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Gravitational waves provide dose of reality about extra dimensions
- kristianp 8y agoSo, is that a strike against string theory's extra dimensions, or is it another type of extra dimension being ruled out?
- whatshisface 8y agoIt is a strike against the idea that over long distances, the spherical shell of "expanding and thinning" gravity can go down faster than 1/r^2 (which would come about from A=pi*r^2, the area of a sphere in three dimensions), achieving that goal by slipping in to dimensions higher than three. However now we have found that gravitational waves are about as strong as we expect it looks like gravity isn't leaving the world of 3+1D.
- skh 8y agoSphere's have surface area and volume. Neither of those quantities is given by the formula for the area of a circle. I’m not a physicist and can’t speak to the merits of the rest of your post but this error of yours makes me doubt your post's veracity. Did you make a mistake in your parenthetical remark? Maybe it edit if you have.
- dfmooreqqq 8y agoI assumed it was a reference to the surface area of a sphere - which is 4pir^2. The point would still hold.
- skh 8y agoMakes sense. That’s what I would have thought if not for the area of a sphere in three dimensions remark.
- empath75 8y agoArea = surface area not volume. Physicists tend to drop constant factors when they do calculations, I’ve noticed.
- skh 8y agoA sphere has surface area and volume. I didn’t claim or imply that I thought area meant volume. It does not have area and the formula for the surface area of a sphere is not pi r^2. I mentioned this in my original post. Then there is the remark that whatshisface made: A = pi r^2, the area of a sphere in three dimensions. That is wrong and most likely a mistake. When talking about dropping a constant factor are you referring to the 1/r^2 part or dropping a 4 in the statement A=pi r^2. Because if the latter then why keep pi? That’s a constant.
- whatshisface 8y agoThe constants perfectly divide out in this case, adding the pi was an afterthought so that people would recognize it as an area. Yes... It should have had a 4 as well. Too late to edit now.
- Razengan 8y ago[Complete layman here.] Could it also not mean that gravity is "just" a 3D (or 3+1D) phenomenon? For example, what does gravity mean in 2D? Can we rule out that our 3D† universe is not a rarity in a larger, 4D universe, much like how true-2D planes are a rarity in our 3D universe? † D as in spatial dimension, excluding time.
- whatshisface 8y agoIf there are extra dimensions that gravity doesn't exit into, this experiment won't detect them. It is not curtians for string theory.
- crazygringo 8y agoAre there particular theories of quantum mechanics or string theory that are now any less likely? Or is this just confirming what everyone expected all along?
- deleted 8y ago[deleted]
- T-A 8y agoIt's yet another limit on the general idea of large extra dimensions: https://en.wikipedia.org/wiki/Large_extra_dimension https://en.wikipedia.org/wiki/Large_extra_dimension It doesn't say anything about compact extra dimensions (the tiny curled up ones you usually hear about in string theory).
- crazygringo 8y agoThanks! That's exactly the context that was totally missing from the article.
- blind 8y ago'Scientists still confusing themselves by taking a 200 year old interpretation of an experiment as scripture.'
- make3 8y agothe title of the article implies that there is new evidence for the existence of multiple dimensions, while the content of the article is about the opposite being the case
- Arnavion 8y agoHard to see how. "Dose of reality" usually means evidence to the contrary. Even if there was some ambiguity, the first line of the article as well as the caption of the opening image make it clear.
- mullikine 8y agoIt's probably more cryptic than ambiguous, due to the decorative language. A lengthy terms and conditions is not ambiguous, yet it's very vexatious. This is neither ambiguous nor vexatious. Perhaps though, it's a little bit cryptic. A simple man must try a little harder. Spock would probably say the statement is intrinsically ambiguous due to the metaphor, however.
- mirimir 8y ago> The gravitational waves from the collision reverberated in LIGO the morning of Aug. 17, 2017, followed by detections of gamma-rays, X-rays, radio waves, and optical and infrared light. If gravity were leaking into other dimensions along the way, then the signal they measured in the gravitational wave detectors would have been weaker than expected. But it wasn't. Do they really mean to say "followed by detections"? Don't gravitational waves propagate at c? Also, I'm puzzled by "weaker than expected". One issue is calculating expected intensities for gravitational waves vs electromagnetic radiation. The other is calibration of LIGO signals. It's not obvious that resulting uncertainties are small enough for drawing reliable conclusions about gravity leakage.
