One of the most well-accepted physical theories makes no logical sense. Quantum mechanics, the theory that governs the smallest possible spaces, forces our human brains to accept some really wacky, uncomfortable realities. Maybe we live in a world where certain observations can force our universe to branch into multiple ones. Or maybe actions in the present influence things earlier in time.
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A team of physicists did some thinking, and realised this latter idea, called retrocausality, is a consequence of certain interpretations of quantum mechanics, and therefore, certain interpretations of the nature of reality. Their new paper is more of a "what-if", an initial look at how to make some of those quantum mechanical interpretations work. Some people I asked thought the work was important, some thought it didn't matter. Others felt the authors' interpretation of quantum mechanics avoids the problems posed by the new paper. But no matter what, quantum mechanics will force us to make some uncomfortable conclusions about the world.
"The foundations of quantum theory are very controversial. We all agree how to use the theory but there's no consensus about the reality it gives us," study author Matthew Leifer from Chapman University told Gizmodo. This is an unusual situation for a theory in physics, since other theories are mostly based on intuitive things we can see and test. Quantum mechanics' maths, and its predictions, describe the world perfectly, but it's sort of impossible to fully grasp what's actually happening beyond the equations.
Quantum mechanics starts with the observation that at the smallest scale, stuff, whether it be light or a piece of an atom, can act simultaneously like a wave and a particle. That means that scientists deal with some level of probability when it comes to tiny things. Send one electron through a pair of parallel slits in a barrier, and you'll see it hit the wall behind the barrier like a dot. But if you send many electrons, you'll see a striped pattern as if they travelled like a light wave. You can't predict exactly where one electron will hit, but you can create a list of the most likely spots.
The trouble is, describing particles with probabilities leads to some messy stuff. If you have two particles interacting and one's innate physical properties relies on the other's, then their associated probabilities, and therefore their identities, are intertwined. As an example, let's say there are two bags, and each has one of two balls, red or green. You give a bag to your friend. Quantum mechanics only gives the probabilities that your bag contains either ball colour, and that's all you know before making the observation. At human scales, each bag already contains a red or green ball. But on the particle scale, quantum mechanics says both balls are red and green at the same time — until you look.
That's weird on its own, but it gets worse. If you look at your ball, the other ball automatically takes on the other colour. How does the other ball know that you looked? Maybe there is hidden physics, or faster-than-light travel that allows the information to be communicated. One popular interpretation is that we live in a multiverse. In that case, the probabilities don't say anything about the ball, but about which universe we live in. Seeing a certain ball colour just means that you're in the universe where your bag had the green ball. In the other universe, you saw a red ball.
Quantum mechanics is weird as hell, where the rules of the world you experience don't apply. Even at distances a thousand kilometres apart, particles seem to be able to communicate with each other instantly, for example.
So, researchers want to know which of these interpretations is correct. In their new paper, they specifically tackled cases where observing the first ball directly influences the ball in the other bag, through some form of communication. At first glance, this requires information to travel faster than the speed of light. And that sucks, because there's already a theory that says nothing can travel faster than light. But that's OK, say the researchers, if things can influence other things back in time. Forwards in time, I'd look at my red ball, then your bag would mysteriously contain a green ball. The retrocausality case says that backwards in time, we already know both ball colours, and my ball must be red because you already knew your ball was green. Then, the balls go hidden into the bag where they become red and green simultaneously. Basically, in this case, you can't run an experiment where you can control for the effects the future has on the past.
This idea of events in the present influencing things in the past is a mathematical consequence of a pair of the author's assumptions. The first assumption is that quantum mechanics should satisfy their definition of time-symmetry, like a lot of other physics theories. That means that particles should behave the same way both forward and played in reverse — a billiard ball hitting a stationary ball looks the same no matter how you play the tape. The theory should also be "real", as Leifer says. This means that the particles are more than a list of numbers, but are instead actual things that behave the same yesterday as they will tomorrow, and have properties that are innate, whether or not the experimenter is able to observe them.
Add the maths, and according to the new paper published in Proceedings of the Royal Society A this past week, boom. If you want your theory to be "time symmetric" and work the same every day, retrocausality is required.
Most would say this is horrible, of course. If things can influence other things in the past, then who cares about all of science? Why test something at all if the result could be causing the cause? Leifer does offer a solution — a sort of block universe, where events in space and time don't cause one another, but instead fit together like a jigsaw puzzle. But this idea hasn't been developed into a mathematical theory, yet.
Basically, if retrocausality is true, then cause-and-effect is an illusion due to the fact that humans only see things in one direction. The paper is only dealing in what-ifs here, and doesn't get into the specifics of how this effect would manifest, aside from in experiments. But the effect would be built into the very fabric of the universe.
Some physicists didn't find this idea compelling. Christopher Fuchs from the University of Massachusetts, Boston told me that these so-called block universes "are neither living nor forced nor momentous for me". He takes these terms from the philosopher William James, and means that the hypothesis doesn't sound like a genuine possibility. It doesn't force him to make a decision one way or the other, and essentially, it isn't groundbreaking. "In my mind a far more viable path has already been blazed through very different considerations," treating the observer of the universe as the most important agent, and sort of avoiding the impossible-to-observe.
Physicist Sean Carroll from CalTech thought the new paper was interesting, but he happens to like the already-strange many worlds theory, that says different results manifest in different universes described under the same probabilistic description. That's the one where, in the red/green ball case, there are actually two universes, one where I saw the red ball and one where I saw the green ball. "It is perfectly time-symmetric and reversible under the conventional definitions," he said. "And it certainly doesn't require retrocausality. So as usual, if you are willing to take seriously the many worlds inside the wave function... much less weirdness is implied by quantum mechanics in other ways." Essentially, he's willing to trade the weirdness of retrocausality for the weirdness of many worlds.
But another expert I spoke to with was far more forgiving, and instead thought of this work as an important go/no-go idea for this line of thinking. "This paper makes a mathematical statement around retrocausality," said Renato Renner from ETH Zurich in Switzerland. "It says maybe we need it if we want time symmetry," a theory that still works if you play the physics in reverse. He thought this paper was one of the first pieces of research make such a well-defined statement about that concept.
So now, researchers have sat and wracked their brains about a solution to a problem that only arises if they assume certain things about the world — in other words, it's a new idea, it's only a requirement of the universe if you assume certain other things, and it's kind of fringe. But as of now, no matter how you want to understand the fabric of the universe, you're going to need to accept something that feels ridiculous, be it a multiverse, faster-than-light communication, or maybe even a world where the future influences the past.
"There's a substantial group of people trying to understand the question of what's really going on, and can we construct a theory based on stuff that really exists out there," said Leifer. "The more different approaches we can think of and try out the better."