Little things can come together to make big things. And those big things can often be successfully described by an approximate theory that can be qualitatively different from the theory of the little things. We say that a macroscopic approximate theory has “emerged” from the microscopic one. But the concept of emergence is a bit more general than that, covering any case where some behavior of one theory is captured by another one even in the absence of complete information. An important and subtle example is (of course) how the classical world emerges from the quantum one.
And here is the Q&A video. Sorry, I hadn’t realized that comments were showing up here on the blog! I have a crack team rushing to get that fixed.
In the video I refer to a bunch of research papers, here they are:
Mad-Dog Everettianism: https://arxiv.org/abs/1801.08132
Locality from the Spectrum: https://arxiv.org/abs/1702.06142
Finite-Dimensional Hilbert Space: https://arxiv.org/abs/1704.00066
The Einstein Equation of State: https://arxiv.org/abs/gr-qc/9504004
Space from Hilbert Space: https://arxiv.org/abs/1606.08444
Einstein’s Equation from Entanglement: https://arxiv.org/abs/1712.02803
Quantum Mereology: https://arxiv.org/abs/2005.12938
Professor Carroll giving a lecture about a paper he mentioned in the Entropy Q&A
Quantum Circuit Cosmology – S. Carroll – Workshop 1
https://www.youtube.com/watch?v=srL8bZ-JLQk
Would like to be able to share comments, Mr. Carroll. Two point type is difficult for me.
Great series of lectures!
In your last diagram I think that if the microscopic theory and the macroscopic theory have overlapping domains in reality, they must also have an overlap in predictions.
Looking at AlphaGo, which category of emergence does it match? Or is it emergence at all? Taking apart its bowels, we will see nothing but off-the-shelf microprocessors made of silicon and obeying the rules of electromagnetism and the instructions of some ingenious programmers. But still, in certain aspects, we are tempted to compare its behaviour with a human brain: neuronal network, self-learning capability, challenging the world’s best Go players.
Very enlightening series, thank you.
Just to see if I get this straight, would it be correct to say that fields are basic and particles are emergent?
If we think of emergence in such way, that it’s already entangled – where to draw borders? Can we think parts of wave function having only curvatures of connection?
Weak and strong emergence:
The substrate is what it is but doesn’t substrate independence hint at a a game-changing cut-off between the substrate and the emergent phenomena? I’m thinking of natural selection, a simple mechanism that doesn’t seem to be present in “elementary” physics and which we could imagine working on a very different substrate. Lee Smolin (I think) used the concept to speculate about the evolution of universes and NS is routinely modelled on computers.
This debate makes me think of the philosophical question in the foundations of maths: where do numbers and maths come from?
Don’t want this series to end
Hello Prof Carroll,
Can we consider Quantum Mechanics as emergence of two separate theory.
– Particle Phyics from Classical Realm
– Wave Theory from Classical Optics.
Hence, Quantum Mechanics is two distinct subsets- almost disjoint. Localised wavefunction with wavelength in consideration is small so its particle otherwise wave. Because as per my limited knowledge QM give solution to duality under limit of wavelength
Would you like to say something about space/time/space-time as emergent properties?
Don’t think its related to emergence but I don’t have a picture in my head for SR – but the maths is “easy”. The picture for GR of space warping is fine but the maths! Can you give me a picture for SR?
Thanks again for some provocative thoughts.
I’m thinking that the Classical Hall Effect and the Quantum Hall Effect might be an example of the Ehrenfest theorem? Plus, it seems that it makes use of “wave packets” and possibly “kinks”. Here’s a nice, short video.
https://www.youtube.com/watch?v=lVorIGNOtsg
Does quantum entanglement mean that we are variables in the emergence equation?
I think you said that a classical theory of bosons (Sine-Gordon) and a classical theory of fermions (Thirring) after quantization yield the same quantum theory. How can we have classical theories of bosons and fermions? Aren’t bosons and fermions fundamentally quantum mechanical (the result of quantum statistics & spin quantization)?
Brilliant! Thank you for making this lecture. Could you please recommend further reading on emergence in the physics literature. I find the philosophy literature on this topic not too enlightening.
Well done Sean, as always.
Since it is so hard to quantize gravity and make it fit with the standard model, and since we have things such as the AdS/CFT correspondence, could gravity in our own 3+1 dimensional space time possibly be emergent somehow from a gravity-free quantum theory in another space?
