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).
In the latter, Baggott mentioned gravity waves and that generated a Brain Bubble.
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.
March 22nd, 2022 at 11:25 am
Here is the talk Jim Baggott gave at the RI about LQG:
March 22nd, 2022 at 11:25 am
And here is the Q&A session afterwards:
March 22nd, 2022 at 11:26 am
Here is the other Jim Baggott RI talk I watched last night. It’s about the concept of mass:
March 22nd, 2022 at 11:27 am
And here’s the Q&A session from afterwards:
March 22nd, 2022 at 11:34 am
I highly recommend The Royal Institution for great science talks of all kinds. If you’re interested in science, it’s a great YouTube channel to follow (one of my favorites).
March 22nd, 2022 at 11:22 pm
Watching another RI talk brought up another aspect that seems to make gravity different from the other three forces and associated quantum fields.
In quantum field theory (QFT), each “particle” — fermion or boson — is a ripple in its associated field. Electrons are ripples in the electron field, photons are ripples in the EM field, up quarks are ripples in the up quark field, and so on.
Gravitons, if they exist, would be ripples in the gravity field, but the gravity field is spacetime itself. Which seems rather different from all the other quantum fields.
There is also that, as with sound and water waves, which are disturbances in some medium, gravity waves are also disturbances in the medium of spacetime (which is what LIGO is measuring).
So, it really seems as if gravity is a very different beast.
March 23rd, 2022 at 1:55 pm
Very interesting! I think I’m ultimately a “unitarity” gal, always searching for the connection points and the insight that transcends contradictions. But at the same time I’m also happy enough with a messy multiplicity. Ambiguity doesn’t bother me! 😀
March 24th, 2022 at 10:18 am
In the human sense of the word, I couldn’t agree more! The notion of Yin-Yang is a cornerstone of my worldview; few things in life are just one thing.
Switching to my pedant’s cap, in the quantum world, unitarity has a precise mathematical definition that basically amounts to the notion that (quantum) information is never lost. It can be dispersed and impossible to recover, but it’s never lost. As such, it’s one of the foundations of the idea that physics is, at least in principle, reversible. Unitarity is a key article of faith in quantum physics, but it’s one I’ve begun to question.
(The black hole information paradox, for instance, challenges the idea that information is never lost. Which, in turn, challenges the idea of unitarity.)
March 24th, 2022 at 10:42 am
I would make a bad scientist because I just blithely assume that the unitarity answer is out there and we simply haven’t found it yet. 😄
March 24th, 2022 at 11:05 am
Quite to the contrary, an abiding belief in realism and that we can discover and understand nature would make you an excellent scientist!
March 24th, 2022 at 12:07 pm
April 7th, 2022 at 9:52 am
To paraphrase an old bit: “If gravity waves, wave back!” 😀
(The old bit is, “If the sea waves, wave back!”)
June 28th, 2022 at 2:54 pm
Here’s part of why we don’t know if gravitons exist:
Any measurement of one of the three quantum forces, as mentioned in the post, always localizes a wave to an apparent “particle”. OTOH, although we don’t directly experience the weak or strong forces, we do experience the EM force as light. And it seems smooth and wave-like to us. As does gravity.
But we can detect the photons in an EM wave. (Understand that the wave is always emitted from a “single-photon” source but is so weak that it can only trigger a single-photon interaction occasionally. A stronger EM wave triggers multiple photon interactions on a continuous basis. The sole difference is the strength of the EM wave.) We generally detect the strong and weak force by detecting what those “particles” decay to, usually some kind of hadron. (Which would involve multiple linked quark waves.)
So why not the graviton? Or its decay products?
Because of the huge strength gap between the three QM forces and gravity. As I noted in the post, the gravity of the entire Earth loses to the EM force of a small magnet. It requires a delicate mousetrap to catch a photon — a very sensitive trigger. And this is a domain with lots of noise; plenty of both failed and false detections. (Researchers often use a strategy of multiple detectors and only simultaneous detections to peer through the noise.)
Graviton interactions would be vastly below that scale. The noise would be huge, for one, and the detector sensitivity beyond anything we can currently envision. Graviton interactions are just too weak to detect. By any instrument we can make, gravity continues to look smooth and wave-like.
Yet I still think there are strong arguments against the graviton. I continue to wonder to what extent quantum gravity means recognizing general relativity as the smooth spacetime background to quantum interactions. Maybe they coexist. Maybe we can’t boil them down to a single theory.