Русский New site

Advanced search

[ New messages · Forum rules · Members ]
Page 38 of 64«1236373839406364»
Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
midtskogenDate: Tuesday, 21.06.2016, 20:18 | Message # 556
Star Engineer
Group: Users
Norway
Messages: 1666
Status: Offline
Quote steeljaw354 ()
Artificial ones?

"Well five million years ago the galactic economy collapsed, and seeing that custom built planets is something of a luxury commodity." -Slartibartfast





NIL DIFFICILE VOLENTI
 
HarbingerDawnDate: Tuesday, 21.06.2016, 20:39 | Message # 557
Cosmic Curator
Group: Administrators
United States
Messages: 8711
Status: Offline
Quote steeljaw354 ()
Artificial ones?

Nobody is saying that they can't possibly exist. What I'm saying is that there's no particular reason to believe that they DO exist.





All forum users, please read this!
My SE mods and addons
Phenom II X6 1090T 3.2 GHz, 16 GB DDR3 RAM, GTX 970 3584 MB VRAM
 
steeljaw354Date: Tuesday, 21.06.2016, 21:40 | Message # 558
World Builder
Group: Users
Pirate
Messages: 862
Status: Offline
Well, we don't even know what planets are in other galaxies. There could be a massive civilization in Andromeda that formed 100000 years ago, and theres no logic to just assume these things don't exist because we haven't seen them yet.

Edited by steeljaw354 - Tuesday, 21.06.2016, 21:45
 
HarbingerDawnDate: Tuesday, 21.06.2016, 21:42 | Message # 559
Cosmic Curator
Group: Administrators
United States
Messages: 8711
Status: Offline
Quote steeljaw354 ()
Well, we don't even know what planets are in other galaxies.

Exactly.





All forum users, please read this!
My SE mods and addons
Phenom II X6 1090T 3.2 GHz, 16 GB DDR3 RAM, GTX 970 3584 MB VRAM
 
MosfetDate: Tuesday, 21.06.2016, 22:16 | Message # 560
World Builder
Group: Users
Italy
Messages: 709
Status: Online
Quote steeljaw354 ()
Well worlds with the density of cotton candy probably do exist.

Never said they don't.





"Time is illusion. Lunchtime doubly so."
Douglas N. Adams
My mods
Asus x555ub: cpu i5-6200u - ram 4gb - gpu nvidia geforce 940m 2gb vram
 
WatsisnameDate: Tuesday, 21.06.2016, 22:18 | Message # 561
Galaxy Architect
Group: Global Moderators
United States
Messages: 2604
Status: Offline
Wow, a lot got covered in a short time. biggrin I'll go back to these:

Quote Alek ()
If singularities are points, is there any way to really determine their exact location without rounding errors? Or are they small enough to have quantum effects and only be a small cloud of probability?


They're definitely small enough for quantum effects to be important. In classical general relativity, they are infinitesimal points, but in reality...?

Quote FastFourierTransform ()
The merger doesen't complete for an external observer?

In terms of the space-time, the black holes merge without any hesitation. Actually, they merge as if with anticipation. The event horizon is defined by the geometry of the space-time. A point lies inside a true event horizon if all null geodesics emitted from that point end at the singularity, and a point lies outside the horizon if at least one can make it to future null infinity -- i.e. it is causally connected to the rest of the universe.

During the merger, the event horizons reach out towards each other and join together. (This always looks kind of creepy to me.) We can define the moment that they touch as the moment of the merger. For a head-on collision it looks like this. And here's one of an in-spiral.

Now if we refer to the sedimentary shell structure of the material approaching the horizons, then these smear around each other as the merger unfolds. But, just as like matter asymptotically approaching the horizons and getting redshifted to invisibility on the same timescale, this smearing of the apparent horizons together happens so quickly that to an observer, it looks like they blend into a single black hole. The resulting black hole rings like a bell as it sheds off any irregularities as gravitational waves, settling down into its final shape -- this is called the "ringdown". We actually saw this process with the LIGO detections!

Quote steeljaw354 ()
How do we know what happens if nobody has experienced falling into a black hole?


By applying physics.

