On Einstein's explanation of the invariance of c

A real train is only partly rigid. There is change in a real train's length.

Wait, is it possible we're postulating the existence of 'material, rigid' bodies that in actuality have no real existence, and all for the benefit of a thought experiment?

I suppose the ansatz must be significant enough that one is obliged to accept their existence, necessarily, even though it's clearly an approximation of the real world? (ellipsis needed here)

 

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An inertial train doesn't change length, and if you say it does than you need to measure it and prove to yourself that it doesn't. Light will always take the same amount of time to travel the length of an inertial train, period! Light travel time is length. Look up the definition of a meter.

 

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There is no change to an inertial train. An inertial train is as rigid as they come!

 

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Bro...do you think "inertial" means that it is not moving relative to us? Inertial just means it isn't undergoing acceleration. In order to perceive a length contraction we would have to generate a relative velocity difference of a large percentage of the speed of light, so if you're trying to understand Physics by walking down to your local train yard with a ruler I think I know wherein the problem lies.

Also, "rigidity" of the train is absolutely irrelevant. We aren't talking about some force that "smooshes it like an accordion" which a really, really stiff train would be able to avoid. We're talking about something much more fundamental than that...if you remained on the train being "smooshed" you would also be smooshed, as would your ruler. Therefore, you would continue to measure the train's length as unchanged.

 

Inertial means not accelerating. What is acceleration? The rate of change of velocity. So if the train is inertial, it isn't accelerating, which means it is traveling a constant velocity. If it is traveling a constant velocity, light will always take the same amount of time to travel the train. Are you implying that light takes a different amount of time to travel the length of constant velocity train each time you test? Has your cheese slid off your cracker??

 

So if a real train is motionless, how does it get to a constant velocity relative to say, an observer on a platform.

Wait, I think I know this one--the train accelerates. The 'inertial' train is non-inertial after all.

 

The answer is that the time to travel the length of a constant velocity train will remain constant, HOWEVER, the time to travel the length of the train will change if its velocity is changed. It will also change if you are on the train vs off the train.

Listen man, I understand why this makes you frustrated. It isn't "my" cracker which has lost its cheese, though...it's Nature's cracker. I'm only explaining SR as I understand the explanation given to me.

 

We are talking about inertial trains, nothing but inertial. I fully understand how acceleration works, and how train's increase and decrease velocity, and how changing the velocity of a train changes the time it takes light to travel from end to end. I am talking about INERTIAL trains, and so is Einstein.

Again, inertial trains are RIGID, and light takes a specific amount of time to travel the length of an inertial (constant velocity) train.

I understand you are just relaying what was relayed to you. It's too bad such bad information was disseminated. I hope someday you understand it.

 

Which means an inertial train is rigid, does it not?

 

Another educational relativity thread hijacked by someone who thinks he knows it all, and that everyone else has the IQ of a squirrel.

This is old ground. MD has two previous threads left hanging:

[post=2541415]The Relativity of Simultaneity[/post]:
MD agrees that according to his model of space and time, anyone with a good laboratory and laser can find measure how fast the Earth is moving through absolute space.
I point out that when tthose measurement are performed, the result is always what you'd expect to get if Earth was at absolute rest.
No response from MD.

[post=2623909]What time is it?[/post]
MD again says that absolute speed can be measured. He'd do it himself if he had the precise equipment.
People who have done the measurement with precise equipment, and always measured an absolute speed of zero "are mistaking", according to MD.

Also in [post=2623922]What time is it[/post], MD describes in a vague way the mathematics required to calculate absolute velocity. When asked if he's done the exercise, he only says "it's proprietary information".
I've offered to show him the simple arithmetic (including time dilation and length contraction, of course), but no response from MD.

 

Well, it means an inertial train is an idealization, for one. Einstein admits this in his original paper.

A better way to say it is that a train moving at constant velocity is a good approximation of an inertial frame of reference. This highlights a 'problem' with SR, which postulates a rigid universe in which light travels in straight lines with no acceleration. SR is somehow included in the real universe, along with relative motions that aren't straight, and time-varying rather than constant velocities.

 

Pete, measuring with two way light is an exercise in futility, as the velocity of the object will not be noted due to the AVERAGE time it takes light to go there and back. Try measuring with one-way, and stop averaging the time it takes light to go there and back.

Here is what you are doing:

(10+2)/2=6
(8+4)/2=6
(6+6)/2=6

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Ignoring your previous agreements again, I see.

You're in a lab on Earth with all the precise equipment you need.
Describe how to measure the one-way speed of light.

Caution: if you use synchronized clocks, be sure to explain how you synchronize them.

 

The actual speed doesn't matter as much as the concept. Saying light travels at a constant speed has many consequences. I've outlined many of them in my 100's of posts on this site.

Do you agree that it takes light the same amount of time to travel the length of an inertial train each and every time you test? Do you understand what that means?

Do you agree that light will take more time to travel the length of a constant velocity train (rear to front) as you raise the velocity of the train each time you test?

Do you agree that light will take less and less time to travel the length of a train (front to rear) as you increase the velocity each time you test?

Do you agree that light takes less time to travel from front to rear than rear to front, on a constant velocity train?

You better think long and hard about those answers before you answer them.

 

Yes, you have. But the consequences you spell out don't match up with real experiments. Your map disagrees with the ground. Your map is wrong.

Measurements of the time it takes depends on how you set up your clocks.

Measurements, MD.
At any constant speed, if you synchronize two clocks in the centre of the train and move one to each end, then using those clocks to measure the speed of light from one end to the other will always give you the same result.

If you don't synchronize the clocks between each test, but accelerate without resynchronizing, then you will measure a diferent speed of light each time.

Done the maths yet?

 

Light will only take more time to traverse the length of a constant-velocity train if the length of the train increases. Likewise it will take less time if the length decreases, that's basic physics.

So if the length of the train is constant at different velocities, then light will take the same time to travel along it, de facto.

Unless the speed of light changes as the speed of the train does, but we all know the speed of light doesn't change (in a constant, isotropic and homogenous medium like air).

 

Yours is flat and the Earth is round.

No, the duration of light travel is what determines length.

Not if you measure rear to front, and then check it front to rear. The ONLY way those will be the same is if the train has an absolute zero velocity.

 

Wrong, light takes more time from rear to front as the train's velocity increases each time you test. As a matter of fact, if a train has a velocity greater than zero, it takes more time for light to travel from rear to front than from front to rear, on the same constant velocity train.

 

What rubbish. The ONLY way the measurements can be the same is if the speed of light is the same in BOTH directions, and the train stays the same length at ALL times.

I think it's been mentioned that the constant length of the train is assumed to be the case for train velocities much less than light's (single) velocity IN THE SAME frame.

 

Read up on the experiments, MD. Reality says you're wrong.

Duration means time, MD. Measuring time requires synchronized clocks.

Wrong again. Remember that simple exercise in arithmetic? Have you done it yet?

Hint: Be sure to consider the effect of time dilation on the clocks as you move them from the centre of the train to the ends.