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The episode jumps right into SG-1 testing out the alterations Carter has made to Merlin's alternate phase machine, which can shift the matter within a certain distance to another phase. I was a little jarred by the beginning - I almost felt like some smoother opening had been cut off. SG-1 seemed way too happy compared to the despondency at the end of the previous episode. SG-1 is sent to a planet populated by former Jaffa who are the imminent targets of an Ori takeover.
The inhabitants of the planet are ready to fight the Ori if necessary, but they are clearly overmatched and would have no chance, so they are depending on the alternate phase machine. A test of the machine used to hide the entire village is successful. However, when the Ori finally arrive, the machine fails, at least initially, and the inhabitants and SG-1 must fight. It's quickly obvious that they are losing badly.
During the fight, Carter is seriously wounded by an Ori soldier, but manages to kill him. Mitchell finds her, and Carter manages to show him how to quickly repair the alternate phase machine. When he turns it on, the entire building they are in disappears from view, and the Ori soldiers can walk right through it without seeing or feeling it. (My problems with this are discussed at length below.) Only Carter and Mitchell are out of phase, leaving the rest of the inhabitants and SG-1 to the mercies of the Ori.
It turns out that Tomin, Vala's Ori husband, is the commander of the Ori troops on this mission. He has apparently moved significantly up the ranks and appears to be second only to the Prior on this mission. He orders Vala to be sent up to his ship, and then callously murders all of the villagers that had been firing weapons againsst them. This is a very chilling scene because of how quickly and easily the order is given to kill the people. At this point, the building Mitchell and Carter are in disappears, but the Ori troops cannot get any explanation from the remaining villagers.
Upon the Ori ship, we learn that the Prior has ordered Tomin to convert her to the Ori. Throughout the rest of the episode, Vala tries in her usual annoying way to show Tomin the error of the Ori's ways. Tomin tells her a traditional Ori story of a "line in the sand" that separated a person from the love of the Ori that eventually widened into a chasm. It appears that the gap in understanding between Vala and Tomin has also widened. However, Vala begins emphasizing the fact that the message of the Book of the Ori does not say it's acceptable to kill innocent people and plays on Tomin's guilt for having become a mass murderer.
I have to say that I don't understand what Tomin sees in Vala, but he clearly still cares for her, and he has just enough basic human goodness that he listens to her. Eventually, at the end of the episode, Tomin returns Vala to the planet just before the Ori leave. However, he refuses to leave the Ori. I wonder just what his punishment will be for releasing Vala. I also wonder if he will begin to try to counteract some of the Ori's evil, much in the way that Bra'tac counseled Teal'c to moderate Apophis's actions as his First Prime (seen in flashbacks in season 5's Threshold).
So, while Tomin is on the ship, Vala is trying to persuade him to go against the Ori. While Tomin is on the planet, he questions the inhabitants about the building that went missing. Some of the inhabitants decide to cooperate with the Ori, but even so, they cannot defeat the alternate phase machine. Teal'c's identity is revealed; whenever that happens, I expect a "shol'vah" pronouncement, but I forget the Ori are not the Goa'uld. Eventually the Prior decides that they will not be able to find the building and the inhabitants are unsavable, and thus to bomb the entire site from orbit. The Ori abandon all the inhabitants and return to their ship.
During all of that, Mitchell and Carter are in the building that is in the alternate phase of reality. Carter's wound is extremely painful, and I found her plight very realistic. She doesn't think she's going to survive, and is mostly concerned about ensuring that the alternate phase machine doesn't fall into Ori hands. Mitchell works continuously to keep up her her morale and to enlist her help in adjusting the machine to again encompass the entire village. We get some slightly humorous scenes as Ori soldiers and the Prior walk through the building to no effect. This would be funnier if Carter wasn't so badly hurt and if the machine didn't annoy me.
By the end of the episode, Mitchell and Carter get the machine working, just in time to shift the entire village to the alternate phase before the Ori bomb it. When Vala is sent to the planet, she arrives in a bomb crater, then the village un-phases and everyone is safe. Carter is returned to the SGC, where she should heal without complications.
