Two statements from Special Relativity are:

  1. The Light Clock measures time.
  2. Time is what a clock measures.

This seems somewhat circular, but I wonder if it is possible to use the light clock as a role model for other physical processes and would be interested how far this transfer works to understand time better.

When I say "time runs slower" (or "faster") I will of course always be referring to the difference between a stationary and an moving observer.

So time runs slower for the moving observer, let it be my friend Joe and his rough "clock" is his heartbeat. His heartbeat runs slower as if he would stay next to me. But how? What are the fundamental processes that take longer in the same way as the Light Clock's light beam has to travel farther in the moving system (length D instead of just length L in the referenced Wikipedia article)? This is what I guess so far:

  1. For the heart to beat, a lot of chemical reactions are going on. Is it correct to say that these require the exchange of (at least virtual) photons. And this exchange is slowed due to the longer path these photons have to travel? If yes, then what about the following:
  2. What about weak interaction Bosons W+, W-, Z. Are they "hampered" in their mediating the interaction due to the longer path to travel in the same way as the photon in the light clock.
  3. What about strong interaction's particles?
  4. Gravitons?
  5. ...?

I know this is mechanistic and intuitive, so I would be happy to at least know how far this intuitive view of time (running slower) has a grain of credit or is utter nonsense.

EDIT: This question is marked as a duplicate of What is time dilation really?, but I cannot see how the answers refer in any way to my question. Rather the duplicate-suggesters seem to interpret more into my question than there is. I can try to rephrase it, but I am not sure I can make it clearer.

An often cited description of the the Light Clock makes time dilation intuitive by referring to the fact that the light in the moving frame, as compared to the non-moving frame, has to travel a longer path. Is this complete nonsense? If not, does the same explanation hold for other fundamental forces (strong, weak, gravitation) in analogy?

  • $\begingroup$ A light clock does not measure time. An observer measures time. So you would measure a dilation in Joe's heart rate because you are at rest relative to him. But in Joe's frame, he measures your heart rate dilated and his normally. $\endgroup$
    – zh1
    Commented Jul 15, 2018 at 12:49
  • $\begingroup$ Time does not 'move slower': the distance between two timelike-separated events (that is, two events one of which is in the past (respectively: future) of the other) is longer on some timelike curves than it is on others. Those timelike curves are the paths objects follow through spacetime. $\endgroup$
    – user107153
    Commented Jul 15, 2018 at 14:14
  • $\begingroup$ Hi Harald. Have a look at What is time dilation really?. I wrote this specifically to help with these sorts of questions. $\endgroup$ Commented Jul 15, 2018 at 16:39
  • $\begingroup$ Possible duplicate of What is time dilation really? $\endgroup$
    – Jon Custer
    Commented Jul 16, 2018 at 15:30

4 Answers 4


My understanding of this is that you should not try to assign a specific mechanism for time running slower on some reference frame because this time dilation effect does not happen at one or other specific part of the system (e.g. the virtual photons on chemical reactions). If we assume, for example, that the time dilation applies only to electromagnetism, someone may easily came out with some paradox about how part of the system has some dilation and other not. The light clock is not some fundamental peace of instrument, it is just the easier way for us to visualize the fact that time runs differently in different reference frames.

You may think in terms of the basic postulates of special relativity:

  1. Speed of light is the same in all reference frames (=c)
  2. The laws of physics are invariant over changing between inertial reference frames.

The second statement also tells us that one can not discriminate between two inertial frames of reference, one moving with a constant velocity with respect to the other.

You may use a light clock to see how time dilation happens using the first statement (one may argue that it is not a fundamental choice, it is just the fact that the light clock refers directly to a postulate, so it is easy to see how things go). Then, you may use the second postulate to argue that this conclusion about the light clock is general: if you have a mechanical clock and a light clock, both need to exhibit the same dilation, because if they didn't we could use this difference to determine whether we are moving with constant velicity with respect to some reference frame or not.

I have a light and a mechanical clock in synchrony. If I fall asleep and wake up to find out that they aren't, now I know I am moving with constant speed. But the second postulate says I should not be able to know that by any experimental means. Therefore, this difference must not exist and everything must slow down just as the light clock, from the mechanical clock to my heartbeat.

