Gravity Brainbugs: Galactic Alignment

[CaptainChewbacca]

Now, let me preface this by saying I'm generally a pretty smart person. I've got some degrees, and I even teach earth science. But, I guess I'm running into the problem of hearing something repeated often enough that I am starting to worry its true.

I'm in my earth science class today, and the students start talking about 2012 and the galactic alignment. Here's what I told them:

*There's no such thing as a sudden 'galactic alignment' that will shift the earth. There is ALWAYS a straight line between the earth and the galaxy's core. The pull of other parts of the galaxy on earth is a trivial influence compared to the sun's gravitational pull.

*The galactic plane is so thick you can't 'suddenly' pass through it and experience gravitational shear. Passage through would take years and would produce gradual results, not a rapid change.

*There is no hard evidence to indicate the earth's axis has ever suddenly shifted without being struck by an incredibly massive object.

Now, just to soothe my poor and troubled brain, are these three points I made all true?

[Samuel]

*There's no such thing as a sudden 'galactic alignment' that will shift the earth. There is ALWAYS a straight line between the earth and the galaxy's core. The pull of other parts of the galaxy on earth is a trivial influence compared to the sun's gravitational pull.

There is a straight line once a year between the Earth the Sun and the core (that is generally what people mean by alignment).

As for the pull of the rest of the galaxy on Earth... the entire solar system is under the same influence so we don't notice it. I'm not sure how strong actually is.

*The galactic plane is so thick you can't 'suddenly' pass through it and experience gravitational shear. Passage through would take years and would produce gradual results, not a rapid change.

Well, it is about a thousand lightyears thick so yeah.

*There is no hard evidence to indicate the earth's axis has ever suddenly shifted without being struck by an incredibly massive object.

Do we have evidence the Earth's axis has ever been suddenly shifted?

[General Zod]

Funnily enough, people made the same claims about the planetary alignment in 2000 and nothing happened.

[Lord of the Abyss]

*There is no hard evidence to indicate the earth's axis has ever suddenly shifted without being struck by an incredibly massive object.

Do we have evidence the Earth's axis has ever been suddenly shifted?

I'm no expert but not that I know of; Earth's axis is stabilized by the Moon. As I understand it the tidal forces would have to be overcome for the Earth to tilt much, and the galaxy just doesn't exert that level of force.

I recall reading some years ago that Mars, or possibly just its crust HAS had its axis tilt. The old equator can be identified from the ancient elliptical craters from captured debris that fell there when its orbit decayed; they form a ring of sorts where it was. But it has no giant moon to stabilize it; IIRC it was theorized that a large build up of cooled lava in one region may have been enough to cause that slightly more massive region to shift to the equator.

[Surlethe]

More importantly than the Moon, the Earth's axis is stabilized by its angular momentum. For the axis to shift, the Earth's angular momentum would have to change, which would require mass leaving the system. As far as the "galactic alignment", the Milky Way has a mass on the order of 5e11 M_O. It is 50,000 ly = 3e19 AU in radius. Assuming the Sun is about halfway from the galactic center and that most of the mass is concentrated in the core, we have the Sun exerting a force of 3.5e22 N, and the galaxy exerting a whopping 7.9e-5 N. That's right: the force difference between the center of the galaxy at superior conjunction and inferior conjunction is, for all intents and purposes, 0.

Edit - N.B.: order of magnitude calc. When the numbers say the Sun exerts a thousand trillion trillion times more force on the Earth than the galaxy, it doesn't matter if they're off by a factor of ten or a hundred.

