If my memory from last year serves me, this is less a wormhole and more of a "magnetically-shielded tunnel". Cool and potentially useful, but I wish they didn't use the word wormhole.
Taking a hot physics buzz-phrase (wormhole, black hole, Hawking radiation, etc.) and applying it to a mathematically analogous (but physically unrelated) system is currently fashionable in physics. Personally I find it kind of dishonest, but it seems to be a popular strategy for focusing attention on research that is otherwise only marginally interesting.
You stated the valid reason why physicists do this naming. The mathematics is analogous. This means that by studying the different system (eg. the wormhole in the lab), the physicists are probing the mathematical structure of the theory of the original system- except that studying the different system is easier than the original system. Actual wormhole physicists then might learn from this research to understand how wormholes might behave.
Why this is a recent trend is because only in the past few decades have experiment technologies improved to the point where exotic systems can be physically simulated in the lab using simple systems. My own research is partly of this sort and so we have fancy names too, but I assure you that, at least in my group, no one cares one bit about our research being published in pop-sci magazines
> This means that by studying the different system (eg. the wormhole in the lab), the physicists are probing the mathematical structure of the theory of the original system
I just don't buy that argument, because it is essentially the same argument that was used to support the ether hypothesis. Both sound and light are governed by a wave equation. Since sound is observed to have a preferred frame (the rest frame of the medium), the above argument asserts that light should also have a preferred frame, since the mathematics is the same. But that line of reasoning is obviously wrong, as we know now.
I don't think that observing some wormhole-like feature in a metamaterial provides any information about how wormholes may or may not behave in GR. The entire endeavor of analogue gravity seems specious to me.
Science going wrong (ether) is not a bad thing. In fact, if scientists were not routinely wrong, we would suspect there was something wrong with our scientific method.
Let me challenge your argument more directly. Postulating ether after observing that sound and light have similar properties (wave behaviour) is not stupid. It is the logical thing to do. It was also a hypothesis. Scientists did well-designed experiments and eventually realized that light does not a need a medium to travel. The scientific process correctly identified falsehood. Outcome: we are clear on which properties light and sound share (wave equation) and which they don't (medium or lack thereof)
I haven't read the paper, but let me make a statement about metamaterial wormholes. Metamaterial experiments in the lab might be able to simulate a type of wormholes. If they can then we know that within another theory that shares enough math with EM will also have wormholes of that type. The question then is if GR has and EM share enough of that mathematical structure. If they do then these types of wormholes will also exist in GR. This is a different way of investigating GR wormholes then directly looking at the theory of GR. The differences between EM theory and GR might make it easier to discover wormholes in the former than in the latter.
> The question then is if GR has and EM share enough of that mathematical structure.
And how do we answer that question, other than directly looking at GR theory? Moreover, even if we do find EM evidence in favor of wormholes, how do we know quantum gravity doesn't prevent their formation?
Yes. We will have to directly look at GR to verify. But as I said before, it might be easier to do the investigation first with EM and then compare to GR rather than look at GR directly.
We don't know what quantum gravity says. The job of the theorist to form self-consistent theories and find the strongest predictions of these theories. When we get experimental data we will find out which theories are falsified.
> the above argument asserts that light should also have a preferred frame, since the mathematics is the same. But that line of reasoning is obviously wrong, as we know now.
Even while we know that it is wrong, the mathematical similarities between sound and light allow for the exploration or derivation of phenomena in one from the other.
For example, we are all familiar with lasers. The first laser was developed in 1960. Well people took the mathematics involved and constructed sound equivalents, for example the sound equivalent of the photon is the phonon.
It was then possible to construct and research the SASER - the sound equivalent of lasers and based on much the same mathematical principles.
So even though at a fundamental level sound and light are entirely different, it is possible to study the mathematics and behaviour of one and have it be directly applicable to the other.
It's not clear in this case that what they've built is mathematically analogous to GR wormholes. This seems more on the "it's kinda conceptually similar" rather than "this works exactly the same as a wormhole under the proper translation of variables"
If they had such a correspondence, than I would definitely say this was an awesome accomplishment, but I do not believe that they do.
