On the idea of time in physics-relativity

[Pete]

(I've resized the diagrams for compactness, and added gridlines)

Yes, I think you put aether type equations into Minkowski Diagrams. The diagrams assume that an observer on the train will recieve the flashes of light from the opposite side of the train at different times.

No, the diagrams assume the Lorentz transform. Everything else follows.
If I use aether type equations, (ie a Galilean transform), then the speed of the light flashes (ie the gradient of the red and yellow lines) would change with reference frame; it would only be c in the aether rest frame.

The arrival times of the front of the train change a lot more than the arrival times of the back of the train when it increases in velocity.

Yes, the difference in clock readings between the two frame is a function of time and distance from the origin (look up the Lorentz Transformation)
At t=0, x=0, the clocks in both reference frames reads 0. The difference between clocks in the two frames increases the further (in both time and distance) you go from there.

For the observer on the train, the arrival times from opposite ends of the train should remain the same. It is just like saying that your bedroom is in motion but you don't know about it. So then you wouldn't notice anything strange about flashing a light in one direction in the room or the other. In your room you would say that it takes just as long for the flashes to go in each direction.

The train frame diagram tells the train observer's story.

Flash 1 starts as the back of the platform passes the front of the train, and it propagates at light speed.
Flash 2 starts as the front of the platform passes the back of the train, and it also propagates at light speed.
Both flashes take the same time to travel the length of the train.
Flash 1 takes from t'=-7.5 to t'=17.5
Flash 2 take from t'=7.5 to t'=32.5

So the train observer notices nothing unusual.

In the platform reference frame diagram, the flashes take different times to travel the length of the train, but that's what the platform observer expects, since in their rest frame, the train is moving.

It would have gauge invariance.

I don't understand gauge theory; the maths is too advanced (Lie algebra?? Lagrangian??). But it's not relevant here since we're not considering interaction with any fields.

 

[origin]

and that is why no two scientist can agree on the same interpretation of the TE simultaneously.

Ok, then are you lying, amazingly ignorant, or trolling? Those are the only options for making such an absurd statement like that. It is funny to think that scientists would even discuss such a basic idea - this is high school physics.

 

[origin]

It means that this discussion has been beaten into the ground already many times over. Last time I had seen this discussion was ended by the mods on another web site just saying that you cannot say there is relative motion to a photon, end of story, deal with it. They didn't even care about trying to explain that with the TE. I don't think it would be correct to assume that someone doesn't understand relativity, because they don't agree with the relative observered motions of photons in different frames. Not following what the relativity of simulataneity has to say on this subject could be a good thing. The theory doesn't agree with the behaivor of light. It doesn't simply add up to a relative velocity like there is some kind of aether. It is just beating an old horse.

It just amazes me that this is so far over your head. It has been explained to you about 5 or 6 different ways and still - nada. Like I said, it is rather tricky trying to teach something to a box of rocks.:shrug:

 

[ash64449]

I think I understand it, it is just that I don't think there is really a different relative velocity to the flashes of light as they are shown in diagrams of this thought experiment. I think it is a much deeper issue than people really put on, and that is why no two scientist can agree on the same interpretation of the TE simultaneously.

Did you watch the two videos that i have linked.. It is clearly explained.. No need of this long discussion... Please watch it and then reply...

 

[Prof.Layman]

I don't understand gauge theory; the maths is too advanced (Lie algebra?? Lagrangian??). But it's not relevant here since we're not considering interaction with any fields.

It is basically like saying that if you where in the box car of a train, and the box car was a perfectly square box, then if you stood in the middle of it and shined a flashlight, then you would end up measuring the beam of light to reach each side of the box car at the same time, even when the train was in motion. It is basically the thing that I have been trying to say this whole time. The photons in the box car in the example would be guage invarient. I think it is relavent since the photon is the carrier of the electromagnetic force.

"The electromagnetic field can be understood as a gauge field, i.e., as a field that results from requiring that a gauge symmetry holds independently at every position in spacetime."

The photon is a Gauge Boson .

"The Standard Model of particle physics recognizes three kinds of gauge bosons: photons, which carry the electromagnetic interaction; W and Z bosons, which carry the weak interaction; and gluons, which carry the strong interaction."

 

[Pete]

It is basically like saying that if you where in the box car of a train, and the box car was a perfectly square box, then if you stood in the middle of it and shined a flashlight, then you would end up measuring the beam of light to reach each side of the box car at the same time, even when the train was in motion.

I'll add such a reflected flash to the diagram when I get a chance.

 

[Prof.Layman]

I'll add such a reflected flash to the diagram when I get a chance.

