In Pursuit of the Unknown (42 page)

In fact, there is a reason why we wouldn't see an egg unscrambling, even if the world did run backwards. Because we and our memories are part of the system that is being reversed, we wouldn't be sure which way time was ‘really' running. Our sense of the flow of time is produced by memories, physico-chemical patterns in the brain. In conventional language, the brain stores records of the past but not of the future.
Imagine making a series of snapshots of the brain watching an egg being scrambled, along with its memories of the process. At one stage the brain remembers a cold, unscrambled egg, and some of its history when taken from the fridge and put into the saucepan. At another stage it remembers having whisked the egg with a fork, and having moved it from the fridge to the saucepan.

If we now run the entire universe in reverse, we reverse the order in which those memories occur, in ‘real' time. But we don't reverse the ordering of a given memory in the brain. At the start (in reversed time) of the process that unscrambles the egg, the brain does not remember the ‘past' of that egg – how it emerged from a mouth on to a spoon, was unwhisked, gradually building up a complete egg . . . Instead, the record in the brain at that moment is one in which it remembers having cracked open an egg, along with the process of moving it from the fridge to the saucepan and scrambling it. But this memory is exactly the same as one of the records in the forward-time scenario. The same goes for all the other memory snapshots. Our perception of the world depends on what we observe
now
, and what memories our brain holds,
now
. In a time-reversed universe, we would in effect remember the future, not the past.

The paradoxes of time-reversibility and entropy are not problems about the real world. They are problems about the assumptions we make when we try to model it.

 

 

 

 

Matter contains energy equal to its mass multiplied by the square of the speed of light.

The speed of light is huge and its square is absolutely humongous. One kilogram of matter would release about 40% of the energy in the largest nuclear weapon ever exploded. It's part of a package of equations that changed our view of space, time, matter, and gravity.

Radical new physics, definitely. Nuclear weapons. . . well, just maybe – though not as directly or conclusively as the urban myths claim. Black holes, the Big Bang, GPS and satnav.

J
ust as Albert Einstein, with his startled mop hairdo, is the archetypal scientist in popular culture, so his equation
E
=
mc
²
is the archetypal equation. It is widely believed that the equation led to the invention of nuclear weapons, that it comes from Einstein's theory of relativity, and that this theory (obviously) has something to do with various things being relative. In fact, many social relativists happily chant ‘everything is relative', and think it has something to do with Einstein.

It doesn't. Einstein named his theory ‘relativity' because it was a modification of the rules for relative motion that had traditionally been used in Newtonian mechanics, where motion
is
relative, depending in a very simple and intuitive way on the frame of reference in which it is observed. Einstein had to tweak Newtonian relativity to make sense of a baffling experimental discovery: that one particular physical phenomenon is not relative at all, but absolute. From this he derived a new kind of physics in which objects shrink when they move very fast, time slows to a crawl, and mass increases without limit. An extension incorporating gravity has given us the best understanding we yet have of the origins of the universe and the structure of the cosmos. It is based on the idea that space and time can be curved.

Relativity is real. The Global Positioning System (GPS, used among other things for car satnav) works only when corrections are made for relativistic effects. The same goes for particle accelerators such as the Large Hadron Collider, currently searching for the Higgs boson, thought to be the origin of mass. Modern communications have become so fast that market traders are beginning to run up against a relativistic limitation: the speed of light. This is the fastest that any message, such as an Internet instruction to buy or sell stock, can travel. Some see this as an opportunity to cut a deal nanoseconds earlier than the competition, but so far, relativistic effects haven't had a serious effect on international finance. However, people have already worked out the best locations for new stock markets or dealerships. It's only a matter of time.

At any rate, not only is relativity not relative: even the iconic equation is not what it seems. When Einstein first derived the physical idea that it
represents, he didn't write it in the familiar way. It is not a mathematical consequence of relativity, though it becomes one if various physical assumptions and definitions are accepted. It is perhaps typical of human culture that our most iconic equation is not, and was not, what it seems to be, and neither is the theory that gave birth to it. Even the connection with nuclear weapons is not clear-cut, and its historical influence on the first atomic bomb was small compared with Einstein's political clout as
the
iconic scientist.

‘Relativity' covers two distinct but related theories: special relativity and general relativity. I'll use Einstein's celebrated equation as an excuse to talk about both. Special relativity is about space, time, and matter in the absence of gravity; general relativity takes gravity into account as well. The two theories are part of one big picture, but it took Einstein ten years of intensive effort to discover how to modify special relativity to incorporate gravity. Both theories were inspired by difficulties in reconciling Newtonian physics with observations, but the iconic formula arose in special relativity.

Physics seemed fairly straightforward and intuitive in Newton's day. Space was space, time was time, and never the twain should meet. The geometry of space was that of Euclid. Time was independent of space, the same for all observers – provided they had synchronised their clocks. The mass and size of a body did not change when it moved, and time always passed at the same rate everywhere. But when Einstein had finished reformulating physics, all of these statements – so intuitive that it is very difficult to imagine how any of them could fail to represent reality – turned out to be wrong.