At least they didn't try to connect it to a marginally-related bit of pop culture. Remember when some Berkeley guys published a way to bend light around an object, and every goddamn headline included the words "Harry Potter invisibility cloak"?
> a tunnel for magnetic fields acting as if was outside the usual 3D space.
I didn't understand what they meant by "acting as if it was outside the usual 3D space".
If I did understand it then, fair enough, I'd accept the thing as a magnetic analogue of a wormhole, just as there are physically interesting hydrodynamic analogues of black and white holes. (https://arxiv.org/abs/1203.6505)
The interesting part of wormholes is that they introduce topologies that connect two extrinsically "distant" points, yes? This research doesn't appear to do that; it seems like it just "hides" the magnetic field for a distance. The field still moves slower than light.
Well yes, but if you introduce the kind of topologies you can get with wormholes, things can take the shorter path through the wormhole, so it "looks like" they are moving faster than light.
Hmm...I am not an expert in GR, but absent some other constraint, lorentz invariance is a local theory, right? Not a global one...
I mean, I send two light beams simultaneously, one directly to you, one off angle to a mirror which reflects and then reaches you too[0]. The direct one reached first because it had a shorter route, they both travel at c...we don't violate lorentz.
I think you're referring to a topology that would allow closed timelike loops which would imply grand father paradox issues? That doesn't violate lorentz which is local and doesn't care about global structure, but it leads to logical inconsistencies (which is global, I think...)? Enlighten me. Again, my expertise is not GR.
[0] If you want to be pedantic, we are both in the same lorentz frame...needs not to be said, but just in case :)
> No. You can't go faster than light. There are no loop holes.
That's what I just said. Your objection is silly.
You're basically saying "Look, if light follows an extrinsically curved geodesic that goes into a black hole it takes 10 minutes to reach the singularity. But if we shoot it straight into the black hole, the math says it only takes 9 minutes. Impossible! That's science fiction! It's going faster than light!"
Information traveling through a wormhole is emphatically not going faster than light. It's just taking a shorter route to get to the other side. You can't necessarily violate causality like this; you can just bring objects closer together.
The mathematics on wormholes is quite solid and appears to admit traversable wormholes in principle. There are many reasons why it might not actually feasible to build or use one, but your objection isn't one of them.
Wormholes are not a shorter path through space time. That's the mistake you're making. The whole topology/folded paper thing you keep mentioning has already been described as science fiction.
There are several resolutions to this complaint. One popular suggestion is that any causality-violating wormholes would cause a feedback loop of virtual particles to accumulate inside the wormhole and destroy it.
> It might be mathematically interesting but that's it
That's exactly what they said about photons and antimatter. Both were discovered mathematically before they were discovered physically. "Oh, this is just a mathematical fluke, this couldn't exist in the real world."
The best physical theory we currently have admits the existence of wormholes. If you have a problem with that, then feel free to try to come up with an alternate physics that explains the world better than we can right now.
Time travel from our perspective is impossible. What people talk about when discussing wormholes affecting things before they've happened are on a quantum level.
Quantum particles decay and change and do stuff to each other and to understand it you need an equation to explain that, but some of those equations require the state of those decayed particles to provide their state to earlier parts of the equation, before they were created through quantum decay or their state determined by waveform collapse. This is sometimes described as time travel but it's not really, time has no relevance at that level of understanding. WE live in space-time and quantum physics is the foundation of that fabric. So the solution doesn't really violate the laws of time, it happens in a place where space and time don't have the same relevance.
If you somehow did create a feedback loop in this situation we'd never know. The concepts of it taking forever or being instantaneously resolved don't apply, because as far as we're concerned we only get the end product. We're not even sure of coherence is a good way of telling how much work has been done in a twisty, strange quantum problem like that.
I mention coherence because at the quantum level it's the only way of knowing which direction everything is happening in. It's not the same as time, but it's a bit more like very very fast entropy at a quantum level.
All of these things come together to give you space-time.
So when we talk about wormholes we're talking about the transmission of information on a quantum level, not a feature of space-time. The space and time parts are only relevant later.
