# Time Dialation Question

1. Sep 28, 2005

### afbla

Hi I was wondering about how fast do you have to go before time dialation comes into effect

P.S. I'am no Qauntum physics professor so I don't know alot about relativity

2. Sep 29, 2005

### Tom Mattson

Staff Emeritus
This has nothing to do with quantum physics, so I'm moving it out of the Quantum Physics Forum.

It's not as though relativistic effects "kick in" at some critical speed. They are always applicable. It's just that they aren't noticeable at everyday speeds.

Now, if you want to know when relativistic effects are noticeable, that would depend on how sensitive your instruments are.

3. Sep 29, 2005

### pervect

Staff Emeritus
To give a very rough idea, at .001 times the speed of light, time dilation effects are about half a part per million. That's 3*10^5 meters/second.

Cesium clocks have accuracies on the order of a few parts per 10^14, so they can detect time dilation at speeds on the order of slightly over 10^-7 c, say 50 meters/sec, which is only around a hunderd miles/hour.

This comes from gamma = 1/sqrt(1-(v/c)^2) ~ 1 + (1/2)*(v/c)^2 for small v via a taylor series expansion.

4. Sep 29, 2005

### Staff: Mentor

Time Dilation Formula

The measured rate of a moving clock is given by the following formula, where $\Delta T_0$ is the time interval according to the moving clock itself and $\Delta T$ is the time interval as measured by clocks in the "stationary" frame ($v$ is the speed of the clock according to the stationary frame):
[tex]\Delta T = \frac{\Delta T_0}{\sqrt{1 - \frac{v^2}{c^2}}}[/itex]

Thus a time interval measured on the moving clock is observed to take longer according to the stationary frame; this is the meaning of time dilation and the statement "moving clocks run slow".

You can use this formula to see how time dilation depends on the speed of the clock ($v$) compared to the speed of light ($c$).

5. Sep 29, 2005

### Vast

afbla, there’s a nice little relativity calculator here especially for time dilation. Where it says “input” just put in a number close to the speed of light such as 290000 km/s and click on km/second. You’ll notice it gives a Relativistic Change Factor, which means, if it’s 3.944802246249386 for the above input of 290000 km/s, 1 year of your time on a spaceship traveling at such a speed would be 3.944 years on earth. Also notice that for significant time dilation to take affect the speed needs to be at least 0.9 the speed of light.

6. Oct 3, 2005

### Sam Woole

Does "3.944 years on earth" mean the earth planet has gone round the sun for 3.944 times?

7. Oct 3, 2005

### Staff: Mentor

Yes, that's what it means.

8. Oct 3, 2005

### pervect

Staff Emeritus
More or less, though picking nits, there are a number of different definitons of "year", depending on how one measures the earth going around the sun. One usual definition is the time between vernal equinoxes, however there are a number of subtle isues here.

To a level of 4 significant figures, though, the statement will be correct by most any of the possible subtly different definitions of "year".

As far as timekeeping goes, though, our atomic clocks keep much more accuarate time than astronomical motions do, and have become our primary time standard.

Atronomical motions will also not be very useful for a hypothetical space traveler as far as measuring time - he will not base his units of time by the astronomical motions of a distant planet around a distant sun (he won't even be able to observe them in real time). He will base his time on the atomic clocks that he caries with him.

9. Oct 4, 2005

### Sam Woole

Thank you, Doc Al. Maybe you knew that my question has something to do with time dilation. The whole sentence reads: "1 year of your time on a spaceship traveling at such a speed would be 3.944 years on earth." When the twin on the spaceship met his twin brother on earth, it means both twins have spent an equal time interval t, from departure on earth to meeting on earth. There could not have been two time intervals. Then a question has arisen. In this same time interval t, what has the planet earth done? Has it gone round the sun 1 time, or 3.944 times? Which is right, 1 or 3.944?

Undoubtly there can be only one right, the 3.944.
The 1 year on the spaceship is wrong. Does this mean that time dilation is nothing but false? It cannot be justified any way we try.

10. Oct 4, 2005

### Staff: Mentor

This means that folks on earth would measure 3.944 years passing on earth while the folks on the spaceship only experienced 1 year. Note that this is according to the earth. Things get interesting--and unambiguous--when the spaceship is able to make a round trip.

Again you have to realize that time is not an absolute, it really does depend on the relative motion of the frame doing the measuring. When the ship returns to earth, the two brothers will really be different ages!

If you are really interested in learning about time dilation and relativity, stick around and ask questions. (You aren't ready to understand the traveling twins quite yet--you first need to understand the relativity of simultaneity!) But please don't start up again with the accusations of lying, cheating, and claiming that relativity is "nothing but false". It's tiresome.

11. Oct 4, 2005

### JesseM

Time dilation is based on what would be read by clocks moving along with each observer--in this case, the earth-twin's clock will say that 3.944 years have passed when they reunite, while the travelling twin's clock will say 1 year has passed. The two twins don't disagree about what the other twin's clock reads--the travelling twin agrees that 3.944 years have passed on the earth-twin's clock, and the earth-twin agrees that 1 year has passed on the travelling twin's clock. The earth going around the sun is just like another type of "clock" that stays at rest relative to the earth-twin, so of course everyone agrees it elapses 3.944 years as well. If the travelling twin carried a copy of the earth and sun along with him on the trip, then the duplicate earth would have only completed 1 orbit when the two twins reunited, and both twins would agree that this was true.

12. Oct 5, 2005

### Sam Woole

JesseM, my understanding of your words above is: if both twins departed at the age of n, when they united both twins agreed they were both (n + 3.944) years old, the same age according to the clock kept by the earth twin. On the other hand, according to the clock kept by the traveled twin, both agreed that both were (n + 1) years old, the same age. That is to say, whichever way we looked at it, there is no differential aging, no time dilation.

