Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [47]
The space-time of the Universe can be thought of as a vast map. All events—from the creation of the Universe in the Big Bang to your birth at a particular time and place on Earth—are laid out on it, each with its unique space-time location. The map picture is appropriate because time, as the flip side of space, can be thought of as an additional spatial dimension. But the map picture poses a problem. If everything is laid out—preordained almost—there is no room for the concepts of past, present, and future. As Einstein remarked: “For us physicists, the distinction between past, present, and future is only an illusion.”
It is a pretty compelling illusion, though. Nevertheless, the fact remains that the concepts of past, present, and future do not figure at all in special relativity, one of our most fundamental descriptions of reality. Nature appears not to need them. Why we do is one of the great unsolved mysteries.
E = mc2 AND ALL THAT
The special theory of relativity does more than profoundly change our ideas of space and time. It changes our ideas about a host of other things too. The reason is that all the basic quantities of physics are founded on space and time. If, as relativity tells us, space and time are malleable, blurring one into the other as the speed of light is approached, then so too are the other entities—momentum and energy, electric fields and magnetic fields. Like space and time, which merge into the seamless medium of space-time, they too are inextricably tied together in the interests of keeping the speed of light constant.
Take electricity and magnetism. It turns out that, just as one person’s space is another person’s time, one person’s magnetic field is another person’s electric field. Electric and magnetic fields are crucial to both generators that make electrical currents and motors that turn electric currents into motion. “The rotating armatures of every generator and every motor in this age of electricity are steadily proclaiming the truth of the relativity theory to all who have ears to hear,” wrote the physicist Leigh Page in the 1940s. Because we live in a slow-motion world, we are hoodwinked into believing that electric and magnetic fields have separate existences. But just like space and time, they are merely different faces of the same coin. In reality there is only a seamless entity: the electromagnetic field.
Two other quantities that turn out to be different faces of the same coin are energy and momentum.
5
And in this unlikely connection is hidden perhaps the greatest surprise of relativity—that mass is a form of energy. The discovery is encapsulated in the most famous, and least understood, formula in all of science: E = mc2.
1 Strictly speaking, each runner will also appear to rotate, so the spectators will see some of the far side of each of them—the side facing away from the grandstand, which would normally be hidden. This peculiar effect is known as relativistic aberration, or relativistic beaming. However, it is beyond the scope of this book.
2 To be precise, a stationary observer sees time slow down for a moving observer by a factor γ, where γ = 1/√(1 – (v2/c2)) and v and c are the speed of the moving observer and the speed of light, respectively. At speeds close to c, γ becomes enormous and time for a moving observer slows almost to a standstill!
3 To be precise, a stationary observer sees the length of a moving body shrink by a factor γ, where γ = 1/√(1 – (v2/c2)) and v and c are the speed of the moving observer and the speed of light, respectively. At speeds close to c, γ becomes enormous and a body becomes as flat as a pancake in the direction of its motion!
4 Actually, there is a subtle flaw in this argument. Since