The dumb questions thread

[Wiploc]

No, because every point was that central point.

For analogy, ask yourself which cell in your body was the original fertilized egg cell. The answer is that every one of them was. Each cell grew from that original cell, just as each place in the universe was the original place.

It's hard to imagine space growing so that multiple places come from one, but that is the answer you are always going to get.

 

[horseyhorsey]

Ok, but if the universe is continuing to expand, hypothetically, could a line not be drawn following the reverse trajectory of every object, which would converge at some 'point'? If there is no central point, but objects in the universe continue to spread apart, in what sense are they spreading? Could two rocks floating in parallel be said to be moving apart? A single point of origin seems necessary in order for every object to be moving away from every other object.

 

[beero1000]

Ok, but if the universe is continuing to expand, hypothetically, could a line not be drawn following the reverse trajectory of every object, which would converge at some 'point'? If there is no central point, but objects in the universe continue to spread apart, in what sense are they spreading? Could two rocks floating in parallel be said to be moving apart? A single point seems necessary in order for every object to be moving away from every other object.

Blow up a balloon. Points on the balloon spread apart, but no point on the balloon is the 'origin' point.

 

[horseyhorsey]

Ok, but if the universe is continuing to expand, hypothetically, could a line not be drawn following the reverse trajectory of every object, which would converge at some 'point'? If there is no central point, but objects in the universe continue to spread apart, in what sense are they spreading? Could two rocks floating in parallel be said to be moving apart? A single point seems necessary in order for every object to be moving away from every other object.

Blow up a balloon. Points on the balloon spread apart, but no point on the balloon is the 'origin' point.

That's a good analogy, and it bends my mind. But what about this one: We're looking at a night sky. It's cloudy so there are no visible stars, only blackness, and no spatial points of reference (assume we lack peripheral vision). A firework is launched. When it explodes we see the flash and particles fly in every direction. While it's true to say that at the exact moment of the explosion there was nothing to give the origin of the explosion any positional reference, it seems we *can* subsequently trace (with a slow capture camera) a real spatial origin, using the line convergence trick I mentioned earlier. Indeed, if we do, we'll find some interesting gases kicking about at that point which could tell us more about the explosion itself.

I can feel where I'm going wrong with this, but please indulge me.

 

[beero1000]

Blow up a balloon. Points on the balloon spread apart, but no point on the balloon is the 'origin' point.

That's a good analogy, and it bends my mind. But what about this one: We're looking at a night sky. It's cloudy so there are no visible stars, only blackness, and no spatial points of reference (assume we lack peripheral vision). A firework is launched. When it explodes we see the flash and particles fly in every direction. While it's true to say that at the exact moment of the explosion there was nothing to give the origin of the explosion any positional reference, it seems we *can* subsequently trace (with a slow capture camera) a real spatial origin, using the line convergence trick I mentioned earlier. Indeed, if we do, we'll find some interesting gases kicking about at that point which could tell us more about the explosion itself.

I can feel where I'm going wrong with this, but please indulge me.

You're implicitly embedding the 2-dimensional balloon/firework surface into a higher 3-dimensional space. That is one place where the balloon/firework analogy breaks down. It's more accurate to imagine the perspective of an ant on the surface of the balloon. To the ant, there is no third dimension, no external observer to the balloon, no extra space to expand into. The surface of the balloon is all there is. All of the points of the balloon are getting farther away from all of the other points, and there is no point that is the center of that expansion.

Now, we live in 3 dimensions of space, so in order to find a center for the expansion that we see, there would need to be a 4th dimension of space (not time). As far as I'm aware, there is no evidence that a 4th space dimension exists. There are hypotheses that postulate more spatial dimensions, and they might predict a center of expansion, but that's currently just conjecture.

 

[Kharakov]

new 1) Wouldn't a black hole singularity be a torus (mmmmhhh, forbidden donut) instead of a dimensionless point because of angular momentum conservation?

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I would think that it wouldn't be flat, like a Kerr metric singularity because there would be a slight bit of angular momentum around the ring (over the donut through the donuthole).

