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Will we ever understand the universe?
Stuart Clark
Lecture given on 14th March 2013
Part of the Santander Foundation Science and Society Lecture Series
Madrid, Spain
One hundred and thirty-four years ago today, on 14 March 1879, Albert Einstein was born
in Ulm, Kingdom of Württemberg, in the German Empire. He stands today as an icon of
science, a man who gave us a way to understand the universe as never before.
The discipline of cosmology starts with Einstein and his theory of General Relativity
because, for the first time in history, he gave scientists a way of writing down an equation
that encapsulated the whole universe. All the individual objects in the universe could be
reduced to a single mathematical term, and the behaviour of the universe calculated from
it.
The future of the universe and its past could be calculated if we know the precise
state of it today. In centuries past, such knowledge would have been seen as that of the
gods. It was a work of supreme self-confidence and mathematical competence.
At an after dinner speech in Einsteinʼs honour, the English playwright George
Bernard Shaw encapsulated the progressive nature of science when he said, “Ptolemy
made a universe, which lasted 1400 years; Newton also made a universe, which has
lasted 300 years.” Then he quipped, “Einstein has also made a universe and I canʼt tell
you how long that will last.”
As we shall see, it is possible that Einsteinʼs General Relativity will not even make it
to its centenary in 2015. The question is: what will replace it? Will that be a final theory or
will it be another incremental step? In short, will we ever understand the universe?
At the time when general relativity was born, Europe was a desperate place. The First
World War was raging and Einstein was becoming an increasingly isolated figure. His
outspoken views against nationalism were an embarrassment to his colleagues and to
Germany. Although born there, he had taken Swiss citizenship in 1900.
Removed from the war effort, with very little support, he worked on his pet theory to
extend the work by Isaac Newton and find a more complete description of gravity.
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Newtonʼs work had been a landmark, a way of describing how objects would move
in the presence of each other through the force of gravity. It explained why the planets
move across the sky, why dropped objects fall to the floor yet the Moon in the sky doesnʼt.
At the time, it was called a ‘System of the World’ – the seventeenth century phrase
for what we would now call the theory of everything. British astronomer Edmond Halley,
who was a friend of Isaac Newton, even described the work as the perfection of
astronomy.
By the end of the nineteenth century, this view had become so entrenched in some
quarters of the scientific establishment that science was thought to be essentially
complete.
In 1900, the great British scientist Lord Kelvin addressed the meeting of the British
Association for the Advancement of Science. He famously said, ‘There is nothing new to
be discovered in physics now. All that remains is more and more precise measurement.’
What he forgot was that scientific advances are often made through better
measurement. Only when you develop better instruments, do movements and readings
that do not fit the current theory become obvious. If they persist with repeated observation,
then it is science’s job to explain them.
By 1900, one such anomaly was clearly apparent: the movement of Mercury. It was
drifting away from where Newton’s law of gravity said it should be. Initially, it was thought
that an undiscovered planet was pulling it off course but Newtonian gravity could not
provide an adequate solution to where the unseen planet was, and observational searches
for it during total eclipses were not finding it either.
Working in an office with a portrait of Newton on the wall, Einstein solved the
problem in 1915. At the moment of his triumph, he experienced heart palpitations.
Obviously he survived but imagine for a moment the legend of Einstein if he had died with
the shock of success!
Conceptually, Einstein’s universe is not too difficult to grasp but, to appreciate the
big difference, we have to look first at Newton’s universe. To him, space and time were
fixed. They were a rigid framework within which to measure things. Einstein allowed this
framework to be distorted by the masses of the celestial objects it contained. Space and
time were warped in the presence of matter. These distortions create the effect of gravity
and explain why things accelerate in the presence of a gravitational field.
Despite finding the correct mathematical framework in 1915, it took four more years
before it came to the attention of the rest of the world. This was because the mathematics
is difficult to work through. The theory needed to be verified by somebody independently.
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Luckily for Einstein, this was possible because the theory made a clear prediction about
the effect of gravity on a beam of light. It would be bent by a precise amount.
By 1919, the world was in the aftermath of the First World War; countries and
empires were collapsing. A new political landscape was being drawn. Change was in the
air and a British astrophysicist was on a small African island, setting up his telescope. His
work would turn Einstein into an icon.
