Every now and then, it is worth pausing for a second and giving thanks to the many, ever so slightly different versions of you that exist in parallel realities. It is these alternative selves that help to keep these universes in balance.
At least, that is what鈥檚 going on if you happen to subscribe to the many-worlds interpretation of quantum theory. First proposed more than 65 years ago, the idea is that reality is constantly splitting off into parallel paths, due to subtle interactions at the level of quantum particles. Though it may boggle the mind, it also smooths over some devilishly tricky problems in physics and, for that reason, plenty of clear-eyed physicists believe it to be true.
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But now this strange idea might be facing a huge challenge, thanks to physicists Sandu Popescu and Daniel Collins at the University of Bristol, UK. They initially set out to solve a 100-year-old puzzle in quantum theory, but ended up undermining the idea of parallel universes. 鈥淲e鈥檝e essentially demolished one of the arguments for it,鈥 says Collins.
It might sound like a destabilising development, but it may actually prove to be a shot in the arm for quantum theory. Already, Popescu and Collins鈥檚 work is helping to resolve other long-standing quantum paradoxes and, in the eyes of some theorists, it points to a fresh way of thinking about the cosmos as a singular quantum reality built from the inside out. 鈥淭his is something deep and new. I think it could become really important,鈥 says Nicolas Gisin, who researches the foundations of quantum theory at the University of Geneva in Switzerland.
The laws of conservation
The story of these outlandish results starts with a basic principle that has run through the bedrock of physics since long before quantum theory was conceived. That principle is called conservation and it applies most famously to energy. It simply says that certain things, including energy, are always conserved. This means they can鈥檛 be destroyed, only converted into different forms: slam on your car鈥檚 brakes, say, and the kinetic energy doesn鈥檛 vanish, it is merely converted into heat and sound energy in the brake discs, pads, wheels and tyres.
In theory, the laws of conservation don鈥檛 just apply to large objects like cars, but also to all the smaller things governed by quantum rules, including atoms and subatomic particles such as photons and quarks. Quantum theory should be subject to them too. But there has always been a problem.
To begin unpacking that problem, let us imagine we set up an experiment where an electron is fired towards 10 boxes and might end up in any one. Quantum theory gives the probability of the electron being found in each box. Depending on the electron鈥檚 trajectory and the relative position of the boxes, that probability might be different for different boxes.
We fire the electron, see where it lands and repeat the process 99 times. The number of times it turns up in each box will match the theory鈥檚 prediction 鈥 quantum theory triumphs.
But let鈥檚 say we do the experiment just once: now there is no way to predict the outcome because quantum theory has nothing to say about single events, only averages. So what does this really mean?
In the traditional view of it 鈥 as put forward by its founding father Niels Bohr 鈥 the system is in a 鈥渟uperposition鈥 of all possible states before the measurement, so in our thought experiment, the electron effectively exists in all 10 boxes. This is a huge problem, especially if you take the idea to its logical conclusion, as Erwin Schr枚dinger did in his famous thought experiment involving a cat in a sealed box, which, through a sequence of events that are subject to quantum laws, is both alive and dead until someone opens the box, 鈥渕easures鈥 its state, and either the dead or alive version disappears.
That鈥檚 strange enough, but if we tweak our scenario, an even deeper problem arises. Let鈥檚 now imagine we are measuring the electron鈥檚 momentum, rather than its position. Unlike position, momentum is subject to the laws of conservation, meaning it can鈥檛 just appear from nowhere. But the superposition state of the possible values of momentum before the measurement will be a totally different kind of quantity to the final measured value. Some momentum does seem to appear (or disappear). That is a violation of the law of conservation of momentum. 鈥淪ince we cannot know what it was at the start, it seems to have jumped,鈥 says Collins. 鈥淭his seemed impossible to avoid.鈥
In other words, quantum theory makes a mockery of conservation laws 鈥 and physicists have been wrestling with the implications for a century. Some gloss over this paradox by saying that perhaps things are just different in the quantum world, so it isn鈥檛 fair to expect the theory to comply with standard conservation laws. 鈥淪ince quantum mechanics is so counterintuitive and seemingly paradoxical, people have been perhaps far too ready to accept any strange behaviour,鈥 says Collins.