  • $\begingroup$ I do not question that SR is symmetric for the observers. Nor do I question that other mechanisms underlie the same time dilation. On the contrary: I am wondering if time dilation, or its effect, if you wish, is as intuitively trivial for other processes (which?) as for the light clock's longer path explanation. $\endgroup$
    – Harald
    Commented Jul 15, 2018 at 13:52

To measure time actually means to measure the time interval between events. According to a stationary (inertial) frame, a moving (inertial) frame will have the spacetime axes rotated by a hyperbolic angle - in such a way that the speed of light remains constant for both frames.

enter image description here

You can see geometrically that the time interval between two events will not be the same in both frames because of that rotation. So the time is not literally ticking slower for moving objects in the sense that they experience everything in slow motion, what happens is that the time interval between two distinct events is greater for a frame that is moving relative to an observer than it is for that observer.

  • $\begingroup$ The events for the Light Clock are start-at-lower-end and getting-back-to-start for the light beam. Leaving aside all the indisputably correct Minkowsky math, the "longer path" explanation is extremely simple and intuitive. My question is whether this intuitive understanding is complete nonsense or, if not, whether it holds water for other phenomena than electromagnetic waves. $\endgroup$
    – Harald
    Commented Jul 15, 2018 at 14:58

Maybe it looks like “mechanistic and intuitive”, but it is interesting to note that approximately the same ideas come to the head of different people, so maybe it is not far away from the truth.

Let me introduce you a simple model in aquatic medium. Using the example of floating ships and simplest method of classical mechanics it simulaties kinematic effects of the entire theory of special relativity.


The model simulates time dilation, length contraction, reciprocal Lorentz transformations, relativistic velocity addition, relativistic (longitudinal and transverse) Dopper effect, twin paradox, Bell‘s spaceship paradox, barn and ladder paradox, symmetry of relativistic effects etc.

Please find a shuttle boat, that moves with velocity V. Motion of these shuttle boats determines the pace of processes on floating ships. Motion of these shuttle boats is very much like oscillation of light in a light clock, isn’t it? Please note, how distance between different barges in a group of floating barges is kept. Do you find parallels with your own ideas? Doesn’t it look like motion of force carriers inside material bodies?

Good to note, that exactly the same model leads to reason of finitiness of the speed of light. As soon as we allow the speed of light „inside“ material bodies, speed of light “outside“ material becomes finite and unachievable for material bodies. Simply because we cannot send neither material body nor massless particle to distant point and back in such a way (immediately), that no events happen "inside us " since this „amount“ of events depends on the speed of force carriers inside our body or "our clock", i.e on the speed of light.


Note: physical explanation for Lorentz contraction is also set out by Feynman in his Lectures on Physics, Vol II, Sec. 21-6 on retarded potential effects. The only physical constraint needed to explain relativistic length and clock/time effects in is that light is the fastest signal in empty space.

Also good note by Trevor Morris is here: About the Ether Theory acceptance


Perhaps an analogy will help you think about what’s “fundamental”:

You’ve got a complex system of colliding balls. You can analyze it either with conservation of momentum, or via detailed looks at individual collisions using forces and Newton’s laws. Which is more fundamental?

Of course, neither is. Sometimes one approach is more useful, sometimes another, but they’re equally correct.

Conservation of energy/momentum and the space-time features of special relativity are (seemingly, always subject to experimental falsification) large-scale truths about how the Universe behaves. The forces in some collision, the ticking of a particular clock, and the reaction rates in chemistry are specific happenings within that Universe. They’re all of a piece, they’re all consistent because they’re all part of the same thing.

Sometimes we can’t see how momentum and/or energy is conserved when we work through a situation, but that’s a flaw in our understanding: what the universe is actually doing will (subject to the conservation laws being experimentally disproved) end up being consistent with conservation.

Same is true for the nature of events in space-time: if a thought experiment (not a real experiment) doesn’t find a consistent result, it’s probably a statement that we haven’t properly analyzed the situation, because both the large and small scale properties of the universe are consistent.


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