**Ask a monkey a physics question thread**

[MadMojoMonkey]

Sure, but the structural integrity of the building went from 100% to 0% in a very short amount of time. That does not support the idea that there was a gradual weakening, which one would expect under the circumstances. It implies that once the stress reached critical point, that once collapse was inevitable, it was almost instantaneous. We also have the problem of what should be an uncompromised steel structure below the impact level and the range of the fire completely failing to support the collapsing mass, not even to the point of tipping it.

What do you mean by "very short amount of time?"
Note: the collapse of each building took tens of minutes from impact to critical failure.

***
F = ma = dp/dt

dp/dt is change in momentum with respect to time.

When a single floor collapses, it slams down into the floor below. It delivers significant momentum in a short time, causing a spike in the reaction forces which support the floor it slams into. The delivers a wave of stress through the entire structure of the building. The wave moves at the speed of sound through the various materials. If any parts of the structure are pushed beyond their yield stress, either due to prior weakening or simply the magnitude of the stress, then they will permanently deform. This displaces the stress from those supports to other supports, which are now bearing more load. If those supports were previously weakened, by whatever means, then they are notably more likely to yield, themselves. Rinse and repeat.

If that lower floor later falls, then the effect is repeated, but multiplied by the added weight of 2 falling floors worth of debris. This usually leads to complete collapse of the entire building in a process called - I'm not making this up - pancaking. It is extremely likely that the process will destroy the structural core of the building where the floors are collapsing, which drops the whole top of the building, at which point... it's all over.

It is very true that once the collapse is begun, it happens quite rapidly. The collapse of the structure due to overloading travels through the structure at the speed of sound in steel. This will look "instantaneous" on a seconds-based time scale. Slow it down enough, and you'd see a wave of failures pass through the structure, each failure making the next failure more severe.

I find it intruiging that an educated physician can accept such a uniform and rapid collapse of a reinforced steel stucture due to impact and fire at the top of the structure. It just seems so implausible. Especially given we're supposed to believe a passport managed to emerge from the rubble intact.

I mean come on.

[...] removed 'cause not physics-related

Physicist - not physician. Unless you're talking about someone else besides me.
I only speak for myself, not any club or group or otherwise.

It's not like the entire building went up in flames. Much of the structures never burned, just fell.

It would be implausible if there wasn't a fair amount of debris that was mostly unscathed by the fall due to the randomness of it all and some bits would just have "gotten lucky" in how they came down. It's perfectly reasonable that a bit of paper could have been sandwiched in between 2 things which prevented Oxygen from getting to it, and therefore, it couldn't burn.


What's intriguing to me is that I can offer you a step by step reason for why every step of the process is plausible. I can do so in ways that you can personally test to determine whether or not I'm fooling you. Yet, you do not perform the tests, and you say I'm fooling you, and that it is not plausible.

But then I can talk GR where I'm hard pressed to present you any way you could test it yourself, and you accept what I tell you and do not accuse me of fooling you.

*shrug*
People are strange.

[OngBonga]

I think you're letting your emotions guide you, here.

No doubt. Not very scientific of me. Sorry, I forgot it was the physics thread.

[MadMojoMonkey]

No doubt. Not very scientific of me. Sorry, I forgot it was the physics thread.

-.-
Not sure if snarky.

[MadMojoMonkey]

afaik the beams didn't melt, they were just weakened by heat, which happens at well under the melting point.

I doubt that any steel I-beams melted clean through. The question as to whether ANY steel could be melted is what I tried to answer.

I tried to make the point about the reduction in structural integrity due to simply heating the steel. How's this for a practical example:

Consider sword making. The blacksmith heats the steel so that it is glowing a reddish-orange color due solely to its own temperature. This puts the Steel in a temperature range close to 1,500 - 2,000 C (based on the observed color, so wide error bars). When the steel is this temperature, it is weak enough to be shaped by the blacksmith.

