You don’t have to be much of a science enthusiast
to have heard that you can’t go faster than light. Well, I’m going to tell you at least one
way that two real, honest-to-god objects, that exist in our universe, can move away
from one another faster than the speed of light. Now, don’t get super excited. I’m talking about ways this can happen without
breaking the laws of physics. To be ultra clear, I’m not talking about
something that will lead to this. Or this. Or this. We're gonna have to go right to- ludicrous
speed! Ludicrous speed? Sir, we've never gone that fast before! I mean, I totally want to know how to go ludicrous
speed. But, unfortunately, and I hate to have to
tell you- these are all fiction. So, if it’s possible to go faster than light,
what exactly do I mean by the speed of light? I mean the speed of light in a vacuum in a
location where spacetime is not bent or changing. Under those conditions, the speed of light
is about 186,000 miles per second or, for the metric crowd, precisely 299,792.458 kilometers
per second, or fast enough to circle the globe 7.5 times in a single second. Light is like really, really, fast. When physicists use the symbol “c” to
denote the speed of light, it’s this speed that they’re talking about. So, how can you go faster than light? I can think of three examples, although they
all aren’t quite what you probably think. The first is cheating. When light enters a transparent medium like
water or glass or something, it slows down. In glass, light travels at about two thirds
the speed it does in a vacuum. In water, it’s about three quarters. You can see the effect of this change in velocity
by simply sticking a pencil in a glass of water. The pencil doesn’t really bend, but appears
to because of this effect. While light slows down in a transparent medium,
other particles do not. That means that if you shoot an electrically
charged particle like an electron or a muon traveling at near the speed of light at that
same medium, the charged particle travels faster than light in the medium. Cool, huh? When this happens, the charged particle emits
blue light like you see here. This light is called Cherenkov light after
its discoverer, Pavel Cherenkov. Explaining just how Cherenkov light is formed
is, well, tricky- and maybe I’ll make a video about it. But the blue light you see here proves that
it is possible for objects to move faster than light in a transparent medium. In this example, the light is formed when
radioactive material emitting highly energetic particles is immersed in water. It’s all way cool. So that’s maybe a cheat. Particles move faster than light, but it’s
not because particles got faster, but because light got slower. Are there other examples? Well there is another instance of information
traveling faster than light which I’ll mention only briefly and this involves a topic called
quantum entanglement. Quantum entanglement is a category of quantum
mechanics, which is known for its bizarre predictions- cats both alive and dead and
all that. It is not possible to explain it in detail
here. That would actually require not just one video,
but an entire series, but here I can give the highlights. In quantum mechanics, probability rules. Anything that is possible can happen, governed
by the probabilities of that particular situation. As an example, a subatomic particle can have
a spin of plus or minus. You can’t know which of those spins it has,
until you actually measure it. It’s important to understand that this isn’t
a simple case of ignorance. It’s not that the spin is plus or minus
and you just don’t know. It’s both plus and minus and it becomes
plus or minus when you measure it. Now suppose you take two particles and set
them up so that they have opposite spins. If one is plus, the other is minus, and vice
versa. When physicists do this, they say that we've
entangled the two particles. You can’t know in advance which particle
is plus and which is minus. You then separate the two particles by a large
distance and look at one of them. Say you find that it's a plus spin. If you look at the other particle, you’ll
find it's a minus spin- every single time. And this will be true even if you look at
the second particle so quickly that you see it before a signal arrives from the first
particle traveling at the speed of light. Einstein called this a “spooky action at
a distance” and it says that the information in quantum mechanics can travel faster than
light. Nobody understands this, but it’s well established
and it's just a true effect. So that’s a case of something traveling
faster than light, but you can’t use it to send a message and it’s still not the
same as an object moving faster than light like the Starship Enterprise or the Millennium
Falcon. So let’s talk about a third situation where
things actually can travel faster than light. And that’s the expansion of the universe. Now I should be cautious. When you hear people saying “the universe
expands faster than light,” it’s obviously a statement that requires some care, because
your first question should be “what does that mean?” Well, obviously the universe can’t mean
our planet, our solar system, or even our galaxy. After all, none of them are expanding much,
if at all- and certainly not at the speed of light. In 1929, American astronomer Edwin Hubble
combined measurements taken by several people and found that distant galaxies are moving
away from Earth and, the further away they are, the faster they're moving. This is now understood to be evidence that
the universe is expanding. Using modern numbers, a galaxy a megaparsec
away is moving away from us at 70 kilometers per second. A megaparsec is million parsecs, which is
3.26 million lightyears by the way, but astronomers use megaparsecs, so I will too. If a galaxy a megaparsec away is moving away
at 70 kilometers per second, a galaxy two megaparsecs away is moving away at 140 kilometers
per second. Three megaparsecs means 210 kilometers per
second, and so on. So we know that the speed of light is 300,000
kilometers per second, so we can figure out how far away we have to go to have a galaxy
moving away from us at the speed of light. That turns out to be 4,296 megaparsecs or
just shy of 14 billion lightyears. This means that the surface of a sphere, centered
on the Earth, and with a radius of about 14 billion lightyears is moving away from us
at the speed of light. It also means that bigger spheres are moving
away from us at faster than light. A sphere with a radius 28 billion lightyears
across is expanding at twice the speed of light. So, what does this mean? Does it mean that there are galaxies moving
away from us at speeds faster than light? Yeah. Yeah, it does. Of course, it also means that we can never
see them. If objects move away from us faster than light,
then that means that light emitted by them never get to our eyes. So, we can never see the light emitted by
anything currently further away than 14 billion lightyears. The true number is a little different because
of details of how the expansion speed has changed over time. To get the number right means we have to take
a more nuanced approach than I’m doing here, but those details don’t change the big message. It’s also incredibly important to be super
careful about how we envision it. The reason is that it’s not precisely accurate
to say that these galaxies are moving away from us faster than light. Yes, the distance between us is increasing,
but it’s because space is expanding, not because the galaxy is moving away in space. It’s kind of like putting a rubber duck
in a river. The duck isn’t moving as far as the water
is concerned. It’s the water that’s carrying the duck
away. Or like drawing dots on a balloon that’s
inflating. The dots don’t move on the surface of the
balloon, but the distance between them is increasing because the balloon is stretching. So it’s entirely fair to say that there
exist galaxies that move away from us faster than light, but only in the sense that the
expansion of space makes it happen. Those galaxies are stationary, or at least
nearly stationary with respect to their own space. They’re not moving through space. I’ve described three examples of the phenomenon,
but none of them are something that those of us who hope to explore the cosmos would
like to see. According to our best understanding of the
laws of physics, light obeys the ultimate speed limit. Now, our understanding of the rules that govern
the universe are constantly improving and it’s okay to hope that we’ll discover
some new principle that makes it possible to go faster than light. Although unlikely, it may be that we will
eventually find some new phenomenon that changes our prospects for exploring the galaxy, and
then, and only then, we will have finally figured out a way to go- LUDICROUS SPEED! People say that it’s impossible to go faster
than light, and, as a practical matter, it’s probably true. But we’ve learned in this video about a
few ways in which we can at least kind of break that rule. So now you have some tidbits to use at your
next cocktail party. You’re welcome. If you like what you’ve seen, remember to
be sure to like, share and comment. We want to know what you think. And, as always, remember- physics is everything.
