Physicists Say It's Possible to Send Messages to the Past

[00:00] Sending messages into the past just got easier.  Or, as the headlines put it “We have figured out   a new way to send messages into the past”  and “Physicists Say It’s Possible to Send  

[00:12] Messages Backward 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  

[00:29] study. But it’s quite interesting indeed, and no  it does not 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.

[00:42] 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 factor in that Einstein tells us there  is no objective way to tell what happens at  

[00:57] 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  

[01:14] 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  

[01:27] they look at is loosely speaking this. You  have a receiver device that you have built at   some time and that is 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  

[01:44] and you let it do its quantum thing. And  once the quantum state has done its thing,   there is 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  

[02:01] if there is noise in this communication channel.  Amazingly enough, if you want to send messages   into the past, the noise is less of a problem  than if you want to send it into the future.

[02:13] referring to a future work from 1985  co-authored by E L Brown and M McFly. Ok, but leaving aside the noise, just what do they  say we must do to send messages into the past?  

[02:32] They are using what is called “postselection”.  This basically means that after you have done   the measurement of the quantum state,  you only look at specific outcomes. You see, in quantum physics we cannot  normally predict the outcome of the  

[02:48] experiment. But they postulate that we  basically live in a universe with only   one outcome. And then the message that you get  in the past is… the outcome which will happen.

[03:00] Ok, 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  

[03:14] 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.  

[03:29] But did you get a message from the  future or is your machine just good   at predicting the stock market? There is  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  

[03:43] discuss until the end of time whether  it really does 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 quantum  

[03:59] measurements 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 X-twitter account and post  a hundred thousand messages predicting the next  

[04:17] major LA 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  

[04:32] traveller. Works on X, doesn’t work in reality. So I have to give this paper a 8 out of 10 on   the bullshit meter. The maths works out, but their  interpretation is highly questionable. That said,  

[04:45] 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  

[04:57] think are much more complicated than the  simple cause-effect idea that we’re used to. I promise I’ll have a word with my past  self about questionable hairstyle choices.

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[06:13] go and check this out. Thanks  for watching. See you tomorrow.