I bring this up to help you understand in an intuitive way that the steel doesn't need to be heated to melting in order to have significantly lower structural rigidity.


Another factor is the loading. As one structural element weakens or is deformed, it displaces that stress to nearby structural members. These members which take on the new load are now more likely to fail, since they are now closer to their max loading. This can cause a chain reaction through a rigid structure.

And the molten metal that conspiratards were talking about wasn't steel, but aluminum, which has a lower melting point than steel, i believe.

Disclaimer: I'm not researching conspiracy theories on this and the wackos who latch onto the path of "IDK -> it must be" with no steps in-between.

Confirmed that Aluminum has a lower melting point than Steel. Aluminum cans can be melted in most camp fires, as any beer-swilling camper can affirm. Steel cans don't usually melt in a normal campfire, but you can certainly do it with a little knowledge about air-flow. Something like a Dakota fire hole can be used to allow controlled oxygen feeding to the fuel, and a rudimentary blast furnace can be implemented in this way.

A Dakota fire hole is basically a U-shaped hole in the ground, with fuel in one hole, and nothing in the other. The empty hole provides Oxygen feeding to the base of the fire, keeping all the air flow through the combustion chamber moving upward. This 2nd hole can have a fan or bellows placed over it to make a simple blast furnace, increasing the combustion rate.

[OngBonga]

I just tend default towards a "bullshit" position when it comes to 9/11.

And no, not snarky, I started rambling like it was the random thread!

[MadMojoMonkey]

I just tend default towards a "bullshit" position when it comes to 9/11.

I'm all in favor of skepticism. It's what drives the scientific process.

There are things about 9/11 that don't make sense to me, but the collapse of the WTC towers isn't among them.

This really isn't the place to talk about politics, though. I'm still happy to answer any physics questions you may have... whether or not they are related to conspiracy theories.

[OngBonga]

Professor Brian Cox, demonstrating that he's capable of changing his views based on new information...

For all those who contacted me to suggest the International Space Sation is fake, you are the highest order of shitwits.

It has been brought to my attention that the correct term for people who don't believe in the space station is fucknozzle, not shitwit.

[MadMojoMonkey]

[MadMojoMonkey]

Rain of Balls

A demonstration to show that a series of impulses is quite similar to application of a constant force.
This is broadly demonstrative of air pressure, as well.

A tube filled with bearing balls is fed via a screw into a dropper so that the balls fall at a constant rate. They fall onto a mechanical balance from a height of 1 meter. A laser is reflected off of the balance and calibrated so that the scale in the background represents grams.



That pony-tailed nerd is me.

[MadMojoMonkey]

Often, when my housemates get back from work, they open the front door (inwards), which in turn causes a shift in the air pressure in the house, which travels upstairs and causes my bedroom door to slowly creak open. Is this similar to what's described here?

Yes (assuming your door opens inward). What causes your door to push open is a pressure differential (different in one place than another), which is caused by the collision of air molecules on one side of your door delivering more net force than the the collision of air molecules on the other side of your door.

There is a shock wave (a weak one, but I still think it's the right word) as the pressure increase from the door opening passes through the interior volume of the home. As with all shock waves, there is a difference in pressure and density across the shock wave. So as the wave passes over things (which are thick compared to the width of the shock wave), there is a moment where the surface experiences a different pressure on one side than the other. This results in a net force being applied and F = ma, so where there's a net F, there's a net a.

This shock wave was caused in the first moments of opening the door before the thickness of the door has cleared the door frame. This is the part of the door opening which slightly compresses the air in the path of the door. Once the thickness of the door clears the frame, the increased pressure on the inside of the door has an easy path around to the decreased pressure on the other side of the door.

The result is a pulse which travels in the same manner as a sound wave. I think that it could be perfectly described as a subsonic pulse, or a "sound" wave that is of too low frequency to be heard by human ears.

