# B Ideal demonstration of time dilation

1. Mar 18, 2017

### Andrew Wright

In thousands of years time (when we have the technology to do almost anything) what would be the ideal demonstration of time dilation? Imagine I am a child.

ps.
Sorry if this post is over speculative. Feel free to move it if that helps.

Last edited: Mar 18, 2017
2. Mar 18, 2017

### Andrew Wright

Does anyone know if any of the NASA clocks used in time dilation experiments still exist? Are they in a museum somewhere and do they still show different numbers on them?

3. Mar 18, 2017

### robphy

With high-precision wristwatches, we could easily observe the clock-effect/twin-paradox.

4. Mar 18, 2017

### BvU

5. Mar 18, 2017

### robphy

although the OP as a "child" might not appreciate this.

6. Mar 18, 2017

### Andrew Wright

So a futuristic factory could synchronise a set of high-precision watches when they are made. The watches could show the difference against other similar watches if they meet up again after say, a plane flight or space flight? (Without cheating by just using some sort of calculation).

7. Mar 18, 2017

### PeroK

They wouldn't need any calculations. Different people could compare their wristwatches and confirm the effects of gravitational and relative-velocity-based time dilation after a flight, say. But, these differences would be tiny fractions of a second.

In order to see the significant results of time dilation, you would need long-term space flights. If some astronauts went off on a 20-year high-speed mission (Earth time) where they potentially only aged 10 years, then they would be quite clearly younger than their contempraries upon their return. Potentially, for longer high-speed space missions, the astronauts could return and meet their children as old people with grandchildren of their own. Or, even when everyone they ever knew was long dead and gone.

Whether we ever achieve this capability is another matter.

8. Mar 18, 2017

### nitsuj

if you accept the posit that c is invariant....then pretty much the existence of spacetime...or ability to perform comparable measures / physics should be acceptable as ideal enough lol

that said, asymmetric time dilation is most noticeable...for everyone :D gravitational time dilation

for a child or anyone really Pythagoras theorem & "light clock" is imo a great method...there is also MIT's slower speed of light game linked here

9. Mar 18, 2017

### robphy

https://www.nist.gov/news-events/ne...hes-new-us-time-standard-nist-f2-atomic-clock
https://www.wired.com/2014/04/nist-atomic-clock/
“NIST-F2 is accurate to one second in 300 million years,”... $1.1\times 10^{-16}$

I would hope that in a thousand years, I could have this precision in my wristwatch.

For 1 m/s, $(\gamma-1)=5\times10^{-18}$... that is the time-dilation factor is $1.00+(5\times10^{-18})$
http://www.wolframalpha.com/input/?i=1/sqrt(1-(1/3e8)^2)-1

If I am interpreting the numbers correctly, crudely... do a roundtrip at about 1 m/s for about 20seconds...

10. Mar 18, 2017

### Mister T

What each person measures on their wrist watch is called their proper time. So this would be a comparison of proper times. That is not the same thing as time dilation.

The easiest way to see this is that proper time is the time that elapses between two events that occur at the same place. In the language of relativity, this means that since your watch doesn't move relative to you, it's always in the same place relative to you.

Time dilation occurs when you compare the time between two events that occur in the same place (proper time) to a time that elapses between between those same two events, but those events occur in different places.

An example would be muon decay. Imagine a clock that the muon carries with it, and when it reaches an elapsed time of $2.2\ \mu s$ the muon decays. A physicist sets up two clocks, one on top of a mountain and another in a valley at the base of the mountain. These clocks are not located in the same place so he must synchronize two spatially separated clocks, and to do so he must follow some agreed-upon procedure. The muon passes the first clock and the time of passage is recorded on both the mountain clock and the muon clock. Then when the muon passes the valley clock the time on the muon clock and the time on the valley clock are recorded. The difference in the clock readings on the muon clock is a proper time. But the difference in the readings on the physicist's clocks is not a proper time. It is in fact larger than the elapsed time measured on the muon clock. It's dilated.

Note the lack of symmetry: The muon needs only one clock and the physicist needs two. On the muon's clock less than $2.2\ \mu s$ elapses, but on the physicist's clocks more than $2.2\ \mu s$ elapses. But the moun hasn't decayed!

11. Mar 18, 2017

### nitsuj

So basically it doesn't matter how old other people measure you to be, what matters is how much faster you move EDIT: err the greater the comparative speeds ...at least as far as physics is concerned.

Last edited: Mar 18, 2017
12. Mar 18, 2017

### Mister T

How old I am is a measure of the amount of proper time that's elapsed on my clock. You'll agree on that regardless of how fast you move relative to me.

13. Mar 18, 2017

### Andrew Wright

Perhaps one day ordinary people will be able to travel 1000 years into the future to find out what is there.

14. Mar 18, 2017

### Andrew Wright

No I haven't done the maths, and I'm sure that looks basically impossible

15. Mar 18, 2017

### robphy

With time dilation and the clock effect, one could travel at high speed and return to the earth when it is a thousand years from now and see what they have come up with, without you having aged a thousand years.
Unfortunately, you can't return to those you left at the start of your trip [and find them at the same age when you left] to tell them what you saw.

16. Mar 18, 2017

### Andrew Wright

I wasn't expecting much when I downloaded this, just a few dull graphs and a maths game. It is awesome. Thank you.

17. Mar 18, 2017

### Chris Miller

I believe that if your GPS didn't take into consideration its satellites' velocities and adjust for time dilation, they wouldn't work. No need to wait 1000 years.

18. Mar 18, 2017

### Staff: Mentor

The adjustment is actually done on the satellites; the time stamps they put into their signals are determined by applying a correction to their onboard clocks to account for both the satellites' velocities and their altitudes (the latter correction is actually larger and of the opposite sign, so the overall effect is to make the corrected clock rate, the one corresponding to Earth clocks, slower than the natural clock rate on the satellites). But yes, GPS would not work if SR and GR were not correct.

19. Mar 18, 2017

### robphy

While GPS is an everyday application of time dilation, it's probably not an ideal demonstration of time dilation for a child to understand... other than saying that without it, we wouldn't have the precision we enjoy today.

20. Mar 18, 2017