# Laser and speed

1. Mar 5, 2009

### mrzero

Hello everybody I have a question about light (a laser) and speed. Lets assume a object (space probe) traveling in a constant motion that haze a vacuum inside itself.
Now there are 4 lasers groped in pairs of 2 in south a way that both lasers in a group are connected with both their ends together (The lasers are identical ). Now group A is pointing along the vector of movement of the probe and group B is flipped 90 degrees from group B.

I have picked lasers because they emit a farley non dispersing light compared to a flashlight. Now What is going to happen if the probe is traveling ½ c ? What will the lasers indicate ?
I assume that group A would indicate the exact speed of the probe because the blue shift of the laser light would be if a observer would look in the direction of the movement and a red shift if he would look in the opposite way. And the group B would do the same thing because if you would measure with a ruler from the one of the lasers to the top or bottom of the probe you would see that the lasses dot is a little of and this would correspond to the direction and speed of the probe. Are my assumptions correct ? Thank you for your responses.

2. Mar 5, 2009

### JesseM

Are you asking about the blueshift/redshift of the lasers, or are you asking about where the laser dots hits the top and bottom of the probe? If you're asking about the laser dots, if the lasers are oriented perpendicular to the ceiling and floor, then the dots will always appear directly above and below the laser (in the frame where the ship is moving, the laser beams will have to be emitted at an angle in order for this to be true--see the light-clock thought experiment. If you're asking about blue shift/red shift, this depends on what frame you're using--there will be no shift in the frame where the ship is at rest.

By the way, do you understand that all speed is relative in relativity, so you can only say the probe is traveling at 0.5c relative to some other object like the Earth, but you can equally well take the perspective of a frame in which the probe is at rest and this other object is moving at 0.5c? Also, do you understand that the laws of physics work exactly the same way in all inertial frames, so that if two observers perform experiments on board windowless ships that are moving inertially relative to one another, both experimenters are guaranteed to get the same result? (for example, if both orient their lasers the same way they'll both see the dots at the same position on their ship)