General relativity admits traversable (i.e. macroscopic) wormholes. This is what we're discussing.
> you need an equation to explain that
Which equation? Schroedinger? The SE has time as a variable and doesn't do any of the "time travel" stuff you're talking about, so you must be referring to something else.
> One popular suggestion is that any causality-violating wormholes would cause a feedback loop of virtual particles to accumulate inside the wormhole and destroy it.
I would imagine a more likely solution is, wormholes either can't exist or can't transmit information.
> The best physical theory we currently have admits the existence of wormholes.
Just because the maths says it can exist in one set of equations, doesn't mean it can/does exist in our universe, especially lacking any evidence.
I'm not saying to stop researching the idea, but anyone sensible knows the outcome.
They will not be possible not matter how much you want them to be true. Just because something can exist doesn't mean it's either likely or not crazy.
> wormholes either can't exist or can't transmit information.
This would contradict known physics. Again, please provide a better solution.
> Just because the maths says it can exist in one set of equations
That "one set of equations" is everything we know about the universe.
> but anyone sensible knows the outcome.
You mean like Kip Thorne, Leonard Susskind, Mat Visser, David Deutsch, and any other physicist who's studied wormholes? Because all those people agree that either we're wrong about physics or wormholes are theoretically possible. Causality violation, probably not, but wormholes don't violate causality just by existing.
> They will not be possible not matter how much you want them to be true
I have no strong opinion one way or the other. However, I accept that, based on the best physical theories we have available to us, wormholes are physically viable.
It seems like, for some reason, you really want them to be false. Perhaps it's your sense of "sensibility", which does not appear to be based in fact (unless you know something physicists don't).
My point is, lots of things are still unproven but we know the outcome. This includes in mathematics. Things remain unproven but mathematicians know the outcome.
It might piss the purists off to think like this but tough luck this is how the world works.
You seem happy to throw out causality? but not the faster than light bit. This seems biased.
> You seem happy to throw out causality? but not the faster than light bit.
Nope, going faster than light violates causality. As I have said N times, wormholes do not allow you to go faster than light. I really can't be any clearer on that. What they do is alter the topology of space so that you can create a shorter route between two locations that used to be "far away" in the old topology.
Yes, but if you entering the wormhole at one end and leaving it at the other are events with spacelike separation, then those events have no absolute order, which means that there is at least one frame of reference in which you leave the wormhole before you enter it. That's time-travel, and that raises the possibility of causality violations.
The paper bending, sticking a pencil through explanation
is science fiction.
There is nothing science fiction about a curved 2D surface in a 3D space and travelling to a different location on the 2D surface via the 3rd dimension. I can both demonstrate the 'fiction' on my kitchen table and mathematically describe the system, showing that something seems to have moved from A to B faster-than-light.
It's entirely unclear why you believe this would be impossible if our observable 4D universe is curved and embedded in one or more additional spatial dimensions.
Then what makes a worm-hole special? I always thought it was that a worm-hole made a smaller path between two points than could normally exist in plain space.
It's a hypothetical mechanism for the transmission of information. It's not taking a shortcut, it's just a different mechanism for getting there, in the same way entanglement propagation can't exceed the speed of light.
And that's why this technique is being described as a wormhole - it's a relatively good simulation of one in terms of some of the measurements they want to make and can be used to carry one of the properties we think a wormhole should be able to carry.
The same math that describes the creation of an ERB, also makes them unsuitable for transit of even a single particle. There's also probably some question as to whether it would even connect to a distant part of our universe, since the math describes two different, asymptotically flat universes.
The other interesting thing about wormholes is they don't exist, and can't exist without basically rewriting every law of physics we have. They violate almost every conservation law there is.
They are a mathematical curiosity, not a thing that could actually exist.
If you could rewrite the laws of physics there are lots of cool things you could do, wormholes would just be the start......
The laws of physics are the mathematical description, and the wormhole concept arises as a curious edge case of those laws. If wormholes are actually impossible, the laws of physics already need to be rewritten, not the other way around.
By pushing hard enough to equalize the GPR you're removing by going into space?
Consider an obvious example: a wormhole which connects two points on earth separated by 1m of real distance, at the same height.
