system
Given the following post, respond as if you're commenting on Reddit:
human
[Stargate (all?)] How exactly do the stargates "work" at the practical level?

My s/o has had me watching SG: Atlantis recently, since I could not be arsed to get through all of SG-1. I've seen a fair few seasons of SG-1 already, plus of course the film, if that has any bearing. I don't mind necessary spoilers for explaining points, just try to steer clear of huge arc- or reveal-type spoilers if possible.

We've been idly (and occasionally more fervently) discussing and debating the specifics of what's plausible but just "more advanced tech" (such as various naquada-based effects just being akin to "more potent uranium", unless I'm terribly mistaken) and what's literally hand-waved "the Ancients did it"-type magic that is made to appear like tech.

One of the early episodes in SG:A features a conundrum related to their stargate's operation, re: getting stuck mid-transit. Now, I recall SG-1 and so on already revealed that anythin stuck halfways across the event horizon when the gate powers down gets cut off (at the molecular/atomic level?). But how exactly, point by point, does gate-transit work?

As far as I know, the event horizon has some properties that allows them to open gates into e.g. hard vacuum without the air from their end rushing through the wormhole, and it generally acts like a "wormhole-based fax-machine", not totally unlike the basic idea behind Crichton's *Timeline* transporters, right?

The above-mentioned SG:A episode mentions that the gate only transmits "discrete entities" (as opposed to acting like e.g. the Aperture Science Handheld Portal Gun, where travel is instantaneous and the affected surfaces are just made spatially interchangeable). But what does this actually work? I think my main question is:

- How does the stargate "know" what a "discrete entity" is?

I've some basic understanding of hard sciences, but it just seems so baffling that the gates can "sense" what a discrete entity is, and how it avoids e.g. leaving vacuum in people's internal cavities when they're transported. Like, how is the differentiation between each "entity" handled, physically speaking? Does it act like modern image-manipulation software, where "contrast tendencies" between molecules or something is used to define an outline, akin to how Photoshop can approximate shapes in a complex image?

Of course there's a lot of "the Ancients just made *that* advanced tech" involved, but the physical possibility of differentiating between "entities" feels off, including the plot-convenient transit of whatever gear you're wearing and whatever air you're containing, despite all of that technically being separate entities or non-entities.
gpt
SG-2's Science Officer reporting in. 

When the Stargate activates, it creates the unstable vortex, colloquially known as the "Kawoosh". This has 2 purposes: 

First, it disperses the excess energy used to establish the wormhole. This is the primary reason. 

The other reason for this is the clear the area immediately in front of the gate so the gate can determine baseline environmental conditions. It uses this to determine how much force to exert outward to prevent atmosphere/water/vacuum from entering the wormhole. 

In effect, it creates a weak force field at the event horizon. 

With sufficient force (generally very little), objects can enter the event horizon. When objects are partially through it, the gate can detect the disruption to the field at that point. When the object has passed through completely, there is no longer a disruption to the field locally and the gate instead senses normal environmental conditions. This is how the gate knows to send data.