If the holograms are really advanced like the hologram of Snoke in The Force Awakens and they are haptic, which means you can feel them, can they be used to make an immersive virtual reality enviornment, where you can and not limited to, have sex with any girl you want. Can holograms in theory get sophisticated enough?
The question currently has the hard science tag. I've attempted to link to sources where I can, but most of this falls outside the range of current science so sources necessarily don't exist.
The Star Trek holodeck has three primary technologies required to get the desired results, along with a few secondary technologies.
The most important technologies are non-existent to impossible, while some of the least important technologies are in use right now. In general, I don't think it's likely to be possible as shown in Star Trek.
An alternative to holograms and forcefields is some type of programmable matter such as claytronics. While this would require a tremendous amount of computation and isn't currently feasible, there don't seem to be any physical barriers to the concept.
I believe that between a 3D background and a foreground comprised of programmable matter, along with some type of moving floor to keep the user centered in the room, a reasonable simulation could be constructed. As such, I've put these three sections first.
Following these sections are sections on simulating acceleration (potentially feasible with a high-end centrifuge, but unnecessary for your use case) and on Star Trek-style "holograms" that I don't believe are feasible in real life.
The background of the holosuite is a 2D image projected such that it appears to be 3D. While necessary for full immersion in a Star Trek Skyrim game, this is the least important aspect for simpler tasks like interactive porn, where the major parts of the simulation are within a few feet of the user and the background can be simple or nonexistent with little practical issue.
The simplest form of this already exists in the real world. Using methods such as shutter glasses, and tracking the current location of the two eyes in the room, the walls can simply be made of high-resolution monitors that display adequately-realistic 3D imagery using methods similar to modern computer games. In this case, the background strobes a "left eye" image while the right eye is blocked by an opaque shutter, then the "right eye" image is strobed while the left eye is blocked. In order to avoid flicker, this needs to be done at 60 to 100 frames per second per eye (possibly as high as 500 fps, but it depends on the technology used to generate images). Difficult, but certainly not insurmountable.
Using advanced technology, you could likely build contact lenses (or internal implants) that act as shutter glasses. These would perform just like existing glasses, but be virtually unnoticeable. While we don't have these yet, there are patents in place for implanted cameras and bionic lenses, leading me to think shutter glasses are very plausible.
A different way would involve the use of tiny laser beams located at each pixel. With two beams at each pixel, one pointed at each eye (again requiring proper eyeball tracking), the image could be constructed like a traditional LED monitor, but the collimated light would restrict the left/right images to the correct eye.
Physically constructing these tiny laser beams might be very hard, so it would likely work better to use tiny, adjustable mirrors and rapidly strobe some more traditional lasers across them from the corners of the room. I've found research on microscopic mirrors used in conjunction with hologram production, but these require a lens between the wall and the user (and use interferometry which is just cool).
These technologies could be expanded to allow multiple people to see simultaneously. Essentially, you just create an image for every eyeball in the room and display it rapidly enough that each person sees a smooth framerate.
The human eye has an angular resolution of about 1 arcminute, or 0.02°. If we assume the room is cube-shaped with square walls, the room is large enough that the user stays roughly in the center of the room at all times, and that the pixels on the wall are of equal size across the wall, we can compute the pixel resolution needed at about 6900 x 6900, which is about 5.75 times as many pixels as a modern 4k screen, or about 30 4k screens to do four walls and a ceiling. While these numbers are high, this is completely plausible with reasonable advances in technology.
Programmable matter (linked above, but again here and here for convenience) is a theoretical concept with a decent basis in reality. The basic idea is to build a large number of tiny robots that can be programmed in real-time to move, deform, assemble and disassemble in a way that mimics other objects.
So the 3D image on the wall would assemble into a physical object as the virtual object moved closer to the user than the wall. Likewise, an object moving beyond the wall would disassemble itself as it approached the wall, becoming a purely virtual object again.
The robots would have to be capable of changing their surface textures (mainly between rough and smooth), changing the stiffness between robots (to replicate either soft muscle tissue or a solid metal bracelet), and changing their color (to give visual texture). Even better if they could change their conductivity (mainly heat transfer, so aluminum would feel cooler than cotton, for example), reflectivity (so a wet surface would appear different than a dry one), and temperature to more accurately reflect the objects they represent.
In order to keep the simulation convincing, you'd ideally want to keep the user approximately in the center of the room. This way the user can walk from room to room, or run across a field, without running into the wall, or having to break the immersion by manually returning to the center of the room before the simulation continues.
In addition to giving the illusion of solid objects, advanced force fields could be used to slowly move the user back to the center of the room without being overtly noticeable. However, as above, such force fields may not be possible.
Alternately, the floor could be setup as a treadmill that works in any direction. There are things like the Virtuix Omni, but they require you to essentially strap on the entire treadmill.
A better method would involve floor plates that can be programmatically moved around the room with edges that disassemble as they move too far away and reassemble as they get close to the user. I don't know of any real-world research on this, but it doesn't seem particularly impossible.
In order to simulate constant acceleration in something like a flight sim, you'd need some kind of artificial gravity. In your example use case, this would be totally unnecessary unless you're going for "extreme porn while falling off a really long waterfall with no terminal velocity" or something equally off the wall.
The best way would involve something like a graviton emitter (essentially, a device that creates real gravity just like an LED creates light), but these are completely fiction as far as we currently know.
As Anonymous Anonymous' answer shows, it may be possible to accomplish a similar result with very large magnets using diamagnetic levitation. Current magnets that might have the strength to levitate a human only have a bore size of about 1 inch, so we'd need huge advances in the size of the magnets, but it's within the realm of plausibility.
An alternate method involves putting your entire holosuite in a multi-axis centrifuge with a lot of power. It wouldn't be able to perfectly simulate all possible accelerations, but with sufficiently advanced technology (and energy), you could probably do a pretty good job. Plus, this is a technology we already use to some extent in real simulations.
People use tilting platforms to simulate sideways acceleration for racing and flight sims today. It's not perfect, since it always has exactly 1 gee of acceleration, but it can still be fun. Additionally, we have centrifuge simulators in use for professional training.
The best case scenario here would involve putting your centrifuge in space so you could simulate anything from free fall to whatever gee-force limits you feel are safe. On Earth, the only way to simulate free fall is to put your centrifuge 30 stories up and actually drop the user. The more stories you have, the longer of a free fall you could simulate. You can get decent simulations for racing sims with very small motions, but flight sims might need excessively large drops to get perfect.
The most important aspect of the Star Trek holosuites are the high-resolution force fields that push back on the user's skin. To my knowledge, such things are completely outside the range of current science.
There are various "force field" technologies being studied to protect military targets from explosions (see here, for example). However, these work by turning the air between the explosion and the target into plasma, which reflects the blast wave.
It's possible to create micro plasma bursts that you can feel (these guys did it), but I don't see how this could be used on a local level for anything that would push on your body hard enough to feel like a solid object.
The visual portion of the various objects and actors within arms reach are made of free-space holograms. (I don't think free-space holograms technically count as "holograms", but everyone who's seen any sci-fi movie knows we're talking about visual projections to mid-air.) I'm not sure how likely it is that we'll really succeed here, but it's looking promising.
These guys (links to the same article as the micro plasma bursts above) have created a form of holographic images by making tiny explosions in the air. Right now, the images are safe to touch but rather loud, and monochromatic. If this technology can be improved to sufficient resolution and color reproduction we might have viable method of achieving this.
Alternately, there's a question here with more discussion on the subject.
In general, it seems unlikely we'll be able to render these holograms in such a way that they'd appear to be solid from multiple discrete angles. Any light on the front of an object would be visible through the back of the object. So multiple people in one room wouldn't work very well, though you could use multiple rooms and project different users into other user's rooms.
An alternative method would involve some type of floating mirror dust that could reflect external laser beams towards the user's eye while allowing background light to pass through unhindered and being totally safe for the user. I'm not sure how feasible this is, but it would have better appearance, since light on the front of an object would be reflected only towards eyeballs that should see the front.
This is just an addition to MichaelS' answer regarding acceleration/artificial gravity.
While not really acceleration, electromagnetic forces may be used to create a pull on the subject's body.
How you would do this:
- First - you invent a very strong and focussed electromagnet (A Bitter Solenoid may be used)
- Second - You construct, cool and power a few of them.
- Third - You should make sure your subject doesn't wear too much ferromagnetic stuff, otheewise it may end up rather uncomfortable.
- Fourth - You're done! Readjust the magnets a few times the second (in case your subject moves) and you should be ready to go!