Theres no violation of conservation of energy because you did not gain or lose gravitational potential energy, and your start and end momentum is conserved.
Susskind's ER=EPR posits that entanglement is actually a form of proto-wormhole, and you could build a bridge through spacetime with two entangled black holes... it works pretty well without rewriting physics. though you couldnt send information from one side out the other. Einstein-Rosen bridge
A wormhole with the same "trick" that entanglement does, where you can't actually send information/energy through it, would indeed not violate any laws. It would also be useless.
But that's not what people mean when they say wormhole.
To be more clear, the ER bridge would actually allow people on different ends to jump in and meet in the 'middle', they just couldnt then go back out (going from one side out the other). This was the closest thing to a wormhole i can think of (given the ER=RPR conjecture is true)
Wormholes for matter and energy are not incompatible with physics. We just probably won't ever see one like the one he proposed, because creating it would require manipulation of matter or gravity at extreme scale.
But, it is wrong to say it is impossible. It is theoretically possible.
What's there to argue with? He doesn't claim wormholes can exist.
> because creating it would require manipulation of matter or gravity at extreme scale.
Not extreme scale, using a type of energy that doesn't actually exist. i.e. it's not about the quantity of energy, it's the type. It's a mathematical result, not an actual thing.
> But, it is wrong to say it is impossible. It is theoretically possible.
No, it is not possible. See my other replies in this thread.
"...calculations in semiclassical gravity suggest that quantum effects may be able to violate this condition in curved spacetime. Although it was hoped recently that quantum effects could not violate an achronal version of the averaged null energy condition, violations have nevertheless been found, so it remains an open possibility that quantum effects might be used to support a wormhole."
It's the creation of wormholes that would violate physics, though, right? AFAIK our current model of physics would work just fine to describe a universe that came into existence with permanent, stable wormholes already in it; that'd just be part of its topology.
He has it backwards. Our current physics allow for wormholes. They probably don't exist but if they were literally impossible, then we would need to rewrite our current physics.
Wormholes are more than likely impossible because to hold one open for travel, you would need some sort of negative matter/negative energy. That isn't forbidden by physics, but it's unlikely that it exists since we haven't seen any. Without negative energy/matter to hold a wormhole open, even if you could create one, just a single proton or electron would cause the wormhole to become unstable and collapse in on itself, and doing so faster than the proton could travel through.
Say you are in a gravity well with one side of the wormhole, and the other side is outside the well.
You could "lift" matter outside the gravity well without spending energy.
There are other problems as well: Gravity has no discontinuities, as objects move, the gravitational field moves with it. You never have a situation where gravity just suddenly shows up. (The same is true for an electrical field BTW - you can never create one, you can only move an existing one (i.e. split up the positive and negative parts). A magnetic field can be created, but that's because you always create a north and south at the same time. If magnetic monopoles existed you could not create just one, you could only separate the two halves.)
Anyway, a wormhole would allow you to have a gravitational discontinuity and that's impossible, it would create a gravitational wave with an infinite frequency. As you know a true square wave is physically impossible - the fourier transform requires an infinite frequency - but that's what you are doing by moving matter via a wormhole.
> You could "lift" matter outside the gravity well without spending energy.
How is that? Wouldn't the gravitational force just propagate across the wormhole, making for a continuous gravitational gradient?
Maybe we're talking about separate conceptions of what a "wormhole" is here. For one example of what I'm picturing, you could have a one-wormhole universe as a three-dimensional hologram along the surface of a 4D hypertorus. You can go "through the doughnut hole" to get somewhere quicker than you could by going "around the doughnut", but that doesn't mean that there's a discontinuity anywhere on the surface of the hypertorus. Gravitational waves from one point-source would be propagating both "into" the wormhole (along the inside of the doughnut) and "through regular space" (along the outside of the doughnut) and eventually encountering themselves at the destination and maybe constructively or destructively interfering, but they wouldn't be forced into a square wave at any point.
> How is that? Wouldn't the gravitational force just propagate across the wormhole, making for a continuous gravitational gradient?
That would be even worse. Let a strong gravitational force go through the wormhole and drop something near it (not through it).
