We all know about the ultimate speed limit... the speed of light.

If person A stands on Earth and shoots his pistol, he observes the bullet to fly directly away at 1000 mph. Person B is standing right next to him (not in front) and watches this event, and agrees that the bullet flies directly away at 1000 mph.

Let's change the situation and say that B is in a spaceship, and A is in a different (and very long) spaceship with lots of windows. B's ship is hovering in space (no thrusters/acceleration). A's ship is approaching from a distance and is going to pass B's ship (very close) but at incredible speed. Make careful note that A's ship is NOT thrusting or accelerating at all, it is "coasting". In fact, A's ship is moving, relative to B's ship at 10 mph less than the speed of light. WOW!

A stands in the middle of his ship and points his gun directly forward (in the direction of travel), and fires the same pistol at the exact moment that he is passing B.

The questions are: How fast does he observe the bullet leave the gun? How fast does B observe the bullet leave the gun?

How do your answers change (if at all) if A aims backwards when he fires?

(In reply to re: What a doppler effect! by rerun141)

You are absolutely right and so is Charlie. I don't suggest that A doesn't have a normal frame of reference where the bullet would appear to travel 1000mph in either direction the gun is fired. The point of near light speed travel is that your entire spaceship's internal clock slows to a crawl relative to the outside universe. Lets say you were travelling between planets which were 1 light year apart at the aforementioned 10 mph less than the speed of light. To an external observer this trip would appear to take just over 1 year. But for someone aboard the ship the trip would seem to take less than a second.

I am basing this on Charlie's numbers since I have no tool to manipulate that quantity of significant digits. But basically if the bullet appears to be travelling .00030 mph relative to A from B's point of view, then over the course of the year the bullet will have gone .2628 miles (.000030*365*24) relative to A - which from A's frame of reference only takes (.2628 miles/1000 mph = .0002628 hours = .0002628hrs*3600sec/hr=.94 seconds).

The classic NASA wonderbyte is that Niel Armstrong, Buzz Aldrin and the like are younger than anyone else born at the same time as them because they have experienced "relativistic speeds".

Oh, and Penny, you rock!

Posted by Eric on 2003-11-28 22:46:36