# Speed of Light question

1. Feb 14, 2008

### osiris774

Hi im new to understanding the theory of relativity so please excuse me. I can make sense of most of it. The one thing im having a hard time grasping is:

If i travel at half the speed of light in one direction, light is still going by me at light speed in the same direction.

It takes 8 minutes for light from the sun to reach earth. So can't i theoretically pass that light leaving from the sun if i can make that same trip in 4 min?

please explain in Lamen's terms so i can understand.

2. Feb 14, 2008

### Staff: Mentor

In order for you to make the trip in 4 min, you'd have to move at twice the speed of light. No can do. (Assuming you mean 4 minutes as seen from earth.)

3. Feb 14, 2008

### osiris774

I know its not possible to travel that fast...i said hypothically speaking of course.

4. Feb 14, 2008

### Staff: Mentor

Since that would violate currently accepted physical law, I'm not sure what kind of answer you want. It's like saying "Ignoring physics, what would happen if I do such and such...." Beats me!

5. Feb 14, 2008

### osiris774

well they say if i could travel half the speed of light lets say....light would still be going by me at the speed of light in the same direction. Ignoring that fact that i can't travel half the speed of light, why would light still go by me at it's normal speed?

6. Feb 14, 2008

### Staff: Mentor

There's nothing wrong with going half the speed of light (at least in a thought experiment). Going at twice the speed of light is a problem.

As to why the speed of light is always the same with respect to any observer, I don't know how to give a satisfying answer to that. One thing to point out is that high speeds do not add in the same manner as low speeds. For example: If you ride a train moving at 50 mph, and you throw a baseball towards the front of the train at 50 mph, the speed of the baseball with respect to the ground is 50 + 50 = 100 mph. But if the train was moving at half the speed of light, and you threw the "baseball" at half the speed of light, the speed of the baseball with respect to the ground would not be 1/2 + 1/2 = the speed of light. Instead, you'd have to use the relativistic rule for the addition of velocity:

$$V_{a/c} = \frac{V_{a/b} + V_{b/c}}{1 + (V_{a/b} V_{b/c})/c^2}$$

Thus the speed of the baseball with respect to the ground would only be 0.8 times the speed of light.

7. Feb 14, 2008

### robert Ihnot

Einstein asked the question if he was going faster and faster on a bicycle chasing a light beam, well, could he look in the rear view mirror and see himself?

8. Feb 14, 2008

### osiris774

ok thx doc.

9. Feb 14, 2008

### CaptainQuasar

osiris774 you might be interested in [THREAD=215019]this conversation[/THREAD] that has been going on recently.