Hmm, can’t see any other comments..? Oh well, here goes nothing.
Should we take dualities like AdS/CFT to be evidence that space may be emergent in our universe? I would guess that people have been looking for dualities for our spacetime but haven’t found any yet?
Thanks!
Physicists place an awful lot of faith in Leplace’s demon, but is such faith warranted? That is, is reality deterministic without remainder? Yes, the reductionist program has achieved remarkable success, but when we reach the social sciences, we find that reality is parsed in ways radically different from how they are in physics. Take, for instance, economics. Money can be an electronic value in the bank’s computers, a piece of paper, or hard specie. Take medicine. The human body is continually ingesting air and nutrition, repairing its machinery, and excreting waste products. The human body replaces itself every seven to ten years. In short, in the social sciences, things are parsed according to our human world, and in physics, things are parsed in terms of wave functions and particles. Even if we could completely calculate the wave function of the entire universe, at the level of physics our calculations would not yield anything of interest, because of how differently reality is parsed. Put aside, for a moment, your faith in a unified world in which God knows all the truths of the world. The truth is that it is we humans who have created the various sciences, one at a time, and we created them haphazardly. We did not create them so that they can be reduced one to the other. Humans created the human world to get things done. In this world, notions like the self, awareness, free will are essential. Physicists created physics to have models of microscopic and cosmological reality. In this worldview, phenomena are local, unaware, and purposeless. If the world of physics and the human world really are so disparate, down to the level of how the very things in their ontologies are parsed, if there really is no God insuring that all our theories are conformal, then strong emergence ought to be a real possibility. In short, things like the self, awareness, and free will are strongly emergent. The philosopher Dan Dennett does a wonderful job of explaining how the human world emerges, via cranes, from the biological world (see his From Bacteria to Bach and Back). I would say that this emergence is strong emergence. I hope that this makes sense.
Can there exist two viable micro theories that are completely independent of each other — or as independent as possible — where they connect to each other only in the emergence of a macro theory? This macro theory would — if elegance rules — be equally dependent on both micro theories.
I like to browse through the comments ahead of the Q&A episode but they’ve all disappeared. (?)
……..including the one I just posted.
Hi doctor Sean! Thanks for the great video!
Please, let me share my view about emergence.
I guess its understandable that the physics of fluids emerges from the physics of atoms, since the only thing atoms can do is attract or repel each other, and it’s the same with fluids, and how it interacts with everything.
But you cant make a car out of fire, or gravity, since the car needs to move and be solid, and fire is just a state of matter, it can’t be solid.
The thing is: an emergent phenomenon has to have the same “essence” as the thing it emerges from.
We cant derive consciousness from neurons turning on and off, or even from atoms attracting or repelling each other. Their nature, their essence, are different.
Like, let’s think that, instead of atoms, the universe were made of giant iron balls that repel and attracts each other at different rates. We can think of a car emerging from it, but it would be nonsensical to believe that, somehow, when this balls attracts or repels each other in some specific way, suddenly we would have the taste of vanilla.
Of course there’s an very intimate correlation in the neurons firing and consciousness, but correlation doesn’t mean causation.
some more speculative theories discuss space and time as emergent phenomenon. could you perhaps touch on what this means?
Hi Sean,
Thanks for the great video! I have a few questions:
– does micro/macro emergence always define a version of entropy, like molecules/gases do with Boltzman entropy?
– is the reverse true: do the various definitions of entropy somehow implies emergence from something more elementary?
– if so, what’s emergent in entanglement entropy?
Hi doctor Sean! Thanks for the Q&A video! I am inspired by your thoughts on how to drive the world from the mad dog everettian perspective and I started to wonder what would take to a theory of quantum mechanics to allow free will… I guess that it should be something about controlling the synapses of the neurons… and that should be about controlling the ion pumps in the neurons… Maybe some kind of field that collapses the state of the ions pump into open or closed… Like if the probabilistic nature of quantum mechanics became violated and gets a definitive value due to the will power or something…
Anyway, thanks for the video!
Sean, may be it’s time to talk about inertia?
The Q&A became a “bonus episode”. Thanks so much.
At some point, could you comment on the “hierarchy problem” and “naturalness” as higher order, guiding principles for theory? I suppose you could throw “beauty” in there too… Wouldn’t Nathaniel Craig be an interesting Mindscape guest?
Thanks again.