On the idea of time in physics-relativity

[Pete]

If the light beams are reduced to single photons and having mirrors at the ends, you end up with the photons traveling in two directions at once. What I mean is that if the person in the middle of the train fires two photons from his photon gun, one to the front and one to the rear, do the photons reflect from the end walls at different times such that depending on where you are looking at the situation, in one situation the photon is still heading toward the reflector but in another it is already turned around and heading back?
I said something about this yesterday and it is the speed of light that is the physics in question. The speed of light is the same in all frames. So the only way you can measure the speed of light is to send light there and back, a two way speed of light.
So it isn't that the person in the train sees light hit both ends at the same time, but the light returning to him comes back at the same time. A two path. There is no way he can determine if the photons hit the end walls at the same time as it says in the article.

You're right. There's no direct experimental way to measure the one-way speed of light without synchronizing separated clocks.
The thought experiments are exploring consequences of the assumption the light propagates at the same speed in all frames, which is how the train observer draws their conclusion.

In the Einstein case the lightning strikes were simultaneous but not sensed as simultaneous by the person in the train because he was moving but he does NOT realize he is moving, so he assumes the lightning struck the front first.

You could just as validly say that that the strikes were NOT simultaneous, but were sensed as simultaneous by the person on the platform because he was moving but he does not realize he is moving.

The whole point of relativity (since Galileo) is that there is no way to choose which is actually not moving.

The platform is moving past the train.
The train is moving past the platform.
There is no way to decide which is absolutely true, so it makes sense to conclude that neither is absolutely true. They are relative statements.
Compare with these easier relative statements:
The platform observer says the platform is here and the train is there.
The train observer says the train is here and the platform is there.

This is the simplest model that is consistent with the experimental findings.
It's true that a Lorentzian model, where there is an absolute but undetectable rest standard, is also consistent with the experimental findings (in flat space, at least), but what's the point of including an unnecessary, undetectable entity?.

 

[Robittybob1]

The only thing that can be established is the relative speed. As long as the difference in velocities when added together give the right amount of relative speed, that is about all we can tell, the range of speed is from -c to c..

 

[Prof.Layman]

The only thing that can be established is the relative speed. As long as the difference in velocities when added together give the right amount of relative speed, that is about all we can tell, the range of speed is from -c to c..

Do you think that would that be relative to the train of the platform that is at rest? Or both at the same time?

The train could assume that it is at rest and that only anything can travel less than the speed of light just relative to it.

The man in the ticket both near the platform could say that the people on the train are wrong and nothing can travel faster than the speed of light relative to the platform.

The train is traveling at a relative speed to the platform. How can they both be right? I don't think things just not being simultaneous would be able to cut it in this case.

 

[Neddy Bate]

Do you think that would that be relative to the train of the platform that is at rest? Or both at the same time?

The train could assume that it is at rest and that only anything can travel less than the speed of light just relative to it.

The man in the ticket both near the platform could say that the people on the train are wrong and nothing can travel faster than the speed of light relative to the platform.

The train is traveling at a relative speed to the platform. How can they both be right? I don't think things just not being simultaneous would be able to cut it in this case.

It sounds like you are starting to discover that velocities wouldn't simply "add up," but instead would have to be "composed" together with all of the relativistic effects accounted for.

Here is a link to Einsteins' 1905 paper where he derives all of the relativistic effects, including the "Compostion of Velocities" in section 5.
http://www.fourmilab.ch/etexts/einstein/specrel/www/
It's an old scientific paper, so it is not very easy to read.


His 1920 book, Relativity, the Special and General Theory is a little easier to digest. Section 13 deals with the composition of velocitites:
http://www.bartleby.com/173/

 

[ash64449]

It sounds like you are starting to discover that velocities wouldn't simply "add up," but instead would have to be "composed" together with all of the relativistic effects accounted for.

Here is a link to Einsteins' 1905 paper where he derives all of the relativistic effects, including the "Compostion of Velocities" in section 5.
http://www.fourmilab.ch/etexts/einstein/specrel/www/
It's an old scientific paper, so it is not very easy to read.


His 1920 book, Relativity, the Special and General Theory is a little easier to digest. Section 13 deals with the composition of velocitites:
http://www.bartleby.com/173/

This is the exact book that i have read!!! Though my book has 5 appendix...

 

[Robittybob1]

I found that really hard to understand too. If something is doing .999 c, an other object can go past it at the speed of light (according to it's frame!)
But even the thing going the speed of light going past another already doing .999 c, is still not going faster than the speed of light compared to anything else either..

