How do probes return signals through the stargate

[Coronach]

Could it have anything to do with how fast light and radio waves travel (i.e. both travel at the speed of light)?

[K^2]

I don't think so. Matter must be sent through the portal either way. And if matter can travel from source to destination, nothing would stop it from going in reverse, in principal. It's either the problem with sending matter streams in both directions at once, as someone mentioned, or a design decision driven by some strategic factors.

[Coronach]

I don't think so. Matter must be sent through the portal either way. And if matter can travel from source to destination, nothing would stop it from going in reverse, in principal.

I know very little about real-world theoretical physics, so I won't speculate too much here. However, the show has stated otherwise, so even if it is the case in real life, I'm just offering up a possible solution to explain this.

It's either the problem with sending matter streams in both directions at once, as someone mentioned, or a design decision driven by some strategic factors.

Both directions at once I could conceivably see as being a problem, but on the show, it's specifically that they can't go through a wormhole what was dialed in. They must first dial out of that gate to go through it.

As per the second comment, if we're speculating on design decisions, it could indeed be the case that the Ancients intended for matter to only travel one way, but let electromagnetic waves travel both ways. However, that would still beg the question as to how they set up this restriction, which could still be explained away by speed of the material entering it (i.e. solid matter vs light/radio waves).

See where I'm coming from? I'm not proposing to know anything about real-world physics in this regard (i.e. wormhole physics). I'm merely trying to give a SG-verse explanation as to why this may be the case.

[K^2]

Where does the show state that matter can't travel both ways? Objects can't travel both ways, because it gets atomized. But the high energy plasma state that it becomes is perfectly capable of traversing the wormhole. Considering Carter's statement about light mesons having time to reconstitute themselves, the gate probably sends matter through as quark-gluon plasma.

Both directions at once I could conceivably see as being a problem, but on the show, it's specifically that they can't go through a wormhole what was dialed in. They must first dial out of that gate to go through it.

I think you missed my point with that. Lets say that collision of matter streams causes problems. So you can't allow two objects to go through in both directions at once. How do you prevent that? Simple, you simply allow objects to travel in one direction only.

Both sides have buffers. Object goes in origin gate's buffer, and once it is fully inside, it is sent to the destination gate's buffer via matter stream + information on how it should be reconstituted. While the object exists the destination side's buffer, particles are still going both ways. Blood flow, etc. But they are kept in the buffer.

If a person steps through the destination gate and attempts to go backwards, he gets trapped in the buffer, which is flushed when connection is dropped.

This is 100% consistent with canon.

[Coronach]

Where does the show state that matter can't travel both ways?

Keep in mind the context of both examples I'm about to give. You could argue that these are not good enough as evidence, but it's the most verbatim examples I could find:

MARTIN
Matter can only move one way through an open wormhole.

*then later*

O'NEILL
Great show! Hey, Marty.

O'NEILL
Umm, what makes you so sure matter can only travel one way through a wormhole?

MARTIN
I don't know. I must have read it somewhere.

and

CARTER
Maybe because that's what it is. An intriguing idea. But it's based on way too many unfounded assumptions to make a valid theory. For example, you assume that matter can travel both ways through an open wormhole.

HAILEY
So?

CARTER
So, how do you know?

HAILEY
Well, until somebody shows me a real wormhole that can only go one way…

CARTER
That's not the point. You made an assumption.

I think you missed my point with that. Lets say that collision of matter streams causes problems. So you can't allow two objects to go through in both directions at once. How do you prevent that? Simple, you simply allow objects to travel in one direction only.

Then...apparently we don't disagree. I didn't think I missed your point, but I get it now I guess.

Both sides have buffers. Object goes in origin gate's buffer, and once it is fully inside, it is sent to the destination gate's buffer via matter stream + information on how it should be reconstituted. While the object exists the destination side's buffer, particles are still going both ways. Blood flow, etc. But they are kept in the buffer.

What to you mean by this? This portion confuses me a little, so, help me out? I understand the part about the buffers though.

If a person steps through the destination gate and attempts to go backwards, he gets trapped in the buffer, which is flushed when connection is dropped.

I can see this as being conceivable, as it's still the case that the matter that just entered cannot then travel backwards to the origin gate. Do we actually disagree on something here? I'm confused now

[K^2]

What I mean is, lets say you are stepping out through the wormhole. That takes a bit of time. During that time, some matter in your body is moving in reverse fast enough to cross back into the gate. If this wasn't accounted for, it'd be same as being cut in half. Yes, only for a brief moment, but how long do you think you can survive if you were cut in half.

So objects can go in and out of the buffer on both ends without harm. This is also demonstrated pretty well in one episode where Jack puts his arm into the gate from destination's end to keep the connection open.

Maybe because that's what it is. An intriguing idea. But it's based on way too many unfounded assumptions to make a valid theory. For example, you assume that matter can travel both ways through an open wormhole.

That's not just an assumption. This is based on very fundamental principles. Wormhole allowing matter to travel only one way would allow for violation of momentum conservation. And if we through that out of the window, then that's pretty much it for Physics. And I don't mean just modern physics, but the principle of studying the world in general. It's difficult to explain to a non-scientist how important it is, but a show that would suggest a violation of such principle could no longer claim to be a science fiction. Show must be believable, and something this fundamental is a requirement.

So I'd call this a mistake on Carter's part. And believe me, people with Ph.D. in physics do make mistakes of that kind. I've heard my share. Difference is that these people realize it fast when called on it.

