Theoretical Time Travel in Quantum Physics

Sending messages into the past just got easier. Or, as the headlines put it,

we figured out a new way to send messages into the past, and physicists say it's possible to

send messages backwards in time. Even more amazing, these headlines are based on a paper

published in PRL, the top journal in the field. As you've guessed correctly, no one's yet figured

out how to actually send messages into the past. This is a theoretical study,

but it's quite interesting indeed. And no, it doesn't involve a DeLorean, a time fairy,

or a black hole behind a bookshelf. It just requires a little bit of quantum physics.

Let's have a look. Quantum physics.

has an uneasy relation with causality. This is because once you accept that particles can do

multiple things at the same time, and factoring that Einstein tells us there's no objective way

to tell what happens at the same time, then quantum mechanics also tells you that generally

you can't tell what happened in which order. Causality becomes as uncertain as the rest

of quantum physics. And of course, the first question you might have is, can I make money

with it? Who wouldn't want to tell their younger self to buy NVIDIA stocks without having to grow

a wormhole in the basement? The new paper is a continuation of an earlier work. It comes from

an MIT group.

and the scenario they look at is loosely speaking this. You have a receiver device that you've

built at some time and that's ready to receive messages from the future. Before you build the

device, you can't get any messages. Then you have a special quantum state that you prepare and you

let it do its quantum thing. And once the quantum state has done its thing, there's a sender who

measures the quantum state and that, so the idea, injects information in the message receiver in

the past. In the new paper they show that this still works if there is noise in this communication

channel. Amazingly enough, if you want to send messages into the past, the noise is

is less of a problem than if you want to send it into the future. The authors finish their

paper in style, referring to a 1985 paper co-authored by E.R. Brown and M. McFly.

OK, but leaving aside the noise, just what do they say we must do to send messages into the past?

They're using what's called post-selection. This basically means that after you've done

the measurement of the quantum state, you only look at specific outcomes. You see,

in quantum physics, we can't normally predict the outcome of an experiment,

but they postulate that we basically live in a universe

with only one outcome. And then the message that you get sent into the past is the outcome which

will happen. Okay, but wait. How is this not just making a prediction? Well, to me,

this is the key point of the paper. In general, you can't tell these two things apart. Forget the

quantum stuff for a moment. Imagine that you just sit there with your device that can receive

messages from the future. Or so you think. It has a screen and noise flickers on the screen most of

the time. Then suddenly it says, buy NVIDIA. You go buy NVIDIA and a year later you're rich. But

did you get a message from the future or is your machine just good at predicting the stock market?

market. There's no way to tell these two things apart. I don't know about you, but if I had

such a device, I wouldn't care. Physicists could discuss until the end of time whether

it does really receive messages from the future or whether it's just crazy good at making

predictions. I'll be sitting at the pool. Cheers.

That said, in today's reality, we can't predict the outcome of a quantum measurement and post-selection

isn't something you can magically enforce on our universe. This means that for practical

purposes their device works more like this. Suppose I set up an ex-Twitter account and

post a hundred thousand messages predicting the next major element.

earthquake for any possible day until 2100. Then, once it happens, I delete all messages

but one, celebrate the correct prediction and claim that it was given to me by a time

traveler. Works on X, doesn't work in reality. So I have to give this paper an 8 out of 10

on the bullshit meter. The maths works out, but their interpretation is highly questionable.

That the real story here isn't that physicists have invented a working telephone to yesterday.

The story is that physicists are slowly rethinking temporal relations,

which I think are much more complicated than the simple cause-effect idea.

that we're used to, and if they finally work out how to send messages into the past,

I promise I'll have a word with my past self about questionable hairstyle choices.