Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics.
Where is the contradiction between quantum physics and Einstein’s gravity?
Rμν−12gμνR=8πGT^μν.
This is Einstein’s field equation. Essentially, this equation is general relativity. The left-hand side represents the geometry of spacetime. The right-hand side, the energy, momentum, and stresses of matter.
What this equation describes, in the words of Wheeler, is this: Spacetime tells matter how to move; matter tells spacetime how to curve.
But look closely. That T
on the right-hand side. It has a hat.
It has a hat because it is a quantum-mechanical operator. Because we know that matter consists of quantum fields. So it is described by operator-valued quantities (Dirac called them q-numbers). They are unlike ordinary numbers. For instance, when you multiply them, the order in which they appear matters. That is, when you have two operators p^
and q^
, p^q^≠q^p^
most of the time. So they are definitely not like numbers.
When Einstein wrote down his field equation over 100 years ago, the T
did not have a hat. But that’s because they didn’t know about operator-valued quantities at the time. Now we do. So I have to put the hat there.
But there are no hats on the left-hand side. And because of that, my equation might as well say something like, some apples = some oranges. It makes no sense. The stuff on the left-hand side (which consists of numbers) can never equal the stuff on the right-hand side (which definitely does not consist of numbers.)
I can make it work, though. I can replace that operator with its so-called expectation value:
Rμν−12gμνR=8πG⟨Tμν⟩.
This is called semiclassical gravity. And it works well, very well indeed. A little too well, as a matter of fact. Gravity is so weak, quantum effects are so irrelevant, this equation accurately describes Nature everywhere we can look. But we still don’t like it, because using that expectation value trick is a cheat, a cop-out.
Now you might wonder, why don’t I put hats on top of the things on the left-hand side? I would… if I knew how to quantize spacetime. That is, how to turn the numbers that describe gravity into quantum-mechanical operators.
But I do not. And nobody does. The standard methods all fail, leading to equations that make no sense at all.
So we are kind of stuck… we don’t know how to quantize gravity, and our observations don’t help us, don’t offer any hints as to how to get beyond semiclassical gravity. Theorists keep trying to come up with new ideas (or recycle old ones) but basically, we’ve been pretty much just spinning our wheels for decades.