Not according to what I've read. It's counterintuitive, but there's no rule against a point particle having angular momentum. Electrons manage it, as far as we can tell.

There are solutions for the rotating black hole having a flat ring singularity. I was extending the flat ring singularity (Kerr metric) to a torus, with rotation through the donut hole as well.

Dumb question inspired by your answer: What if electrons are rotating around all of space which is why they appear as a point particle from within space?

old 2) I still want to know where the radiation/particle pressure from a galaxy matches that of intergalactic space (post #133 above).

And now that we've established intuition is unreliable ...

...can't do the calculation; but intuitively, since radiation falls off according to an inverse square law, the high-order term in the radiation pressure from the rest of the universe will be whatever comes from the nearest big source. So if you head out of the Milky Way toward Andromeda, you can expect to reach the point where half your light comes from the Milky Way and half from everything else just a little before the point where Andromeda is just as bright as the Milky Way, i.e. when you're almost half way there. Of course in other directions it will be a lot further since there's no nearby Andromeda in most directions.

Isn't the sheer number of sources from all directions going to make it a bit closer to a galaxy (like a sphere around the galaxy, although maybe a dent in the sphere closer to other galaxies, such as your Andromeda/ MW example), despite the inverse square law?

Or is there too much space?

 

[horseyhorsey]

Thanks for the answers, fellas. I think I understand it, even if I cannot visualise it. I guess that's the point, we don't visualise in enough dimensions.

 

[Kharakov]

Thanks for the answers, fellas. I think I understand it, even if I cannot visualise it. I guess that's the point, we don't visualise in enough dimensions.

Words help, sometime concepts snap into place. Lol... the wind chime just made a melodic pattern. How unusual.

 

[Wiploc]

Ok, but if the universe is continuing to expand, hypothetically, could a line not be drawn following the reverse trajectory of every object, which would converge at some 'point'? If there is no central point, but objects in the universe continue to spread apart, in what sense are they spreading? Could two rocks floating in parallel be said to be moving apart? A single point of origin seems necessary in order for every object to be moving away from every other object.

Imagine a grid of galaxies, as if one were at each intersection of lines on a sheet of graph paper. And let's extend that sheet of graph paper to infinity. No edges.

Now we circle one galaxy arbitrarily, and say, "We are here. This is us." And we mark imaginary axes, horizontal and vertical lines, thru our chosen home. And label ourselves 0,0 (zero up, zero right).

Then we, in that galaxy, get our telescopes and prisms, and note that galaxy 1,0 is moving away from us at speed 1. Galaxy 2,0 is moving away at speed 2. Galaxy 188,0 is 188 away from us, and is moving away from us at speed 188. In fact every galaxy (for the purpose of this illustration, we're ignoring the local cluster) is moving away at a speed proportional to its distance.

It's as if they know where we are, and they don't like us.

So we certainly look like the center of the universe. The problem is that everybody else does too. We think galaxy 2,0 is moving up at speed 2, but, to them, we look like we are moving down at speed 2.

So how do galaxy 3,0 and 0,4 look to each other? We can find their speed relative to each other by using the Pythagorean theorem. (I put them on a grid for a reason, you know.) Galaxy 3,0 sees galaxy 0,4 as moving down and right at speed 5. Galaxy 0,4 sees itself as stopped, and sees galaxy 3,0 as moving up and to the left at a speed of 5.

Every galaxy, then looks like the center of the universe. Each galaxy sees the other galaxies as departing at a speed proportional to their distance.

In other words, there's nothing special about our location, or any other location.

So there came to be a term for this symmetry, this assumption that things would look the same from wherever you were. (Perhaps someone will tell us the term?) It became so ingrained an habitual that, for awhile, we assumed it applied to time as well as space. Hence, the steady state theory.

A firework is launched. When it explodes we see the flash and particles fly in every direction.

That firework expands in existing space. But, with the universe, what expands is space itself.

Imagine that you are holding the largest imaginable tape measure. Luke Skywalker (galaxy far far away) is holding the end for you, and you are holding the roll. You note that the tape is not running out; that is, he is not, in the usual sense, getting farther away. But you also note that the marks on the tape are getting closer together. The numbers are getting bigger. When you first looked at the tape, it said Luke was far far away. But now, now it says he is far far away plus 42.

