Gravitational waves provide dose of reality about extra dimensions • Hacker News Board

Re: Gravitational waves provide dose of reality about extra dimensions

#41

Post by whatshisface » Mon, Sep 17, 2018, 11:34 AM UTC

skh wrote: post #11

empath75 wrote: post #10

Earlier quoted context omitted.

Area = surface area not volume. Physicists tend to drop constant factors when they do calculations, I’ve noticed.

A 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 r…

The 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.

Re: Gravitational waves provide dose of reality about extra dimensions

#42

Post by rotorblade » Mon, Sep 17, 2018, 11:35 AM UTC

I'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.

Re: Gravitational waves provide dose of reality about extra dimensions

#43

Post by whatshisface » Mon, Sep 17, 2018, 11:43 AM UTC

Razengan wrote: post #37

whatshisface wrote: post #4

Earlier quoted context omitted.

It 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 wo…

[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.

If there are extra dimensions that gravity doesn't exit into, this experiment won't detect them. It is not curtians for string theory.

Re: Gravitational waves provide dose of reality about extra dimensions

#44

Post by whatshisface » Mon, Sep 17, 2018, 12:07 PM UTC

k__ wrote: post #38

How come there are people saying there are higher dimensions and people saying it's all holographic and the truth is 2D

Holography 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.

Re: Gravitational waves provide dose of reality about extra dimensions

#45

Post by dogma1138 » Mon, Sep 17, 2018, 12:12 PM UTC

rotorblade wrote: post #42

I'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 n…

Gravitational 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.

Re: Gravitational waves provide dose of reality about extra dimensions

#46

Post by BurnGpuBurn » Mon, Sep 17, 2018, 12:55 PM UTC

Animats wrote: post #16

Nice. 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.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

Re: Gravitational waves provide dose of reality about extra dimensions

#47

Post by foobarbecue » Mon, Sep 17, 2018, 1:25 PM UTC

BurnGpuBurn wrote: post #46

Animats wrote: post #16

Nice. 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.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

I don't think that time shifting the gravity vector has a measurable effect on the orbit. Indeed, the vector is 1au*c late in real life, but we didn't know that for sure until the recent neutron star collision observation.

Re: Gravitational waves provide dose of reality about extra dimensions

#48

Post by pavel_lishin » Mon, Sep 17, 2018, 1:29 PM UTC

BurnGpuBurn wrote: post #46

Animats wrote: post #16

Nice. 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.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

Why would being 8 minutes out of date result in an unstable orbit?

Re: Gravitational waves provide dose of reality about extra dimensions

#49

Post by mikeash » Mon, Sep 17, 2018, 1:42 PM UTC

BurnGpuBurn wrote: post #46

Animats wrote: post #16

Nice. 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.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

“Instant” is not a concept that makes sense with relativity. One observer’s “instant” is another observer’s “back in time.”

I’m not totally sure why this doesn’t result in spiraling into the sun as you describe, but I think it’s because the sun’s gravitational field is basically constant, and it’s changes which propagate at the speed of light.

Re: Gravitational waves provide dose of reality about extra dimensions

#50

Post by SketchySeaBeast » Mon, Sep 17, 2018, 1:50 PM UTC

BurnGpuBurn wrote: post #46

Animats wrote: post #16

Nice. 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.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

As you're always working with a fixed temporal distance, wouldn't that basically negate any instability? There'd never be a gravitational "correction", as that 8 minute offset would never shift.