Our experience of time seems uniform and unidirectional, but relativity allows it to be a little fuzzier than we might expect.
For example, in a strong gravitational field, time flows more slowly than time further away – as if gravity itself is pulling it along.
Likewise, if you travel fast enough, time will pass more slowly for you than for someone who stays put.
In these examples, time only flows in one direction.
But beyond relativity, in the field of quantum mechanics, another possibility appears: one in which time could not only slow down, but actually go backwards.
But what happens when this happens? A new mathematical exploration of this question has yielded a surprising answer: the quantum environment itself imposes a limit on the effective distance over time.
And it’s extremely short.
For the simplest model explored by physicists Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research, time could only effectively go backwards for about a femtosecond – or a quadrillionth of a second.
Their work is detailed in a preprint uploaded to arXiv.
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This isn’t Aharonov’s first time-reversal rodeo.
In 1990, he and his colleagues proposed a theoretical device called a quantum time translation machine, which could exploit the strange properties of quantum mechanics to make a system behave as if time had accelerated, stopped, or even gone backwards.
The idea relies on a quirk of quantum mechanics called superposition, in which a system can exist in a combination of different possible states at once.
Imagine having a clock in such a superposition state, simultaneously occupying two different positions in a gravitational field – with a difference in gravitational force large enough to affect the rate at which time passes.
At a deeper position in the gravitational field, time passes more slowly. In the other, further away, time passes faster.
Quantum mechanics allows both possibilities to coexist. And when the two arms of the clock are brought together, they can interfere with each other.
This is where things get weird. By selecting a particular outcome of this interference – a process called post-selection – physicists can theoretically make the clock behave as if it had experienced a period of time outside of either possibility.
This could mean more time than has passed for either branch. That might mean no time at all. Or it could mean a negative duration – as if the clock had gone backwards.
Today, 36 years later, Aharonov and Elitzur return to the idea of investigating a particularly thorny question: how far could this negative period really go?
Because the concept remains theoretical rather than realized experimentally, the two men turned to mathematics to explore four slightly different configurations of the machine.
But before they could begin, they had to answer a deceptively simple question.
Suppose you manage to turn back time. How the hell can you prove it?
And that’s where a little atomic disintegration steals the talk.
When an atom absorbs energy, one of its electrons can move to a higher energy level, putting the atom in what is called an excited state.
But atoms are a bit lazy – they tend to return to their (relative) unexcited state. Eventually, the electron can drop back to a lower energy level, releasing the excess energy in the form of a photon. This process is called spontaneous emission, and under ordinary circumstances it only goes one way.
The researchers therefore proposed to start with a set of atoms that had already started to disintegrate and see if their theoretical machine could reverse the process.
Imagine you have 100 excited atoms. You wait until some of them have released their energy, leaving only 80 still excited.
Now suppose you put these 100 atoms through the theoretical time machine.
If you get more than 80 excited atoms, the machine has effectively reversed some of the decay that had already taken place.
However, if time actually went backwards, the emitted light would have to return to the atoms, returning them to their excited states – which poses a new problem. The photons may have already traveled or been absorbed by something else.
To explain all this, Aharonov and Elitzur imagined enclosing the atoms in a completely sealed cell, with the light they emitted and walls that could absorb it.
The entire cell would then run through the theoretical machine, allowing researchers to determine whether the entire process could indeed be reversed.
And this is where a rather draconian barrier appeared.
Even inside a sealed cell, atoms do not exist in isolation. By emitting light, they interact with the surrounding electromagnetic field, leaving traces of what happened.
For the simplest version of the machine, decay reversal depends on these interactions retaining the quantum coherence necessary for the interference of the two possible histories.
But this window doesn’t stay open for long.
In the researchers’ optical example, this lasts about a femtosecond.
Beyond that, the environment has effectively retained too much information about the emissions for the simplest two-limb machine to produce the desired inversion.
The limit is therefore not imposed by time, but by the memory of the light in the environment.
More elaborate versions of the machine could theoretically reverse longer intervals, but only by introducing many more quantum branches, with increasingly unlikely chances of success.
But what about, you may now be wondering, about the second law of thermodynamics? A salient point, wise reader, and one to which the researchers directly responded.
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According to the second law of thermodynamics, the total entropy of an isolated system can never decrease. Natural processes tend to move in one direction, from more ordered states to less ordered states.
As Sir Arthur Eddington so succinctly put it: “If your theory proves contrary to the second law of thermodynamics, I can give you no hope; there is nothing to do but collapse in the deepest humiliation. »
But if time could flow backwards, wouldn’t that violate the second law? Well, not necessarily.
The flaw lies in the post-selection. The apparent reversal of time only occurs in a very small fraction of possible outcomes, carefully selected from all others.
When all results are considered together, the system as a whole still obeys the second law of thermodynamics. Entropy didn’t magically start decreasing across the board.
Thus, Aharonov and Elitzur’s theoretical machine can produce something resembling a return of time without overturning one of the most fundamental laws of physics – thus avoiding Eddington’s threat of humiliation.
But why go so far to risk an egghead?
Well, because the question isn’t really whether we can build a machine that sends things back in time. This is why time seems to move in only one direction in the first place.
Many fundamental equations of physics work backwards as well as forwards. Yet the world we live in is full of processes that seem impossible to undo.
By identifying exactly where quantum time reversal breaks down, Aharonov and Elitzur’s work could help physicists understand how the environment gives rise to the irreversible processes that define our experience of time.
“The arrow of time enters exactly where the machine stops,” the researchers write.
The research is available on the arXiv preprint server.
This article was fact-checked by Carly Cassella and edited by Carly Cassella. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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