PFE015: Mirage

This was inspired by driving down the highway (I've been driving around a lot lately).

If you have ever been on a long road trip on a sunny day and you've looked ahead on the highway, you may have noticed a shimmer or a reflection. It sort of looks like there is water on the road, but even as you're driving 70 miles per hour, you can't seem to reach it. Where does this phenomenon come from?

It turns out to be a trick of the eyes.

Before we can discuss this, let's talk about the way light and our minds behave. First, imagine you are seeing something both in a mirror and normally. We know that the light from the same object hits your eyes twice, after traveling along two different paths. Since I assume that light travels in a straight line it looks like there is a second object behind the mirror. Luckily, I (along with most people and some chimps) have experience with mirrors and know that the light is really bouncing off it.

There is another fascinating property of light, and that is that it bends as it passes through different substances. Consider a (straight) straw in a glass of water. The straw actually looks bent at the point where it enters the water even though we know it is actually straight. This is because the speed of light actually changes in the water creating a bending effect. "But, I thought the speed of light was always constant?" you protest. Once again, your teachers have lied misinformed you. The speed of light is constant in a vacuum, but is slower in other things like glass, water, even air a little bit. And as light changes speed, it bends.

Now we know all of the physics to understand the glimmer at the edge of vision. On a sunny day (it doesn't necessarily have to be warm) the sun will heat up the pavement which will in turn heat up the air. But this effect has a limit in that only the air up to about a foot or two will be significantly warmer than the rest of the air. This difference in temperature, you guessed it, causes the light to bend. But unlike with the water where there's a kink, the bend is smoother and curvier because the temperature of the air changes smoothly.

(I think I have one of those awards coming for my graphic artistry.)

So there appear to be two images of the car, the normal one, straight ahead, and another one from below. But since we naturally assume that light travels in a straight line, our eyes see the second image as a reflection.

Desert mirages are actually the same thing, but they should be differentiated from hallucinations. Mirages are actual images (they show up on a camera) that remind us of water. You don't have to be crazy to see them. The other kind, the hallucinations, does require some loss of sanity.

That's a mirage.

PFE014: LHC Part 2 - The Big Picture

Now that you know how the LHC works, I can talk a little bit about some of things happening there and explain some things you may have read in the media.

The LHC is at CERN. CERN is the European Organization for Nuclear Research and is in Geneva on the border between France and Switzerland (hence the misleading acronym). It has been a center for high energy physics research for some time. Recently the began work on the LHC, the large hadron collide. The fact that it's 17 miles should explain the large part. A hadron is a type of particle. There are so many different particles and so many classifications that it is often referred to as a particle zoo. Protons are hadrons (and the primary particle collided at the LHC). Collider should also be pretty clear, although it is interesting to note that there are 4 collision points in the LHC and that only a small fraction of the particles in the beams actually collide at these points.

On to the media. Google news gives 150 news stories for "god particle" in the last year alone including another one picked up by all the major news outlets just yesterday. This one irks me the most because it is entirely a media construction. The particle in question is the Higgs boson. The Higgs hasn't been seen even though it was first predicted some 45 years ago. The Higgs is supposed to be a way to describe how gravity works (yeah, we still don't really know how gravity works. I know, lame, right?) and since everything feels the gravity of everything else it is said, in some sense to be everywhere. So not only is the particle a sort of holy grail, a way to complete a nearly complete model that has been sitting for decades, but would also, in some sense, exists everywhere. Somewhere along the way a journalist misinterpreted a physicist comments and dubbed the particle the "god particle". Since the name is edgy in an article about science the media seems to love it, but it should be clear that the particle has nothing to do with any god of any sort. My main fear here is that if the LHC sees the Higgs, the papers are going to scream that physicists have proven god's existence with sections poorly explaining the actual physics.

The next media fiasco tied to the LHC is the fear that it will destroy the world (see here and here). There were several attempts to sue the United States government to shut down the LHC before it turned on (one such opinion can be found here (pdf)). Needless to say such claims are preposterous and baseless (you don't have to worry about the world ending from the LHC. 2012 is up to you though). Essentially the fears stem from a particularly bizarre theory taking off in a really unfortunate way (things like microscopic black holes or strange matter). On the one hand, there's no a priori reason to believe that these things can't happen. The Tevatron has been running for decades and nothing has happened. Not only has nothing happened, they haven't even glimpsed anything to suggest that something unheard of might occur. Maybe because the LHC will collide particles with 7 times as much energy these new phenomena will show up? Again, maybe. But particles with these energies (and higher) have been striking the earth's atmosphere forever and the earth is still here. While the frequencies are significantly lower than in a particle accelerator, these collisions do happen very regularly all the time and all around us.

