Physics In the Playground

[Yora]

what tidal effects? Of course, this depends a lot on the radius of the orbit and the actual mass of the star, I assume.

[factotum]

The biggest problem is likely to be tidal effects as all the mass isn't spread out.

There would be no greater tidal effects at a typical orbital distance than there were before--as said above, a uniform sphere behaves as if all its gravitational attraction acted from a point at the centre, regardless of its size. The reason tidal forces might become significant with a black hole is that it's smaller, so you can get much closer to the thing!

[Exachix]

There would be no greater tidal effects at a typical orbital distance than there were before--as said above, a uniform sphere behaves as if all its gravitational attraction acted from a point at the centre, regardless of its size. The reason tidal forces might become significant with a black hole is that it's smaller, so you can get much closer to the thing!

That's very true. I was trying to work out in my head why accretion disks are formed and what matter would have formed them. Apologies for my misinformation.

[noparlpf]

Question: When a massive star colappses into a black hole, it has the same gravity since it has the same mass. (Actually less, since it loses some mass during its lifetime and supernova stage.)

However, assumed that you stay in a stable orbit around a massive star while it colappses and the star does not lose any mass during the supernova. Would gravity at the given orbit not increase? After all, half of the stars mass has now moved closer to you as it moved into the center.
And as I type it now, it becomes obvious that the other half of the mass is now more distant from the orbit as it move to the center, that should cancel each other out.

Still, I leave this question here, to get a confirmation that I have the facts right now. Or is there still something wrong about it?

Assuming the entire mass of the star remains as a black hole: The center of mass of the star and of the black hole should be at roughly the same location, and the mass would be the same, so planets' orbits wouldn't be affected.

[Yora]

Very often you have binary stars and even a supernova does not do a lot damage to very close nearby stars. After that, the black hole can suck off matter from the companion star.

Which still doesn't answer why the star didn't do that before the supernova?

[Form]

Very often you have binary stars and even a supernova does not do a lot damage to very close nearby stars. After that, the black hole can suck off matter from the companion star.

Which still doesn't answer why the star didn't do that before the supernova?

Sometimes they do. Sometimes binaries are so close to each other they can transfer mass from one to the other.

There's also the evolutionary stage of the stars to consider. The companion of a star that becomes a black hole could enter the red giant stage and increase in size only after formation of the black hole. Then there could be mass transfer only after the black hole has been formed, simply because the other star wasn't large enough before.

[Douglas]

Which still doesn't answer why the star didn't do that before the supernova?

I believe the answer to this one is "actually, it did." The difference is that the matter being sucked off tends to get stuck in a close orbit and slowed down by time dilation instead. With a star instead of a black hole, paths that would have that result around a black hole instead impact the star because the radius of the close orbit is smaller than the star.

The reason accretion disks exist is that black holes are so ridiculously tiny relative to their mass that the matter their gravity sucks in tends to miss the black hole and swing around outside of it. If the entire planet Earth were collapsed into a black hole, it would be less than a single inch across. Even the Sun would be a mere 6 kilometers (3.8 miles) wide. That presents a target for in-falling matter that requires an incredibly precise trajectory to actually hit rather than merely getting close. Even for the matter that does hit the perfect course or gets slowed down by bumping into other matter in the disk, time dilation approaches infinite at the event horizon. I don't think anything ever does actually cross a black hole's event horizon; instead, matter that enters a black hole does it by being nearby when the event horizon expands, which would happen when a whole lot of matter is really close but not quite there.

[hamishspence]

Even for the matter that does hit the perfect course or gets slowed down by bumping into other matter in the disk, time dilation approaches infinite at the event horizon. I don't think anything ever does actually cross a black hole's event horizon; instead, matter that enters a black hole does it by being nearby when the event horizon expands, which would happen when a whole lot of matter is really close but not quite there.

That only applies from the frame of reference of a person looking at at from. A long way away. From the frame of reference of the matter falling in, time passes as normal- and it's the rest of the world that's behaving differently.

[noparlpf]

I believe the answer to this one is "actually, it did." The difference is that the matter being sucked off tends to get stuck in a close orbit and slowed down by time dilation instead. With a star instead of a black hole, paths that would have that result around a black hole instead impact the star because the radius of the close orbit is smaller than the star.

