6 Crazy Things You (Probably?) Didn't Know About Light
If we here at [Havoc Mantis' one-man blog] know one thing, it's that the universe is weird. Really weird. Every day, scientists are discovering new things about seemingly unexplainable mysteries like gravity, magnetism, and people who pay for porn. But of all the crazy things in this crazy universe that we call home, light just might be the craziest. Here's why:
6. Moving at the speed of light requires infinite energy
It's pretty common knowledge that nothing can move faster than light. But if you think about it from the perspective of classical physics, it doesn't make any sense for there to be a universal speed limit. In classical mechanics, pushing on an object causes it to gain kinetic energy, which is related to its velocity by the equation KE = (1/2)mv^2. If you push something hard enough, and for long enough, you can give it as much kinetic energy as you want. And since velocity squared is directly proportional to kinetic energy, this should mean that you can make the object go as fast as you want, with no limits.
But the equation KE = (1/2)mv^2 is kind of like the physics equivalent of the Christopher Columbus story that they told you in elementary school: a convenient lie told to hide the horrific truth. What in physics could possibly be as horrific as genocide and enslavement? Well, say hello to the relativistic equation for kinetic energy.
|This, uh, actually still doesn't really quite measure up to genocide, if I'm being honest.|
To see why this prevents objects from going at the speed of light, try plugging in c, the speed of light, for v, the velocity. The part on the bottom of the fraction becomes 0, which means that you end up dividing by zero. And one of the most fundamental rules of physics is that you never divide by zero. The technical physics term for dividing by zero is a "catastrophe", and I'm not even sure I'm joking about that. If you start plugging in values for the kinetic energy, and solving for velocity, you'll find that no matter how high the kinetic energy is, as long as it is finite, the corresponding velocity is always less than c. So, to reach c, you need an amount of energy that is not finite. Infinite, you could even say.
5. It always moves impossibly fast
Nothing can go faster than light, so it's a good thing that it's pretty damn fast, with a blistering top speed of 299,792,458 meters per second. (about 670 million miles per hour). But that's not just light's top speed. It's also its only speed. To really understand what this means, and why it's so crazy, you have to know a little bit about relativity. But don't worry; you won't need any math beyond addition and subtraction to understand this.
Suppose you're outside in the park on a lovely summer day. I know that this might be a bit of a stretch for some of my readers, but please bear with me. Amidst of the peaceful chirping of birds and the wonderful aroma of budding flowers, you are suddenly and viciously kicked in the 'nads (lady-nads?). The perpetrator then sprints away from you at 10 miles per hour. You are still recovering from the surprise groin kick you just received, so you are unable to give chase for a few seconds (or maybe a few minutes, no one's judging.) During this time, the assailant is moving at 10 miles per hour, relative to you. After you recover for a bit, you run after him at 3 miles per hour. Now, he is moving at 7 miles per hour relative to you, because velocities add and subtract exactly how you expect them to. This is Galilean relativity, a fundamental rule of the universe.
But light doesn't play by the universe's rules. The universe plays by light's rules, and light is like that annoying neighbor who plays Monopoly with weird house rules that screw everyone else over. But instead of only insisting that there's a cash prize for landing on Free Parking when he lands on it, light makes us play by the rules of Special Relativity. In special relativity, adding velocities is much more difficult, and the rules are carefully arranged so that nothing can go faster than light, and light always goes the same speed from every reference frame. This means that if a man made of light ever kicks you in the nuts, then you're shit out of luck. Even if you can instantaneously accelerate to 0.8c (80% of the speed of light), and then fire a bullet at a 0.8c relative to you. According to Galilean Relativity, he should move away from you at 0.2c, and the bullet should move towards him at 0.6c, because 0.8c + 0.8c – c = 0.6c. But according to Special Relativity, the light man moves away from both you and the bullet at exactly c, utterly disregarding the relative motion between the two. Even if you run away from him, while Galilean relativity says that he should be moving at 1.8c away from you, special relativity ensures that he always moves at exactly c. I guess what I'm trying to say is that you shouldn't make an enemy of a man made of light.
|"I shall become more powerful than you can possibly imagine", indeed|
4. It doesn't experience Time or Space
Galilean Relativity says that all velocities are relative, with no one frame of reference that is absolutely correct. Special Relativity goes a step further, saying that time and space themselves are relative, with no absolutely correct frame of reference. So, while a clock may take 1 second to tick in one frame of reference, it could take 2 seconds to tick from a different frame of reference, and they're both equally correct. Similarly, a meterstick can appear to have a length of half a meter if viewed while going fast enough. Surprisingly, scientists say that this has nothing to do with why a single Monday seems to last longer than an entire weekend.
|This phenomenon is still being investigated by top minds in the field.|
These phenomena are called time dilation and length contraction, and they're entirely dependent on how fast you're going. When you go faster, external events seem to happen more quickly, and distances seem shorter. As you approach the speed of light, the time for external events to happen approaches 0, and distances also approach 0. So, the next time someone tells you that the light from distant stars is millions of years old, tell them that that's only true from your perspective. From the perspective of the light, it took less time than it took for you to stop reading this for being too boring, and the distance was less than the distance you had to move your mouse over to click over to a porn tab.