Is this proof that the LHC won't destroy the world? No. It is very hard to prove that something won't happen. We can show that something has happened, or that something won't happen up to a certain probability. This is incredibly unsettling to some people. But our lives are ruled by random events. A random solar flare in just the right place can knock out half our satellites. No GPS, no satellite communications, in an instant. Or on a highway. The driver next to you can lose concentration and swerve into your car. These events, and their effects on us are probabilistic. We can plan for some eventualities, and put in place measures to limit these probabilities, but this doesn't mean that we shouldn't use cars or take advantage of satellites.

To be more precise on topics like these is impossible simply because no one understands them. If we did, we wouldn't need huge machines like the LHC to sort them all out.

That's the LHC.

PFE013: LHC Part 1 - The Basics

Over the last several years, the LHC has been in the news a lot. Enough to hit critical mass in the media. Apparently, when it comes to science that no one understands, this means that it's okay to write stories based on a bizarre theory someone came up with, write about it as though it's widely accepted, and then include a sentence at the end explaining that it hasn't been proven yet.

Before I talk about these things, I think an understanding of how such a monstrous machine works is helpful to keeping up with a large portion of physics in the news.

A particle accelerator may be used for a variety of different things. Accelerators like the LHC, the Tevatron, or SLAC are used to study basic physics. But accelerators like this account for a very small percentage of all accelerators. There are accelerators for manufacturing electronics, medical research, and medical treatment. Most of this post will focus on the higher energy physics based accelerators, but it all applies to medical, manufacturing accelerators too.

But we have all seen particle accelerators in our everyday lives. A battery is a device that accelerates electrons. It is doing essentially the same thing as the LHC! Just on a scale about nine trillion times smaller. So an accelerator is any mechanism that creates a stream of particles going very quickly (or, more usefully, with more energy).

Particle accelerators can be classified into two main types: circular accelerators, and linear accelerators. Each with its own advantages and disadvantages.

Circular accelerators have three main parts: magnets, rf-cavities, and detectors. Since the particles that are accelerated are charged, magnets are used to bend them in a circle. In fact, there are typically two beams of particles moving in opposite directions. A simple relation can be used to show that how strong the magnets need to be increases as the speed and energy of the particles increases and decreases as the size of the circle increases. Since more new physics can be seen at higher energies, and the limiting factor is often the size of the magnets, these machines can end up being as large as 17 miles around.

The next important part is the rf-cavities. The first thing to know is that magnets can't be used to make particles go faster, they can only change their direction. To get the particles going this fast, you need something else to accelerate them. And the methods used are similar to how microwaves work. The best way to imagine how an rf-cavity works is to think of surfing. The cavity creates waves of energy moving through a chamber, and, if the particles enter the cavity at just the right point on the wave, it will be pushed through the cavity and will get a touch more energy. The major advantage of circular accelerators is that  one rf-cavity can be used many times to accelerate a particle. So particles can gain as much energy as we want, up to infinity, right? Sadly, no. As the particles are bent around the circle, energy is lost. The more energy the particles have and the sharper the curve, the more energy is lost. So eventually the amount of energy lost will equal the amount of energy the cavity can add and the particle has reached its maximum energy.

The final part is the detector. There are a number of monitoring devices to keep track of where everything is. Now they use all kinds of fancy equipment, but a story passed down to me from the early days of accelerators was that to check if the particles were in the pipe, they would stick their head in and actually look. The particles would create a blue light inside their eyeball and they would know that the machine was working properly. The main detectors are where the particles collide. At these points on the ring, the magnets bend the two beams into each other and a bunch of massive collisions (hopefully) happen. Particles are sprayed out in all directions and huge detector measures what happens to all of them, before the next particles collide, an instant later. Then, computer software figures out what happened at the collision point.

A linear accelerator operates in largely the same fashion as a circular accelerator. As it turns out, the energy lost as particles are bent around in a circle is much more for some particles than others (it goes by m^-4 for those interested). So for these sorts of particles (typically electrons) it is more efficient to line a bunch of rf-cavities and either smash two such beams or hit a stationary target. This takes more rf-cavities, but you don't need huge magnets to bend it in a circle and energy isn't lost from doing so.

I should emphasize that as much as I have covered here is only a small portion of the actual mechanics of particle accelerators. There are a number of topics that I glossed over (or simply ignored), so please ask to expand on anything that's confusing or unclear.

That's accelerators.