TG: In 2004, you won the
Nobel prize in physics for developing
the theory of asymptotic freedom. Can you tell me about that?
DG: When I started graduate school... theorists really had no
clues, no deep understanding of what was going on inside the
nucleus.
Shortly after I got out of graduate school, I went off to a
postdoctoral fellowship, from Berkeley to Harvard, and there
were some wonderful experiments going on.
[In these experiments,
the goal] was to shoot electrons, which we understand very well,
onto protons at very high energies, and look at the various
scatterings of these electrons... to essentially have a
microscope that looked inside the proton.
These experiments were very surprising, and they seemed to
indicate that the proton was made out of some point-like
particles, [with] no structure.
That had at least been observed
at short distances and over short times, and that was pretty
mysterious.
I'd been working on this and making predictions of what might
happen if you made various outrageous assumptions.
And it looked
like these particles were consistent with being what are called
quarks, which were hypothesized earlier as mathematical objects
to explain the patterns of the particles that were being
produced.
But this experiment revealed that they were real and somehow
moving freely - which made no sense at all, because then they
would easily be knocked out of the proton if you hit it hard
enough. Nobody had ever seen the quark.
And so I got obsessed with that, which led to the discovery of
asymptotic freedom and then quantum chromodynamics.
Asymptotic
freedom is this property that the force between the quarks gets
weaker when they get closer together, which is counterintuitive
and unlike any other theory that we knew.
The force gets weaker when they get closer, the force gets
stronger when they get farther apart, and maybe strong enough so
that you can never pull them apart, which seems to be the case.
So that was the watershed moment for the theory of the
strong
nuclear force.
In the same years - in the early '70s - the
theory of the
weak nuclear force was also being constructed,
again, in a different setup, but the same kind of generalization
of electrodynamics.
And by the middle/end of the '70s, we
completed what we call the Standard Model, the standard theory
of particle physics:
what makes up matter, what are the forces
that act between them.
TG: At that point, it seems like we united three of the forces,
but there's this outlier, gravity, right? So from there you move
on?
DG: I couldn't move on immediately.
Once we had a theory in
which you could calculate nuclear phenomena... one could
calculate, make predictions and test the theory.
Quantum chromodynamics is a very deep and long and complicated
and beautiful story that goes on today in full force.
At short
distances, when the quarks are close, it's easy because the
[strong] force gets weaker and weaker, so you can calculate
easily - and people now have extended those calculations over
50 years to incredible accuracy.
But what I was most interested in was trying to understand, is
it really true that quarks are completely confined, and how does
that work? And how do you control the theory when the forces
become strong? That's much harder.
Many questions are open. But I got tired of it because it was
hard, and I couldn't really solve it.
And besides that, as you say, there were indications within the
standard theory that, if you pushed it to the extreme - to very
high energies and very short distances - it failed because
gravity came in.
So that was a sign that we should try to unify
all the forces with gravity.
And that led to
string theory, which I've been mostly working on
ever since.
TG: Can you explain a little bit about string theory and what
you're working on?
DG: Questions that we ask [in string theory] are even more
ambitious than unifying all the forces.
Gravity is, according to
Einstein, in our understanding, the dynamics of
space-time,
right?
Now we're beginning to understand that we're going to have to,
once again, like many times in the history of physics, modify,
improve our understanding of space-time.
What is space-time made of, and how does it behave at short
distances?
How did the universe evolve?
We don't understand much of that.
But we especially don't
understand the beginning, and that's where all of our ideas
break down - even, so far, attempts to use string theory - but
string theory still offers the best hope of trying to address
the question of how the universe began.
TG: So one of the roadblocks is that you have all these
[unified] theories, but then to test them, you need experiments,
and the energy regimes where you could test them are extreme?
DG: It's very hard to directly test them.
So, in the 19th
century, chemists and physicists hypothesized the existence of
atoms.
But nobody had ever seen an atom or had any direct way of
probing what an atom is made out of, or even if there are atoms
and so on. So it was a similar situation.
And then breakthroughs or the real advances in understanding
that the atomic structure of ordinary matter and of the atom
happened in the 20th century - they weren't anticipated, and
many people regarded atoms as,
"OK, some kind of mathematical
gimmick to construct theories' but they weren't really real."
That happens over and over again [in science], and of course,
the great thing is that experiments can settle the issue.
That
happened with atoms, with
Brownian motion [the random motion of
particles, which was elucidated by Einstein] and Rutherford
[whose gold foil experiments showed atoms were mostly empty
space with densely-packed nuclei].