3. It's just a wiggle of electricity
When I say “Electromagnetic Radiation”, what's the first thing you think of? Concrete pillars that belch billowing clouds of steam? Devastating weapons with the power to level entire cities? Spider bites with improbably beneficial side effects? Well, in reality, Electromagnetic radiation is just the scientific name for light, including not just the visible spectrum, but infrared, ultraviolet, X-rays, and so on. Electromagnetic radiation is made of waves, and to understand these waves you must understand electric fields.
|Piece of cake, right?|
Every charged particle exerts an attractive or repulsive electrical force on every other charged particle, much like how all massive particles exert a gravitational force on one another. This force is described by an electric field that is emitted by the particle. When the particle moves, the electric field also moves. BUT, no information can travel faster than light, so the change in the electric field propagates at the speed of light, causing it to bend. If a charged particle were to oscillate, or wiggle back and forth, it would cause the electric field to similarly wiggle up and down, like wave. When the electric field changes, it induces an accompanying magnetic field that is perpendicular to the electric field. This wiggle of electric and magnetic fields is what makes up electromagnetic radiation, all of the light that we see and don't see. And now is the time when you can feel free to admit that you have no idea what I'm saying, because I'm honestly only about 60% sure that what I'm saying makes any sense. But I'm about 90% sure that it's really cool, and I like those odds.
2. Except it's actually a particle
The only problem with the above description of light is that light is a particle, called a photon. And if it's a particle, then it clearly doesn't make any sense that it could also be a wave, right? But this is where we get into the dangerous realm of quantum mechanics, and the first rule of quantum mechanics is that if something makes sense, it isn't true. So light can be a particle and a wave at the same time, and this property is called particle-wave duality. And since I said all that stuff earlier about it being impossible for a massive particle to move at the speed of light, it must follow that photons have no mass. So if you try to use the earlier formula to calculate the energy of a photon, something weird happens. The mass is 0, so you have a 0 on the top, and it's going at light speed, so you also have a 0 on the bottom, giving you 0/0. 0/0 is one of the strangest expressions in all of math, because it can be any number. To see why this is the case, consider the definition of division.
If you have some number c = a/b, then c is the number such that b*c=a. The quotient is something that, when multiplied by the bottom number, gets the top number. So 0/0 is a number that, when multiplied by 0, gets 0. You may recognize this as every number ever. To find the actual value, you have to do some neat tricks with limits, but this isn't the place to talk about that. What's important now is that this only happens if the photon is traveling at the speed of light. If you were to somehow slow down a photon to below the speed of light, (like if it were to, I don't know, pause to kick someone in the balls?) then the energy just becomes 0, and the photon ceases to exist. This means that light will die if it ever slows down; for this reason, scientists call light “The sharks of the air”.
|As illustrated by XKCD|
1. The wave and particle descriptions seem to disagree
Suppose you have a photon gun; not a very useful weapon, but great for demonstrating how weird light is. Let's say you set it to fire at a frequency of 1 photon per second. Thanks to wave-particle duality, this means that it is also firing a wave of light. It seems reasonable to think that the frequency of the wave, or how many times it wiggles up and down in one second, would be the same: 1 wiggle per second. Even if this is not the case, it should be obvious that they are dependent on one another; increasing the rate at which the photon gun fires should make the wave wiggle up and down faster. Similarly, increasing the amount of energy per photon should increase the amplitude of the wave, or how big it is. After all, in classical mechanics, the energy of a wave is determined by its amplitude. It's just common sense.
|Thomas Paine: Brilliant political theorist, terrible quantum physicist|
And that's quantum mechanics' cue to come crashing in like a particularly unKool-Aid man and explain why everything that makes sense is wrong. It turns out that those correlations are actually reversed. Increasing the energy per photon will increase the frequency of the wave, and increasing the number of photons per second will increase the amplitude of the wave. So the frequency of the photons is the amplitude of the wave, and the "amplitude" of the photons is the frequency of the wave. Now I need you to act really surprised and amazed by this, because when I told all my physics friends, the general response was a resounding "Yeah? So what?"
I can only hope that you learned as much from this experience as I did. While you may have learned a thing or two about light, the universe, and how bad I am at explaining these things, I definitely learned that it was a good idea for me to not write about science on my blog for a year and a half. I think I'll probably just go back to doing that.