The interaction between Carter and Mitchell while Carter is injured is much more meaningful when nearly the reverse of this situation is shown in the following episode "The Road Not Taken". Mitchell does seem to have become closer to Carter than to the other members of SG-1 in his time as commander. The other nice build-up for the next episode is Carter's ability to make the machine work on a village-size-scale; it will be employed on a wider scale later.
What implications does this episode have? The title "Line in the Sand" implies that Earth and its allies are drawing that line in the sand and saying "no farther" to the Ori. However, Earth's forces were largely ineffective in this episode. The plan to save the village did not initially work; without last-minute efforts by Mitchell and Carter, everyone would have died. That's not much of a line in the sand. However, now that the machine is working, its potential is enormous.
Possibly the "line in the sand" refers to Tomin's attitudes toward the Ori, and they have crossed the line with him. That would imply that he will now try to work against the Ori to some extent. We do not know.
Yet again, Teal'c doesn't have much to do in this episode besides try to keep up the morale of the villagers. Vala has been rejected again by her husband, but possibly on more hopeful terms. Carter and Mitchell have presumably forged a closer bond with Carter's near-death.
For the purposes of this discussion, let's assume that as seen in this episode, the alternate phase machine shifts all of the matter within the building and the building itself to the alternate phase. That includes the building, the people inside, the equipment inside, the air inside, the bacteria inside, etc.
Here are the problems and contradictions I have found with this idea, just thinking about it for about a day:
If the shifted matter no longer causes spacetime to curve - that is, it no longer exerts gravitational force on other matter - then the building and everything in it would travel according to their inertia. As the Earth spins, they would not "stick" to the surface of the Earth, but would travel tangentially away from the surface into space. (Note: the people and buildings would not fall apart, because they are held together by molecular and electromagnetic forces, not gravity. Do those forces still work in the alternate phase? Even I'm not going to try to reason through that.)
I have no reason to think that shifted matter wouldn't cause spacetime to curve (since the matter is still in our spacetime, i.e., our universe), so let's assume the shifted matter still exerts a gravitational force on normal matter. In that case, all of the shifted matter should fall through the Earth! First, normal matter can pass through shifted matter. Second, the mass of the Earth and the mass of the shifted matter exert a gravitational force pulling them together. Since the shifted matter is in another phase, there is no "floor" to stop the gravitational force from drawing them together immediately. The shifted matter building and its contents should be drawn toward the center of the Earth.
Now, I realize that this is a common problem with "alternate phase" stories, such as in season 3's "Crystal Skull". To some extent I can suspend my disbelief. However, when added to the problems I discuss below, I have some serious issues suspending disbelief.
The atmospheric pressure of the Earth's atmosphere (as well as other planets' atmospheres) is created by the gravitational force of the Earth pulling the air against its surface. The amount of atmospheric pressure we feel is based on the strength of gravity and the amount of gas in the atmosphere. In the episode, once the building and the air inside of it is shifted, there is nothing to prevent the air inside the building from leaking out and encircling the Earth in an extremely thin layer. (This is assuming that for some reason the "normal matter" surface of the Earth prevents the shifted matter from falling into it, as per my gravity discussion above.) There is no way the building is sealed enough to prevent air leakage, and there is no shifted matter air outside the building to push back on the air inside the building and keep it from moving. This would be analogous to a spacecraft leaking in space. This effect should happen quite rapidly.
OK, so let's assume that for some reason all the shifted air stays inside the building. In that case, Mitchell and Carter will eventually run out of oxygen. They have a limited supply of oxygen - the oxygen in the building - and they cannot (maybe I should say "should not") be able to breathe normal air, because it would pass right through them. Now, there's probably lots of shifted air in the building for just two of them, but they don't even mention the issue.
One final issue: once the building and everything inside it is shifted, there is a vacuum left in "normal space" where the building was. Air from the surroundings should rush in extremely quickly and fill in the hole. For a building-sized hole, I would think this would cause very strong winds for a brief period, as well as a thunder-clap of sound. (And, of course, once the shifted matter is returned back to normal, all of that air, plus any solid objects, will have to be displaced. Even I'm not going to deal with that issue at the moment.)
If you could come up with some bizarre reason why the shifted matter people can hear the normal matter people, then why don't the normal matter people hear the shifted matter people? I don't see why it wouldn't work both ways if it works one way.
First, a quick explanation of light and how we see things. An object emits or reflects light waves, which are waves that are made up of electric and magnetic fields and do not have any mass. Normally, those waves travel into a person's eye, where the molecules in the eye absorb the light waves; when this happens, the energy from the light causes the molecule that absorbed it to become excited.
The first question is: can shifted matter absorb light waves? Since the light waves do not have matter in them, I suppose they would not be affect by the phase shifting. In addition, since the shifted matter is still matter, I suppose the eyes of the shifted people could absorb the light. That's what's shown in the episode.
So here's the first problem: when Mitchell, for example, sees the Prior walk through the shifted building, his eyes are absorbing light reflected from the Prior. Once Mitchell's eyes absorb that light, it no longer exists. A normal matter person standing on the other side of Mitchell would see a gap in the light that the Prior reflected, the gap being the light that Mitchell absorbed.
Now, this doesn't seem like a big issue, because Mitchell's eyes are small, so the gap would be small. However, the light reflected by the Prior also hits Mitchell's clothes and body, which then reflect the light (by absorbing and re-emitting) in other directions. So the normal matter person standing on the other side of Mitchell from the Prior would see a "shadow" of missing light where Mitchell was. The same thing would happen for all the other shifted matter in the building - they would cast shadows. This would make the shifted matter immediately locatable, even if the normal matter couldn't affect it directly.
The second problem: if shifted matter people can see light emitted/reflected by normal matter people, why can't normal matter people see light emitted/reflected by shifted matter people? There is no physical reason for them not to, given the assumptions above that were made to match up with Mitchell actually being able to see normal matter people. This is another case of the writers trying to have their cake and eat it, too.
The third problem: since we have established that shifted matter people can see the light emitted or reflected by normal matter people, then we can assume that shifted matter people should be able to "see" all types of light that normal matter objects exist: radio waves, ultraviolet light, infrared light, etc. At the end of the episode, the Ori bomb the village. We don't know whether the bomb was conventional or nuclear, but both types involve the emission of lots of light: infrared light (as heat), visible light, and potentially x-ray or gamma-ray light. The light that the bomb emitted should have toasted the shifted matter people to a crisp, since their shifted matter molecules could absorb it. However, they would not be hit by shrapnel.
What I Think They Should Have Done
I realize that science fiction requires suspension of disbelief, and I am very willing to cooperate with that, just not to the extent required by the alternate phase machine. What would I have suggested?
The gravity issue, namely falling through the Earth's surface, is pretty difficult to overcome. If the writers just ignored it (like they do) and had none of the other issues, I would be happy. I don't see a plausible way to get around this without employing lots of technobabble.
The air issue is tougher. I would suggest that the alternate phase machine shifts matter by creating a "bubble" around the objects that are shifted; the interface between the bubble would stop or slow down the flow of air. In this way, the air would not escape, but the prospect of running out of oxygen would still exist. I think that could actually add some suspense and realism to the proceedings - the characters would have to consider how long they are going to be shifted and will they have enough air? They would also have to deal with the fact that shifting a small volume of space would result in a very limited supply of air. However, this solution would prevent normal matter people from walking through the shifted matter space.
The sound and light issues should be completely changed, in my opinion. I do not think the shifted matter people should be able to hear the sound or see the light from normal matter people. Again, I think that this would only serve to heighten suspense. Once the people shifted, then they would be cut off from contact with the normal matter world. They would have to decide if and when they thought it was safe to return to normal. This change in how the phase machine worked would also protect the shifted people from the light-effects of items like bombs.
What About Shifting an Entire Planet?
In the next episode, "The Road Not Taken", Carter increases the alternate phase machine's capabilities to be able to shift the entire Earth (and everything on its surface and in its atmosphere) to an alternate phase. Operating the machine on this large size scale makes many of my objections go away.
The gravity issue doesn't come up, because the whole Earth is shifted, so gravity will still operate normally on everything on its surface. The air issue isn't a problem, because the entire atmosphere is shifted. Sound and light transmission between people and objects on the surface of the Earth would also work like usual.
However, any normal matter person in space and not on the Earth should not be able to see or hear things from the Earth, and vice versa. The hearing is a non-issue, since there is the vacuum of space in between them, anyway. However, I still maintain that the shifted matter people on Earth should not be able to see the normal matter objects in space.