# Eintein's theory of relativity

1. Jul 30, 2009

i would like to know in context to Eintein's theory of relativity if it is actually possible if someone was in a train travelling at the speed of light, and they held up a mirror is it possible that, that person would be able to see their own refelction?

umm also in a paradox, if one twin was to travel into space at the speed of light for 10 years theoritically they would have aged 50 years,, would they look different to their other twin after travelling at the speed of light for 10 years?

thankyou

Last edited: Jul 30, 2009
2. Jul 30, 2009

### Nabeshin

No massive object can travel at the speed of light so both of these questions are pointless.

3. Jul 30, 2009

umm its a theory IF it could happen is it possible

4. Jul 30, 2009

### stovepipe

Einstein's theory of special relativity postulates that light travels at the same rate in all inertial reference frames. If you are on a train going nearly the speed of light with respect to another reference frame, you would still see light in your frame as traveling at the speed of light. In other words, YES you would see your reflection, AND it wouldn't seem any different from looking into a mirror on Earth.

5. Jul 30, 2009

### Pengwuino

You need to place some comma's in your sentences. Neither situation is theoretically possible. It's like asking what's North of the North pole (as a member here once described it). The second situation is better posed (in reverse, as your question was posed incorrectly) as what would happen to the twin if he was sent off at such a speed approaching the speed of light that he experienced 10 years in the stationary twin's 50 years of time. The answer is yes, the, stationary twin will appear 40 years older then the twin who was jetted off.

6. Jul 30, 2009

### stovepipe

I don't really understand this question. As you accelerate to near the speed of light you start to age slower than an observer at rest. So if your twin stays home, and you travel on a rocket at near the speed of light, your twin will be older when you get home.

7. Jul 30, 2009

yea sorry i mis-worded that question, i was referring, if one twin was accelerating to the speed of light for 10 years, whilst the other twin remained in earth for the same peroid of time would the twins still look the same?

cause i heard therotically, 10 years travelling at the speed of light is equivalent to 50 years spent on earth.

8. Jul 30, 2009

### Pengwuino

It is impossible to travel at the speed of light if you have mass. Period. End of Statement.

Time dilation is dependent on the speed the twin is traveling. The passing of 50 years in the stationary frame while 10 years passes for the twin traveling with a constant velocity is true for only 1 velocity which is easily calculable but i'm too lazy to do it.

9. Jul 30, 2009

### vin300

T'=t(rt.1-v^2/c^2)
10=50(rt.1-v^2/c^2)
v^2/c^2=24/25
v=[4.89/5]c
v=0.97c

10. Jul 30, 2009

### stovepipe

Traveling at 0.9798c will make you age 10 years while the twin on Earth ages 50.

$$t_0 = \frac{t}{\gamma}$$
$$\gamma = \frac{1}{\sqrt{1-\left(\frac{v}{c}\right)^2}}$$

t0 is your time, t is Earth time, v is your velocity, c is the speed of light.

11. Jul 30, 2009

### Bob_for_short

12. Jul 30, 2009

### HallsofIvy

Do you realize you are saying "If Einstein's theory were wrong, what would Einstein's theory say about this situation?" The situation you envision is theoretically impossible in relativity so, even theoretically, it cannot happen.

You can say, "suppose a man traveling at 99.999999% the speed of light relative to a given observer were to look into a mirror".

But the whole point of "relativity" is that all motion is relative. In his own frame of reference, the man is not moving at all. Of course, he can see his own reflection.