But the tape hasn't pulled out. It is space itself that has changed. Is this a clue to the meaning of life, the universe and everything?

Not much of one, but it is one layman's attempt to illustrate the difference in a traditional (firework-like) explosion within space, and the expansion of space itself.

 

[horseyhorsey]

Great response, Wiploc. Bringing Star Wars into your analogy shows you appreciate the level of intellectual maturity you're dealing with.

Okay, so the universe is expanding, by which we mean the space itself within which objects exist is expanding. The result is that it appears from any position as though other objects are moving away, no matter from where you observe, and at a constant rate. So, are we saying that the appearance of the movements of two galaxies away from one another is illusory, or just that it is meaningless to talk in terms of their spatial position relative to one another...or am I getting lost again?

Let's take all the objects out of space. Is it still expanding?

 

[Wiploc]

Great response, Wiploc. Bringing Star Wars into your analogy shows you appreciate the level of intellectual maturity you're dealing with.

Okay, so the universe is expanding, by which we mean the space itself within which objects exist is expanding. The result is that it appears from any position as though other objects are moving away, no matter from where you observe, and at a constant rate.

At a rate proportional to their distance. If X is five times as far away as Y, that is because X is moving five times as fast as Y.

In The Fifth Element, Lawrence Krauss said that something is speeding things up. X is going away faster than it used to be. The cosmological constant isn't constant after all.

Hence all this talk about dark energy. Who has his foot on the accelerator?

So, are we saying that the appearance of the movements of two galaxies away from one another is illusory, or just that it is meaningless to talk in terms of their spatial position relative to one another...or am I getting lost again?

You're lost again. Join the club.

Let's take all the objects out of space. Is it still expanding?

This I believe I can address even though I am a layman: If you took the objects out of space, there would be no space.

Let's do a thought experiment: Picture a void (no time, no space, no energy, no pop bottle rockets, nothing). Now stick a particle into that nothingness. Suddenly you have a location. And, from the moment that thing arrives, stuff starts shooting out of it, gravitons or photons or whatever. And this ejecta moves at the speed of light. And every place the stuff reaches becomes place, it becomes space. So, a year after the particle popped into existence, space would be two light-years across. Because the gravitons bring spaceness with them. They cause space.

Matter and space are not two separate things. Matter isn't just something that distorts space, it is the distortion of space. Tie a knot in space: poof, you've got an electron. Take away all the knots: you don't even have space.

Note: I've given you more information than I have. The fact that I cope with incomprehensibilities better than some ignorant people doesn't mean I'm not one of the ignorant people. I am musing outside my area of competence.

 

[Bomb#20]

Not according to what I've read. It's counterintuitive, but there's no rule against a point particle having angular momentum. Electrons manage it, as far as we can tell.

There are solutions for the rotating black hole having a flat ring singularity. I was extending the flat ring singularity (Kerr metric) to a torus, with rotation through the donut hole as well.

Okay, then you know more than I do, so I'll bow out.

Dumb question inspired by your answer: What if electrons are rotating around all of space which is why they appear as a point particle from within space?

Hmm. Offhand, that sounds like a change of coordinates without observable consequences, and the new coordinate system will be hairier to use.

And now that we've established intuition is unreliable ...

...can't do the calculation; but intuitively, since radiation falls off according to an inverse square law, the high-order term in the radiation pressure from the rest of the universe will be whatever comes from the nearest big source. ...

Isn't the sheer number of sources from all directions going to make it a bit closer to a galaxy (like a sphere around the galaxy, although maybe a dent in the sphere closer to other galaxies, such as your Andromeda/ MW example), despite the inverse square law?

Or is there too much space?

Hey, like I said, I'm just going on intuition here, so what do I know? But imagine the Milky Way and its satellite galaxies all wink out of existence, except for you and your space suit. Look around. You're going to see essentially nothing at all, except for Andromeda. Is the too-diffuse-to-see-anything black background actually collectively brighter than the one object you're detecting photons from? This seems doubtful. (Also, theoretically, I'd think galaxies would have to be uniformly distributed in order for light from the distant ones to outweigh the close ones, instead of galaxies being grouped into a hierarchy of clusters.)

