Quantum mechanics is unquestionably a robust and successful theory — so far, all its predictions have held, and scientists can build powerful technologies based on it. Yet, understanding what it tells us about the nature of reality and how we experience it has proven tricky. Physicists and philosophers have been grappling with it for a century, ironing out some of the early ambiguities, but some conceptual problems remain. And the non-intuitive nature of quantum physics makes it fertile ground for misunderstandings. Here, six physicists explore the origins of widespread myths about quantum history, theory and applications.
MARIA VIOLARIS: Has quantum physics made time travel possible?
If you’ve been following quantum-science announcements in the past few years, you might think that experiments have managed to send quantum particles back in time. But despite intriguing theoretical proposals and experimental studies, that has not been achieved (yet).
The idea relies on exploiting quantum ‘time loops’ — hypothetical twists in space-time that allow a particle, or anything else, to come out of the loop at an earlier time than when it went in. These loops could exist in the Universe, for example through tunnels in the fabric of space-time.
A century of quantum physics
The recent proposals were based on quantum teleportation of qubits, in which the state of a qubit is transported from one location to another, without physically moving between them. This can be done by using an entangled pair of qubits, one at each location.
However, to avoid violating core principles, such as no faster-than-light communication, quantum teleportation can be successful at most only one-quarter of the time. For the remainder, the receiver needs to correct its teleported qubit using information from the sender. But researchers are looking into an alternative approach, in which they discard these failed cases, keeping only the successful one-quarter.
This selective version of teleportation has been proposed as a model for a quantum universe that allows time travel. Such a Universe could have laws of physics that automatically discard any paradoxical outcome arising from something changing the past. By following a similar protocol, but instead manually discarding certain measurement outcomes, researchers have achieved a quantum advantage in the field of metrology (the science of making precise measurements).
The experimental results look identical to those that would come from a real time loop, but the behaviour has been engineered from quantum entanglement. So, no one has really sent a particle to the past quite yet. But the general theory of relativity allows for time travel — and quantum models give promising ways to resolve its paradoxes. Quantum mechanics therefore could yet make time travel possible — but I’d need to read a paper sent back from the future to be sure.
ESTELLE INACK: Can quantum computers guarantee speedier calculations?
The promise of quantum computers and their abilities to solve a host of intensive computational problems — from how the quantum behaviour of electrons affects chemical reactions to optimizing routes in logistics — has spurred a booming industry that is attracting billions of dollars of investor cash. As excitement has grown, so has a misunderstanding about how quantum computers work, why they are potentially so powerful and fast at making calculations and what their limitations might be. It’s one thing to have a quantum computer, but another to extract the right answer for a complex calculation out of it. And it won’t simply speed up every existing application — we are not likely to need ‘quantum Word’ or ‘quantum Zoom’. Instead, they are promising tools for exploring very complex systems.
Quantum devices are sometimes said to offer power and speed by relying on quantum bits (qubits) that are both 0 and 1 at the same time; by contrast, classical bits are either 0 or 1. This is misleading. What happens instead is that a qubit exists in a superposition of 0 and 1 classical states. And each time a measurement is taken, it has a probability of being measured as either 0 or 1.
Could the Universe be a giant quantum computer?
When putting many qubits together, say N of them, to form a quantum computer, their quantum superposition spans the same mathematical space as 2N classical bits; this is often referred to as quantum parallelism with exponential speed-up. When a quantum computation is performed, the system outputs one single state from those 2N possible ones.
The computation must be repeated many times (although fewer than 2N times, which would be impossible when N is large) to build a probabilistic picture of the system: the outcome with the highest probability gives you the correct answer. This overhead could reduce the advantages of quantum computers over classical computers. Algorithms that increase the probability of obtaining the correct (most likely) outcomes from each calculation are crucial.
Another limitation of quantum computers is that quantum states are very fragile and need to be protected from interactions with their environment, which can disrupt them. Researchers are exploring clever ways to do this through error-mitigating algorithms.
Thus, quantum computers are indeed powerful machines that rely on quantum superposition and parallelism — but innovations in algorithms, hardware and software are also needed to harness their full potential.