The many worlds interpretation
Others insist that it does matter, and this is where the many worlds interpretation (MWI) comes in. Not only does it explain what happens to the dead-and-alive cat 鈥 one version persists in another universe 鈥 it also appears to solve the problems with conservation laws. If you consider all the universes together, then no momentum has been created or destroyed after all.
and weren鈥檛 as relaxed as some physicists about the seeming violation of conservation laws. In a published last April, the pair showed that momentum is, in fact, conserved across any single quantum measurement event 鈥 and for surprising reasons. The result is part of a along with colleagues at Chapman University, California and at Ben-Gurion University of the Negev, Israel. 鈥淲e have taken it much deeper,鈥 says Popescu.
They begin with the simple case of a particle moving in a circle and then imagine measuring its angular momentum 鈥 another conserved quantity. This gives a definite outcome, yet the particle was in a superposition before, so something has clearly changed. Where could this angular momentum change have come from?
First, they considered the measuring device. 鈥淵ou know it is interacting, so you think surely something passes between the measuring device and the system,鈥 says Collins. But their calculations told them this wasn鈥檛 the case.
Next, they considered the apparatus that puts the moving particle into its superposition, called the preparer. This revealed a quantum version of robbing Peter to pay Paul. If, after the measurement, you add up the angular momentum of the particle and the angular momentum of the preparer, you will find that the total quantity remains the same as it was when the preparer and the particle first interacted. In other words, the preparer is actually part of the superposition and keeps everything precisely in balance.
In real-world experiments, the preparer might be a set of laser photons that knocks a trapped ion into its superposition state before the measurement is taken. Collins and Popescu found that the laser field and the ion would have a 鈥渞esidual entanglement鈥. That is, they are bound together in the superposition, the outcome of which is yet to be decided. And whatever the momentum change of the ion turns out to be, there is always a balancing change in the angular momentum of the laser photons.
This innovation in conservation gives us a completely unprecedented view of quantum processes. We have never before been able to talk meaningfully about the numbers behind a single quantum event. 鈥淭his is a change in one of the most basic assumptions about the laws of quantum mechanics,鈥 says Popescu.
One of the immediate implications is, of course, for the many-worlds interpretation. If conservation laws are obeyed in this universe, that undermines the need to invent others. In which case, the new work resolves the paradox that the MWI has recruited to its cause. 鈥淲hat we show is that, in each individual branch, you have conservation for the individual cases,鈥 says Popescu. 鈥淪o, your argument that many worlds helps, doesn鈥檛 help.鈥 Quantum theorist Renato Renner at the Swiss Federal Institute of Technology in Zurich agrees. 鈥淚t offers the possibility that even without many worlds, we can have a consistent view,鈥 he says. 鈥淚t鈥檚 one reason less to believe in many worlds.鈥
That said, , a staunch advocate of the MWI, is unperturbed by Collins and Popescu鈥檚 result. A theorist at Tel Aviv University in Israel, he argues that the role of conservation laws is overblown. 鈥淚 was not particularly concerned about the so-called paradox,鈥 he says. In Vaidman鈥檚 view, conservation laws would hold within each world described by the MWI anyway, which is in line with Collins and Popescu鈥檚 finding.
I'm left in an uncomfortable superposition of agreeing and disagreeing with the finding
For his part, Collins regards this all as a cautionary tale to avoid ideological stances when it comes to interpretations in quantum mechanics: assuming the MWI is valid makes you stop thinking about the problem, he says. 鈥淵ou would have never discovered the role of a preparer or any of this. You would have just thought that everything is fine.鈥
Now, it seems these new insights into quantum conservation laws could open avenues for grasping the core of quantum mechanics. For example, it points to the importance of an idea called reference frames, when making quantum measurements. You can think of reference frames as a kind of point of view from which the physics is observed. Collins and Popescu say that the reference frame for a quantum measurement is set by the attributes of the preparer apparatus 鈥 and that knowledge of this frame of reference is an essential part of establishing conservation in individual quantum measurements.