[Junghalli]

I'd think a simple application of the inverse square law should be enough to show how ridiculous this "galactic alignment" bullshit is. Just tell them to take the number of AU to the nearest star (I'm too lazy to figure it out but it's in the ballpark of 250,000) and write down two zeros for every decimal place in that number. That's roughly the orders of magnitude that Alpha Centauri's gravity at Earth is weaker than the sun's gravity at Earth (OK, maybe divide it by 2.5 or something to account for Alpha Centauri being a triple star system, the number's still going to be very long). Now ask them how the hell they think the positions of other stars are going to have anything more than a negligeable effect on Earth's orbit or rotation when their gravitational effect on Earth is trillions or quadrillions of times less than that of the sun and the moon. Oh, and if they start bleating about the supermassive black hole in the middle of the galaxy a simple calculation using the inverse square law, the number of AU in a light year, the distance between it and us in light years, and its estimated 4.1 million sun mass should show it too exerts piss-weak force on Earth compared to the sun and moon. Oh yes, and if they start bleating about the combined gravitational force of all the stars in the galactic equator have fun combining all of them into one giant black hole and dropping it X light years from Earth to demonstrate that at respectable galactic range even that kind of mass is going to exert fuck all significant force on Earth compared to the sun. I'm too lazy to do that calculation myself, but while you're at it also introduce them to the fact that the galactic plain is ~1000 light years deep, the galaxy is ~100,000 light years across, the volume of a sphere increased by 8X every time its radius doubles while that of a circle increases by 4X, and there are 15000 stars within 100 light years - i.e. the vast majority of that mass is going to be a long ass way away from us.

The galactic rotation is also slow as hell compared to human time. The sun is something like 15-90 light years "north" of the galactic equator according to Wikipedia, and it's moving at 220 km/s (ref), so even if it was moving in a polar orbit of the central black hole and headed straight for the galaxy's equator it'd take, oh, let's just say 15-90,000 years to get there because I'm feeling really lazy and the math is easier if you just idealize its speed as .001 c (~300 km/s). So even if there was any merit to this stupid-ass theory we wouldn't have to worry about anything for at least 15,000 years.

As I just demonstrated, if you knew anything whatsoever about science you can make this bullshit look ridiculous with zero intellectual effort. These kinds of theories are only not ridiculous to people who know absolutely nothing about the scientific field in question and are impressed by the sciency sounding buzzwords like "galactic alignment".

Actually I've heard theories that the passage of the sun through the denser parts of the spiral arm might be correlated with mass extinctions, so this might actually be one of those especially insidious psuedoscience theories that steal just enough stuff from real science to make it sound like respectable science supports it to somebody with zero knowledge of the subject. Although it may also just be a case of even a broken clock being right twice a day. To quote Hanlon's Razor "never attribute to malice that which can be adequately explained by stupidity", although in cases like this I'm partial to the version that adds "but don't rule out malice".

[Simon_Jester]

Now, let me preface this by saying I'm generally a pretty smart person. I've got some degrees, and I even teach earth science. But, I guess I'm running into the problem of hearing something repeated often enough that I am starting to worry its true.

I'm in my earth science class today, and the students start talking about 2012 and the galactic alignment. Here's what I told them:

*There's no such thing as a sudden 'galactic alignment' that will shift the earth. There is ALWAYS a straight line between the earth and the galaxy's core. The pull of other parts of the galaxy on earth is a trivial influence compared to the sun's gravitational pull.

*The galactic plane is so thick you can't 'suddenly' pass through it and experience gravitational shear. Passage through would take years and would produce gradual results, not a rapid change.

*There is no hard evidence to indicate the earth's axis has ever suddenly shifted without being struck by an incredibly massive object.

Now, just to soothe my poor and troubled brain, are these three points I made all true?

1) is true, because you can always draw a line between the Earth and the galactic core, or any other point in the universe; that's simple geometry. And because the inverse square law works over interstellar distances.

2) is true simply because of the distance scales involved; the galaxy is many light years thick, so any significant geometric parameter it has will also be blurred out over light years, and the Earth does not move through the galaxy at high-relativistic speeds.

3) is true, and it is extremely easy to demonstrate that by all sanity it MUST be true.

[Surlethe]

I recall reading some years ago that Mars, or possibly just its crust HAS had its axis tilt. The old equator can be identified from the ancient elliptical craters from captured debris that fell there when its orbit decayed; they form a ring of sorts where it was. But it has no giant moon to stabilize it; IIRC it was theorized that a large build up of cooled lava in one region may have been enough to cause that slightly more massive region to shift to the equator.

