8 ms·

Humanly traversable wormholes (2020)

92 points by nabla9 5y ago

nabla9 5y ago
Published in: Phys.Rev.D 103 (2021) 6, 066007 Published: Mar 9, 2021 DOI:10.1103/PhysRevD.103.066007 >. We have not given any plausible mechanism for their formation. We have only argued that they are configurations allowed by the equations.
- DennisP 5y ago
  Also only "in some previously considered theories for physics beyond the Standard Model," and > we engage in some “science fiction”. Namely, we will introduce a dark sector with desirable properties for constructing macroscopic traversable wormholes. Seems unsurprising that if you purposely introduce hypothetical physics with desirable properties for making traversable wormholes, then it turns out the physics supports traversable wormholes.
  - emteycz 5y ago
    Sure. But it still seems interesting to see in what ways you must bend the rules to make it possible. Consider that the goal here is not really to produce working human-traversable wormholes, but rather to learn more about various models of physics and how they behave in extreme conditions.
    - Ansil849 5y ago
      > Consider that the goal here is not really to produce working human-traversable wormholes Then the paper should not have the clickbaity title "Humanly traversable wormholes".
      - Rusky 5y ago
        I dunno, this feels like a "no fun allowed" sort of rule. The target audience clearly has some background in this area to tell the difference, at which point this is just a fun joke-y title.
  - at_a_remove 5y ago
    I have typically framed this as "Once you allow for one impossible thing, other impossible things follow."
  - zachf 5y ago
    I can see why it might look like that from the outside, but the set of mathematical results that led up to it actually come from the angle of, “let’s try to prove that traversable wormholes and the negative energy densities required to create them are impossible.” In trying to prove that, you learn a lot of interesting things along the way: 1. Negative energy densities are a universal prediction of all quantum field theories, and therefore are not as outrageous to think about as one might naively believe. [0] 2. The amount of negative energy density permitted by quantum field theory is NOT enough to support traversable wormholes, as long as you make some mild assumptions about the behavior of spacetime. [1] 3. Those mild assumptions seem NOT to be required by string theory, and string theory supports some solutions with traversable wormholes. [2] 4. Those solutions of string theory appear to be self consistent in an unusual and novel way, which is why string theorists like Juan Maldacena find them interesting. [3] Every step is all super nontrivial and tells us something new about the mathematics of spacetime. Of course they’re models, maybe they all rest on some faulty assumption about nature that will later turn out to be wrong. So the final pillar of the story, the one that’s hard to communicate without spending years of your life studying it, is that every remaining assumptions about quantum gravity that goes into these arguments appear to be impossible to get rid of without having horrible consequences where spacetime can’t become smooth or relativistic at large scales. So these models are at least very plausible even though the ultimate truth will have to come from experiments. [0] Proof appears many places, I like the one in arxiv:1803.04993 on page 11 [1] arxiv:1010.5513 [2] arxiv:1608.05687 [3] the paper linked by OP :)
    - DennisP 5y ago
      Thanks, that does sound more plausible than I expected.
29athrowaway 5y ago
Maybe if you enter a wormhole, this happens: https://www.youtube.com/watch?v=doyaw8ipQpk https://www.youtube.com/watch?v=doyaw8ipQpk
- TedDoesntTalk 5y ago
  I can’t believe Spore is 10 years old.
  - messe 5y ago
    More than that. It's 14 years old.
    - 29athrowaway 5y ago
      It aged very well as a game. It is still very playable today, and best of all, runs out of the box on Proton.
netfl0 5y ago
Solution for traffic.
shireboy 5y ago
I wasn’t aware of the constraint that travel within wormhole must take longer than travel between the mouths. Given that, is there any real practical use for them? In scifi they are usually presented as shortcuts, but this constraint makes that seem less likely to ever be true.
- beecafe 5y ago
  This constraint is needed because having a shortcut would allow time travel, like all FTL does.
  - codethief 5y ago
