String theory advances philosophy. No, really.

I have a soft side, and I don’t mean my Snoopy pants, though there is that. I mean I have a liking for philosophy because there are so many questions that physics can’t answer. I never get far with my philosophical ambitions though because the only reason I can see for leaving a question to philosophers is that the question itself is the problem. Take for example the question “What is real?” What does that really mean?

Most scientists are realists and believe that the world exists independent of them. On the very opposite end there is solipsism, the belief that one can only be sure that one’s own mind exists. And then there’s a large spectrum of isms in the middle. Philosophers have debated the nature of reality for thousands of years, and you might rightfully conclude that it just isn’t possible to make headway on the issue. But you’d be wrong! As I learned on a recent conference where I gave a talk about dualities in physics, string theory indeed helped philosophers to make progress in this ancient debate. However, I couldn’t make much sense of the interest in dualities that my talk got until I read Richard Dawid’s book which put things into perspective.

I’d call myself a pragmatic realist and an opportunistic solipsist, which is to say that I sometimes like to challenge people to prove me they’re not a figment of my imagination. So far nobody has succeeded. It’s not so much self-focus that makes me contemplate solipsism, but a deep mistrust in the reliability of human perception and memory, especially my own, because who knows if you exist at all. Solipsism never was very popular, which might be because it makes you personally responsible for all that is wrong with the world. It is also the possibly most unproductive mindset you can have if you want to get research done, but I find it quite useful to deal with the more bizarre comments that I get.

My biggest problem with the question what is real though isn’t that I evidently sometimes talk to myself, but that I don’t know what “real” even means, which is also why most discussions about the reality of time or the multiverse seem void of content to me. The only way I ever managed to make sense of reality is in a layer of equivalence classes, so let me introduce you to my personal reality.

Equivalence classes are what mathematicians use to collect things with similar properties. It’s basically a weaker form of equality, often denoted with a tilde ~. For example all natural numbers that divide evenly by seven are in the same equivalence class, so while 7 ≠ 21, it is 7 ~ 21. They’re not the same numbers, but they share a common property. The good thing about using equivalence classes is that once defined one can derive relations for them. They play an essential role in topology, but I digress, so back to reality.

Equivalence classes help because while I can’t make sense of the question what is real, the question what is “as real as” makes sense. The number seven isn’t “as real as” my shoe, and the reason I’m saying this is because of the physical interaction I can have with my shoe but not with seven. That’s why, you won’t be surprised to hear, I want to argue here the best way to think about reality is to think about physics first.

As I laid out in an earlier post, in physics we talk about direct and indirect measurements, but the line that separates these is fuzzy. Roughly speaking, the more effort is necessary to infer the properties of the object measured, the more indirect the measurement. A particle that hits a detector is often said to be directly measured. A particle whose existence has to be inferred from decay products that hit the detector is said to be indirectly measured. But of course there are many other layers of inference in the measurement. To begin with there are assumptions about the interactions within the detector that eventually produce a number on a screen, then there are photons that travel to your retina, and finally the brain activity resulting from these photons.

The reason we don’t normally mention all these many assumptions is that we assign them an extremely high confidence level. Reality then, in my perspective, has confidence levels like our measurements do, from very direct to very indirect. The most direct measurement, the first layer of reality, is what originates in your own brain. The second layer is direct sensory input: It’s a photon, it’s the fabric touching your skin, the pressure fluctuations in the air perceived as sound. The next layer is the origin of these signals, say, the screen emitting the photon. Then the next layer is whatever processor gave rise to that photon, and so on. Depending on how solipsisitic you feel you can imagine these layers extending outside or inside.

The more layers there are, the harder it becomes to reconstruct the origin of a signal and the less real the origin appears. A person appears much more real if they are stepping on your feet, rather than sending an image of a shoe. Also, as optical illusions tell us, the signal reconstruction can be quite difficult which twists our perception of reality. And let us not even start with special relativistic image distortions that require quite some processing to get right.

Our assessment of how direct or indirect a measurement is, and of how real the object measured appears, is not fixed and may change over time with technological advances. It was historically for example much topic of debate whether atoms can be considered real if they cannot be seen by eye. But modern electron microscopes now can produce images of single atoms, a much more direct measurement than inferring the existence of atoms from chemical reactions. As the saying goes “seeing is believing.” Seeing photos from the surface of Mars likewise has moved Mars into another equivalence class of reality, one that is much closer to our sensory input. Doesn’t Mars seem so much more real now?

[Surface of Mars. Image Source: Wikipedia]

Quarks have posed a particular problem for the question of reality since they cannot be directly measured due to confinement. In fact many people in the early days of the quark model, Gell-Mann himself included, didn’t believe in quarks being real, but where thinking of them as calculational devices. I don’t really see the difference. We infer their properties through various layers of reasoning. Quarks are not in a reality class that is anywhere close to direct sensory input, but they have certainly become more real to us as our confidence in the theory necessary to extract information from the data has increased. These theories are now so well established that quarks are considered as real as other particles that are easier to measure, fapp - for all practical physicists.

It’s about at the advent of quantum field theory that the case of scientific realism starts getting complicated. Philosophers separate in two major camps, ontological realism and structural realism. The former believes that the objects of our theories are somehow real, the latter that it’s the structure of the theory instead. Effective field theories basically tell you that ontological realism makes only sense in layers, because you might have different objects depending on the scale of resolution. But even then, with seas of virtual particles, and different bases in the Hilbert space, and different pictures of time-evolution, the objects that should be at the core of ontological realism seem ill-defined. And that’s not even taking into account that the notion of a particle also depends on the observer.

For what I can extract from Dawid’s book it hasn’t been looking good for ontological realism for some while, but it’s an ongoing debate and it’s here where string theory became relevant.

Some dualities between different theories have been known for a long time. A duality can relate theories that have a different field content and different symmetries. That by itself is a death spell to anything ontological, for if you have two different fields by which you can describe the same physics, what is the rationale for calling one more real than the other? Dawid writes:

“dualities… are thoroughly incompatible with ontological scientific realism.”

String theory now not only has popularized the existence of dualities and forced philosophers to deal with that, it has also served to demonstrate that theories can be dual to each other that are structurally very different, such as a string theory in one space and a gauge-theory in a space of lower dimension. So one is now similarly at a loss to decide which structure is more real than the other.

To address this, Dawid suggests to instead think of “consistent structure realism” by which he seem to mean we need to take the full “consistent structure” (ie, string theory) and interpret this as being the fundamentally “real” thing.

For what I am concerned, both sides of a duality are equally real, or equally unreal, depending on how convincing you think the inference of either theory from existing data is. They’re both in the same equivalence class; in fact the duality itself provides the equivalence relation. So suppose you have convincing evidence for some string-theory-derived duality to be a good description of nature, does that mean the whole multiverse is equally real? No, because the rest of the multiverse only follows through an even longer chain of reasoning. You either must come up with a mechanism that produces the other universes (as in eternal inflation or the many worlds interpretation) and then find support for that, or the multiverse moves to the same class of reality as the number seven, somewhere behind Snoopy and the Yeti.

So the property of being real is not binary, but rather it is infinitely layered. It is also relative and changes over time for the effort that you must make to reconstruct a concept or an image isn’t the same I might have to make. Quarks become more real the better we understand quantum chromo dynamics in the same way that you are more real to yourself than you are to me.

I still don’t know if strings as the fundamental building blocks of elementary particles can ever reach a reality level comparable to quarks, or if there is any conceivable measurement at all, no matter how indirect. Though one could rightfully argue that in some people’s mind strings already exist beyond any doubt. And if you’re a brain in a jar, that’s all that matters, really.