Travelling through a wormhole in space

Umbran · Jun 15, 2015

freyar said:
But I don't know anything about neutronium and time machines.

Tipler, 1974, "Rotating Cylinders and the Possibility of Global Causality Violation". Bascially, if you get a long cylinder of neutronium, and you spin it on its long axis fast enough, and fly around it, you can get closed timelike curves - a time machine. Nothing less than neutronium has the mass to do frame dragging to produce the effect.

Tipler maintained that you could do this with a cylinder of finite length if it was spinning fast enough. Hawking held that anything less than an infinite cylinder would violate the weak energy condition, and thus need something with negative energy density to do the trick.

Neither one, of course, includes any quantum effects.

Umbran · Jun 15, 2015

freyar said:
As to "why wormholes," my understanding is that people went looking for them. Let me explain more precisely. The Einstein equations of general relativity take in sketch form "geometry" = "matter-energy." Usually, what we do is start with some configuration of matter-energy and ask what the corresponding geometry of spacetime is. That's not what people have done in the case of wormholes. People wanted to study a specific geometry, namely a tunnel between two regions of space, so they plugged that into the Einstein equations and figured out what kind of matter-energy is required.

Kip Thorne came at it this way, yes. But there's history further back that gave him the thought to look.

The original concept of wormholes goes back to 1916, I think. Take the basic idea of an eternally-existing black hole. Now, take the idea that, aside from the gravitational singularity, that spacetime does not have any "edges" to run into - you should be able to trace the path of any particle infinitely into the past or future. This leads to weirdness - the "black hole/white hole", which has a black hole connecting to another universe, falls out of this. It was shown that for this original formulation, the bridge between universes is unstable. This bridge does not exist for black holes made from collapsing stars - it only exists for black holes that have existed since the beginning of time. And they weren't *looking* for bridges - they were doing some of the initial considerations of what black holes were like, and what spacetime in and around them would have to be, and happened to stumble upon it.

Then comes Kerr, in 1963, who shows that within a *spinning* black hole (and a couple years later, a spinning black hole with charge) there are even more clear paths that look like you can jump between universes if you enter a black hole. And this is, if I recall correctly, more stable.

These are the first "wormholes" - they call for entering a black hole, and coming out in some other universe.

Thorne shifted this to, "Well, can we do this without a black hole? What does that look like?"

freyar · Jun 15, 2015

Jhaelen said:
Actually, the way I understood it, the existence of dark matter was posited to avoid having to introduce new math. I.e. if there was no dark matter, we'd have to revise our math to match our observations.

Likewise dark energy: if our math is correct, then dark energy must exist, otherwise our observations make no sense. Again the alternative, if I understood it correctly, is that our current math is simply incorrect (when applied to the universe at large).

That's the essence of it. If you don't like dark matter or dark energy, you are forced to change the laws of gravity. You then have to ask whether you can get the effects you want even by changing gravity. In the case of dark energy, it seems possible, but it's also possible that all the modified gravity theories that mock up dark energy are mathematically inconsistent (there is some diversity of opinion on this). In the case of dark matter, you can pretty easily reproduce its effects in galaxies by modifying Newtonian gravity, but even very few proponents of modified gravity have claimed they can reproduce observations of the early universe (which were predicted by dark matter).

freyar · Jun 15, 2015

Morrus said:
I think that would be fun!

Cool, hopefully I can start something like that soon (assuming work gets a little less busy).

freyar · Jun 15, 2015

Umbran said:
Tipler, 1974, "Rotating Cylinders and the Possibility of Global Causality Violation". Bascially, if you get a long cylinder of neutronium, and you spin it on its long axis fast enough, and fly around it, you can get closed timelike curves - a time machine. Nothing less than neutronium has the mass to do frame dragging to produce the effect.

Tipler maintained that you could do this with a cylinder of finite length if it was spinning fast enough. Hawking held that anything less than an infinite cylinder would violate the weak energy condition, and thus need something with negative energy density to do the trick.

Neither one, of course, includes any quantum effects.

Interesting. Though I thought negative energy densities are generally from quantum effects (like Casimir effects). In any case, Hawking is generally right about this kind of stuff. There is also a commonly held view (though certainly not proven) that closed timelike curves (at least ones not shielded by event horizons) shouldn't be allowed in a final theory of gravity. So you have to wonder how they'd be forbidden in this case.