SABINE HOSSENFELDER: Did Einstein reject the idea of entanglement?
You might have heard that what Albert Einstein referred to as ‘spooky action at a distance’ is technically known as ‘entanglement’, and that he insisted that entanglement couldn’t exist. Neither is true.
The ‘spooky action’ quote is a direct translation of the German phrase spukhafte Fernwirkung, which Einstein wrote in a 1947 letter to fellow physicist Max Born. He was referring to an idea that had long intrigued him — how to interpret the measurement process in quantum mechanics, which he had earlier described as relying on a “peculiar mechanism of action at a distance” (G. Bacciagaluppi and A. Valentini Preprint at arXiv https://doi.org/p2ns; 2006).
Mathematically, the measurement process in quantum mechanics is instantaneous. Say you want to measure the position of a particle. Before you do so, the equations allow the particle to be in several places at the same time. Observe or measure it, however, and suddenly it is in only one place.
Don’t believe the hype — quantum tech can’t yet solve real-world problems
This issue of reality apparently suddenly materializing out of uncertainty when you observe it is known as the measurement problem. The update happens faster than light, seemingly violating Einstein’s special theory of relativity, which says that no signal can exceed light speed. Of course, Einstein didn’t like it. That is why, together with physicists Boris Podolsky and Nathan Rosen, Einstein argued in 1935 that quantum mechanics must be an “incomplete” theory (A. Einstein et al. Phys. Rev. 47, 777; 1935), in which measurement is just a probabilistic description of an underlying physical reality.
That same year, Erwin Schrödinger coined the term ‘entanglement’ to describe a correlation between two or more objects about which one has incomplete knowledge. You could, for example, have two particles, one on the left and one on the right, that can each have a state (usually physicists consider the property of ‘spin’, but it could be something else, such as momentum) of either +1 or –1, and both values must add up to 0. So, either the left particle has spin –1 and the one on the right spin +1, or the other way around.
In an experiment, you can flip the spin of one particle, say the left one, even without knowing what it is. If it was –1, it is now +1; if it was +1, it is now –1. If you do that, what happens to the particle on the right side? Nothing. The other particle itself has not changed, and the two particles are still entangled — just the correlation between them has changed. You have changed an entangled system into a different, also entangled system. There is no ‘spooky action’ in entanglement, no exchange of information that is faster than the speed of light.
I think the reason why even some physicists get this mixed up is that in their 1935 paper, Einstein, Podolsky and Rosen used what we now call ‘entangled particles’ to illustrate the problem with the instantaneous update of a system on measurement. The two concepts — measurement and entanglement — became entangled, so to speak.
Einstein never claimed that entanglement, or quantum physics itself, is wrong. What he did was question the physical interpretation of the measurement: that a quantum system seems to exist in several possible superposed states but updates to a different state as soon as you observe it. That is an issue that still hasn’t been resolved.
Illustration: Sandro Rybak
NORMA SANCHEZ: Is general relativity irreconcilable with quantum physics?
Physicists have devised two grand theories to understand reality. The general theory of relativity dominates how things happen at large scales, such as across the cosmos. Quantum mechanics, meanwhile, covers atom-sized forces or smaller. Many physicists quibble that they might never be reconciled — although we have no real indication that it is not possible. In the past few years, progress and the potential for new observations, such as those of gravitational waves, gives me hope that we won’t need a completely new theory to encompass both.
In their current form, these theories produce pictures that are completely at odds with each other, impractical or unintelligible. Gravity, for example, is well explained by the general theory of relativity as a curvature of space-time in the presence of massive bodies. But because this formalism considers particles to have non-zero mass concentrated to a single point (with zero volume), following it at subatomic scales would make gravity infinite, which makes no sense.
The ‘quantum’ principle that says why atoms are as they are
There have been many attempts to reconcile the two frameworks. One is string theory — in which particles and forces arise from the vibrations of tiny one-dimensional ‘strings’. But this theory has run into problems: it does not explain the observed expansion of the Universe, or its structure, and no direct experiments have supported it. Other approaches that start with classical gravity and try to ‘quantize’ it have not succeeded either.
