Stellarator wrote:

Source of the post Ignoring for the moment the viability of that technology, and assuming that the Einstein-Rosen Bridge interpretation of that possible phenomena is correct, how visible would such a spatial structure be? 

Let me first get out of the way that current thinking is that wormholes probably cannot exist even artificially, because of exotic energy requirements or that they may lead to causal paradoxes.  That being said, in the realm of mathematical solutions to general relativity they are very interesting objects and we may like to study their properties and appearance.  That leads to ways they could be found in observations, which seems to be what you're most interested in, and we'll cover how that works in some detail.

So the short answer is that a wormhole would be detectable due to how it bends light around it.  From close by, the gravitational lensing effects would be very obvious (the visuals for the wormhole in Interstellar come to mind).  But even from a very great (interstellar or even intergalactic) distance, the effect could still be visible as gravitational microlensing, which shows up in photometry for the light curve of a bright source passing behind the lens.  This works because a lens can magnify or demagnify an image and change its brightness.  Gravitational microlensing is also a method we use for detecting exoplanets, and potentially even rogue planets.

The biggest issue with microlensing events is that because they depend on a chance alignment of the Earth, lens, and a background source, they are generally one-time-only observations unless the lens lies in a sufficiently tight orbit in a bound system.  Otherwise, we may know where the lensing object is on the sky, but not necessarily how far away it is with great precision, or which direction it is moving, and it may be completely invisible unless it happens to pass in front of another bright object again.

The upshot is that the microlensing signature of a wormhole should be very unique, as we'll see shortly.

Stellarator wrote:

Source of the post at what distance from Earth would it be invisible to our observations using radio telescopes or other detective instruments?

This depends on the size/mass of the wormhole and the particular type (metric) it obeys.  We could imagine a "supermassive" wormhole with a lensing power equivalent to a supermassive black hole, for which the strong lensing effects could be directly imaged with a network of telescopes like EHT from many kiloparsecs away.  But a "more reasonable" sized wormhole 100s of kilometers across could be detected through microlensing.

As it turns out, there are numerous studies on how different types of wormholes would appear by microlensing, and even a survey to look for evidence of (natural, intergalactic) wormholes in gravitational microlensing events of gamma ray bursts.  Obviously, they didn't find any conclusive evidence for them (or we'd all have heard of it already), but instead this serves to place observationally-derived limits on their numbers and masses.  But more importantly for our purposes, it serves to show that the technique to look for them exists in a functional way with our current technology!

What wavelengths of electromagnetism would the presence of the wormhole be apparent on? And how would it's size (in terms of the physical aperture from which matter and energy comes in and out of) effect visibility?

These are intimately related.  Any wavelength that is much smaller (say by at least a factor of 10 or so) than the diameter of the wormhole's mouth will be useful for observing it by gravitational lensing or microlensing.  This is because those photons will simply trace geodesics through the curved spacetime around the wormhole.

Wavelengths that are comparable to the diameter will instead be strongly diffracted, and wavelengths much larger than the diameter would basically ignore it, like infrared light passing through dust.)


What would the light curve of a microlensing event by a wormhole look like?

It depends on a number of things, but in most cases it is very different from the lensing behavior of a typical mass (the Schwarzschild lens such as by a planet or black hole).

The classical wormhole has two mouths that each have an effective negative mass, which causes light rays to be bent away from it rather than toward it.  Therefore if we image an object far behind the wormhole, the image of that object will be displaced toward the wormhole (whereas lensing by a positive mass like the Sun displaces images away from it).  Here are some simulations of how a background object gets lensed by a wormhole, from Safanova et al. (2001):


















I also recommend this paper's introduction which has a pretty good review of the previous literature on wormholes and their detectability, including a lot of the above material.

If there exists a wormhole from one galaxy to another a billion lightyears apart, would that have implications for what we observe about the universe at cosmological scale? (like dark energy behaviour.)  Or in other words, are there observations we can make to rule out the existence of such "long" wormholes?

Wow Watsisname - that was a great answer! You certainly answered my questions satisfactorily - the addition of the hyperlinks to those papers (two of which I've already read) are most helpful, and will further my research into this.

midtskogen wrote:

Source of the post If there exists a wormhole from one galaxy to another a billion lightyears apart, would that have implications for what we observe about the universe at cosmological scale? (like dark energy behaviour.)  Or in other words, are there observations we can make to rule out the existence of such "long" wormholes?

I second this question.

In addition, since there are microblackholes or Kugelblitz blackholes, could there be such a phenomena/technology like a micro-wormhole? Perhaps it could be used to covertly transport microscopic and/or quantum matter? Or would the small size and mass of such an object render it inherently unstable?

Stellarator wrote:

Or would the small size and mass of such an object render it inherently unstable?

