Science and Astronomy Questions

10 Jan 2019 16:27

Gnargenox wrote:
Source of the post Thinking negatively, is Dark Matter just static electricity?

Ah, no. Dark matter cannot have electric charge, or else it would interact with photons and we would see it.

Gnargenox wrote:
Source of the post Created by the gravitational pull and spin created by normal matter coalescing on a galactic scale

We know that the formation of the dark matter halos preceeds the formation of galaxies, as this affects the appearance of the CMB anisotropies, and also the formation of the cosmic web. There is about 5 times more dark matter than regular matter in the universe, and the small density variations in the dark matter during the early universe gravitationally collapse first. Then the regular matter falls in to the gravitational wells produced by the dark matter. Regular matter collides with itself and can also radiate its potential energy away as light, thus enabling it to collapse into stars and galaxies.

The gravitational wells produced by the dark matter affect the appearance of the CMB by curving the paths of the CMB photons through it, and blueshifting (or redshifting) them as they fall into (or climb out of) the well. Since dark matter does not collide with itself, the size and shape of its gravitational well is different than the regular matter, so this makes an observable difference in the CMB which we can compare with simulations. There's an excellent video lecture explaining this physics in more detail, which I link here in the cosmology thread.

Gnargenox wrote:
Source of the post I guess magnetic fields stretch as far as gravity does too.

Yes, the magnetic field has infinite range. But (at large distances from the source), it drops in strength according to the inverse cube of the distance rather than the inverse square. This is because magnetic fields arise from currents, and far from the source we can model that source as a "dipole". So in addition to the inverse square law decay which follows from geometry (area of spheres increase as radius squared), there is an additional drop due to the two "magnetic charges" of the magnetic dipole appearing to get closer together and cancelling out as you get farther away, just as with the electric field from an electric dipole:

Gnargenox wrote:
Source of the post But every time I get a change to ask Richard Feynman "Why do magnets work?", he starts punching the wall

Feynman is ranting about the problems of asking "why" something happens in physics, or "how something works". It often comes across as very angry and arrogant, but I think he is actually making an extremely important point:

Actually Feynman is incorrect here when he describes why ice is slippery (the pressure is insufficient to melt it, if we compute and compare with water's phase diagram), but that's besides the point.

The point is, in the way I like to think about it, is that if I say I "understand" something then that means I know something about what's happening at least one level deeper than that. For example, I can say I understand that time passes more slowly near a massive object, because I know how space-time geometry is modified by a gravitational field, and I know how to use space-time geometry to compute time intervals measured by clocks carried by different observers. But I cannot say I understand why gravitational field modifies the space-time, other than by going through the derivation of the equations of general relativity.

Why do magnets work? I can honestly say that I don't understand why they work. I know how to compute the magnetic field, and I know how that field affects charges in its vicinity. I also know that magnetic fields are generated by moving charges. And I know that electric and magnetic fields are interrelated by special relativity. Why do they work that way? I don't know. "They just do, as shown by experiments." I similarly cannot explain what electric charge even is. All I can say is that it's a conserved quantity that generates electric field and causes predictable and measurable effects on other charges.

What is an electric or magnetic field, anyway? They are mathematical models we have built to describe and predict observed electromagnetic phenomena. But if you want to know why do those phenomena work that way? I cannot give you a satisfactory answer!

Gnargenox · 10 Jan 2019 16:33

Still, you always nail it!

Stellarator · 10 Jan 2019 20:25

Watsisname wrote:
Source of the post But if you want to know why do those phenomena work that way?

Dohhh... because math

?

10 Jan 2019 20:55

I would not say that math is a tool which explains why things work the way they do. Rather, they work the way they do, and math helps you describe and predict what happens.

For example, suppose someone asks,

"Why do things fall when I drop them?"
Because there is a gravitational force.

"What is a gravitational force?"
It is a model we've built to describe this particular interaction we call gravity, which makes masses attract each other.

"How does the gravitational force work?"

Do these equations help you understand why masses attract each other? I don't think that they do (even assuming that the math is understood).

The Newtonian model definitely doesn't help. It says nothing about the nature or origin of the force. It simply allows you to calculate it.

The general relativistic model goes a step further. It replaces a force with a deviation of paths in space-time, and describes how the space-time curvature is produced by matter and energy. But it still doesn't explain why gravity works this way. That is not its power. Its power is that it lets us predict it.

midtskogen · 11 Jan 2019 00:19

Here's Newton's take on the cause of gravity:

Translated:

► Show Spoiler

Original text:

► Show Spoiler

Newton's idea of doing science was to observe, then derive the math describing what was observed. Done. Math is simply something that generalises observations through induction. According to Newton, there is no place for guesswork in science.

