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The Average Null Energy Condition (ANEC) makes it so there can not be a traversable wormhole joining two otherwise disconnected regions of spacetime. So by cons

The Average Null Energy Condition (ANEC) makes it so there can not be a traversable wormhole joining two otherwise disconnected regions of spacetime. So by construction, any infinite null geodesic (a light ray) which makes it through the wormhole must be chronal (causally connected). This means there can be no traveling faster than light over long distances by going through one.

FakeComments 7y ago
You don’t really explain how the assumption leads to the conclusion — and I’m left with the impression you used many words to say “they just assumed you can’t”. Could you elaborate? Edit: Looking into energy conditions further, they are literally assumed restrictions on the equations because physicists felt some predictions were unphysical. I’d really like if someone could explain if there’s any justification to what I was responding to beyond “well, because we assumed it should work that way”. I think it behooves the physics community to be honest which claims are conclusions and which are their assumptions, and the specific reasoning that leads from assumption to conclusion.
- Filligree 7y ago
  Without negative matter, it's impossible to build a wormhole that's shorter than the distance through normal space. This piece of math shows that it may still be possible to build a wormhole which isn't shorter than that distance. Obviously that wouldn't be much of a shortcut, but the potential research and (maybe) real-world applications are still impressive.
  - FakeComments 7y ago
    That’s not what I was asking. I was asking about the particular usage of the average null energy condition as justification to rule out wormholes: why isn’t that just begging the question by assuming your conclusion? and how does that particular assumption actually lead to the conclusion there can’t be wormholes? It’s interesting the downvotes for asking someone to support a scientific claim, and be clear where they’re making assumptions versus reaching empirical conclusions.
    - lupire 7y ago
      That's right. When you have a mathematical theory, there are often extremal cases that predict strange things that have never been observed. At this point you have a choice to make: 1) Assume the mathematical theory is too permissive, and rule out the things you have no reason to exist, and hope to find a more elegant theory (on the controversial metaphysical assumption that simpler/elegant theories are more likely to be correct) 2) Assume that the mathematical theory is pointing you in a direction to search for a new phenomenon, and build things like superconducting supercolliders to search for empirical evidence. With wormholes, we're a bit stuck in that we are decades to centuries away from empirically testing the theories, so physically the Average Null Energy Condition is moot -- it's fine math to do, as groundwork/scaffolding for future physics, but it doesn't say anything physically until we get empirical evidence for or against it.
      - FakeComments 7y ago
        Okay — and just to be clear, there’s nothing problematic to me about simplifying assumptions or effective theories. Both are important tools for making predictions tractable. But when we lose sight of what are conclusions, what are strongly justified assumptions, and what are simplifying assumptions we don’t have justification for (or even know to be untrue), we begin to create fundamentally inaccurate models or wrongly shut down others’ avenues of inquiry. This happens in economics and business quite often, but simplifying assumptions become orthodox truth with surprising frequency in hard sciences like physics, as well.
    - empath75 7y ago
      If that condition didn’t hold, we would expect to see a lot of phenomena that we don’t see, which makes it a poor model for reality.
- eigenschwarz 7y ago
  I definitely get your point. One difference between theoretical physics and just math is we have since we use math as just a tool to describe the world, we still have to input physical assumptions to make any sense of what we see. There are many instances of things being mathematically "OK" but we don't think physically exist. See "White Hole" for instance. I will try to give a better explanation later today! Funny enough, I am off for the section for the QFT2 that Daniel teaches right now, hah. I can also ask him personally questions later in the week.
  - lupire 7y ago
    Who is the "we" who thinks White Holes don't exist? https://en.wikipedia.org/wiki/GRB_060614 https://en.wikipedia.org/wiki/GRB_060614
    - nine_k 7y ago
      Black holes are hard to notice, except by side effects, but we know great many of them, end even portrayed one. White holes would be relatively hard to miss, because they must be shining very brightly. One of the current theories suggests that the big bang (or "a big bang") was a white hole: every black hole is a white hole producing a big bang a parallel universe. We've already had ours, and are lucky enough to still register its echo as the CMB.
piker 7y ago
So that's it, then? Some thing, say, a million light years away from another thing is in practice entirely inaccessible to civilizations existing on that other thing, requiring a minimum of a million years to reach?
- BartBoch 7y ago
  As always, soon we will discover another way, or there will be a different theory that will allow bending the rules.
  - lupire 7y ago
    Not always. Faster than light travel has never been bent.
    - jcoffland 7y ago
      Shadows can move faster than light.
      - saagarjha 7y ago
        Shadows are an abstract concept (namely, the absence of light), not a physical thing.
        smoyer 7y ago
        I would argue that shadows are a very physical effect.
      - TotempaaltJ 7y ago
        That's an interesting assertion... Why would shadows be able to move faster than light?
        virgilp 7y ago
        It's perfectly correct and does not contradict physics. Think about it in the opposite direction - say you have a perfect laser, with perfect pointing ability. You pick two planets that are far away from each other but at roughly the same distance from Earth, and make an angle of 10 degrees with it; it will take you a few seconds to "move" the laser 10 degrees and thus move the spot from one planet to the other - in the process making the spot "travel" millions of light years in a few seconds. However, the light itself has not traveled at faster-than-light speeds (it still takes a lot of years for the light from your laser pointer to reach any one of those two planets).
hinkley 7y ago
I'm kind of relieved by this. The last wormhole conversation I had with someone, I imagined a weapon using a short-distance wormhole with the ends opposed 180 degrees, and using a star's own gravity to tear chunks out of it. Everybody dies from massive solar flares. Of course time travel books, wormhole researchers, and myself all make the same mistake over and over: If you made a wormhole or traveled in time, why do we assume that the frame of reference of the system is our star? Sol is whirling around our galaxy and an alarming rate, and that's moving through the universe at a huge velocity. Why would the hole you're trying to make in space move along with our solar system? If I traveled back to five minutes ago I'd die in hard vacuum. I'd have just enough time to realize how stupid I am. Similarly, every time I try to use the same wormhole it would be farther from where I am and the other end farther from where I want to be.
- olemartinorg 7y ago
  Presumably, if you're smart enough to make a time machine, you'd be smart enough to calculate the target position in both time _and_ space.
  - hinkley 7y ago
    I suspect if you had that much understanding of orbital mechanics, you'd be a billionaire already and not worry so much about time travel.
    - TeMPOraL 7y ago
      No sensible protagonist or villain use time travel to become a billionaire. There are usually better pursuits involved.
      - VRay 7y ago
        Except Biff of course
- andrewflnr 7y ago
  AIUI wormhole mouths don't have a direction. They're spherical. The picture of a wormhole as a bell-shaped indentation in a sheet, like the graphic on this article, is an artifact of trying to explain 4-d concepts in 3-d shapes (in 2-d images). For a being in the 2-d sheet, the wormhole is a circularly symmetric spot of weird-shaped space. In real 3-d space, a wormhole is a spherically symmetric spot of weird-shaped space.

FakeComments 7y ago

You don’t really explain how the assumption leads to the conclusion — and I’m left with the impression you used many words to say “they just assumed you can’t”. Could you elaborate? Edit: Looking into energy conditions further, they are literally assumed restrictions on the equations because physicists felt some predictions were unphysical. I’d really like if someone could explain if there’s any justification to what I was responding to beyond “well, because we assumed it should work that way”. I think it behooves the physics community to be honest which claims are conclusions and which are their assumptions, and the specific reasoning that leads from assumption to conclusion.

Filligree 7y ago
Without negative matter, it's impossible to build a wormhole that's shorter than the distance through normal space. This piece of math shows that it may still be possible to build a wormhole which isn't shorter than that distance. Obviously that wouldn't be much of a shortcut, but the potential research and (maybe) real-world applications are still impressive.
- FakeComments 7y ago
  That’s not what I was asking. I was asking about the particular usage of the average null energy condition as justification to rule out wormholes: why isn’t that just begging the question by assuming your conclusion? and how does that particular assumption actually lead to the conclusion there can’t be wormholes? It’s interesting the downvotes for asking someone to support a scientific claim, and be clear where they’re making assumptions versus reaching empirical conclusions.
  - lupire 7y ago
    That's right. When you have a mathematical theory, there are often extremal cases that predict strange things that have never been observed. At this point you have a choice to make: 1) Assume the mathematical theory is too permissive, and rule out the things you have no reason to exist, and hope to find a more elegant theory (on the controversial metaphysical assumption that simpler/elegant theories are more likely to be correct) 2) Assume that the mathematical theory is pointing you in a direction to search for a new phenomenon, and build things like superconducting supercolliders to search for empirical evidence. With wormholes, we're a bit stuck in that we are decades to centuries away from empirically testing the theories, so physically the Average Null Energy Condition is moot -- it's fine math to do, as groundwork/scaffolding for future physics, but it doesn't say anything physically until we get empirical evidence for or against it.
    - FakeComments 7y ago
      Okay — and just to be clear, there’s nothing problematic to me about simplifying assumptions or effective theories. Both are important tools for making predictions tractable. But when we lose sight of what are conclusions, what are strongly justified assumptions, and what are simplifying assumptions we don’t have justification for (or even know to be untrue), we begin to create fundamentally inaccurate models or wrongly shut down others’ avenues of inquiry. This happens in economics and business quite often, but simplifying assumptions become orthodox truth with surprising frequency in hard sciences like physics, as well.
  - empath75 7y ago
    If that condition didn’t hold, we would expect to see a lot of phenomena that we don’t see, which makes it a poor model for reality.
eigenschwarz 7y ago
I definitely get your point. One difference between theoretical physics and just math is we have since we use math as just a tool to describe the world, we still have to input physical assumptions to make any sense of what we see. There are many instances of things being mathematically "OK" but we don't think physically exist. See "White Hole" for instance. I will try to give a better explanation later today! Funny enough, I am off for the section for the QFT2 that Daniel teaches right now, hah. I can also ask him personally questions later in the week.
- lupire 7y ago
  Who is the "we" who thinks White Holes don't exist? https://en.wikipedia.org/wiki/GRB_060614 https://en.wikipedia.org/wiki/GRB_060614
  - nine_k 7y ago
    Black holes are hard to notice, except by side effects, but we know great many of them, end even portrayed one. White holes would be relatively hard to miss, because they must be shining very brightly. One of the current theories suggests that the big bang (or "a big bang") was a white hole: every black hole is a white hole producing a big bang a parallel universe. We've already had ours, and are lucky enough to still register its echo as the CMB.

piker 7y ago

So that's it, then? Some thing, say, a million light years away from another thing is in practice entirely inaccessible to civilizations existing on that other thing, requiring a minimum of a million years to reach?

BartBoch 7y ago
As always, soon we will discover another way, or there will be a different theory that will allow bending the rules.
- lupire 7y ago
  Not always. Faster than light travel has never been bent.
  - jcoffland 7y ago
    Shadows can move faster than light.
    - saagarjha 7y ago
      Shadows are an abstract concept (namely, the absence of light), not a physical thing.
      - smoyer 7y ago
        I would argue that shadows are a very physical effect.
    - TotempaaltJ 7y ago
      That's an interesting assertion... Why would shadows be able to move faster than light?
      - virgilp 7y ago
        It's perfectly correct and does not contradict physics. Think about it in the opposite direction - say you have a perfect laser, with perfect pointing ability. You pick two planets that are far away from each other but at roughly the same distance from Earth, and make an angle of 10 degrees with it; it will take you a few seconds to "move" the laser 10 degrees and thus move the spot from one planet to the other - in the process making the spot "travel" millions of light years in a few seconds. However, the light itself has not traveled at faster-than-light speeds (it still takes a lot of years for the light from your laser pointer to reach any one of those two planets).

hinkley 7y ago

I'm kind of relieved by this. The last wormhole conversation I had with someone, I imagined a weapon using a short-distance wormhole with the ends opposed 180 degrees, and using a star's own gravity to tear chunks out of it. Everybody dies from massive solar flares. Of course time travel books, wormhole researchers, and myself all make the same mistake over and over: If you made a wormhole or traveled in time, why do we assume that the frame of reference of the system is our star? Sol is whirling around our galaxy and an alarming rate, and that's moving through the universe at a huge velocity. Why would the hole you're trying to make in space move along with our solar system? If I traveled back to five minutes ago I'd die in hard vacuum. I'd have just enough time to realize how stupid I am. Similarly, every time I try to use the same wormhole it would be farther from where I am and the other end farther from where I want to be.

olemartinorg 7y ago
Presumably, if you're smart enough to make a time machine, you'd be smart enough to calculate the target position in both time _and_ space.
- hinkley 7y ago
  I suspect if you had that much understanding of orbital mechanics, you'd be a billionaire already and not worry so much about time travel.
  - TeMPOraL 7y ago
    No sensible protagonist or villain use time travel to become a billionaire. There are usually better pursuits involved.
    - VRay 7y ago
      Except Biff of course
andrewflnr 7y ago
AIUI wormhole mouths don't have a direction. They're spherical. The picture of a wormhole as a bell-shaped indentation in a sheet, like the graphic on this article, is an artifact of trying to explain 4-d concepts in 3-d shapes (in 2-d images). For a being in the 2-d sheet, the wormhole is a circularly symmetric spot of weird-shaped space. In real 3-d space, a wormhole is a spherically symmetric spot of weird-shaped space.