**How Much Do We Really Know About The Universe?**

Stephen
Hawking, the British physicist who is the topic of a 2014 biopic, makes a
surprising statement in his 1992 book, Black Holes and Baby Universes, about
the extent to which physics is almost complete. He says: “Although we have not
found the exact form of all [the physical laws], we already know enough to
determine what happens in all but the most extreme situations.” Hawking adds
that he gives it a 50-50 chance that we will find the exact laws in the next
twenty years.

We know
already that his second statement hasn’t come to pass: 2012 passed and we’re
not very close to a complete theory of everything. I’m not trying to pick on
Hawking’s predictions, armed with the benefit of hindsight. Rather, what I want
to highlight is how little we really do know about the universe, even at the
fundamental physical level, and how little we can predict with any certainty,
despite Hawking’s statements to the contrary.

We can
obviously point to every social science, such as sociology, psychology, or
economics, and recognize immediately that predicting the future is a futile
task. Experts distinguish themselves by being able to talk intelligently about
theory and the future but few are foolish enough to make firm predictions
because these experts know that such predictions are impossible given our
present state of knowledge, and perhaps impossible in principle.

But the
problem of prediction—the sine qua non of science because it allows for
testability of theories and thus their possible falsification—goes far beyond
the “soft” social sciences. It’s also inherent to physics, the model of
firmness in science.

Let me
highlight a well-known problem to illustrate my point. Isaac Newton, the
British physicist and mathematician who almost singlehandedly developed
classical physics, included at the heart of his system the basic equation of
what we now call Newtonian gravity. This simple equation shows that gravity
declines between two bodies with the inverse square of their distance. So if
we’re traveling in a spacecraft away from Earth the farther we go the smaller
the gravitational attraction between the spacecraft and the planet, and it
drops off pretty quickly but never disappears entirely. Newton’s famous
equation, which showed that gravity was a universal force that applied in the
realm of falling apples as equally in the realm of planets orbiting a star,
only works for two bodies. In my example, it was the spacecraft and our planet.

What happens
when we try to solve the equation for three bodies? Well, it gets exponentially
more difficult. In fact, Henri Poincaré famously showed in a 1902 paper that
the “three-body problem” couldn’t be solved at all. Huh? Why not?

Well, it
turns out that even introducing one extra body to the gravitational situation
trying to be analyzed introduces such sensitivity to initial conditions that it
becomes impossible to make accurate predictions over the long-term. (A nerdy
aside: some solutions are possible to this problem and it is now recognized
that there are 16 families of solutions; however, these are very limited cases
and the general problem is recognized as having no solution, in principle).

Hawking’s
point about knowing the physical laws of our universe seems to ignore even this
obvious example of the limits to our knowledge. Hawking surely knows about this
example because he has, after all, for many decades now occupied the Lucasian
chair at Cambridge that Newton himself occupied in the 17th Century.

So was
Hawking referring to, rather than our ability to make firm predictions, our
ability to instead deduce the relevant equations that govern the universe (even
if those equations can’t be solved in many cases)? Even if we interpret his
statement in this manner it seems clear that he is also more optimistic than
the facts warrant. In fact, it seems far more clear that we know very little
about the laws that govern our universe.

General
relativity leads to similar problems as we just saw in Newtonian gravity
because solving Einstein’s gravity equations, a set of eight inter-linked
equations, is fiendishly difficult in real-world situations. This is why
Newtonian gravity is usually used in practice rather than general relativity.
Many solutions to the relativistic equations have been found but solving the equations
for three or more bodies is actually even more difficult than in Newton’s
equation. Again, it’s impossible, in principle, to solve the “n-body problem”
for general relativity in a general sense: only certain limited solutions are
possible.

**The Big Problems In Physics**

Lee Smolin
discussed in his excellent 2006 book, The Trouble With Physics, five major
problems that modern physics faces. There are, of course, far more than these
problems facing modern physics, but Smolin was highlighting the big ones, which
include.

1) Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature (“quantum gravity,” “grand unified theory,” or the “theory of everything”).2) Resolve the problems in the foundations of quantum mechanics, either by making sense of the theory as it stands or by inventing a new theory that does make sense.3) Determine whether or not the various particles and forces can be unified in a theory that explains them all as manifestations of a single, fundamental entity.4) Explain how the values of the free constants in the standard model of particle physics are chosen in nature.5) Explain dark matter and dark energy. Or, if they don’t exist, determine how and why gravity is modified on large scales. More generally, explain why the constants of the standard model of cosmology, including the dark energy, have the values they do.

