Is Quantum Mechanics Stopping Aliens From Contacting Us?

0:06 "Why hasn't extraterrestrial life contacted us yet?" is

0:08 the core question of the Fermi Paradox- in short,

0:11 the observable universe contains so many millions of billions of stars

0:15 and planets that the chances of intelligent

0:17 life arising elsewhere (and probably many,

0:19 many, many places elsewhere) seems almost inevitable.

0:21 And yet we haven't heard from any aliens.

0:24 One possible explanation is that we’re overlooking quantum mechanics.

0:27 Here’s the thing: to communicate long distances in space,

0:30 it may be that you need to be using quantum communication.

0:33 A fundamental problem with interstellar communication

0:34 is the speed of light- the distances

0:36 between stars are just so big and light takes so long to get there,

0:39 you have to be really efficient with your communication- you

0:41 don’t want to waste a decade to hear back “sorry, you cut out for a moment,

0:44 can you say that again?” And it turns out

0:46 that quantum communication- where you send photons that are

0:48 entangled or in a carefully superimposed quantum state- quantum

0:51 communication can be incredibly more

0:53 information dense than non-quantum communication:

0:55 in one protocol [superdense coding], any number of quantum bits can be

0:57 used to transmit twice as many non-quantum bits,

0:59 and in another setup [hidden matching

1:00 problem] it takes exponentially more non-quantum

1:02 bits than quantum bits to transmit

1:04 a particular type of information [Onscreen note:

1:05 Plus quantum communication allows certain computations and levels

1:05 of security impossible to replicate with classical communication].

1:06 Though both of these quantum efficiency

1:07 gains have caveats that we’ll explain later.

1:09 But regardless, quantum communication being more efficient doesn’t on its

1:12 own explain Fermi’s “where are all the aliens?” paradox.

1:15 Quantum communication, it turns out,

1:17 has some very particular requirements and it's

1:19 those that might/can help solve the fermi paradox.

1:21 Specifically, to send quantum information to someone else,

1:23 you can’t just broadcast a signal out in all

1:25 directions like you can with radio waves or whatever.

1:27 The laws of physics require the person receiving the quantum information to get

1:30 over 50% of the photons you send in order to reconstruct the info,

1:34 so instead of broadcasting a quantum signal,

1:35 you have to narrowcast- I mean, focus it.

1:37 The equation for this tells us that the size

1:39 of the transmitting and receiving telescopes must be really really

1:42 big- exactly how big depends on how far you are

1:45 sending the message and what wavelength of light you’re using.

1:47 And the 50% receipt requirement means you have to send

1:49 a quantum signal using photons with wavelengths that will pass

1:52 undisrupted both through the earth’s atmosphere and across interstellar space

1:55 rather than getting scattered off of interstellar dust or whatever,

1:58 and the numbers work out such that if you want to do

2:00 quantum communication with Alpha Centauri (the nearest star system to us),

2:03 you’d need a telescope around ~100km in size,

2:06 and a bigger one for more distant locations.

2:07 And this is not like the Very Large Array where they

2:09 have radio dishes really far apart to simulate having a large telescope,

2:12 in this case you actually have to build the whole telescope

2:14 because you have to get more than 50% of the photons.

2:16 Interstellar quantum communication is a hard thing to do.

2:19 Perhaps now you can start to see how this explains the Fermi paradox:

2:22 suppose that widespread alien civilisations are using

2:24 interstellar quantum communication with huge oversized telescopes.

2:27 And for their quantum communications to work,

2:29 each transmission has to get the majority

2:31 of the transmitted photons into the receiving telescope.

2:33 Which means that very few would stray off

2:35 by happenstance to be detected by our tiny/undersized telescopes.

2:37 And because we wouldn’t receive more than 50%

2:39 of the photons from a transmission (both because

2:41 we don’t have a big enough telescope

2:42 AND the signal wouldn’t be pointed at us anyway),

2:45 it wouldn’t be possible to understand the information we received.

2:48 Basically- aliens communicating quantumly could be sending messages whizzing

2:51 past us all the time and we wouldn’t know.

2:53 PLUS any aliens that had telescopes powerful

2:56 enough for quantum communication would also consequently have

2:58 telescopes good enough to see that we

3:00 DON’T have telescopes powerful enough for quantum communication,

3:02 so they would know that sending us a message is futile,

3:05 and therefore perhaps wouldn’t bother.

3:07 In summary: our theoretical and experimental

3:09 understanding of quantum information suggests that aliens

3:11 might be likely to use quantum

3:13 interstellar communication (rather than non-quantum communication),

3:15 which means aliens would be sending

3:17 narrowcast messages that would mostly pass us,

3:19 and even if we could intercept them we wouldn’t

3:22 have the capacity to intercept enough to receive the message.

3:24 And therefore, we would expect not to hear anything!

3:27 And that’s the Fermi paradox, solved.

3:30 Kind of.

3:30 There are still some open questions, like:

3:32 If aliens wanted to communicate with us

3:34 and could tell we couldn’t receive a quantum message,

3:36 why wouldn’t they just send us a regular signal,

3:38 like with radio waves or something?

3:39 And also If aliens had travelled through the galaxy

3:41 to set up a network of giant telescopes, why couldn’t they just come visit?

3:44 …but answering those is beyond the scope of this video.

3:47 Whether you are an alien learning

3:49 to build an interstellar quantum communication network,

3:51 or simply a human like me wanting to excel at learning science,

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4:30 Ok, now for the caveats:

4:32 the protocol called superdense quantum coding allows you to send

4:35 2 non-quantum bits of information from each quantum bit you send,

4:38 which seems like a 2x speed up,

4:40 except it requires the receiver to already be in possession of an entangled

4:43 quantum bit from the sender in order to receive the message.

4:45 SO, you’d either have to bring a big supply of pre-entangled

4:48 quantum bits with you when you built the receiving telescope,

4:50 or the sender would still have to send

4:52 two quantum bits to transmit two non-quantum bits.

4:54 In which case they could just send the non-quantum ones.

4:57 The other procedure, called the Hidden Matching Problem,

5:00 which requires exponentially more non-quantum bits to be sent than quantum ones,

5:03 this procedure ALSO requires the receiver

5:05 to already be in possession of some information,

5:07 and it’s not clear exactly how it

5:08 would be turned into a general communication protocol.

5:10 But it has been proven that this procedure

5:12 is exponentially faster when you communicate quantumly, than non-quantumly.