 

[Prof.Layman]

It sounds like you are starting to discover that velocities wouldn't simply "add up," but instead would have to be "composed" together with all of the relativistic effects accounted for.

I was actually hoping it could lead to other people here making that discovery for themselves. Relative velocities to photons don't just simply "add up". It seems like all these diagrams people claim as proof just show how the relative velocity to photons just simply "add up". If it would just simply "add up", then that would have meant that there is an aether. But, because they don't just simply "add up" then there is no aether. It takes a relativistic equation to describe it.

 

[ash64449]

I was actually hoping it could lead to other people here making that discovery for themselves. Relative velocities to photons don't just simply "add up". It seems like all these diagrams people claim as proof just show how the relative velocity to photons just simply "add up". If it would just simply "add up", then that would have meant that there is an aether. But, because they don't just simply "add up" then there is no aether. It takes a relativistic equation to describe it.

Yes. Relative velocities to photons doesn't add up. You are right. But still you cannot understand Einstein's thought experiment?? Actually when we use Einstein's Equations we can see that in actual sense Velocities don't add up!!!

I will give you some links to videos given by minutephysics. His method is easy to grasp..
http://www.youtube.com/watch?v=ajhFNcUTJI0 (Special Relativity)
http://www.youtube.com/watch?v=IM630Z8lho8 (Velocities don't add up)

 

[Pete]

Do you think that would that be relative to the train or the platform that is at rest? Or both at the same time?
The train could assume that it is at rest and that only anything can travel less than the speed of light just relative to it.
The man in the ticket both near the platform could say that the people on the train are wrong and nothing can travel faster than the speed of light relative to the platform.
The train is traveling at a relative speed to the platform. How can they both be right?

Here is the Minkowski diagram of the train passing the platform, with the platform at rest.
Light flashes are emitted as the front of the train passes the back of the platform, and as the back of the train passes the front of the platform, propagating in both directions.

There are eight wordlines shown - one for each end of the train and platform, and two (one forward, one backward) for each light flash.

Time is on the horizontal axis, distance on the vertical axis.
The gradients of the worldines indicate speed.
Light flash worlines have a gradient of c.
I have the train moving at 0.8c, so the gradient of the train worldlines is 0.8c
The platform is stationary, so the platform wordlines are flat.

Here is the diagram transformed (by the Lorentz Transform) so the platform is passing the train, with the train at rest.
Notice that:

• The platform is shorter (the vertical separation between green lines)
• the train is longer (the vertical separation between black lines)
• the time for one end of the platform to pass the length of the train is longer (the horizontal span of a green line between the two black lines)
• the time for one end of the train to pass the length of the platform is shorter (the horizontal span of a black line between two green lines)

Here is an animation of how the diagram changes as it is transformed by different velocities:

The Lorentz transform is like a diagonal squeeze and stretch of the spacetime diagram. It keeps lightlike worldines at the same gradient, ie always at a speed of c. Other worldlines (with a gradient between -c and c) will stay between -c and c.

This might be clearer in this animation from Wikipedia:

 

[origin]

I found that really hard to understand too. If something is doing .999 c, an other object can go past it at the speed of light (according to it's frame!)
But even the thing going the speed of light going past another already doing .999 c, is still not going faster than the speed of light compared to anything else either..

It is really pretty simple if you just keep in mind that light is always measured at a speed of c (which is in a vacuum), mass cannot exceed c and no frame is prefered.

Assume a set up where we have a few space ships flying about and we have an observer in a position that is stationary to the space ships. If a ship is flying at .99967c and a pulse of light is sent from behind the ship, the ship will read the speed of light as c. The stationary observer will see the speed of light slowly catch up and pass the ship at about 100 km/s.

If there are 2 ships traveling at .99967c heading straight towards each other, as they pass they will measure the speed of each other as just under c about .9999c. The stationary observer will see the closing velocity of the 2 ships as about twice the speed of light. Neither ship of course is exceeding c. For the stationary observer a light pulse passing the ships will appear to pass one ship at a relative velocity of about 100 km/s and the other ship at a relative velocity of about 2c. The speed of the light pulse as measured on both ships will be c. At no time does any of the observers measure the speed of light as anything other than c and at no time does any observer measure the speed of an individual ship at c or above.

 

[Robittybob1]

It is really pretty simple if you just keep in mind that light is always measured at a speed of c (which is in a vacuum), mass cannot exceed c and no frame is prefered.

