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<blockquote data-quote="freyar" data-source="post: 6639629" data-attributes="member: 40227">Let me collect some of the comments from the thread first:In this case, TomB has the right idea.&nbsp; Neutronium is really just normal matter in extreme conditions.&nbsp; The &quot;exotic matter&quot; required for wormhole spacetimes very specifically must violate what is known as the Null Energy Condition (NEC); this is actually proven mathematically.&nbsp; Neutronium doesn't do that, nor does any kind of matter we know about.&nbsp; However, some quantum effects (such as the Casimir effect) and some objects in string theory (which is hypothetical, of course) do violate the NEC in ways that lead to sensible physics, so it's not possible to rule out wormholes on this basis.&nbsp; Other modified gravity theories can also mimic violations of the NEC.&nbsp; However, usually if you include violations of the NEC in physics, you get some kind of pathology like instability of the vacuum, causality violation (superluminal propagation), or negative probabilities.&nbsp; It's not clear that the NEC violation you need for a wormhole is a &quot;safe&quot; kind.&nbsp; So it may exist or it may not.Incidentally, tachyonic fields to violate the NEC.&nbsp; As Umbran says elsewhere in the thread, they represent an instability.&nbsp; Specifically, a tachyon field at zero value is like a ball sitting on the top of a hill --- if it moves a tiny amount, say due to a quantum fluctuation, it falls off the hill and rolls into a valley.&nbsp; And when the tachyon settles in the valley, it's no longer tachyonic.&nbsp; This is indeed like the Higgs field of the Standard Model.Anyway, the murky status of NEC violation is probably why wormholes are still kind of a niche subject.&nbsp; I mean, there's a reasonably sizeable literature on them but not a big one, and there seem to be a few authors who have done most of the work on them.But I don't know anything about neutronium and time machines. <img src="http://www.enworld.org/forum/images/smilies/blush.png"&nbsp; class="smilie" loading="lazy" alt=":blush:" title="Blush&nbsp; &nbsp; :blush:" data-shortname=":blush:" /></blockquote>

[QUOTE="freyar, post: 6639629, member: 40227"] Let me collect some of the comments from the thread first: In this case, TomB has the right idea. Neutronium is really just normal matter in extreme conditions. The "exotic matter" required for wormhole spacetimes very specifically must violate what is known as the Null Energy Condition (NEC); this is actually proven mathematically. Neutronium doesn't do that, nor does any kind of matter we know about. However, some quantum effects (such as the Casimir effect) and some objects in string theory (which is hypothetical, of course) do violate the NEC in ways that lead to sensible physics, so it's not possible to rule out wormholes on this basis. Other modified gravity theories can also mimic violations of the NEC. [B]However[/B], usually if you include violations of the NEC in physics, you get some kind of pathology like instability of the vacuum, causality violation (superluminal propagation), or negative probabilities. It's not clear that the NEC violation you need for a wormhole is a "safe" kind. So it may exist or it may not. Incidentally, tachyonic fields to violate the NEC. As Umbran says elsewhere in the thread, they represent an instability. Specifically, a tachyon field at zero value is like a ball sitting on the top of a hill --- if it moves a tiny amount, say due to a quantum fluctuation, it falls off the hill and rolls into a valley. And when the tachyon settles in the valley, it's no longer tachyonic. This is indeed like the Higgs field of the Standard Model. Anyway, the murky status of NEC violation is probably why wormholes are still kind of a niche subject. I mean, there's a reasonably sizeable literature on them but not a big one, and there seem to be a few authors who have done most of the work on them. But I don't know anything about neutronium and time machines. :blush: [/QUOTE]

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