13. Oct 5, 2005

### Staff: Mentor

Both twins agree that only 3.944 years have elapsed on earth. But what counts as far as aging goes is the time elapsed on the clocks that move along with each twin. Both twins will agree that the traveling twin is physically younger than the stay at home twin.

The twins themselves are biological clocks. To make the difference more apparent, increase the speed so that 100 years go by on earth while only a year passes on the ship. When the twin returns to earth, he'll find his brother long dead.

14. Oct 5, 2005

### Staff: Mentor

And both twins agree that one year has elapsed on on the traveling twin's spaceship. (just to make this point clear)

For a worked-out numeric example that demonstrates how the twins can arrive at this agreement, see posting #3 in this thread.

15. Oct 5, 2005

### JesseM

I don't know how you got that conclusion from my words. What I said was: "The two twins don't disagree about what the other twin's clock reads--the travelling twin agrees that 3.944 years have passed on the earth-twin's clock, and the earth-twin agrees that 1 year has passed on the travelling twin's clock." So if they departed at the age of n, this sentence tells you that the travelling twin would agree that the earth-twin was n+3.944, and the earth-twin would agree that the travelling twin was n+1.

16. Oct 5, 2005

### Sam Woole

While I do apologize for my offensive charges, on the other hand I do believe that there must be something wrong with the relativity theory, as can be deduced from your words above.

When the traveling twin left, he was in his inertia frame. The clock he carried was at rest in his frame. His clock therefore would work exactly like any other clocks in inertia frames such as the one carried by the earthbound twin. Don't you agree?

If you do, then both clocks would registered one identical departure time D when the twins departed; then both clocks would register one identical arrival time A when they met again. (A - D) would give one time interval, which means both twins are of the same age.

If you don't agree, then show me why the clock carried by one twin would work differently from the clock carried by the other twin.

17. Oct 5, 2005

### Staff: Mentor

All you can deduce is that relativity does not agree with your preconceptions about time.

All clocks do work the same way. They just don't work the way you think they do!

As measured from any inertial frame, the rate at which a moving clock operates depends on its speed. As long as the two clocks remain in their single inertial frames, they both can equally claim that the other's clock runs slow--perfect symmetry. But if the travelling twin makes a return trip to earth he cannot possibly remain in a single inertial frame; he must accelerate and thus change frames. (To really understand this you'll have to learn some relativity.)

It would be a problem if the motion of the twins were perfectly symmetric and yet their clocks read different times when they reunited. But their motion is not symmetric! One remains in an inertial frame; the other accelerates.

If you like, you can arrange for the two clocks to read the same at the start of the trip. But once you do, you'll find that they don't read the same when they are reunited. The only way you can deduce that the clocks would read the same is if you ignored what relativity has taught us about how moving clocks--and time itself--actually work and just assumed that time flows at the same rate for everyone, regardless of relative motion.

It's not that they work differently, it's that they were moved differently. One accelerates; the other doesn't.

18. Oct 7, 2005

### Sam Woole

I believe you were contradicting not only yourselves but also Einstein. Here you said "One accelearates; the other doesn't" This directly contradicted the principle of relativity, which means (to me at least) all motions are relative. If the spaceship accelerates relative to earth, earth is also accelerating relative to the spaceship. Based on this known principle, the earthbound twin would see the clock on the spaceship to have registered 1 year while his own has done 3.944 years. Similarly the spaceship twin would see the clock on earth has registered 1 year while his own has done 3.944 years. Your words : "As long as the two clocks remain in their single inertial frames, they both can equally claim that the other's clock runs slow--perfect symmetry. " Namely, it is always the other guy's clock running slow, not mine, acceleration or not. The fact that it is the other guy's clock running slow also agrees with the physical phenomenon as we know it, that light needs a time interval to reach the earth from the spaceship, say 2.944 years, and vise versa. When the two clocks come together, united on earth, both certainly will read the samething, no time dilation, no differential aging.

Not only you appeared contradicting yourselves, but also you were using languages that don't agree with convention. JesseM said the space twin carried a copy of the sun-earth. He also used the word "duplicate". If it was a duplicate, it must duplicate the number of orbits the earth has done. If it did not duplicate the number of orbits, then it was not a copy of the solar system. It was completely a different system, completely a different kind of clock, not the clock of identical construction specified by Einstein. In such a contradictory confusion people like me certainly cannot learn relativity. I do not understand how could some have.

If it were true that speed would make us younger, it must also be true that life forms on earth will never die, because the earth is always moving at c relative to light. But we are all dying. It is obvious to me that there is no time dilation.

19. Oct 7, 2005

### ZapperZ

Staff Emeritus
This is wrong. You can ALWAYS do an experiment to detect that you are accelerating. You cannot do an experiment to detect if you're moving without using another frame as a reference. The accelerating frame can always tell that it is accelerating, and can tell that another frame isn't.

Zz.

20. Oct 7, 2005

### Staff: Mentor

You need to learn to distinguish an inertial frame (non-accelerating) from an non-inertial frame (accelerating). The principle of special relativity can be written as "All inertial frames are equivalent for all experiments; no experiment can measure absolute velocity". If one frame accelerates, things are very different.

Wrong again. A solar system like sun-earth works exactly like any other clock. You just mistakenly think that the operation of a clock is independent of its speed with respect to the frame observing it.
It seems to me that you much prefer the comfort of your preconceptions.

Wrong again. This is exactly the opposite of what relativity actually says. The principle of relativity says that I age at the usual rate according to my clocks, and that it would be a violation of physics if it were any other way. And you (on that uniformly moving spaceship) age at the usual rate according to your clocks. You can't tell that you are "really" moving (at least not by how your own clocks work).