## Time dilation

A forum designed mainly for high school physics students in New South Wales, Australia.
r.jenkins
Posts: 11
Joined: Sat May 01, 2010 10:43 pm

### Time dilation

Hi,
With twins paradox they say that the person flying at relativistic speeds away from Earth will 'age less' - understandable! But why then, if this person was to watch an event happening on earth, does it appear to go slower then it would in their 'spacecraft'? Don't these contradict each other?

Thanks
Mini
Posts: 66
Joined: Sat Mar 06, 2010 10:33 am

### Re: Time dilation

tv=t0/sqrt(1-v^2/c^2)

The person in the spacecraft measures t0. So t0 < tv. Therefore in the spacecraft's frame of reference, an event takes a shorter amount of time to occur compared to the Earth's "stationary" frame of reference.

Suppose that the person in the spacecraft had an identical sibling. Before one of the twins set off to travel in the spacecraft that would travel at relativistic speeds, they were of the same age. When this twin returns from his voyage he/she will be younger. This is because he/she measures t0 whilst the counterpart left on Earth would've measured tv. (The spacecraft is not an interial frame of reference because it turns around to come back to Earth)
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Time dilation

Thanks Mini.

Mr/s Jenkins:
This is a subtle but well discussed point and the superficial paradox is known as the twin paradox. We give a detailed treatment on
http://www.phys.unsw.edu.au/einsteinlight/

Best
Joe