# The Relativity of Simultaneity

Discussion in 'Physics & Math' started by Motor Daddy, May 12, 2010.

1. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,166
I'm spelling it out for you because I can't seem to make myself understood.
You quoted me in post 73:
"If two light flashes started at the same time, ..."
I mentioned synchronized clocks to be absolutely sure that you understand what "at the same time" means. If you think that's changing the scenario, then I don't know what you're thinking.

You can put it that way if you like. I'd rather stick with light flashes emitted at the same time, like we established, but whatever floats your boat.

The time for light from the front, and the time for light from the back.
Do they measure the same times, or different times?
If different, which time is shorter?
What does this tell the train observer (if anything) about their absolute motion?

Sorry, I don't know what you mean.

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3. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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Which way, that the clocks appear to remain synchronized from the midpoint of the train, or that they appear to be out of sync? Which one?

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5. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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If the train were to have a velocity, and two lights simultaneously emitted, would the train observer at the midpoint remain at the point where the two light spheres will meet?

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7. ### PeteIt's not rocket surgeryRegistered Senior Member

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The light flashes are emitted at the same time. The clocks are synchronized. No one needs to look at the clocks, they just trigger a timer. Have the guy do whatever process you need to satisfy yourself that he can accurately measure the time taken for light to get from each end to the middle.

Last edited: May 13, 2010
8. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,166
No. But, relativity says that the question of whether the train "has a velocity" is arbitrary. It's a matter of choice, not an absolute thing.
But, right now I'm trying to establish what you think.

So, the train observer measures:
The time for light to to go fromt he front of the train to the middle, and the time for light to got from the back. of the train to the middle.
Do they measure the same times, or different times?
If different, which time is shorter?
What does this tell the train observer (if anything) about their absolute motion?

9. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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5,105
You don't care what they look like? Certainly you do, as when you look at the night sky you do realize that what you are seeing is the past? So you are not concerned with redshift or blue shifted light, it's all the same to you?

Come on, you're pulling my leg, of course you care. Do the clocks appear to be synchronized when viewed from the midpoint of the train?

Also, if the observer initially synchronized 3 clocks, put one at each end and kept one for himself at the midpoint, if the train had a zero velocity would the clocks at each end of the train agree with the midpoint observer's clock from his perspective, and also, and most importantly, if the midpoint observer viewed the end clocks as being out of sync with each other, would that mean the train had a velocity?

Last edited: May 13, 2010
10. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,166
:bugeye:
Sorry MD, the clocks are internal to the flash apparatus. You can't see a reading with the cover closed.

The guy sets the timers when they are together. The flashes are set to fire at a specific future time. Is that enough for you? Why is it not sufficient to say that the flashes are emitted simultaneously?

11. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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I want to know your answers to my questions. I edited the previous post with an additional question. Can you please just answer my questions to the best of your ability?

12. ### PeteIt's not rocket surgeryRegistered Senior Member

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This started with me asking you a simple question. You're side tracking.

So, the train observer measures:
The time for light to to go from the front of the train to the middle, and the time for light to go from the back of the train to the middle.
Do they measure the same times, or different times?
If different, which time is shorter?
What does this tell the train observer (if anything) about their absolute motion?

13. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

Messages:
5,105
Pete, I have answered all your questions to the best of my ability. I expect you to do the same. Please, if you want to continue an honest conversation then please answer the questions to the best of your ability, in all honesty. You are beating around the bush because you know what I am getting at, and it doesn't help your case. As a matter of fact, it blows your ship out of the water!

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14. ### PeteIt's not rocket surgeryRegistered Senior Member

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MD, right now I'm not exploring what I think relativity says. I want to know what you think. I honestly don't know. it seems to me you've said two completely opposite things, and I want to resolve what you meant.

If you think it's important, you tell me. Do the clocks look like they're synchronized?

When you've got that off your chest, how about addressing the simple question I posed more than ten posts ago?

The train observer measures:
The time for light to to go from the front of the train to the middle, and the time for light to go from the back of the train to the middle.
Do they measure the same times, or different times?
If different, which time is shorter?
What does this tell the train observer (if anything) about their absolute motion?

15. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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5,105
First and foremost, light takes time to travel, regardless how small the distance. Light does not travel instantly. Given that fact, the observer at the midpoint of the train with his clock will not see the end clocks as reading the same as his, even though he just synchronized them and knows the clocks keep accurate time. It is simply a matter of distance, the end clocks are a distance away from the midpoint clock. An observer at the midpoint can not see his clock as reading the same as the end clocks.

Point two, and most importantly, if the two end clocks appear to be out of sync to the midpoint observer, the train MUST have a velocity. The ONLY way the two end clocks will appear to be in sync with each other, to the midpoint observer, is if the train has an actual zero velocity. Being inertial only means "not accelerating" and that says nothing about the velocity of an object, for an object could be inertial and have a velocity.