Can you find this for me? Off the top of my head, that makes no sense - total angular momentum is in the direction of the axis of rotation, and it is conserved, so the axis will not shift without mass entering or exiting the system. If mass shifts around inside the system, it can change the rate of rotation but not the axis.

[CaptainChewbacca]

Mars doesn't shift relative to its' axis, rather Mars' axis tumbles in its angle relative to its orientation towards the sun. The north pole on mars is always the north pole.

[Kuroneko]

The real issue isn't even inverse-squared, but inverse-cubed. As Samuel implied, a constant gravitational force induces zero stress on the accelerating body, and is in fact locally completely undetectable. The tidal forces, on the other hand, are ∝1/r³. More specifically, the Sun's orbit has a period of about P = 7.5E15s, so by Kepler's laws, μ/a³ = (2π/P)² = 7.0E-31 1/s². Even at the scale of the solar system (much less Earth's diameter), that's nothing: 2(7.0E-31/s²)(1AU) = 2.0E-19 m/s².
Edit: fix silly unit mistake and factor of two.

Off the top of my head, that makes no sense - total angular momentum is in the direction of the axis of rotation, and it is conserved, so the axis will not shift without mass entering or exiting the system. If mass shifts around inside the system, it can change the rate of rotation but not the axis.

Only the total angular momentum is conserved; the planet could exchange orbital angular momentum for spin angular momentum or vice versa. Whether this makes sense in this case is not clear to me, however.

[Surlethe]

Off the top of my head, that makes no sense - total angular momentum is in the direction of the axis of rotation, and it is conserved, so the axis will not shift without mass entering or exiting the system. If mass shifts around inside the system, it can change the rate of rotation but not the axis.

Only the total angular momentum is conserved; the planet could exchange orbital angular momentum for spin angular momentum or vice versa. Whether this makes sense in this case is not clear to me, however.

The only mechanism I know of for exchanging orbital angular momentum for spin angular momentum without mass leaving the body is tidal interaction; are there others? Also, the nebular hypothesis describing the formation of the solar system implies that orbital and spin angular momentum will both have the same direction (right?), so without collisions happening first, exchanging one for the other will not alter the spin axis.

[TheLostVikings]

Actually I've heard theories that the passage of the sun through the denser parts of the spiral arm might be correlated with mass extinctions, so this might actually be one of those especially insidious psuedoscience theories that steal just enough stuff from real science to make it sound like respectable science supports it to somebody with zero knowledge of the subject. Although it may also just be a case of even a broken clock being right twice a day. To quote Hanlon's Razor "never attribute to malice that which can be adequately explained by stupidity", although in cases like this I'm partial to the version that adds "but don't rule out malice".

In a similar vein if you apply Occam's razor to the "earths axis will reverse" BS I think it's most likely that they heard about how the earths magnetic poles switch places every "few" hundred thousands years or so. And when they think about that they actually imagine it as if the geographic poles somehow switch places, i.e. the earth literally flipping upside down.

Sure, it makes absolutely no sense whatsoever if you stop to think about it, but then again, most of these people doesn't seem to think all that much to begin with.

[Simon_Jester]

They don't have the right context to think IN. When you think in terms of angular momentum it's obvious that you can't just take a giant spinning ball and flip it upside down by tapping it gently, but what if you've never heard of angular momentum? Or have only heard of it as a quantity it's desirable to calculate on exams without knowing in your bones what it really means?

If you don't have the right starting point to work from, you may have no freaking clue what is or is not possible, and when you try to figure it out you're at a high risk of hitting a bullshit conclusion. Most scientific facts, when described in words, do not have an obviously higher a priori probability than corresponding absurd bullshit. It's only when you analyze the science in terms of a framework of known math and physics that you can tell the truth from the lies... and the framework takes years to learn even when you have the aptitude for it. People without the aptitude are screwed, even if they have largely functional brains in other areas, unless they happen to stumble onto a good teacher who actually cares about clearing the bullshit from their head.

