(ORDO NEWS) — In 2022, the Nobel Prize in Physics was awarded for experimental work showing that the quantum world should destroy some of our fundamental ideas about how the universe works.
Many look at these experiments and conclude that they challenge “locality” the intuition that distant objects need a physical intermediary to interact.
Indeed, the mysterious connection between distant particles may be one way to explain these experimental results.
Others instead see experiments as challenging “realism” the intuition that our experience is based on an objective state of affairs.
After all, experiments are hard to explain only if our measurements are thought to match something real.
In any case, many physicists agree with what is called the “experimental death” of local realism.
But what if both of these intuitions could be preserved at the expense of a third?
A growing group of experts believe that instead we should abandon the assumption that current actions cannot affect past events. This option, called “retrocausality,” claims to save both locality and realism.
Causality
What is a causal relationship? Let’s start with a well-known line: correlation is not causation. Some correlations are causal, but not all. What is the difference?
Consider two examples. (1) There is a correlation between the barometer needle and the weather – that’s why we learn about the weather by looking at the barometer.
But no one thinks that the barometer needle causes the weather. (2) Drinking strong coffee correlates with increased heart rate. Here it seems correct to say that the first causes the second.
The difference is that if we “wiggle” the barometer, we will not change the weather. The weather and the barometer needle are controlled by a third thing, barometric pressure, which is why they correlate.
When we control the arrow ourselves, we break the connection with air pressure, and the correlation disappears.
But if we intervene to change someone’s coffee intake, we usually change their heart rate as well. Causal relationships are those that remain in effect when we change one of the variables.
These days, the science of finding these reliable correlations is called “causal discovery.” That’s a big name for a simple idea: to find out what else is changing as we rock things around us.
In ordinary life, we usually take it for granted that the effects of the wiggle will show up later than the wiggle itself. This is such a natural assumption that we do not notice that we are doing it.
But nothing in the scientific method requires this to happen, and it is easily abandoned in fiction. Similarly, in some religions, we pray that our loved ones will be among the survivors of, say, yesterday’s shipwreck.
We imagine that what we do now can affect something in the past. This is retrocausality.
Quantum retrocausality
The quantum threat to locality (that remote objects need a physical intermediary to interact) stems from an argument by Northern Irish physicist John Bell in the 1960s.
Bell considered experiments in which two hypothetical physicists, Alice and Bob, obtain particles from a common source.
Everyone selects one of several measurement settings and then records the measurement result. Repeated many times, the experiment generates a list of results.
Bell realized that quantum mechanics predicted that there would be strange correlations in these data (now confirmed).
They seemed to imply that Alice’s choice of setting has a subtle “non-local” effect on Bob’s outcome, and vice versa, even though Alice and Bob may be light years apart.
Bell’s argument is said to pose a threat. with Albert Einstein’s special theory of relativity, which is an important part of modern physics.
But that’s because Bell suggested that quantum particles don’t know what dimensions they’ll encounter in the future.
Retrocausal models suggest that Alice’s and Bob’s choice of measurements affect particles back at the source. This could explain the strange correlations without violating special relativity.
In a recent paper, we have proposed a simple strange correlation mechanism – it involves a familiar statistical phenomenon called the Berkson bias (see our popular summary here).
There is now a thriving group of scientists working on quantum retrocausality. But this is still invisible to some experts in the wider field. It is confused with another point of view called “superdeterminism”.
Superdeterminism
Superdeterminism is consistent with retrocausality, according to which the choice of measurement and the underlying properties of the particles are somehow correlated.
But superdeterminism treats this as a correlation between the weather and the barometer needle.
There is supposed to be some mysterious third thing a “superdeterminant” that controls and correlates both our choices and particles, just as barometric pressure controls both the weather and the barometer.
Therefore, superdeterminism denies that the choice of dimensions are things that we can wiggle at will, they are predetermined. Free fluctuations would break the correlation, as in the case of the barometer.
Critics counter that in this way superdeterminism undermines the basic assumptions needed to conduct scientific experiments.
They also say that this means the denial of free will, because something controls both the choice of dimension and the particle.
These objections do not apply to retrocausality. Retrocausalists make scientific causal discoveries in the usual loose, tortuous way.
We say that people who reject retrocausality forget the scientific method if they refuse to follow the evidence where it leads.
Proof
What is the evidence for retrocausation? Critics demand experimental evidence, but this is the simplest: the relevant experiments have just won the Nobel Prize.
The difficulty lies in showing that retrocausality provides the best explanation for these results.
We mentioned the possibility of eliminating the threat of Einstein’s special theory of relativity.
In our opinion, this is a rather important hint, and it is surprising that it took so long to study it. The confusion with superdeterminism is mostly to blame.
In addition, we and others have argued that retrocausality better explains the fact that the particle microcosm does not care about the difference between past and future.
We do not mean that everything is going smoothly. The biggest concern about retrocausality is the ability to send signals into the past, opening doors to the paradoxes of time travel.
But to make a paradox, you need to measure the effect in the past. If our young grandmother cannot read our advice not to marry grandfather, which means that we will not be in the world, there is no paradox in this.
And in the quantum case, it is well known that we will never be able to measure everything at once.
However, there is still work to be done to develop specific retrocausal models to meet this limitation that you cannot measure everything at once.
So we end with a cautious conclusion. At this point, retrocausality is holding the wind in its sails, so hurry up for the biggest prize of all: saving locality and realism from death by experiment.
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