I will be going out of town tomorrow, but when I get back, if you could make one like that I would like to see it.

 

[Robittybob1]

It is basically like saying that if you where in the box car of a train, and the box car was a perfectly square box, then if you stood in the middle of it and shined a flashlight, then you would end up measuring the beam of light to reach each side of the box car at the same time, even when the train was in motion.

Well for my sake I hope you are wrong. It is a two way light path that we notice . The light heads out for the sides and strikes the reflective sides and then returns. Even if you are in the middle of the box the light will take longer to reach the forward side but then have a shorter return leg.
So in all cases the light returns at the same time, so the motion of the box, if any, becomes undetectable.
That is my view, and I hope I'm backed up by Pete's animations.

 

[Robittybob1]

Relativity of simultaneity video

It looks like from watching this video that the velocity of the train then changes the observed speed of light. It assumes that the trains movement then allows the flash from lightning in the front of the train to be seen before the flash behind the train because of the trains velocity.

The trains velocity should not change the measured speed of light!

http://www.bing.com/videos/search?q...6148D889A6A3E5E4213561&view=detail&FORM=VIRE2
There is a mistake in that video. The commentator says "as the person in the center of the train aligns with the person on the platform, the person on the platform see two bolts of lightning strike the front and rear of the train."

Now that set up is physically impossible.
If he sees the two lightning flashes at the same time that means he was at the midpoint of the train when the lightning struck, so when he sees them the train must have moved on forward so he will no longer be opposite the passenger at the midpoint on the train.
What he could mean: "As the two people eyeball each other (out of sight 2 bolts of lightning strike) and when they arrive to him they are simultaneous so he calculates they must have hit the ends of the train at the very instant the two were looking at each other, in some mystical way, the three events were simultaneous.
Now he wonders if the Earth moved for her, on the train, in the same way? Did she get the same sense of coincidence?
But alas she hears the lightning at the front of the train long before she notices the strike at the rear. He was just another bystander on some distant platform.

 

[Robittybob1]

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.

It is possible that they both could see the lightning strikes at the same time and also to eyeball each other at the same time. I think this can happen if the passenger got up earlier for a toilet stop and when she returned to what she thought was her seat but really it was somewhat further back in the train. The seat looked the same.
So when the train was exactly halfway past the person on the station lightning strikes both ends of the train.
When the light has traveled to him the train is now in a position where both can gaze into each other's eyes. Both hear the lightning strikes at exactly the same time. It seems the world has conspired these events for them.

 

[Pete]

Well for my sake I hope you are wrong. It is a two way light path that we notice . The light heads out for the sides and strikes the reflective sides and then returns. Even if you are in the middle of the box the light will take longer to reach the forward side but then have a shorter return leg.
So in all cases the light returns at the same time, so the motion of the box, if any, becomes undetectable.
That is my view, and I hope I'm backed up by Pete's animations.

You're both correct, in different ways.

The flashes reach the ends of the train simultaneously as measured by clocks moving with the train.
The flashes reach the ends at different times as measured by the clocks of any other reference frame.

In all frames, the flashes return to the train observer at the same time.

Diagrams. Dotted lines are observers in the middle of the train (black) and platform (green). Blue lines are flashes emitted as the train observer passes the platform observer that reflect off the ends of the train. Red and yellow lines are as before, flashes of light emitted as the front of the train passes the back of the platform and vice versa.
Units are chosen such that c=1 (eg years and lightyears, or microseconds and microlightseconds).

 

[origin]

Well for my sake I hope you are wrong. It is a two way light path that we notice . The light heads out for the sides and strikes the reflective sides and then returns. Even if you are in the middle of the box the light will take longer to reach the forward side but then have a shorter return leg.

There is no shorter leg. It is not an academic excersize to say that you can say you are at rest on a moving train. It is a fact. There is no possible way for you to say that you are moving and the country side is at rest. So the light hits both walls at the same time for the person on the train.

For the person on the platform he will see the light hit the front wall first.

The 2 different observers will see different timed events. It is really obvious if you agree the speed of light is constant and there is no prefered frame. You may not like the results but it is really obvious that based on the postulates this is what happens.

 

[Robittybob1]

There is no shorter leg. It is not an academic excersize to say that you can say you are at rest on a moving train. It is a fact. There is no possible way for you to say that you are moving and the country side is at rest. So the light hits both walls at the same time for the person on the train.

For the person on the platform he will see the light hit the front wall first.

The 2 different observers will see different timed events. It is really obvious if you agree the speed of light is constant and there is no prefered frame. You may not like the results but it is really obvious that based on the postulates this is what happens.