    But that's not a problem per se. The speed of light is only a limit locally but globally there is no such thing and in General Relativity there is nothing preventing time travel (closed timelike curves) at a global level, even though the existence of such curves is rather unlikely. The paper notes: > Interestingly, they are allowed in the quantum theory, but with one catch, the time it takes to go through the wormhole should be longer than the time it takes to travel between the two mouths on the outside. Does anyone know why exactly "quantum theory" would impose such requirements? A priori to me it sounds like quite a stretch to take a local theory like quantum mechanics to make claims about the global topology of the universe, given that QM and GR haven't been unified yet. Unless of course by "quantum theory" Maldacena actually means "string theory" or "AdS/CFT" – which wouldn't surprise me at all.
    - ben_w 5y ago
      IIRC (I’m not a physicist), if you combine a closed timelike curve with quantum mechanics, then a random fluctuation in the photon field (which happens all the time) appearing in the middle of it will go all the way around, meet itself, and now there are two of them, then four, then eight, … But as this is in a closed loop of time, from the outside it goes to infinity instantly.
      - zozbot234 5y ago
dark-star 5y ago
And, as always with these kinds of papers, everything only works out in ADS space, which has nothing in common with how our own universe works...
- sdoering 5y ago
  ADS space? I have not yet heard that acronym. If anyone else never heard of the "Anti-de Sitter space" [0] here a short description from Wikipedia: > In mathematics and physics, n-dimensional anti-de Sitter space (AdSn) is a maximally symmetric Lorentzian manifold with constant negative scalar curvature. Anti-de Sitter space and de Sitter space are named after Willem de Sitter (1872–1934), professor of astronomy at Leiden University and director of the Leiden Observatory. Willem de Sitter and Albert Einstein worked together closely in Leiden in the 1920s on the spacetime structure of the universe. Sadly this is so far outside of my level of understanding that I still don't have a clue. Would love an ELI5. [0]: https://en.wikipedia.org/wiki/Anti-de_Sitter_space?wprov=sfla1 https://en.wikipedia.org/wiki/Anti-de_Sitter_space?wprov=sfl...
  - zachf 5y ago
    Think about 2D surfaces. Which ones are the most symmetric? A flat plane is a very nice space: every point is as good as any other (there’s nothing intrinsic to any point to distinguish any point from any other, except arbitrarily), and no direction is particularly special either. A space like that has a lot of symmetries. A sphere also has a lot of symmetries, it also has no directions or points which are distinguished until you declare, “this is my North Pole” arbitrarily. (The earth isn’t a perfect sphere of course and we can use the imperfections as the way we define north and south.) The last type of symmetric space looks like a saddle (like on a horse). It bends one way in one direction and bends the other way in the other direction. An idealized saddle also has no distinguished directions or points. The analogs of these things in higher dimensions, and where one of the directions is time, are important in general relativity. The analog of the plane is called “flat space” or “Minkowski space”. The analog of the sphere is “de Sitter space”. Finally, the analog of the saddle is “anti-de Sitter space” (usually abbreviated AdS, with a lowercase d). It’s a bit of an odd space in a lot of ways. When you look at what space looks like at any given time, it’s a bit like M.C. Escher’s “Angels and Devils”. Surprisingly, Anti-de Sitter space is the easiest space to understand quantum aspects of gravity in. That’s because anti-de Sitter space is curved in such a way that the complicated stuff can be neatly separated from the easy stuff. You can start from something you understand well and turn on the complexity piece by piece. Roughly speaking it’s because the gravitational stuff becomes less important as you go farther and farther away from any matter you’re considering, in a way which is even faster than this happens in flat space or de Sitter space. It turns out that we can exactly understand everything in this gravitational theory by mapping the physics one-to-one to a nongravitational model which we understand really well. There’s a lot of evidence that the map works perfectly. This is called the AdS/CFT correspondence. A lot of work goes into testing the correspondence and attempting to prove it, and this is a big research area. de Sitter space doesn’t have the same desirable properties. Nevertheless there has been great progress in understanding quantum properties of de Sitter in the last year [0]. These results would not have been possible without understanding AdS first. Flat space quantum gravity remains challenging, although again some progress has been made recently too [1]. [0] arxiv:2110.14670 [1] arxiv:1905.09809 and many others
Einstant 5y ago
