not show that time actually slows down in effects of time dilation. I don't think it could ever show that there is time dilation becaue it doesn't assume the constant speed of light. It would more accuratly describe when lightning or electrons would arrive somewhere. I don't think this could be a cause of different observers, measuring particles to be at different locations at the same time either. So then by saying that it is lightning, Einstein has prevented himself from being wrong in this respect.

I have heard that relativity of simultaneity proves that time dilation doesn't actually slow down time, that is in contrary to what I have read about the subject, so then I question its validity. It doesn't assume the constant speed of light, if it doesn't assume the constant speed of light then I would agree that the relativity of simultaneity does not show that time actually slows down in effects of time dilation. I don't think it could ever show that there is time dilation becaue it doesn't assume the constant speed of light. It would more accuratly describe when lightning or electrons would arrive somewhere. I don't think this could be a cause of different observers, measuring particles to be at different locations at the same time either. So then by saying that it is lightning, Einstein has prevented himself from being wrong in this respect.

You "heard" wrong. Time dilation is not significant for observers at non-relativistic velocities, but the relativity of simultaneity is calculated using Lorentz transformations, which necessarily includes even inappreciable time dilation. The relativity of simultaneity very much does assume the constant speed of light. This is the whole reason that time dilation and length contraction are evoked.

The relativity of simultaneity is a direct consequence of the two postulates of SR, and as such necessarily assumes the constant speed of light, seeing how that is one of the postulates. And who said anything about "different observers, measuring particles to be at different locations at the same time"?