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I'm creating a universe where ftl space travel is possible by folding space. You can only travel point to point with direct line of sight.

The example I plan to give in the book, to clarify, is that day you are a circle on a sheet of paper and you want to get to a square on the other side. Instead of travelling through the entire sheet of paper you fold the paper over in the third dimension and travel vertically next to the square. The paper then returns to its previous state and the objects in the teleported location swap places.

I was wondering if it would be reasonable to do a blue shiftish thing since an object is approaching so quickly. I'd like the sky's colour to drastically change in an observable way, but over a bit of time so that the protagonist thinks it's weird. Does this sound too far fetched or is there a scientific way to explain this?

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    $\begingroup$ Seems like a teleporting ship would not blue-shift at all. Only matter travelling the long way would blue-shift, as only that matter would meet and reflect light along the way. Consider having the act of teleportation do something, like generate a big electric fields or flash brightly for a moment. $\endgroup$ – user535733 Apr 18 '17 at 0:07
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    $\begingroup$ If it appears 'in' Earth's atmosphere it should displace a sizable portion of air- which I imagine would make a audible disturbance. $\endgroup$ – Friendlysociopath Apr 18 '17 at 0:24
  • $\begingroup$ @Friendlysociopath if the ship swaps places with the air in its new location, the would be no sound at all from the teleportation at all. You would suddenly have a very powerful drive holding up a ship though. $\endgroup$ – Asher Apr 18 '17 at 0:39
  • $\begingroup$ @Asher wouldnt that require a device, even it is on the sending side that will do technobabble to the "matter" of the target site (and the shape of the object would need to be removed). As far as I can reason teleportation out the issue if mass/energy/matter etc. at the target site is a pretty big technobabble OR just don't mention it ever. $\endgroup$ – Enigma Maitreya Apr 18 '17 at 0:48
  • $\begingroup$ In one sense your envisioning the method of travel from "Dune" and all its sequels. You could look there and see how they dealt with all that, but I am pretty sure you will find they didn't the ship simply was there. $\endgroup$ – Enigma Maitreya Apr 18 '17 at 0:50
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Your space-folding teleportation is effectively generating a wormhole connecting the two points in space-time.

For a simplified notion of a wormhole, space can be visualized as a two-dimensional (2D) surface. In this case, a wormhole would appear as a hole in that surface, lead into a 3D tube (the inside surface of a cylinder), then re-emerge at another location on the 2D surface with a hole similar to the entrance. An actual wormhole would be analogous to this, but with the spatial dimensions raised by one. For example, instead of circular holes on a 2D plane, the entry and exit points could be visualized as spheres in 3D space.

Source: Wikipedia

Around the wormhole itself there might be disturbances in the fabric of space-time itself. This could create optical effects like rippling distortions or lensing effects of focusing light sources.

There are most probably quantum effects associated with the wormhole. This could lead to spectacular effects as the wormhole itself collapses after the passage of a spacecraft. Since this collapse takes place in a fictional world it is not unreasonable to 'bend' the science a little to allow the wormhole collapse to take some time. This could be tiny fractions of a second to, possibly, minutes. The collapsing wormhole could produce radiation like light, particles and exotic matter. This would resemble a spectacular lightshow above the Earth's atmosphere.

Most people seeing an unexpected eruption of light above the atmosphere would think it's weird. This answer 'bends' the science of wormholes a bit, but no more than most science-fiction striving for a reasonable amount of scientific verisimilitude.

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    $\begingroup$ +1 to the lensing effect and wormhole collapse. And the complete answer also depends on the viewer position. If you are perpendicular relative to the wormhole plane, then you will see a "window" opening with a lensing effect. If you are viewing from the side, you will only see disturbance and sudden appearing of the spaceship $\endgroup$ – Vylix Apr 18 '17 at 7:56
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    $\begingroup$ @AdiNugroho Quite right. The question appears to assume someone looking upwards (well, that was my interpretation), but depending on your perspective the effects will, more or less, vary. Although if the wormhole mouth has a spherical structure the effects should look the same in all directions. Whereas if it was a flat, two-dimensional wormhole mouth, then its appearance will be very different according to the viewer's position relative to the teleporting ship. Effects will vary with the geometry of the wormhole mouth. The OP can choose their preferred lightshow. $\endgroup$ – a4android Apr 18 '17 at 9:11
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The method of teleportation you are describing is the same method used in that much-maligned book, Battlefield: Earth.

In it, interdimensional teleportation is done by "swapping coordinates," the teleporters define cubes of space and then "swap out" the space inside temporarily. So, anything that was within the space of the "sending" teleporter appears inside the space of the "arriving" teleporter.

The key here is that teleportation is instantaneous; space is either THERE or HERE, but never half-way in between. There's a loud bang, a flash of light, and then it's DONE.

Your FTL drive sounds like this. You fold space so that an object HERE appears THERE. I'd expect this to either be:

1) Instantaneous, or 2) Look like Hyperspace from Star Wars.

The thing is, there's no blue shift because there's no actual MOVEMENT happening. The ship leaves THERE and arrives HERE via some higher dimensional wrangling.

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Your paper analogy is good and it may give some very interesting effects. As other comments have pointed out there would be little to no blue shift (although, light can do some pretty weird things when you you start talking about bending space), but there may be some very visible effects from bending space. Remember that your not actually bending a sheet of paper, it's more like rapid rotation of (at least) 3 dimensions around some fixed reference in a higher dimension. The visual effects, depending on what else must be moved through to fold space over could look like a a black splotch, a spiralling window to deep space, etc. The main thing here is to consider the speed of your folding process, and then how the ship is folded.

The speed is important because it determines how gradually the image will change.

The mechanism is important because it dictates how the ship appears. Is it a leaf in book that's being turned to the right page? Is it fractionally present across 4D and slowly dialing in on the right 3D location, or is it just sitting just outside the folding contact region waiting to fly through when the messy inter-dimensional work is done. There is a lot of leeway here but the big takeaway is that space isn't paper. Check out the wiki on hypercubes (https://en.m.wikipedia.org/wiki/Tesseract) and view the animation. It might help to visualize.

Hyperspace can be fun. Hope this helps!

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This idea has no basis in physics, but would allow you some visual effects and possibly other effects.

Say the two pieces of space you intend to bring together allow electromagnetic radiation to traverse between them in the time before the juncture is enough to allow matter to swap places. It may not necessarily be complete - not a portal in the sky. But if an increasing percentage of sunlight entering a given area of the sky is scattered into a different 3d space rather than down to your eye, that area will seem darker and darker - probably darker blue, like the sky at dusk. If there is visible light on the other side an increasing amount of that will come through - and so in addition to the dusk effect of local light not leaving that space you will have an increasing percentage of light from the distant space entering your space. Light from a distant red sun mixed with a dusk effect on your side would make a purplish combination which would be pretty cool.

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You could treat faster-than-light-travel like faster-than-sound-travel and have some kind of 'photonic boom' (like a sonic boom) involved, where you get all the light emitted previously by the ship still catching up to it. This is already an existing effect that can (only) happen in non-vacuum, where light actually travels slower than light speed.

Say a ship would jump in from a light-year away, you would get light coming your way for a full year, slowly fading as the 'apparent source' moves away from you at light speed.

You could also do all kinds of stuff with the rotation of the earth and the earth moving in orbit: once a year there's this weird blue star that's visible for a short time in a certain constellation (where the ship came from), and every year it's getting dimmer.

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