Is time universal? NO (and its proof)

[Pete]

Which implies that the clocks in his frame would need to be synchronized both before and after the acceleration.

Interesting... it hadn't actually occurred to me that a simultaneous-in-embankment frame stop would desynchronize the train clocks... but you're right, of course.
The oblivious-to-change observer also implies that the proper length of the train is unchanged after acceleration, right?

Neddy,
How are you coming along with Raphael's instant-stop situation?
Note that he's now effectively specified that the length of the train after it stops in the embankment frame is it's full length.

So, in what order must each clock be stopped in the embankment frame?
What about in the train frame?
Note that he's now specified (in effect) that all parts of the train stop simultaneously in the train frame

 

[Raphael]

Interesting... it hadn't actually occurred to me that a simultaneous-in-embankment frame stop would desynchronize the train clocks... but you're right, of course.
The oblivious-to-change observer also implies that the proper length of the train is unchanged after acceleration, right?

It implies that the observer on the train would measure the length of the train with a measuring device on the train as the same value before and after the acceleration.

Note that he's now specified (in effect) that all parts of the train stop simultaneously in the train frame

In effect, it is not I who specified the condition but relativity. I mearly echoed a principle of relativity.

 

[Neddy Bate]

</em>Interesting... it hadn't actually occurred to me that a simultaneous-in-embankment frame stop would desynchronize the train clocks... but you're right, of course.
The oblivious-to-change observer also implies that the proper length of the train is unchanged after acceleration, right?

The simultaneous-in-embankment frame-stop sounds analogous to my suggestion of a "linear" disk-brake closing on the train at a preset time in frame E (according to the synchronized alarm clocks). I thought that illustrated a problem in which the train length would have to change at the same time as it was being squeezed.

</em>Neddy,
How are you coming along with Raphael's instant-stop situation?

Not very well I guess. I never expected the train passengers to notice the train length as anything other than its proper length. I was focused more on the embankment frame where it looked to me as if the train would have to grow.

</em>
Note that he's now effectively specified that the length of the train after it stops in the embankment frame is it's full length.

Yes, in the embankment frame the train increases in length as it stops, correct?

</em>
So, in what order must each clock be stopped in the embankment frame?

I thought the embankment frame stop was timed by the alarm clocks to be synchronized in that frame.

</em>
What about in the train frame?

Not synchronized in this frame, I thought.

</em>
Note that he's now specified (in effect) that all parts of the train stop simultaneously in the train frame

And once again I seem to have everything backwards?

 

[Pete]

Actually, I'm having a difficulty with the details.
I can't find a way to bring the train to an emergency stop (in which any chosen part of the train accelerates to the embankment frame in a vanishly small proper time) in which the train clocks stay synchronized and the train's length remains unchanged... or maybe I'm just getting confused.

I'll be back after some thought and analysis.

 

[Aer]

I can't find a way to bring the train to an emergency stop (in which any chosen part of the train accelerates to the embankment frame in a vanishly small proper time) in which the train clocks stay synchronized and the train's length remains unchanged

I believe this is the correct conclusion. Such a setup would not be possible.

 

[Pete]

You're right.

Raphael,

I would like to assume a common thought experiment condition in which a person asleep in the frame in which the acceleration occurs would be unaware of a change in velocity when he awoke. Which implies that the clocks in his frame would need to be synchronized both before and after the acceleration.

I don't think this is possible in a simple way. I can't see any simple process for stopping the train that ends up with the trainclocks synchronized in the Earth frame.

 

[Raphael]

You're right.

Raphael,

I don't think this is possible in a simple way. I can't see any simple process for stopping the train that ends up with the trainclocks synchronized in the Earth frame.

Are we saying that the accelerated frame must demostrate a physical change in the measurements taken in its frame before and after the acceleration?

 

[Pete]

Yes, I think so.
At least, I personally can't see a way to retain all its measurements.

 

[Raphael]

Yes, I think so.
At least, I personally can't see a way to retain all its measurements.

Perhaps you should consider what it means for clocks to be synchronized.

 

[Pete]

Please explain?
Perhaps I'm being dense, but I don't see your point.

 

[Aer]

Perhaps you should consider what it means for clocks to be synchronized.

They read the same time at t₀ and some time after at t₁?

 

[Raphael]

Please explain?
Perhaps I'm being dense, but I don't see your point.

I'm not really sure of what my point was either.

What the observer on the train is left with is a situation in which his clocks tick simultaneously before and after the acceleration only the displayed values are not the same.

