Being retired, along with doing all my TV watching via streaming services, has the consequence of almost completely disconnecting me from the weekly rhythm. Weekends mean nothing when every day is Saturday. To create some structure, I follow a simple schedule. For instance, Mondays I do laundry and Thursdays I buy groceries.
More to the point here, Monday (and sometimes Tuesday) evenings are for YouTube videos, many of which are science related. Last night I watched Jim Baggott give two talks at the Royal Institution, one about mass, the other about loop quantum gravity (LQG).
It all begins with one of my favorite topics, light. As something that most humans and animals experience, it has a long history of thought attempting to explain the various phenomena associated with it. A key question throughout history has been whether it is wave-like or composed of particles (or, in early eras, “corpuscles”).
By the 19th century, work by Faraday and Maxwell convinced most scientists that light was a wave. But in the beginning of the 20th century, in the 1905 paper that won him his Nobel prize, Einstein demonstrated that light was a particle.
Ever since, we’ve been stuck with the mutually exclusive complementary duality that light has the properties of both a wave and a particle. Louis deBroglie (“deh-Broy”) ran with that ball and suggested the wave/particle duality applies to all matter. Experiments ever since have proved him correct. One of the most famous experiments in science demonstrated that individual electrons acted like waves and interfered with themselves.
So, we’re left with the unresolved conundrum that, when we’re not looking, matter acts like a wave, but when we do look, it acts like a particle. (It would be more accurate to say that matter waves have point-like interactions with other matter waves.)
Which brings us to gravity, which also has a long history of thought behind it.
For Newton, it was an unspecified but well documented and described force. (His arch-rival, Leibniz, called it “occult”.) At the time, it was thought to be instantaneous, but his theory was good enough to predict the existence of the planet Neptune. (However, it failed to predict the precession of the perihelion of Mercury’s orbit.)
Einstein (him again), with general relativity, equated gravity and acceleration, and his theory (in one of the greatest emotional experiences of his life) did accurately predict the precession in Mercury’s orbit. (Today, GPS requires his theories of relativity to be accurate.)
General relativity says that gravity is due to the acceleration we feel when mass warps spacetime. One consequence of this is that a moving mass can create gravitational waves, and in 2015 the LIGO collaboration detected the waves made by two black holes spiraling into each other and merging. They have since detected many more such occurrences and started a new kind of astronomy.
In quantum physics, matter (fermions) interacts via force particles (bosons), the most well-known is the photon, which carries the electromagnetic (EM) force. Many posit that the graviton should be the carrier of the gravity force.
If gravity is indeed a force along with the three acknowledged forces of quantum physics: the EM force, the weak force, and the strong force. But under general relativity, gravity is the warping of spacetime and not a force like those three.
So, the question is whether there is such a thing as a graviton.
And, hence, my Brain Bubble.
As mentioned above, when quantum waves interact, they do so as “particles” — the interaction localizes the wave as an apparent point. But classical waves, such as sound or water waves, interact as waves (and general relativity is a classical theory).
Which got me to wondering what it would look like to localize a gravity wave into an apparent graviton. If Einstein was right, it seems we would expect gravity waves to act like classical waves and never localize. And it’s hard to imagine what a localized gravity interaction might be. LIGO, for instance, is detecting waves, not gravitons.
One problem is that gravity is such a weak effect. The gravity of the entire Earth loses against the pull of a small magnet. Even the static force imparted to a balloon from rubbing it on your hair and sticking it to the wall is enough to defy gravity. At the scale of “particle” interactions, gravitons would be extremely difficult, if not impossible, to isolate.
The holy grail of quantum physics is a theory that unifies general relativity and quantum physics. As Baggott mentioned in his talk (and Lee Smolin wrote a book about), there are three paths: start with quantum physics and bring in general relativity; start with general relativity and bring in quantum physics; or find some entirely new theory.
What’s vexing is that both general relativity and quantum physics are amazingly accurate at predictions in their respective regimes. No one yet has anything better than a wild guess at how to unify them.
I’ve long liked the realism of general relativity, so I’ve hoped that Einstein’s theory would prove the more correct. One thing that strikes me is that our classical theories tend to be non-linear, but linearity is a key aspect of quantum mechanics. That makes me suspect that QM may not be the right path. I’ve also been suspicious of unitarity, but these are topics for another post.
Stay classical, my friends! Go forth and spread beauty and light.