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In Star Trek: Generations when the crew are in the holodeck for Worf's promotion ceremony at the start, Captain Picard gives a command for the exit to appear. The exit appears in the middle of the holodeck, and he then walks out and leaves.

The exit is surrounded on all sides by people on the holodeck.

Previously, when the holodeck is shown the exit appeared on a wall somewhere, or it made it clear it was on the wall of the holodeck.

Given that the holodeck is an advanced hologram used in conjunction with an omnidirectional treadmill...how could the exit appear in the middle of a room with people on all sides? When Picard walks out into the hallway, he is in the same space as people in the holodeck.

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The people behind the exit are not just programs, as there were crewmen standing on all areas of the ship before this scene.

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    I would have to watch the scene again to be sure, but I am pretty sure everyone "behind" the arch was just a simulation. – Xantec Jun 27 '13 at 16:55
  • @Xantec, even if that's the case, the crew was standing behind where the holodeck appeared, making it seem as though the exit is in the middle of the holodeck room. Indeed at one point Deanna is on one side of the arch, and Will on the other. – Sonny Ordell Jun 27 '13 at 17:07
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    I thought the Holodeck used tractobeam technology to simulate the sensations of movement. If it was a treadmill how would it be possible for two people to walk at different speeds, or one to walk and another to stand still? – Monty129 Jun 27 '13 at 17:08
  • Correction: It uses forcefield technology – Monty129 Jun 27 '13 at 17:13
  • Actually there is another problem with this scene, pointed out in The Nitpicker's Guide. The floor of the corridor and the holodeck are level with the deck of the holographic ship. So when people are seen falling overboard into the water they seem to be falling below the floor of the holodeck. I guess the holodeck can lift the people and their setting upwards when people fall overboard and they don't really fall through the floor. But it really seems like the creators assumed the holodeck was magical and had no limitations. – M. A. Golding Dec 8 '17 at 20:28
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The other answers have the right spirit, but aren't exactly canonical. Here is an answer that takes into account only what is currently known about Holodeck technology of that era.

First, why the era matters. If you remember, way back in episode #1 of TNG, Data tosses a rock at the Holodeck wall and it hits. This early Holodeck tech used a variety of techniques to create the illusion of there being more space than there really was. However, it was not yet able to create enough space to prevent Data from striking the wall.

As the show went on, the holodeck received upgrades. Ultimately, at the end of TNG, they were able to trick Geordi's visor and to create convincing illusions of large spaces. (cave networks, large distances, so on)

Then, by the time of Voyager, the holodecks were capable of doing even more astounding feats. Such as being a venue for skydiving or space flight (gravity manipulation, wind control) and in one case, being linked together into a giant network of holodeck rooms to simulate a Nazi occupation.

The events of Generations takes place during Season One of Voyager, so the holodeck technology of Generations is roughly equivalent to Voyager's level. These are the most sophisticated holodecks shown in all of ST history. And the trick we are trying to achieve is no more complicated than projecting holograms on the walls of the room.

Holograms, unlike traditional projection, use light wave interference to imbue a flat sheet with different images depending on the angle that the image is viewed from. In this case we are talking about a 3D projection of another part of the holodeck. Clearly, whomever programmed the ship simulation thought it would be cute to put the arch there. As for walking past the arch (and into a wall) that can also be achieved with similar tricks that involve moving people without them knowing.

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    Actually the Holodeck tech in the early seasons of ST:TNG was replicating solid matter using replicator tech, the later seasons changed it to manipulating photons and using forcefields to give the feelings of weight and mass. The rock was real as long as it stayed in the Holodeck, that's why it hit the bulkhead. – Monty129 Jun 27 '13 at 19:10
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    @Monty129 I think it did both all throughout - it's generally small, simple objects that get replicated (stuff that's not worth the processing power), while more complex ones are simulated. Don't know if it's ever been officially stated (like in a technical manual or something), but it matches what we've seen. – Izkata Jun 27 '13 at 23:47
  • If I remember correctly, there's one TNG episode, where Wesley(?) and others throw snow out of the Holodeck and/or leave while being covered in some snow (or being wet). It's been a while, but there have been at least some instances where stuff didn't just disappear at the door. – Mario Jun 28 '13 at 0:52
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    @Mario you are correct, anything created inside the holodeck (with the exception of food) in the later seasons could exist for a very limited time outside the holodeck before it reverted to energy and discorporated. – Monty129 Jun 28 '13 at 18:48
  • Just remembered something else being interesting and perfectly fitting into this "evolution"/idea: In one of the last episodes of Voyager, Seven sets up a holodeck program for her rendezvous with "holo Chakotay". Her otherwise visible implants disappear while the holodeck program is active. In earlier instances this hasn't been possible and characters always weared real clothes they brought with them, but at the same time this might be just something special, because I could imagine Seven manipulating her own view? Although that would be rather odd as well. – Mario Jun 28 '13 at 21:46
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There are many times within the holodeck where individuals are in separate simulated locations [1]. This would strongly imply that that the holodeck is taking into account the individual perspectives of each observer, and projecting an environment unique to each observer. So from Picard's (or the camera's) perspective people could be behind the arch, but that might not reflect the actual physical location relative to Picard.

  • [1] For example in 'Elementary, Dear Data' Pulaski is captured and separated from Data/Geordi though all three entered the same holodeck.
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6

If you think about the holodeck for a while you have to accept that it's playing around with people's positions and distances from each other. If we stipulate the reality that the holodeck space is not actually behind the door, and that thus the people are not actually behind the door, you have to accept that they're actually physically somewhere in the room ahead of the exit. That they appear to be behind the exit is an illusion.

But real people inside the holodeck can touch each other - this means that after Picard enters the room near the door, he must actually be physically close enough to the others irrespective of holodeck trickery. So, the only conclusion you can reach is that the holodeck is probably constantly moving people around with tractor beams to keep up the illusion. You would ordinarily feel it if you're being shuffled around a room, so there also has to be an inertial dampening component.

Out-of-universe - the writers probably didn't think about it, but thought it would look cool, which it did.

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Actually, there are several technologies that were progressively integrated in holodecks. Initially it was mainly a holograpgic technology within the normal space occupied by the holodeck. But the new understanding of the continuum that resulted from the technical work to improve the warp drive also led to the discovery of side-space, which can be opened and connected to our normal space, without being in our space. Then the holographic technolofy operating in side space allowed for more space to be available. People in the side-space holographic scenery were no longer in the holodeck of normal space. Connectivity between side-space and normal space was a very complex operation as can be seen in the nazi episode of voyager.

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    This "side-space" you're referring to seems to be made up. If it does actually exist within Star Trek, can you provide a reference/example? (And no, in The Killing Game, the walls between multiple holodecks were torn down so that they could be linked together to create one large holodeck. If this "side-space" exists, that was not an instance of it.) – Izkata Jul 7 '13 at 21:59

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