This is not how science is done today. Unlike Newton, we're not happy with simply "the maths works".

A-L-E-X · 11 Jan 2019 02:25

The fascinating thing I found about passing through the singularity is that from what I have read, as you get closer and closer towards the center, the area from which you can see the universe becomes more and more limited until it looks like you are looking through a straw or a pinhole? I guess when the field of view starts getting smaller and becomes distorted the person would realize they are doomed!

The ever fading image you describe that others would see almost sounds like how the movies portray ghosts of dead people as something that is there and not there at the same time and usually fading away into nothingness- it's as if the energy "evaporates"!

It would be fascinating if an alien species had actually done something with a black hole to make a planetary system habitable. I wonder how close to the black hole a planet could be and still be habitable (in other words how large could the hole appear to be in the sky of a habitable planet)?

Question about micro black holes if we ever discover them, how much of a risk would they pose? Since they also have event horizons could one cause damage if it was close enough? How small would a black hole have to be not to cause any damage and be safe enough to "handle"?

Just saw the more recent posts about the electromagnetic force and gravity and had some additional thoughts. I had heard that the elusive magnetic monopole had been discovered- was this the particle that was being sought after or was that something else? If it was, that would be amazing.

Also, I see we're using math to try and explain why something happens and coming up short because it shows how something works but does not explain why it works that way. Does a unifying theory like string theory work better in that regard and also explain why something happens and the true nature of it? It had an elegant explanation for why gravity is so much weaker than the other three forces and so much harder to unify with them and has successfully been applied to other areas of physics and find deep connections between different areas. Maybe it also can explain the true nature of mass and energy.

ps Newton would have hated theoretical physics and cosmology lol

Stellarator · 11 Jan 2019 02:45

Watsisname wrote:
Source of the post Rather, they work the way they do

I was joking of course, but wouldn't it be unscientific to say something "just is" without an explanation? I could easily see a 'god of the gaps' argument formulating there.

A-L-E-X wrote:
Source of the post It would be fascinating if an alien species had actually done something with a black hole to make a planetary system habitable. I wonder how close to the black hole a planet could be and still be habitable (in other words how large could the hole appear to be in the sky of a habitable planet)?

Quite close. A few AU even, depending on the mass of the blackhole. The funny thing about blackholes is that they are surprisingly manageable when it comes to stellar engineering. It is in fact a blackhole's accretion disk that really sucks matter in, by virtue of accelerated gravitational processes. The blackhole on it's own acts rather like a super-massive star, and thus can be orbited to some degree.

A-L-E-X wrote:
Source of the post Question about micro black holes if we ever discover them, how much of a risk would they pose? Since they also have event horizons could one cause damage if it was close enough? How small would a black hole have to be not to cause any damage and be safe enough to "handle"?

There is no 'safe' size for blackholes. Even microscopic ones are dangerous under any circumstances, though they are incredibly short-lived. Kugelblitz blackholes, ones we have discussed earlier, would be the most stable and viably useful blackhole we could use for technological purposes. These are usually very small. But bear in mind that any blackhole can be contained if suitably positioned. Even stellar mass blackholes could have Dyson swarms built around them if they were only fed a trickle of matter at a time. It would be an interesting physics project, to be sure.

A-L-E-X · 11 Jan 2019 02:51

The thing I would be worried about though is any harmful radiation that didn't fall into the black hole might skirt the edges of the event horizon and be propelled outward in "jets"- that could cause problems for habitability. This might be a problem specific to a charged or spinning (or both) black hole?

Maybe an atmosphere could evolve to protect against that?

Ha I like the idea of making lab black holes! Remember when the LHC scare about creating micro black holes was happening? I hope one day we can create them in a lab, I would say hopefully we should be able to do that in our lifetimes! Hopefully?

Stellarator · 11 Jan 2019 02:54

A-L-E-X wrote:
Source of the post This might be a problem specific to a charged or spinning (or both) black hole?

Yes, only blackholes that are being 'fed' a lot of matter at once produce jets, like the one at the center of our galaxy (Sagittarius A*) or distant quasars. What you see as a jet is, in most cases, a hiccup or 'burp' of matter being discharged by the blackhole.

A-L-E-X · 11 Jan 2019 02:55

Good so an isolated stellar black hole shouldn't do that, and even the ones in binary or multiple star systems wouldn't do it unless their partner(s) were too close?

Stellarator · 11 Jan 2019 03:03

Astrophysical jets made by blackholes are often those made by the supermassives, not stellar-mass blackholes. So yes, most binary-type stellar-mass blackholes that do not reside in a young galaxy OR near a galactic core will not produce ejection jets. That being said, there are extreme examples of some smaller 'holes spitting out 'indigestible' bits of stars or whatever they tried to suck in.