But the truth is you are just saying these things, for if you say that I'm wrong you need more than words to make me change my mind.
If the light is seen from the outside, "the light hit the front wall first" that is what is happening on the inside too, it is the same photon and you can't have the same photon going forward in one frame and reversed in the other, or can you in your version of reality?

 

[Robittybob1]

You're both correct, in different ways.

The flashes reach the ends of the train simultaneously as measured by clocks moving with the train.
The flashes reach the ends at different times as measured by the clocks of any other reference frame.

In all frames, the flashes return to the train observer at the same time.

Diagrams. Dotted lines are observers in the middle of the train (black) and platform (green). Blue lines are flashes emitted as the train observer passes the platform observer that reflect off the ends of the train. Red and yellow lines are as before, flashes of light emitted as the front of the train passes the back of the platform and vice versa.
Units are chosen such that c=1 (eg years and lightyears, or microseconds and microlightseconds).

I'll have to learn how to read those diagrams. I'm part way there. Do they pass the single photon test? Are you having the same photon being seen moving in 2 directions at the same time? That to me is not logical if it does.
When I say above and talk about a shorter leg I am meaning a shorter time leg not distance.
So when Origin says "There is no shorter leg." I'm not sure if he is talking talking distance or time, even though with light they are about the same thing as it can only ever go the same distance in the same length of time, so shorter distance implies shorter time, but the distance is the distance light moves, not the physical distance. A moving light clock slows down as the light distance increases, but the distance between the mirrors doesn't change.
Tip the light clock over and how do you account for the additional light distance now?

 

[Robittybob1]

If you have two identical light clocks side by side and they are synchronized, put one on a spacecraft and go on a trip, the extra distance the traveling one has done will slow down the number of ticks it makes, so we say time has slowed. In this double clock situation the photons in each light clock will be out of phase. But if there was only one light clock shared between the moving craft and the Earth then when time slows for the moving craft, the photon under consideration is seen moving in one direction or the other in both frames but not out of phase.

Like I believe it is impossible for the moving light clock to be slow in the moving frame and also seen to be keeping normal time for those on Earth.

 

[origin]

But the truth is you are just saying these things, for if you say that I'm wrong you need more than words to make me change my mind.

It is not just an idea - the invariant speed of light has been shown to be confirmed in ALL experiments. That is what the experiments show, if you want to pretend this is not true that is your right - why you want to do that is beyond me.:shrug:

If the light is seen from the outside, "the light hit the front wall first" that is what is happening on the inside too, it is the same photon and you can't have the same photon going forward in one frame and reversed in the other, or can you in your version of reality?

oops - if I said front wall I misspoke I meant the back wall first.

Listen carefully. It is really simple. Experiments prove the speed of light is invariant. It is always measured at the same speed independent of the speed of the source or the observer.

That means for the observer on the train the speed of the photons moving towards the front of the train will have exact same speed as the photons moving to the back of the train. Since they travel the same distance for the observer on the train they will hit the front and back wall simultaneously. This is not a guess or a theory this is shown to be true by experiemnt.

For an observer on the platform the photons moving towars the front of the train will be moving at exactly the same speed as the photons moving to the back of the train BUT the train is moving forward that means that the distance the photons travel to the back of the train is less than the front, so the photons will hit the back wall first! In other words the photons do not hit the walls simultaneously. Again this is because the speed of light is invariant - it is an experimental fact.

So you might ask, "Then which observer is right, did they hit at the same time or not?" The answer is they are both right. That is the whole godamn point!!! We can not say for certainty if 2 events are simultaneous, because it all depends on your inertial frame.

This is reality, sorry. If you don't feel comfortable with this reality, it is no problem; there is almost no chance it will have any affect on your life one way or the other.

 

[Pete]

I'll have to learn how to read those diagrams.

It does take a bit of effort, but it's not too complicated. I'll help, if you like.

Always remember that the diagrams plot position (vertical axis) against time (horizontal axis).
Focus on one line at a time.
Try the exercises below.

Look at the dotted green line, the platform observer. It is a horizontal line at vertical coordinate zero.
This means every point on that line has a position coordinate of zero.
So, the platform observer is at rest at position zero in this diagram.
So - A horizontal line represents something at rest.

Now look at the solid green lines, the front and back of the platform, and see if you can find:
What are their position coordinates?
What does this tell you about the length of the platform?

Now look at the black dotted line, the train observer.
This line is sloped. As time (the horizontal coordinate) increases, the position coordinate increases.
This means that the train observer is moving is the positive direction. The line is straight, so their speed is constant.
So - A sloping straight line represents something moving at constant speed.