We are,
unfortunately, far from solving any of these problems. Smolin’s book discusses
in depth the problems with string theory, which attempts to resolve the first
question by reconciling quantum theory and general relativity under a single
framework. That these very large problems remain unsolved weighs heavily
against Hawking’s optimism.

Marcelo
Gleiser, a physicist at Dartmouth University in Vermont, supports my point in
his 2014 book, Island of Knowledge: The Limits of Science and the Search for
Meaning, stating in the prologue to his book:

From our past successes we are confident that, in time, part of what is currently hidden will be incorporated into the scientific narrative, unknowns that will become knowns. But as I will argue in this book, other parts will remain hidden, unknowables that are unavoidable, even if what is unknowable in one age may not be in the next one. We strive toward knowledge, always more knowledge, but must understand that we are, and will remain, surrounded by mystery.

Taking an
even deeper look at the nature of knowledge in our modern world, Nancy
Cartwright examines in her 1999 book, The Dappled World: A Study of the
Boundaries of Science, how little we know about the universe. The dappled world
she refers to is the patchwork of physical laws and theories that work pretty
well in some limited situations. But her point is that there are vast gaps in
our understanding that remain and our ability to predict outcomes is terrible
in all but the most simple of situations.

**Are There Even Bigger Problems Remaining In Physics?**

A major
problem that Smolin alludes to but doesn’t include in his top five list is
this: there is another important integration and reconciliation of different
physical theories that has yet to happen. Going one step beyond reconciling
quantum mechanics and general relativity, we need to reconcile thermodynamics
with these two other pillars of modern physics. The nature of time is at the
heart of this reconciliation. The problem is that most modern physical theories
include a reversible concept of time. This means that the equations can be used
to look backwards or forwards in time and there’s no basic difference between
these two temporal directions. This is a problem because when we look at the
world around us, near or far, we see irreversible processes everywhere,
including the stubborn fact that eggs don’t unbreak themselves spontaneously,
cream doesn’t unmix itself from your coffee when you stir the spoon the other
way, and stars don’t unform gradually as gas drifts away slowly. All of these
processes are irreversible despite the fact that our equations are often
reversible.

By
recognizing that irreversible processes are common in nature we should also
recognize that time itself is fundamentally asymmetrical and irreversible. This
notion of time allows us to make progress with the big problem of reconciling
the concept of irreversible time in thermodynamics with the concepts of time in
quantum mechanics and general relativity.

The
Belgian-Russian physicist Ilya Prigogine made this point in a series of books
and articles over a long career that ended with his death in 2003. He won the
1977 Nobel Prize in chemistry for his work on non-equilibrium thermodynamics,
which is all about irreversible processes. Prigogine has this to say on the
nature of time in his most readable book, The End of Certainty: Time, Chaos,
and the New Laws of Nature (p. 19):

[A]ccording to the fundamental laws of physics, there should be no irreversible processes. We therefore see that we have inherited two conflicting views of nature from the nineteenth century: the time-reversible view based on the laws of dynamics and the evolutionary view based on entropy. How can these conflicting views be reconciled? After so many yeas, this problem is still with us.

Prigogine’s
many decades of work is all directed at resolving this problem and his solution
is to call for a comprehensive re-working of modern physical theories to
incorporate an irreversible/asymmetrical concept of time. In other words,
modern physics has yet to incorporate the concept of evolutionary time and an
evolving universe. This is a big job, to be sure, but it has to be done if we
are going to make real progress on the Theory of Everything that Hawking and
many others wish to see happen.

**Evolving Time, Evolving Views**

Things
change and maybe Hawking now agrees with me anyway. He stated in a 2004 talk:
“Up to now, most people have implicitly assumed that there is an ultimate
theory that we will eventually discover. Indeed, I myself have suggested we
might find it quite soon. However, [new developments in quantum gravity have]
made me wonder if this is true. Maybe it is not possible to formulate the
theory of the universe in a finite number of statements.” Hawking is here
recognizing that perhaps his dream of a simple equation or set of equations
that can explain and predict the entire universe is an impossible dream.

He adds at the end of this interesting talk:

Some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles. I used to belong to that camp, but I have changed my mind. I’m now glad that our search for understanding will never come to an end, and that we will always have the challenge of new discovery.

Hear hear,
and kudos to Mr. Hawking for allowing his views to change and acknowledging
that process of evolutionary change.

## Post A Comment:

## 0 comments: