# About time measurement vs measurement of movement

1. Jan 22, 2009

### rabbetussa

hi.
as a non-physicist reading physicsbokks i have a problem understanding how, in richard feynmans words, "time itself", is measured. feynman seems to move from clock-measurement to "time"-measurement without a justification. as tsr clairly states time-dilation i have a problem of understanding the terminology. do we talk of "time going slower" or of "clocks going slower"? this might seem ridiculous, but it is a point i simply do not understand when reading feynmans "six not so easy pieces" and other semi-introductory descriptions of the theory of special relativity.

2. Jan 22, 2009

### A.T.

Clocks measure time
Slower than what? Time dilation is always relative. It is one clock going slower than another identical clock.

3. Jan 22, 2009

### rabbetussa

thanks a.t but i fear that my question is not completely clear. you say "clocks measure time", but it appears to me as if clocks refer to movement in time, not to time itself (as succession). our clocks are based on the regular movement of planets, and regular movement of atoms if you wish, but they are anyway based on regular movement. my problem is that if movement occurs in time and space, there has to be a separation between movement and space and time. but a clock appears to measure only movement. so: is there no separation between clocks and time in tsr? if so; might i just as well say clock-dilation as time-dilation?

4. Jan 22, 2009

### A.T.

Not only movement, but all physical processes. You could build a clock based on radioactive decay rate.
No.

5. Jan 22, 2009

### rabbetussa

thank you a.t.. thats all i need to know.

6. Jan 22, 2009

### matheinste

Hello rabbetussa.

Think of a clock as something periodic (it ticks). the ticks mark the progress of time and counting them gives us the amount of time. Of course we require this ticking rate or period to be constant for an observer at rest with the clock.

Matheinste.

7. Jan 22, 2009

### Staff: Mentor

The idea that clocks slow down (instead of time itself) is essentially Lorentz's aether theory, and it is experimentally indistinguishable from SR. The reason that we typically prefer the idea that time "slows down" is because it is simpler (Occhams razor). The alternative is to explain the several individual mechanisms by which light clocks, mechanical clocks, quartz clocks, atomic clocks, particle half lifes, etc. each coincidentally slow down by the exact same amount.

8. Jan 23, 2009

### rabbetussa

thank you dalespam. both for the info and for the will to help. it amazes me how helpful and nice people are in this site.
your answers help me a lot in understanding the topic. would it be possible (not trying to pose a new theory here, but to rephrase into a viable metaphysical language) to say that: "in a moving system, all internal prosess slows down by the same ratio as a function of the relative speed of the system".? if it is then maybe i'm finally getting it. (my major difficulty is that i am normally studying metaphysics of the natural sciences and the terminology is a bit different concerning the consepts of time and space).

9. Jan 23, 2009

### matheinste

Hello rabbetussa

First of all you cannot just say “in a moving system” as there is no absolute rest system. If you have two frames moving inertially relative to each other, call them A and B, then an observer in A will observe all processes in B to be running slower than his own and vice versa. The amount of slowing is a function of the relative speed. This is of course after allowing for light transmission times. There is no objective truth as to who is keeping correct time. Each sees the others time as running slower than his own.

Matheinste

10. Jan 23, 2009

### Phrak

This sort of question come around like clockwork. No one seems to care that rulers measure distance. Rulers don't come around like clockwork. There may be something profound here.

Maybe rulers questions are just spaced out at a characteristic distance---say 45000 miles.

11. Jan 23, 2009

### matheinste

Hello Phrak

I think that people feel more at home with distances. Time seems much more mysterious.
Of course there are fundamental issues with length measurement as well.

Matheinste

12. Jan 23, 2009

### Staff: Mentor

Maybe, maybe not. I've been in discussions before where people argue that certain types of physical processes might be affected by motion or movement through a gravitational field (ie, a pendulum clock ticks at a different rate on the moon vs on the earth). The person was undeterred by the point that there are different ways to measure time that use different types of physical processes and all show the same time dilation.

13. Jan 23, 2009

### Staff: Mentor

It's just because they hear about time dilation first! We have plenty of people in here who question length contraction, but they have to get past time dilation to get to it!

14. Jan 23, 2009

### Phrak

I'd like to hear them. But maybe later on, so as not to trounce on the OP's original question.

15. Jan 23, 2009

### Staff: Mentor

I'm sorry, but I don't understand how the concept you describe here differs from Lorentz's aether theory. Can you explain a bit (preferably avoiding gravity if possible so that we can stick to SR)?

16. Jan 23, 2009

### Staff: Mentor

In Lorentz's theory you could indeed say "moving system" meaning moving wrt the aether which would be the "absolute rest system". Unfortunately, because of the way the aether distorted clocks and rulers there is no way to detect it or determine your velocity wrt it. And, as you mentioned, two observers always see the other clock running slower even if in the aether frame one is "really" running slower. So, although in Lorentz's theory there is an objective truth about what is moving and which clock is keeping correct time, there is no way to experimentally determine it. This is another reason to abandon the aether, it is fundamentally undetectable.

17. Jan 23, 2009

### Naty1

All internal processes in one inertial frame (one moving system) will appear slower when viewed by an observer in another inertial frame moving relative to the first.

18. Jan 23, 2009

### Staff: Mentor

The gravity example is the easiest because it is true (which helps to confuse crackpots), whereas the argument's I've heard from crackpots about SR are false. I've heard crackpots argue that there is something about motion that makes certain types of clocks tick slower than others. Ie, if you sent a different type of clock up in a GPS satellite, it might act differently. This differs from Lorentz aether theory (or relativity) because in reality all clocks tick at the same rate (in their own frame) regardless of their speed relative to another object.

I say this only because the OP's line of questioning seemed to be going in that direction - implying to me that if speed affects clocks, that it might affect different clocks differently.

Last edited: Jan 23, 2009
19. Jan 23, 2009

### Staff: Mentor

A pendulum clock on the earth does tick at a different rate than a pendulum clock on the moon, but a pendulum clock at rest on the surface of the earth ticks at the same rate as a pendulum clock moving at a uniform (non-relativistic) velocity wrt the earth.

20. Jan 24, 2009

### Phrak

I think I see a problem with this, Dale. The rate of clock on the Earth would be the same as the rate of a clock at assymtotic infinity. The rate of the clock at assymtotic infinity would be the same as the rate of a clock on the moon...