Is the speed of light an absolute limit?

Sabine Hossenfelder contends that faster-than-light
communication is possible. Steven French provides an analysis.

Over at her YouTube Channel in her recent video ‘Why The Speed of Light Is Not An Absolute Limit’, Professor Sabine Hossenfelder offers one of the oddest solutions to the Fermi Paradox that I’ve come across so far. Put simply, the paradox asks, given that it’s unlikely we’re the only sentient beings in the entire universe, how come we haven’t yet made contact with any alien species? Numerous answers have been proposed, some more plausible than others (see the Wikipedia page at the previous link).

Hossenfelder’s suggestion is that it might be because the extraterrestrials are using superluminal (faster than the speed of light) means of communication, which we are ill-equipped to receive because of our commitment to the speed of light as the absolute limit on information transfer. Now, one might speculate that any civilisation sufficiently advanced to have developed something akin to the ansible in Ursula K. Le Guin’s novel The Dispossessed and her other Hainish cycle books, might also have the wit to realise that not all sentient creatures have reached that level of scientific progress and so would find a way of slowing things down enough for us to receive their messages: assuming of course they’d even want to communicate with us in the first place (but if we are going to go down that route, we do not need faster-then-light gizmos to answer the paradox).

So why is Hossenfelder so keen to give up on a principle which most physicists regard as a fundamental component of a theory – Einstein’s relativity – that has been experimentally confirmed time and time again? The answer to this question has to do with her interpretation of quantum mechanics.

At the very core of quantum theory lies a mathematical device known as the ‘wave function’ which is usually taken to represent the state of a system, such as a particle like an electron, say, as a superposition of all its possible states. That wave function evolves in time quite deterministically but when a measurement is performed on the system, something happens and only one of those possible states is realised, with a certain probability. What that ‘something’ is depends on the particular interpretation of the theory that is adopted. According to one set of views, the superposition is said to collapse, whether through the intervention of a conscious observer (not a popular view these days, despite what many pop-science commentaries would have you believe) or through the spontaneous action of a ‘collapse field’ of some sort. According to the so-called Many Worlds Interpretation, on the other hand, there is no collapse at all; rather, upon measurement, or indeed any kind of interaction, all the possible states encoded in the wave function are realised, but in different ‘branches’, from which alternative worlds then emerge.

All these interpretations come at a cost. In the case of the Many Worlds approach, it is not really the unbelievably large number of worlds created every second that’s the problem – after all, the theory itself states that they’re all inaccessible from one another (apologies to all the authors of multiple world-hopping science fiction stories [such as Pratchett and Baxter's Long Earth series, Carey's Infinity Gate, Gibson's Extinction Game, McAuley's Cowboy Angels and all the rest]). The real issue is to account for the probabilistic nature of quantum physics when all possible outcomes are actualised. Recent work has suggested ways of handling that but at the cost of redefining what we mean by probability.

Hossenfelder’s preferred option is a variation of what is known as a ‘hidden variables theory’. Again, putting things simply, such approaches posit that underlying quantum mechanics is a deeper theory involving variables which, being deeper, are ‘hidden’, and represent the actual state that the system has all along and which is then simply revealed upon measurement. The problem is, for these hidden factors to do what they are introduced to do, they have to act faster than the speed of light. And that tends to generate an immediate ‘thanks but no thanks’ from most physics commentators. Hence, the attempts by advocates of such hidden variable theories like Hossenfelder to throw out the principle that the speed of light represents an absolute limit.

At this point, the science savvy SF² C reader will no doubt bring up Bell’s Theorem. Proposed by CERN physicist John Bell in 1964, this apparently rules out these hidden variable approaches on the grounds that they can’t reproduce the results predicted by ‘standard’ quantum mechanics. And experimental tests have all come down on the side of the latter. However, the theorem depends on a number of assumptions, some explicit, others, not so much. One such is that the person measuring the state of the system can freely choose which property to measure; that is to say that they have free will.

Two particles may interact in such a way that their states are said to be
‘entangled’, in the sense that the state of one cannot be described independently
of the state of the other. So measurements made on one are perfectly correlated
with measurements on the other. Can these correlations be explained by some
underlying ‘hidden’ variable? Bell’s Theorem states that they can’t.
© NASA, used under its non-commercial copyright terms

Hossenfelder’s twist on this hidden variables line is to appeal to something called ‘superdeterminism’ which holds – again, putting things simply – that the observer has no such free will and that her choice of what to measure is just as determined as the state of the system. And with one bound she is able to leap through the loophole in Bell’s Theorem!

Her explanation as to why the rest of the physics community haven’t followed her is that, again to put it bluntly, they’re all ‘sheeple’ or as she declaims, in the grip of ‘group thinking’. Actually, I think the explanation is much simpler. First of all, most physicists don’t think of this stuff at all. They’re all too busy wrangling the mathematics or a recalcitrant piece of equipment or the latest draft of their grant proposal. And of those that do think about these sorts of things tend to weigh up the costs and, on balance, an increasing number find that accepting many (many!) worlds and tweaking our notion of probability is, overall, less expensive than abandoning the speed of light as a limit and, moreover, swallowing the idea that we have no free will.

Until physicists find a way of testing these various alternatives, this kind of cost-benefit analysis is the best we can do, Hossenfelder’s admonitions notwithstanding.

Steven French is a member of the SF²Concatenation book review panel. He is also a historian and philosopher of science (now retired) with a background in physics. His latest book is A Phenomenological Approach to Quantum Mechanics: Cutting the Chain of Correlations, Oxford University Press, 2023. He is also co-editor, with Juha Saatsi, of Scientific Realism and the Quantum, Oxford University Press, 2020, which presents a series of essays by leading philosophers of physics on explanation in the context of different interpretations of quantum mechanics.