Today I want to explain how we know that the
way Einstein thought about gravity cannot be correct. Einstein’s idea was that gravity is not
a force, but it is really an effect caused by the curvature of space and time. Matter curves space-time in its vicinity,
and this curvature in return affects how matter moves. This means that, according to Einstein, space
and time are responsive. They deform in the presence of matter and
not only matter, but really all types of energies, including pressure and momentum flux and so
on. Einstein called his theory “General Relativity”
because it’s a generalization of special relativity. Both are based on “observer independence”,
that is the idea that the laws of nature should not depend on the motion of an observer. The difference between General Relativity
and Special Relativity is that in Special Relativity space-time is flat, like a sheet
of paper, while in General Relativity it can be curved, like the often named rubber sheet. General Relativity is an extremely well-confirmed
theory. It predicts that light rays bend around massive
objects, like the sun, which we have observed. The same effect also gives rise to gravitational
lensing, which we have also observed. General Relativity further predicts that the
universe should expand, which it does. It predicts that time runs more slowly in
gravitational potentials, which is correct. General Relativity predicts black holes, and
it predicts just how the black hole shadow looks, which is what we have observed. It also predicts gravitational waves, which
we have observed. And the list goes on. So, there is no doubt that General Relativity
works extremely well. But we already know that it cannot ultimately
be the correct theory for space and time. It is an approximation that works in many
circumstances, but fails in others. We know this because General Relativity does
not fit together with another extremely well confirmed theory, that is quantum mechanics. It’s one of these problems that’s easy
to explain but extremely difficult to solve. Here is what goes wrong if you want to combine
gravity and quantum mechanics. We know experimentally that particles have
some strange quantum properties. They obey the uncertainty principle and they
can do things like being in two places at once. Concretely, think about an electron going
through a double slit. Quantum mechanics tells us that the particle
goes through both slits. Now, electrons have a mass and masses generate
a gravitational pull by bending space-time. This brings up the question, to which place
does the gravitational pull go if the electron travels through both slits at the same time. You would expect the gravitational pull to
also go to two places at the same time. But this cannot be the case in general relativity,
because general relativity is not a quantum theory. To solve this problem, we have to understand
the quantum properties of gravity. We need what physicists call a theory of quantum
gravity. And since Einstein taught us that gravity
is really about the curvature of space and time, what we need is a theory for the quantum
properties of space and time. There are two other reasons how we know that
General Relativity can’t be quite right. Besides the double-slit problem, there is
the issue with singularities in General Relativity. Singularities are places where both the curvature
and the energy-density of matter become infinitely large; at least that’s what General Relativity
predicts. This happens for example inside of black holes
and at the beginning of the universe. In any other theory that we have, singularities
are a sign that the theory breaks down and has to be replaced by a more fundamental theory. And we think the same has to be the case in
General Relativity, where the more fundamental theory to replace it is quantum gravity. The third reason we think gravity must be
quantized is the trouble with information loss in black holes. If we combine quantum theory with general
relativity but without quantizing gravity, then we find that black holes slowly shrink
by emitting radiation. This was first derived by Stephen Hawking
in the 1970s and so this black hole radiation is also called Hawking radiation. Now, it seems that black holes can entirely
vanish by emitting this radiation. Problem is, the radiation itself is entirely
random and does not carry any information. So when a black hole is entirely gone and
all you have left is the radiation, you do not know what formed the black hole. Such a process is fundamentally irreversible
and therefore incompatible with quantum theory. It just does not fit together. A lot of physicists think that to solve this
problem we need a theory of quantum gravity. So this is how we know that General Relativity
must be replaced by a theory of quantum gravity. This problem has been known since the 1930s. Since then, there have been many attempts
to solve the problem. I will tell you about this some other time,
so don’t forget to subscribe.
