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Why is the light clock accurate?

  1. Aug 11, 2011 #1
    My friend and I have debated about this for some time and, because the vast majority of the scientific community backs him, I presume him to be correct but still do not understand why. I have far from an extensive knowledge of relativity, but I will try not to make a complete fool of myself. :smile:

    From what I understand, some or much of the theory of Time Dilation rests upon the model of and experiments concerning the light clock, or the clock that has an interval of time equal to the amount of time it takes for a beam of light to strike one photosensitive plate, "bounce" off, and strike another. It is relatively simple to understand how this clock is affected by time dilation. If an astronaut were to have a clock such as this aboard his static spaceship, it would keep time rather well both for an equally static outside observer and the astronaut himself. However, were the astronaut to fire up his futuristic ship and go 0.99c, then the clock would still keep time for anyone aboard the ship, but to the previous static observer, if he could somehow detect electromagnetic radiation, the light would be forced to travel further in order to reach the other panel and would look as though it were traveling diagonally. Because c is constant, it would take longer, causing the observer to perceive more time relative to the astronaut according to the clock.

    That much I know (or think I know. It is possible my confusion comes from a poor understanding of what I tried to explain above), but here is where I am confused.

    Why is it not just an error in the method of keeping time? For example, while a light clock would have these problems, why would two analog clocks somehow perfectly synchronized before the journey give separate readings after reaching 0.99c? As I said before, this example appears to only account for an error in man's attempt to keep time, not in time itself as measured by the universe (I.E. cellular degradation, planetary orbits, birth and death of stars). Where am I going wrong? Any and all help would be greatly appreciated.
  2. jcsd
  3. Aug 11, 2011 #2


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    First off, your understanding of time dilation is perfectly correct, and your explanation is very good.

    The best simple answer I can give regarding other kinds of clocks is that they are also operating with a mechanism similar to light. What is light? It is the interaction between charged particles that are far away from each other. How do chemistry and mechanics work? They are the interaction of charged particles that are close to each other. That's not a rigorous answer but it may be simple enough to satisfy you.

    But in terms of it being an error, the problem is to know which clock is correct and which one is in error. If the universe is keeping time by her own clock, she isn't telling us which one it is, so we have to settle for what we can know.
  4. Aug 12, 2011 #3


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    Although most clocks depend on the constancy of electromagnetism, some do not. Time as measured by radioactive decay depends only on the nuclear force. The length of the year as measured by the Earth's path around the sun depends only on the gravitational force. And the elapse of time as measured by the Earth's rotation about its own axis depends on neither.
    Sorry, but your explanation of time dilation is incorrect. Time dilation is an intrinsic property of moving objects and is not related to the travel time. Following your logic, what if the object was moving toward you.
  5. Aug 12, 2011 #4
    First time I have ever read that. So in absense of moving objects there is no time dilation, following your logic.

    I found the OPs description very clear and congruent with what I have read before.

    Travelling towards you? It would be length contracted in that direction no? i.e. the reflecting mirrios would appear closer together for the non-inertial observer. Making that part of the calculation for c.
  6. Aug 12, 2011 #5
    I think perhaps Bill K misunderstood what I was saying. I was not saying that the way a light clock works is the way time dilation works, but rather is just an evidence for it. I was then wondering why it was that this evidence, which to me appears specific only to the form of measurement, had an effect upon the universe. I used the planets not to point out what sort of measurement they would give (one based upon gravitation. I.E. Rotation around the sun) but rather why would they be affected by time dilation if the form of measurement were different.

    Thanks everyone for your replies.
  7. Aug 12, 2011 #6
    nope...it's just a way to visualize it in a way that relates to our everyday (low speed) existence. It's an illustration.

    You can read Einstein's original RELATIVITY online....his special relativity does not depend on a light clock model...but does depend on light speed being "c" is all interial frames.

    Of course the evidence is specific to the form of measurement.....but it's just an example of a way to think about time dilation.

    Einstein's general theory shows that time dilation also occurs due to gravitational potential...and is reflected of course by any measuring devices of sufficient accuracy, like atomic clocks.
  8. Aug 12, 2011 #7


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    You have to consider the whole meaning of the statement "c is constant". What this really means is that c is invariant. Put another way everyone measures c as having the same value relative to themselves.

    Imagine that you have two observers passing each other at some speed. At the instant they pass, a light pulse is emitted from the point where they meet. If you ask each observer what happens to the pulse, they will say that it expands out from them at c in all directions. Each will say that they remain at the center of the expanding sphere of light.( though they will not say that this is true fro the other observer.)

    Now take this to the light clock example. Each observer has his own light clock by which he counts seconds. The light bouncing back and forth between the mirrors set at a given distance fro a set number of times equals 1 sec. That is what each measures watching their own light clock. However when they compare their light clock to the other observers light clock, they note that the light travels a longer distance. Since the light for both clocks must travel at the same speed, this means that the other clock ticks slower when compared to their own. The time that one observer measures as 1 sec, the other observer measures differently.

