Relativity Question

1. May 28, 2010

Procrastinate

A physicist decides she would like to remain 21 for 10 years. What is the minimum constant speed relative to earth at which she would have to take a rocket trip into outerspace and back in order to achieve this?

Let t=10 and $$t_{o}=1$$

$$t=\frac{t_{o}}{\sqrt{1-\frac{v^2}{c^2}}}$$

$$10=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$

$${\sqrt{1-\frac{v^2}{c^2}}}=\frac{1}{10}$$

$$1-\frac{v^2}{c^2}=\frac{1}{100}$$

$$\frac{v^2}{c^2}=\frac{99}{100}$$

$$v^2=8.91\times10^{16}$$

$$v=298496231.1=0.9949874371c$$

This was the correct answer when I looked up the solutions.

What I do not understand is why the normal time is considered to be 1 year since ten years of proper time has actually past. If I do this problem the other way, a negative answer is obtained. Could someone please explain why this occurs, or have I just not grasped the concepts of what is the proper time and what is the relative time?

Last edited: May 28, 2010
2. May 28, 2010

Staff: Mentor

No, her proper time is only one year, not ten. She only wants to age 1 year (proper time) while 10 years pass on earth.

3. May 28, 2010

Procrastinate

Since t represents the interval between two events as measured in a reference frame moving with speed v with respect to the first reference frame, wouldn't this be 1? This is because the physicist is travelling at high velocity and thus time would be shorter relative to earth.

This doesn't seem to coincide with the values I put as t and $$t_o$$.

4. May 28, 2010

Staff: Mentor

The physicist represents the "clock" (her body) that we are interested in. t is the time between events (her birthdays, perhaps) as seen on earth. t0 is the proper time, which is measured from a frame in which the clock is at rest. t0 is the time that the physicist measures in her rocket ship; t0 is her actual age.
The time as measured by her will be shorter. Only 1 year compared to 10 years on earth. Seen from a moving observer (the earth), her biological clock appears to run slow: she only ages 1 year when 10 years have passed on earth clocks.

Last edited: May 28, 2010