# A Train, Three Clocks, and an Observer

Discussion in 'Physics & Math' started by Motor Daddy, May 14, 2010.

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

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A test of absolute motion.

An observer in a train synchronizes three clocks. He places one clock at one end of the train, one clock at the other end of the train, and returns to the midpoint of the train with one clock.

1. The observer looks at each end clock and sees the clocks as out of sync with his own clock. That is due to the fact that the end clocks are a distance away from his clock at the midpoint. Light takes time to travel, hence the observer will see the clocks on the end as reading differently than his own, even though the clocks actually remain synchronized.

2. If the train were to have an absolute zero velocity, the two end clocks would appear to the observer at the midpoint as being in sync with each other, but just reading differently than his own clock at the midpoint.

3. If the train were to have an absolute velocity of greater than zero, the two end clocks will appear to the observer at the midpoint as being out of sync with each other, and reading differently than his own clock at the midpoint.

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3. ### CptBorkRobbing the Shalebridge CradleValued Senior Member

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By your definition of the experiment, and your definition of "absolute velocity", the observer on the train will measure an "absolute velocity" of zero every time, and this result will not change even as you vary the speed of the train.

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5. ### QuarkHeadRemedial Math StudentValued Senior Member

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Gosh, how bored one gets with trains, clocks and light flashes. But let's play the game....
Define " actually". But OK, let's play.

I can make little sense of this. How can two clocks "appear to the observer as being in sync" and yet read different times? I am lost (perhaps not for the first time)

It seems to me you are claiming that at something called "absolute zero velocity" light signals travel instantaneously? This is implied by your (1) and (2) above

So let me get this straight.

All the above suggests you are claiming that at "absolute rest" light travels faster than light speed, but in "absolute motion" it doesn't?

I suggest a class in logic dear boy

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

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Not so. There are three different situations as I already explained. Do you agree the observer at the midpoint can view each end clock to read the same, but different than his own clock? Do you agree that the midpoint observer can view each end clock to be different from each other, and different from his own clock?

Last edited: May 14, 2010
8. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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He views the two end clocks as reading the same time, but different than his own clock.

Nobody said anything about light traveling instantaneously. Just the opposite as the example shows clocks are synchronized, and yet due to light travel time due to the distance between the clocks, the clocks can not be seen as being synchronized from any one point.

Light travels faster than light speed? How do arrive at that conclusion?

9. ### rpennerFully WiredRegistered Senior Member

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Detailed calculations indicate that this is not the best synchronization procedure. Better would to rig a pulley system such that all three clocks start at the middle of the train and the two clocks at the center and simultaneously cranked out to the ends of the train at the same rate. Also note that you never utilize the clock at the center, so it appears superfluous to your setup.

Already you have assumed absolute time in violation of physically established principles and conflate questions of simultaneity with geometrical signal propagation times. But if you use the improved synchronization procedure above, both the Absolutist and the Relativist agree that the clocks at the end of the train are synchronized with each other in the viewpoint of an observer at the center of the train.

Here you make a claim without any reasoning, and reasoning is crucial. Here you assert that in the absolute rest frame that the speed of light is equal in both directions, therefore the geometrical signal delay (of the two clocks that we have agreed are synchronized) is equal. The Relativist believes based on 150 years of evidence that this is true in any inertial frame, not just the putative frame of absolute rest.

Again, you reason in an Absolutist sense than the velocity of signal propagation to is be composed with the absolute velocity of the train as $v = \frac{v_1 \pm v_2}{1 \pm K \times v_1 \times v_2}$ and assert that K = 0. Yet, $K = c^{-2}$ is also self-consistent and is better supported by 150 years of experiment. If $K = c^{-2}$. as the Relativist will believe, then $v = \frac{c \pm u}{1 \pm \frac{c \times u}{c^2}} = c \times \frac {1 \pm u/c}{1 \pm u/c} = c$ in both directions, and so no failure of synchronization is observable.

This was spelled out in a lengthy post which was referred to you, but it looks like it was not read.

You have failed to argue for your viewpoint, but simply asserted it.

http://www.sciforums.com/showthread.php?p=2039656#post2039656

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

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No need for a pulley setup, as there is not a requirement to place the clocks on each end "simultaneously." The clocks remained synchronized at all times. You can place the clocks on the ends of the train however you want, run to one, crawl to the other, etc, and once returned to the midpoint, the clocks are an equal distance away from the midpoint observer, all the while remaining synchronized. The clock at the center is crucial to the experiment. It allows the midpoint observer to observe the difference between the time on his clock, and the time he sees on the other two clocks at any given time.

Last edited: May 14, 2010
11. ### Janus58Valued Senior Member

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As rpenner has alluded to, this would have been considered a valid argument 150 yrs ago. Since then, we have learned otherwise.