    This is the whole point of time dilation that observers in different frames of reference actually measure time differently from each other.
  9. Aug 12, 2011 #8
    Special Relativity is generally taken to be derived from two propositions: (1) that c is invariant, and (2) the Principle of Relativity, that any inertial reference frame is as good a rest frame as any other and the laws of physics are the same in all of them.

    Proposition (2) implies that what is true for the light clock must be true for any clock. Otherwise, a scientist in a windowless lab would be able to determine his absolute state of motion by comparing the light clock to some other clock. For in that situation, if the clocks agree, they must be at rest. If they disagree then they must be in motion, with a velocity derivable from the difference in times the two clocks keep. The existence of an experiment whose outcome depends on one's absolute motion contradicts proposition (2).
  10. Aug 12, 2011 #9
    Just a few precisions:
    Actually, such experiments have only been done after the theory was already drawn up.
    See: http://en.wikipedia.org/wiki/Kennedy–Thorndike_experiment
    Yes you correctly applied the second postulate to the "static" system: the speed of light is assumed to be constant, independent of that of the source.
    I concur with ghwellsjr. And as also Zikzak indicated, the principle of relativity can only hold if these effects occur for all physical processes.

    Best regards,
  11. Aug 12, 2011 #10


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    Sorry if I did. Here's the issue I thought I was responding to, and it has nothing to do with time dilation. Reference was made to "other types of clocks" including analog clocks. The question was: what are the ways of measuring time locally, in a single rest frame, and whether all methods of time measurement are guaranteed to stay in agreement. I can think of the following:

    Methods which depend only on the intrinsic properties of spacetime:
    Bouncing photon
    Rotation of an object

    Methods which depend on the strength of electromagnetism:
    Quartz crystals
    Atomic transitions

    Methods which depend on the strength of the gravitational constant:
    Orbital motion about the sun

    Methods which depend on the strength of the nuclear strong and/or weak force:
    Radioactive decay

    My point was that although no disagreement between these methods has yet been observed, it is possible in principle, and requires experimental verification.
  12. Aug 12, 2011 #11
    I think I understand what you mean. But wouldn't that mean that, if they were not necessarily to stay in agreement, then one type of clock may register a different level of time dilation than another even though each type of clock (assuming they are in the same category) should be considered equally valid as seen by an outside observer?

    I would agree with you but for the fact that a light clock is calibrated according to the reference frame of the maker. This seems to imply that the frame the clock was made in is, for all intensive purposes, the designated rest frame which means that Prop. 2 seems to support the fact that the method of a light clock may not provide any evidence for time dilation. The scientist would only be able to determine his speed relative to where and at what velocity the clock was made.

    Here is my (possibly flawed) logic. Because, in this instance, time dilation is measured by a clock that forces light to travel further according to a relatively static observer, the light must go further for them because of Prop. 2. Because c is constant, time must shift. However, if the clock were to involve something that travels and sub-light speeds, for example the revolution of an object that spins once a second, c wouldn't be involved in such a way that time would need to be distorted because the measurement of time doesn't involve the progress of light.

    I understand that light would still need to be moving at a constant rate and, when two objects are moving at varying speeds, something must give in order for that rate to be maintained; however, while this may provide evidence for time dilation in a different sense, why is it that the light clock is used as a demonstration? As the title states, why is it accurate?
  13. Aug 12, 2011 #12
    It makes no difference whatsoever where/when the clock is made. Clocks don't remember their birthdays. The only thing that matters is the motion of the plates and the photon relative to the observer.

    The light-clock is used as a demonstration because it is easy to calculate. We know that the light-clock keeps the same time as all other clocks because otherwise absolute motion would be detectable.
  14. Aug 12, 2011 #13


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    It doesn't matter if you use an object that travel at less than c, you could use a bouncing ball and you would get the same result. For example, assume you do have such a bouncing ball clock right next to your light clock. For every time the ball bounces once, the light pulse reflects "n" times. Now someone flies by you at some fraction of c relative to you. He has a light clock and a identical "ball clock". He counts n reflections of his light clock for each bounce of his ball clock also. By the light clock experiment, we know that he will count fewer reflections of your light per given period then he will of his own. HE also has to agree with you that that their are n reflections to your light clock for every bounce of your ball clock.(anything else would lead to physical contradictions if you were ever to meet up.) Thus, he must note that your ball clock bounces more slowly then his. This holds for any type of device used to measure time.
    The light clock is used because it is the easiest way to demonstrate the principle. But don't get to hung up on the fact that it is using light. What is important here is the speed c and not the light itself. Light just happens to travel at c which makes it convenient to use in examples. The importance of c, is that it is invariant, or more importantly just such a thing as an invariant speed even exists. It is the mere existence of an invariant speed that leads to the consequences of time dilation and other Relativistic effects. For both of us to measure light to travel at c relative to ourselves regardless of our relative velocity with respect each other or the source of the light, requires us to measure time and space differently, and that difference extends to all objects, even those moving at sub-light speeds.
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