# Laws of time and space

1. Apr 12, 2005

### InvariantBrian

The laws of motion are not independent of time and place. Time and space are values that can be different depending on one’s frame of reference. Moving clocks run slower than stationary clocks and moving rulers are shorter than stationary rulers. This means that two people can get different results when measuring the time and distance of moving objects and they both can be right. Hence, the type of invariance that physicists anticipate is not that the laws of motion will be independent of time and place, but the principles governing temporal and spatial variation will remain unchanged.

Is this true?

Last edited: Apr 12, 2005
2. Apr 12, 2005

### HallsofIvy

You seem to be making a distinction between "laws of motion" and "principles governing temporal and spatial variation". What exactly do you mean by "principles governing temporal and spatial variation"?

The fundamental laws of motion must be independent of the frame of reference- that's one reason laws of motion in relativity theory are written as tensor equations- any tensor equation that is true in one frame of reference is true in all.

3. Apr 12, 2005

### chroot

Staff Emeritus
No, it's not really accurate. "Moving clocks run slow" is essentially a bastardization of the theory of relativity.

- Warren

4. Apr 13, 2005

### InvariantBrian

Ok maybe I should state it this way:

The time and distance of objects in motion can be measured differently depending on one’s frame of reference. This means that two people can get different results when measuring the time and distance of moving objects and they both can be right. The type of invariance that physicists anticipate is not that the measurements of objects in motion will always be the same for all observers, but the principles governing temporal and spatial variation will be the same for all observers.

Last edited: Apr 13, 2005
5. Apr 13, 2005

### chroot

Staff Emeritus
InvariantBrian,

That sounds about right. Time and distance appear differently to different observers, but there is only one set of physical laws, which apply equally to all observers.

- Warren

6. Apr 13, 2005

### Kirk Gregory Czuhai

' . "Moving clocks run slow" is essentially a bastardization of the theory of relativity.
' !!!
gee whiz !!!
well, I NEVER !!!
T = [ t *(1/[1-r]) r=(v^2/c^2)
why would this be a "bastardization?"
at least "locallly" the equations of Physics are Lorentz invariant,
at least "classically they seem to obey Einstein's General Relativity Theory,
except at quantum distances,
why would this be a "bastardization?"
LOL!
peace and love,
and,
love and peace,
(kirk) kirk gregory czuhai
http://www.altelco.net/~lovekgc/kirksresume.htm [Broken]

Last edited by a moderator: May 2, 2017
7. Apr 13, 2005

### DaveC426913

I think it can be boiled down to: "The application of 'invariance' in the universe is not in *what* we measure, but in *how* we measure."

And yes, that is true.

In Greene's 'Fabric o/t Cosmos', he explains how a highjumper in New York can expect that when he goes to the Lunar Olympics, he can expect gravity to behave the same way. It does not mean he *gets the same numbers*, but he can expect that, taking the laws of gravity as we know them, the gravity on the Moon will *behave as it should*.

8. Apr 13, 2005

### jdavel

Kirk,

"....T = [ t *(1/[1-r]) r=(v^2/c^2) why would this be a 'bastardization?'...."

There's nothing wrong with the equation, but it's not equivalent to the statement "moving clocks run slow".

If two clocks are in relative motion, which one will "run slow"?

Last edited: Apr 13, 2005
9. Apr 13, 2005

### chroot

Staff Emeritus
"Moving clocks run slow" is a misconception. The proper way to express time dilation is "clocks appear to run slowly to observers with high relative velocities." The two statements are, in fact, very different.

- Warren

10. Apr 13, 2005

### InvariantBrian

I'm confused..I thought time moves differently for objects moving at different velocities. Do a clock appear to run more slowly or does it 'really' run more slowly?

11. Apr 13, 2005

### chroot

Staff Emeritus
InvariantBrian:

Your wristwatch will always appear to tick at the same rate to you, no matter how fast you're moving with respect to other objects in the universe. To other observers, however, it might appear to tick slowly.

The debate over what's "real" and what's "perception" is a philosophical one, but most physicists are local realists: what you can measure is what's real.

