What is quantum gravity?
This subject comes from ShannonKellie, who asked about the Gravity Probe B. I told her that I hadn’t heard of it, but it turns out that I do know something about what it’s trying to measure. Check out ShannonKellie’s wonderful books of poetry.
Gravity Probe B was launched by NASA to measure the Stress-energy tensor for the flux of energy and momentum in the spacetime curvature around Earth. You have to wonder what the heck is going on here, and why would NASA be doing something this esoteric. The answer is simple. They want to know.
The Stress-energy tensor comes from Einstein’s equations of general relativity. His idea is that spacetime curves noticeably around massive objects like planets and the sun. This Stress-energy tensor measures the effect of gravity on the distribution and motion of matter in space. Measuring this effect near Earth would be another verification of Einstein’s equations. The probe is designed to measure the geodetic effect on spinning gyroscopes. Essentially, what this boils down to is trying to determine a dragging or drifting effect on four gyroscopes relative to a distant star caused by the curvature of spacetime near Earth as predicted by Einstein’s general relativity theory. The math for this is mind-boggling. The result so far is a frame-dragging rate of -37.3 + or - 7.2 mas/yr. Mas/yr is milliarc-seconds, per year, which is equivalent to 2.778 time 10 to -7 degrees per year. This is a very tiny amount. Einstein’s equations would predict -39.2 mas/yr. That’s pretty close as far as I am concerned, but nitpickers might disagree.
But, what does this have to do with quantum gravity? The answer is elementary, my dear Watson. An understanding of quantum gravity is needed to reconcile Einstein’s General Relativity theory with the principles of quantum physics, which deal with the other three fundamental forces of nature, but not gravity. One way that this could be done is to introduce the idea of a graviton. A graviton is a theoretical massless boson particle with a spin of -2. The other three forces have messenger particles so why shouldn’t gravity? Good question!
The reason why a graviton is assumed to be massless is because it has infinite effect. In other words gravity works everywhere. This massless spin -2 particle must interact with the stress-energy tensor like the Newtonian gravitational field does. If this particle could be found, it would wed quantum theory with gravity, which would be a huge step forward for science.
The problem with this idea is that individual gravitons cannot be captured with any reasonable detector because they’re massless and have a very low cross section. A detector with the mass of Jupiter placed in an orbit around a neutron star, which has enormous gravity, would only detect one graviton particle every ten years and neutrinos would interfere with the measurement. So, don’t expect any physical evidence for a graviton’s existence anytime soon.
Thanks for reading.