Physics Help Forum Light Clocks and Relative Motion
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 Aug 5th 2018, 01:40 AM #1 Junior Member   Join Date: Aug 2018 Posts: 2 Light Clocks and Relative Motion I have couple questions about the time, measured by 2 light clocks in a frame, which moves at a constant velocity - where one clock is perpendicular and second one is parallel to the direction of motion: https://www.pitt.edu/~jdnorton/teach...ods/index.html 1. First, the parallel one - according to SR, distance which is being passed by a photon is in this case reduced due to lenght contraction. When the photon is moving in the same direction, as the entire frame, lenght contraction cancels out the increased distance, caused by motion of the clock (as light is "chasing" the mirror) - this is why, the same time can be measured between the "ticks" by the parallel and perpendicular clocks. For now, everything seems to be OK... Problems begin, when the photon is moving back to the first mirror (left one). Due to motion of the entire clock, mirror and the photon are moving in opposite directions and decrease the distance between them. Even without the lenght contraction, photon which moves in the direction, opposite to the clock's motion, will hit the mirror much faster, than in the case of perpendicular clock. If we will add the lenght contraction, this time gets even shorter... The only solution, I can think of, is to use the relativity Doppler's effect, instead of Einstein's lenght contraction - what would result in decreased distance in the direction of motion (just like in the case of lenght contraction) and increased distance, for the opposite direction... Is there any other explanation? 2. Clock, which is perpendicular to the direction of motion - there seems to be a lot of disagreement between people, as for the behavior of light, which is being emitted perpendiculary to the source's motion. According to officially approved knowledge, motion of the source doesn't affect the velocity and direction of emitted light. We simply can't treat photon as a bullet, which was fired from a speeding car - where perpendicular components of motion are adding. Some people say, that the motion of ligt source affects the angle, at which photons are emitted, but this can be easily disproven, by extending the path of light inside the moving source. We can for example make a kind of tube, inside which photons will move perpendiculary to the frame's motion, before they will move further into space - similar to the image in the middle, where path of light is not parallel to the orientation of perpendicular source (what is probably incorrect). Here are some links, where this problem is being discussed: The light Clock An Analysis of the Light Clock by Miles Mathis https://books.google.pl/books?id=fRR...motion&f=false Besides velocities below c are completely relative, so according to the rules of relativity it is completely wrong to tell, that photons emitted from a source, which moves perpendiculary to the direction of propagation, pass a longer distance, than light emitted from a stationary source. All we have to do, is to change the point of view - and source, which was stationary, will become a moving one, extending the path of light, just because of the relativity of motion and without any changes of velocity ... Here are 2 movies, which show the same scenario from 2 different perspectives: I will send 50$to anyone, who will tell me, for which ball time is flowing slower due to it's velocity... Good luck! This shows, that the idea of time dilation due to constant velocity is totally inconsistent with the basic laws of relativity and breaks the symmetry of relative velocities. The only possible solution, which I figured out, is to use (once again) the relativity Doppler's effect, which in this case would decrease the distance in the direction of source's velocity vector, leaving only the perpendicular component of photon's motion. However this would be in disagreeent with SR, which tells that for a moving frame, light is passing longer distance, than for a stationary one, what is causing the time dilation (it doesn't bother me too much, as I don't like the idea of time dilation due to a constant velocity). Sorry for my english - it's not my native language...  Aug 6th 2018, 07:10 AM #2 Physics Team Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,347 Let's start with the case of the light clock that is operating perpendicular to the direction of motion. Your animation is fundamentally incorrect because the two photons are shown moving at different velocities. They appear to both have the same vertical component of velocity, but the one moving diagonally also has a horizontal component of velocity, and hence its total velocity is greater than the photon from the stationary source. This means in your animation that the two photons are traveling at different velocities. But a principal foundation of SR is that light always travels at velocity c as measured by all observers, regardless of the relative motion of the source and observer. You need to change the animation so that the velocity of the photon moving diagonally is the same as the velocity of the photon moving vertically. When you make that change, you will see that the photon moving diagonally takes longer to complete its path than the one moving vertically, because it has to travel a greater distance. Hence the clock using the moving source (relative to the observer who considers his own source to be stationary) appears to run slower than the clock with stationary source (relative to the same observer). This is true for both observers - in other words the observer located with the red source sees the yellow clock running slower than his own red one, and the observer with the yellow source sees the red clock running slower than his yellow one. Both observe the other person's clock running slower than his own. This is a critical aspect of SR - two observers moving relative to each other will disagree as to which clock is slow. Now consider the light clock operating in line with the direction of travel. Given that we know the path length the photon must travel is different for both observers, and given that the velocity of the photon is constant c for both observers, and given that the moving clock runs slow relative to a stationary clock, the only explanation for the rate of relative slowness of the moving clock is if length contraction has occurred. Hope this helps. Please send the$50 to your favorite charity. topsquark likes this. Last edited by ChipB; Aug 6th 2018 at 07:13 AM.
Aug 6th 2018, 09:33 AM   #3
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 Originally Posted by ChipB Please send the \$50 to your favorite charity.
Remember that the Forum takes it's usual 30% cut.

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Aug 6th 2018, 02:38 PM   #4
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 Originally Posted by topsquark Remember that the Forum takes it's usual 30% cut. -Dan
Hey, why does he get off so lightly?

You told me it was 300%

Aug 6th 2018, 04:00 PM   #5
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 Originally Posted by studiot Hey, why does he get off so lightly? You told me it was 300%
Ummmmmm.....I'll give you a discount next time?

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