How do space, time and gravity emerge from quantum physics?

In an earlier post I wrote how, according to the Copenhagen interpretation, not just matter but time is also created by the measurement.

Today I want to present to you my comments on a particularly interesting interview Steven Strogatz had with Sean Carroll which was published in Quanta Magazine on May 4, 2022. Steven Strogatz is a mathematics professor, Sean Carroll is a quantum physicist who studies quantum gravity. Carroll discusses the creation of time and space.

Einstein’s description of curved space-time doesn’t easily mesh with a universe made up of quantum wavefunctions. Theoretical physicist Sean Carroll discusses the quest for quantum gravity with host Steven Strogatz.

I found Carroll’s statements in this interview extremely captivating. It reveals a lot about the changing view of nature of quantum physicists. I therefore want to comment on some of his statements. You can find the full interview here. He has also published a book on the subject – the emergence of spacetime from the quantum world: Something Deeply Hidden.

Incidentally, Sean Carroll is also an advocate of the many-worlds interpretation of quantum physics. A hypothesis that is not mine. Read my post “Multiversa and the Double Slit“.

Relativity and quantum physics

To begin with, Carroll puts the importance of Einstein’s theory of relativity into perspective from the position of quantum physics:

C: ‘Yeah, you know, we think of relativity, the birth of relativity in the early 20th century, as a giant revolution in physics. But it was nothing compared to the quantum revolution that happened a few years later’.

Yet the relationship between time and space, between energy and matter became established by the special theory of relativity in an extremely revolutionary way, even though the theory is still considered part of classical physics. Time and space are elastic and relative to the observer. That elasticity can certainly no longer be called classic. The important role of the observer is already apparent, although Einstein has denied it. You could say that relativity has paved the way for the even more shocking message of quantum physics.

Quantum physics is the real fundamental physics – on all scales

C: ‘We’ve accepted that quantum mechanics is a more fundamental version of how nature works. Quantum mechanics is the theory of how the world works. What happens at small scales is that classical mechanics fails. So, you need quantum mechanics. Classical mechanics turns out to be a limit, an approximation, a little tiny baby version of quantum mechanics, but it’s not the fundamental one.’.

Finally someone who says it plainly. Quantum physics is not limited to the world of the atomic, it is a fundamentally more correct description of the world at any scale. Classical physics is the special case, which predicts very well on the limited scale of our senses.

C: ‘And we kind of tend to think of the world in classical terms. Classically, things have positions, and they have locations — positions and velocities. Quantum mechanically, that’s not true.

The experience of the world on the scale of our biological senses determines how we think about the world, about what we can imagine. There are things, permanently occupying a position in space. But it is the wrong picture. The world as we experience it, doesn’t really exist like those permanent things.

There is no procedure that will take you safely from classical physics to quantum physics

C: ‘So there’s supposed to be, in some sense a map from the space of classical theories to quantum theories, okay? The quantization procedure. This is all a complete fake. I mean, it sort of is a kludge that works sometimes, but this purported map from classical theories to quantum theories is not very well-defined’

We are still trying to understand quantum physics departing from a classical basis. That of concrete things. We used and still use a procedure to convert the classical description to the quantum physical, the quantization procedure. For example, physics students learn to translate to quantum physics from the classical basis they have learned before . But it’s not right at all. Quantization produces infinities in your equations. We were able to normalize these with mathematical tricks for the electromagnetic forces. But with gravity, those tricks no longer work. Quantization produces complete nonsense with gravity.

C: ‘But then there’s a whole set of more deep conceptual issues, not only do you not know what to do, you don’t know what you’re doing. Because, with everything else, every other theory other than gravity, it’s very clear what’s going on. You have stuff inside space-time. The stuff has a location, right? It has a point in space, it’s moving through time. Even if you have a field, it has a value at every point in space, etc.’

As long as you continue to use the classical concepts, such as objects in space and time, things will go wrong. You don’t really know what you’re doing. You don’t understand.