Umbran said:
Kip Thorne came at it this way, yes. But there's history further back that gave him the thought to look.

The original concept of wormholes goes back to 1916, I think. Take the basic idea of an eternally-existing black hole. Now, take the idea that, aside from the gravitational singularity, that spacetime does not have any "edges" to run into - you should be able to trace the path of any particle infinitely into the past or future. This leads to weirdness - the "black hole/white hole", which has a black hole connecting to another universe, falls out of this. It was shown that for this original formulation, the bridge between universes is unstable. This bridge does not exist for black holes made from collapsing stars - it only exists for black holes that have existed since the beginning of time. And they weren't *looking* for bridges - they were doing some of the initial considerations of what black holes were like, and what spacetime in and around them would have to be, and happened to stumble upon it.

Then comes Kerr, in 1963, who shows that within a *spinning* black hole (and a couple years later, a spinning black hole with charge) there are even more clear paths that look like you can jump between universes if you enter a black hole. And this is, if I recall correctly, more stable.

These are the first "wormholes" - they call for entering a black hole, and coming out in some other universe.

Thorne shifted this to, "Well, can we do this without a black hole? What does that look like?"

Oh, yes, wormholes through black holes came up earlier in the thread. I think the "wormholes" in the rotating/charged black hole case are still unstable, though. At least that's what my notes from GR class say.

Umbran · Jun 15, 2015

freyar said:
Interesting. Though I thought negative energy densities are generally from quantum effects (like Casimir effects).

Well for many, the idea of "exotic matter", as in actual *physical* material that you can gather up and manipulate, is a bit far fetched. Those same folks sometimes don't have as much of a problem with invoking quantum effects (like the Casimir effect) that produce the same results without having any odd physical materials around.

The point, though, is that Tipler wasn't calling on any such to make his cylinder work.

In any case, Hawking is generally right about this kind of stuff.

Yes. As if we needed Hawking to prove that a light-year long cylinder of neutronium wasn't going to be able to allow anyone to move through time

There is also a commonly held view (though certainly not proven) that closed timelike curves (at least ones not shielded by event horizons) shouldn't be allowed in a final theory of gravity. So you have to wonder how they'd be forbidden in this case.

You can forbid them by noting that you probably can't get enough neutronium in one place to build the thing without it collapsing into a singularity.

As for the commonly held view - that sounds like assuming the conclusion, if you ask me. We don't get to determine whether closed timelike curves should be allowed in a final theory. Either they are possible, or they are not, no matter our desires on the issue.

Oh, yes, wormholes through black holes came up earlier in the thread. I think the "wormholes" in the rotating/charged black hole case are still unstable, though. At least that's what my notes from GR class say.

Which is why I noted them as "less unstable", as opposed to actually unstable.

The Kerr solution is also for a black hole with no beginning or end, not one formed by a collapsing star, in any event. And it isn't like anything other than a super-massive black hole has an event horizon we can cross without being destroyed. Though, admittedly, galactic black holes are likely to be rotating, and may well be charged. So, if someone really wants to check it out, they can just go a few tens of thousands of light years away.

Not that the rest of the universe will even know about it, as you don't get to come back and tell us if you reached another universe. The Kerr solution is a one-way trip.

Bullgrit · Jun 15, 2015

the existence of dark matter was posited to avoid having to introduce new math. I.e. if there was no dark matter, we'd have to revise our math to match our observations.

Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete. Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea.

Isn't this how aether theory was believed for a while.

Bullgrit

Umbran · Jun 15, 2015

Bullgrit said:
Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete. Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea.

Any halfway decent scientist knows that taking a measurement is really taking a sample. It is thus subject to potential sampling error. Scientists need to be honest with themselves about when they might be introducing such errors.

Dark Matter arose as a theory thusly: We looked at distant galaxies, estimated their masses, and realized that their motion was not consistent with their masses. By a *long* shot.

The models of motion in question were based on Newtonian mechanics and Einsteinian relativity - a couple of theories whose areas of validity are very well known, tested, and verified. It is difficult to arrange a correction to these that would allow for the galactic motion observed that didn't bollix up other times when they have been proven to work quite well. There are some that do so, but to me they sort of cherry pick how they are applied, and lead to an inelegant solution.