[Steel]

Earth's angular momentum would have to change, which would require mass leaving the system.

Not so!

Angular momentum goes as r x mv, so infinitesimally small mass can carry away arbitrarily high angular momentum. This mechanism is important in astrophysics with accretion discs, and is the reason why they (eg the rings of Saturn) are so perfectly circular (and thin). The disc seeks to minimise angular momentum subject to fixed total energy. Its because the disc transports angular momentum outwards while retaining its energy, which results in circular trajectories.

Hmmm... might have fucked up the reasoning on that, don't have my notes from the course at the moment, anyway when I did the analytics the fact is that you can shed arbitrary AM without dropping mass.

Its also important in magnetohydrodynamics for stars, as their solar wind can carry away arbitrary angular momentum via its interaction with the magnetic field of the spinning star, giving a massive braking force slowing the rotation. Although I didn't take that course as its notoriously tricky and even with the best models on the fastest computers your solutions evolve in just-a-bit-slower-than real time meaning you can predict the future just after it happens

[Junghalli]

In a similar vein if you apply Occam's razor to the "earths axis will reverse" BS I think it's most likely that they heard about how the earths magnetic poles switch places every "few" hundred thousands years or so. And when they think about that they actually imagine it as if the geographic poles somehow switch places, i.e. the earth literally flipping upside down.

That sounds very plausible.

They don't have the right context to think IN. When you think in terms of angular momentum it's obvious that you can't just take a giant spinning ball and flip it upside down by tapping it gently, but what if you've never heard of angular momentum? Or have only heard of it as a quantity it's desirable to calculate on exams without knowing in your bones what it really means?

Exactly. That's much of the reason obvious bullshit theories like this thrive. They're laughable to anyone who knows anything about the subject, but they sound plausible enough to somebody who knows absolutely nothing about it, and scientific illiterates are a huge percentage of the population (I suspect the majority of it by a hefty margin).

It doesn't help that some fundamental concepts like the inverse square law (application of which is one of the quickest ways to show this particular theory to be absurd) are somewhat counterintuitive. It's natural to assume that if you get twice as far away from something the force its gravity exerts on you drops off by 1/2, not 3/4.

[Surlethe]

Earth's angular momentum would have to change, which would require mass leaving the system.

Not so!

Angular momentum goes as r x mv, so infinitesimally small mass can carry away arbitrarily high angular momentum. This mechanism is important in astrophysics with accretion discs, and is the reason why they (eg the rings of Saturn) are so perfectly circular (and thin). The disc seeks to minimise angular momentum subject to fixed total energy. Its because the disc transports angular momentum outwards while retaining its energy, which results in circular trajectories.

Hmmm... might have fucked up the reasoning on that, don't have my notes from the course at the moment, anyway when I did the analytics the fact is that you can shed arbitrary AM without dropping mass.

That makes no sense. An infinitesimally small mass would have to have an arbitrarily high velocity in order for the cross product to be nonzero. Sure, accretion disks will minimize angular momentum subject to the energy constraint, but when my astrophysics prof mentioned it he gave the impression that a significant amount of mass is carried away in order for that to happen.

[PeZook]

The sad thing about 2012 doomsday prophecies is that it doesn't take a genius to figure out what is actually going to happen according to the Mayan calendar: it's a simple mathematical change, like from 1999 to 2000. It's not exactly rocket science.

Then again, maybe that's why people invent all those pseudoscientific theories to justify their insane doomsday beliefs, because the matter is so mundane if you really look into it.

[Vendetta]

it's a simple mathematical change, like from 1999 to 2000. It's not exactly rocket science.

Except in a bizarre combination of base 20 and base 18.

People get confused over the Mayan creation myth, which said that the previous world ended on 13.19.19.17.19 and the current one began. None of the myths mention this happening again.

