So, in short, yes, this is going to get a bit physicsy. I realise most of my 2 readers don't have a strong background in physics but I'm hoping to explain things in a way that most people should understand.
So! To understand the speed of light, we first need to understand what exactly light is. So..
What is Light?
The short answer is, light is a wave. Well, technically, a wave-particle but the particle bit isn't important here, so we'll ignore that. So what's a wave? The short answer there is, it's a disturbance in something that travels through space, transferring energy. You've probably seen water waves, or heard sound waves or anything like that. How do we know that light is a wave?
Waves have a kinda interesting thing they do called "interference". Basically, two or more waves combine and affect each other. As much as I want to keep maths out of this, it's easiest to show with a graph. Or, three:
Of these, the second one seems the most nonsense-ey, surely you can't actually make two sound waves in such a way that they don't make any sound? Well, it turns out you can and it's something that concert hall designers need to take into consideration when they build. You can also see it in wave pools at swimming pools and that sort of thing.
Now, obviously, I wouldn't be bringing this up if I weren't about to tie light into it. Because, light can do it too! Unfortunately, it's not terribly easy to do because, the problem with light is that it tends not to come in nice, neat wave-y forms and when it does, light waves are really really short so trying to get them to match up nicely is damn near impossible. But fortunately, SCIENCE! has the answer. Specifically, lasers and CDs.
Basically, lasers are cool because laser light does come in nice, neat, wave-y ways, which also makes it really easy to split up in a way that you can make it do cool things. So, what you want to do, is reflect the light off the back of a CD on a wall. Done right, you'll see the reflection of the laser beam, plus a few extra dots. These dots are the first graph, the dark space in between are the second.
So, what's the point of all this? Light is a wave, so what? Look at the examples of waves I gave earlier. Sound waves travel through air. Water waves travel through, well, water. Seismic waves travel through the earth. Waves are a disturbance in something, they need a thing to travel through. So what does light travel through?
The Ether
This was a big problem for physicists 200 years ago or so, because, well, light needs something to travel through! What they decided on was what they called the "ether". Basically, it was an invisible, undetectable and still ether that existed everywhere in the universe and was what light went through. It makes sense, if you think about it. Light needed to travel through something, we just had to detect it.
Luckily, two guys, Michelson and Morley came up with a way to do it. See, if there was an ether in all of space, our planet was moving through it.
When a wave travels through something, it moves straight outwards in a circle. If the medium is moving, the wave moves with it. If you're moving through the medium, it looks like the medium is moving and that's what the Michelson-Morley experiment was supposed to measure. If light was moving through the ether, and we were also moving through the ether, what they could do was measure the speed of light in one direction, then rotate their equipment a quarter turn, measure it again and there should be a difference. So, they did this and found... no difference. Which kinda disproved the ether hypothesis. This leaves us with a problem, of what exactly does light travel through?
Magnets, how do they work?
Maxwell is a sadly, kinda unheard of physicist, which is a shame because he had one of the biggest impacts out of anyone on electromagnetism, when he wrote down his four equations that describe how electricity and magnetism work:
- ∇·E = ρ/ε0
- ∇·B = 0
- ∇×E = -∂B/∂t
- ∇×B = μ0J + μ0ε0 ∂E/∂t
These equations are very cool from a mathsy point of view. I'm sure they're meaningless to most people but basically...
The first one says that an electric charge puts out an electric field. Like, say, an electron or proton will put out an electric field.
The second one says there are no magnetic monopoles (Or, none that we've found so far. Basically, you know how every magnet has a north or south pole? We have yet to find one that's only a north pole or only a south pole)
The third one says that a changing magnetic field makes an electric field. This is how you're probably reading this blog, since most power plants work by turning magnets in a way that makes electricity.
The fourth one says that a changing electric field makes a magnetic field. That's how magnets work, Insane Clown Posse. All magnets are made by changing electric fields.
Now, if you're clever like Maxwell, you'll notice something kinda funky about the last two. Namely, a changing magnetic field will make a changing electric field will make a changing magnetic field will make a changing electric field. Normally, all this does is make it a little harder to generic an electric or magnetic field, but it's, in theory, possible to make a self-sustaining electromagnetic wave. Basically, a wave that is an electric field that generates a magnetic field that generates an electric field and it continues like that, well, forever.