So, buffers on both ends support objects going in and out. We also know that the basic physics of the wormhole would allow matter to travel either way. I really don't see what else could arise that would forbid travel both ways through an open wormhole, just as long as it isn't at once.

can see this as being conceivable, as it's still the case that the matter that just entered cannot then travel backwards to the origin gate. Do we actually disagree on something here? I'm confused now

I'm saying that travel in reverse is possible in principle, but is forbidden by the programming, if you will, of the gates. It's a safety precaution that is useful for exploration through the gates, which Ancients would have anticipated, and for making absolutely sure that two matter streams don't collide in transfer.

You can view this as pedantic, but I can't bring myself to say that the gate can't send things both ways. It's that the safety features prevent it from doing so. Kind of like nail gun refusing to fire if you don't press it against the wall. If you disable the safety system, it's more than capable of it.

[Coronach]

What I mean is, lets say you are stepping out through the wormhole. That takes a bit of time. During that time, some matter in your body is moving in reverse fast enough to cross back into the gate. If this wasn't accounted for, it'd be same as being cut in half. Yes, only for a brief moment, but how long do you think you can survive if you were cut in half.

So objects can go in and out of the buffer on both ends without harm. This is also demonstrated pretty well in one episode where Jack puts his arm into the gate from destination's end to keep the connection open.

I think I understand. Things passing in and out of the buffer makes sense for the reason you provided. Isn't this a bit different than going back and forth through the wormhole itself though?

That's not just an assumption. This is based on very fundamental principles. Wormhole allowing matter to travel only one way would allow for violation of momentum conservation. And if we through that out of the window, then that's pretty much it for Physics. And I don't mean just modern physics, but the principle of studying the world in general. It's difficult to explain to a non-scientist how important it is, but if a show that would suggest a violation of such principle could no longer claim to be a science fiction. Show must be believable, and something this fundamental is a requirement.

I promise you, I'm not a non-scientist . I am aspiring to get a PhD in some biological field...I just have to decide what I'm most interested in. Still two years of undergrad left though. Because of this, though, I don't understand much about a lot of the physics behind wormholes and such. I certainly do understand fundamental principles of science though. Would you mind explaining to me how a restriction to one-way travel violates momentum conservation? I promise I'll do my best to understand .

So I'd call this a mistake on Carter's part. And believe me, people with Ph.D. in physics do make mistakes of that kind. I've heard my share. Difference is that these people realize it fast when called on it.

I see. If what you say is true, I can accept this.

So, buffers on both ends support objects going in and out. We also know that the basic physics of the wormhole would allow matter to travel either way. I really don't see what else could arise that would forbid travel both ways through an open wormhole, just as long as it isn't at once.

I guess the only clarification I need is why the basics of wormhole physics allow this, but that ties in with the question I asked above. You can feel free to PM me with that if you want though, as it might be considered off-topic here.

[EDIT]

I'm saying that travel in reverse is possible in principle, but is forbidden by the programming, if you will, of the gates. It's a safety precaution that is useful for exploration through the gates, which Ancients would have anticipated, and for making absolutely sure that two matter streams don't collide in transfer.

You can view this as pedantic, but I can't bring myself to say that the gate can't send things both ways. It's that the safety features prevent it from doing so. Kind of like nail gun refusing to fire if you don't press it against the wall. If you disable the safety system, it's more than capable of it

I actually completely understand what you mean now. Once I hear your explanation about why it's possible in principle (as I mentioned above), I'm totally set.

Also, you could maybe argue that Carter did know what she was talking about, and knows that the restriction on two-way gate travel is due to something like what you're suggesting. But she simplified it by just saying what she did, as it's pretty much always the case when using wormholes via a stargate?

Just an idea.

[K^2]

Could be.

Purely theoretical explanation is simple. To each symmetry in nature there corresponds a conservation law. Momentum conservation is due to space symmetry. Space that allows matter to travel in one direction is not symmetric, ergo, momentum conservation fails.

A simpler way to think of it as a somewhat stretched thought experiment. Imagine I place two electrons really close to each other in such space. Their initial momentum is zero. They repel from each other. Now, there is a finite chance that they separate in direction along the wormhole. One electron will be able to move forward, but the other will be stuck, as it cannot go back. Final momentum is some quantity along the wormhole.

I know there are a lot of holes in this example, but it's only meant as an illustration to the explanation from pure theory. Symmetry of space would be broken, and that is very, very bad.

[Quadhelix]

A simpler way to think of it as a somewhat stretched thought experiment. Imagine I place two electrons really close to each other in such space. Their initial momentum is zero. They repel from each other. Now, there is a finite chance that they separate in direction along the wormhole. One electron will be able to move forward, but the other will be stuck, as it cannot go back. Final momentum is some quantity along the wormhole.

Just throwing this out here, but what about the possibility that the "space" inside the wormhole isn't static, but is instead "moving" along the inside of the wormhole at some significant fraction of c (say, 0.2 c). This would still allow electromagnetic radiation to travel both ways relatively unimpeded, but would require matter to be extremely energetic to travel "against the flow," as it were. I may be completely off base on this, but it is just a thought.

[K^2]

You are describing frame dragging. While it may quite possibly occur during formation of the wormhole (kind of like little eddies in the water after you move the oar through) it doesn't change the fact that simply reversing momentum of particles will send them back along the exact same trajectory they just came by. Yeah, you might have just stepped up the energy requirement to launch them, but keeping in mind that the extra energy needed is equal to the energy boost that the particles going to destination will get, and that this boost is just a fraction of total energy required to create the wormhole, I would say that it is safe to assume that the gate can provide sufficiently more energy than that, and if it was necessary, reversing the energy stream would not be difficult.

It's an interesting idea, though. I haven't really thought about frame dragging in this context before.