Or, we could just realize that we might not be estimating how much matter was in these galaxies properly. How could that estimate be off? Well, for one thing, our estimate only included mass that we could *see* - that glowed brightly enough to be seen over these extreme distances.

What's more likely? That Newton and Einstein need to be completely thrown out, or that maybe there's just mass out there that doesn't glow brightly?

It isn't like that was a lightweight question at the time, by the way. It was a major point of contention. It was hashed over a *lot*. It really only clarified when it was shown that the presence of dark matter implied some things about microwave background radiation... and that measurements were consistent with the presence of this mass, and not consistent with it's absence. So, the presence of dark matter has been put through the usual wringer, requiring a new theory to make a prediction we can test.

freyar · Jun 15, 2015

Umbran said:
As for the commonly held view - that sounds like assuming the conclusion, if you ask me. We don't get to determine whether closed timelike curves should be allowed in a final theory. Either they are possible, or they are not, no matter our desires on the issue.

Absolutely. It's just that causality, the inability to create time-travel paradoxes like killing your parents before you're conceived, is expected to be a important principle.

I have to run off to a conference but will hopefully answer Bullgrit re: dark matter tonight.

tomBitonti · Jun 15, 2015

Edit: Sorry, a bit of a ramble. I'll try to clean this up tonight.

Bullgrit said:
Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete. Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea.

Isn't this how aether theory was believed for a while.

Bullgrit

In detailed literature, there are collations of explanations for phenomena (what we observe), with an exporation of different explanations, and the necessary adjustments to current known laws for a particular explanation to work. Those necessary adjustments can lead to predictable results in associated phenomena, which can pretty much wreck a candidate explanation.

I found this sort of collation googling explanations for the red shift a while back. All sorts of explanations have been explored, and discarded due to solid evidence, leaving, more or less, universal expansion as the current best explanation.

I would think that that exhaustive searches are ongoing for explanations, with folks looking in all sorts of directions. For example, one might try to explain unexpected measurements of radial velocities of observable matter in galaxies by postulating a modification to the inverse square law of gravity. That would have a measurable consequence in other areas, which will be looked at, and the scientific community would, over time, make a judgement of whether the explanation was compelling.

At this level of detail, there would end up being an enumeration of explanations, with qualifications attached to each as the quality of each explanation, with a sense of "this is it" applying to one or another explanation only after enough evidence was gathered.

I would guess that all of the following, and more, are or have been considered:

Measurement errors (somehow, we aren't measuring the phenomenon accurate);

Analysis errors (we measured it correctly, but aren't reasoning through the data correctly)

Errors in currently accepted physical theories (some equation which is being used has a necessary modification which we aren't making)

Errors in our understanding of the underlying physical reality (our basic idea of what is there to explain is missing an important detail)

I'm thinking that a modification to a widely held law (say, the inverse square law of gravity) would be looked at pretty hard, since an observably provable modification would be a very big deal, but somewhat on the fringe, since fundamental changes are pretty rare things.

These provides a little information:

https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Though dark matter is by far the most accepted explanation for the resolution to the galaxy rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives, the most notable is Modified Newtonian Dynamics (MOND), which involves modifying the laws of gravity.

And:

Alternatives to dark matter

There have been a number of attempts to solve the problem of galaxy rotation curves by modifying gravity without invoking dark matter. One of the most discussed is Modified Newtonian Dynamics (MOND), originally proposed by Mordehai Milgrom in 1983, which modifies the Newtonian force law at low accelerations to enhance the effective gravitational attraction. MOND has had a considerable amount of success in predicting the rotation curves of low-surface-brightness galaxies, the baryonic Tully–Fisher relation,and the velocity dispersions of the small satellite galaxies of the Local Group. These results are surprising in the context of dark matter, which does not predict the same things as MOND without considerable fine-tuning.

I would be very very careful before accepting the above quotes as being mainstream within the the scientific community. This is, I'm thinking, one of several areas where non-scientific literature does a poor job of representing the general scientific viewpoint, at least so far as presenting the current prevailing thoughts. A particular exciting alternate theory can garner a lot more attention than it deserves, and because of the extra attention, seem to have a lot more support than it really does.

Thx!

TomB