Science Sunday with John Duffield: Quantum Gravity

You hear a lot about quantum gravity. For example in New Scientist you can read how it could take the singularities out of a black hole. That’s good. But in the same article you can read about black holes being portals to another universe. That isn’t.

There’s other things about quantum gravity that aren’t good. Like the confusion between curved space and curved spacetime, along with the idea that the speed of light varies with frequency. It’s as if some people don’t understand relativity or how gravity works. It’s as if they’ve never heard about the wave nature of matter or that objects fall at the same speed. And worse still, they don’t seem to understand the graviton:

Image Credit Julie Peasley

Image Credit Julie Peasley

The problem is that people think it’s a real particle. It isn’t. It’s a virtual particle. Check out the Wikipedia article on the graviton and search for the word “virtual”. There’s only one mention, and it’s tangential. Wikipedia is good, but it draws upon authoritative sources such as CERN and Fermilab. And they don’t make it clear that the graviton is a virtual particle. Or that the problem goes deeper than just the graviton. See where Wiki says this?

“The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by the gluons, and the weak interaction by the W and Z bosons”.

Sounds fair enough. But if you read what I said about dark energy you might recall Matt Strassler saying this: A virtual particle is not a particle at all”. That’s right, virtual particles aren’t real particles. They aren’t short-lived real particles that pop in and out of existence like magic. Electromagnetism isn’t mediated by photons, it’s mediated by virtual photons, and they aren’t photons. Instead they’re field quanta. It’s like you divide the field up into little chunks and say each is a virtual photon, even though there are no actual chunks. Then you do your calculations and get the right answer. Think of virtual photons as something like accounting units, like pennies in your bank account. When you get paid there is no storm of real pennies flitting through the air from your place of work to your bank branch. In similar vein there is no actual swarm of real photons flitting back and forth between the electron and the proton in the hydrogen atom. The virtual photons of QED are related to the evanescent wave which is a standing wave also known as the near field. But they aren’t actual photons. Hydrogen atoms don’t twinkle, magnets don’t shine, electromagnetism just doesn’t work like that.

Did somebody mention the strong interaction? It’s mediated by gluons, and guess what? See the Wiki gluon article, and note this: “as opposed to virtual ones found in ordinary hadrons”. Yes, gluons are virtual too. And how about the weak interaction? See the Wiki neutron article. A free neutron decays in about 15 minutes via the weak interaction, mediated by the W-boson. Only you don’t have to be the Brain of Britain to notice that the neutron mass is 939MeV while the W-boson mass is 80GeV. You don’t have to know too much about conservation of energy to know there can’t be many of them twinkling around inside a neutron. The bottom line is that all the “messenger particles” are virtual particles because it’s quantum field theory not quantum penny-storm theory. The same goes for the graviton, which is why gravitons aren’t pouring out of black holes. Nothing can get out of a black hole. Gravitons are just field quanta, like you divide the gravitational field into little chunks, even though there are no actual chunks.

To visualise these chunks, start with a real photon. You doubtless already know that It’s a singleton electromagnetic wave with energy E=hf and a small gravitational effect. And hopefully you now know that it isn’t surrounded by a swarm of other particles popping in and out of existence. But did you know that potential is more fundamental than field, and that Feynman wished he’d been taught it? Follow the clues and look at electromagnetic radiation on Wiki. Read the Derivation From Electromagnetic Theory section, and you’ll see this: “the curl operator on one side of these equations results in first-order spatial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field, is first order in time”. Then see The Role of Potentials in Electromagnetism by Percy Hammond and look at the sentence near the end-note: “We conclude that the field describes the curvature that characterizes the electromagnetic interaction.” See that derivative and curvature? Now draw the photon’s sinusoidal electromagnetic waveform. That’s your field, which is the derivative or slope of your potential. Then draw the potential underneath, grid-style, dividing space into little chunks like so:


Looking at the curved grid, the photon is wherever there’s curvature, hence many-paths. Any square that’s a skewed is a virtual photon. It’s spin1 because it isn’t a perfect parallelogram and you need to rotate it 360 degrees to look the same. Next imagine you’ve got a whole train of photons one behind the other, overlapping each other. This time when you draw the potential, there’s no electromagnetic curvature any more:



This is inhomogeneous space, which is what Einstein talked about in his Leyden Address. It’s a gravitational field. Any square that’s shortened is a virtual graviton. It’s spin2 because it looks the same when you rotate it 180 degrees. Now look at the curved grid again. Any square that’s skewed is a virtual photon, any square that’s shortened is a virtual graviton… and skewed squares are shortened! So there’s an overlap between virtual photons and virtual gravitons. Hence a particle like an electron doesn’t have an electromagnetic field that’s totally separate to its gravitational field. They’re just two aspects of the greater whole.


Anyway, that how I reckon it works. Sure it’s a bit of a rough sketch, and I might not have got it quite right. But I don’t think it’ll turn out to be a million miles off.


Time will tell.

4 comments on “Science Sunday with John Duffield: Quantum Gravity

  1. Mots
    December 1, 2013 at 10:21 am #

    My brain hurts…
    I think you are saying that forces overlap and are (potentially) all part of the same thing, yes ?
    What are the implications of this ?

  2. duffieldjohn
    December 1, 2013 at 12:29 pm #

    Yes. The implications are unification. Reconciling QM with GR. Putting gravity into the Standard Model.

    It’s a bit like what happened with electromagnetism. That’s why I used the phrase “greater whole”, which is in the Wiki Electromagnetic Field article. Take a look at Minkowski’s Space and Time, and see this bit near the back:

    “In the description of the field caused by the electron itself, then it will appear that the division of the field into electric and magnetic forces is a relative one with respect to the time-axis assumed; the two forces considered together can most vividly be described by a certain analogy to the force-screw in mechanics; the analogy is, however, imperfect.”

    It’s THE field, but there’s two forces. Electromagnetic field interactions result in linear “electric” force and/or rotational “magnetic” force. See Forensic Physics 101 for more about it:

  3. shorelark
    December 2, 2013 at 2:11 pm #

    Hi DJ

    I have a question for you. The Lienard-Wiechert potentials describe the action of one moving charge on another in terms of the retarded time; that’s to say the the time it takes for the effect to be transmitted, at speed C, from one to the other, and vice versa. That sounds like a photon to me; I may be wrong, but since I have no idea what a “virtual photon” might be, I’m backing the photon.

    The same goes for the graviton. The people at LIGO aren’t looking for “virtual gravitons” but the spin-2 counterparts of the spin-1 photon or the spin-1/2 neutrino.

    “Hence a particle like an electron doesn’t have an electromagnetic field that’s totally separate to its gravitational field. They’re just two aspects of the greater whole.”

    OK, but if you – collectively – are so smart, why aren’t you rich? I’ve heard NASA would pay quite a bit for a gravity screen or gravitational field generator.


    • duffieldjohn
      December 2, 2013 at 5:56 pm #

      Shorelark: if you accelerate a charged particle it emits a photon. There’s no problem with that. But check out the wiki article on the Lienard-Wiechert potentials. See where it says “an electron around an atom does not emit radiation in the pattern predicted by these classical equations”. The electron isn’t emitting any electromagnetic waves. There aren’t any actual photons twinkling back and forth between the electron and the proton in the hydrogen atom. Virtual particles called virtual particles because they aren’t real particles. They’re just arbitrary portions of a field, and that field can be perfectly static. Sure when there’s some kind of field change it can be likened to a photon, but you don’t get a burst of light when you turn on an electromagnet.

      In similar vein there aren’t any actual gravitons rattling back and forth between your pencil and the Earth.

      I’m not rich because I’ve spent my time doing paid IT work instead of thinking about how I can do something more dramatic.


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