But if you're up for doing the calculation, here's the data you need. Column "m" is the one you want. Raise 2.512 to the power -m for each galaxy, skipping the satellites of the Milky Way. Add up the numbers for Andromeda and its satellites. Multiply by 4 to allow for going half way to Andromeda. Then add up the numbers for everything else, and see if it looks like the infinite series will converge to something comparable to the Andromeda sum.

 

[Kharakov]

There are solutions for the rotating black hole having a flat ring singularity. I was extending the flat ring singularity (Kerr metric) to a torus, with rotation through the donut hole as well.

Okay, then you know more than I do, so I'll bow out.

Not likely- you're the mathematician. I just thought that a very thin torus seemed more likely, with the poloidal angular momentum maintaining a minimum diameter for the tube circle of the torus.

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And now that we've established intuition is unreliable ...

...can't do the calculation; but intuitively, since radiation falls off according to an inverse square law, the high-order term in the radiation pressure from the rest of the universe will be whatever comes from the nearest big source. ...

Isn't the sheer number of sources from all directions going to make it a bit closer to a galaxy (like a sphere around the galaxy, although maybe a dent in the sphere closer to other galaxies, such as your Andromeda/ MW example), despite the inverse square law?

Or is there too much space?

Hey, like I said, I'm just going on intuition here, so what do I know? But imagine the Milky Way and its satellite galaxies all wink out of existence, except for you and your space suit. Look around. You're going to see essentially nothing at all, except for Andromeda. Is the too-diffuse-to-see-anything black background actually collectively brighter than the one object you're detecting photons from? This seems doubtful. (Also, theoretically, I'd think galaxies would have to be uniformly distributed in order for light from the distant ones to outweigh the close ones, instead of galaxies being grouped into a hierarchy of clusters.)

I was hoping for something along the lines of: the Andromeda galaxy occupied 20% of the background, the other 80% would have more photons coming from it because of things like DEBRA ( Diffuse_extragalactic_background_radiation), which includes WHIM ( warm hot intergalactic medium), etc.

I forgot to mention something very important when I brought up this old question...

I originally wanted to know where the radiation pressure from the galaxy was approximately equal to or less than the gravitational influence of the galaxy + other radiation pressure from all directions. The gravitation probably will change the answer quite a bit.

But if you're up for doing the calculation, here's the data you need. Column "m" is the one you want. Raise 2.512 to the power -m for each galaxy, skipping the satellites of the Milky Way. Add up the numbers for Andromeda and its satellites. Multiply by 4 to allow for going half way to Andromeda. Then add up the numbers for everything else, and see if it looks like the infinite series will converge to something comparable to the Andromeda sum.

Yeah, I think it will be a bit more complicated- I think the apparent magnitudes are calculated from a standard distance, rather than the actual distance. Not too mention, it doesn't include the magnitude of the WHIM/ DEBRA, so that's a problem as well, since those 2 come from many directions (not just active galactic sources).

This question might require a bit of digging, and asking some astronomers.

 

[skepticalbip]

.................
This question might require a bit of digging, and asking some astronomers.

Your question is one that I have never heard astronomers consider, not meaning that they haven't just not one I have heard. The closest similar interest I've heard is in describing the galactic bow shock but that is more the galactic magnetic field interacting with the intergalactic medium than radiation pressure.

You may also consider that the radiation pressure from distant sources and the general background would essentially cancel itself since it would be coming equally from all directions. A particle being "pushed" equally from all directions would experience no net "push". So it seems that only the radiation pressure from local galaxies would be all that would be of concern.

 

[Bomb#20]

Okay, then you know more than I do, so I'll bow out.

Not likely- you're the mathematician. I just thought that a very thin torus seemed more likely, with the poloidal angular momentum maintaining a minimum diameter for the tube circle of the torus.