Trying to Make sense of: “with an AdS3 radius which is of order of the AdS5” Maldacena Feel free to correct me, and also if you think there’s something terribly wrong with the formula, feel free to share along. Now, AdS3 is a three-dimensional space with negative curvature, which is used in string theory as a model for the three-dimensional world. The AdS5 radius is the distance from the center of AdS3 to its boundary. Maldacena is saying that the AdS3 radius is of the same order as the AdS5 radius, meaning that they are both large compared to the Planck length. Is this supported by empirical evidence? No, this is not supported by empirical evidence. This is based on the fact that the AdS3 radius is much larger than the Planck length, while the AdS5 radius is only slightly larger than the Planck length. This means that the two spaces are of the same order of magnitude, and so the AdS3 radius is of the same order as the AdS5 radius. The Gibbons-Hawkings metric is a metric on the space of all Riemannian manifolds. It is defined where S is the action of the manifold and R is the Ricci scalar curvature. If we consider S to be an action under the Gibbons-Hawkings metric, where R is the Ricci scalar curvature of the manifold, In that case, S would be the action of the AdS3 manifold, which is where R is normed curvature of the AdS3 manifold. This is the Ricci scalar curvature: R = \frac{1}{2} \mathrm{tr}\left( R_{\mu \mu} \right) = \frac{1}{2} \mathrm{tr}\left( \frac{\partial{{\partial x^{\mu} \omega_{ {\mu} - \frac{\partial{\partial {x^{\mu} \omega_{\mu} + \omega_{\mu} \omega_{ u} - \omega_{ u} \omega_{\mu} \right) where R_{\mu u} is the Ricci tensor and \omega_{\mu} is the connection 1-form. This is the Ricci scalar curvature: R = \frac{1}{2} \mathrm{tr}\left( R_{\mu u} \right) = \frac{1}{2} \mathrm{tr}\left( \frac{\partial}{\partial x^{\mu}} \omega_{ u} - \frac{\partial}{\partial x^ u} \omega_{\mu} + \omega_{\mu} \omega_{ u} - \omega_{ u} \omega_{\mu} \right) There is not enough empirical evidence to determine whether or not the AdS3 radius and AdS5 radius are of the same order of magnitude. However, based on the results of the calculation, it seems that the AdS3 radius is of the same order of magnitude as the AdS5 radius.
teleforce 5y ago
Most of the people and even Muslim do not aware there is a chapter (chapter 70) with a title Al-Maarij in the Quran with the literal meaning of Maarij "the door to ascent" that some modern scholars refer to this as the wormhole [1]. According to one of the best English translations The Clear Quran, Maarij is translated as "pathways of heavenly ascent" [2]. In the 4th verse of the chapter it's mentioned "through which the angels and the holy spirit will ascend to Him on a Day fifty thousand years in length". That means based on our (human) reckoning it will be fifty thousand years but for angels (or those travel through this pathway or door) it will only takes one day of journey. [1] https://en.wikipedia.org/wiki/Al-Maarij https://en.wikipedia.org/wiki/Al-Maarij [2] https://quran.com/al-maarij https://quran.com/al-maarij
sdfasg34253 5y ago
just thanks for this, opened a couple of think paths. Could be productive at some point in the future (no pun intended).
virgildotcodes 5y ago
I think it was an article posted on HN a little while back that got me thinking about the time travel paradoxes that wormholes could enable. There was one thought experiment about a wormhole with one end on your front lawn, and the other in a spaceship that leaves with your partner, accelerating away at 1g to eventually reach the speed of light, and at some point turning around and coming back. The way I envisioned it is that the wormhole would act like a Zoom call, where you could talk/interact at "normal" speed of time during the entire voyage. Through the wormhole on your front lawn, you'd see your partner arriving back in your front yard and greeting future you despite the fact that their ship was still years away from arriving back on your yard from your frame of reference. Not to mention that you could also step through the wormhole at that point and join the future you and your partner, and then the partner that would arrive on your original lawn would find you missing. Obviously this turns into spaghetti quickly. Now, this post is the first time I learn that travel within the wormhole must take longer than travel between the mouths of the wormhole. This got me thinking about whether my concept of looking through the wormhole was correct. Would you actually see your partner moving at "normal" speed, or would their movements appear to you to begin to slow down as they approached the speed of light, with your movements appearing to them to accelerate dramatically? It seems as if their EM radiation towards you would eventually shift to infrared or radio and yours towards them would shift to gamma? Have I made a total mess of my understanding of all of this? Apologies for the amateurish questions.

