You don’t have to be much of a science enthusiastto have heard that you can’t go faster than light.
Well, I’m going to tell you at least oneway that two real, honest-to-god objects, that exist in our universe, can move awayfrom one another faster than the speed of light.
Now, don’t get super excited.
I’m talking about ways this can happen withoutbreaking the laws of physics.
To be ultra clear, I’m not talking aboutsomething that will lead to this.
We're gonna have to go right to- ludicrousspeed! Ludicrous speed? Sir, we've never gone that fast before! I mean, I totally want to know how to go ludicrousspeed.
But, unfortunately, and I hate to have totell 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 alocation where spacetime is not bent or changing.
Under those conditions, the speed of lightis about 186, 000 miles per second or, for the metric crowd, precisely 299, 792.
458 kilometersper 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” todenote 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 theyall aren’t quite what you probably think.
The first is cheating.
When light enters a transparent medium likewater or glass or something, it slows down.
In glass, light travels at about two thirdsthe speed it does in a vacuum.
In water, it’s about three quarters.
You can see the effect of this change in velocityby simply sticking a pencil in a glass of water.
The pencil doesn’t really bend, but appearsto because of this effect.
While light slows down in a transparent medium, other particles do not.
That means that if you shoot an electricallycharged particle like an electron or a muon traveling at near the speed of light at thatsame medium, the charged particle travels faster than light in the medium.
Cool, huh? When this happens, the charged particle emitsblue light like you see here.
This light is called Cherenkov light afterits discoverer, Pavel Cherenkov.
Explaining just how Cherenkov light is formedis, well, tricky- and maybe I’ll make a video about it.
But the blue light you see here proves thatit is possible for objects to move faster than light in a transparent medium.
In this example, the light is formed whenradioactive 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’snot because particles got faster, but because light got slower.
Are there other examples? Well there is another instance of informationtraveling faster than light which I’ll mention only briefly and this involves a topic calledquantum entanglement.
Quantum entanglement is a category of quantummechanics, which is known for its bizarre predictions- cats both alive and dead andall that.
It is not possible to explain it in detailhere.
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, governedby the probabilities of that particular situation.
As an example, a subatomic particle can havea 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’ta simple case of ignorance.
It’s not that the spin is plus or minusand you just don’t know.
It’s both plus and minus and it becomesplus or minus when you measure it.
Now suppose you take two particles and setthem up so that they have opposite spins.
If one is plus, the other is minus, and viceversa.
When physicists do this, they say that we'veentangled the two particles.
You can’t know in advance which particleis plus and which is minus.
You then separate the two particles by a largedistance and look at one of them.
Say you find that it's a plus spin.
If you look at the other particle, you’llfind it's a minus spin- every single time.
And this will be true even if you look atthe second particle so quickly that you see it before a signal arrives from the firstparticle traveling at the speed of light.
Einstein called this a “spooky action ata distance” and it says that the information in quantum mechanics can travel faster thanlight.
Nobody understands this, but it’s well establishedand it's just a true effect.
So that’s a case of something travelingfaster than light, but you can’t use it to send a message and it’s still not thesame as an object moving faster than light like the Starship Enterprise or the MillenniumFalcon.
So let’s talk about a third situation wherethings 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 universeexpands faster than light, ” it’s obviously a statement that requires some care, becauseyour first question should be “what does that mean?” Well, obviously the universe can’t meanour 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 Hubblecombined measurements taken by several people and found that distant galaxies are movingaway from Earth and, the further away they are, the faster they're moving.
This is now understood to be evidence thatthe universe is expanding.
Using modern numbers, a galaxy a megaparsecaway is moving away from us at 70 kilometers per second.
A megaparsec is million parsecs, which is3.
26 million lightyears by the way, but astronomers use megaparsecs, so I will too.
If a galaxy a megaparsec away is moving awayat 70 kilometers per second, a galaxy two megaparsecs away is moving away at 140 kilometersper second.
Three megaparsecs means 210 kilometers persecond, and so on.
So we know that the speed of light is 300, 000kilometers per second, so we can figure out how far away we have to go to have a galaxymoving away from us at the speed of light.
That turns out to be 4, 296 megaparsecs orjust shy of 14 billion lightyears.
This means that the surface of a sphere, centeredon the Earth, and with a radius of about 14 billion lightyears is moving away from usat the speed of light.
It also means that bigger spheres are movingaway from us at faster than light.
A sphere with a radius 28 billion lightyearsacross is expanding at twice the speed of light.
So, what does this mean? Does it mean that there are galaxies movingaway from us at speeds faster than light? Yeah.
Yeah, it does.
Of course, it also means that we can neversee 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 byanything currently further away than 14 billion lightyears.
The true number is a little different becauseof details of how the expansion speed has changed over time.
To get the number right means we have to takea more nuanced approach than I’m doing here, but those details don’t change the big message.
It’s also incredibly important to be supercareful about how we envision it.
The reason is that it’s not precisely accurateto 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 duckin a river.
The duck isn’t moving as far as the wateris concerned.
It’s the water that’s carrying the duckaway.
Or like drawing dots on a balloon that’sinflating.
The dots don’t move on the surface of theballoon, but the distance between them is increasing because the balloon is stretching.
So it’s entirely fair to say that thereexist galaxies that move away from us faster than light, but only in the sense that theexpansion of space makes it happen.
Those galaxies are stationary, or at leastnearly 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 wouldlike to see.
According to our best understanding of thelaws of physics, light obeys the ultimate speed limit.
Now, our understanding of the rules that governthe universe are constantly improving and it’s okay to hope that we’ll discoversome new principle that makes it possible to go faster than light.
Although unlikely, it may be that we willeventually find some new phenomenon that changes our prospects for exploring the galaxy, andthen, and only then, we will have finally figured out a way to go- LUDICROUS SPEED! People say that it’s impossible to go fasterthan light, and, as a practical matter, it’s probably true.
But we’ve learned in this video about afew ways in which we can at least kind of break that rule.
So now you have some tidbits to use at yournext cocktail party.
If you like what you’ve seen, remember tobe sure to like, share and comment.
We want to know what you think.
And, as always, remember- physics is everything.