- Baeocystin 8y agoYes. Even if the creation events of each part of the spectrum are at the exact same time (which is unlikely) there is going to be matter in between the event and us. That matter will slow the rate of light's propagation.
- mirimir 8y agoAh. Didn't think of that. Edit: And I didn't see Animats' comment that EM radiation was delayed ~2 seconds. But also that the speed difference was very small.
- kakarot 8y agoThis conjecture also assumes that our understanding of thermodynamics applies in any other hypothetical dimension. It's entirely possible that displacement in these dimensions corresponds to an increase in energy in the system that offsets the "lost" energy.
- kawakiole 8y agoAnything is possible, but how would you falsify that hypothesis?
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- Animats 8y agoNice. A related result from the neutron star collision is that the speed of light and the speed of gravity are the same, to within something like 1 part in 10^15. Both light and gravity waves from an event 130 million light years away came in less than 2 seconds apart.
- ComputerGuru 8y agoBut what explains that difference? Based on current understandings, shouldn’t they have arrived at exactly the same time?
- Baeocystin 8y agoLightspeed is that number in a vacuum. Interstellar space is not a perfect vacuum. There will have been some matter to slightly slow the photons.
- lifeformed 8y agoIs there nothing that will slow gravity down? Does that mean that gravity is essentially faster than light? I mean not technically, but for all practical cases it would be slightly faster?
- Baeocystin 8y agoRegarding your first question, the only way we could even determine if it a meaningful question to ask is if we demonstrated conclusively if gravitons exist or not, which would require a quantum theory of gravity that we currently just don't have. Great question though. Regarding your second question, yes. And as we increase our number of gravity wave followed shortly by EM wave detections, I'm willing to bet that the difference between the two will become a very useful tool in the cosmic distance ladder toolbox. https://en.m.wikipedia.org/wiki/Cosmic_distance_ladder https://en.m.wikipedia.org/wiki/Cosmic_distance_ladder
- andrewla 8y agoElectromagnetic radiation is never actually slowed down, but the effective speed of light in a medium is more a function of a statistical averaging effect as some radiation is absorbed and re-emitted. So a wave packet that might be very narrow, representing a distant pulse, will tend to spread out as it travels through a medium. The net effect will be that the maximum of the impulse will happen further after the (undetectable) leading edge of the packet, leading to the apparent slow-down of electromagnetic radiation. For gravitational waves the same effect will happen, as the waves pass through matter with its own stress-energy that will absorb some of the incoming radiation and re-emit it. But gravitation interaction with matter is so much weaker that it would take a significant amount of concentrated stress-energy or order to have the same effect. Sparse matter in interstellar space is too diffuse to experience tidal effects even at the small magnitudes of gravitational waves.
- k__ 8y agoHow come there are people saying there are higher dimensions and people saying it's all holographic and the truth is 2D
- konschubert 8y agoThe discussion about how many dimensions "really" exist is pointless because there is no other way to define "real" than to define it as the reality that I experience. And I experience 3 dimensions. Physics is a collection of models expressed as formulas, plus recipes of how to translate these formulas to the reality that we experience. If I have a formula with 5 dimensions in it that tells me how to build a teleporter then that's great but it doesn't change the fact that the world that I experience is 3 dimensional in space.
- whatshisface 8y agoHolography attempts to explain 3D in terms of 2D, but that doesn't make 3D go away. People are still around after it was found out that we were made of cells.
- rotorblade 8y agoI'm confused over the size of these extra dimensions... They say that they do not rule out small compact extra dimensions, but only large ones, the number they give is ~100 km. But this must surely be ruled out by ordinary gravitational experiments that establish the 1/r^2 law for gravity, since I would assume that that law would be seriously broken for extra dimensions of those sizes for even everyday physics. The number that is quoted in some textbooks on String theory (but I do not have the refs.) is that gravity is 1/r^2 down to sizes of ~1 cm for extra dimensions.
- dogma1138 8y agoGravitational waves are the best litmus test we have for gravity on a large scale which is why they put to rest most of the modified gravity theories. Newtonian gravity on large scales wasn’t “proven” explicitly in fact we have had to introduce dark matter to make it work to match observations, there is also the issue of empty space having weight which is attributed to various favtors depending on the theory in question.