Gravity is a fundamentally quantum phenomenon?

The opening of an article posted earlier this week sets the stage:

“A few month’s ago, Erik Verlinde at the the University of Amsterdam put forward one such idea which has taken the world of physics by storm. Verlinde suggested that gravity is merely a manifestation of entropy in the Universe. His idea is based on the second law of thermodynamics, that entropy always increases over time. It suggests that differences in entropy between parts of the Universe generates a force that redistributes matter in a way that maximises entropy. This is the force we call gravity.

What’s exciting about the approach is that it dramatically simplifies the theoretical scaffolding that supports modern physics. And while it has its limitations–for example, it generates Newton’s laws of gravity rather than Einstein’s–it has some advantages too, such as the ability to account for the magnitude of dark energy which conventional theories of gravity struggle with.

But perhaps the most powerful idea to emerge from Verlinde’s approach is that gravity is essentially a phenomenon of information.

Today, this idea gets a useful boost from Jae-Weon Lee at Jungwon University in South Korea and a couple of buddies. They use the idea of quantum information to derive a theory of gravity and they do it taking a slightly different tack to Verlinde.”

Full article here.

The South Korean team returns to a long conundrum in gravitational theory. When a particle crosses the event horizon of a black hole, does the Universe’s entropy increase or decrease? The study of that question – and the application of classical thermodynamic thought to black holes has created a subfield in physics of Black Hole Thermodynamics.

It’s the work in this field that has given rise to the Holographic Principle first proposed by Gerard ‘t Hooft that says in a nutshell:

[T]he entire universe can be seen as a two-dimensional information structure “painted” on the cosmological horizon, such that the three dimensions we observe are only an effective description at macroscopic scales and at low energies.

It’s the question of what happens specifically to particles at the event horizon boundary that led the South Korean group to this line of reasoning:

Jae-Weon and co assume that this erasure process must occur at the black hole horizon. And if so, spacetime must organise itself in a way that maximises entropy at these horizons. In other words, it generates a gravity-like force.

That’s intriguing for several reasons. First, Jae-Weon and co assume the existence of spacetime and its geometry and simply ask what form it must take if information is being erased at horizons in this way.

It also relates gravity to quantum information for the first time. Over recent years many results in quantum mechanics have pointed to the increasingly important role that information appears to play in the Universe.

Now this is way way too new for it to be considered a respectable theory. But it’s very intriguing. And it does what fundamental insights in Physics often do – explain multiple puzzles all at once.

If this idea is true, it explains trivially why physicists working on Unified Field Theory haven’t been able to find a useful way to include gravity into the Standard Model. It’s because Gravity isn’t a fundamental force. It’s a macroscopic experience of a quantum phenomenon. Rather than try to include gravity into quantum physics by looking for a graviton, we include quantum physics into gravity by first principles.

And what I find particularly interesting here, given the discussion I posted a couple of weeks ago, is that this moves the boundary of quantum experience to the very edge of the Universe. Rather than worrying about how to manage the transition between the quantum regime and the classical regime, we now might think of classical physics as a particularly useful calculation trick that allows us to quickly work out the mechanical evolution of macroscopic systems.

And philosophically, does this now mean that determinism and positivism are fully dead? If experimental evidence is showing that absolutism is an illusion, and philosophy is concerned with the pursuit of truth, then what profit is there in following a dead end trail?

Which means that we (the Church) may have come to the end of the line of the Enlightenment’s religious formulation of the Reformation. Calvinists will probably not be happy. The Greek Orthodox and the Process types should be ecstatic…

My point is that there is an awful lot riding on this question of thermodynamic gravity. It’s no where near settled, but it sure is going to be fun to watch it and the attendant philosophical conversation develop.

(And I really do mean to try to write down something about the reason this question of where the boundary between the classical and the quantum realm is found is such a big deal for us in theology… Hopefully after Easter Day.)

Author: Nick Knisely

Episcopal bishop, dad, astronomer, erstwhile dancer...

4 thoughts on “Gravity is a fundamentally quantum phenomenon?”

  1. Let me get this straight: quantum mechanics works at traditional quantum length scales and at cosmological length scales, but we get classical physics in between? Wild.

  2. I know! That’s what’s so amazing about this if it works out. We just happen to live on a classical island length scale.
    Though actually, that’s true no matter whether this theory is accepted or not. Classical physics basically only works on the human length scale. At the very small or very large scale it gives consistently wrong answers.

  3. I don’t claim to understand this, but it makes sense on one level: Classical physics is the physics we developed before having access to any length scales other than human ones. Perhaps it makes sense that we get some surprises now that our horizon extends a bit further.
    I am beginning to imagine the uses of this in science fiction. Instead of traveling at warp speed, once could simply tunnel across the galaxy. Neat.

  4. And, continuing the science fiction idea – you could send an instantaneous binary signal great distances if you simply had two strongly entangled electrons (or photons) that you could flip back and forth.
    Which means Star Trek’s subspace radio would be achievable!

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