7 replies to this topic

[Sir Deimos]

So. If you're travelling at the speed of light in a ship, and that ship were to fire a projectile, what would happen? Would the projectile move faster due to being fired at maximum velocity? Would it not fire at all because the projectile can't go fast enough to be able to even leave the barrel?

[Hana-Nezumi]

I suppose it wouldn't be able to fire.

[Kwicky Koala]

So if a man was thrusting at the speed of light, he'd never ejaculate?

[Kisseena]

Maybe the projectile could still be able to shoot out if you're shooting it the opposite way you're travelling. So it'll go negative the speed of light. That's how it works right?

[Egann]

The thing with time dilation is that it makes apparent speeds different from actual speeds. You can't ever "go the speed of light" at least not in a vacuum, anyway, but if you were going away from me at 99% of the speed of light relative to me and fired a projectile at 99% of the speed of light relative to you ahead of you, I would only see it going away from me at about 99.14% the speed of light because your sense of time is slowed down.

Edited by Egann, 23 April 2013 - 04:47 PM.

[Wolf O'Donnell]

Surely, the reason that nothing can go faster than light is because special relativity says that anything approaching that speed would end up with infinite mass12. The energy required to reach the speed of light also becomes infinite, thus nothing can reach this speed or at least, nothing can go faster than it.

However, it's only the equations used that come up with the answer of infinity. So is the equation valid?

With the infinity that general relativity, physicists expect quantum mechanics to provide answers or would do if quantum physics was compatible with general relativity. What about special relativity? Perhaps there is a new type of physics that lies at that end of the spectrum. I don't know what physicists currently think about that situation. Maybe no one has thought about it, because there's no way to think about it. If so, I don't mind. I'm happy with the answers we currently have, until there is need to question them.

1. Why can't anything go faster than the speed of light?. Curiosity.com from Discovery Channel. Accessed 24/04/2013.
2. What if you traveled faster than the speed of light?. William Harris. Howstuffworks.com. Accessed 24/04/2013.

[arunma]

I didn't check Egann's calculation, but basically what he said. Believe it or not this is actually a really simple question. Well, the answer is simple anyway. Special relativity postulates two things:

1.) The laws of physics are the same in every inertial reference frame. (Inertial means it's not accelerating, and this is actually a principle of Newtonian/Galilean relativity)
2.) The speed of light is the same in every reference frame. <-- This is the important one

So first I'll answer a slightly different question than the one Deimos asked. A ship can't travel at the speed of light, but let's say you're traveling at almost the speed of light. If you turn on a flashlight and emit a beam of light from the ship, then while you're standing on the ship you'll see the light beam move away at the speed of light. Now let's say you're standing on the shore watching all of this happen. From your perspective a ship is moving at almost the speed of light and it turns on a flashlight. The light coming out will still only travel at the speed of light from your point of view.

This doesn't make sense, right? If you were traveling at 10 mph on a ship and you threw a ball at 5 mph, from your perspective the ball is going at 5 mph. But from the perspective of a guy on the shore, it goes at 15 mph. Somehow this idea of the speed of light being the same in every reference frame changes everything. In order to preserve postulate number 2, we have to rewrite the laws of physics.

Like Egann said, time slows down at high speeds. If you're on the ship going almost the speed of light you'll experience time normally. If you look at a clock on the ship, the second hand will move just like you'd expect. But to the guy on the shore, as you go by at almost the speed of light, the second hand will move really slowly. And if people on the ship are walking around, the guy on the shore will see them moving really slow. The ship will also look a lot shorter from front to back than it really is. So if a guy throws a ball or other projectile, it'll move really slowly and will still not exceed or even reach the speed of light. Also the ball will look more like a pancake, since it's likewise compressed along its direction of motion.

Why don't we actually see this happening? It's because at speeds much less than the speed of light, the laws of relativity basically boil down to Newtonian mechanics. Even at 10% the speed of light, not much happens. You need to travel upwards of 90% the speed of light to notice these things. But this theory has been well tested and shown to be true. The crazy part is that it comes out of electrodynamics. I don't want to get into that right now (though I could if people really want). But based on what we know about electric charges and electric currents, the stuff I said above has to be true. Even though electric currents seem to have nothing to do with how fast-moving objects work.

Sometimes truth is stranger than fiction. OK no it isn't, Star Trek is still cooler.

[Egann]

I didn't check Egann's calculation, but basically what he said. Believe it or not this is actually a really simple question. Well, the answer is simple anyway.

Good. I don't think I added velocities correctly. The special relativity calculator I used is kinda clunky.