*Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime*. This was not Dr Carroll's first Secret Science Club Rodeo, so I was prepared to be informed, entertained, and a little baffled by the quantum physics that Dr Carroll studies. After an introduction by Dorian and Margaret, Dr Carroll received a second introduction by Dr Brian Greene, Columbia University physicist and chairman of the World Science Festival.

Dr Carroll began his lecture with a humorous thought experiment, The Universe Splitter, a 'quantum-involved universe bifurcator'. He sent a signal to a particle accelerator in Europe, and the result of a sensor detecting a particle determined whether he would jump to the left or to the right. With the result he received, he ended up jumping to the left, and joked that, according to the Many Worlds interpretation of quantum physics, in another universe he had jumped to the right. He then noted that quantum physicists are good at making predictive models, but that they haven't succeeded in understanding the underlying reality. Regarding quantum physics, Richard Feynman once said: "I think I can safely say that nobody understands quantum mechanics." Quantum mechanics are necessary for the production of semiconductors, transistors, microchips, lasers, and computer memory... we use quantum mechanics though we don't understand quantum mechanics.

The classic Rutherford model of the atom consists of a nucleus surrounded by electrons in discrete orbits, but this model is incorrect. In this model, electrons should lose energy by radiation, which would mean that they spiral down to the nucleus, causing the atom to collapse. Electrons actually function in a wavelike fashion (a phenomenon known as the wave function). Even the single electron of a hydrogen atom should appear as a cloud, rather than a distinct particle. Wave functions over time are described by the Schrödinger equation, which describes the energy of a state and the rate of change- basically, how much energy and how fast it is moving. High energy states entail rapid movement while low energy entails slow movement.

When observed, electrons look like particles- Dr Carroll described wave functions as 'shy', they collapse when observed, appearing to be localized at specific values. This property of being undefined until observed is known as the Copenhagen interpretation, which posits two sets of rules for quantum mechanics- one when nobody is looking, another when there is an observer. This was considered by some physicists unacceptable as a fundamental theory of nature. There are two problems... the ontology problem can be summed up as 'what is wave function?' The measurement problem can be summed up as 'what does observation involve?'

Dr Carroll described Hugh Everett as a 'quantum therapist' who tried to get everyone to 'chill out'. He posited that wave functions don't actually collapse, but obey the Schrödinger equation. It is now known that the Higgs boson can decay into an electron and a positron. The Higgs boson has zero spin, while the resultant electron and positron have either upward or downward spin- when the Higgs boson splits, there is a conservation of spin as the resultant particles' spin cancels each other out. The particles are entangled, and as soon as the direction of spin for one is determined, that of the other can be determined as well. Hugh Everett proposed that there is really on one wave function, a Wave Function of the Universe with only one state, due to entanglement.

Dr Carroll then gave us a brief overview of the Schrödinger's cat thought experiment, comparing various classic and quantum models of the experiment. Everett posited that there was no 'collapse' of wave function on observation- in the experiment, the observer is in a quantum state as well as the cat, and opening the box involves a superposition, most people don't feel that they are in a superposition. Decoherence is the entanglement of quantum particles with the environment, and takes place before measurement can occur. Wave function splits represent different outcomes, and once diverged, will never effect each other again- as Everett described it: "It is as if they were in seperate worlds." When the divergence occurs, the universe isn't 'doubled', the existing 'amount of universe' is divided. Of course, there are objections to the 'many worlds interpretation'... the first is that it would result in 'too many universes'. The second objection is that the theory is not falsifiable, though Dr Carroll characterized falsifiablity as an outdated concept.

There are a couple of 'reasonable questions' regarding quantum mechanics... Why are probabilities of a particle being in a particular place, according to Schrödinger's equation, given by the square of the wave function? How does the classic world emerge from quantum processes? To answer the first question, probability is epistemic, not objective. Even unobserved wave functions can be explained by the equation. In finding the classic world, we still tend to privilege what we see over what is. Reality doesn't start off with the classic model and veer off into the quantum, it starts off in the quantum world.

Dr Carroll noted that interactions are local in space, that there is no real 'spooky action at a distance'- things interact with things that 'they bump into'. Space can be defined as the set of variables in which these interactions occur. Space is curved, and the distortion in space by objects is known as gravity. While there is a so-so 'quantum theory of gravity', Dr Carroll suggested that researchers not try to 'quantize' gravity, but to find gravity within quantum mechanics.

Quantum field theory posits that the universe is best modeled as a system of interacting fields, not particles. The relative proximity of objects is determined by their entanglement, and as more fields become entangled, geometry emerges... Dr Carroll advised us that this is just a hypothesis. The amount of entanglement of two systems is related to the entropy of either one. As particles are added to a system, entanglements are broken, increasing the energy and the entropy of the system. The geometry of spacetime can emerge from entanglement and the quantum wave function.

After this heady lecture, Dr Carroll advised us, "Stop doing whatever you are doing, and try quantum physics."

The lecture was followed by a Q&A session. One funny question involved a 'conservation of embarassment'- is there a theory which is less 'embarrassing' than the many worlds interpretation, which doesn't involve other worlds. Dr Carroll told the questioner, 'Don't worry.' There's no need to be embarrassed because many theories have fallen by the wayside. A question regarding the heat death of the universe received a quick, jocular, 'Are you waiting for it?' The universe is fourteen billion years old, the future may be infinite, but with increasing emptiness. Dr Carroll advised, 'Live your life now, you only have ten to the fifteenth years left.' Regarding quantum mechanics and consciousness, Dr Carroll quipped, 'Nobody really understands either.' Regarding the 'simulation hypothesis', Dr Carroll talked about how theorizing about the construction of artificially conscious creatures led to the question, 'Could we be such creatures?' He dismissed it with a curt, 'I don't buy it.'

A question regarding the probability of wave action being the wave function squared was answered with the assertion that wave functions can interfere with each other, and action tends to follow the path of least resistance.

Another question involved the possibility of freaking people out with such esoteric subject matter as the many-worlds interpretation, and Dr Carroll joked, 'Freaking people out is a feature, not a bug.' The narrative becomes weirder because it deviates more from the traditional views of existence. He noted that he is not existentially worried about the existence of other worlds, and that how you live your life should be no different if there are many worlds or just one.

The last question was a simple 'What about time?' That, the good doctor said, was another lecture.

I didn't get a question in during the Q&A session, but I did ask Dr Carroll about quantum models of dark matter and dark energy during the post-lecture book signing. He said that dark matter is boring, it'll probably be just another particle eventually discovered, regarding dark energy, he tantalizingly noted that there is a section about it in the book.

Once again, the Secret Science Club has dished out a fantastic lecture. Kudos to Dr Carroll, Margaret and Dorian, and the staff of the scintillating Symphony Space for delivering the goods. It was a night of heady subject matter, but Dr Carroll was able to cover it cogently and coherently. Here's a video of the good doctor discussing the many worlds interpretation:

Pour yourself a refreshing beverage and soak in that SCIENCE!

## 3 comments:

Very nice summary, well-written, well told. Wish I still lived up at 97th & CPW! Woulda' been there. Or maybe I was in another world

Thanks, Jim. It's a nice neighborhood. I have a family friend who is selling a townhouse up there for $4.9 million...

Have the Asian dogwood fruits ripened in your neck of the woods? It should be a week more before they are ready to eat here.

Haha. No idea about the fruits. But the blackberries are about the best ever. Cheers!

Post a Comment