# 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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How do you explain the fact that if you sync three clocks, take one to NY, one to Florida, and one to Virginia (Mid point between clocks) that each observer will see their own clock as running ahead of the others, and that the Virginia clock observer will see the other two clocks in sync with each other, but behind his own clock?

So previously, it was agreed that each observer at each clock would send a light signal at precisely 12:00:00.

The clocks all strike 12:00:00 simultaneously, and the light signals are sent.

During the time of light travel, the observer in Virginia travels a distance away from his original mid point position, so the light from Florida reaches him at an earlier time (according to his clock) than the light from NY reaches him.

So since the observer in Virginia traveled a distance during the time of light travel, it is impossible for him to see the other clocks as in sync with each other, and different than his own.

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3. ### Janus58Valued Senior Member

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Yeah, that does seem to be the last nail in the coffin of his mental stability and intellectual honesty.

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

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If the clocks are sync'd on the satellite and the receiving unit, and they remain in sync at all times, if a signal with a time stamp is sent from the satellite to the receiver, it is impossible for the receiver's clock to read exactly as the time stamp when it is received, ie, if the signal is punched like a punch clock with a given time at time of emission, when it is punched at the receiver, the times do not match if the clocks were in perfect sync at all times, because light takes time to travel. The difference between the two punched times is the time of travel, which, if it was light traveling indicates the distance traveled, as distance is defined by light travel time.

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7. ### rpennerFully WiredStaff Member

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Confusing statements of relativity with statements of geometry does not help your case.

8. ### Janus58Valued Senior Member

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Because he is closer to the Florida clock than he is to the New York clock. He has moved relative to those clocks. This is entirely different from the situation where he remains at the midpoint of the Train and does not move relative to the end clocks.

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

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It has nothing to do with Florida's position after emission, it has to do with the light from Florida. Once the light is emitted, the source is irrelevant. This situation is the same as the train. The end clocks position is irrelevant to the example after the light is emitted. So on the train, the mid point observer and the end clocks move as one, but since the end clocks don't matter after emission, it is simply the mid point observer moving relative to both lights that were already emitted.

The light is timestamped at time of emission, you can't change that time stamp, you can however change the position of the midpoint observer in relation to the light before the light reaches him, which changes the light travel time of impacting the midpoint observer.

Last edited: May 19, 2010
10. ### Janus58Valued Senior Member

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Yes, we all know what you believe happens. But there is nothing that requires it to happen that way, and in reality, it doesn't.

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

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It has to, that is the reality of light travel time and synchronized clocks. There is no two ways around it, traveling takes time, regardless of how fast you travel.

Here's an interesting tidbit:

If all three clocks were perfectly synchronized and a light signal was emitted simultaneously from each, if the midpoint observer were to start traveling towards Florida at that time of emission, there is no speed he could travel to get to Florida before the light signal was sent. That's right, regardless if he traveled 10 times faster than the speed of light, he would not get there in time. If he traveled at the speed of light, he would meet that signal half way between Virginia and Florida. Twice the speed, half that distance, and such, but it would be impossible to get there at emission or before.

12. ### Jack_BannedBanned

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Wow, it seems you have it all figured out.

Can you help me.

Can you explain please why the earth's rotational sagnac is in GPS but not the earth's orbital sagnac.

This answer is crucial to this thread. The rotational sagnac supports the OP's argument that light is an absolute constant speed in space at c and thus any motion of an object is relative to that absolute constant.

13. ### rpennerFully WiredStaff Member

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No it doesn't since ring laser gyroscopes measure rotation, not velocity.

The laboratory is attached to the rotating Earth, and it is this angular change with time that is measured, not the radius of the Earth. Likewise if you got a telescope mount programmed to remain orientated at the Sun, a ring laser gyroscope would be rotating at the rate of 1 rev per orbit, and this would measure the orbital sagnac effect. Likewise, if you had the same rig programmed to follow Sirius, you would not be able to measure any Sagnac signal from the gyroscope, even though from the laboratory (which is attached to the Earth) the rig would have the appearance of (slow) rotation.

14. ### FarsightValued Senior Member

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Motor Daddy, it sounds like I'm the only one who will help you here. What you're missing is this: what is the train made of? This is the conversation we need to have:

Farsight: MotorDaddy, what is the train made of?
Farsight: OK, so what is the steel made of?
MotorDaddy: Iron atom and carbon atoms.
Farsight: And what are those atoms made of?
MotorDaddy: Electrons, protons, and neutrons.
Farsight: So what are electrons, protons, and neutrons made of? Let's start with the electron. What is the electron made of?
MotorDaddy: We can't say, it's a fundamental particle.
Farsight: Not so. You can make an electron, and a positron, via pair production. The electron is quite literally made from light.
MotorDaddy: Hmmn. What about the proton and the neutron?
Farsight:Low-energy proton-antiproton annihilation typically results in two neutral pions which decay in a femtosecond to gamma photons. That's light. A free neutron undergoes beta decay and turns into an electron, a proton, and an antineutrino after about 15 minutes. The antineutrino isn't light, but it has very little mass and travels very fast, so it's not too dissimalir for you to get the drift: in essence, your train is made of light. Now try out your scenario from scratch with a train that's made out of light.

