# The Relativity of Simultaneity

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

1. ### rpennerFully WiredRegistered Senior Member

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Obviously, but assumes absolute space unnecessarily as this is trivially true in Galilean and Special Relativity also, and works with any two objects at any two speeds provided the event where the objects coincide is in the future.

i.e. You aren't doing physics but a trivial description of motion.

Better to write: light is always observed to travel at c, independent of the relative motion between observer and source and observer and destination, and independent of the measurements of any other observer who may also be in relative motion with the original observer and yet will still measure the speed as c.
Again, you say absolute, but cannot possibly measure absolute.And in fact the only physical examples you give are of relative motion, not absolute.
... as measured by a particular observer.

It's only when you consider the viewpoint of a different observer that the core of Relativity can be tested.

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

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And again, the radius of the light sphere increases at the rate of ~186,000 miles per second. It doesn't matter what the motion of the source is, or what the motion of any other object or observer in the universe is. The radius of the light sphere increases at the rate of ~186,000 miles per second, period.

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

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Relative to what, MD?

You think there's some absolute rest frame, that light moves at c onl relative to "absolute space". Guess what - when people look for "absolute space", it's not there. Any reference frame is as good as another.

You're arguing with reality, and it's really getting very boring. Do you want a discussion that involves some learning, or do you just want to dictate how the universe should work? Do you think it will listen?

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

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I'm not saying how the universe should work, I'm saying that light travels a specific distance in a specific duration, independent of observers or objects.

Space means simply volume. The universe is an infinite volume. In that volume there are objects of mass, each with a motion. Light travels in the dimensions of the volume, regardless of what the motion of each mass is.

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

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MD, if the moving train observer sets off two flashes at the front and rear of the train at the same time, and he measured the speed of those using his moving rulers, you assert that he would measure the front-to-back light to go faster, right?
You say that the front-to-rear light would go from one end of his meter stick in less time than the rear-to-front flash. Right?

So, if we do the same experiment on Earth using Earth-based rulers, and if Earth is moving in absolute space, then we should also measure light to go faster in one direction than another because our rulers are moving, right?

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

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If a moving train has two lights, one at the front and one at the rear, and the lights emit light simultaneously, the light from each light sphere will have the same radius when they meet. If the train had a velocity during that time of light travel, the sources will not be an equal distance from the meeting point when they meet. Two light spheres are being emitted, the velocity of the sources has no affect when those spheres will meet.

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

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Do you agree that my previous post is an accurate statement of your position?

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

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No, and I explained why in my response.

Light speed is not dependent on an observer's motion. Two light spheres a specific distance apart will always meet in the same amount of time if emitted simultaneously, regardless of the motion of the sources after emission.

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

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Your previous post is consistent with what I said.

Try it this way:
The train observer is trying to figure out whether the train is moving, and in which direction.
They have a light, a detector, and some very precise timing equipment.
They measure the time it takes light to travel from the front of the train to the back, and from the back of the train to the front.

What do they measure?
Same times?
Different times?

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

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Same times.

They are only measuring the speed of light, not the train's motion.

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

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Are you sure? Great if you are, but it doesn't seem to mesh with what you were arguing earlier.

So, Light going from the front to the back takes the same time as light going from the back to the front?

If two light flashes started at the same time, one from the front and one from the rear, you say that they would also reach the other end of the train at the same time?

Is that what an embankment observer would see as well if they could peek in the window as the train went past?

Last edited: May 13, 2010
15. ### Janus58Valued Senior Member

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Yes. you are, because your very argument is contrary to real physical observation of the universe shows.
That distance and duration is observer(frame) dependent.

You need a frame of reference to measure any motion. And there is no preferred frame of reference. Every one of those masses can make an equal claim to being at rest. But the masses themselves are not important. only the frame of reference (you don't need a physical object or observer for a frame of reference. There are an infinite number of frames of reference, all with different velocities with respect to each other, and each one measures the speed of light as 186,00 mps relative to itself.

to use your Sun analogy. For someone for which the Sun is moving, the Sun leaves the center of the expanding sphere of light, but for the Sun, it always remains at the center of the sphere of light that it emitted.