Arthur Eddington was born in Cumbria, England, and educated at Cambridge. An
exceptional mathematician, he was certainly aware of the scientific importance of
Einsteinʼs work and argued that if Newtonʼs theory of gravity was to be overthrown, then
an Englishman should be involved in doing it.
Eddington waited until the eclipse of 1919 and measured the deflection of starlight
around the Sun. The amount of this deflection, caused by the distortion in space and time
around the Sun, was clearly predicted by Einsteinʼs General Relativity and also
substantially differed from the value offered by Newtonʼs work.
Eddington measured a deviation consistent with General Relativity, announced the
result in early November 1919 and Einstein became world famous. The New York Times
declared it to their readers with the amazing headline:
Lights All Askew in the Heavens
Men of Science More of Less Agog Over Results
of Eclipse Observations
Einstein Theory Triumphs
Stars Not Were They Seemed or Were Calculated to be
but Nobody Need Worry.
A book for 12 Wise Men
No More in all the World Could Comprehend It, Said Einstein When
His Daring publishers Accepted It
But Einstein himself, like Newton before him, knew that from the lofty pinnacle of
achievement, they saw not a final solution but a whole new landscape of possibilities
stretched out before them. Newton expressed it best when he wrote, “I do not know what I
may appear to the world, but to myself I seem to have been only like a boy playing on the
seashore, and diverting myself in now and then finding a smoother pebble or a prettier
shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
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General Relativity extended human knowledge into a whole new realm of understanding.
Now it was possible to compute the behaviour of the whole universe. The ability of space
and time to be distorted meant that the universe could be in a state of overall expansion or
contraction. Belgian Georges Lemaître predicted this expansion in 1927, two years before
American Edwin Hubble ʻdiscoveredʼ it.
It is in the interest of historical justice that we must work harder at giving Lemaître
the credit he deserves. He also used General Relativity to predict a beginning to the
universe. We now call this the big bang, yet Lemaître called it ʻThe Day Without
Yesterdayʼ.
In June 1966, word reached Lemaître on his deathbed that a spectacular discovery
had been made. The universe was filled with microwaves – so many that they
outnumbered the atoms by a billion to one. Extraordinarily, the existence of these
microwaves had been mathematically predicted by Ralph Alpher, an American
cosmologist. You could think of the microwaves as the remains of the fireball that
accompanied the universeʼs birth. So there had indeed been a day without yesterday.
The discovery of the microwave background was a stunning vindication of scienceʼs
great belief that mathematics is the language in which the universe is written. In 1623,
Galileo Galilei wrote one of the most famous expressions of this belief. It occurs in his
book The Assayer. He wrote, “The universe cannot be read until we have learned the
language and become familiar with the characters in which it is written. It is written in
mathematical language, and the letters are triangles, circles and other geometrical figures,
without which means it is humanly impossible to comprehend a single word. Without
these, one is wandering about in a dark labyrinth.”
But why should numbers describe the universe so well? Does it mean that reality is
mathematical or are we being fooled by fitting imprecise theories to approximate
observations?
If so mathematics is just a tool rather than a fundamental property of the universe.
Yet even if that is the case, maths can still be useful.
The man who originally proved the value of mathematical astronomy was German
Lutheran, Johannes Kepler. Indeed to George Bernard Shaw’s list of ‘universes’ I would
add one more: Kepler’s mathematical description of planetary motion in the early
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seventeenth century. Kepler was the first astronomer in history to show that heavenly
motion could be been distilled into general mathematical formulae.
His three laws of planetary motion hold not just for all the planets in the solar
system that he knew about, but the three that were subsequently discovered. They are
also true for all the asteroids and comets that circle that Sun, and the almost 1000 planets
that have been found around stars other than the Sun during the last 17 years.
In his laws, Kepler discovered a piece of universal truth so profound that it applies
across the universe. For astronomers it was the herald of a new way of working and gave
them every reason to believe that the universe was rational and could be captured in
mathematics.
If something is possible mathematically then, goes the reasoning, it stands a good
chance of being true in reality. By the twentieth century, such mathematical predictions
were truly paying off. As well as Lemaître’s anticipation of the expanding universe, Karl
Schwarzschild predicted the existence of black holes, for which there is now overwhelming
evidence.