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

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It has nothing to do with what the train is made of. This example is enforcing the standards of distance and time. It's the way distance and time are defined, and it applies to all measures of them.

Take for example the speed of your car relative to the ground. Say your car travels at a constant 60 MPH from Florida to NY. Another person is also driving the same type of car that also travels at a constant 60 MPH from NY to Florida. Say the midpoint of that distance is in Virginia with another person and car that is capable of traveling any speed the operator desires. You all have synchronized clocks and agree that the NY car and the Florida car will start traveling at exactly 12:00:00 towards Virginia. The cars leave at exactly 12:00:00 traveling towards each other. If the midpoint observer stays in place, each other car travels exactly the same distance and time to reach the midpoint. If the midpoint car drives 30 MPH towards Florida before the cars reach him at the midpoint, the Florida car will reach him in less distance in less time. The NY car will have to travel a much greater distance and time to reach the observer at his new position. If the midpoint observer travels towards Florida at 12:00:00, there is no speed he could travel to get to Florida when the Florida car leaves or before that time. He could travel a million times the speed of light and still not be there, as traveling a distance takes time. You have to understand, Distance and time are standards, and those standards must be used properly to get correct results, or you get erroneous information and all kinds of weird paradoxes. There is no paradox using this method!

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

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So as in my example, two cars won't meet at the midpoint if they each drive the same speed and start traveling towards each other simultaneously?

18. ### rpennerFully WiredStaff Member

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They will meet at the midpoint if
1) The road is all at rest in reference to itself.
2) The road itself is moving inertially, without acceleration
3) They travel at constant speed with respect to the road
4) Either of Galilean or Special Relativity applies to the universe of discourse.

So this example does not depend on experiments which have different experimental outcomes depending if Galilean or Special Relativity is correct.

19. ### Jack_BannedBanned

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I am really lernin' from you.

No it doesn't since ring laser gyroscopes measure rotation, not velocity.

How exactly do ring laser gyroscopes measure rotation? What change are they measuring????

Well. mathpages does not agree with you and neither do I.

http://www.mathpages.com/rr/s2-07/2-07.htm

Angular velocity is one variable but so is the radius which is the distance from the sun to the earth.

So, your argument falls apart.

Is there any way you can make your argument consistent with the mathpages argument? If not, I can help you.

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

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Two light sources a distance away emit light simultaneously. Each light travels the same amount of time to reach each other. That light travel time defines the distance of light travel.

Comparing light to another object's motion has no affect on the light travel time (distance) in which the light travels, as light's motion is not defined relative to other objects.

If those lights were emitted above Earth, and the Earth rotated during the time of light travel, the fact that the Earth rotated doesn't change the time or distance that the light travels to reach each other. If the Earth has an absolute zero rotational velocity, the midpoint on the Earth at time of emission will be where the lights impact each other. If the Earth moved during the time of light travel, that midpoint on Earth will not be the place where the lights impact, but the Earth's absolute motion is what determines that, not the light's distance and time traveled.

Again, light's distance traveled is defined by light travel time, by definition, it has nothing to do with another object.

Last edited: May 21, 2010
21. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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Look at it this way, every point in the universe is a synchronized clock, all beating as one. If you are at one point, and you travel to a different point, the time you leave is stamped on you, it is a starting time. The time you arrive at the new point is also stamped on you, it is a finish time. The difference is the time of travel, and if you are light, that light travel time defines the distance of light travel. It really is as simple as that, and there is no paradox, or way of violating causality, as traveling takes time.

Say for instance you are an observer at the midpoint of a train, and a light strikes you from each end at different times. There are different possibilities as to what really happened, one that the train was in motion and the lights could have been emitted from each end of the train simultaneously, hence reaching the observer at different times, or that the train was at an absolute zero velocity, and that the lights were emitted at different times. To assume the later is absurd, as when the lights hit you, the light has a time stamp of when it was emitted. If two lights hit you at different times, but have the same departure times stamped on them, guess what???

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

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So as in my New York, Virginia, Florida example, if two cars leave from NY and Florida simultaneously and travel towards the midpoint Virginia observer, and the midpoint Virginia observer moves before the cars get to him, why does he insist that the NY and Florida based cars must have started traveling at different times, just because they reached him at different times? Is he really that stupid to not see that he changed his position and that caused the cars to reach him at different times? Why does he insist he was at rest the entire time? Some people's kids, I tell ya.

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23. ### phytiRegistered Senior Member

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Using Einstein's synchronization method, after the distant
clocks are positioned, from the midpoint you send a light
signal to both, and record the transit time t.
Next you send a signal to both to set each at current time +
t/2. The distant clocks are now synched to the middle clock
for all measurement purposes. Even though the observer may
have a motion relative to a fixed frame of reference in an
absolute sense, it doesn't matter. The method doesn't require
you to know it, because it uses the 2-way path of light. The
observer will receive the same earlier readings from both
clocks, which he attributes to propagation time.
Since this method works for all uniform motion, and is
independent of speed, case 3 will not happen.