So Maxwell did a little mathsing to find out exactly what the conditions are for a self-sustaining electromagnetic wave and found that it needs to satisfy an equation:
c = 1/√ε0μ0
It's a fairly simple equation there. Since you're probably not all nerds like me, the things in it are kinda meaningless. Well...
c is the speed the wave travels at. Obviously.
μ0 is the permeability of free space, it's basically how much a vacuum will accept a magnetic field, and it's equal to 4π×10-7Am.
ε0 is the permittivity of free space and is basically how much a vacuum will accept an electric field. It's equal to 8.854×10-12 F/m.
It's a pretty simple equation, and Maxwell plugged in the numbers and got a wavespeed out: 299,792,458 m/s.
That's a number that should jump out at any first year physics student and it certainly jumped out at Maxwell. For those who don't recognise it, it's the speed of light. Maxwell had, kinda accidentally, stumbled across what exactly light is. It's a self-sustaining electro-magnetic wave and, by the nature of electromagnetic fields, it doesn't strictly need an ether or any other medium to travel in. Light's just fine without one of those, but unfortunately, that starts causing a few more problems, but it really took until one Albert Einstein came along to uncover exactly what was about to happen.
Relativity
I'm gonna make a brief side-track here to talk about relativity, since it's where the light-speed speed limit comes from. How many people know what the theory of relativity is? Most people would probably say e=mc2, which is pretty wrong. That equation comes from it, but it's not the theory of relativity. Wanna know what it is? It's pretty simple really, it's one pretty basic statement:
The laws of physics are the same in any reference frame.
Basically, what it's saying is no matter how fast you're going, or if you're accelerating or decelerating or turning or anything like that, the laws of physics are the same. That seems really obvious, right? Why would the laws of physics break down because you're going fast? And why should that sort of a statement cause any issues?
Go back to that equation that Maxwell used to find the speed of light. There's something missing from it. Go on, look for it. I'd be surprised if you find it. Give up? It's missing a source velocity. Basically, it means that the speed of light doesn't depend on how fast the source is going. Again, that doesn't seem too odd. It doesn't quite show why relativity is about to cause problems.
Relativity is called that because speed on its own is kinda meaningless. You need a speed relative to something else. It's easy enough to say you're running at 10 km/h, because people assume you mean relative to the surface of the Earth. What if you're flying through space at 100 km/s? Is that relative to the earth? Or to the sun? Or to the moon?
So when light is flying through space at 299,792,458 m/s, what's that relative to? It'd be easy if the ether had turned out to be real, it'd be 299,792,458 m/s relative to the ether and, as long as we know how fast we're travelling through the ether, we can find out fast light is travelling relative to us.
Unfortunately, the ether wasn't real, which brings us back to Einstein's statement. See, the laws of physics are the same, no matter how fast you're going and that includes Maxwell's equation for the speed of light. Basically, the speed of light is always exactly 299,792,458 m/s, no matter how fast you're going, or the source of the light is going, because light cannot go another speed.
Again, that doesn't seem like too much of an issue because we, as humans, have a lot of trouble perceiving speeds that quick. So here's a wee thought experiment. Imagine you're standing on the side of the road and you throw a ball down the street at 10 km/h, while I demonstrate amazing MS Paint skillz:
Bored with just throwing balls, you decide you'd rather hop in a car, drive down the street at 50 km/h and throw the ball instead. Now, relative to you, the ball is still going at 10 km/h, but relative to the earth, the ball is going at 50 + 10 = 60 km/h.
But oh noes! Someone's driving towards you at 30 km/h! Relative to them, the ball is going at 50 + 10 + 30 = 90 km/h!
Note that these speeds are all relative. Obviously, the ball is travelling at one speed, it just depends on your point of view exactly what the speed is. The difference between the ball and light, is that the speed of light is always exactly the same. Using our ball metaphor, it's as if, from everyone's point of view, the ball is always travelling at 10 km/h.
This is going to need a little more explanation, so we're gonna get out of our cars and head into space and look at some spaceships. This time, we're sitting on a space station while two space ships come flying towards us (Not on a collision course). You bust out your speedo and measure that both ships are travelling at 70% of the speed of light:
But oh no! The red spaceship suddenly doesn't like the blue one and has decided to fire a laser at it! Which is travelling at the speed of light, what with it being light and all that:
See, here's where things get messy. From the red spaceship's point of view, the laser is travelling at 299,792,458 m/s. From your point of view on the space station, it's also travelling at 299,792,458 m/s. Obviously, the laser is going to reach the blue spaceship before the red one does.