"Poloidal angular momentum" -- that's a new one for me. Seems to be a concept from tokamak engineering. Well, here's my intuition again, worth no more than 2 cents. I don't think poloidal angular momentum is a conserved quantity. And I don't see how it could persist through the process of a star collapsing into a black hole, since it involves stuff going up the outside of a torus, heading inward toward the geometric center, going down the inside of the donut hole, and then back outward along the bottom of the torus to complete the circuit. It's that last step that's throwing me -- stuff doesn't go outward once it's inside the event horizon of a black hole.

According to the book I read on black holes (thirty-odd years ago!), a black hole is completely characterized by only three properties: mass, charge and standard angular momentum. This would seem to imply that whatever poloidal angular momentum the star starts with goes to zero during the collapse or else is blown out into space with the supernova ejecta.

I was hoping for something along the lines of: the Andromeda galaxy occupied 20% of the background, the other 80% would have more photons coming from it because of things like DEBRA ( Diffuse_extragalactic_background_radiation), which includes WHIM ( warm hot intergalactic medium), etc.

Oh, I see. I have no idea how energetic that radiation is, or, for that matter, how radiation pressure scales with wavelength.

I forgot to mention something very important when I brought up this old question...

I originally wanted to know where the radiation pressure from the galaxy was approximately equal to or less than the gravitational influence of the galaxy + other radiation pressure from all directions. The gravitation probably will change the answer quite a bit.

That, I might be able to help with. There is no answer, because it's an apples/oranges comparison. The radiation pressure from a source depends on the surface area and the reflectivity and transmittance of the illuminated object; the gravitational influence depends only on its mass. So at the same distance, the radiation pressure on a meteoroid from a galaxy can be less than its gravitational influence while the radiation pressure on an equally massive solar sail will be greater.

Yeah, I think it will be a bit more complicated- I think the apparent magnitudes are calculated from a standard distance, rather than the actual distance.

The apparent magnitudes are (more or less) measured, not calculated. It's the absolute magnitudes that rely on a standard distance. But yes, the summation I described will be thrown off by inaccuracy in distance measurements; and there's a lot of uncertainty even with nearby galaxies.

 

[Kharakov]

.................
This question might require a bit of digging, and asking some astronomers.

Your question is one that I have never heard astronomers consider, not meaning that they haven't just not one I have heard. The closest similar interest I've heard is in describing the galactic bow shock but that is more the galactic magnetic field interacting with the intergalactic medium than radiation pressure.

You may also consider that the radiation pressure from distant sources and the general background would essentially cancel itself since it would be coming equally from all directions. A particle being "pushed" equally from all directions would experience no net "push". So it seems that only the radiation pressure from local galaxies would be all that would be of concern.

Yeah, both you and Bomb#20 appear spot on in your reasoning- but my question asking skills were/are a bit off:

I had originally forgotten to mention the galaxy's gravitation's effects when I re-brought up this question.

Radiation pressure (a term I am using loosely) falls off faster that gravitation because of interstellar and intergalactic medium (inverse square law for both, accept gravitation is increased by the interstellar/ intergalactic medium and radiation pressure is decreased/diffused).

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In addition, I hadn't yet read (on wikipedia... so take it as you will) that nearly 50% of the stars in our local cluster of galaxies are thought to be extragalactic, or rogue stars that exist outside of galaxies. This would imply 50% of the starlight in the Virgo cluster would be coming from diffuse sources.

The reason I asked this question was to determine if there was some form of non-inertial matter that could ride on radiation pressure to form a sphere around the galaxy where gravitation and radiation pressure ~cancel one another out.

A photon can interact with this exotic matter like a photon does with other matter, but instead of being partially absorbed and re-emitted, it imparts all of its momentum to the matter until another photon interacts with the matter, at which time the original photon goes on its way, and the new photon carries the matter along for a while in whatever direction the new photon is traveling.

If the exotic matter & photon pair runs into ordinary matter, the photon interacts with the ordinary matter, and the exotic matter starts falling back into the gravity well of the galaxy until it is picked up by another photon going in any direction.


