# Time Dilation

1. Jan 4, 2008

### mihais18

hello. I have read a lot of information about time dilation, the twin paradox, the doppler effect and the lorentz transform, but, because I am not a physicist, I have to confess that I understand time dilation only partially.

On the internet there are lots of examples that go with the theoretical explanations. (eg. http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm or http://www.walter-fendt.de/ph11e/timedilation.htm [Broken]). There is this example of the 2 clocks that are synchronized. One of them stays on the earth, the other is placed on a spaceship that travels at near-lightspeed. Both of them work with a light beam that bounces off a mirror. The basic idea is that the clock on the spaceship ticks slower, because it takes the lightbeam more time to bounce off the mirror. There is also the case of the twin paradox that is brought into the discussion.

Now you’ll have to excuse my childish ignorarace: for me these examples only demonstrate that at relativistic speeds a light beam clock ticks slower, not that time itself goes slower. As for the twin paradox, why does the twin brother who travels on the spaceship age slower than the one on earth? In what way are the biological processes slowed down? Is that because the particles of the atoms that make up the human body are also slowed down, just like the light beam?

it is known that for an object that travels at a certain speed time goes slower than for an object that stays still. So I would like someone to explain to me the relationship between speed and time dilation. If this has also to do with light or the speed of light, then I would like to get an explanation about the relation between time dilation and speed of light…

Last edited by a moderator: May 3, 2017
2. Jan 4, 2008

### Staff: Mentor

It is important to note that several different types of physical processes have been tested and all show the predicted time dilation. So there is no basis to theorize that it affects only certain physical processes and not time itself.
Well that's just it - it makes no sense to theorize that the speed somehow affects certain physical processes. It is time itself that is affected. Even in Galileo's version of physics, there was an overall postulate of "relativity" that stated that the laws of physics are the same in any inertial frame of reference. This means there is no experiment you can do inside a spaceship with no windows that will tell you how fast you are going. Einstein's version just extends that.

Last edited: Jan 4, 2008
3. Jan 5, 2008

### Staff: Mentor

So, from this comment it seems that you understand time dilation for light beam clocks. Remember that special relativity is founded on two postulates.

1) that all the laws of physics are the same in all inertial reference frames
2) that the speed of light is the same in all inertial reference frames

So the part that you understand, the time dilation of a light clock, is primarily based on the second postulate. But to understand how we make the jump from light clocks to time in general you need to consider the first postulate:

Imagine that we have a light clock, an atomic clock, a piezoelectric quartz clock, a windup spring clock, and a rat with a really steady heartbeat. All are clocks working on different physical principles. Because the laws of physics are the same in all inertial reference frames (first postulate) if they all beat at the same rate in one frame they must all beat at the same rate in any other frame. So, in an inertial frame where the clocks are moving at relativistic velocity, because the light clock slows down the other clocks must also slow down or the laws of physics would be different. Therefore, because any physics expression with a "t" in it must slow down, we say time slows down.

Last edited: Jan 5, 2008
4. Jan 7, 2008

### paw

That's one of the most concise and understandable explanations I've read. Should this be in the FAQ's? I think so. BTW, the rat is a nice touch.

5. Jan 7, 2008

### Staff: Mentor

Thanks! I put the rat in since people often don't connect biological age to more physical or mechanical clocks.

6. Jan 12, 2008

### vsandel

The scientific concept of time assumes that there is a proportional relationship between all rates of change. In relativity, where time is malleable according to ones frame of reference, it is assumed that the rate of change of clocks used to measure the velocity of light in a vacuum (c) will vary in such a manner that they come up with the same value for "c" irrespective of the state of motion of that reference frame. Considering two reference frames that are in motion relative relative to each other, clocks in one frame will run slower than the other. The assumption is that every other rate of change in that reference frame will slow proportionately. Is that assumption justified? It seems to me that all the conformations of special relativity have involved particles on the quantum level of size and mass since only those can be accelerated to near the speed of light. These quantum particles exhibit many characteristics not shared by macro objects. Could it be that changes in macro objects do not experience this proportionality?

Last edited: Jan 12, 2008
7. Jan 12, 2008

### belliott4488

Amen!! I agree on both counts!

8. Jan 12, 2008

### Staff: Mentor

That is correct. This is a result of the first postulate which is, as you say, an assumption.
It seems to be a good assumption for objects at least as large as GPS satellites as well as for quantum particles.

If it did happen that the "t" in one physical law varied differently than the "t" in another law then it would be fairly simple to build a device that would detect inertial motion.

9. Jan 13, 2008

### vsandel

Quote: It seems to be a good assumption for objects at least as large as GPS satellites as well as for quantum particles.

