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posted by martyb on Friday October 05 2018, @09:55PM   Printer-friendly
from the 4-dimensions-ought-to-be-enough-for-anybody dept.

Scientists have managed to constrain the possible number of dimensions of our universe to 3+1 (3 spatial and 1 time).
According to a new paper on Arxiv https://arxiv.org/abs/1801.08160
A recent merger of neutron stars that was observed in the visible spectrum, as well as with gravity waves, was used to determine that there are no extra dimensions for gravity to leak into. This reinforces our current models based on 3+1 to an extremely high degree of certainty and essentially rules out any theory that requires extra dimensions in order to function.

Quoting the paper:

The observation of GW170817 in both gravitational and electromagnetic waves provides a number of unique tests of general relativity. One question we can answer with this event is: Do large-wavelength gravitational waves and short-frequency photons experience the same number of spacetime dimensions? In models that include additional non-compact spacetime dimensions, as the gravitational waves propagate, they "leak" into the extra dimensions, leading to a reduction in the amplitude of the observed gravitational waves, and a commensurate systematic error in the inferred distance to the gravitational wave source....

The short of it was that there was absolutely no evidence for electromagnetism and gravity to be propagating through a different number of dimensions than the expected 3+1.

These are just some really cool and unexpected results; in my case I was a big supporter of brane theory until this result came out. Now, I don't know what to think. Gravity is too weak to make any sense at all. What do you think?


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  • (Score: 2) by Immerman on Saturday October 06 2018, @02:19AM (3 children)

    by Immerman (3985) on Saturday October 06 2018, @02:19AM (#744923)

    Actually, I believe string theory still predicts that gravity is weakened due to propagating through additional dimensions. The difference being that the assumption is that those other dimensions loop back on themselves at very small scales - so you don't expect to see anomalous weakening over long distances, only at distances shorter than the size of the largest "dimensional loop". Which, if I recall correctly, is currently constrained to a maximum size on the sub-millimeter scale to remain consistent with historical observations.

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  • (Score: 2) by DrkShadow on Sunday October 07 2018, @01:07AM (2 children)

    by DrkShadow (1404) on Sunday October 07 2018, @01:07AM (#745291)

    This would not be a "dimension". This may be localization of effects, but consider what a dimension is.

    It's an index into the universe. You have a dot. Zero-dimensional. You can't go up, can't go left, you have a dot and you have a dot.

    Add a dimension. Now you go, Hey! I can move! Cool! ... put a pebble on that line. Oh, crap -- something's blocking me. I can't get through that block.

    Add a second dimension. Now you're like, .... ok, I can go around this rock. That's cool. Hey, this second dimension is nifty. The second dimension is the floor flan of your house -- where there's perhaps few places where you can look from any given far corner to the door without seeing _anything_ between you (no couch, no coffee tables, ..). The second dimension allows you to go around them.

    But wait! I'm going to pour a salt line across the room! Hah! You're trapped!! You can't get through my impervious wall spanning infinitely through the second dimension, you can only go up to my wall in your first dimension! And so add a third dimension. Hop. Now you're over the line. You went upward in the third dimension to bypass an obstacle that was in the second dimension. You used dimension 3 index 1 instead of dimension 3 index 0. And so it is. Maybe there's something in index 5 (the ceiling, watch out for your head).

    Dimensionality is an index.

    But what good are three dimensions? You can move around things, but really... these dimensions are just placement of "things". They don't move. They're there, and ... they're there. That's it. It's an index into a dimension, and dimension 2 has a salt line. Period. The end. It's there.

    Add a dimension. Dimension 4. We have that index of all three prior dimensions. Brush that salt line aside a bit. In dimension 4 index 0, you have a salt line in your three dimensional room because someone tried to block your two dimensional workaround to that one dimensional space. Each dimension is infinite in every direction, because it's just an index.

    So you brush the salt line. Dimension 4, index 1 has a hole in the salt line. Dimension 4 index 2 has a bigger hole in the salt line. Dimenison four could contain _completely_different_ things from dimension 3 in the same three-dimensional coordinates. It's the exact same first three dimensions, but each progressive dimension is an "index into a selection of the n-1 dimensional organization." We call dimension 4 "time". Imagine three dimensions where your home were fixed. Everything is where it is, forever, unchanging. Only with dimension 4, "Which state of the home", do we have change -- and that change is considered time.

    Add a dimension. We now have five dimensions. Hey cool! But what does that __MEAN__??? It's nonsense! -- or just iterate. It's an index into the line, the floor, the room, the change in the room. It's an offer of how the room could be changed differently at the same time. It's the "divide" between choice A and choice B in time. If you can only make one choice, then you only have one time dimension. If you can choose a different "time", then you have a fifth dimension, where the same 3d space is occupied either the same or differently, and time progresses differently. (We call this the "multiverse".)

    So what if we add a dimension? A sixth dimension. An index of multiverses. What would you do with a collection of different multiverses? A whole card collection of all the possible states of all the possible timelines, now you have different collections of them. Like you've got a bootleg multiverse, a shadow copy of all of your possible copies of the different developments of the 3d universe.