You can howl and scream how this is Absurd or impossible all you want, this is how the universe works, As has been verified by actual experiment.

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

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Positive. They are only measuring the speed of light.

Light travels at c, regardless of "direction."

They are measuring the speed of light. The speed of light is the same from the front to the back or the back to the front.

The embankment observer would see them measuring the speed of light, and laugh at them.

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

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So, our train observer in the middle of the carriage can have a light flash at each end, pass him in the middle at the same time, and continue to the other end, regardless of the train's motion.
Excellent. I'm glad we agree, and I'm glad you've changed your mind since post 42 when you suggested that this could only happen for an observer with zero absolute velocity.

Good night.

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

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You can not measure motion while it is happening. The time has to have already elapsed for you to know the distance that was traveled. There is no motion at that point, the stop watch has already stopped.

19. ### funkstarratsknufValued Senior Member

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Nature doesn't agree. End of story.

(Why does this forum keep attracting cranks?)

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

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What makes you think that if the lights were emitted simultaneously that they would always meet the observer simultaneously? That can only happen if the observer has a true zero velocity. Changing the observers velocity means he is "hastening" towards one light while moving away from the other, which mean the two lights will not impact the observer simultaneously, as he has changed positions since the lights were emitted.

Good night. While you sleep, you realize time keeps ticking, correct? I mean, time actually continues to pass even though you are not observing it, believe it or not.

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

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You just agreed that for the train observer with unknown velocity, if two light flashes started at the same time, one from the front and one from the rear, they would also reach the other end of the train at the same time because the speed of light is fixed.

Now, you're saying that no, the front-to-rear light reaches the middle of the train before the rear-to-front light.

Let's try once again:
The train observer (who doesn't know whether the train is moving) is standing in the middle of the train.
They use synchronized clocks to trigger simultaneous light flashes from the front and rear of the train.
They measure how long it takes the light flashes to reach them in the middle of the train.
What do they measure for the two flashes?
Same times?
Different times?

Can the train observer use these measurements to measure their velocity in absolute space?

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

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You did not say anything about synchronized clocks. You are changing the scenario and then saying I changed my answer. Tisk tisk.

So let's talk about the term "synchronized" shall we? You mean, two clocks were at the center of the train, reading and keeping the same time, correct? OK. Now, you place one clock on each end of the train and return to the center of the train, correct? When the clock's light from each clock reaches the center of the train, do the clocks appear to remain synchronized, or does one clock appear to be ahead of the other?

They measure the time it takes the light to reach them at their current position on the train? So if the train did posses a velocity, that midpoint position would continuously change relative to the speed of light, correct??

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

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Cranks??

Is that the part of the internal combustion engine that converts reciprocating motion into a more useful rotary motion? And how would you measure the motion of said "crank?" How about we measure the revolutions per minute, you know, using the standard second as the interval of time. So we measure the RPM of the crank at a constant rotational velocity at say 3,000 RPM. Now, we need to know more about that motion, as the crank could be turning a 3000 lb-ft load at 3,000 RPM, or it could be turning a 30 lb-ft load at 3,000 RPM. The term RPM doesn't quite describe the motion in its entirety. Luckily, torque is here to save the day. Yes, that's right, we can use the unit of measure called horsepower (HP) to measure the power by simply measuring the RPM and the torque on the crank at that RPM. You see, the unit of measure of HP is equal to 550 ft-lbs of work per second, or 33,000 ft-lb of work per minute. So, if you have a 1 lb load on the end of a 1 ft bar (1 lb-ft of torque) and you spin it 1 RPM, the 1 lb load travels 6.2832 ft per minute, which is 6.2832 ft-lb of work per minute. Since 1 HP is 33,000 ft-lb of work per minute, a 1 lb load spinning (33,000/6.2832) 5252 RPM is equal to 1 HP, or HP=torque*RPM/5252.