Quantum physics is relative too
Issue #55
It’s very popular these days to emphasise differences between quantum physics and the theory of relativity. Some people even maintain that some of these are irreconcilable, which is why – or so proponents of this view maintain - we will not be able to quantize gravity (gravity being captured by the general theory of relativity).
Here, I want to counter such sentiment and instead discuss one crucial similarity between the two theories. In fact, if it were not for this similarity, probably the theory of relativity would not make sense in the first place! The similarity I have in mind is that things in quantum physics are observer-dependent too.
A few comments are in order. If you’ve read my previous blogs, you will know that I don’t think that observers are needed anywhere in physics. I haven’t changed my mind; it’s just that I am using the concept of “observers” simply as an educational tool in this particular blog. Everything could be said (but in a less transparent manner) without observers. Secondly, everyone knows that in Einstein’s relativity, it is space and time that are relative to the observer (namely, they depend on the speed of the observer). The key point here is that the concept of simultaneity is relative to the motion of the observer. In other words, two events that appear to happen at the same time as far as I am concerned will not be happening at the same time for another person traveling at some velocity with respect to me.
So far so good, but what is the relativity in quantum physics? The answer is that the state that we ascribe to a physical system is relative to the observer in quantum physics. For instance, I may see an atom here or there, but for you, the atom may still be in a superposition of the two states or even a mixture of the two states of here and there. Just think about Schrödinger's cat experiment. As far as the cat is concerned, she may well be either dead or alive (i.e., in a definitive state), but a person who has not interacted with her will have to accept that the state is a mixture of one or the other.
Now, the crux of what I’d like to communicate and discuss is that if things in quantum physics were not relative in this way, we would have a problem with relativity in the theory of relativity too. The reason is that we should be able to think of getting a definitive outcome in quantum physics as a relativistic event. Something happens when we make a measurement and obtain an outcome. The outcome itself constitutes a relativistic event, and we can attribute a space and a time coordinate to it (dependent on the observer's state of motion as before).
So, if in quantum states of physical systems were absolute (a dead cat is dead as far as everyone in the universe was concerned), this would be a problem for relativity because what’s simultaneous for one observer is not so for other observers. Take the famous example in which two observers are moving away from each other at a constant speed. Each observer thinks that the other one ages less! Pushing this to its most dramatic conclusion, in each reference frame, the observer in that frame will die before the other observer has died. If quantum physics were not relative in the same way as relativity, we could never have a unified theory of quantum physics and special relativity (called quantum field theory, a topic I covered at great length in the past).
So, we may well think that both quantum physics and relativity are counterintuitive (and they certainly are) but their main counterintuitive features are exactly the ones that make them compatible. Whether the same will turn out to be true for general relativity is an open question; however, as far as I am concerned, this is a real possibility.
In my conclusions, I'd like to get more philosophical. We have two main interpretations of quantum physics and two main interpretations of the special theory of relativity. The quantum interpretations I have in mind are the Copenhagen interpretation and the Many Worlds interpretation (or what I would more generally like to call “Everything is a Quantum Wave” interpretation). The interpretations of relativity are the Einsteinian and the Lorentzian ones.
The remarkable thing (at least to me) is the similarity between Einsteinian relativity and the Many Worlds interpretation on the one hand and between the Lorentzian relativity and the Copenhagen interpretation on the other hand.
Here is what I have in mind. The issue in quantum physics is whether we think of it as applicable at all scales. The same is true for relativity. If you think the answer is yes to both, you subscribe to the Many Worlds interpretation and Einsteinian relativity. In this case, you think that there is no place for an absolute classical world outside of the quantum domain, and you also think that all motion at constant speed is relative, in the sense that there is no absolute frame of reference anywhere in the universe with respect to which to measure your own speed. In either case, there is no place for absolute observers (the ones whose outcomes are true everywhere and everywhen).
On the other hand, if you believe that an outside classical context is needed to make sense of quantum measurements, you are a Copenhagen supporter (or you believe in the collapse of the quantum superpositions, but this is then not any interpretation, it’s a different theory to quantum physics altogether). As far as relativity is concerned, if you think that the universe contains an absolute frame (what used to be called aether, but many things could play this role such as the cosmic Microwave background), then you are a Lorentzian as far as the interpretation of relativity.
Of course, your beliefs could also be crisscrossed. You could be a Lorentzian Many Worlder or an Einsteinian Copenhagen supporter. These are interesting alternatives as the choice seems bipolar: you think that quantum physics applies to everything, but the principle of relativity stops at some point where the absolute frame starts; or, you think that quantum physics needs a classical boundary, but the fact that everything is relative is universal. Both are logically possible.
These questions are not just interpretational. If we establish experimentally that some objects are only classical and that some frames of reference are absolute (establishing such things could be challenging, but – at least in principle – doable), then it is clear that Copenhagen and Lorentz would be vindicated.
It is also possible, as I have been advocating, that the two issues in quantum physics and relativity are connected. Namely, the objects that we prove behave only classically could also provide the absolute frame in relativity! This option is interesting to contemplate, as it would reduce two mysteries to one; however, it would force us to look for new theories that upgrade both quantum physics and relativity. Win-win, I’d say.
Take care of yourselves,
Vlatko
Hello Vlatko, good afternoon.
I've read your (and others') article on QGEM (Quantum Gravity Entanglement between Masses) and I think it's a very interesting experiment to determine whether gravity is quantum or not. I hope you can carry it out in the coming years.
I'm pleased to inform you that I've ordered your new book, "Portals to a New Reality," from Amazon UK. It will be published in October and will be delivered on November 9, 2025. I'll let you know what I think once I've read it. Although I live in Spain and some friends and I form a group analyzing new developments in Quantum Physics, your blog articles and those already published are very interesting. I encourage you to continue advancing in this important field.
Best regards.
There is a more satisfactory cross-hypothesis. It reconciles the two seemingly contradictory positions — it also opposes determinism (classical observer) and indeterminism (no observer, which is annoying: who runs this blog?).
The hypothesis is that of the whole/parts principle, the 'whole' having an existence as concrete as the 'parts'. It is an independent existence, not reduced to the sum of the parts. This independence can only arise in an additional dimension of reality, which is the complex dimension. A new physical framework to consider, but one which has the enormous advantage of integrating all the others.
Applied to the problem you cite, Vlatko, the solution in this framework becomes simple. The superimposed quantum states are the parts, the 'whole' is the deterministic configuration of these associated probabilities. Each set of field excitations constructs its own 'observer,' which is the whole corresponding to the configuration of the probabilities of the different states of the system.