Why Does E=mc2_ - Brian Cox [21]
Before we discuss an experiment that settles the argument, let us pause to reflect upon the result that we just uncovered. Let’s look once again at the thought experiment from the point of view of a passenger on the train sitting beside the clock. For the passenger, the clock is not moving and the light simply bounces up and down, just as it would have for a person sitting with the same clock in a café in the station. The passenger must see the clock tick once every 6.67 nanoseconds and 150 million times for every heartbeat, because she is perfectly correct in deciding that the clock is not moving relative to herself, in the spirit of Galileo. Meanwhile, the person on the platform says that the clock on the train took a little longer than 6.67 nanoseconds to perform one tick. After 150 million ticks of the moving clock, his heart will therefore have made slightly more than one heartbeat. This is astonishing: According to the person on the platform, he is aging faster than the passenger sitting on the train.
As we have just seen, the effect is a tiny one for real trains, which don’t travel anywhere near as fast as the speed of light, but it is real nonetheless. In an imaginary world where the train whizzes along a very long track at close to the speed of light, the effect gets magnified and there would be no doubt about it: The person on the platform would age quicker from his perspective.
In real experiments, if we are to test this breakdown in absolute time, then we need to find a way to investigate objects that can move close to the speed of light, for only then will the time-stretching factor γ become measurably larger than 1. Ideally we’d also like to study an object that has a lifetime, that is to say that it dies. We could then look to see if we could prolong the lifetime of the object simply by making it move fast.
Fortunately for scientists, such objects do exist; in fact, scientists themselves are built out of them. Elementary particles are tiny subatomic objects that by virtue of their smallness are easy to accelerate to vast speeds. They are referred to as elementary because, as far as we can tell with our current technology, they are the smallest building blocks of everything in the universe. We will have much more to say about elementary particles later in the book. For now, we would like to describe just two: the electron and the muon.
The electron is a particle to which we are all indebted, because we are built out of them. It is also the particle that flows through electric wires to light our bulbs and heat our ovens; the electron is the particle of electricity. The muon is identical to the electron in every way, except it is heavier. Why nature should have chosen to give us a copy of the electron that appears to be redundant if all you want to do is to build planets and people, is not something physicists really understand. Whatever the reason for the existence of the muon, it is very useful indeed to scientists wishing to test Einstein’s theory of relativity because it has a short lifetime and it is very small and easy to accelerate to very high speeds. As far as we can tell, electrons live forever, whereas a muon placed at rest beside you would live for something like 2.2 microseconds (a microsecond is one-millionth of a second). When a muon dies, it nearly always turns into an electron and another pair of subatomic particles called neutrinos, but that is extra information that we don’t need. All we need here is that the muon does die. The Alternating Gradient Synchrotron (AGS) facility at Brookhaven National Laboratory on Long Island, New York, provides a very nice test of Einstein’s theory. In the late 1990s, the scientists at Brookhaven built a machine that produced beams of muons circulating around a 14-meter-diameter