But from the blue spaceship's point of view? The laser is still travelling at 299,792,458, while the red space-ship is apparently travelling 40% faster than that. In other words, the red space-ship will reach the blue spaceship before the laser.
This is a contradiction here. Either the laser is travelling faster than the spaceship or the spaceship is travelling faster than the laser. It can't be both, that's a nonsense statement and something's gotta give here.
Time Dilation and Length Contraction
Before we find out what exactly, we're gonna do another wee experiment. Someone on the blue spaceship is doing a physics experiment and fires a laser from one side of the ship to the other:
You're a bit creepy, so you look in the window while they do that and watch the beam move from one end of the ship to the other. The problem is, from your point of view, by the time the beam reaches where it came from, the ship has moved a lot:
If you have any common sense, you won't like the answer. Because the answer is, we're both right. It takes 1.4ms for the laser to make its return trip from our point of view and 1ms from the blue ship's point of view. Which is completely bat-crap insane. How can that possibly ever make any sense?
It's an effect called "time dilation". Basically, the faster you're going, the slower time travels for you. Relativity causes a lot of other strange things too. For example, length contraction, where things that are moving towards you become shorter. It's not a "they seem shorter", it's a literal shrinkage from your point of view. There's also the fact that what might happen simultaneously to you might not happen simultaneously for someone moving towards you.
Which brings us back to our space-ships. See, when the red spaceship fires its speedometer at the blue spaceship, it doesn't say that it's going at 140% of the speed of light, it says that it's going at 94% of the speed of light. Which is still slower than the laser, which solves our contradiction from earlier.
I realise this seems a little abstract and off the point a bit here, and if you're utterly confused, don't worry, that's intended. I'm trying to prove a point here. My point?
The speed of light is the universal speed limit and the universe does not want you going faster than it. The universe will completely screw with common sense notions of space and time to prevent you going faster than it. There's simply no way you can go faster than the speed of light because insane, common sense defying science says otherwise.
And yet, CERN have apparently found particles which are doing just that.
Now, at this point, you're probably thinking "Oh well, maybe relativity is wrong and the speed of light isn't the universal speed limit!"
The thing is, relativity really isn't wrong. Sure, you can say "But we can't measure that! We can't fly spaceships at close to the speed of light and watch things length contract and watch time dilate!" Well, yes we kinda can. Just with subatomic particles. They've been launched at close to the speed of light, then measured to see the effects. Little particles called muons have a well known decay rate, and a formed in the upper atmosphere by cosmic rays and fall to the earth and speeds close to the speed of light. With their decay rate, we know how many should decay by the time they land, except that amount is much much less because, from our point of view, time has dilated and less have decayed, and from their point of view, the length they fall has contracted and less have decayed.
You don't even need anything as abstract as that. Ever used GPS? It's powered by satellites, as is implied by the name, which fly at rather fast speeds compared to us (Not to mention suffering less gravity, which also causes time to go screwy) and these cause relativity to mess up their clocks, which needs to be compensated for or else they'd lose accuracy at an alarming rate (As in, hundreds of metres of accuracy per day). In short, if relativity doesn't work how we think it does, GPS systems worldwide would simply not work.
You see my point? We've hit another contradiction here. These neutrinos seem to be travelling faster than the speed of light and we have no idea why. Either relativity actually is wrong, these neutrinos are doing something really funky or CERN's data is wrong somehow and, well, to be frank, CERN are basically the smartest people in the world.
I realise this is probably my longest blog post to date, and hopefully you're all a bit more enlightened about the nature of light and relativity, but really, this is potentially very exciting news for physics. Sure, the practical implications are pretty minimal, but hey, the potential for new knowledge, if it does turn out it's possible for neutrinos to screw with physics, are absolutely incredible.
My thoughts? Well, I dunno. I doubt it'll turn out to be a problem with relativity, and I'd be surprised if CERN has made that colossal a screw up, these neutrinos just seem to be doing something funky.
I can say one thing for certain though. The universe is, and will always be, an infinitely amazing, wonderful and surprising place and we will never know everything about it for certain. And that, quite simply, is fantastic.
I must admit this is very exciting. I sure as hell hope that neither Einstein nor CERN were wrong.
ReplyDelete