Assume a set up where we have a few space ships flying about and we have an observer in a position that is stationary to the space ships. If a ship is flying at .99967c and a pulse of light is sent from behind the ship, the ship will read the speed of light as c. The stationary observer will see the speed of light slowly catch up and pass the ship at about 100 km/s.

If there are 2 ships traveling at .99967c heading straight towards each other, as they pass they will measure the speed of each other as just under c about .9999c. The stationary observer will see the closing velocity of the 2 ships as about twice the speed of light. Neither ship of course is exceeding c. For the stationary observer a light pulse passing the ships will appear to pass one ship at a relative velocity of about 100 km/s and the other ship at a relative velocity of about 2c. The speed of the light pulse as measured on both ships will be c. At no time does any of the observers measure the speed of light as anything other than c and at no time does any observer measure the speed of an individual ship at c or above.

What you have said above fits in with whatI understand relativity says except for the bit about closing speed.
I was instructed earlier that just as you can't add speeds you can't add closing speed either. So this next bit is in question:

The stationary observer will see the closing velocity of the 2 ships as about twice the speed of light. Neither ship of course is exceeding c. For the stationary observer a light pulse passing the ships will appear to pass one ship at a relative velocity of about 100 km/s and the other ship at a relative velocity of about 2c.

I am wondering if someone could confirm this?

 

[origin]

What you have said above fits in with whatI understand relativity says except for the bit about closing speed.
I was instructed earlier that just as you can't add speeds you can't add closing speed either.

You absolutely cannot add the speed if you are in either one of the moving inertial frames. That would violate relativity because that would result in a speed in excess of c. For the observer in the stationary frame all he sees is 2 ships traveling at <c, no problem there at all. If he did not understand relativity he might incorrectly conclude that the ships would measure their closing speed in excess of c, but he would be wrong because that is not possible.

 

[Prof.Layman]

Here is an animation of how the diagram changes as it is transformed by different velocities:

IDK if this diagram was done completely correctly. It shows that Flash 2 changes more rapidly from different locations in the trains (top?) wordline. I guess this would be one side of the train?

 

[Prof.Layman]

Yes. Relative velocities to photons doesn't add up. You are right. But still you cannot understand Einstein's thought experiment?? Actually when we use Einstein's Equations we can see that in actual sense Velocities don't add up!!!

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.

 

[origin]

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

Two scientist can't agree on the interpretation of TE at the same time? What in the hell is that suppose to mean?

 

[Prof.Layman]

Two scientist can't agree on the interpretation of TE at the same time? What in the hell is that suppose to mean?

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.

 

[Robittybob1]

Two scientist can't agree on the interpretation of TE at the same time? What in the hell is that suppose to mean?

I thought it was a play on the ideas of relativity. No two frames agreeing on simultaneity. I think TE means Thought Experiment, is that right?

 

[Prof.Layman]

I thought it was a play on the ideas of relativity. No two frames agreeing on simultaneity. I think TE means Thought Experiment, is that right?

Yes, that means thought experiment. Took me a minute too, I just saw it used earlier in this thread, so I assumed they knew what it means.

 

[Pete]

IDK if this diagram was done completely correctly. It shows that Flash 2 changes more rapidly from different locations in the trains (top?) wordline. I guess this would be one side of the train?

Not sure what you mean, Layman, but I'll explain some stuff, we'll explore, and see what we can discover.

In both diagrams, the origin (t=0, x=0) is when the middle of the platform meets the middle of the train.
Units are chosen so that c=1. In the platform frame, the train and platform are both 20 units long. The train moves at v=0.6c relative to the platform.

The top train worldline is the front of the train.
The top platform worldine is the back of the platform.
Flash1 is the flash that is emitted as the front of the train meets the back of the platform.
Flash2 is the flash that is emitted as the front of the platform meets the back of the train.

The intersection of the Flash2 worldline with the front-of-train worldline is a particular event - it's when the Flash2 arrives at the front of the train.
In the platform rest frame, this event occurs at t=50, x=40, ie at a time 50 units after the middle of the train passes the middle of the platform, and at a distance of 40 units from the middle of the platform.
In the train rest frame (a boost of v=0.6, or gamma=1.25 from the platform rest frame), this event occurs at t=32.5, x=12.5, ie at a time 32.5 units after the middle of the platform passes the middle of the train, and at a distance of 12.5 from the middle of the train.

Make sense? Can you see any problems or mistakes?

 

[Prof.Layman]

Make sense? Can you see any problems or mistakes?

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. 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. 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. It would have gauge invariance .