Ethan Siegel addresses a question on whether spacetime is real.
But there’s more to the Universe than the objects within it. There’s also the fabric of spacetime, which has its own set of rules that it plays by: General Relativity. The fabric of spacetime is curved by the presence of matter and energy, and curved spacetime itself tells matter and energy how to move through it. But what, exactly, is spacetime, and is it a “real” thing, or just a calculational tool?
After going through a quick grand tour of special and general relativity, as well as other physics, he comes to the conclusion that science can’t really provide an answer.
This question about whether something is real or merely a mathematical accounting convenience, is one that comes up all the time in science, and has throughout its history. When Copernicus published his theory of the Earth moving around the Sun instead of the other way around, many were willing to accept his mathematics since they made astronomical predictions easier, but insisted it was only a mathematical convenience, not reality. Max Planck, when he first introduced energy quanta into physics, only considered them a mathematical tool.
But for spacetime, I think Siegel actually answers the question in the quote above by paraphrasing John Wheeler (a physicist known for coming up with quick snappy terms and phrases): “Spacetime tells matter how to move; matter tells spacetime how to curve.” In other words, spacetime, whatever it is, has causal effects on things we can measure. It can affect and be affected by matter and energy. That, to me, is enough to consider it real in some sense.
That doesn’t mean it’s necessarily something fundamental. A number of physicists think it might be emergent from other things, such as time perhaps emerging from entropy, or space emerging from quantum entanglement. But just because something is emergent doesn’t mean it doesn’t exist. If it did mean that, then nothing would exist above quantum fields, and maybe not even them.
In this view, all that’s necessary for us to productively consider something real is that it participate in the causal chain that eventually effects what we measure.
This is why, in quantum physics, I generally consider the wavefunction to be modeling something real. Something causes the measured interference effects, and the various formalisms for modeling the wave dynamics accurately predict those effects. (Even if they only provide probabilities for particle positions.) That doesn’t mean the wavefunction is necessarily the complete story, or that it’s real in every respect, only that the overall phenomena is something that participates in the causal chain.
But this is also why I’m not a Platonic realist, someone who believes that abstract objects exist independently of the mind. In the Platonic view, abstract concepts are supposed to exist outside of time and space, be unchanging, and causally inert. Platonic objects, in and of themselves, do not participate in the causal chain. If we consider them to not exist, there’s nothing about them that forces us to reconsider that judgment. Any actual causal power they might have, only seems to happen through our mental models of them and the relations in the world that encourage us to form those models.
So, if something has causal effects, it is, at least in some manner, real. If it has no detectable effects, or at least theoretical ones, we can’t say conclusively that it isn’t real, but it may effectively not be real for us.
What do you think of the causal criteria? Does it miss anything real? Or does it include anything we commonly would say isn’t real?