Modern Mind: An Intellectual History of the 20th Century (147 page)

In the 1970s Hawking was invited to Caltech, where he met and conferred with the charismatic Richard Feynman.
²¹
Feynman was an authority on quantum theory, and Hawking used this encounter to develop an explanation of how the universe began.
²²
It was a theory he unveiled in 1981 in, of all places, the Vatican. The source of Hawking’s theory was an attempt to conceive what would happen when a black hole shrank to the point where it disappeared, the troublesome fact being that, according to quantum theory, the smallest theoretical length is the Planck length, derived from the Planck constant, and equal to io
^–35
metres. Once something reaches this size (and though it is very small, it is not zero), it cannot shrink further but can only disappear entirely. Similarly, the Planck time is, on the same basis, 10
^–43
of a second, so that when the universe came into existence, it could not do so in less time than this.
²³
Hawking resolved this anomaly by a process that can best be explained by an
analogy. Hawking asks us to accept, as Einstein said, that space-time is curved, like the skin of a balloon, say, or the surface of the earth. Remember that these are analogies only; using another, Hawking said that the size of the universe at its birth was like a small circle drawn around, say, the North Pole. As the universe – the circle – expands, it is as if the lines of latitude are expanding around the earth, until they reach the equator, and then they begin to shrink, until they reach the South Pole in the ‘Big Crunch.’ But, and this is where the analogy still holds in a useful way, at the South Pole, wherever you go you must travel north: the geometry dictates that it cannot be otherwise. Hawking asks us to accept that at the birth of the universe an analogous process occurred – just as there is no meaning for
south
at the South Pole, so there is no meaning for
before
at the singularity of the universe: time can only go forward.

Hawking’s theory was an attempt to explain what happened ‘before’ the Big Bang. Among the other things that troubled physicists about the Big Bang theory was that the universe as we know it appears much the same in all directions.
²⁴
Why this exquisite symmetry? Most explosions do not show such perfect balance – what made the ‘singularity’ different? Alan Guth, of MIT, and Andrei Linde, a Russian physicist who emigrated to the United States in 1990, argued that at the beginning of time – i.e.,
T
= 10
^–43
seconds, when the cosmos was smaller even than a proton – gravity was briefly a
repulsive
force, rather than an attractive one. Because of this, they said, the universe passed through a very rapid inflationary period, until it was about the size of a grapefruit, when it settled down to the expansion rate we see (and can measure) today. The point of this theory (some critics call it an ‘invention’) is that it is the most parsimonious explanation required to show why the universe is so uniform: the rapid inflation would have blown out any wrinkles. It also explains why the universe is not completely homogeneous: there are chunks of matter, which form galaxies and stars and planets, and other forms of radiation, which form gases. Linde went on to theorise that our universe is not the only one spawned by inflation.
²⁵
There is, he contends, a ‘megaverse,’ with many universes of different sizes, and this was something that Hawking also explored. Baby universes are, in effect, black holes, bubbles in space-time. Going back to the analogy of the balloon, imagine a blister on the skin of the balloon, marked off by a narrow isthmus, equivalent to a singularity. None of us can pass through the isthmus, and none of us is aware of the blister, which can be as big as the balloon, or bigger. In fact, any number may exist – they are a function of the curvature of space-time and of the physics of black holes. By definition we can never experience them directly: they have no meaning.

That phrase, ‘no meaning,’ introduces the latest phase of thinking in physics. Some critics call it ‘ironic science,’ speculation as much as experimentation, where there is no real evidence for the (often) outlandish ideas being put forward.
²⁶
But that is not quite fair. Much of the speculation is prompted – and supported – by mathematical calculations that point toward solutions where words, visual images and analogies all break down. Throughout the twentieth century physicists have come up with ideas that have only found experimental support much later, so perhaps there is nothing very new here. At the moment,
we are living at an in-between time, and have no way of knowing whether many of the ideas current in physics will endure and be supported by experiment. But it seems unlikely that some ever will be.

Another theory of scientists like Hawking is that ‘in principle’ the original black hole and all subsequent universes are actually linked by what are variously known as ‘wormholes’ or ‘cosmic string.’
²⁷
Wormholes, as conceived, are minuscule tubes that link different parts of the universe, including black holes, and therefore in theory can act as links to other universes. They are so narrow, however (a single Planck length in diameter), that nothing could ever pass through them, without the help of cosmic string – which, it should be stressed, is an entirely theoretical form of matter, regarded as a relic of the original Big Bang. Cosmic string also stretches across the universe in very thin (but very dense) strips and operates ‘exotically.’ What this means is that when it is squeezed, it expands, and when it is stretched, it contracts. In theory at least, therefore, cosmic string could hold wormholes open. This, again in theory, makes time travel possible, in some future civilisation. That’s what some physicists say; others are sceptical.

Martin Rees’s ‘anthropic principle’ of the universe is somewhat easier to grasp. Rees, the British astronomer royal and another contemporary of Hawking, offers indirect evidence for ‘parallel universes.’ His argument is that for ourselves to exist, a very great number of coincidences must have occurred, if there is only one universe. In an early paper, he showed that if just one aspect of our laws of physics were to be changed – say, gravity was increased – the universe as we know it would be very different: celestial bodies would be smaller, cooler, would have shorter lifetimes, a very different surface geography, and much else. One consequence is that life as we know it can in all probability only form in universes with the sort of physical laws we enjoy. This means, first, that other forms of life are likely elsewhere in the universe (because the same physical laws apply), but it also means that many other universes probably exist, with other physical laws, in which
very
different forms of life, or no forms of life, exist. Rees argues that
we
can observe our universe, and conjecture others, because the physical laws exist about us to allow it. He insists that this is too much of a coincidence: other universes, very different from ours, almost certainly must exist.
²⁸