For the particle physicists, their mathematical theories of the way atoms and their
constituent particles interact were predicting previously unknown particles of nature. More
than that, experiments designed to capture these fleeting things were achieving results.
Antimatter, predicted in 1928, was found in 1932; neutrinos, proposed in 1930, were
discovered in 1956.
By the 1970s, the particles physicists were starting to believe that they could find an
equation for everything. It would explain why there were four fundamental forces in the
universe today and how they are linked together. However, such a theory requires further,
as yet undiscovered particles. They are predicted to be different from atoms, hardly
interacting with normal matter at all except through their gravity. If so, the astronomers
should be seeing movement in the universe that they can’t explain.
As it turned out the astronomers had been struggling with some strange motions...
The trouble all started because as measurements became more and more precise,
so the reach of science became greater. Astronomers could peer out into the depths of
space and measure great motions that had been rendered small only by their distance.
That’s when they realised that things were not as they seemed.
Prickly Swiss astronomer Fritz Zwicky emigrated to America in 1925 to work at the
California Institute of Technology. By 1933, he had convinced himself that there was more
to the universe than meets the eye. He was studying a collection of a thousand galaxies
that were all bound to each other through the gravity they generated. Each galaxy was
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home to a hundred billion or more stars yet Zwicky’s analysis showed that this was not
enough. If the stars alone made up the majority of matter in the galaxies, then the cluster
could not be generating sufficient gravity to keep it together. Yet there it was. And it was
not alone. Ever more powerful telescopes were showing clusters of galaxies spread
throughout the universe. Something was keeping them together.
Zwicky concluded that there must be extra reservoirs of matter providing an
additional gravitational force. Because these reservoirs were not easily visible, they could
not be emitting any light. He published his original paper on the subject in German in a
paper to the Swiss Physical Society and referred to the unseen stuff as dunkel Materie,
dark matter.
Initially, dark matter was thought to be clouds of ordinary atoms that had so far
escaped detection because they had not collapsed into stars. Decades of searching,
however, turned up very little. Radio telescopes and infrared telescopes increasingly found
stocks of once invisible atoms but not in nearly the quantities that were needed. By the
1970s, things were looking grim for the astronomers. So the particle physicists’ suspicions
of undiscovered particles of nature, linked to a theory of everything, were something of a
magic bullet.
Not only do most astronomers think that dark matter holds clusters of galaxies
together, they also think it provides the gravitational glue to stop individual galaxies flying
apart. The only problem is that, so far, no one can find a single piece of direct evidence
that it actually exists. Awkward.
Things got worse. The astronomers weren’t done with their revelations. The
universe was hiding not just a big secret from us but the biggest secret. By the late 1990s,
astronomers were ready to make the announcement, and they could have pinched the
opening line of Star Wars to do it. A long time ago, in a galaxy far, far away...
A star exploded. The light from that cataclysm reached Earth billions of years later
and astronomers analysed the light to compare it to their mathematical expectations based
on General Relativity.
Astronomers expected to find that the expansion of space, sparked by the explosive
creation of the universe in the big bang, would be slowing down as gravity tried to pull
everything back together. That’s why they had gone looking for distant exploding stars to
measure the effect from then to now. Instead, they saw that the universe was accelerating.
Something was resisting gravity. But what?
It could be an energy or a force. No one knows. Astronomers have called it dark
energy to signify its mystery. Their mathematical estimates are growing ever more
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sophisticated and show that dark energy constitutes almost three-quarters of the universe.
The putative dark matter makes up almost another quarter, and the normal atoms are the
puny four percent left over.
In this view, the atoms are nothing but celestial froth. Everything we see around us
is the most insignificant part of the universe.
Then the particle physicists who were working on the theory of everything suffered
a setback. Instead of a definitive mathematical description of the universe, physicists
found a number of possible ones, each one seemingly as valid as the others. These were
named string theories because they shared the common foundation of transforming the
particles of nature from point-like entities into wiggling knots of subatomic energy.