Black holes are remarkably simple objects compared to other things we might be interested in. Simply solve Einstein's field equations for a point source, and you have yourself a complete mathematical description of a black hole. How do we know this solution is physically meaningful? Because we can prove that any distribution of mass that lies within its own Schwarzschild radius must form a black hole. We can study the collapse of the cores of massive stars, and find that they can easily satisfy those conditions. And when we look out in the universe, we see lots of objects that behave exactly how black holes should behave.

How do we know what happens to you if you fall into one? Well, we don't know the entire story of what happens, but we do know the most important parts. That is, we know what the journey would look like, we know that going beyond the horizon is a one-way trip, and we know that it is fatal. How? Because the space-time geometry only allows inward motion once inside the horizon. Even light is pulled inwards. The curvature, and therefore tidal forces, also increase as you get closer. We don't know what happens at the singularity, but it doesn't make any difference as far as someone falling in should be concerned about. I'm pretty sure the singularity destroys whatever fundamental particles are left of you. I'm pretty sure the singularity doesn't give you cheese and crackers. smile





 
steeljaw354Date: Tuesday, 21.06.2016, 22:56 | Message # 562
World Builder
Group: Users
Pirate
Messages: 862
Status: Offline
Well what is inside a black hole? A portal? All the matter it has sucked up through it's life time?
 
apenpaapDate: Tuesday, 21.06.2016, 23:01 | Message # 563
World Builder
Group: Users
Antarctica
Messages: 1063
Status: Offline
A singularity. A single, infinitely dense point into which all the mass it has absorbed has collapsed. Although I believe the situation is even stranger if it's a rotating black hole.




I occasionally stream at http://www.twitch.tv/magistermystax. Sometimes SE, sometimes other games.
 
WatsisnameDate: Tuesday, 21.06.2016, 23:57 | Message # 564
Galaxy Architect
Group: Global Moderators
United States
Messages: 2604
Status: Offline
Yeah. For a non-rotating black hole, everything goes to a point at the center -- a singularity. When we examine the equations for a rotating black hole, the point becomes a ring. However, this is unphysical -- an artifact of our assumptions.

The equations we use to describe a rotating black hole begin by assuming all the mass is at the central singularity, with some angular momentum. This predicts that the singularity must be a ring, and it also splits the horizon structure into two (and does some further interesting things to the horizon shape and exterior space-time). The outer horizon behaves as you expect. It is a region you cannot escape from because space is flowing into it at the speed of light. But the spin of the black hole produces a centrifugal force just like antigravity, so the inward flow of space starts slowing down as you get closer, and drops below the speed of light at the inner horizon.

This makes weird predictions for things that fall in to a spinning black hole. They never hit the singularity. Instead, they pile up (violently!) near the inner horizon. This contradicts the assumption that all the mass is at the center, so the space-time is no longer described by the same equations as what we started with. Furthermore, the region inside the inner horizon, near the ring singularity, allows for closed time-like curves. That means an observer could follow a slower-than-light trajectory and return where they started in time. This is time travel. This is a no-no in physics because there are causal paradoxes. The rotating black hole's interior cannot behave this way -- the solution is unphysical.

We know (or are at least very sure based on current observations; further observations coming very soon will give us a lot more!) that the solution is good for everything outside the outer horizon. It correctly describes the frame-dragging due to the hole's rotation, the location of the inner-most stable circular orbit, production of the jets, and so forth. But we don't really know what happens deep inside. There are a lot of really interesting hypotheses. smile





 
SalvoDate: Wednesday, 22.06.2016, 06:32 | Message # 565
Star Engineer
Group: Local Moderators
Italy
Messages: 1400
Status: Offline
Quote Watsisname ()
we don't really know what happens deep inside

Can't we do that someway with sound? I mean, if we could manage to create a rotating "sound hole" (?) could we observe what the sound does in the inner of the "sound horizon"?

I'm not sure that creating such a thing would be much easier than creating a short-living black hole, by the way.





The universe is not required to be in perfect harmony with human ambition.

CPU: Intel Core i7 4770 GPU: ASUS Radeon R9 270 RAM: 8 GBs

(still don't know why everyone is doing this...)
 
