Liquid spacetime: A very slippery superfluid, that's what spacetime could be like.
What if spacetime were a kind of fluid? This is the question tackled by theoretical physicists working on quantum gravity by creating models attempting to reconcile gravity and quantum mechanics. Some of these models predict that spacetime at the Planck scale (10^-33cm) is no longer continuous, as held by classical physics, but discrete in nature. Just like the solids or fluids we come into contact with every day, which can be seen as made up of atoms and molecules when observed at sufficient resolution. A structure of this kind generally implies, at very high energies, violations of Einstein's special relativity (an integral part of general relativity).
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(Score: 3, Funny) by wonkey_monkey on Thursday April 24 2014, @02:55PM
http://en.memory-alpha.org/wiki/Fluidic_space [memory-alpha.org]
systemd is Roko's Basilisk
(Score: 2) by davester666 on Thursday April 24 2014, @05:56PM
ew. imagine you are all covered in space fluid, and right before you have to meet your girlfriend's parents.
(Score: 3, Funny) by MrGuy on Thursday April 24 2014, @03:04PM
http://xkcd.com/171/ [xkcd.com]
s/strings/fluid
(Score: 4, Interesting) by Covalent on Thursday April 24 2014, @03:17PM
In the very early universe, the density of matter was so high that "sound" could exist. We've seen the effects of this sound in the CMB. When I first heard that, it seemed incredible. But the universe seems to always follow the same rules on a grand scale that it does on small scales (with the possible exception of dark matter / energy). So what we see in the lab maps onto what we see through telescopes surprisingly well.
So the fact that the granularity of the universe would cause it to behave in a fashion similar to other things that are made of small particles seems pretty likely to be true. If nothing else, investigations of this kind are likely to lead (eventually) to the reconciliation of quantum physics and general relativity. We "know" that both of these theories are correct, and also that they are contradictory in some instances. The solution to this quandary must, therefore, not really be "quantum" or "relativistic". I would be extremely happy if the real answer were classical, particularly if it allowed us some insight as to how to get around the pesky limitations of relativity.
You can't rationally argue somebody out of a position they didn't rationally get into.
(Score: 2, Insightful) by MozeeToby on Thursday April 24 2014, @04:32PM
What's interesting to me is that if space is discrete, that means that it's interactions should be computable. You can quantify how those discrete pieces of space-time interact with each other and simulate it. The fact that universe can be mathematically described to the tiniest detail is so pervasive that most people don't even think about how incredible it is.
It always seems to me the more we know about the universe, the more it looks like it's all a giant computation.
(Score: 2) by Covalent on Thursday April 24 2014, @05:03PM
Excellent point! Now, simulating many trillions of trillions of trillions of voxels of spacetime per cubic nanometer of space will be difficult, but theoretically possible. This is especially true if there are no quantum weirdnesses (?) inherent in those voxels.
You can't rationally argue somebody out of a position they didn't rationally get into.
(Score: 2) by melikamp on Friday April 25 2014, @05:57AM
(Score: 0) by Anonymous Coward on Thursday April 24 2014, @11:11PM
"I would be extremely happy if the real answer were classical, particularly if it allowed us some insight as to how to get around the pesky limitations of relativity."
That's a contradictory stance. Classical physics has no way around the "pesky" limitations of relativity. There is no way whatsoever that one can travel faster than light, not least because paths through spacetime are split into timelike, null and spacelike, and never the trio shall meet. We follow timelike geodesics -- those that move in a way to increase whatever you have chosen as a time coordinate, and so do all bodies with positive mass. Photons follow null geodesics -- those that move in a way that their proper distance measured along the path always remains zero, as do any other particles with zero mass. If such thing as a tachyon existed (hint: it doesn't) it would follow a spacelike geodesic -- one could always choose a time coordinate that did not change along the path. That's ultimately a statement of where classical physics has ended up.
In quantum physics, however, there is no such issue, even when one considers particles propagating on curved backgrounds. We integrate across all possible paths of, say, an electron, and those paths include some that would be "faster than light" when interpreted with classical physics. As the theory turns out, those paths are heavily suppressed and the maximum probability tends to focus on the classical path (or what have you, depending on situation -- certainly not acausal, as an FTL would be) but still the quantum theory opens you to that possibility.
"In the very early universe, the density of matter was so high that "sound" could exist."
It is now, too -- I can hear my fingers tapping this keyboard remarkably clearly. To be a bit less facetious, yes, though I don't entirely see what the great shock is: we know that "sound" propagates through any collisional medium, so even in the present universe there are times we should expect to see its effects, and in the early universe we should be pretty bloody shocked if it isn't there. That isn't to say that seeing the impact of those early sound waves on the CMB, and then seeing the imprint of the *exact same sound waves* but about 12bn years later in the ripples left on the large-scale structure of matter is genuinely impressive and says something instructive about cosmology beyond "collisional matter propagates perturbations".
