# Faster than light travel…? [duplicate]

So -

Black holes possess a gravitational force so great it can pull anything in. Including light. So placing these side by side, If I were to shoot a laser pointer out of a cannon at a black hole, and also be standing stationary pointing a laser at a black hole, conventional logic would tell me that the light from the cannon-shot laser would be traveling faster than light once captured in the gravitational field of the black hole, as opposed to the light from my stationary laser in the same gravitational field. Am I right? or am I confusing some part of the theory relativity in here?

Thanks!

• It would be an odd "conventional logician" who knows about black holes but not about the invariance of light speed. – WillO Jun 9 '16 at 3:38
• Instead of light appearing to move faster the space it is travelling through would contract so that the speed is still C not more than C however the contraction in space would happen only for your frame of reference, weird? Such are the joys of quantum physics. It is why Einstein called universe having space-time and not space and time since space-time are one entity according to relativity. – user1062760 Jun 9 '16 at 4:26
• Also i recommend you watch the science tv series "through the wormhole with morgan freeman" it will answer a lot of your questions – user1062760 Jun 9 '16 at 4:30
• "Conventional logic" is a formal procedure that uses algebraic rules to derive complex expressions from simple ones. I don't see you doing that here. I am quite skeptical that you know what "conventional logic" is and how it works. – CuriousOne Jun 9 '16 at 5:45

You don't need to get confused with black holes. Just stand outside a train and shine a laser, and have your brother inside a train shime another laser. Your light would be traveling at speed c with respect to you. If the train was going at a speed c/2, youd conclude also that his light was going again speed 3c/2, faster than you.

As @WillO implied, if you don't know that relativity says that the speed of light is the same in all coordinate frames, meaning as seen by any observer (say inertial ones for the simpler special relativity), then you don't know any relativity. The answer above is that you would your brother's light also going at speed c. In relativity speeds don't add simply. And one c plus another c/2 winds up adding to c.

Black holes are more complex, you need general relativity, but locally the same thing is true: observers will always measure c in their local coordinates.

If you're getting interested in these kinds of things it's good. You have gone beyond getting any value from reading simple popular articles on physics. Take a couple serious courses, even online, there's a lot basic stuff that is useful to then understand black holes. I think that what @WillO was hinting at, much more clearly and concise than I ever could.

• So to sum this up- – DMK564 Jun 9 '16 at 4:03
• Yes, confusing things – Bob Bee Jun 9 '16 at 4:07
• So to sum this up- Light all travels at the same speed, regardless if it was shot out of a moving object, and because of this fact, light will not travel faster than 'Moving' light. The gravitational well of a black hole will not alter this. Is this right? Also any good ideas for free online physics and calc courses? Thanks in advance! – DMK564 Jun 9 '16 at 4:11
• Your interpretation of the answer is right. For online courses, try the free Susskind's lectures. He's got a series called the Theoretical Minimum, pick his lectures on what interests you (you'll learn more) but include some basics. If you think you sort of know classical physics take a lecture or 2 on Special Relativity, then something on quantum mechanics and later quantum field theory, and General Relativity, and then you can start flying. He's got other courses, all good and fun, but at some point you'll want to try someone else, he tends to go slow sometimes – Bob Bee Jun 9 '16 at 4:37
• On calc, if you mean math, depends on where you are. If you mean calculus, yes, you need one, I can no The help you which, there's got to be millions and at least 5 good ones. Somebody else may give you an answer. – Bob Bee Jun 9 '16 at 4:43