One of the things I like about particle physics
is its ability to describe the behavior of matter under every experimental condition
we’ve ever investigated. We call our very successful theory the Standard Model of Particle
physics. I made another video that describes this incredible theory. While the Standard Model covers most of the
known fundamental forces, specifically electromagnetism, the strong and weak nuclear forces and the
Higgs field, this model says absolutely nothing about the force that literally binds the universe
together – the force of gravity. The reason for this is simple. Gravity is
incredibly, ridiculously, weaker than the other known forces. On the size of, say…
about the size of the atomic nucleus, the other forces all have kinda sorta the same
strength, with the weak force being about 100,000 times weaker than the strong force. Now that last statement probably sounds kind
of silly, because 100,000 sounds like a big difference, like comparing something, oh,
four inches tall to Mount Everest, but gravity is unfathomably weaker still. It is about-
wait for it- a hundred, thousand, trillion, trillion, trillion times weaker than the strong
force. That’s like comparing the tiny proton to the size of the visible universe. It’s
a huge difference. Since gravity is so weak in the quantum world,
there is no chance that we will ever see any effect due to gravity in a particle physics
experiment. In fact, if all we had to go on was the data from particle physics experiments,
we wouldn’t even know gravity existed. The reason that we know of gravity is because
it has an infinite range and up to size scales of the Milky Way or even clusters of galaxies
that we can see that it works basically like Isaac Newton predicted 350 years ago. It takes
the mass of asteroids or planets or stars to see gravity at all. But I don’t want to talk about the gravity
of the big, which is the domain of astronomy or cosmology, but rather I want to talk about
the nature of gravity in the realm of the very small. But I just told you that at sizes comparable
to that of a proton, gravity is very weak. So what the heck am I talking about? Well gravity, even if weak, must apply in
the microworld. That’s not a very profound thought, but it’s true. And, since our best
theory of gravity is Einstein’s theory of general relativity, the most obvious thing
to do is to just apply that theory to the subatomic realm. As an illustrative example,
let’s imagine an electron orbiting a nucleus. If you do that, you find that Einstein’s
theory predicts that the electron would lose energy by the emission of gravity waves and
then spiral down into the proton. A similar prediction using classical electromagnetism
led to the invention of familiar, or at least well-known, quantum mechanics. This same chain
of reasoning suggests that gravity must also have some kind of quantum nature. Another reason to suspect that gravity must
have a quantum nature is because A, we definitely have a quantum theory for the other forces,
and B, general relativity is a classical theory. It is impossible to seamlessly wed a quantum
and classical theory and this is taken as additional evidence that there should exist
a theory of quantum gravity. Otherwise, we’ll not be able to write a theory that accurately
describes everything in the world of the very small. So if we accept the idea of quantum gravity,
what do we know? Well, there are some basic conclusions we can make that are true for
all such theories. One such conclusion is that there should be a particle called a graviton.
In just the same way that a quantum theory of electromagnetism predicts that a photon
exists, quantum gravity predicts that a graviton should exist. Now we’ve never seen a graviton, which means
that you shouldn’t believe in it. But, if it exists, in order to agree with both Newton’s
and Einstein’s theory of gravity, the particle must have certain properties. To have gravity’s
infinite range, the graviton must be massless. To be only an attractive force, the graviton
must have a quantum mechanical spin of 2, which is different from the electron’s spin
of 1/2 and the photon’s spin of 1. The graviton must also be electrically neutral. So this all seems pretty simple. The theory
predicts a particle with very specific properties. So it would seem that the next step would
be to go out and find it. I mean, my colleagues and I do that sort of thing all the time,
right? Of course, the problem is that gravity is
so weak. And, because it’s weak, it’s essentially impossible to make a graviton
in a particle physics experiment. To all intents and purposes, there is no chance that we’ll
ever find a graviton even using the accelerators we might imagine building with the technology
of a hundred years from now. There is one small possibility we might see
a graviton someday soon, but that’s only if the universe is much different than it
appears. If the universe has additional tiny dimensions beyond the familiar three, it’s
possible that we might find gravitons and even possibly find massive gravitons as well.
But this possibility is dependent on these small extra dimensions existing. Frankly,
while it’s possible, it’s a long shot. If you’re interested, take a look at my
video on the idea. So, getting back to the more basic idea of
quantum gravity, has there been any theoretical progress on the subject? Well, yes, and no. There have been a couple quantum gravities
theories proposed that are kind of successful. And, by successful, I mean that they are still
possible. One is superstring theory, which says that the very smallest building blocks
of matter are actually very tiny strings. This theory has been very popular for many
years, although some have criticized it for not making testable predictions. If you’re
interested in the idea, check out my video on the topic. Another idea that’s been floating around
for a while is called “loop quantum gravity.” The mathematics of this theory is pretty complex
and goes by the confusing name of “spin networks,” but the basic idea is that there
is a smallest quantum of space and time. Now, this is a pretty bizarre idea. It means
that unlike ordinary sizes, in which you can cut an object a meter long into two objects
a half a meter long, when you get to a certain size, you literally no longer can make smaller
objects. The physical dimensions of this smallest space
and time are too small to test in particle physics experiments, although they might have
some testable consequences in observations of very distant astronomical objects. The
jury is still out on these studies, but so far there is no evidence that confirms these
ideas. So there is no confirmation of quantum gravity,
but if the idea is true, it has some real consequences that will change how you think
about such cool things such as the center of black holes and the universe right before
the Big Bang. If you have even a casual knowledge of physics,
you’ve no doubt heard that scientists think that before the Big Bang all of the matter
of the universe existed in a single mathematical point with zero size. Similarly, the center
of a black hole is said to hold all of its mass of the parent star compressed to zero
size. These tiny concentrations of enormous mass
are called singularities. And singularities are unphysical. They don’t exist. If a theory
predicts them, then this is a sign that the theory has been pushed hard enough that it
is broken. Now I don’t- I do not- want you to think
that this means that black holes don’t exist or that the Big Bang never happened. Nor do
I want you to think that huge concentrations of matter in tiny, tiny, volumes aren’t
real. All of these things really exist. So don’t send me some anti-relativity email. But what I am telling you is that as matter
gets compressed into smaller and smaller volumes that gravity becomes more important and that
the theory of quantum gravity starts to dominate. Quantum gravity is what protects against a
singularity. And what this really means is that we will never understand the details
of the beginning of the universe or the center of a black hole until someone works out a
theory that blends gravity and quantum mechanics. So I hope that this conversation gives you
a sense of the complexities involved in a quantum theory of gravity. Realistically,
solving this problem will take a long time, but it’s a fascinating topic and one that
we’ll need to solve before we finally have a theory of everything.
