# Ultimate speed limit

1. Mar 27, 2009

### kidsmoker

Hi,
i'm just revising for my special relativity module and this bothers me:

http://img259.imageshack.us/img259/1482/17053293.jpg [Broken]

If the spaceship was going at slightly less than c then you would still see a completely different thing on earth to what you would see on the ship. On earth, it would appear as if the light was moving a little ahead of the ship, whereas on the ship, the light would travel away with speed c. What is more special about the contradiction they derive?

Thanks!

Last edited by a moderator: May 4, 2017
2. Mar 27, 2009

### Fredrik

Staff Emeritus
Suppose that Alice and Bob are on the ship, and that Alice is closer to the front than Bob. Now Bob fires a very powerful laser in her direction. If the ship travels at c, she dies in one frame and survives in the other. That's a contradiction.

If the ship travels at 0.9999999999999999999999999c, Charlie on Earth may disagree with Bob about at what time Alice died, but they both agree that she dies. So there's no disagreement about what actually happens at that event, or any other. They only disagree about what numbers (coordinates) to assign to the event.