This would allow exotic matter shells to form outside of galaxies, which contribute to the gravitational fields of the galaxies, without having an impact upon electromagnetic radiation. In globular galaxies, or galactic hubs, the exotic matter particles would interact with photons going every which way, so wouldn't escape, which would mean there would be more exotic matter in globular galaxies and galactic hubs, while spiral galaxies would have exotic matter in a halosphere around them.

 

[Kharakov]

And I don't see how it could persist through the process of a star collapsing into a black hole, since it involves stuff going up the outside of a torus, heading inward toward the geometric center, going down the inside of the donut hole, and then back outward along the bottom of the torus to complete the circuit. It's that last step that's throwing me -- stuff doesn't go outward once it's inside the event horizon of a black hole.

Funny- I've been reading a bit more about  Kerr metric  ring singularity  black holes, and they do have a toroidal "time travel" area, that is supposed to run into an "inflationary instability" problem (towards the bottom of the last link is a bit on ring singularities).

I was envisioning that the black hole singularity's rotation could not exceed the speed of light, so the increase in rotational velocity as the BH collapsed would set a minimum radius for the singularity (which could shrink due to frame dragging). There also might be some poloidal momentum around the ring (which could collapse to a filament if the rotation was small enough, but even an almost infinitesimal amount of poloidal momentum would be enough to sustain a tiny bit of thickness).

That, I might be able to help with. There is no answer, because it's an apples/oranges comparison. The radiation pressure from a source depends on the surface area and the reflectivity and transmittance of the illuminated object; the gravitational influence depends only on its mass. So at the same distance, the radiation pressure on a meteoroid from a galaxy can be less than its gravitational influence while the radiation pressure on an equally massive solar sail will be greater.

I was using radiation pressure as an average quantity of photons traveling through a space, rather than in the classical sense, which further confused the issue. I fucked up this particular question rather badly, leaving out gravitation, and using radiation pressure in such a loose manner.

The apparent magnitudes are (more or less) measured, not calculated. It's the absolute magnitudes that rely on a standard distance. But yes, the summation I described will be thrown off by inaccuracy in distance measurements; and there's a lot of uncertainty even with nearby galaxies.

I started thinking about this last night: that these questions might be premature for the level of information we can gather from within our solar system, within our galactic halo.

 

[spikepipsqueak]

I am reading a work of fiction in which the science is otherwise accurate.

It was published in 1975, and says that material obtained in the moon landings proved conclusively that the Earth and moon were never part of the same mass.

Until a couple of minutes ago I thought that the theory about the moon splashing out of the molten earth, after a collision with a third party, was a done deal.

Anybody know for sure?

 

[bilby]

I am reading a work of fiction in which the science is otherwise accurate.

It was published in 1975, and says that material obtained in the moon landings proved conclusively that the Earth and moon were never part of the same mass.

Until a couple of minutes ago I thought that the theory about the moon splashing out of the molten earth, after a collision with a third party, was a done deal.

Anybody know for sure?

I don't know for sure. But I think they are both right - for a given value of right.

If I recall correctly, one of the leading theories in the 1960s was that the molten proto-earth 'span off' some material that became the moon. This was shown to be false by isotopic analysis of the Apollo samples - they are a fairly close match for Earth's mantle, but not close enough to fit the 'no-collision' hypothesis that existed prior to Apollo.

This left the question of where the large Moon we see today did come from; and the best fit to the data we have today are collision theories, which involve a Mars sized object colliding with the proto-earth, to produce a Moon with the observed makeup, while still allowing for the observed slightly different makeup of the Earth.

The collision hypothesis was considered highly implausible in the 1960s, and institutional inertia made such ideas unpopular through the 70s despite increasing evidence that this had occurred.

Even now, a non-negligible minority of opinion opposes the collision hypothesis as such a collision is unlikely - planet sized objects don't collide easily; a near-miss leading to one or both being shifted significantly - even to the point of being ejected from the system entirely - is far more likely.

But as all of this is based in half-remembered details from a huge range of sources of variable reliability, I would advise against taking my word for it.

 

[Kharakov]

How much longer until the moon no longer fully eclipses the sun?