The end of theoretical physics

You might think from the discussion in the previous section that
the end of theoretical physics is in sight. Maybe M-theory will
answer all our questions, including questions which have until
now been thought to be beyond the realm of science. Maybe we
lesser mortals should just sit tight whileWitten and his colleagues
sort out the details of M-theory over the next few years. Then all of
fundamental physics would be known. I for one do not subscribe
to this view. This is in part due to the fact that I amnot an expert in
superstring theory or M-theory, and therefore cannot share in the
sense of excitement that the practitioners in these fields must feel.
But there is another, more justifiable reason for my scepticism.While many physicists firmly believe we already have a
theory of quantum gravity in the shape of the multi-dimensional
superstrings or M-theory, there are otherswhoare not so confident.
They draw parallels with the state of physics at the end of the
last century when it was thought that the end was in sight and
that all the laws of nature had been unravelled and understood.
Then came the discoveries of x-rays and radioactivity, Max Planck
suggested that energy came in discrete packets, or quanta, and
Einstein overthrew the Newtonian view of space and time. One
hundred years ago no scientists in their wildest dreams could have
anticipated what was to happen over the next quarter of a century.
So why should we be so confident today? In fact, historians of
science point out that back then scientists were probably more
justified in thinking that the end of physics was in sight than we
are today. Some of the world’s leading physicists, such as Roger
Penrose and David Deutsch, firmly believe that before quantum
mechanics and relativity can be united into a theory of quantum
gravity, one or even both of these two theories might need major
surgery.
Some ideas in physics stand the test of time and evolve slowly
as experimental evidence in their support accumulates. Gradually
we understand them more and grow confident that they are a
correct description of the physical Universe. Other ideas burst
upon the scene suddenly because of an individual moment of
genius or a surprising experimental result. But many theories
are consigned to the scrap heap when they fail the test of closer
scrutiny.
A few successful theories cause a revolution in the way we
view the world—known as a paradigm shift in our view. This was
the case with Einstein’s theory of relativity when he suggested that
there was no need for the ether through which light waves would
propagate. This led immediately to the conclusion that a beam of
light travels at the same speed whether we are moving towards the
source of the light or away from it. In turn, this led inevitability to
the fact that time runs slower for different observers.
But surely quantum mechanics cannot be wrong, can it? Its
predictive power is not in doubt, it’s been around for seventy five years and now underpins so much of modern science. Of course,
anyone who has learnt something about quantum mechanics will
acknowledge just how weird it suggests the microscopic world
is, but the standard argument goes like this: The mathematical
formalism is right, it is just what the equations mean that is not
properly understood, and that is a matter of philosophy not
physics. The majority of physicists today believe that despite
there being a whole host of different interpretations of quantum
mechanics on the market, all of which are equally valid, the
underlying mathematical framework is correct and it is purely
a matter of personal taste which interpretation an individual
subscribes to. Whether you believe the Copenhagen interpretation
in which nothing exists until it is observed, or the manyworlds
interpretation in which the Universe splits into an infinite
number of copies, or the Bohmian interpretation in which signals
travel faster than light or even, more recently, the transactional
interpretation in which signals travel backwards in time, it doesn’t
matter. No experiment has yet been devised that can discriminate
between these rival views. The only thing we are sure about is
that quantum mechanics does not have a simple common sense
explanation.
In my opinion, to say that the meaning of the mathematical
equations that describe reality at its most fundamental level is
not important, and that all we should be concerned with are the
numbers we obtain by solving these equations, is a cop-out. Over
the past ten years or so I, and a growing number of physicists,
have become convinced that not all interpretations of quantum
mechanics can be right. Nature behaves in a certain way and
the fact that we have yet to figure out what is really going on is
somethingwehave yet to properly address. For instance, either the
Universe splits into many copies of itself or it doesn’t. It is tough
luck for us if we cannot find out whether this happens or not, but
we should not stop trying. We may never succeed in finding what
is really going on, but something is going on. I believe that one
day we will find out.
As a research student, my hero was the late John Bell, an Irish
theoretical physicist and one of the twentieth century’s leading experts on quantum mechanics. He was also, as far as I am
concerned at any rate, the voice of reason when it came to the
interpretation of quantum mechanics. During the 1920s, the two
giants of physics, Neils Bohr and Einstein, had a long running
debate about the meaning of the then new theory. Einstein argued
that quantum mechanics could not be the last word and that there
had to be something missing, while Bohr claimed that quantum
mechanics told us all we could ever know about nature. Bohr was
convinced that physical theories do not describe reality directly
but only what we can know about reality. His version of quantum
mechanics became known as the Copenhagen view since that was
where his institute was based. Einstein, on the other hand, felt
that a good theory had to be ontological in that it described how
reality really is. Quantum mechanics should not be any different.
It is generally acknowledged that Bohr won that debate and since
then generations of physicists have followed the Copenhagen
view.
I ama regular visitor to the Neils Bohr Institute in Copenhagen
where much research still goes on today. Fromthe outside it looks
like a rather quaint collection of small buildings dwarfed by the
nearby large hospital. On the inside, however, it is easy for visitors
to become lost in the myriad of tunnels and passages that link the
buildings together underground. My real inspiration, however,
comes from walking in the park behind the Institute where Bohr
and the other giants of early twentieth century physics would
spend so much time trying to figure out the strange implications
of the new quantum mechanics.
John Bell was of a later generation. I heard him lecture on a
number of occasions where he always said he felt that those who
adhered to the Copenhagen view were like ostriches with their
heads in the sand, not daring to question the deeper meaning of
quantum mechanics, but satisfied to blindly follow its rules which
worked so well. This troubled Bell since he felt that physics should
be about trying to understand the deeper meaning of what was
going on in nature.
However, Bell was by no means on the fringes. He has been
one of the most respected figures in world physics since the early 1960s and has made some of the most important discoveries in
modern physics. I bumped into him for the last time at a meeting
of theAmerican Physical Society in Baltimore in 1989, a year before
he died. I had attended a fringe meeting on the foundations of
quantum mechanics in which the speaker had proposed some new,
and clearly dubious, interpretation. I noticed that John Bell was
also in the audience. Later that morning, I found myself alone with
Bell in a lift going to the cafeteria on the top floor of the conference
centre. In order to strike up a conversation with the great man, I
asked him what he thought of the last talk.
“Oh, he’s clearlywrong” he smiled, “he is obviously not aware
of the helium problem”.
“Obviously not,” I laughed eagerly, wondering what the hell
the helium problem might be, but keen to make sure he realized I
was in complete agreement with him.
I remember once asking Bell a question after a lecture he gave
at Queen Mary College London. He had just argued that he was
quite a fan of David Bohm’s interpretation of quantum mechanics
which describes the whole Universe as being interconnected on
the quantum level so that something happening to an atom here
on Earth might instantaneously affect another atom in a different
galaxy. This type of connectivity between all the particles in the
Universe is known as non-locality, or action-at-a-distance, and
would require some sort of signalling that must travel faster than
light. But surely, I asked Bell, this violates Einstein’s special theory
of relativity. He replied that he would rather give up special
relativity than reality itself, which is literally the price one has to
pay if the Copenhagen view is to be believed. You see, according
to Bohr nothing even exists in the quantum world until we have
measured it and observed it, and since everything is ultimately
made up of quantum objects anyway, then nothing (not even the
next page of this book) exists until we look at it. Bell maintained
that if this were not the case, where would we draw the line
between the microscopic world that obeys the quantum rules and
the macroscopic world of everyday life?