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1 {photo of aftermath of a car collision}
1 Caption: Ka-BLAMM!
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Not actually the remains of my car, thankfully. |
Cars are designed these days to crumple on impact. It wasn't always like that. Cars used to be designed to be as solid and strong as possible. At first thought, that makes sense. You don't want you car to be crushed like a paper bag when it hits something, as that obviously isn't safe, so going to the opposite extreme and making it as hard and rigid as possible seems appealing. Until you look at the physics of what happens in a collision.
There are two physical quantities that are conserved in any interaction of matter. The first is energy, which we've talked about a fair bit already. In a collision between, say, two cars, the kinetic energy of the cars moving at their pre-collision speeds is converted into other forms of energy. The cars may still be moving immediately after the collision, so some of the kinetic energy remains as kinetic energy, but much is lost as heat and, most noticeably, sound. Some of the energy is "used up" in doing the work necessary to deform the body panels of the cars. This energy goes into breaking some of the chemical bonds that give structural integrity to the panels. Or pieces of plastic and glass as well if you're particularly unlucky.
The second quantity that is preserved is momentum, which I've talked about before too. Momentum is the product of mass and velocity, and has a direction. So if two cars of equal mass and speed collide head-on, the total momentum before is actually zero, since the directions cancel. After the collision, the momentum of the stationary pile of metal is also zero. The energy, however, has gone from huge to zero, and the release of all that energy is what causes the damage.
Accelerating a car down a road. Creative Commons Attribution image by Nikos Koutoulas. |
Crashing cars tend to go from a high speed to a low speed within a very short space of time. This means a very high acceleration, because acceleration is equal to the change in velocity divided by the time it takes the change to occur.[2] If a car is travelling at 60 km/h and brakes normally, it can come to a stop in about 3 seconds. The acceleration is 60/3 = 20 km/h per second. If the same car hits an oncoming car and stops in half a second, the acceleration is 60/0.5 = 120 km/h/s.
And here's where things get dangerous. The parts inside a car and the human body can only withstand so much acceleration. If you slow down at 20 km/h/s, that's normal; you do that in traffic every day. If you slow down at 40 km/h/s, that's like slamming the brakes on hard to avoid a collision. You get jerked against your seatbelt. If you slow down at 120 km/h/s, you get slammed violently against the seatbelt, the steering wheel, the dashboard, the windscreen, and possibly the road if you weren't wearing a seatbelt. Modern cars have inflatable airbags to absorb some of this impact between your body and bits of the car, but it's still not exactly what you'd call safe.
The easy way to generate flint sparks is with a lighter. Creative Commons Attribution-Share Alike image by Chris Isherwood. |
Collisions of course don't just occur between cars. They happen when things fall and hit the ground, when animals fight or attack one another, when waves smash against rocks, and when objects in space crash into one another. One type of collision led to a major technological breakthrough in human history: the collision of flint with steel.
Flint is a very hard stone composed of very small crystals of quartz. Steel is a refined form of iron, with a controlled amount of carbon impurity to make it harder than pure iron. Despite this, flint is harder still, and when a piece of flint strikes a piece of iron, the collision scratches off small particle of iron. If done forcefully enough, the energy of the collision goes into heating up the flakes of iron to a point where the burn, combining rapidly with oxygen from the air to form iron oxide (commonly known as rust). These burning flakes of iron form sparks, and if one strikes a suitable inflammable substance such as dry tinder, it can set it alight, and so begin a fire. If the sparks strike somethign more volatile, like black powder, they can set off an explosive reaction that can fire a bullet - this is how flintlock guns work.
The Crab Nebula pulsar imaged by NASA's Chandra satellite. A significant source of cosmic rays impacting Earth. Public domain image from Wikimedia Commons. |
Now if these particles and gamma rays collided with your body, you'd be in a similar amount of trouble as if you were in a car crash, though the problem would be very different in nature. Because when a high-energy particle or gamma ray hits animal tissue, it dumps all of its energy into the tissue, which rips the electrons off many of the atoms there by overcoming the electron binding energy. This disrupts chemical bonds, breaking down important substances like the proteins and fats that make up your body. If you get exposed to enough radiation, the raw damage to your tissue can be enough to make you sick or even kill you. But even worse, a much smaller dose can damage the DNA and other structures inside your cells (which I discussed here). If this happens, your cells can "lose their programming". The instructions encoded in the DNA for making proteins may be corrupted, or the ribosomes and enzymes may be corrupted, resulting in the cell being unable to make the chemicals necessary to grow and function properly. Worse, the cell may begin making things your body doesn't need. If such a damaged cell can still manage to divide and multiply, this leads to the disease we know as cancer.
Fortunately for us, we live not on the very outside of Earth, but buried roughly 100 km inside its atmosphere. The atmosphere protects us by intercepting almost all of the cosmic rays. What's actually occurring are innumerable collisions. An incoming cosmic ray will usually hit a molecule of the Earth's atmosphere somewhere between 50 and 100 kilometres above sea level. Some make it lower, but the atmosphere rapidly gets thicker, and by the time sea level is reached there are very few incoming cosmic rays left.
