Tag Archives: physics

Review: Carlo Rovelli, Reality is Not What it Seems

Carlo Rovelli is a big fan of loop quantum gravity, and of physics in general, and this book recaps the whole history of modern physics, at least partly in order to show how elegantly loop quantum gravity fits into place as a reasonable extrapolation. It’s an interesting and believable history, and the case for the plausibility of loop quantum gravity looks convincing to me. But then, I think I was an easy mark — since I already agreed with a series of strange (from the layperson’s point of view, at least) assertions Rovelli makes about known physics.

Rovelli inserts helpful diagrams every so often to summarize the history (and sometimes potential future) of “what there is” in the physical world according to physics. I can’t quite do justice to them so I use a table (please read it as one table).

NewtonSpaceTimeParticles
Faraday MaxwellSpaceTimeFieldsParticles
Einstein 1905SpacetimeFieldsParticles
Einstein 1915Covariant fieldsParticles
Quantum mech.SpacetimeQuantum fields
Quantum gravityCovariant quantum fields

In the transition from special relativity (1905) to general (1915), fields and spacetime are absorbed into “covariant fields”. This is because spacetime, Rovelli asserts (and I instinctively agree), is the gravitational field. So other fields like the electromagnetic field are covariant fields – fields that relate to each other in circumscribed ways. The curvature of spacetime depends on the energy (e.g. electromagnetic) present, and the behavior of electromagnetic fields depends on that curvature.

Rovelli likes to sum up some key features of each theory, and these summaries are very helpful. For QM, Rovelli lists three key principles:

  • Information is finite;
  • There is an elementary indeterminacy to the quantum state;
  • Reality is relational (QM describes interactions).

As a fan of Everettian QM, I don’t think we really need the indeterminacy principle. But it’s still true that we face an inevitable uncertainty every time we do a quantum experiment (it’s just that this is a kind of self-locating uncertainty).

Loop quantum gravity refines the “information is finite” principle to include spacetime as well. Not only are energy levels discrete; spacetime is also discrete. There is a smallest length and time scale. Rovelli identifies this as the Planck length (and time).

Rovelli explains loop quantum gravity as the quantization of gravity, deriving from the Wheeler-DeWitt equation. This equation can only be satisfied on closed lines aka loops. Where loops intersect, the points are called nodes, and the lines between nodes are called links. The entire network is called a graph, and also a “spin network” because the links are characterized by math familiar from the QM treatment of spin. Loop quantum gravity identifies the nodes with discrete indivisible volumes, and each link with the area of the surface dividing the two linked volumes.

Rovelli is at pains to point out that the theory really says what it’s saying. For example: “photons exist in space, whereas the quanta of gravity constitute space itself. … Quanta of space have no place to be in, because they are themselves that place.” This warning might seem too obvious to be necessary, but that’s because I didn’t reproduce the graphs of spin networks in Rovelli’s book. (I lack the artistic talent and/or internet skillz.) You know, graphs that sit there in space for you to look at.

OK, that’s space, but what about time (and aren’t these still a spacetime)? This deserves a longish excerpt:

Space as an amorphous container of things disappears from physics with quantum gravity. Things (the quanta) do not inhabit space; they dwell one over the other, and space is the fabric of their neighboring relations. As we abandon the idea of space as an inert container, similarly we must abandon the idea of time as an inert flow, along which reality unfurls.

[…] As evidenced with the Wheeler-DeWitt equation, the fundamental equations no longer contain the time variable. Time emerges, like space, from the gravitational field.

Rovelli, chapter 7

Rovelli says loop quantum gravity hews closely to QM and relativity, so I assume we get a four-dimensional spacetime which obeys the laws of general relativity at macroscopic scales.

In a section of Chapter 11 called Thermal Time, Rovelli uses thermodynamics and information theory to explain why time seems to have a preferred direction, just as “down” seems to be a preferred direction in space near a massive body. When heat flows from a hot zone into the environment, entropy increases. Since entropy reductions of any significant size are absurdly improbable, these heat flows are irreversible processes. And since basically everything in the macroscopic world (and even cellular biology) involves irreversible processes, time “flows” for us. Nevertheless, at the elementary quantum level, where entropy is undefined (or trivially defined as zero – whichever way you want to play it) time has no preferred direction. All of this will be familiar to readers of my blog who slogged through my series on free will. This is the key reason scientific determinism isn’t the scary option-stealing beast that people intuitively think it is.

There was one small section in Chap. 10 on black holes that seemed to fail as an explanation. Or maybe I’m just dense. Since spacetime is granular and there is a minimal possible size, loop quantum gravity predicts that matter inside the event horizon of a black hole must bounce. The time dilation compared to the outside universe is very long, so an observer would see no effect for a very long time, but then the black hole would “explode”. But surely “explode” is not the right word? Intuitively it would seem that any bouncing energy should emerge at a comparable rate to that at which it entered, at least for matter entering during a period of relatively stable Schwarzschild radius. Maybe by “explode” Rovelli just means the black hole would “give off substantially more energy than the usual Hawking radiation”?