92 points by nabla9 5y ago

Published in: Phys.Rev.D 103 (2021) 6, 066007 Published: Mar 9, 2021 DOI:10.1103/PhysRevD.103.066007 >. We have not given any plausible mechanism for their formation. We have only argued that they are configurations allowed by the equations.

DennisP 5y ago
Also only "in some previously considered theories for physics beyond the Standard Model," and > we engage in some “science fiction”. Namely, we will introduce a dark sector with desirable properties for constructing macroscopic traversable wormholes. Seems unsurprising that if you purposely introduce hypothetical physics with desirable properties for making traversable wormholes, then it turns out the physics supports traversable wormholes.
- emteycz 5y ago
  Sure. But it still seems interesting to see in what ways you must bend the rules to make it possible. Consider that the goal here is not really to produce working human-traversable wormholes, but rather to learn more about various models of physics and how they behave in extreme conditions.
  - Ansil849 5y ago
    > Consider that the goal here is not really to produce working human-traversable wormholes Then the paper should not have the clickbaity title "Humanly traversable wormholes".
    - Rusky 5y ago
      I dunno, this feels like a "no fun allowed" sort of rule. The target audience clearly has some background in this area to tell the difference, at which point this is just a fun joke-y title.
- at_a_remove 5y ago
  I have typically framed this as "Once you allow for one impossible thing, other impossible things follow."
- zachf 5y ago
  I can see why it might look like that from the outside, but the set of mathematical results that led up to it actually come from the angle of, “let’s try to prove that traversable wormholes and the negative energy densities required to create them are impossible.” In trying to prove that, you learn a lot of interesting things along the way: 1. Negative energy densities are a universal prediction of all quantum field theories, and therefore are not as outrageous to think about as one might naively believe. [0] 2. The amount of negative energy density permitted by quantum field theory is NOT enough to support traversable wormholes, as long as you make some mild assumptions about the behavior of spacetime. [1] 3. Those mild assumptions seem NOT to be required by string theory, and string theory supports some solutions with traversable wormholes. [2] 4. Those solutions of string theory appear to be self consistent in an unusual and novel way, which is why string theorists like Juan Maldacena find them interesting. [3] Every step is all super nontrivial and tells us something new about the mathematics of spacetime. Of course they’re models, maybe they all rest on some faulty assumption about nature that will later turn out to be wrong. So the final pillar of the story, the one that’s hard to communicate without spending years of your life studying it, is that every remaining assumptions about quantum gravity that goes into these arguments appear to be impossible to get rid of without having horrible consequences where spacetime can’t become smooth or relativistic at large scales. So these models are at least very plausible even though the ultimate truth will have to come from experiments. [0] Proof appears many places, I like the one in arxiv:1803.04993 on page 11 [1] arxiv:1010.5513 [2] arxiv:1608.05687 [3] the paper linked by OP :)
  - DennisP 5y ago
    Thanks, that does sound more plausible than I expected.

29athrowaway 5y ago

Maybe if you enter a wormhole, this happens: https://www.youtube.com/watch?v=doyaw8ipQpk https://www.youtube.com/watch?v=doyaw8ipQpk

TedDoesntTalk 5y ago
I can’t believe Spore is 10 years old.
- messe 5y ago
  More than that. It's 14 years old.
  - 29athrowaway 5y ago
    It aged very well as a game. It is still very playable today, and best of all, runs out of the box on Proton.

netfl0 5y ago

Solution for traffic.

shireboy 5y ago

I wasn’t aware of the constraint that travel within wormhole must take longer than travel between the mouths. Given that, is there any real practical use for them? In scifi they are usually presented as shortcuts, but this constraint makes that seem less likely to ever be true.

