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

Re: Gravitational waves provide dose of reality about extra dimensions

#31

Post by kakarot » Mon, Sep 17, 2018, 8:42 AM UTC

kawakiole wrote: post #27

kakarot wrote: post #20

Earlier quoted context omitted.

This 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.

Anything is possible, but how would you falsify that hypothesis?

Good question and I don't know the answer, but this unknown still helps to define the limits of what we can definitely say is and isn't taking place.

We can't say "we're definitely not experiencing displacement in X dimensions" if we don't definitively know the rules of displacement are homogeneous in every dimension. Since we don't definitively know the number of dimensions, we also can't exhaustively test these rules.

Re: Gravitational waves provide dose of reality about extra dimensions

#32

Post by pps » Mon, Sep 17, 2018, 8:44 AM UTC

kakarot wrote: post #22

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.

The fact that the event was 130 million light years away makes me imagine this chaotic, turbulent network of gravitational waves just ripping through pockets of interstellar space... I wonder how close you would need to be to "feel" the gravitational wave pass through you. Imagine being in the epicenter of a cluster of them.

https://stuver.blogspot.com/2012/06/q-what-would-gravitation...

https://www.reddit.com/r/askscience/comments/45n0sz/how_clos...

Re: Gravitational waves provide dose of reality about extra dimensions

#33

Post by kakarot » Mon, Sep 17, 2018, 8:45 AM UTC

AstralStorm wrote: post #29

kakarot wrote: post #22

Earlier quoted context omitted.

The fact that the event was 130 million light years away makes me imagine this chaotic, turbulent network of gravitational waves just ripping through pockets of interstellar space... I wonder how close you would need to be to "feel" the gravitational wave pass through you. Imagine being in the epicenter of a cluster of them.

I think 0.1G of acceleration difference would be felt as center of inertia shifts from center of mass. Presumably while you're moving. Even more so if weightless. If the wave is long enough or small amplitude enough you wouldn't feel it. For something like the neutron star collision, it would be relatively far but not far enough to not get fried by EM radiation.

> not far enough to not get fried by EM radiation

Aww, this is not the answer I was hoping for. :(

Re: Gravitational waves provide dose of reality about extra dimensions

#34

Post by lifeformed » Mon, Sep 17, 2018, 9:17 AM UTC

Baeocystin wrote: post #19

ComputerGuru wrote: post #17

Earlier quoted context omitted.

But what explains that difference? Based on current understandings, shouldn’t they have arrived at exactly the same time?

Lightspeed is that number in a vacuum . Interstellar space is not a perfect vacuum. There will have been some matter to slightly slow the photons.

Is 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?

Re: Gravitational waves provide dose of reality about extra dimensions

#35

Post by Baeocystin » Mon, Sep 17, 2018, 9:44 AM UTC

lifeformed wrote: post #34

Baeocystin wrote: post #19

Earlier quoted context omitted.

Lightspeed is that number in a vacuum . Interstellar space is not a perfect vacuum. There will have been some matter to slightly slow the photons.

Is 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?

Regarding 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

Re: Gravitational waves provide dose of reality about extra dimensions

#36

Post by mullikine » Mon, Sep 17, 2018, 9:55 AM UTC

Arnavion wrote: post #13

make3 wrote: post #12

the 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

Hard 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.

It'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.

Re: Gravitational waves provide dose of reality about extra dimensions

#37

Post by Razengan » Mon, Sep 17, 2018, 10:19 AM UTC

whatshisface wrote: post #4

kristianp wrote: post #2

So, is that a strike against string theory's extra dimensions, or is it another type of extra dimension being ruled out?

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.

Re: Gravitational waves provide dose of reality about extra dimensions

#38

Post by k__ » Mon, Sep 17, 2018, 10:51 AM UTC

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

Re: Gravitational waves provide dose of reality about extra dimensions

#39

Post by lilgreenland » Mon, Sep 17, 2018, 10:56 AM UTC

ComputerGuru wrote: post #17

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.

But what explains that difference? Based on current understandings, shouldn’t they have arrived at exactly the same time?

The neutron stars were orbiting each other very fast to make the gravity waves. After they collided the gravity waves stopped and light waves were produced.

Re: Gravitational waves provide dose of reality about extra dimensions

#40

Post by konschubert » Mon, Sep 17, 2018, 11:05 AM 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

The 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.