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So why can people and other objects just go in the one direction while radio signals can go both ways? Is that ever explained or is it just fact?

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When matter (e.g. a human) enters the gate, it's converted to energy, that energy is transmitted via the wormhole between the gates, and the receiving gate converts the energy back into the matter it originally was. However, the wormhole can carry electromagnetic energy without conversion. This means radio waves can traverse the wormhole. When a gate dials out, it then takes in matter and transmits it, the receiving gate will not receive matter and transmit it, so it's a one-way affair. This prevents messes that would happen if a person were to be stepping through a gate at the same time someone was coming out from the other side.

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    This prevents messes that would happen if a person were to be stepping through a gate at the same time someone was coming out from the other side. chunky salsa
    – Xantec
    Apr 21, 2011 at 22:19
  • ok sounds plausible
    – suicide
    Apr 21, 2011 at 22:43
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    This is mostly covered early in S1 of SG-1, when they're establishing the rules of the universe. If you haven't seen it, I highly recommend it.
    – user1027
    Apr 22, 2011 at 1:25
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    Awesome Answer, awesome question
    – Mister IT Guru
    Apr 22, 2011 at 9:41
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    @RemyLebeau In Stargate Atlantis (s1e4) the team is in the back end of a jumper which got stuck half way through the event horizon. One of the side characters does ask why the front half of the ship does not appear in Atlantis, although it was already submerged in the gate. The Stargate tries to keep everything in one piece. It waits until an object of physical matter is completely disintegrated by the outgoing gate before transmitting the data through the wormhole to the receiving gate. This is a safety precaution to ensure that upon reintegrating nothing is left behind.
    – user68849
    Jul 14, 2016 at 17:09
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They never explain why, but they do note that it's only some things that can go both ways. Typically, these things are electromagnetic phenomenon, though we've also seen gravitic phenomenon do the same.

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There is actually a great plausible physical explanation for this they could have used (I've never seen this canonically explained, though).

Matter is made of fermions, subject to the exclusion principle; radio signals are made of photons, which are bosons (not subject to the exclusion principle). You could have argued that the stargate can only transfer fermions in one direction due to some unspecified law of nature, but that it can transfer bosons both ways (maybe because they can overlap, which fermions can't). This jives well with the fact that black holes can have a gravitational influence on the initiating side of the gate, since gravitons are also bosons.

A simpler explanation (maybe more justifiable to a lay audience of the show, but less physically sensible) could be that sending radio signals only transfers information, not matter, which you can do just by - uh - mirroring to both sides all pulses at the surface of the event horizon; so you are not really "transferring anything" across the wormhole and it's not affected by the directionality rule. That makes it sound like radio acts like sound, but we don't know what surface of the open gate "is made of", so you could make it work.

Far harder to explain is why you can send radio signals through, but not visible light (the surface "shines" but it's always with the same intensity so it's not just light from the other side, and you can't "see" what's on the other side, in either direction. Why?)

In practice, of course, the reason is that it makes for a good framework for storytelling.

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    It probably sends light through just as well as any other EM wave but it produces noise all across the EM spectrum. Human eyes would interpet the noise overlaid on the actual photons as shimmering light because our corneas detect the amplitude after discarding phase information. A tuned radio transmitter/reciever actually works through random noise just fine, the random phases of the noise cancel each other out rather than add up like in our corneas making the signal come through just fine.
    – John Meacham
    Jul 15, 2014 at 11:35
  • @JohnMeacham Also, the visible light would probably be horribly distorted anyway - it's mentioned many times that the wormhole is sub-atomic sized. For all we know, the "event horizon" is actually light from the other side, but unrecognizable due to the spacetime geometry involved. Advanced civilizations are shown to do a lot of things SGC considers impossible with the gates - it's quite possible someone could build a snooper capable of showing at least a hint of what's on the other side (similarly to how we seem to be able to reconstruct the radio signal).
    – Luaan
    Aug 30, 2022 at 8:46
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The stargate universe seems a little confused on this fact. In the first episode the Goa'uld (sp?) traveled through the Stargate to Earth and then back again without disconnecting the wormhole or dialing out (pretty lucky for them, because with no DHD they may have been a little confused). But in season four carter talks with Cadet Hailey at the academy and implies it only goes one way, which seems to be the case throughout the rest of the season. I'm thinking they may not have had all the kinks out in episode 1 but I think it's a funny mistake either way. :)

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    in the first episode the Goa'uld do not go back through the same session of the gate, the wormhole closes soon after Apophis comes through to Earth. Both the humans and the Goa'uld know how to manually dial a gate (without using a DHD or the computer gateworld.net/wiki/Manual_dialing )
    – user2195267
    Jun 12, 2013 at 9:29
  • stuff from the first 3 or 4 episodes of stargate need to be taken with grains of proverbial salt. there were no established rules yet for gate travel.
    – acolyte
    Jun 12, 2013 at 13:20
  • This also plays into the idea that gates can be remote dialed, so Apophis could have just remote dialed back into Earth from his own gate (though of course the idea of remote dialing hadn't been established yet when that episode was created).
    – Remy Lebeau
    Jul 30, 2015 at 2:42
  • @RemyLebeau If someone can remote dial them how would the other party know or is not one way? is there some sort of traffic light system that matter is incoming or outgoing?
    – 1.21 gigawatts
    Apr 8, 2021 at 21:04

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