Three clocks all synchronized placed in a train, one at each end and one at the midpoint of the train with an observer. If the train has a velocity the two end clocks will appear to the midpoint observer as being out of sync with each other. Cut and dry.

Last edited: May 13, 2010
16. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,166
Lovely. So (according to you) we can measure absolute motion by measuring light signals (or looking at synchronized clocks).

So, what is this absolute motion? If it can be measured, then surely anyone with a good laboratory and laser can find measure how fast the Earth is moving through absolute space, right?

17. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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I'll leave that to the experts, which doesn't include me.

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18. ### Jack_BannedBanned

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Look. Here's a 1972 experiment that measured the speed of light in a laboratory to within 1 m/s. They didn't adjust for the Earth's rotation or orbit. This measurement has been repeated and improved upon. No one has found it necessary to adjust for Earth's motion, yet they all get the same results.
What does that tell you?

It tells me you have no idea what you are talking about.

http://relativity.livingreviews.org/Articles/lrr-2003-1/

Look at chapter 5. You will note a nercessary Sagnac correction based on the rotation of the earth of GPS would be inaccurate.

So, there needs to be an adjustment and you are wrong.

19. ### Jack_BannedBanned

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1,383
Here you are again.

The reason we can detect distance star light aberration is because the earth moves relative to a constant speed of light in space.

In fact, this was used to calculate the speed of the earth's orbit around the sun by testing the earth's relative motion to this constant absolute light speed.

20. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,166
There was no adjustment, and you are *still* wrong about the Sagnac effect.

...and still making shit up. We can measure stellar aberration because the Earth's motion relative to any given star changes over the course of a year.

21. ### PeteIt's not rocket surgeryRegistered Senior Member

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Well, the experts have found that it can't be done. Try as you might, you can't mesaure light in a laboratory and work out your absolute speed - the result is always what you'd expect to get if Earth was at absolute rest.

What do you make of that?

22. ### rpennerFully WiredRegistered Senior Member

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4,833
You also fail to understand when the Sagnac effect $\Delta t = \oint \frac{2 \bf{v} \cdot d\bf{s}}{c^2 - \left( \frac{\bf{v} \cdot d\bf{s}}{| d\bf{s} | } \right)^2}$ does apply. Nor do you work out any of the details. http://sciforums.com/showthread.php?p=2526994#post2526994

The Sagnac effect does not affect the speed of light, it affects roundtrip time in an experiment when part of the light path is in relative motion, such as a rotating arrangement of mirrors or a fiber optic conveyor belt or when the receiver of a GPS system is in motion relative to the assumed ECI frame.

Neil Ashby writes that by working in a coordinate frame which does not rotate every 24 hours, the Sagnac effect of a path about the Earth (hundreds of nanoseconds) need not be accounted for. In the same way, a conventional light speed measurement which uses linear paths that enclose no area have zero Sagnac effect, even if they are rotated or carried about.

Last edited: May 14, 2010
23. ### Jack_BannedBanned

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1,383
Glad to see you take a stand.

Your integral is logically equivalent to taking the
path length * ( 1/(c-v) - ( 1/(c+v) )

2πr( 1/(c-v) - ( 1/(c+v) ) you will find the answers to be the same.

Hence, you are of the crowd that believes an enclosed path is a necessary condition for sagnac just by the way the equations looks. Looks can be deceiving.

When you write the equation correctly, my way the nature of sagnac is revelaed. A closed path is not necessary.

Let's check GPS to see you are wrong.

The Sagnac effect can be regarded as arising from the relativity of simultaneity in a Lorentz transformation to a sequence of local inertial frames co-moving with points on the rotating earth.
http://relativity.livingreviews.org/Articles/lrr-2003-1/

See chapter 2 and 5. You will note, an enclosed path does not exist between the satellite and the GPS unit.

So, you are wrong.

Let's get some terms for your training.

There is the speed of light in space and there is the measured speed of light.

I was talking about the measured speed of light.

Pete's proposed experiment claimed light measured c in all directions regardless of the speed of the rotating earth. That was the context.

These terms can be tricky.

I posted GPS which proves light is not always measured c in the rotating earth frame as he contended.

Hence, his experiment is false and it is a well know physics experiment that is stupid and false.

I assume you realize the space-time coords sent to the ground based receiver are in the rotating earth frame.

This is correct. But, you did not read it completely. If ALL coords of GPS where registered in the non-rotating earth frame, meaning an artificial frame of an earth that does not rotate, then and only then would sagnac be ignored. That is absolutely true.

This is similar to the sagnac experiment in the lab that does not rotate. It does not see sagnac.

So if all coords of the rotating frame in the lab were converted to the stationary non-rotating frame, then sagnac would not be picked up.

Oh, by so doing, the conversion to that frame would inherently contain the sagnac adjustment by using c+v and c-v to convert to that non-rotating frame.