    So what if you add a dimension again, you greedy bastard? What is a seventh dimension? It's really getting weird. You use it in programming all the time. Every variable is, itself, a dimension. You can change any one of them without affecting the other variables, and you end up with a different result. I'm just sorry, but we don't have a name, or a readily-available concept for this one.

    So. Dimensions are indexes. When you talk about an index of possible 3d spaces, you don't go, "And this 3d space could be 3mm wide." That's nonsense. You could have a wall at 3mm out, but the space itself is... an index into all the possible two-dimensional floors. You don't say that a building can only have three floors because its third dimension only goes 30 feet up.

    Theories saying there are multiple dimensions that "fold in on themselves" are misnomers. I don't necessary know what they're talking about, but it's not dimensionality.

    • (Score: 0) by Anonymous Coward on Sunday October 07 2018, @07:55PM

      by Anonymous Coward on Sunday October 07 2018, @07:55PM (#745614)

      5th could be 'parallel universes' where one minor change or choice cascaded to all future events. And 6th could be collections of multiverses which started with different seed values. If you take the example of procedurally generated game universes into consideration, it would not be impossible that the 3+1 constraint is real, from our perspective, but that parallel universes of every possible choice also exist, or even multiverses entirely different from ours founded on different physics or universal seed values that make it operate entirely differently from our own.

      In theory none of these possibilities would be testable if the system containing them was perfect. They would all run in parallel, there would be no way to test interaction between them, etc. If by some chance the system was IMPERFECT however (think Spectre/Meltdown on metaphoric hardware containing all dimension 5 entities if not also dimension 6 containers) there might be cornercases that would allow us to view data from the other side, or mutually exchange data through the boundary, via mutual inference attacks against the container. Considering the difficult with those attacks on real hardware, and the usage of hardware much larger than the original processors to develop and prove an attack, that doesn't mean it is actually possible for us to do it with only the resources available within our universe.

    • (Score: 2) by Immerman on Monday October 08 2018, @04:06PM

      by Immerman (3985) on Monday October 08 2018, @04:06PM (#746002)

      I prefer the description of a dimension as "a direction-line at right angles to all other labeled direction-lines", I find it a bit clearer.

      Pick a direction line(dimension), call it North-South. Add an additional dimension, East-West, and you can travel along that direction without moving in the original dimension, and vice versa. Add a third dimension, call it Up-Down, now you can move along U-D without moving at all in either either E-W or N-S.

      Superstring dimensions are exactly that sort of dimension, but you have to introduce one more concept for them to make sense - the idea of "open" or "closed" dimensions. In an open, or "flat" dimension you can travel an infinite distance and just keep getting further from where you started. A closed dimension though loops back on itself - traveling far enough in a straight line will eventually bring you back to your starting point, with the distance traveled being the circumference of the closed dimension.

      There is some serious debate as to whether the normal 3 space dimensions are actually open or closed - space appears flat (on average) to the limits of our measurement ability, but may be closed on a scale spanning galactic clusters at least, and very possibly spanning the entire observable universe - it's non-trivial in something like the Hubble Ultra-Deep Field image to tell for certain whether every galaxy is unique, or if the further galaxies are actually younger versions of the closer ones, with the light having "looped around" the universe one or more times before it reaches us.

      And that's how the extra dimensions in superstring theory work - there *are* other directions, perpendicular to the three normal ones, but they're all closed, and loop back on themselves over a very short distance. If you were to travel along one of them you'd only have to travel, at most, a fraction of a millimeter to get back to where you started. If you were a bacteria locked in a 3-dimensional salt-box, you could potentially travel a 0.1mm along one of those extra dimensions, then travel outside the limits of the box along "normal" dimensions, and then travel back along the other dimension to get back into the normal world, outside the box.

      Of course that assumes that both the salt-box and bacteria really only exists in three dimensions - which they don't, because they're made of N-dimensional superstrings that exist along all these other dimensions as well. Even then it might be possible if the "strange-dimension thickness" of the two were considerably smaller than the circumference of the strange dimension. That gets into a lot of unknowns, but the most immediately relevant being just how big are these other dimensions. As I said above, experiments have limited them to no larger than the sub-millimeter scale, but they could be far, far smaller. Even Plank-scale, which is to say many orders of magnitude smaller than a subatomic particle.

      That is to say, they could be so small as to simply not matter in any way for most practical purposes - the exceptions being that they still provide additional degrees of freedom for superstring rings to vibrate in, allowing for the harmonics necessary to get the "magic numbers" that govern Quantum Mechanics, and that gravity would (could?) still "leak out" in these other directions. It would obviously leak back in almost immediately as it get back to its starting point, but would have to constantly "fill up" the other dimensions as it goes, offering an explanation for why it's so much weaker than the other fundamental forces.

      That also allows for one of the very few ways in which superstring theory makes (potentially) testable predictions distinct from other QM theories. *If* the circumference of the largest of the many other looped dimensions is larger than the shortest distance we can test the force of gravity over, then at distances smaller than the largest dimensional loop, gravity should start falling off as 1/r^3 instead of 1/r^2, since at that scale it would be propagating through 4 dimensions instead of only three.