What evidence is there that the proportionality holds for objects as large as GPS satellites? The GPS system uses atomic clocks where the timing is a resonance frequency between energy states of an atom. This seems to me to be a quantum particle, no? I know of no evidence that the proportionality holds for macro objects. I would like to hear of any evidence that it does. I am a skeptic.

10. Jan 13, 2008

### Staff: Mentor

A single atom does not make an atomic clock. Yes, there is a quantum resonance involved, but the clock itself is a macro assembly not a quantum entity. At a minimum there is a clear proportionality between the "t" in the quantum resonance and the "t" in the electronic circuitry that turns it into a clock.

I am also a skeptic. Have you any evidence that the proportionality does not hold?

11. Jan 13, 2008

### Staff: Mentor

It isn't a logical skeptic position to believe that while (at least) half a dozen different processes show the predicted dilation, others would not. There just isn't any reason to believe that it would be different and no evidence to imply it.

12. Jan 13, 2008

### yogi

It isn't confined to macro objects - it happens in GPS satellites, in airplanes flown on extended flights around the world, it is a consquence of the unification of time and space as per Minkowski - clocks simply measure the amount of time difference -

Last edited: Jan 13, 2008
13. Jan 13, 2008

### belliott4488

I don't know what you mean by multiple "rates of changes" in one reference frame, but it sounds as if you are suggesting that you could have one rate of time for subatomic processes and another for what you call "macro" processes. How would we observe such "macro" processes? Would a pendulum-driven clock count? If so, then this would lead to a contradiction, as follows.

Suppose a stationary observer watches two clocks on a moving train, one an atomic clock and the other a pendulum clock. Let's suppose as well that he has an identical pair of clocks in his own frame, which are synchronized to run at the same rate as each other. You're suggesting that only the atomic moving clock will appear to run more slowly than the stationary atomic clock, whereas both pendulum clocks will run at the same rate. Since his own clocks run at the same rate, then he sees the moving atomic clock running more slowly than the moving pendulum clock. Since those clocks are in the same frame, however, then an observer on the train will also see his atomic clock running more slowly than his pendulum clock.

To the observer on the train, however, the other observer (on the embankment) is moving, so he must see the other atomic clock running more slowly than his own, by the Lorentz symmetry. Since he sees both pendulum clocks running at the same rate (by your assumption), however, then that means that the atomic clock on the embankment is running more slowly than its partner pendulum clock, which contradicts the assumption above.

In short, the symmetry between inertial frames demands that the time dilation work in "both directions" (i.e. with either frame considered to be at rest), which requires that it is the passage of time, not just the rate of physical processes, that is affected.

14. Jan 16, 2008

### Magic Man

That does obviously require you to accept that the observed action of the light clock slowing means that time itself has slowed...

15. Jan 16, 2008

### belliott4488

Exactly so.

16. Jan 16, 2008

### vsandel

Relativistic velocities lead to mass increases. The mass of particles (atoms, molecules, etc.) does affect the rate of diffusion, chemical reactions, and many other processes. Since the photon does not have mass it is not effected by relativistic mass. Therefore I would expect that there would not be a strict proportionality between all rates of change at relativistic velocities. Thus the assumption that time dilation affects all processes proportionately seems to me to be wrong.

17. Jan 16, 2008

### Staff: Mentor

I recommend that you check out the FAQ and some of the other threads that are currently going on about "relativistic mass". Your understanding of that concept is way off. The short version is that photons do have relativistic mass and that relativistic mass is generally not considered a useful concept.

In any case, the weak and strong forces are compatible with the electromagnetic dilation, and gravitational time dilation also coincides with electromagnetic time dilation. So what else is left?

Frankly, chemical reactions are primarily goverened by QM and EM processes anyway which, by your own admission, are already shown to be compatible with relativity. I think you need to look for a physical law that doesn't involve EM, QM, or gravitation in order to even have a chance of having a rational argument for your position.

18. Jan 17, 2008

### belliott4488

I think one thing that might be confusing you is the idea that "relativistic velocities lead to mass increases." That's true, if you're talking about the masses that would be measured in a frame of reference in which the particles are moving relativistically. In the moving frame, however, the masses have their normal values. In other words, in the moving frame, Physics behaves just as it does in any other inertial frame. In particular, there are no frames where atomic clocks run more slowly than pendulum clocks, so all these scale-dependent time systems you're imagining are the same.

Did you read my earlier post (#13) about this? I believe it show that your suggestion of different rates of time leads to a contradiction. Or, to use your words, the assumption that time dilation does not affect all processes proportionately is wrong (mathematically, according to SR).