The reason accretion disks exist is that black holes are so ridiculously tiny relative to their mass that the matter their gravity sucks in tends to miss the black hole and swing around outside of it. If the entire planet Earth were collapsed into a black hole, it would be less than a single inch across. Even the Sun would be a mere 6 kilometers (3.8 miles) wide. That presents a target for in-falling matter that requires an incredibly precise trajectory to actually hit rather than merely getting close. Even for the matter that does hit the perfect course or gets slowed down by bumping into other matter in the disk, time dilation approaches infinite at the event horizon. I don't think anything ever does actually cross a black hole's event horizon; instead, matter that enters a black hole does it by being nearby when the event horizon expands, which would happen when a whole lot of matter is really close but not quite there.

These things provide a simple example. On a very small scale. So stuff would eventually swing down into the black hole, but it takes friggin' forever. The coins in this thing hit the center pretty quickly because it's a closed system on a very small scale.

[Yora]

Question remains what would cause a planet in a stable orbit to leave the orbit and move towards the star.

However, there are two things to consider: Massive stars have shorter lifetimes and it gets shorter the greater the mass is. Sometimes only in the tens of millions of years. At that time, there could still be a number of planets in less than stable orbits that will eventually fall into the star or the black hole.
The other is, of course, the supernova event.
No idea if planets can actually survive that without being flung out into space, but if their orbits change, their interaction with each other might then push some of the smaller planets towards the center.

Or maybe as a third option material ejected during the supernova that did not reach a velocity to completely escape gravity and eventually is pulled back in.

[factotum]

Question remains what would cause a planet in a stable orbit to leave the orbit and move towards the star.

During the events that formed the black hole, an awful lot of gas will have been expelled from the star's surface...that gas would cause drag on the planet in its orbit, slowing it down and potentially sending it starward.

[Urpriest]

I don't think anything ever does actually cross a black hole's event horizon; instead, matter that enters a black hole does it by being nearby when the event horizon expands, which would happen when a whole lot of matter is really close but not quite there.

This is a bit inaccurate. While an object that falls into a black hole indeed slows to a standstill from the point of view of an outside observer, from the point of view of the object itself the transition from outside to inside the event horizon is perfectly smooth and feasible, and once inside it takes a finite (set, only dependent on black hole mass) amount of time for the object to reach the singularity.

[Douglas]

This is a bit inaccurate. While an object that falls into a black hole indeed slows to a standstill from the point of view of an outside observer, from the point of view of the object itself the transition from outside to inside the event horizon is perfectly smooth and feasible, and once inside it takes a finite (set, only dependent on black hole mass) amount of time for the object to reach the singularity.

I was talking about the viewpoint of an outside observer. I thought that was obvious enough I didn't need to specify it.

Unless you're claiming something in that frame different from what I said, "inaccurate" is the wrong choice of word.

[Yora]

Is it more or less accurate to treat the event horizon as a uniform object? Does it matter how the mass is distributed inside it in regard to object in orbit?

[Urpriest]

I was talking about the viewpoint of an outside observer. I thought that was obvious enough I didn't need to specify it.

Unless you're claiming something in that frame different from what I said, "inaccurate" is the wrong choice of word.

Looks like I misunderstood your statement then, my apologies.

Is it more or less accurate to treat the event horizon as a uniform object? Does it matter how the mass is distributed inside it in regard to object in orbit?

There's actually a theorem that it specifically doesn't matter how mass is distributed inside the event horizon. Outside the event horizon you only need to know three things about a black hole to know everything about it: mass, charge, and angular momentum.

[Yora]

What about the axis? Since black holes do have acretion disks, they have to spin. And acretion disks are the result of the difference in "effective gravity" above the equator and the poles of a spinning body. So it can't be entirely uniform.

[aldeayeah]

What about the axis? Since black holes do have acretion disks, they have to spin.

That's what the angular momentum's for.

[Urpriest]

That's what the angular momentum's for.

Yup. To put it another way, remember that angular momentum is a vector, not just a number. The vector tells you the axis.