Then move the wormhole (and the gravitational force) elsewhere, lift the object, and bring the wormhole back.
Normally the thing that generates gravity has inertial mass, so it costs energy to move. Not with a wormhole. If you speculate that it should have inertial mass, then how much?
The solution to this is almost certainly "moving the wormhole takes energy and solves this 'paradox'". The mass-energy of the wormhole could be from gravitational field energy, for example.
How much energy? The wormhole doesn't have an inherent mass, so how much energy does it take? After all you can put an arbitrary amount of mass through it.
You're looking at it the wrong way. If you can extract energy from dropping the rock through the wormhole, it takes energy to pull the rock away from the wormhole (and, by symmetry, it takes that much energy to move the wormhole away from the rock).
I would guess that that would depend on the situation.
I would think that whatever method you have of moving a mouth of the wormhole, would have some interactions with the masses providing the gravitation.
Also, I'm not 100% sure that a wormhole would not have something like an inherent mass, because iirc there are solutions where a black hole mass is entirely because of the way spacetime is set up, without any particles needed? I'm not sure about that, but I think that is what I read..
I'm kind of lost by your contruction. Is the something we drop the same as the object we "lift"? What does it mean to lift the object? Try removing the pronouns, and being more specific with your steps.
Is this a thought experiment you came up with on your own, or do you have a paper you're citing? If the latter, do you have a link?
> Try removing the pronouns, and being more specific with your steps.
That would be even worse. Let the strong gravitational force of the Earth go through a wormhole located on a small platform in deep space, and drop a rock near the wormhole (not through the wormhole).
Then move the wormhole (and the gravitational force) elsewhere, lift the rock off the platform, and bring the wormhole back.
You're assuming the gravitational field can't permeate the wormhole. In your first scenario, the gravity well would extend out of the other side of the wormhole.
A wormhole is in no way a gravitational discontinuity, the toplogy for them is well defined. The creation of a wormhole would be a bit tricky, as this does require a discontinuity, but there's no reason to think that's impossible. After all, a gravitational singularity is also a discontinuity, and physicists have no problem with that.
> You're assuming the gravitational field can't permeate the wormhole. In your first scenario, the gravity well would extend out of the other side of the wormhole.
> A wormhole is in no way a gravitational discontinuity
A movable one certainly is, and a non movable one doesn't make any sense from a relativity point of view.
> Ellis Wormhole
Did you read your own link? "and there is no gravity"
There's a reason they say that. And there is no place in the universe without gravity. It's a mathematical solution, not a representation of the physical world.
> and a non movable one doesn't make any sense from a relativity point of view
Do you agree that you can't "leave the universe" in the sense of travelling past the information-propagation wavefront of the Big Bang—even in a case where our universe begins to shrink and that wavefront starts to come toward us?
If yes, then a non-movable wormhole can be justified exactly like that: it's part of that same wavefront—part of the topological outer surface of the universe, not an object "in" the universe.
Can you define what you mean here by discontinuity? I meant that the function of the gravitational field is continuous everywhere under the topology of the manifold, the mathematical definition. I see no discontinuity, but perhaps you're using a different definition?
The "no gravity" line there means the stress tensor is zero, and this is a so-called "vacuum solution." Essentially all closed form solutions to the field equations are vacuum solutions because it's almost impossible to write a closed form solution otherwise.
i'm not commenting on the validity of what you say, just that it reminds me of the story of max plank and his photon 'hack' for getting round the ultraviolet catastrophe - he thought it was a mathematical tick/curiosity too. then Einstein said 'hang on hang on hang on, maybe not...'. wouldn't be the first time in history a hack turned out to be the real deal.
There are a lot of mathematical things like that - tachyons for example are a mathematical "solution".
The ultraviolet example is the opposite: The universe acted in a certain way, so we looked for math to match, which happened to have a strange property to it.
Here first we found the strong property and are looking for the universe to match.
Well of course we can do the second.
It has happened all the way with the element table in chemistry. Heck we're even manufacturing non-natural matter (as in, probably never generated elsewhere in the universe) to fill the holes that the theory has created.