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

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The definition of a meter is:

http://en.wikipedia.org/wiki/Meter

The metre (or meter), (IPA: ['miːtəʳ] ), symbol m, is the base unit of length in the International System of Units (SI). Since 1983, it is defined as the distance travelled by light in free space in 1⁄299,792,458 of a second.

If the midpoint observer observes his clock to be 1⁄299,792,458 of a second ahead of each end clock, what does that mean to you? Careful, there might be a meter stick on board that train somewhere.

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13. ### rpennerFully WiredRegistered Senior Member

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This is not what experiment shows.

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

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What, that the midpoint isn't really the midpoint? Is that what experiments contradict?

Last edited: May 14, 2010
15. ### funkstarratsknufValued Senior Member

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What's your purpose with these threads?

16. ### rpennerFully WiredRegistered Senior Member

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Specious and insincere -- the troll traits are strong in this one.

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

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To show that when you get stuck in space in a box with no windows and nothing external to relate to, and you can't tell if you are moving or not, that there is a way to tell, using clocks, and the knowledge that light travels at c.

18. ### MontecRegistered Senior Member

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Hello Motor Daddy
Are you alluding to the differences in the time-of-flight for the clock images from the front and rear of the moving train?

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19. ### Jack_BannedBanned

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I wil assume your model except replace the center of the train with a hand held gps unit.

With no loss of generality, I will assume a gps satellite in the east and one in the west.

BTW, this has been done.

The signals are emitted simultaneously from the satellites.

Guess what, they reach the ground based observer simultaneously.

Now, the timing has been corrected for GR effects and the earth's rotational Sagnac as well as ionsphere.

But, we are not seeing the motion of the earth in its orbit around the sun in the GPS timing which is required for your argument to be true.

Hence, your logic is inconsistent with GPS and the earth's orbit.

I left a hole here, let's see if you can find it.

20. ### Janus58Valued Senior Member

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Please drop the act, you aren't that clever.

Yes, light travels one meter in 1/299,792,458 of a second, and this means that the light from the end clocks reach the midpoint clock in 1/229,792,458 sec if they are one meter from the midpoint (according to the clocks on the train) .

The statement that you make that is conflict to what we know to be true is:

What know to be true is that as long as the clocks are in sync, (according to the train observer) the midpoint observer will always see them as being in sync and 1/229,792,458 sec behind his.

You can have a second train identical to your own, but has some velocity relative to yours, and he see the same thing regarding his clocks.

This very fact is the reason that we define the meter as the distance light travels in 1/229,792,458 sec.

You are sitting in that black box you mentioned in another post. You shine a light and make two marks on the floor to mark the distance that it traveled in 1/229,792,458 sec. That is 1 meter. You shine the light in the opposite direction, and repeat the measurement and get the same length.

Now you fire up some rockets to change your velocity. You perform the same two experiments as above. You will get the same result. You will not find the the light traveled a shorter distance relative to the floor of your box when it is shone in one direction than when it is shown in the other. Or put another way, if your box is two meters across, the light will always take 2/229,792,458 sec to cross from one side to the other, no matter how much you change the velocity of your box.

And this is why we define a meter in this way, because we know that all we have to do is measure the distance light travels relative to ourselves in 1/229,792,458 sec to determine the length of a meter without worrying about how fast we are or are not moving.

21. ### Jack_BannedBanned

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This very fact is the reason that we define the meter as the distance light travels in 1/229,792,458 sec.

We humams are so clever don't you think?

The earth's rotational sagnac has been verified by GPS.

Hence, the definition of the meter is directionally based.

Yes, humans are so clever.

22. ### Janus58Valued Senior Member

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To the contrary, how you do this experiment is very important.

If you carry clock all three clocks to 1 end, leave one, walk back to the other end, drop of the second clock and then return to the center with the third clock, you will get different results then if you just carried each end clock to its end by itself and left the third clock at the midpoint.

You will get a different result if you walked at a different speed while carrying one of the clocks than you did with the other.

Even if you are careful to move each clock to the endpoint at exactly the same speed, you will find that the difference between the time you see on your midpoint clock and the end clocks will be slightly more than the distance to the clock divided by the speed of light.

You will get a different result if you move the clocks to the ends of the train and then change the velocity of your train than you will if the change the velocity of your train and then move the clocks to the ends of the trains.

This is the difference between the real universe and the imaginary one you propose in your experiment. In the real universe, it does matter how the clocks got to the ends in terms as to whether or not they stay synchronized during the process.

Last edited: May 15, 2010
23. ### funkstarratsknufValued Senior Member

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In that case nature disagrees.

(I had hoped that you might be doing it to learn where you were wrong. Trust me, there nothing original about your idea.)