[Dr. Bath]

Anyone know how to calculate miller indices in a cubic lattice when you only know the first incident bragg reflection angle? I feel like there should be something simple but nothing seems to work without knowing at least the wavelength.

Blarg.

[noparlpf]

1. Why do planets and asteroid belts mostly form in a rough plane around their stars? Or do they? That's the impression I've been given.

2. Light is a wave. How does it travel in a vacuum? Is it that wave-particle duality business?

[factotum]

1. Conservation of momentum. The initial cloud of gas and dust that collapsed to form the solar system was spinning slowly, and as it collapsed, it began to spin faster and flatten out into a plane, thus resulting in the arrangement we see today.

2. Not 100% sure about this one, but I'm pretty sure that just because something is a wave doesn't mean it has to be travelling through a medium. Light is an electromagnetic wave, and unlike waves in water, you don't need some luminiferous ether for those to travel in!

[Drolyt]

2. Not 100% sure about this one, but I'm pretty sure that just because something is a wave doesn't mean it has to be travelling through a medium. Light is an electromagnetic wave, and unlike waves in water, you don't need some luminiferous ether for those to travel in!

I think the medium it travels through is the electromagnetic field itself.

[The Extinguisher]

I think the medium it travels through is the electromagnetic field itself.

Rather, it propagates itself. The oscillating magnetic field induces and oscillating electric field, which induces and oscillating magnetic field and so on and so on. It's not using a medium at all.

[noparlpf]

1. Conservation of momentum. The initial cloud of gas and dust that collapsed to form the solar system was spinning slowly, and as it collapsed, it began to spin faster and flatten out into a plane, thus resulting in the arrangement we see today.

Okay, that makes perfect sense.

2. Not 100% sure about this one, but I'm pretty sure that just because something is a wave doesn't mean it has to be travelling through a medium. Light is an electromagnetic wave, and unlike waves in water, you don't need some luminiferous ether for those to travel in!

I think the medium it travels through is the electromagnetic field itself.

Rather, it propagates itself. The oscillating magnetic field induces and oscillating electric field, which induces and oscillating magnetic field and so on and so on. It's not using a medium at all.

Okay, we're back into where Physics lost me. I got a 28 on one of these exams. (It was a B-, though, so at least nobody else in the class got it either.)

[Urpriest]

Okay, we're back into where Physics lost me. I got a 28 on one of these exams. (It was a B-, though, so at least nobody else in the class got it either.)

Here's a (maybe) better way to think about it: all that a wave is, mathematically speaking, is a particular sort of behavior in space and time: something goes up and down as time passes, and the disturbance travels along in some direction in space.

The thing that goes up and down could be something obvious and physical, like the height of water. It could also be any other number that can go up and down, though. Sound waves are waves of pressure, and involve pressure going up and down. You could have a wave of temperature, with temperature going up and down. All that light is is a wave of electric field going up and down (plus magnetic field going up and down in a perpendicular direction, but that detail isn't important for the analogy). It doesn't need a medium to travel through because electric and magnetic fields don't need a medium to travel through.

[noparlpf]

Here's a (maybe) better way to think about it: all that a wave is, mathematically speaking, is a particular sort of behavior in space and time: something goes up and down as time passes, and the disturbance travels along in some direction in space.

The thing that goes up and down could be something obvious and physical, like the height of water. It could also be any other number that can go up and down, though. Sound waves are waves of pressure, and involve pressure going up and down. You could have a wave of temperature, with temperature going up and down. All that light is is a wave of electric field going up and down (plus magnetic field going up and down in a perpendicular direction, but that detail isn't important for the analogy). It doesn't need a medium to travel through because electric and magnetic fields don't need a medium to travel through.

I sort of get it conceptually, but my subconscious is trying to reject it out of hand because of the hellish experience of class averages on exams regularly being under 50 points.

[noparlpf]

Guys what is time?

[Pie Guy]

Guys what is time?

A magazine.
Ask me what life is.

[IthilanorStPete]

A magazine.
Ask me what life is.

A cereal, naturally.

[Yora]

Guys what is time?

Care to elaborate? This obvious is more than a dictionary definition question.