And then
quantum mechanics
was developed, and now we understand ordinary material
completely.
In this case [testing string theories], it gets harder and
harder the farther away you get from the human scale. I mean,
the scale we're looking at is so teeny.
It's about as teeny as
you can get.
TG: And this is the Planck scale [1.6X10-35 meters, where
quantum effects are thought to dominate gravity]?
DG: Yes, the Planck scale is the scale where gravity becomes a
very strong force, where the structure of space itself becomes
so complicated that it's probably not a good idea to even think
about space.
TG: To use the word "space" doesn't even make sense maybe at
that scale.
DG: Space is... a picture of the world that we develop as infants
in order to get the toy or the food.
It's how we explain how the
world works.
But it might not be the right explanation; it might be a
coarse-grained or a kind of approximate notion.
And in fact,
that's where we're being led, but we're just beginning to
understand what that could possibly mean and develop the tools
to deal with it.
Titan II,
the largest intercontinental
ballistic missile of its time,
now on display in a museum in
Green Valley, Arizona.
Nobel Laureate David Gross
argues that
the risk of nuclear war
has increased in recent years.
(Image
credit: Michael Dunning via Getty Images)
TG: Do you feel that in 50 years, we'll be closer to having some
kind of unified theory that incorporates all the forces?
DG: Currently, I spend part of my time trying to tell people... that
the chances of you living 50 [more] years are very small.
Due to the danger of nuclear war, you have about 35 years...
TG: Why do you think that we'll blow ourselves up, essentially,
within 35 years, give or take?
DG: So it's a crude estimate.
Even after the Cold War ended,
[when] we had strategic arms control treaties, all of which have
disappeared, there were estimates there was a 1% chance of
nuclear war [every year].
Things have gotten so much worse in
the last 30 years, as you can see every time you read the
newspaper.
I feel it's not a rigorous estimate, that the chances are more
likely 2%. So that's a 1-in-50 chance every year. The expected
lifetime, in the case of 2% [per year], is about 35 years.
[The
expected lifetime is the average time it would take to have had
a nuclear war by then. It is calculated using similar equations
as those used to determine the "half-life" of a radioactive
material.]
TG: So what do you suggest as remedies to lower that risk?
DG: We had something called the
Nobel Laureate Assembly for
reducing the risk of nuclear war in Chicago last year.
There are steps, which are easy to take - for nations, I mean.
For example, talk to each other.
In the last 10 years, there are no treaties anymore. We're
entering an incredible arms race. We have three super nuclear
powers.
People are talking about using nuclear weapons; there's
a major
war going on in the middle of Europe; we're
bombing Iran; India
and Pakistan almost went to war.
OK, so that's increased the chance [of nuclear war].
I would
really like to have a solid estimate - it might be more, and I
think I'm being conservative - but a 2% estimate [of nuclear
war] in today's crazy world.
TG: Do you think we'll ever get to a place where we get rid of
nuclear weapons?
DG: We're not recommending that.
That's idealistic, but yes, I
hope so.
Because if you don't, there's always some risk
an A.I.
100 years from now [could launch nuclear weapons], but chances
of [humanity] living, with this estimate, 100 years, is very
small, and living 200 years is infinitesimal.
So [the answer to] Fermi's question of,
"Where are the civilizations,
all the intelligent organisms around the galaxy,
and why don't they talk to us?",
...is that they've killed
themselves...
You asked me to think about the future, and I am obsessed the
last few years, thinking about that - not the future of ideas
and understanding nature, but of the survival of humanity.
TG: I think in some ways, during the Cold War, it was easier for
people to conceptualize because we had one major enemy. Now
there's chaotic interactions between countries.
DG: There are now nine nuclear powers...
Even three is infinitely
more complicated than two. The agreements, the norms between
countries, are all falling apart. Weapons are getting crazier.
Automation, and perhaps even A.I., will be in control of those
instruments pretty soon.
TG: That scares me too - that a lot of weapons are using
A.I.
systems to make decisions on some level.
DG: It's going to be very hard to resist making A.I. make
decisions because it acts so fast.
If you have 20 minutes to
decide whether to send a few hundred nuclear armed missiles to
both China and Russia for "our dear president," the military
might feel that it's wiser to make A.I. make that decision.
But if
you play with A.I., you know that it
sometimes hallucinates.
TG: The problem feels too big for ordinary people to do anything
about, which is the same thing with
climate change, right?
DG: People have done something about climate. So that's
something scientists began to warn people about 40 years ago.
And they convinced people that's a real danger.
It's a much harder argument to make than about nuclear weapons.
We made them; we can stop them.