 

[Pete]

So we're throwing away the idea that a person asleep in the frame in which the acceleration occurs would be unaware of a change in velocity when he awoke?

What the observer on the train is left with is a situation in which his clocks tick simultaneously before and after the acceleration only the displayed values are not the same.

That's going to be true no matter how the train is stopped, as long as all the train clocks wind up at rest in the embankment frame.

I think we need at least one of the following conditions specified:

• All parts of the train stop simultaneously in the embankment frame
• All parts of the train accelerate to the embankment frame simultaneously in the train's initial inertial frame
• The train's proper length is the same after acceleration as before
• A single superbrake operates stopping the middle of the train at the middle of the embankment
• Some other description of the braking process

 

[Raphael]

So we're throwing away the idea that a person asleep in the frame in which the acceleration occurs would be unaware of a change in velocity when he awoke?

After thinking about it, I think we should. The change of values on the clocks is effectively data collected during the acceleration. Anyone who collects acceleration sensitive data during an acceleration would tend to be able to determine that an acceleration occured. Thus the numbers themselves would not be protected by relativity, but only whether the clocks tick at the same rate before and after acceleration. (If I performed a time sensitive experiment before and after the acceleration the measured change in time must be equal.)

The train's proper length is the same after acceleration as before

This I believe would still be a requirement in order for relativity to hold true.

 

[Pete]

This I believe would still be a requirement in order for relativity to hold true.

It doesn't have to be true, it's quite a specific condition... in most cases, the train would be seriously deformed during the emergency brake (see barn and pole paradox).

If this condition is dictated for our train, it means than the braking is simultaneous in the "averaged" frame... the frame in which the train's initial motion and the embankment's motion are equal and opposite. The braking is not simultaneous in either the train's frame or the embankment frame.

 

[Raphael]

It doesn't have to be true, it's quite a specific condition... in most cases, the train would be seriously deformed during the emergency brake (see barn and pole paradox).

While it doesn't have to be true, the condition is rather specific to relativity. The train at rest should measure the same as the train in motion relative to an observer on the train. Like the clocks measure a synchronous change in time before and after, a ruler should measure a synchronous change in distance.

Unlike our train, the pole (ladder) paradox does not have "physical contact" (our recepits) between the frames prior to the last paragraph, which adds something to our problem that is not explained by this paradox.

The last paragraph on that page under "What if the doors are left shut?" describes in a general sense what would happen to our train if we measured the stop at the front of the train, but nothing prohibits the "spring back to its natural shape". The middle of the train is somewhat different. Regardless of the dynamics, the train would be deformed during the acceleration but should measure the same before and after it.

 

[Pete]

a ruler should measure a synchronous change in distance.

...unless it has been physically stretched or compressed. There are no perfectly rigid rulers.

nothing prohibits the "spring back to its natural shape".

If many superbrakes were applied to stop all parts of the train, that would indeed prohibit further shape-altering.

But that's OK, you're now specifying some more about how the train is stopped... If the train is free to decide its own length, that means that only one part of the train is fixed to the embankment when it is stopped (ie there's only one brake, which I think you place in the middle of the train), and that the train is the super-strong perfectly elastic variety.

The problem just got harder!


In the embankment frame, when the brake is applied:
1) The front half of the train will stretch (and rebound?) until TA is aligned with EJ
2) The back half of the train will compress then rebound until TJ is aligned with EA
3) The behaviour of the clocks is not straightforward. It is even possible with this back-and-forth motion that they could end up fully synchronized.

 

[Raphael]

Feel free to set any conditions which you would like. I was just looking for an accurate count of how many receipts are printed for the TB-EJ meeting.

 

[Billy T]

..(see barn and pole paradox)....

Thanks Pete. I had not heard of this before. If I had, I would not have invented the train with exploding bombs grazing the noses of the line of ground observers, (some 54 pages back!). This paradox proves the point of this thread (see title) better than my train.

 

[MacM]

Thanks Pete. I had not heard of this before. If I had, I would not have invented the train with exploding bombs grazing the noses of the line of ground observers, (some 54 pages back!). This paradox proves the point of this thread (see title) better than my train.

My only objection here is your continued use of the term "Proves". It actually proves nothing. It is all based on untested and untestable assumptions and unsupported stipulations of physics advocated by SRT.

As Aer has correctly stated you have a tendancy to make an assumption and based on that assumption then claim your assumption is proven. They are not.