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So aliens have arrived in the solar system.

Both parties want to converse up close but they cannot come in contact with each other. The location of these interactions does not matter. Neither party can change the charges of their own particles, nor can they manufacture technology with the opposite charge safely.

What are efficient ways for these two groups to communicate?

And if possible, what can allow close interaction (same room) to further scientific partnership between humans and this species without the risk of deadly particle collision?

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    $\begingroup$ Wait, can antimatter ships orbit a normal matter moon? $\endgroup$ – Xandar The Zenon Dec 24 '16 at 16:47
  • $\begingroup$ @XandarTheZenon the gravity of antimatter is not fully understood, but it is possible that it acts in the same way as ours. Assume orbit is possible / gravity won't repel them for simplicity. $\endgroup$ – Zxyrra Dec 24 '16 at 16:48
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    $\begingroup$ While we've never generated enough antimatter to conclusively test it, the general consensus is that the most likely behavior for antimatter will be to be affected by gravity in the exact same way normal matter is. $\endgroup$ – Cort Ammon Dec 24 '16 at 17:54
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    $\begingroup$ An antimatter ship anywhere in the solar system (or indeed, in interstellar space) will slowly disintegrate as it is bombarded by the solar wind. The bombardment will also release energy. Asking on the physics or astronomy sites might get you detailed numbers. $\endgroup$ – jamesqf Dec 24 '16 at 18:41
  • $\begingroup$ @jamesqf I tried to account for that. See the 4th bullet $\endgroup$ – Zxyrra Dec 24 '16 at 19:27
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Depending on the nature of their shields you might even be able to do same-"room" interaction, for certain definitions of "room".

As Zxyrra says, use the electromagnetic spectrum. If for some reason they feel a need to get closer, consider "Childhood's End" movie scene in the room where the protagonist met the alien. Now, in both the movie and the book the divider is one-way glass, but what if we use their shields?

They give us their shield technology. We build a 5-walled room of matter, the open face covered with one of their shields. They build a 5-walled room of antimatter, the open face covered with one of their shields. Move the two as close to each other as the shield technology permits.

I can see no reason other than psychological ones to bother with this, though. The only interchange is still visual. The only fundamental unit still defined by a physical object is not amendable to measurement by the aliens and I don't believe it's needed anyway:

E=mc^2.

m=E/c^2

c is defined in meters per second.

Seconds are defined in terms of a number of oscillations of a photon emitted by a certain atomic transition.

Their anti-atoms should emit the same photos as our posi-atoms. Thus they can build a machine to derive our second.

Meters are defined in terms of the number of wavelengths of a photon emitted by a certain atomic transition. Again, they can derive this.

Thus they can derive c.

Joules can be expressed in terms of a coulomb-volt. A coulomb = a farad-volt.

A volt can be expressed in terms of chemical reactions, they can derive it.

A farad can be expressed in terms of a number of electrons. They have anti-electrons, they can count them.

Thus we have means of communicating every term on the right side of the equation, they can calculate our unit of mass.

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  • $\begingroup$ This is good. Could you explain how to hold both sides of the room together if a field divides it? $\endgroup$ – Zxyrra Dec 24 '16 at 21:21
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    $\begingroup$ @Zxyrra Magnets and anti-magnets work the same. (Although theirs will be backwards as their wires carry positrons rather than electrons.) $\endgroup$ – Loren Pechtel Dec 25 '16 at 6:12
  • $\begingroup$ . . . . . . . . $\endgroup$ – KalleMP Jul 4 '18 at 20:33
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Use the electromagnetic spectrum

Humans already communicate at long distances using radio waves, and see using visible light. Photons have no charge, so it's safe to use visual displays, or any wave on the EMS that isn't ionizing, to converse. Antimatter particles will not be annihilated when coming in contact with photons from matter, and matter won't annihilated when coming in contact with photons from antimatter; it's perfectly fine to "see" as long as you don't "touch".

I don't think it's possible to have same-room interractions; gas particles, even in a near vacuum, make it far too risky.

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    $\begingroup$ plus it turns out matter and antimatter use the same light spectrum. sciencealert.com/… $\endgroup$ – John Dec 24 '16 at 17:00
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    $\begingroup$ This is a good answer because photons are their own antiparticles. $\endgroup$ – fffred Dec 24 '16 at 17:03
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    $\begingroup$ As boring as it might be, light waves and radio waves are literally the perfect tool for the job. $\endgroup$ – Cort Ammon Dec 24 '16 at 17:52
  • $\begingroup$ If they maintain a strong enough EM field to prevent particles from hitting them, you won't be able to use radio to contact. Visual spectrum only. $\endgroup$ – SRM Dec 24 '16 at 21:17
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    $\begingroup$ Light isn't bent by magnetic fields. The only bending caused by those fields is their energy and the effect of that energy on space-time, which, unless these clients live in a neutron star, is going to be minimal at best. physics.stackexchange.com/questions/67464/… $\endgroup$ – ifly6 Dec 28 '16 at 19:57

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