Before quantum physics it was obvious what a measurement was

This is where the major pain points are clearly discussed. In classical physics it was not necessary to describe things like observation and measurement, in quantum physics they are necessary, but we are still not in agreement on what exactly these are.

C: ‘So I don’t think that there is any such thing as a position or a velocity of a particle. I think those are things you observe, when you measure it, they’re possible observational outcomes, but they’re not what is — okay, they’re not what truly exists. And if you extend that to gravity, you’re saying that what we call the geometry of space-time, or things like location in space, they don’t exist. They are some approximation that you get at the classical level in the right circumstances. And that’s a very deep conceptual shift that people kind of lose their way in very quickly.’

There are no things with a position and speed. What may emerge from a measurement is not what already exists. That’s quite a statement, isn’t it? But Bohr and Heisenberg had already said this.

Energence of space and time

Then Carroll talks about his idea that spacetime is emergent in the same way that the macroscopic properties of a gas are emergent and arise from the atomic properties, whereby nothing fundamentally new arises during that emergence. That’s called weak emergence. Spacetime emergence is also a weak emergence according to Carroll. That means that there is a fundamentally different reality beneath macro-reality that we have to master first. So, don’t quantize our classic models, but set up something very fundamentally new. We must therefore say goodbye to the classical idea of locality. Which is the message of entanglement. Entanglement violates locality. Carroll then inverts the question of entanglement and locality, why is there so much locality in the universe that we perceive if it is not fundamental?

C: ‘Locality is just the idea that if I poke the universe at one point in space-time, the effects of that poke will happen at that point, and then they will ripple out…. So then, if you believe that locality is fundamental like that, then you’re sort of asking this question, why does the universe almost violate that but seem to not quite? That’s the puzzle that we have. It’s “why is there locality at all?”’

So, this is the question: can we infer reality as we experience it with its locality in space and time from what we know about quantum physics?

C:’ We just have an abstract quantum wavefunction and we’re asking, can we extract reality as we know it from the wavefunction? Space-time, quantum fields, all of those things’
C: ‘So, in the real world, we have, to a very good approximation, the world is run by what we call quantum field theory. Okay, so, the stuff of the world, the particles and the, you know, the forces, etc., all come from fields that spread all throughout space and time and have a quantum mechanical nature.

Space, time and entanglement

Could we perhaps establish the relationship between non-local entanglement and physical distance in spacetime via the quantum wave function?

C: ’Okay, so, the stuff of the world, the particles and the, you know, the forces, etc., all come from fields that spread all throughout space and time and have a quantum mechanical nature. The quantum state of the fields at these two points in space, is it entangled? And then what you can do is take two different points of space-time, at some distance between them, and because there’s still things there, because there still are fields even in empty space, you can say, is there entanglement between these two points of space? And the answer is yes, it is always going to be entangled. And in fact, more than that, if the points are nearby, the fields will be highly entangled with each other. And if the fields are far away, the entanglement will be very, very low. Not zero, but very, very low. So, in other words, there is a relationship between the distance between two points and their amount of entanglement in the lowest-energy state of a conventional quantum field theory. Let me assume, let me put out there as an ansatz [a mathematical assumption], that when the entanglement is strong, the distance is short. And I’m going to define something called the distance. And it’s a small number when the entanglement is large, it’s a big number when the entanglement is small. But the point is that if we follow our nose, if we say we start not with space, but with entanglement, how should it behave? How should it interact?

In short, the more powerful the entanglement between two points in space, the closer they are to each other. That’s Carroll’s hypothesis. In other words, we experience (measure) distance in space and time by quantum entanglement in the non-local quantum field! So, distance in space and time are no longer fundamental concepts. This makes the elasticity in dimensions in time and space experienced by the observer, which follows from the theory of relativity, a lot more comprehensible as far as I am concerned.

I’m curious where the developments are going. I see a paradigm starting to shift.

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