BBC botches physics in series on free will

The BBC recently came out with a three-part series on free will. Part 2 is about physics. If you’re going to infer lessons from physics, it helps to get the physics right. They don’t. Part 2 of the BBC series can be found here: https://www.bbc.com/reel/playlist/free-will?vpid=p086tg3m

The picture above analogizes a series of physical events to a chain of dominoes, in order to talk about cause and effect. But there’s something odd about this metaphor, if the dominoes are supposed to represent the physical universe: look at that first domino, in black. What makes it tip over? Something from outside the universe, a “god” so to speak, intervenes to set the whole thing in motion. We seem to have jumped from physics to theology.

This would just be a nit-pick, if the negligent treatment of the “start” in the model did not affect the conclusions drawn. But it does, as we will see.

But first let’s look at some additional physics mistakes in the video. Jim Al-Khalili says “When we think we’re making free choices, it’s just the laws of physics playing themselves out.” Well no, the laws of physics alone don’t cause anything. The laws of physics are rather abstract. If you want to understand how a concrete action came about, you need not just laws of physics but also what physicists call “boundary conditions”, AKA concrete reality. Especially bits of concrete reality that heavily interact with the action in question. For example, you. Of course, perhaps Al-Khalili didn’t mean “just the laws of physics” quite so literally. But it matters how you phrase things, especially when you accuse people of only thinking they’re making free choices. Your grounds for calling them mistaken had better not be based on distorted depictions of the physics.

From the “libertarian” side of the philosophical debate, Peter Tse makes a different mistake – or maybe just poorly worded statement: “Patterns of energy don’t obey the traditional laws of physics.” Unless he means “classical physics” (in which case: say “classical”), that’s not true. The Wikipedia article on Lagrangian mechanics is a good resource for seeing just how deeply physics treats patterns of energy. “The kinetic and potential energies still change as the system evolves, but the motion of the system will be such that their sum, the total energy, is constant.”

Block universe as a loaf of bread, from BBC video

Since Einstein, physicists have known that space and time are not independent, but aspects of a single four-dimensional manifold, spacetime. For observers in different inertial reference frames, which direction counts as “time” will differ. A metaphor called the “block universe” is sometimes used to describe this, where we only depict two spatial dimensions and then repurpose the third to represent time. Jim Al-Khalili uses a loaf of bread, with different times being different slices.

The block universe is like a loaf. OK, let’s go with this metaphor: one end of the loaf is very hot (we call it the Big Bang) and the other is cold. There are certain patterns that stretch from one end of the loaf to the other. If we know the pattern (laws of physics) and we know the boundary conditions (full state of any slice) we can derive the state of any other slice. Why say that the hot end caused the cold end to be the way it is? Why not say that the cold end caused the state of the hot end? After all, the mathematical derivation works equally well in that direction. Better yet, why not admit that “causality” is a useless concept at the level of a complete description of the universe, and just look at the bidirectional laws of nature instead? Why not start your analysis in the middle (but nearer to the hot side), and work your way toward both ends? The last option is a lot more practical, since that middling point is where you are.

The idea that the Big Bang is the Big Boss and we are just its slaves has no basis in science. Remember that “god” that tipped over the first domino? He’s creeping back in through the back door of Al-Khalili’s thinking. He thinks the Block Universe is dominated by its early times. You can only get such domination by swapping out a scientific view of time and causality, and sneaking in an intuitive picture of time and causality in its place.

Al-Khalili does that when he says “The past hasn’t gone … the future isn’t yet to be decided.” The narrator does that when she says “every single frame of that animation already exists and will exist forever.” Argh, no! Time is within the loaf! If you’re going to use a metaphor, stick with the structure you used to create it – don’t sneak your intuitive conception of time into the background while leaving scientific time in the foreground, now portrayed spatially.

Al-Khalili says “the future … is fixed, even though we don’t know it yet.” This conclusion would repeal the very laws of physics Al-Khalili was claiming to honor. The future is dependent on us because, to repeat myself, laws of physics must be applied to boundary conditions to derive a prediction about the future, and those boundary conditions include us.

Modern physics does destroy the traditional “solution” to the “problem of free will”. What these commentators don’t seem to notice is that it also destroys the traditional “problem” of free will. When you notice that your intuitive ideas of time and causality conflict with science, you need to figure out the full consequences of the science, not take one point from science and then re-apply your intuitive ideas. The future isn’t set in stone. It’s set in spacetime. And spacetime is lighter than air.