The universe prefers symmetry
This is potentially profound. That鈥檚 because reference frames are an essential part of yet another idea that forms part of the bedrock on which physics is built: symmetry. In physics, this concept of symmetry means that you can transform a system by, say, flipping it or rotating it, and it will remain the same 鈥 and there is a deep connection between these symmetries, conservation laws and new discoveries in physics. 鈥淎ll of physics is symmetry, and from symmetry you get conservation laws 鈥 that鈥檚 why conservation laws are so useful,鈥 says Collins.
The mathematician Emmy Noether was the first to show, in 1918, that conservation laws are a result of the universe鈥檚 preference for symmetry in all of its processes. Since then, whenever physicists found new symmetries, or instances where they are violated, that is where they found something worth investigating. For example, the existence of many particles in the standard model of particle physics 鈥 such as quarks and the Higgs boson 鈥 were predicted in this way. Experimentalists then built billion-dollar machines to go and find them.
With the new work, reference frames, symmetries and conservation laws are now tied together in the quantum world, in the same way they have been in classical physics. 鈥淚t鈥檚 not that they have just solved a complicated equation that no one else has been able to solve before: they really have an insight into the physics,鈥 says .
Already Renner thinks that this connection could help to settle yet another persistent quantum paradox. In the 1960s, Eugene Wigner devised a thought experiment known as Wigner鈥檚 friend. In it, his friend is measuring a quantum system akin to the cat-in-a-box one, in a lab. Meanwhile, Wigner stands outside the lab behind a closed door. The paradox occurs when his friend opens the box to, say, find that the cat is still alive. But from Wigner鈥檚 point of view, the cat is still in a superposition of being alive and dead, while also being entangled with Wigner鈥檚 friend. These realities are mismatched, yet according to quantum mechanics, both are correct.
However, a more precise understanding of reference frames could help here, says Renner. The paradox assumes that the friend exists in a pure quantum state that is distinct from Wigner. But Collins and Popescu鈥檚 result shows that this is impossible, as the friend must also be entangled with the preparer 鈥 which, in this case, is Wigner himself. So, the effects of the preparer, which are usually small and can be ignored, must now be taken into account. 鈥淢aybe these paradoxes can be resolved if you are more careful in the modelling of Wigner鈥檚 friend,鈥 says Renner.
All this is causing Renner and others to lean towards a picture of reality that is starkly different to the MWI. The idea that measurements are relative to observers and their reference frames is baked into two alternative interpretations of quantum mechanics. First, relational quantum mechanics says that objects don鈥檛 exist independently of each other and reality only arises through relational connections. Meanwhile, quantum Bayesianism, or QBism, regards quantum mechanics as a tool observers use to make predictions about the outcomes of measurements. In QBism, reality is defined through the relations between these observers and the measurements they make. Instead of myriad alternative realities continually branching away from our own, these relational interpretations build a singular universe up from within by stitching together many subjective perspectives.
There is still plenty to be done, though, to shore up Collins and Popescu鈥檚 result. So far, they have only theoretically demonstrated conservation of angular momentum and then assumed it applies to conservation laws more broadly. But at Newcastle University, UK, says that it isn鈥檛 clear the result would hold when applied to conservation laws involving energy and time 鈥 especially as the concept of time in quantum mechanics isn鈥檛 well understood.
, a philosopher of physics at the University of Bergamo, Italy, is also cautious about the result. There are subtle but problematic 鈥渓eaps鈥 in the argument, she says. 鈥淭he leaps are in the nature of interaction between the [particle] and the measurement device, or the [particle] and the preparer.鈥
Experimentalists are now thinking about how they could observe each of the conservation laws in practice. 鈥淭hese individual quantum conservation laws could be seen as imposing restrictions on the kinds of quantum states that can be prepared, motivating experiments to try to generate and measure them,鈥 says at the University of Toronto.
When he first saw the work, Steinberg鈥檚 intuition was that it couldn鈥檛 be right. But he has since studied the paper a fair bit and has 鈥渟tarted to find it surprisingly convincing鈥, he says. 鈥淚鈥檓 left in an uncomfortable superposition of agreeing and disagreeing with them.鈥
Whatever pans out, Popescu hopes that his research with Collins challenges the long-held belief that we will never progress in making sense of quantum theory. 鈥淚t鈥檚 a common thing for people to say that nobody understands quantum mechanics,鈥 says Popescu. 鈥淲ell, we鈥檙e trying to build that understanding.鈥
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