They were not totally wrong, of course. If they had been nonsense, then Newton's work would never have got off the ground. The Newtonian picture of the physical universe is an approximation, not an exact description. The approximation is extremely accurate provided everything involved is moving slowly enough, and in most everyday circumstances that is the case. Even a jet fighter, travelling at twice the speed of sound, is moving slowly for this purpose. But one thing that does play a role in everyday life moves very fast indeed, and sets the yardstick for all other speeds: light. Newton and his successors had demonstrated that light was a wave, and Maxwell's equations confirmed this. But the wave nature of light raised a new issue. Ocean waves are waves in water, sound
waves are waves in air, earthquakes are waves in the Earth. So light waves are waves in. . . what?

Mathematically they are waves in the electromagnetic field, which is assumed to pervade the whole of space. When the electromagnetic field is excited – persuaded to support electricity and magnetism – we observe a wave. But what happens when it's
not
excited? Without waves, an ocean would still be an ocean, air would still be air, and the Earth would still be the Earth. Analogously, the electromagnetic field would still be. . . the electromagnetic field. But you can't observe the electromagnetic field if there's no electricity or magnetism going on. If you can't observe it, what is it? Does it exist at all?

All waves known to physics, except the electromagnetic field, are waves in something tangible. All three types of wave – water, air, earthquake – are waves of movement. The medium moves up and down or from side to side, but usually it doesn't travel with the wave. (Tie a long rope to a wall and shake one end: a wave travels along the rope. But the
rope
doesn't travel along the rope.) There are exceptions: when air travels along with the wave we call it ‘wind', and ocean waves move water up a beach when they hit one. But even though we describe a tsunami as a moving wall of water, it doesn't roll across the top of the ocean like a football rolling along the pitch. Mostly, the water in any given location goes up and down. It is the location of the ‘up' that moves. Until the water gets close to shore; then you get something much more like a moving wall.

Light, and electromagnetic waves in general, didn't seem to be waves in anything tangible. In Maxwell's day, and for fifty years or more afterwards, that was disturbing. Newton's law of gravity had long been criticised because it implies that gravity somehow ‘acts at a distance', as miraculous in philosophical principle as kicking a ball into the goal when you're sitting in the stands. Saying that it is transmitted by ‘the gravitational field' doesn't really explain what's happening. The same goes for electromagnetism. So physicists came round to the idea that there was some medium – no one knew what, they called it the ‘luminiferous aether' or just plain ‘ether' – that supported electromagnetic waves. Vibrations travel faster the more rigid the medium, and light was very fast indeed, so the ether had to be extremely rigid. Yet planets could move through it without resistance. To have avoided easy detection, the ether must have no mass, no viscosity, be incompressible, and be totally transparent to all forms of radiation.

It was a daunting combination of attributes, but almost all physicists assumed the ether existed, because light clearly did what light did.
Something
had to carry the wave. Moreover, the ether's existence could in principle be detected, because another feature of light suggested a way to observe it. In a vacuum, light moves with a fixed speed
c
. Newtonian mechanics had taught every physicist to ask: speed relative to what? If you measure a velocity in two different frames of reference, one moving with respect to the other, you get different answers. The constancy of the speed of light suggested an obvious reply:
relative to the ether
. But this was a little facile, because two frames of reference that are moving with respect to each other can't both be at rest relative to the ether.

As the Earth ploughs its way through the ether, miraculously unresisted, it goes round and round the Sun. At opposite points on its orbit it is moving in opposite directions. So by Newtonian mechanics, the speed of light should vary between two extremes:
c
plus a contribution from the Earth's motion relative to the ether, and
c
minus the same contribution. Measure the speed, measure it six months later, find the difference: if there is one, you have proof that the ether exists. In the late 1800s many experiments along these lines were carried out, but the results were inconclusive. Either there was no difference, or there was one but the experimental method wasn't accurate enough. Worse, the Earth might be dragging the ether along with it. This would simultaneously explain why the Earth can move through such a rigid medium without resistance, and imply that you ought not to see any difference in the speed of light anyway. The Earth's motion relative to the ether would always be zero.

In 1887 Albert Michelson and Edward Morley carried out one of the most famous physics experiments of all time. Their apparatus was designed to detect extremely small variations in the speed of light in two directions, at right angles to each other. However the Earth was moving relative to the ether, it couldn't be moving with the same relative speed in two different directions. . . unless it happened by coincidence to be moving along the line bisecting those directions, in which case you just rotated the apparatus a little and tried again.

The apparatus,
Figure 48
, was small enough to fit on a laboratory desk. It used a half-silvered mirror to split a beam of light into two parts, one passing through the mirror and the other being reflected at right angles. Each separate beam was reflected back along its path, and the two beams combined again, to hit a detector. The apparatus was adjusted to make the paths the same length. The original beam was set up to be coherent, meaning that its waves were in synchrony with each other – all having the same phase, peaks coinciding with peaks. Any difference between the speed of light in the directions followed by the two beams would cause their phases to shift relative to each other, so their peaks would be in different places. This would cause interference between the two waves, resulting in a striped pattern of ‘diffraction fringes'. Motion of the Earth relative to the ether would cause the fringes to move. The effect would be tiny: given what was known about the Earth's motion relative to the Sun, the diffraction fringes would shift by about 4% of the width of one fringe. By using multiple reflections, this could be increased to 40%, big enough to be detected. To avoid the possible coincidence of the Earth moving exactly along the bisector of the two beams, Michelson and Morley floated the apparatus on a bath of mercury, so that it could be spun round easily and rapidly. It should then be possible to watch the fringes shifting with equal rapidity.