Actually microscopic black holes tends to evaporate very quickly due to Hawking Radiation irradiating an incredible amount of energy, I think a similar phenomena might occur in microscopic wormholes. So it would be stable for a even smaller time than that of normal wormholes, you should create it, send data and close it

Salvo wrote:

Source of the post Actually microscopic black holes tends to evaporate very quickly due to Hawking Radiation irradiating an incredible amount of energy, I think a similar phenomena might occur in microscopic wormholes.

If a wormhole is traversable both ways, how can it emit Hawking radiation?

Furthermore, artificial Kugelblitz blackholes are stable over relatively long periods (if in a highly monitored environment), due to their artificial nature. I suspect a similar case is true for microscopic wormholes, at least in theory.

Stellarator wrote:

Source of the post In addition, since there are microblackholes or Kugelblitz blackholes, could there be such a phenomena/technology like a micro-wormhole?

As far as I understand, you (probably) can not make stable micro wormholes.
The reason you (probably) can not make big wormholes is that you need negative energy for that. (Which probably does not exist.)
Without negative energy, there are no stable wormholes.
I think micro wormholes emerge and disappear instantly, much like the matter / antimatter particles of the vacuum. (If there are micro wormholes.)

midtskogen wrote:

If a wormhole is traversable both ways, how can it emit Hawking radiation?

I'm not saying it actually emits Hawking radiation, I'm saying it might evaporate like Black Holes but with a obviously different process.
Anyways, this is nothing but pure speculation.

midtskogen wrote:

Source of the post Or in other words, are there observations we can make to rule out the existence of such "long" wormholes?

I don't think so.  We could imagine s wormhole formed as a connection between two points in space that were in causal contact in the very early universe, or even as an initial condition of the spacetime itself, and then the mouths spread apart by the rapid expansion during the inflationary era, so that they are then beyond one another's cosmological event horizons.  

Another method of forming a very long wormhole would be via the ER=EPR conjecture, where two clouds of entangled particles are brought far apart from one another, and then each collapsed into a black hole.  If the particles remain entangled during that process, then the two black holes are joined by a wormhole inside, according to the conjecture.

An interesting (but probably not surprising) feature of wormholes in an expanding universe is that the length of the throat of the wormhole grows in proportion to the universe's scale factor.  So if the size of the universe doubles, the throat grows to be twice as long as well.  This has been shown rigorously in a few papers that combine a wormhole metric with the FLRW metric for an expanding universe.

So I don't think there's an obvious limit to the length of a wormhole in cosmology (or maybe there is that I'm just not aware of).  Instead a serious issue will arise if the two mouths of a wormhole are accelerated to high speeds relative to one another and then brought back together (as in the twin paradox of special relativity).  With a wormhole that yields a time machine, which suggests it should be impossible to do this.

Stellarator wrote:

Source of the post In addition, since there are microblackholes or Kugelblitz blackholes, could there be such a phenomena/technology like a micro-wormhole? Perhaps it could be used to covertly transport microscopic and/or quantum matter? Or would the small size and mass of such an object render it inherently unstable?

There's no size limit on wormholes just as there is no size limit for black holes.  We could imagine making a microscopic one by ER=EPR (though it won't be traversable even for subatomic particles).  midtskogen is also correct that traversable wormholes don't suffer Hawking radiation since they lack event horizons.  The only problem is how to hold them open since the classic Einstein-Rosen bridge type of wormhole is unstable without the support of exotic energy.

Stellarator wrote:

Source of the post Furthermore, artificial Kugelblitz blackholes are stable over relatively long periods (if in a highly monitored environment), due to their artificial nature.

Why?  By the no-hair theorem, the only properties a black hole can possess are mass, spin, and charge, so a black hole doesn't "care" whether it was created artificially or not.  A black hole formed by Kugelblitz should be completely indistinguishable from a black hole formed from gravitational collapse of an equivalent amount of matter.

Stellarator wrote:

Source of the post artificial Kugelblitz blackholes are stable over relatively long periods (if in a highly monitored environment), due to their artificial nature.

I meant that since such tiny masses for such a theoretical phenomena are unstable, it would need to be continually fed a stream of energy tuned to the diameter of the event horizon in order to function, something that Arthur C Clarke in his Imperial Earth book discussed.  It is of course completely speculatory, and very much "Clarktech" stuff, but I do know that Kugelblitz's are similar to other blackholes in that there is no difference between an artificial one and a natural one.

Stellarator wrote:

Source of the post I meant that since such tiny masses for such a theoretical phenomena are unstable, it would need to be continually fed a stream of energy tuned to the diameter of the event horizon in order to function

Ah, okay.   I mistook you for suggesting a Kugelblitz was automatically different somehow, not that it was being maintained.

Watsisname wrote:

Source of the post I don't think so.  We could imagine s wormhole formed as a connection between two points in space that were in causal contact in the very early universe, or even as an initial condition of the spacetime itself, and then the mouths spread apart by the rapid expansion during the inflationary era, so that they are then beyond one another's cosmological event horizons.  