Looking a the diagram, see if you can find:
At what time coordinate does the train observer have the same position coordinate as the platform observer?
At what time coordinates does the train observer have the same position coordinate as each end of the platform?
How long does the train observer take to get from one end of the platform to the other?

Now look at the black solid lines, the front and back of the train.
They have the same slope as the black dotted line. This means that they have the same speed as the train observer.

Look at the diagram, see if you can find:
At t=0, what are the positions of the front, back, and middle of the train?
At t=50, what are the positions of the front, back, and middle of the train?
How long is the train?
How fast is the train moving (change in position / change in time)?

 

[Robittybob1]

It is not just an idea - the invariant speed of light has been shown to be confirmed in ALL experiments. That is what the experiments show, if you want to pretend this is not true that is your right - why you want to do that is beyond me.:shrug:



oops - if I said front wall I misspoke I meant the back wall first.

Listen carefully. It is really simple. Experiments prove the speed of light is invariant. It is always measured at the same speed independent of the speed of the source or the observer.

That means for the observer on the train the speed of the photons moving towards the front of the train will have exact same speed as the photons moving to the back of the train. Since they travel the same distance for the observer on the train they will hit the front and back wall simultaneously. This is not a guess or a theory this is shown to be true by experiemnt.

For an observer on the platform the photons moving towars the front of the train will be moving at exactly the same speed as the photons moving to the back of the train BUT the train is moving forward that means that the distance the photons travel to the back of the train is less than the front, so the photons will hit the back wall first! In other words the photons do not hit the walls simultaneously. Again this is because the speed of light is invariant - it is an experimental fact.

So you might ask, "Then which observer is right, did they hit at the same time or not?" The answer is they are both right. That is the whole godamn point!!! We can not say for certainty if 2 events are simultaneous, because it all depends on your inertial frame.

This is reality, sorry. If you don't feel comfortable with this reality, it is no problem; there is almost no chance it will have any affect on your life one way or the other.

Speed of light has always been measured as the two way speed of light. It is the there and back that they measured and I have no problem with that, but you seem to assume that in all situations the time there always equals the time back and that is where i think you are going wrong. True you can't tell if something is moving, in fact it is safe to assume we are all moving, so the time in the two legs of the two way speed calculation will be different but since they average out to all being the same we say the speed of light is the same in every frame.
Whatever happens the single photon (assuming it can be seen from the inside and outside will hit the walls at the same time the photons will always be going forward in both frames or backward in both frames, never forward in one and backward in another.
But then you have simultaneity???? I'll have to try and think it through - you might be right, but I can't do it now.

 

[origin]

Speed of light has always been measured as the two way speed of light. It is the there and back that they measured and I have no problem with that, but you seem to assume that in all situations the time there always equals the time back and that is where i think you are going wrong. True you can't tell if something is moving, in fact it is safe to assume we are all moving, so the time in the two legs of the two way speed calculation will be different but since they average out to all being the same we say the speed of light is the same in every frame.

You can believe whatever you want. There is lots of info on this. The concept has been explained several different ways -so whatever.

Whatever happens the single photon (assuming it can be seen from the inside and outside will hit the walls at the same time the photons will always be going forward in both frames or backward in both frames, never forward in one and backward in another.

Don't know what you are talking about. Nobody ever said that would happen, that I know of.

But then you have simultaneity???? I'll have to try and think it through - you might be right, but I can't do it now.

Sure.

 

[Neddy Bate]

Speed of light has always been measured as the two way speed of light. It is the there and back that they measured and I have no problem with that, but you seem to assume that in all situations the time there always equals the time back and that is where i think you are going wrong. True you can't tell if something is moving, in fact it is safe to assume we are all moving, so the time in the two legs of the two way speed calculation will be different but since they average out to all being the same we say the speed of light is the same in every frame.

That is a good way of looking at it when you are first trying to understand the basic concepts. But it is an over-generalization of what relativity really does. For example, without time dilation or length contraction, the "average" you refer to above would not come out to be the same in both frames. Imagine that the train car has a standard length, but it is moving at almost the speed of light. In that case, it is going to take a very long period of time for a photon to travel from the center of the train car to the front wall. Then after the light reflects off the front wall, it travels very quickly back to the observer at the midpoint. This makes for a rather long round-trip time for that photon, as measured by the train frame. The platform frame would not measure that same long period of time for an identical train car parked at rest on the platform. It would measure a much shorter round trip time.

So, if you want to explain why the speed of light comes out to be the same in both frames, you need more than just the "average" concept. You need the full apparatus of relativity, including length contraction, time dilation, and relativity of simultaneity.