# faster than light?

• February 11th, 2013, 02:18 PM
Mr. Bow
faster than light?
Maybe a silly question to start with:

man in a spaceship that is one light second in length
shine flashes a light
spaceship going at 0.99999c away from spacestation
and light travels to front of space ship in one second
and spaceship travels one light second (approx) in one second
then light has travelled 2light seconds from spacewstation in one second??

How?
• February 12th, 2013, 04:55 AM
Markus Hanke
Quote:

Originally Posted by Mr. Bow
Maybe a silly question to start with:

man in a spaceship that is one light second in length
shine flashes a light
spaceship going at 0.99999c away from spacestation
and light travels to front of space ship in one second
and spaceship travels one light second (approx) in one second
then light has travelled 2light seconds from spacewstation in one second??

How?

This is not how you add velocities in Minkowski space-time; there is a special relation for this, refer here :

Velocity-addition formula - Wikipedia, the free encyclopedia
• February 12th, 2013, 05:06 AM
Strange
I don't think that is appropriate in this case as one of the velocities is c.

Quote:

How?
The trouble is that you are not taking length contraction and time dilation into account. For example, at this velocity, the spaceship will appear 1/224th of its length, according to the stationary observer. And its clock will appear to be running 224 times slower.

It is hard to answer your question as it is very ambiguous:

man in a spaceship that is one light second in length

One light second in length as measured by who? The man in the ship or the man on the spacestation?

light travels to front of space ship in one second

One second measured by who? The man in the ship or the man on the spacestation?

spaceship travels one light second (approx) in one second

One light-second measured by who? The man in the ship or the man on the spacestation?
One second measured by who? The man in the ship or the man on the spacestation?
And "approx" is not good enough. That is the other place where the apparent paradox is hidden.

light has travelled 2light seconds from spacewstation in one second

Two light-seconds measured by who? The man in the ship or the man on the spacestation?
One second measured by who? The man in the ship or the man on the spacestation?

There are probably some obvious answers to these questions that would allow the proper answer to be worked out. But I will leave that for the moment.