beecafe 5y ago
This constraint is needed because having a shortcut would allow time travel, like all FTL does.
- codethief 5y ago
  But that's not a problem per se. The speed of light is only a limit locally but globally there is no such thing and in General Relativity there is nothing preventing time travel (closed timelike curves) at a global level, even though the existence of such curves is rather unlikely. The paper notes: > Interestingly, they are allowed in the quantum theory, but with one catch, the time it takes to go through the wormhole should be longer than the time it takes to travel between the two mouths on the outside. Does anyone know why exactly "quantum theory" would impose such requirements? A priori to me it sounds like quite a stretch to take a local theory like quantum mechanics to make claims about the global topology of the universe, given that QM and GR haven't been unified yet. Unless of course by "quantum theory" Maldacena actually means "string theory" or "AdS/CFT" – which wouldn't surprise me at all.
  - ben_w 5y ago
    IIRC (I’m not a physicist), if you combine a closed timelike curve with quantum mechanics, then a random fluctuation in the photon field (which happens all the time) appearing in the middle of it will go all the way around, meet itself, and now there are two of them, then four, then eight, … But as this is in a closed loop of time, from the outside it goes to infinity instantly.
    - zozbot234 5y ago

dark-star 5y ago

And, as always with these kinds of papers, everything only works out in ADS space, which has nothing in common with how our own universe works...

sdoering 5y ago
ADS space? I have not yet heard that acronym. If anyone else never heard of the "Anti-de Sitter space" [0] here a short description from Wikipedia: > In mathematics and physics, n-dimensional anti-de Sitter space (AdSn) is a maximally symmetric Lorentzian manifold with constant negative scalar curvature. Anti-de Sitter space and de Sitter space are named after Willem de Sitter (1872–1934), professor of astronomy at Leiden University and director of the Leiden Observatory. Willem de Sitter and Albert Einstein worked together closely in Leiden in the 1920s on the spacetime structure of the universe. Sadly this is so far outside of my level of understanding that I still don't have a clue. Would love an ELI5. [0]: https://en.wikipedia.org/wiki/Anti-de_Sitter_space?wprov=sfla1 https://en.wikipedia.org/wiki/Anti-de_Sitter_space?wprov=sfl...
- zachf 5y ago
  Think about 2D surfaces. Which ones are the most symmetric? A flat plane is a very nice space: every point is as good as any other (there’s nothing intrinsic to any point to distinguish any point from any other, except arbitrarily), and no direction is particularly special either. A space like that has a lot of symmetries. A sphere also has a lot of symmetries, it also has no directions or points which are distinguished until you declare, “this is my North Pole” arbitrarily. (The earth isn’t a perfect sphere of course and we can use the imperfections as the way we define north and south.) The last type of symmetric space looks like a saddle (like on a horse). It bends one way in one direction and bends the other way in the other direction. An idealized saddle also has no distinguished directions or points. The analogs of these things in higher dimensions, and where one of the directions is time, are important in general relativity. The analog of the plane is called “flat space” or “Minkowski space”. The analog of the sphere is “de Sitter space”. Finally, the analog of the saddle is “anti-de Sitter space” (usually abbreviated AdS, with a lowercase d). It’s a bit of an odd space in a lot of ways. When you look at what space looks like at any given time, it’s a bit like M.C. Escher’s “Angels and Devils”. Surprisingly, Anti-de Sitter space is the easiest space to understand quantum aspects of gravity in. That’s because anti-de Sitter space is curved in such a way that the complicated stuff can be neatly separated from the easy stuff. You can start from something you understand well and turn on the complexity piece by piece. Roughly speaking it’s because the gravitational stuff becomes less important as you go farther and farther away from any matter you’re considering, in a way which is even faster than this happens in flat space or de Sitter space. It turns out that we can exactly understand everything in this gravitational theory by mapping the physics one-to-one to a nongravitational model which we understand really well. There’s a lot of evidence that the map works perfectly. This is called the AdS/CFT correspondence. A lot of work goes into testing the correspondence and attempting to prove it, and this is a big research area. de Sitter space doesn’t have the same desirable properties. Nevertheless there has been great progress in understanding quantum properties of de Sitter in the last year [0]. These results would not have been possible without understanding AdS first. Flat space quantum gravity remains challenging, although again some progress has been made recently too [1]. [0] arxiv:2110.14670 [1] arxiv:1905.09809 and many others

Einstant 5y ago

Trying to Make sense of: “with an AdS3 radius which is of order of the AdS5” Maldacena Feel free to correct me, and also if you think there’s something terribly wrong with the formula, feel free to share along. Now, AdS3 is a three-dimensional space with negative curvature, which is used in string theory as a model for the three-dimensional world. The AdS5 radius is the distance from the center of AdS3 to its boundary. Maldacena is saying that the AdS3 radius is of the same order as the AdS5 radius, meaning that they are both large compared to the Planck length. Is this supported by empirical evidence? No, this is not supported by empirical evidence. This is based on the fact that the AdS3 radius is much larger than the Planck length, while the AdS5 radius is only slightly larger than the Planck length. This means that the two spaces are of the same order of magnitude, and so the AdS3 radius is of the same order as the AdS5 radius. The Gibbons-Hawkings metric is a metric on the space of all Riemannian manifolds. It is defined where S is the action of the manifold and R is the Ricci scalar curvature. If we consider S to be an action under the Gibbons-Hawkings metric, where R is the Ricci scalar curvature of the manifold, In that case, S would be the action of the AdS3 manifold, which is where R is normed curvature of the AdS3 manifold. This is the Ricci scalar curvature: R = \frac{1}{2} \mathrm{tr}\left( R_{\mu \mu} \right) = \frac{1}{2} \mathrm{tr}\left( \frac{\partial{{\partial x^{\mu} \omega_{ {\mu} - \frac{\partial{\partial {x^{\mu} \omega_{\mu} + \omega_{\mu} \omega_{ u} - \omega_{ u} \omega_{\mu} \right) where R_{\mu u} is the Ricci tensor and \omega_{\mu} is the connection 1-form. This is the Ricci scalar curvature: R = \frac{1}{2} \mathrm{tr}\left( R_{\mu u} \right) = \frac{1}{2} \mathrm{tr}\left( \frac{\partial}{\partial x^{\mu}} \omega_{ u} - \frac{\partial}{\partial x^ u} \omega_{\mu} + \omega_{\mu} \omega_{ u} - \omega_{ u} \omega_{\mu} \right) There is not enough empirical evidence to determine whether or not the AdS3 radius and AdS5 radius are of the same order of magnitude. However, based on the results of the calculation, it seems that the AdS3 radius is of the same order of magnitude as the AdS5 radius.

teleforce 5y ago

Most of the people and even Muslim do not aware there is a chapter (chapter 70) with a title Al-Maarij in the Quran with the literal meaning of Maarij "the door to ascent" that some modern scholars refer to this as the wormhole [1]. According to one of the best English translations The Clear Quran, Maarij is translated as "pathways of heavenly ascent" [2]. In the 4th verse of the chapter it's mentioned "through which the angels and the holy spirit will ascend to Him on a Day fifty thousand years in length". That means based on our (human) reckoning it will be fifty thousand years but for angels (or those travel through this pathway or door) it will only takes one day of journey. [1] https://en.wikipedia.org/wiki/Al-Maarij https://en.wikipedia.org/wiki/Al-Maarij [2] https://quran.com/al-maarij https://quran.com/al-maarij

sdfasg34253 5y ago

just thanks for this, opened a couple of think paths. Could be productive at some point in the future (no pun intended).

virgildotcodes 5y ago

I think it was an article posted on HN a little while back that got me thinking about the time travel paradoxes that wormholes could enable. There was one thought experiment about a wormhole with one end on your front lawn, and the other in a spaceship that leaves with your partner, accelerating away at 1g to eventually reach the speed of light, and at some point turning around and coming back. The way I envisioned it is that the wormhole would act like a Zoom call, where you could talk/interact at "normal" speed of time during the entire voyage. Through the wormhole on your front lawn, you'd see your partner arriving back in your front yard and greeting future you despite the fact that their ship was still years away from arriving back on your yard from your frame of reference. Not to mention that you could also step through the wormhole at that point and join the future you and your partner, and then the partner that would arrive on your original lawn would find you missing. Obviously this turns into spaghetti quickly. Now, this post is the first time I learn that travel within the wormhole must take longer than travel between the mouths of the wormhole. This got me thinking about whether my concept of looking through the wormhole was correct. Would you actually see your partner moving at "normal" speed, or would their movements appear to you to begin to slow down as they approached the speed of light, with your movements appearing to them to accelerate dramatically? It seems as if their EM radiation towards you would eventually shift to infrared or radio and yours towards them would shift to gamma? Have I made a total mess of my understanding of all of this? Apologies for the amateurish questions.