(Score: 2) by The Mighty Buzzard on Thursday April 24 2014, @03:23PM
My rights don't end where your fear begins.
(Score: 2) by Common Joe on Thursday April 24 2014, @03:52PM
If spacetime is a fluid, then what does that say about Aether [wikipedia.org] which was supposedly debunked?
(Score: 2) by The Mighty Buzzard on Thursday April 24 2014, @05:24PM
My rights don't end where your fear begins.
(Score: 2) by HiThere on Thursday April 24 2014, @08:25PM
It was debunked as in "Not appearing to have the properties described by the theory". Something else with much of the same description but with different properties hasn't been debunked, but might not be improved by giving it the same name.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by The Mighty Buzzard on Thursday April 24 2014, @08:39PM
My rights don't end where your fear begins.
(Score: 1, Interesting) by Anonymous Coward on Thursday April 24 2014, @11:01PM
"If spacetime is a fluid, then what does that say about Aether which was supposedly debunked?"
Literally nothing. The aether was some "fluid" that electromagnetic radiation propogated through, and with respect to which electromagnetic radiation has a velocity equal to that of the measured speed of light in vacuum. This has absolutely nothing to do with any fluid properties of spacetime, not least because on the scales at which spacetime is a "fluid" we're already in the continuum limit, where relativity can be taken as a valid theory, and we have *already* observed, repeatedly, reproducably, and for well over a century, that the predictions of relativity hold.
Further to that, why would it introduce an aether? What is the possible mechanism for doing so? We don't need an aether. In current physics we have photons propagating on a curved spacetime (which can be described by an appropriate action, something like \sqrt{g}F^2) where g describes gravity and F^2 electromagnetism). In a different model we recover that in a continuum and, presumably, will end up with a path summation describing the propagation of photons across discretised units of spacetime, which in a continuum limit would become a familiar path integral.
I don't see any need for an aether in that, myself.
(Score: 2) by threedigits on Friday April 25 2014, @09:15AM
Put another way: aether was described as "something" we we moved through, like ships move in water. That was proved to be not the case by the Michaelson-Morley experiment. A fluid Space/time would be something different, it would be something that we are part of, and moves with us.
(Score: 0) by Anonymous Coward on Friday April 25 2014, @09:30AM
AC above here. Entertainingly the Michelson-Morley experiment (and similar) don't actually say there's no aether -- they say that the Earth is at rest with respect to the universal reference frame. Since that's clearly preposterous the conclusion is that there is no universal reference frame, but I've always amused myself with the alternative explanation, which is that the Earth actually is in the centre of the universe and everything *does* orbit around it. Up until the day we do Michelson-Morley type experiments on other planets, that conclusion is just as valid.
(Score: 1) by justthinkit on Friday April 25 2014, @12:55PM
(Score: 3, Funny) by Foobar Bazbot on Thursday April 24 2014, @04:04PM
Actually, it's more like a big ball of wibbly-wobbly, timey-wimey... stuff.
(Score: 0) by Anonymous Coward on Thursday April 24 2014, @04:08PM
A fluid is normally something flowing in spacetime. But if spacetime itself is the fluid, what does it flow in?
(Score: 2) by Gaaark on Thursday April 24 2014, @05:52PM
You're kind of a 'universe is half full' kind of guy aren't you? :)
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 2) by mhajicek on Thursday April 24 2014, @09:18PM
Just because you can put something into words doesn't mean it makes sense. There may be no "in" there.
The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
(Score: 2) by stormwyrm on Friday April 25 2014, @01:13AM
Both quantum field theory and general relativity developed equations that describe the vacuum that look remarkably like the equations of fluid mechanics. In classical fluid mechanics with the linear approximations to the equations of fluid dynamics, we have the result that moving an object through a fluid faster than the fluid's wave speed results in infinite pressure. That's just another way of saying that it's impossible to break the speed of sound in a medium. Of course everyone knew that the linear fluid dynamics equations were just approximations, and non-linear equations needed to be developed to describe the behaviour of the fluid at transonic and supersonic conditions. Perhaps the Lorentz transformation equations that similarly say that moving an object through the vacuum faster than the speed of light requires infinite energy is in the same way an approximation, and a new theory needs to be developed that describes the behaviour of spacetime at transluminal and superluminal velocities? The article didn't seem to elaborate on the violations of special relativity mentioned in the lead paragraph, which seems a far more interesting consequence of the spacetime fluid hypothesis.
Numquam ponenda est pluralitas sine necessitate.
(Score: 1) by hendrikboom on Saturday April 26 2014, @12:53PM
The loop quantum gravity people have something to say about this.
The gravitational field isn't something in spacetime or on spacetime. Spacetime isn't something that contains the gravitational field.
The gravitational field *is* space. Spacetime *is* the gravitainoal field.
-- hendrik