Apparently, this is becoming a good place for people to get their questions answered! My friend Brian, recently wanted to know what the possibilities were for faster-than-light travel. Specifically, he was interested in it because he wants humans to do it.
Brian: but otherwise how will we ever have a civilization like that in Star Trek?
Presumably, Brian’s goal is to be able to travel nearly instantaneously between any two points in the galaxy. The problem is that there are physical laws we have to obey; we don’t have a choice. One of them is special relativity, which tells us that the maximum speed any object can move through space is c, or the speed of light in a vacuum, also known as 299,792,458 meters-per-second. Furthermore, only things with zero mass can ever travel even that fast (and even then only in a vacuum); everything with a mass always, by the laws of physics, has to travel slower than that.
First off, why is that the case? The easiest explanation is that light always travels at the speed of light. Well, duh, you might say, that’s by its definition! But this is actually profound. What it means is that, no matter how quickly you move, light will always move relative to you at the same speed! Let’s say I shine a light at BatmanTM, and Batman and I are both standing still.
We can both measure the speed of that light, and we’ll both get the same value: c. On the other hand, what if I move towards Batman? If it were anything other than light, like a baseball, a bullet, or an electron (even electrons have mass), Batman and I would measure different velocities. Here’s why; let’s say I throw a baseball at 100 mi/hr, and I run towards Batman at 20 mi/hr. I would measure the baseball to be moving at 100 mi/hr, but Batman would measure the baseball at 120 mi/hr.
But this doesn’t happen the same way with light; regardless of how fast I’m moving or how fast Batman moves, we both always measure the light beam to be moving at the same speed, c, or 670,000,000 mi/hr. If I take something with a mass, like my baseball, into a rocket, and I fly at 400,000,000 mi/hr towards Batman and launch the baseball at 400,000,000 mi/hr, will he observe the baseball moving at 800,000,000 mi/hr? No. Special Relativity tells us that velocities don’t work like that close to the speed of light, and Batman would see the baseball actually moving at 580,000,000 mi/hr, or less than the speed of light in a vacuum. Lots of funny things happen close to the speed of light, but this really boils down to three things:
This last one explains why something with a mass can never move faster than c. No matter how much energy I put into something with a mass, trying to accelerate it faster and faster, I wind up making it heavier close to the speed of light, making it harder to accelerate. It gets so hard as I approach the speed of light, that to actually reach c, anything with a mass requires an infinite amount of energy! And that is why it’s impossible for anything with a mass to travel even as fast as the speed of light in a vacuum.
But, what if we didn’t want to travel through space at a speed faster than c, is there a way to cheat this law? Is there a way that I could let a beam of light go at one place, and arrive at a destination before the beam of light gets there? No, there are two ways! First, you can slow light down. If you’re not in a vacuum, light doesn’t travel at c, it travels slower. They have engineered things so well that they’ve gotten light down to 38 miles-per-hour. Well, *I* can go faster than 38 mi/hr in a car; what happens if I send something moving faster than 38 mi/hr through a material where light only goes at 38 mi/hr? Well, the answer is it continues to move faster than 38 mi/hr, or the speed of light in that medium. So you can break the speed of light as long as you’re not in a vacuum. In fact, if you do, you’ll start emitting light (i.e., Cherenkov radiation) until you slow down to below the speed of light in that medium. Cherenkov radiation usually gives off a calming, blue glow like this:
I’m interested in the funny things you can do with space for another reason: although nothing can move through space faster than the speed of light, space itself can expand without a speed limit. Our Universe is expanding, right now, at 71 km/s/Mpc. That means if I look at something that’s more than 4,300 Mpc away from us, it looks like it’s actually moving away from us faster than the speed of light, because of the expansion of space.
Unfortunately, we don’t know how to control the rate at which space expands, but I’ve always found it fascinating that the way to break the speed limit of the universe isn’t to try to make things go faster, it’s to let space expand and do the work for you!