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Old 04-23-2012, 04:08 AM   #31
vicky_molokh
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Default Re: Wormholes in Space

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Originally Posted by Ulzgoroth View Post
That part is easy to avoid. Orbits, you know.
Orbits are okay, you know . . . in orbit. But
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Originally Posted by Sindri View Post
what are good excuses to keep the wormholes in space rather than on the surface of planets.
a wormhole in orbit at the altitude of a planetary surface will proceed to become a tool of digging and demolition.
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Old 04-23-2012, 04:38 AM   #32
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Default Re: Wormholes in Space

The other likely reason is some reasonable concern about what happens if the wormhole isn't quite as stable as you thought. If collapsing an expanded wormholes would release enough energy to vaporize your planet, you might not want to put it there even if you were pretty sure that would never happen.
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Old 04-23-2012, 05:34 AM   #33
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Default Re: Wormholes in Space

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If collapsing an expanded wormholes would release enough energy to vaporize your planet, you might not want to put it there even if you were pretty sure that would never happen.
See Babylon 5 re: The Bonehead Manuver. (intentionally opening a jump point...inside an already open jump gate.)

Even if people are SURE the technology cannot have an accidental failure...I would not want to put it near anything important...otherwise someone might be tempted to use it as a weapon...
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Old 04-23-2012, 07:36 AM   #34
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Default Re: Wormholes in Space

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Is there really any justification for the "FTL doesn't work this close to a gravity well" thing though? I thought that was an entirely magical restriction.
"FTL works" is already magic. Adding a little more magic won't hurt anything.
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Old 04-23-2012, 08:28 AM   #35
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Default Re: Wormholes in Space

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It's not really much of a handwave. If you can have stable wormholes at all, they are going to be extremely large negative masses. A planet would be repelled by them and they'd be attracted to the planet. It would be a really bad idea to put one anywhere near a planet you want to keep.
In general relativity, you have to be careful about this. All wormholes have regions which have negative energy densities (which is more or less equivalent to negative mass density), but the far-field curvature of space-time commonly matches that of a body of zero or positive mass, so that they would behave like a body with zero or positive mass. There are good reasons to believe that quantum mechanics forbids net zero or negative mass bodies (the statement is more sophisticated than this, but this helps you get the idea). So you end up with an object that essentially has a positive mass.

Now, it is quite likely that the mass will be very large - the mass of Jupiter, the mass of a million suns, something like that. This depends on the particular implementation of the wormhole.
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Old 04-23-2012, 08:36 AM   #36
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Default Re: Wormholes in Space

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Is there really any justification for the "FTL doesn't work this close to a gravity well" thing though? I thought that was an entirely magical restriction.
I suppose you could say that the space warp thingie was very delicate, had a large spatial extend, and thus was strongly affected by tidal forces. The actual depth into the gravity well must be irrelevant, as must the gravitational field, but tidal forces (how fast the gravitational field changes) can definitely affect things. Tidal forces scale with the length of the object being affected, the mass of the gravity source, and the inverse cube of the distance to the gravity source.

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Old 04-23-2012, 08:40 AM   #37
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Default Re: Wormholes in Space

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Now, it is quite likely that the mass will be very large - the mass of Jupiter, the mass of a million suns, something like that. This depends on the particular implementation of the wormhole.
Do you know where I could get information on traversable wormhole mass requirements by implementation?
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Old 04-23-2012, 08:40 AM   #38
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Default Re: Wormholes in Space

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Originally Posted by lwcamp View Post
I suppose you could say that the space warp thingie was very delicate, had a large spatial extend, and thus was strongly affected by tidal forces. The actual depth into the gravity well must be irrelevant, as must the gravitational field, but tidal forces (how fast the gravitational field changes) can definitely affect things. Tidal forces scale with the length of the object being affected, the mass of the gravity source, and the inverse cube of the distance to the gravity source.

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Old 04-23-2012, 08:45 AM   #39
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Default Re: Wormholes in Space

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Natural wormhole pairs are probably really really a)tiny and b) short-lived. They also only occur in "true" vacuum (like any other quantum flux). That's why you need all that negative mass, to pump them up to a size that's useful if you aren't a lepton and make them last longer than an insignificant fraction of a pico-second.
This is one possibility. Another possibility is that the topology of space-time fundamentally cannot be changed. If this were the case, you simply cannot ever create wormholes to another part of this universe, but any wormholes left over as relics of creation (if any) cannot ever fully collapse. They can shrink down to specks on the order of the Plank scale, but if you find one, you can always pump it back up with negative energy stuff and see where the other end is located.

Another nifty thing - if the topology of space-time cannot change, you can pump massive amounts of energy into a tiny speck of it to initiate inflation, and then this universe will always be connected to the universe you just created with a wormhole. Use time dilation tricks to wait until things have quieted down and you have stars and planets and stuff, then open up the wormhole to see where it goes (there are ways to propel and steer wormholes from the other side, making use of time dilation effects to mean everyone at home doesn't have to wait long even though the wormhole on the other side might experience thousands of years of proper time en route to its new home).

At this time, we have no experimental or theoretical justification for preferring topology changing versus non-topology changing physics.

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Old 04-23-2012, 08:51 AM   #40
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Default Re: Wormholes in Space

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Do you know where I could get information on traversable wormhole mass requirements by implementation?
The usual place where this is discussed is the peer-reviewed journal Physical Review D (abbreviated Phys. Rev. D in citations). There is also Matt Visser's text "Lorentzian Wormholes: From Einstein to Hawking." Either of these are only recommended if you are familiar with the workings or general relativity.

Otherwise, note that the Visser wormhole (a "two dimensional" "window" or "gate" wormhole) and its extended generalization, the thin shell wormhole (a "surface" that acts as a wormhole) has exactly zero mass in the far field. This is probably unstable, but from this you can add mass to whatever your story or setting needs (and adding mass will get rid of at least some of the instabilities leading from quantum "Ford- Roman" effects).

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