A-L-E-X · 11 Jan 2019 03:07

I'm thinking of Cygnus X-1 and its massive companion, that must have been quite a pair when Cygnus X-1 was still a living star lol.

I wonder what will happen with Eta Carinae and R136a1, I've seen some beautiful procedural planets orbiting both. Maybe no chances of life there now but maybe after they explode there might be if the habitable black hole system idea is correct?

11 Jan 2019 17:20

A-L-E-X wrote:
Source of the post The fascinating thing I found about passing through the singularity

Do you mean passing through the event horizon? The event horizon you can pass, but you cannot pass through the singularity at the center! Not even in theory. The singularity is defined by being a location at which all allowed paths terminate and cannot be extended further. No paths lead away from it, and there is no "through".

A-L-E-X wrote:
Source of the post as you get closer and closer towards the center, the area from which you can see the universe becomes more and more limited until it looks like you are looking through a straw or a pinhole?

Not quite. That is what it would look like if you slowly lowered yourself down toward the event horizon and hovered there. The distortion is caused by the extreme acceleration required in order to hover there. This is also how Space Engine renders your view of black holes.

But if you instead freely fall into the black hole, then the black disk of the hole will always appear to be ahead of you. Even when you are deep inside of the event horizon and just about to hit the singularity, the black disk still takes up less than half of the sky.

Here's a general relativistically accurate render of what it would look like to fall into a non-spinning (Schwarzschild) black hole. The video is slowed down significantly as you get closer, just so that you can see it. (Otherwise the early portion would take ages and the last portion would happen so quickly you wouldn't even see it).

The event horizon is crossed 33 seconds into the video. At this moment the black disk still appears to be below you. The singularity is met in the final frame. It looks less like being enveloped in darkness and seeing the outside universe shrink away, and more like you are approaching and landing onto a black sphere.

A-L-E-X wrote:
Source of the post I wonder how close to the black hole a planet could be and still be habitable (in other words how large could the hole appear to be in the sky of a habitable planet)?

It depends on the mass of the black hole, for determining at what distances the tidal forces would disrupt a planet (just like the Roche limit). The more massive the black hole, the closer this limit is, with an absolute limit of 3 times the event horizon radius (which is as close as any circular orbit can exist around a black hole.)

Here I've calculated the Roche limits for black holes of masses ranging from 10 to 10 million solar masses, assuming both the rigid case and fluid case for the planet, and for a planet with Earth's mass and density. Plotting this distance in terms of event horizon radii and on a log-log scale turns it into a nice straight line:

And below is the apparent angular diameter of the event horizon as seen from the Roche Limit, using the closer rigid case. Note that gravitational lensing will increase the apparent size of the black disk of the hole, but by only a factor of a few.

So whether we are dealing with a stellar mass or supermassive black hole matters a lot. The closest a planet could orbit a 10 solar mass black hole is tens of thousands of horizon radii, from which the event horizon spans less than an arcminute -- requiring a telescope or good pair of binoculars to resolve it in the sky. But for a supermassive black hole, the orbit could be much closer to the horizon, making it a commanding presence in the sky, such as seen in Interstellar. For SgrA* at our galactic center (4 million solar masses), the closest planetary orbits could be about 10 horizon radii away, from which the horizon would span more than 10 degrees on the sky.

A-L-E-X wrote:
Source of the post Question about micro black holes if we ever discover them, how much of a risk would they pose?

I suggest checking out my more detailed post on microscopic black holes here.

The smaller the black hole, the more intense its Hawking radiation, and for microscopic black holes this can be very destructive.

A-L-E-X · 12 Jan 2019 05:11

Watsisname wrote:
A-L-E-X wrote:
Source of the post The fascinating thing I found about passing through the singularity

Do you mean passing through the event horizon? The event horizon you can pass, but you cannot pass through the singularity at the center! Not even in theory. The singularity is defined by being a location at which all allowed paths terminate and cannot be extended further. No paths lead away from it, and there is no "through".

A-L-E-X wrote:
Source of the post as you get closer and closer towards the center, the area from which you can see the universe becomes more and more limited until it looks like you are looking through a straw or a pinhole?

Not quite. That is what it would look like if you slowly lowered yourself down toward the event horizon and hovered there. The distortion is caused by the extreme acceleration required in order to hover there. This is also how Space Engine renders your view of black holes.

But if you instead freely fall into the black hole, then the black disk of the hole will always appear to be ahead of you. Even when you are deep inside of the event horizon and just about to hit the singularity, the black disk still takes up less than half of the sky.

