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I am working with some possible concepts for my world building project and had the idea for two alien civilizations. One made from regular matter and another made out of antimatter in their own solar systems made from their type of matter.

My question is, would it be possible for these two civilizations to communicate using radio or similar types of communication?

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    $\begingroup$ I wonder if the two civilizations would be able to tell. If not, they might decide to try to meet... $\endgroup$
    – prl
    Dec 5, 2022 at 4:59
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    $\begingroup$ @A.I.Breveleri That's not quite true. It's true that at the subatomic level, an antiparticle resembles a particle moving backwards in time. But at the macroscopic level, time-asymmetric effects like friction and heat transfer go the same way for antimatter as for regular matter. $\endgroup$ Dec 5, 2022 at 11:07
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    $\begingroup$ @Tanner-reinstateLGBTpeople, Re, "...have to send all your messages in reverse." Even supposing there was some alien civilization whose future was our past and vice versa, the idea that we would have to send all of our messages in reverse in order for the aliens to understand them is silly. When the aliens receive our first transmission, they know nothing about us, or how we communicate. All they know is, we sent a pattern. It's up to them to find meaning in it if they can. Time-reversal of the pattern won't make their job any easier or any harder. $\endgroup$ Dec 5, 2022 at 16:24
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    $\begingroup$ @SolomonSlow That's a funny thought! It reminds of one time when I played blind go with a group of people. Contrary to chess, go doesn't have a convention regarding coordinates on the board: we can use letters for columns and numbers for rows, or the other way around, and we can number from top to bottom or from bottom to top. When we were about to start the blind game, someone exclaimed: "Wait! We forgot to agree on which convention we were using for the coordinates!". Someone else laughed and explained there was no need to agree on a convention, since the game would exist only in our heads. $\endgroup$
    – Stef
    Dec 5, 2022 at 20:19
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    $\begingroup$ Of course there is the much bigger problem of having matter and antimatter within reach of each other. What is the rest of the universe made of? Some kind of third state ? Because otherwise the anti-planet is going to be destroyed pretty quickly as everything ranging from particles to meteorites from the surrounding universe crash into it. $\endgroup$
    – komodosp
    Dec 6, 2022 at 8:49

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Yes, for example, the electromagnetic radio wave produced by an antimatter dipole antenna would have an opposite polarity of the field since you could think of positive charge being moved instead of negative charges. But the receiving antenna would still sense the electromagnetic wave and the negative charge in the regular matter antenna would respond to that field. The fact that wave started out with a different polarity doesn't really matter since the relative phase also depends on the distance between the antennas and if you move the antenna a half wavelength distance away it would also change the phase 180 degrees.

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    $\begingroup$ Or, a photon is it’s own anti-particle, so photons are photons (no matter how small?). $\endgroup$
    – Jon Custer
    Dec 4, 2022 at 17:11
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    $\begingroup$ The polarization of a photon is not affected by the charge of the particle that produces it. en.wikipedia.org/wiki/Polarization_(waves) You are possibly referring to the phase, which is not ordinarily observable any way. $\endgroup$
    – BillOnne
    Dec 4, 2022 at 19:39
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    $\begingroup$ @BillOnne Phase of radio photons is easily observable, it just doesn't matter for radio transmission. This isn't wavefunction phase, this is just the phase of the electric field. It is true, however, that phase of high-frequency photons (roughly, infrared and higher frequency) is not possible to measure with current technology. $\endgroup$
    – Hearth
    Dec 5, 2022 at 5:30
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    $\begingroup$ @Hearth Relative phase is observable. Absolute phase is not. Ordinarily even relative phase is not observable for a radio reciever. $\endgroup$
    – Boba Fit
    Dec 5, 2022 at 13:58
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    $\begingroup$ Or, simply speaking, without a common reference system except their radio communication, they have no way to find out if both are of the same type of matter. $\endgroup$
    – Karl
    Dec 5, 2022 at 22:39
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The Photon is its own antiparticle

The radio works by shooting out and receiving streams of photons. The anti-radio works by shooting out and receiving streams of anti-photons. But anti-photons are the same as photons. So not only can the anti-radio communicate with the regular radio, it doesn't need to be modified to work.

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  • $\begingroup$ Surely you would need to either run the anti-radio backwards in time, or parity invert the radio... $\endgroup$
    – Aron
    Dec 5, 2022 at 4:11
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    $\begingroup$ @Aron Not really needed. Experiments with antiparticles (up to whole antiatoms) show pretty much consistent behavior of the antimatter in this regard. $\endgroup$
    – fraxinus
    Dec 5, 2022 at 8:19
  • $\begingroup$ @Aron Yeah the signal will be upside down. But the other radio doesn't care. $\endgroup$
    – Daron
    Dec 5, 2022 at 13:43
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    $\begingroup$ @Aron the idea that anti-matter is "runs backwards in time" is totally incorrect and a misconception. Tanner explains this in a comment under the question. $\endgroup$
    – Fattie
    Dec 5, 2022 at 15:05
  • $\begingroup$ @Fattie CPT symmetry. Reversal of C must (afaik) require an inversion of PT. EM is also reversible, meaning time reversal doesn't actually do anything. Seems you misunderstood my point.. $\endgroup$
    – Aron
    Dec 6, 2022 at 2:25
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In general, it is very hard to tell the difference between matter and antimatter without actually annihilating it. Nearly every chemical could exist the same in both antimatter and matter. However, as previously posted the polarity of light would be reversed for antimatter. This couldn't be detected by a radio however.

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Before gamma ray astronomy, many physicists believed that the universe contained equal amounts of matter and antimatter. There is no way to tell from light or radio waves whether the radiation originates from matter or antimatter.

However, because even intergalactic space isn't a perfect vacuum, if there was much antimatter around we'd see 511 keV annihilation radiation all over the place. We don't.

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In theory, yes. In practice, not that easy.

Sure you can receive the signal, IF you happen to be monitoring the specific frequency used to transmit it and are close enough that your receiver is powerful enough to pick it up.

But that's only the beginning of the problems. How are you going to interpret the signal you just received? And THAT problem is unrelated to whether your receiver and sender are made of matter, anti-matter, or unicorn dust. It's an eternal and universal problem communicating with disparate means. Heck, you probably don't even realise it's not a natural source when you first get the message, and after you do realise it's not a natural source you're going to have to do a lot of guess work, more or less at random, until you find out what encoding it's using, what modulation system is used (if any), what sidebands to consider, etc. etc.. And even after that, if you do happen to have a signal that's what was initially sent out, you're still having to decrypt (possibly) and interpret it. These anti-matter aliens won't speak your language, they won't use the same encoding of data that you use, the clear text message will be utter gibberish to you (and yours to them, they're going through the same problem at no doubt a different pace).

That's a big thing, a very big thing, that most first contact stories (as well as SETI) forget about.

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    $\begingroup$ I disagree with the claim that fiction or reality near-universally disregards this. Starting with constants and building an alphabet from them is a solution that has been anticipated (and attempted, for our own transmissions). Similarly, keeping the encoding simple is something that has been deliberately considered. $\endgroup$ Dec 6, 2022 at 15:25
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    $\begingroup$ Beyond that, detecting that there is something that carries information is something we have practically workable (entropy-measurement) theory and practice for. $\endgroup$ Dec 6, 2022 at 15:26
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    $\begingroup$ Not an answer to the question. $\endgroup$
    – toolforger
    Dec 7, 2022 at 3:39

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