# Several questions

1. May 9, 2014

### Quinapalus

Question #1: Let's say you had a space-ship that could accelerate to the speed light. Once you had attained this speed would it be possible to slow down from that speed? Or would this be impossible because the speed of light is constant? I'm already anticipating that some will say it's impossible to attain the speed of light in the first place. Well, what if your ship is traveling only at a mere 99% of c?

Question #2: What is the future of weaponry? Various sci-fi media like to show our heroes and villains wielding energy-based weapons. But as I understand it, it would be very difficult to harness such a huge amount of energy effectively for a small weapon and isn't really realistic. So how can we realistically expect weapons technologies to develop? (Edited for clarification)

Last edited by a moderator: May 9, 2014
2. May 9, 2014

### micromass

Staff Emeritus
Right. It is impossible for us to travel at the speed of light. We can (theoretically) go very fast, like 99.999999% the speed of light, but never 100%.

One of the things to learn in relativity is that you are always travelling at a certain speed relative to something else. For all we know, we might already be travelling at 99% the speed of light relative to some observer, meaning that some observer on some far away planet might see us travelling at 99% the speed of light already. Of course, we don't see ourselves travelling the speed of light, in fact, we see ourselves not moving at all!

So let me clarify the scenario for you. You are sitting on a spaceship which is stationary on the earth. Then you leave earth and accelerate until you are travelling 99% the speed of light relative to me, who is still sitting on the earth. Then you slow down again until I see you travelling at my speed again. Yes, this is theoretically perfectly possible. I don't know anything in physics that forbids this scenario. It's only an engineering problem. That is, we must find some way to accelerate you to 99% the speed of light. And we must somehow make sure the spaceship doesn't get destroyed by hitting some slower particles. So there are various practical challenges involved which I doubt we will ever solve, but theoretically it's possible.

3. May 9, 2014

### Quinapalus

Thanks, that really helps. :)

4. May 9, 2014

### GTOM

Question #2: TV trope has a section about it, Kineticsarejustbetter.
Otherwise i think infantry weapons will remain kinetics for a long time, vehicles can rather support lasers, and use them to take out inbound missiles, but they might still dont disappear (depending on laser focusing, and target tracking vs jamming efficiency).
Personally i like coilguns. We dont know too much about ball lightning, so i dont know what can be the plausability of SF plasma weapons?

Last edited: May 9, 2014