But if mathematics was capable of providing more than one description of the
universe, how could you decide between them? What did this mean for mathematics and
its relationship to reality? How do we decide between such equals? Perhaps you don’t
have to, suggested American physicist Hugh Everett. Another way to read the string
theories is that there is a multiverse, of which our universe is just one small part. True
reality is a multitude of universes in which all possibilities are played out somewhere. We
just happen to be trapped in one small portion of it.
If this is the case, science will never be able to explain why our universe exists.
Instead, it just does because it can – and so do countless others.
All in all, we seem further from an understanding of the universe now than at any
time since Newton. To turn this around we need to critically look at the assumptions that
underpin the thrust of our research, and we need to look for any puzzling observations that
may provide a clue as to where to go next.
One assumption is that gravity can be linked to the other forces of nature using the
quantum theory, which was developed largely in Germany between the world wars. Here,
everything was reducible to particles. Even forces were carried by particles. But the search
for the quantum theory of gravity, which led to string theory, has all but stalled.
By seeming to be able to describe everything (including universes beyond our own),
string theory loses its power to tell us much about our own. There is currently no real
prediction from string theory that we can test. So how do we continue?
To make progress, we need new leads, and that means new experiments.
They could show us where we are going wrong with string theory or take us in an
entirely different direction.
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One experiment is currently being studied by a working group of scientists and
engineers. To my mind, it is the most important gravitational experiment since Eddington’s
1919 eclipse expedition.
The European Space Agency is building a mission called LISA-Pathfinder. It was
designed solely to test the technology needed for a larger mission called LISA (Laser
Interferometer Space Antenna) but it is now being realised that the mission is capable of
so much more.
An alternative to dark matter and dark energy is to modify the behaviour of gravity.
While many researchers think that this is a long shot, progress is so slow in finding dark
matter or understanding the nature of dark energy that more and more people are willing
to entertain what was once thought to be a wacky idea.
Modifying gravity is not easy. Classic Newtonian and Einsteinian gravity works so
well in the solar system that we must be careful not to destroy this with any tinkering that
we do. So, for example, you cannot make gravity pull a little harder. You have to be subtler
than that.
In the 1980s, Mordehai Milgrom, then at Princeton University, tweaked Newton's
laws so that an object in a very weak gravitational field experiences a slightly stronger pull
than Newton would have predicted. He showed that this revised version of gravity, now
called Modified Newtonian Dynamics (MOND), can neatly describe the observed rotation
of stars in giant spiral galaxies without the need for dark matter.
But how can we test this? We will never be able to send a probe tens of thousands
of light years to the edge of our galaxy. LISA-Pathfinder may be able to do it just a few
million kilometres away from Earth.
The gravitational field of the Sun is overwhelming in the solar system, but there are
places where the gravity of the planets cancels it out. These are called ‘saddle points’. The
one between the Earth and the Sun occurs 260,000 kilometres away. If LISA-Pathfinder
can be sent through this saddle point, its instruments will measure the acceleration due to
gravity so precisely that we will see if MOND – or some other unexpected gravitational
behaviour – is at play.
The working group are still defining the requirements of this mission and will report
back to ESA this year. Although officially slated for a launch in 2015, the launch may slip
to 2017. If so, it means that the saddle-point experiment could take place in 2019, a
century after Eddington’s eclipse confirmed General Relativity.
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Next week, we will learn about our latest clue to the universe. The European Space
Agency reports the results from the Planck spacecraft. This has been mapping the cosmic
microwave background radiation that Lemaître learned about the week before his death.
Spacecraft have studied this radiation before. It is effectively the blueprint for the
universe. The importance of next week is that it is almost certainly impossible to take
better pictures of the microwave background. Although we can build better microwave
detectors, the image itself is blurred on its way through space.
What we see next week is the best image we will ever see of the universeʼs
blueprint. There will be other ways to investigate it in the future but it is sobering to think
that in just over 400 years since the first astronomical use of a telescope, we have gone
from Galileoʼs spyglass to the most precise map of our origins it is possible to take with a
ʻsimilarʼ telescope. The question is: will we be able to decode its message and then test
our hypotheses?
If so, linked with the other experiments that I have talked about, we stand a chance
of taking the next revolutionary leap. But will that be an incremental step or the final
theory? We donʼt know. We canʼt know.
Will we ever understand the universe?
Maybe – but I suspect not for a long time.
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