There’s a lot of discussion about “time” in the world of quantum physics. At the micro level, where waves and particles can behave the same, time tends to be much more malleable than it is in our observable realm of classical physics.
Think about the clock on the wall. You can push the hands backwards, but that doesn’t cause time itself to rewind. Time marches on.
But things are much simpler in the quantum realm. If we can mathematically express particulate activity in one direction, then we can mathematically express it in a diametric one.
In other words: time travel actually makes sense through a quantum lens. Whatever goes forward must be able to go backward.
Related: Google’s ‘time crystals’ could be the greatest scientific achievement of our lifetimes
But it all falls apart when we get back to classical physics. I don’t care how much math you do, you can’t unbreak a bottle, untell a secret, or unshoot a gun.
As Gid’on Lev points out in a recent article on Haaretz, this disparity between quantum and classical physics is one of the field’s biggest challenges.
Per Lev’s article:
Hawking demonstrated that regarding black holes, one of the two major theories leads to an error.
According to his calculations, the radiation emitted by the hole is not a function of the material the hole swallows, and therefore, two black holes that formed by different processes will emit the same exact radiation. This meant that the information on every physical particle swallowed into the black hole, including its mass, speed of movement, etc., disappears from the universe.
But under the theory of quantum mechanics, such deletion is impossible.
Hawking was wrong, then he was right
Lev’s article goes on to explain how Stephen Hawking eventually conceded (he lost a bet) that the information entering a black hole wasn’t gone. He, of course, couldn’t explain exactly where it went. But most physicists were pretty sure it had to go somewhere – nothing else in the universe just vanishes.
Fast forward to 2019 and two separate research teams (working independently of each other) publishedpre-print papers seemingly confirming Hawking’s hunch about the persistence of information.
Not only were the papers published within 24 hours of each other, but the lead authors on each ended up sharing the 2021 New Horizons Breakthrough Prize for Fundamental Physics.
What both teams discovered was that a slight change in perspective made all the math line up.
When information enters a black hole it appears to be lost because, for all intents and purposes, it’s no longer available to the universe.
And that’s what stumped Hawking. Imagine a single photon of light getting caught in a black hole and swallowed up. Hawking and his colleagues knew the photon (and the information that was swallowed up with it) couldn’t be deleted.
But, according to Hawking, black holes leak thermal radiation. And that means they eventually lose their energy and mass and… fade away.
Hawking and company couldn’t figure out how to reconcile the fact that once a black hole is gone, anything that’s ever been inside it appears to be gone too.
That’s because they were looking in the wrong places. Hawking and others were trying to find signs of the missing information leaking out simiarly along a black hole’s event horizon.
Unfortunately, using the event horizon as a starting point never panned out – the numbers didn’t quite add up.
The 2021 New Horizons Prize winners figured out a different way to measure the “area” of a black hole. And, by applying the new lens to measurements over various stages of a black hole’s life, they were finally able to make the numbers add up.
Here’s how this relates to time travel
If these two teams did in fact demonstrate that even a black hole can’t render information irreversible, then there might be nothing physically stopping us from time travel.
And I’m not talking about that hard-to-explain, gravity at the edge of a black hole, your friends would get older while you stayed young kind of time travel.
I’m talking about real-life Marty Mcfly time travel where you could set the dials in the DeLorean for 13 March 1986 so you could go back and invest in Microsoft on the day its stock went public.
Now, much like Stephen Hawking, I don’t have any math or engineering solutions to the problem at hand. I’ve just got this physics theory.
If information can and does escape from black holes, then it’s only logical to assume that other processes which we only see in quantum mechanics could also be explained through classical physics.
We know that time travel is possible in quantum mechanics. Google demonstrated this by building time crystals, and numerous quantum computing paradigms rely on a form of prediction that surfaces answers using what’s basically molecular time-travel.
But we all know that, when it comes to quantum stuff, we’re talking about particles demonstrating counter-intuitive behavior. That’s not the same thing as pushing a button and making a car from the 1980s appear back in the old Wild West.
However, that doesn’t mean quantum time travel isn’t just as mind-blowing. Translating time crystals into something analogous in classical physics would mean creating donuts that reappear on your plate after you eat them or beer that reappears in your glass no matter how many times you chug it.
If we concede that time crystals exist and information can escape a black hole, then we have to admit that donuts – or anything, even people – could one day travel through time too.
Then again, nobody showed up for Hawking’s party. So, either it isn’t possible or time travelers are jerks.