Cosmic ray air shower simulation. Creative Commons Attribution image by Dinoj Surendran and COSMUS Group, University of Chicago. |
Some of the particles produced in these air showers can reach the ground. The most common are neutrinos, which are not a problem because they barely interact with matter at all. Cosmic shower neutrinos generally pass right through people, the ground, and in fact right through the Earth and out the other side, without causing any further damage whatsoever. There are millions passing through your body right now as you read this.
The next most common are muons, which are from the same particle family as the electron and so have similar properties, but are approximately 200 times heavier. These are actually unstable too, and decay with an average lifetime of about 2 microseconds - but this is long enough for some of them to reach the ground at the very high speeds they are travelling when generated by the air shower.[3] These muons can be detected using sensitive instruments, which we use to measure cosmic ray showers. By measuring multiple muons with different detectors at the same time, we can triangulate and determine that they came from the same position high in the atmosphere, thus giving us a way to measure the altitude of the initial cosmic ray collision with the air.
ATLAS detector at the Large Hadron Collider. Creative Commons Attribution-NonCommercial-No Derivatives image by Simon Bisson. |
The reason we have needed to build successively larger (and more expensive) machines to do this is because we want to extend our knowledge of subatomic particle physics to particles of heavier and heavier masses. The only way we know to generate such particles is to make use of Einstein's relation, and provide enough energy in a small enough space that some is converted to mass in the form of the particles we wish to study. And the only way to do that is by more and more energetic subatomic collisions.
Before 2012, one of the greatest successes of this branch of experimental physics was the discovery and confirmation of the W and Z bosons. These were particles predicted in 1968 during the formulation of the theory which provided a combined explanation for electromagnetism and the weak nuclear force, proposed by Sheldon Glashow, Steven Weinberg, and Abdus Salam.
Their theory explained these forces very elegantly, but only if they included a set of new fundamental particles, then unknown. They called them the W and Z bosons. (There are two W bosons, one each with positive and negative electric charge, and one Z boson, with no electric charge.) The problem was that the boson—or particle which "carries" the force—related to electromagnetism was already well known: it is the photon. The W and Z bosons should also have been evident in experiments performed up to that time, if they had no mass like the photon. The problem could only be solved if they posited that the W and Z bosons had mass. This led Peter Higgs to propose a mechanism for why these bosons had mass. Besides explaining why the W and Z bosons were not massless, the so-called Higgs mechanism also predicted the existence of a further particle, called the Higgs boson.
Thankfully these collisions don't produce particle showers. Notice the crumpling of both vehicles to absorb energy. Creative Commons Attribution-NonCommercial-Share Alike image by Eva Ekeblad. |
Fast forward to 2012, and the Large Hadron Collider. The major missing piece of our model of particle physics was the Higgs boson. The problem was its predicted mass: at least 125 GeV - substantially bigger than the W and Z bosons. The LHC was built specifically to try to confirm the existence of the Higgs boson. The collisions it produces are the most concentrated bursts of energy we are capable of producing. If we could find the Higgs boson, it would be the final validation of our understanding of the physics of fundamental particles. If it could not be found, then it would be an indication that there is a problem with the theory. As it turned out, two teams working independently at the LHC found strong evidence of a new particle with a mass of 125 GeV, which displays all the expected characteristics of the Higgs boson in terms of electric charge, spin, and how it decays into smaller particles.
As with all science, this is not enough to prove the newly detected particle is the Higgs boson, and there are still some tests that are planned to rule out other possibilities. But for now the CERN scientists are confident there is indeed a 125 GeV particle, and that there is no evidence that it is not the Higgs boson. As they carry out further experiments over the next few years, our confidence will grow (or be shattered as something contradictory comes to light!). Either way, we will learn something wonderful and new about how our universe works.
And all because of collisions.
[1] Or even sideways or at some other angle. A car moving at a constant speed but turning is accelerating sideways.
[2] Strictly speaking, this is the average acceleration over that period of time. The instantaneous acceleration at any point of time within that period may be different, and is defined in terms of the "rate of change" of speed, which generally requires a bit of calculus to figure out. It's exactly analogous to the concept of average speed versus instantaneous speed: If you travel 10 km across the city in your car and it takes you 20 minutes, that's an average speed of 30 km/h, but at any given time your instantaneous speed (or your rate of change of distance travelled) could be 60 km/h as you move freely along a road, or zero, when stopped at a traffic light, or any value in between or slightly higher.
[3] Observant readers will notice that even moving at the speed of light (c), 2 microseconds is only enough time to travel about 600 metres. However, remember that objects travelling at speeds close to light experience time dilation. The measured speed of cosmic-ray-generated muons at the Earth's surface is from 0.994c to 0.998c. At 0.998c, a muon experiences time flowing approximately 16 times more slowly relative to an observer on Earth, so the muon can travel an average of almost 10 kilometres before decaying. And that's an average - some travel further than this.
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