Like most senior physicists, cosmologists, and mathematicians, Hawking has also devoted much energy to what some scientists call ‘the whole shebang,’ the so-called Theory of Everything. This too is an ironic phrase, referring to the attempt to describe all of fundamental physics by one set of equations: nothing more. Physicists have been saying this ‘final solution’ is just around the corner for more than a decade, but in fact the theory of everything is still elusive.
²⁹
To begin with, before the physics revolution discussed in earlier chapters of this book, two theories were required. As Steven Weinberg tells the story, there was Isaac Newton’s theory of gravity, ‘intended to explain the movements of the celestial bodies and how such things as apples fall to the ground; and there was James Clerk Maxwell’s account of electromagnetism as a way to explain light, radiation, magnetism, and the forces that operate between electrically charged
particles.’ However, these two theories were compatible only up to a point: according to Maxwell, the speed of light was the same for all observers, whereas Newton’s theories predicted that the speed measured for light would depend on the motion of the observer. ‘Einstein’s general theory of relativity overcame this problem, showing that Maxwell was right.’ But it was the quantum revolution that changed everything and made physics more beautiful but more complex at the same time. This linked Maxwell’s theory and new quantum rules, which viewed the universe as discontinuous, with a limit on how small the packets of electromagnetic energy can be, and how small a unit of time or distance is. At the same time, this introduced two new forces, both operating at very short range,
within
the nucleus of the atom. The strong force holds the particles of the nucleus together and is very strong (it is this energy that is released in a nuclear weapon). The other is known as the weak force, which is responsible for radioactive decay.

And so, until the 1960s there were four forces that needed to be reconciled: gravity, electromagnetism, the strong nuclear force, and the weak radioactive force. In the 1960s a set of equations was devised by Sheldon Glashow, and built on by Abdus Salam and Steven Weinberg, at Texas, which described both the weak force and electromagnetism and posited three new particles, W
⁺
, W
^–
and Z
⁰
.
³⁰
These were experimentally observed at CERN in Geneva in 1983. Later on, physicists developed a series of equations to describe the strong force: this was related to the discovery of quarks. Having been given rather whimsical names, including those of colours (though of course particles don’t have colours), the new theory accounting for how quarks interact became known as quantum chromodynamics, or QCD. Therefore electromagnetism, the weak force, and the strong force have all been joined together into one set of equations. This is a remarkable achievement, but it still leaves out gravity, and it is the incorporation of gravity into this overall scheme that would mark, for physicists, the so-called Theory of Everything.

At first they moved toward a quantum theory of gravity. That is to say, physicists theorised the existence of one or more particles that account for the force and gave the name ‘graviton’ to the gravity particle, though the new theories presuppose that many more than one such particle exists. (Some physicists predict 8, others 154, which gives an idea of the task that still lies ahead.) But then, in the mid-1980s, physics was overtaken by the ‘string revolution’ and, in 1995, by a second ‘superstring revolution.’ In an uncanny replay of the excitement that gripped physics at the turn of the twentieth century, a whole new area of inquiry blossomed into view as the twenty-first century approached.
³¹
By 1990 the shelves of major bookstores in the developed world were filled with more popular science books than ever before. And there were just as many physics, cosmology, and mathematics volumes as there were evolution and other biology titles. As part of this phenomenon, in 1999 a physics and mathematics professor who held joint appointments at Cornell and Columbia Universities entered the best-seller lists on both sides of the Adantic with a book that was every bit as difficult as
A Brief History of Time,
if not more so.
The Elegant Universe: Superstrings, Hidden Dimensions and the Quest for the
Ultimate Theory,
by Brian Greene, described the latest excitements in physics, working hard to render very difficult concepts accessible (Greene, not to put his readers off, called these difficult subjects ‘subtle’).
³²
He introduced a whole new set of physicists to join the pantheon that includes Einstein, Ernest Rutherford, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Wolfgang Pauli, James Chadwick, Roger Penrose, and Stephen Hawking. Among these new names Edward Witten stands out, together with Eugenio Calabi, Theodor Kaluza, Andrew Strominger, Stein Strømme, Cumrun Vafa, Gabriele Veneziano, and Shing-Tung Yau, about as international a group of names as you could find anywhere.

The string revolution came about because of a fundamental paradox. Although each was successful on its own account, the theory of general relativity, explaining the large-scale structure of the universe, and quantum mechanics, explaining the minuscule subatomic scale, were mutually incompatible. Physicists could not believe that nature would allow such a state of affairs – one set of laws for large things, another for small things – and for some time they had been seeking ways to reconcile this incompatibility, which many felt was not unrelated to their failure to explain gravity. There were other fundamental questions, too, which the string theorists faced up to: Why are there four fundamental forces?
³³
Why are there the number of particles that there are, and why do they have the properties they do? The answer that string theorists propose is that the basic constituent of matter is not, in fact, a series of particles – point-shaped entities – but very tiny, one-dimensional strings, as often as not formed into loops. These strings are very small – about 10
^–33
of a centimetre – which means that they are beyond the scope of direct observation of current measuring instruments. Notwithstanding that, according to string theory an electron is a string vibrating one way, an up quark is a string vibrating another way, and a tau particle is a string vibrating in a third way, and so on, just as the strings on a violin vibrate in different ways so as to produce different notes. As the figures show, we are dealing here with very small entities indeed – about a hundred billion billion (10
²⁰
) times smaller than an atomic nucleus. But, say the string theorists, at this level it is possible to reconcile relativity and quantum theory. As a by-product and a bonus, they also say that a gravity particle – the graviton – emerges naturally from the calculations.