- Warren

12. Apr 13, 2005

### Staff: Mentor

The key is to realize that different observers will measure the same clock to tick at different rates. It's no illusion; according to all measurements made in a given frame, a moving clock runs slower than a stationary clock. But whether a clock is "moving" or not depends on what frame is observing the clock. Time measurements are frame dependent.

(Saying that moving clocks "appear" to run slow may lead some to think that time dilation is an illusion, not a real physical effect. But just saying "moving clocks run slow" is even more misleading without some elaboration.)

13. Apr 14, 2005

### InvariantBrian

Why will they measure the same clock tick at different rates? The difference in measurement is due to time dialation is it not?

Is it true to say that time flows differently for observers moving at different speeds? For example, if some one is going 100 mph and another is going 1000 mph. Doesn't time move more slowly for the one moving more quick?

Last edited: Apr 14, 2005
14. Apr 14, 2005

### HallsofIvy

Yes, IF you remember to say from which frame they are being observed.

If person A observes person B moving at, say 0.9 c relative to himself, he will observe B's clocks, B's heartbeat, B's aging, much slower than his own.

Of course, person B will observe A moving at 0.9 c relative to HIMSELF (in the opposite direction but time constraction is independent of the direction) and will observe B's clocks, B's heartbeat, etc as much slower than his own.

What do you MEAN by "really running slow" as opposed to "appear to run slow"?

15. Apr 15, 2005

### InvariantBrian

I see the point of your queston. It's relative. fine. but what about the case where one twin ends up older than the other. In that case, time objectively moved more slowly for one of the twins. But I guess that's not really an inertal frame.

I think i get the first part though..
In inertial frames, the movenment of time is relative to ones frame of reference.

16. Apr 15, 2005

### chroot

Staff Emeritus
That's correct. You can't compare the clock readings unless the clocks are brought back together again, and you can't bring the clocks back together unless one of the twins turns around. Turning around necessarily involves acceleration, and acceleration means the frame is no longer inertial.

- Warren

17. Apr 19, 2005

### InvariantBrian

But in the case of intertial frames, is the time dialation effect relative?

18. Apr 19, 2005

### cepheid

Staff Emeritus
I'm sorry that this question has nothing to do with physics, but can someone explain to me why everyone seems to be saying clocks "run slow"? Specifically those two words put next to each other. It doesn't really make sense to me when I hear it. Slow is an adjective right? So you can say that something is slow, such as the rate* at which the clock of a passenger on a moving train ticked, as measured by an observer on the rail embankment.

*in this case, the rate is the thing being described as slow

But if you are to describe the action itself (the act of running, or ticking), wouldn't you have to say, moving clocks run slowly? (adverb)

It's bothering me, because I have even seen the phrase "moving clocks run slow" (horror of horrors!) in print, which is making me second guess all this ^.

InvariantBrian: If by "is the time dilation effect relative", you mean that if two observers are in relative motion, and each one is moving at a constant velocity relative to the other, will each one measure the other's clock to be ticking more slowly than his own?, then I guess the answer is yes.

19. Apr 19, 2005

### chroot

Staff Emeritus
cepheid,

This isn't a grammar forum, but you're right, it should be "moving clocks run slowly." That's still misleading physically, but at least it's grammatical, eh?

- Warren

20. Apr 19, 2005

### cepheid

Staff Emeritus
Yeah, I see how it neatly sweeps away the whole essence of the special theory of relativity because it never specifies -- they run slowly as perceived by whom?! I suppose you could claim that it is implicit in the statement that the clock is moving relative to the observer, otherwise he wouldn't be describing it as a "moving" clock now would he? But I think it's a *bad* idea to state anything implicitly,assuming it will be interpreted the same way by everyone. Special relativity is difficult enough (for me, anyway) without additional ambiguities. The whole idea, as I understand it, is that no statement of time (e.g. a time interval, the duration of an event, whatever) has absolute significance. It is meaningful only when given with reference to the inertial frame of the observer who measured it. This statement about moving clocks just doesn't capture that. In case anyone's interested, I saw "moving clocks run slow" in Griffiths Electrodynamics, and he had put it in bold, inside a box, as if to imply that it nicely summarized the entire paragraph above it. :yuck: