61–70 of 190 posts
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> No information passes thru the event horizon of a black hole. that is only true in one direction though, right?
There is only one direction through the event horizon of a black hole.
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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 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.
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 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.
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Every space launch and every large object in orbit are trivially observable. Even when the NRO launches top secret spy satellites that they barely acknowledge the existence of, we know exactly when the launch is. Thanks to the cold-war risk of nuclear ICBMs, every launch is publicly announced to reduce the risk of false positives in an early warning system. In light of this, your proposition is impossible without als…
unnecessary. visual cloaking. are you watching objects in 5th gen FLIR? zero radar cross section. even USAF has acknowledged craft you can't trivially observe. your point is rubbish.
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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?
As always, soon we will discover another way, or there will be a different theory that will allow bending the rules.
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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 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 e…
Earlier quoted context omitted.
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.
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?
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> No information passes thru the event horizon of a black hole. that is only true in one direction though, right?
From the point of view of something falling in you are correct. From the point of view of something further away watching the thing falling in, it takes forever. I think. Time dilation is easy to lose track of.
it feels almost meme like that from an observer’s point of view, a black hole will have a bunch of stuff “stuck” on its horizon. i haven’t ever seen elaborations of how it would actually look though.
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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.
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’…
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.
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As always, soon we will discover another way, or there will be a different theory that will allow bending the rules.
Not always. Faster than light travel has never been bent.
Also, there are theories of FTL: https://en.wikipedia.org/wiki/Faster-than-light
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> Because the distance to the singularity grows at the speed of light, doesn't our universe expand at the same speed?
:D is the universe expanding, or revealing more of itself? if the universe is expanding at the speed of light, doesn't that imply that just light is reaching us more from further away places? sorry to be off topic but that comment gave me this brainfart.