[PeZook]

Except in a bizarre combination of base 20 and base 18.

People get confused over the Mayan creation myth, which said that the previous world ended on 13.19.19.17.19 and the current one began. None of the myths mention this happening again.

It's no more bizarre than our combination of base 30, 31, 7 and 12. In fact, it's way more elegant than that

It was probably done like that to make a year correspond to an astronomical year, since a pure 20 base system would have one tun (year equivalent) at 400 days. Either way, it's just time-keeping maths.

[PeZook]

Ghetto edit: By "more elegant" I meant "easier to understand by the mathematically challenged". The Gregorian calendar is self-correcting, so it's probably a superior solution than the Mayan one.

[Simon_Jester]

It doesn't help that some fundamental concepts like the inverse square law (application of which is one of the quickest ways to show this particular theory to be absurd) are somewhat counterintuitive. It's natural to assume that if you get twice as far away from something the force its gravity exerts on you drops off by 1/2, not 3/4.

I can't remember a time when I thought the inverse square law was counterintuitive, but I learned it before I was old enough to develop common sense*, so I may not count.

*Common sense: n. the set of prejudices you accumulate by your late teens.

Except in a bizarre combination of base 20 and base 18.

People get confused over the Mayan creation myth, which said that the previous world ended on 13.19.19.17.19 and the current one began. None of the myths mention this happening again.

As PeZook says, there's a good reason to use the base 20/base 18 combination, because the Earth orbits the Sun in ~360 times its own rotation period. Using base 20/base 20 gives you a 400-day calendar year, which doesn't even come close to matching the real year; using base 18 gives you something that diverges from reality by only about 1.5% a year.

Now, you COULD use a base 19 calendar and be fairly consistent, because 19^2 = 361... but 19 is an inconvenient number for many purposes, because it's a prime and doesn't submit gracefully to division in base systems humans would naturally use.

[Junghalli]

The sad thing about 2012 doomsday prophecies is that it doesn't take a genius to figure out what is actually going to happen according to the Mayan calendar: it's a simple mathematical change, like from 1999 to 2000. It's not exactly rocket science.

So apparently the theory's believers seem to be as ignorant of the relevant history as they are of the relevant science. That's not a surprise. I imagine the details of Mayan time-keeping are if anything far less well known than the details of astronomy.

I can't remember a time when I thought the inverse square law was counterintuitive, but I learned it before I was old enough to develop common sense*, so I may not count.

Maybe it's not counterintuitive in the same way Newtonian mechanics is ("what do you mean it takes just as much energy to slow down"), but it's not something that springs to mind intuitively. I suspect if you administered this test to scientific illiterates:

There are two atomic bombs in your vicinity. One of them (bomb A) is 2 km from your position. The other one (bomb B) is twice as far away and three times as powerful, which should I detonate to give you the best chance of survival?

That you'd get a lot of people going with bomb A.

Handing that question out to, say, a high school class might actually be an interesting experiment.

[Scottish Ninja]

I just posed that question to my mom and she says "Bomb A".

[Lusankya]

Maybe it's not counterintuitive in the same way Newtonian mechanics is ("what do you mean it takes just as much energy to slow down"), but it's not something that springs to mind intuitively. I suspect if you administered this test to scientific illiterates:

There are two atomic bombs in your vicinity. One of them (bomb A) is 2 km from your position. The other one (bomb B) is twice as far away and three times as powerful, which should I detonate to give you the best chance of survival?

That you'd get a lot of people going with bomb A.

Handing that question out to, say, a high school class might actually be an interesting experiment.

I would reword that question, to "which one should I detonate to give you the worst chance of survival" . Otherwise you might have people accidentally reading 'detonate' as 'defuse', since most people don't make an intuitive link between 'detonating nuclear bombs' and 'survival'.

Incidentally, the correct answer to the question as written is 'neither'.

[Samuel]

Incidentally, the correct answer to the question as written is 'neither'.

What if it is just a tactical nuke? Or there are hills screening you? You can survive that close then.