WatsisnameDate: Wednesday, 22.06.2016, 08:53 | Message # 566
Galaxy Architect
Group: Global Moderators
United States
Messages: 2604
Status: Offline
Now that's an interesting question. There are analogues of black holes that can be made with sound or the behavior of fluids. These are not perfect analogues (no fluid metric can exactly equal a black hole metric), but they are close and still produce a lot of the same phenomena, like the event horizon, redshift, and even the Hawking radiation.

I wasn't sure how good the whirlpool analogue would be for a rotating black hole. Certainly a whirlpool provides a good analogy for frame dragging, but beyond that I had no idea. It turns out that a such a "draining bathtub" model actually works rather well. This paper focuses mainly on how the whirlpool responds to perturbations, just like black holes do. It also investigates the harnessing of the hole's rotational energy. However, this is only a study of a mathematical model of an ideal rotating fluid. To my knowledge we haven't made an actual rotating acoustic hole in the lab to do these tests. I'd guess that making a whirlpool with a rotational flow exceeding the sound speed is challenging.

The other thing to note is that while acoustic holes are useful for studying analogies of real black hole behavior that are otherwise hard to observe, they don't necessarily help us to study how the black holes are formed, or what happens to the mass distribution that went into making them. For that, I'm not sure of any useful method beyond numerical relativity simulations and assuming our knowledge of relativity is still good under such extreme conditions.





 
AlekDate: Wednesday, 22.06.2016, 20:29 | Message # 567
Pioneer
Group: Users
United States
Messages: 318
Status: Offline
Quote Watsisname ()
and assuming our knowledge of relativity is still good under such extreme conditions.


What if it's not, though? Newtons laws of gravity work quite well for low accuracy, general measurements, but create weird predictions or are outright wrong in some cases. What if relativity begins to fall apart in a similar way except in only such extreme situations as to be ones that we can't measure?





Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
 
WatsisnameDate: Wednesday, 22.06.2016, 20:52 | Message # 568
Galaxy Architect
Group: Global Moderators
United States
Messages: 2604
Status: Offline
We know that general relativity must fail somewhere close to a singularity, and much of current theoretical gravitational physics is trying to figure out a more complete theory of quantum gravitation. It's very probable that these will never be testable near a singularity, but hopefully the theories may have testable implications elsewhere.

There are also still tests that remain to be done to be sure general relativity works in the strong-field regime near event horizons. The gravitational wave detection of black hole mergers was an outstanding test for this, but we can still visually look at the black hole shadow and the material swirling right around the innermost stable orbit to see if those behave the way we expect them to.

Ultimately, any breakdown of a theory in conditions that are fundamentally impossible to test... is a cessation of physics. Physics only works on things that are testable in principle -- even if not presently, then potentially in the future.





 
AlekDate: Wednesday, 22.06.2016, 21:05 | Message # 569
Pioneer
Group: Users
United States
Messages: 318
Status: Offline
Ah, I understand.

Also, concerning quantum gravitation, why have we not been able to detect gravitons yet? Are they actually the background "noise" we get in particle detectors because there are so many but are so weak that it just looks random? Because honestly that sound like the best explanation to me--what IS the background noise if it's not any known particle corresponding to the other three forces? Gravity is everywhere, also, especially on a planet in a solar system full of other planets and a smallish-medium size star.





Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
 
WatsisnameDate: Wednesday, 22.06.2016, 22:34 | Message # 570
Galaxy Architect
Group: Global Moderators
United States
Messages: 2604
Status: Offline
I can guarantee that gravitons are not the source of any noise in any particle detector. smile Gravitons, if they exist, have such an absurdly small cross section that no reasonable detector would be able to see them.

People have done the math on this. A detector with the mass of Jupiter, placed next to a neutron star, could expect to see a graviton about once a century! During that time, any graviton signal would be swamped by neutrinos. The mass of shielding necessary to block the neutrino noise would result in collapsing the detector into a black hole!

So there's just no practical way to detect the graviton directly. They are really, really weakly interacting. Neutrinos are the oft-cited "ghost" particle, but gravitons are orders of magnitude less social. Some have proposed that it may instead be possible to find indirect evidence of them by their imprint on the CMB (just like how we expect the CMB to show evidence of primordial gravitational waves and quantum fluctuations), but even this seems sketchy.





 
Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Page 38 of 64«1236373839406364»
Search: