How can we test a Theory of Everything?
That’s a question I get a lot in my public
lectures.
In the past decade, physicists have put forward
some speculations that cannot be experimentally
ruled out, ever, because you can always move
predictions to energies higher than what we
have tested so far.
Supersymmetry is an example of a theory that
is untestable in this particular way.
After I explain this, I am frequently asked
if it is possible to test a theory of everything,
or whether such theories are just entirely
unscientific.
It’s a good question.
But before we get to the answer, I have tell
you exactly what physicists mean by “theory
of everything”, so we’re on the same table.
For all we currently know the world is held
together by four fundamental forces.
That’s the electromagnetic force, the strong
and the weak nuclear force, and gravity.
All other forces, like for example Van-der-Waals
forces that hold together molecules or muscle
forces, derive from those four fundamental
forces.
The electromagnetic force and the strong and
the weak nuclear force are combined in the
standard model of particle physics.
These forces have in common that they have
quantum properties.
But the gravitational force stands apart from
the three other forces because it does not
have quantum properties.
That’s a problem, as I have explained in
an earlier video.
A theory that solves the problem of the missing
quantum behavior of gravity is called “quantum
gravity”.
That’s not the same as a theory of everything.
If you combine the three forces in the standard
model to only one force from which you can
derive the standard model, that is called
a “Grand Unified Theory” or GUT for short.
That’s not a theory of everything either.
If you have a theory from which you can derive
gravity and the three forces of the standard
model, that’s called a “Theory of Everything”.
So, a theory of everything is both a theory
of quantum gravity and a grand unified theory.
The name is somewhat misleading.
Such a theory of everything would of course
not explain everything.
That’s because for most purposes it would
be entirely impractical to use it.
It would be impractical for the same reason
it’s impractical to use the standard model
to explain chemical reactions, not to mention
human behavior.
The description of large objects in terms
of their fundamental constituents does not
actually give us much insight into what the
large objects do.
A theory of everything, therefore, may explain
everything in principle, but still not do
so in practice.
The other problem with the name “theory
of everything” is that we will never know
that not at some point in the future we will
discover something that the theory does *not
explain.
Maybe there is indeed a fifth fundamental
force?
Who knows.
So, what physicists call a theory of everything
should really be called “a theory of everything
we know so far, at least in principle.”
The best known example of a theory of everything
is string theory.
There are a few other approaches.
Alain Connes, for example, has an approach
based on non-commutative geometry.
Asymptotically safe gravity may include a
grand unification and therefore counts as
a theory of everything.
Though, for reasons I don’t quite understand,
physicists do not normally discuss asymptotically
safe gravity as a candidate for a theory of
everything.
If you know why, please leave a comment.
These are the large programs.
Then there are a few small programs, like
Garrett Lisi’s E8 theory, or Xiao-Gang Wen’s
idea that the world is really made of qbits,
or Felix Finster’s causal fermion systems.
For references, please check the information
below the video.
So, are these theories testable?
Yes, they are testable.
The reason is that any theory which solves
the problem with quantum gravity must make
predictions that deviate from general relativity.
And those predictions, this is really important,
cannot be arbitrarily moved to higher and
higher energies.
We know that because combining general relativity
with the standard model, without quantizing
gravity, just stops working near an energy
known as the Planck energy.
These approaches to a theory of everything
normally also make other predictions.
For example they often come with a story about
what happened in the early universe, which
can have consequences that are still observable
today.
In some cases they result in subtle symmetry
violations that can be measurable in particle
physics experiments.
The details about this differ from one theory
to the next.
But what you really wanted to know, I guess,
is whether these tests are practically possible
any time soon?
I do think it is realistically possible that
we will be able to see these deviations from
general relativity in the next 50 years or
so.
About the other tests that rely on models
for the early universe or symmetry violations,
I’m not so sure, because for these it is
again possible to move the predictions and
then claim that we need bigger and better
experiments to see them.
Is there any good reason to think that such
a theory of everything is correct in the first
place?
No.
There is good reason to think that we need
a theory of quantum gravity, because without
that the current theories are just inconsistent.
But there is no reason to think that the forces
of the standard model have to be unified,
or that all the forces ultimately derive from
one common explanation.
It would be nice, but maybe that’s just
not how the universe works.
Thanks for watching, see you next week.