Here's a general relativistically accurate render of what it would look like to fall into a non-spinning (Schwarzschild) black hole. The video is slowed down significantly as you get closer, just so that you can see it. (Otherwise the early portion would take ages and the last portion would happen so quickly you wouldn't even see it).

The event horizon is crossed 33 seconds into the video. At this moment the black disk still appears to be below you. The singularity is met in the final frame. It looks less like being enveloped in darkness and seeing the outside universe shrink away, and more like you are approaching and landing onto a black sphere.

A-L-E-X wrote:
Source of the post I wonder how close to the black hole a planet could be and still be habitable (in other words how large could the hole appear to be in the sky of a habitable planet)?

It depends on the mass of the black hole, for determining at what distances the tidal forces would disrupt a planet (just like the Roche limit). The more massive the black hole, the closer this limit is, with an absolute limit of 3 times the event horizon radius (which is as close as any circular orbit can exist around a black hole.)

Here I've calculated the Roche limits for black holes of masses ranging from 10 to 10 million solar masses, assuming both the rigid case and fluid case for the planet, and for a planet with Earth's mass and density. Plotting this distance in terms of event horizon radii and on a log-log scale turns it into a nice straight line:

And below is the apparent angular diameter of the event horizon as seen from the Roche Limit, using the closer rigid case. Note that gravitational lensing will increase the apparent size of the black disk of the hole, but by only a factor of a few.

So whether we are dealing with a stellar mass or supermassive black hole matters a lot. The closest a planet could orbit a 10 solar mass black hole is tens of thousands of horizon radii, from which the event horizon spans less than an arcminute -- requiring a telescope or good pair of binoculars to resolve it in the sky. But for a supermassive black hole, the orbit could be much closer to the horizon, making it a commanding presence in the sky, such as seen in Interstellar. For SgrA* at our galactic center (4 million solar masses), the closest planetary orbits could be about 10 horizon radii away, from which the horizon would span more than 10 degrees on the sky.

A-L-E-X wrote:
Source of the post Question about micro black holes if we ever discover them, how much of a risk would they pose?

I suggest checking out my more detailed post on microscopic black holes here. The smaller the black hole, the more intense its Hawking radiation, and for microscopic black holes this can be very destructive.

Yes sorry I meant the event horizon, not the singularity! I forgot to go back and edit that sorry! We dont even know if an actual singularity exists or of a future theory of quantum gravity will eliminate it. I just meant that the warping of space-time as you fall through the event horizon should distort our view of the universe and restrict our field of view until it looks like we are peering through a straw. But not when looking towards the singularity- I meant let's say you were looking from inside the event horizon back towards the event horizon while falling towards the singularity (in other words looking backwards relative to your direction of motion), would the universe not appear to get smaller and smaller as you got further and further away from the event horizon if you were looking in that direction as you get closer and closer to the singularity?
Thanks for those graphs, Wat! They really show what I was thinking of in my mind. But why can the distance a planet can be from a supermassive black hole be closer to that black hole's event horizon than a smaller black hole? Is it because a much larger black hole warps space-time in a much more gentle slope compared to a smaller black hole where you see much more extreme changes to the sloping of space-time? Also could a planet near the center of the galaxy, that close to a supermassive black hole actually be habitable? That reminds me of Trantor from the Foundation series haha. I think we've also discovered intermediate mass black holes, at the center of globular clusters, haven't we? So perhaps a planet could be habitable there also, somewhere inside Omega Centauri, or M13 in Hercules or 47 Tucanae? Would those black holes be somewhere between 10^3 and 10^5 solar masses?

I tried to post to the thread you linked but it sent me back here so I will just add it to this one:

Wat that time might be now! How small does a black hole need to be for it to be harmless, so that one could tie a string around it and use it as a yoyo when walking about (sort of kidding about that, but it would be amusing if one could do that!)

I remember when a micro black hole was being floated about as a theory behind the Tunguska event!

midtskogen · 12 Jan 2019 10:54

A-L-E-X wrote:
Source of the post I remember when a micro black hole was being floated about as a theory behind the Tunguska event!

«Was the Tungus Event due to a Black Hole?» Nature, vol. 245, 14. september 1973, pp. 88-89.
Paywalled, but the idea might have been to explain why no meteorites were found. A black hole, however, should have caused a similar event as it exited the earth, but no such thing was observed. Anyway, I don't think the lack of meteorites is a serious problem. If it was a high speed impact (and lower mass than often assumed), the crash in the atmosphere was really, really hard and only the blast wave reaches the ground. The point is, exotic explanations like black holes are not necessary.

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