I was a bit unclear. What I was having in mind were artificial wormholes within our observable universe, can we rule out that some advanced civilisation has created a wormhole across much of the universe?  The Hollywood illustration for such wormholes is a sheet of paper with two distant points and the points are brought together by bending the entire paper.  If the entire universe is bent in a similar fashion, shouldn't we be able to detect such geometrical oddities?  If this civilisation has created a large network of wormholes in this manner, the shape of the universe must be really odd.

Watsisname wrote:

Source of the post Instead a serious issue will arise if the two mouths of a wormhole are accelerated to high speeds relative to one another and then brought back together

Moving wormholes create all kinds of problems.  Can you send one wormhole through another?  Or through itself?  The easiest way to rule out the existence of wormhole is by an argument ad absurdum.

midtskogen wrote:

Source of the post Moving wormholes create all kinds of problems.  Can you send one wormhole through another?  Or through itself?  The easiest way to rule out the existence of wormhole is by an argument ad absurdum.

Of course you could send a wormhole through a wormhole. Pack a small transportable wormhole into a spaceship and send the ship through a large wormhole. Where is the problem?
But a wormhole through itself? If both ends are the same size (as I suspect), that's no problem. Of course this is not possible. But if one end is smaller than the other? That would be a problem!
(But to fit through the exit end (through itself), it would have to shrink during the journey. )
But just because that seems absurd does not mean that wormholes can not exist. Maybe a wormhole would be destroyed if you tried to send it through itself.

midtskogen wrote:

Source of the post I was a bit unclear. What I was having in mind were artificial wormholes within our observable universe, can we rule out that some advanced civilisation has created a wormhole across much of the universe?  The Hollywood illustration for such wormholes is a sheet of paper with two distant points and the points are brought together by bending the entire paper.  If the entire universe is bent in a similar fashion, shouldn't we be able to detect such geometrical oddities?

Surprisingly, no.  This visualization of "bending the whole universe" in a higher dimension to make the shortcut is actually more helpful than we might initially realize, because it correctly demonstrates that this is a form of extrinsic curvature, which is not detectable in any way by creatures who live on that surface or manifold.  If we take a sheet of paper and roll it up to make opposite sides meet, then we have changed how the paper is embedded in 3D space, but we have not changed the geometry of the paper itself at all.  It is still geometrically flat (Euclidean).  Mathematically sophisticated ants living on the paper won't be able to notice any difference.  Straight parallel lines remain parallel and the sum of angles in a triangle remains 180°.  Likewise, our astronomers can measure the intrinsic curvature of the universe, but they cannot measure how it is embedded in any higher dimension.

midtskogen wrote:

Source of the post If this civilisation has created a large network of wormholes in this manner, the shape of the universe must be really odd.

Lol, yes indeed.  The extrinsic geometry of the universe with a large number of wormholes would be very bizarre.  But the topology need not be very complicated at all (just a number of holes equal to the number of wormholes), and the intrinsic geometry could still be flat everywhere on large scales.  We'd still be unable to notice since all observations we can make only probe the intrinsic geometry.  (Though mapping out the connections made by the wormholes and the time it takes to pass through them would serve to map the extrinsic geometry).

This does lead to more difficult questions, like "how does forming a wormhole make the connection by distorting how the universe is embedded in higher dimensional space?", and "what prevents those connections from getting tangled up or intersecting... or what happens if they do?"  I love the question of what happens if we send the mouth of a wormhole through itself, and I have no idea what the answer would be.  Probably something absurd.

I'm not sure if this path of questioning is the best way to produce an "ad absurdum" argument against wormholes, though. I still think the most efficient way to do that is by thought experiment with using them to create a time machine, and asking if a wormhole could exist, what prevents us from being able to use it as one?  Convoluted explanations exist, but it seems the simplest answer is that the wormhole itself is not allowed.

Watsisname wrote:

Source of the post This visualization of "bending the whole universe" in a higher dimension to make the shortcut is actually more helpful than we might initially realize, because it correctly demonstrates that this is a form of extrinsic curvature, which is not detectable in any way by creatures who live on that surface or manifold.

Yes, I realise that for a flat universe in that particular case, but for a network of wormholes it seems impossible to retain unless the configuration of the mouths exactly mirror each other, which would be less useful.  An if the network isn't properly planned from the beginning, there will easily be all kind of bending to bring new mouths together without breaking existing pairs.  It seems very hard to do this and maintain the flat shape, certainly from the paper analogy.  The bulklanders must look down on our universe and say "look, the volumelanders are curling the volume again" and scratch their heads.
But a network seems unnecessary.  Why not instead create a time machine out of a single wormhole and make a spacecraft out of it capable of highly relativistic travel?