Experiment by having him open the door more and less quickly than usual. Change how wide the gap is when your door is ajar.

I'm guessing that the effect is most pronounced when he opens the door quickly and when your door is nearly (but not quite) closed.

[OngBonga]

Just noticed you replied to this here!

I'm guessing that the effect is most pronounced when he opens the door quickly and when your door is nearly (but not quite) closed.

Well, first of all Lara comes home, then Alex half an hour later. Lara tends to open the door more slowly, so my door creaks open slower when she arrives. Alex burts in very quickly, and thus my door opens faster. I normally have my door slightly ajar.

I thought it was essentially a sound wave. My understanding is that a shock wave that is subsonic is a sound wave, while supersonic shock waves are pressure waves (like when that meteor exploded over Russia and blew out a shit load of windows). But this is through a gaseous medium. What happens to a solid that has a supersonic wave going through it? Can it happen?

[MadMojoMonkey]

Well, first of all Lara comes home, then Alex half an hour later. Lara tends to open the door more slowly, so my door creaks open slower when she arrives. Alex burts in very quickly, and thus my door opens faster. I normally have my door slightly ajar.

This is all as I suspected.

I wonder if the effect is more pronounced if you have a window open in your room, but no other windows open. I suspect that the random wind from outside will make the internal pressure in your room less well-regulated, and give more variance in the results. I wonder if, assuming the air is still, the door would move more than if the window was closed. Since the pressure increase caused by the shockwave traveling through your doorway will increase the pressure in your room, which would bounce the shock wave back at the door from the other side. This would serve to slow your doors motion, or maybe even cause it to close a little bit, after it opened a little bit. If the window is open, then the pressure difference wouldn't reverse itself as much.

I thought it was essentially a sound wave. My understanding is that a shock wave that is subsonic is a sound wave, while supersonic shock waves are pressure waves (like when that meteor exploded over Russia and blew out a shit load of windows).

First off, they're all shock waves... and they're all pressure waves. All pressure moves at the speed of sound through a material, because one way of thinking about the speed of sound is the speed at which pressure information propagates through the medium. So it's kinda the definition of sound that it is a pressure wave.

What blew out the windows is the same thing that pushes your door. If your roommates open the doors fast enough, they will blow out your windows (or break the door... structural integrity in question). Same as if you pointed a large speaker at your window and turned it up to 11 (assuming the speaker has the structural integrity to produce a pressure differential strong enough to break the glass before it breaks itself from internal forces).

Also:
There's a mismatch in terms. I said subsonic to mean below the threshold of human hearing, specifically in terms of frequency, as opposed to volume. The shock wave which pushes your door travels at the speed of sound in air.

When you say supersonic, you're probably referring to an object - like a fighter jet - moving through a medium - like air - faster than the speed of sound in that medium.

These are very different uses of the words subsonic and supersonic which are kinda implicitly antonyms. Here, they are not antonyms. They are talking about being below or above very different thresholds which aren't even on the same scale (one is frequency the other is speed).

Importantly, the shock wave created by the meteor over Russia moved at the speed of sound in air. The object creating that shock wave was moving faster than the speed of sound, but the effects of that motion propagate at the speed of sound.

But this is through a gaseous medium. What happens to a solid that has a supersonic wave going through it? Can it happen?

The wave moves at the speed of sound. The wave itself is not supersonic. That's not possible, as I said above, because the speed of sound is the speed of pressure waves because sound is a pressure wave.

If the solid has another solid move through it above its speed of sound, then problems with definition of solid arise.

Check out Cherenkov Radiation for an example of what happens when a charged particle moves faster than the speed of light through a medium. Recall that the speed of light in vacuum is the cosmic speed limit, but light travels slower through "stuff" than it does through "not stuff" 'cause refractive index.

[OngBonga]

I wonder if the effect is more pronounced if you have a window open in your room

I'll start experimenting now!

There some really interesting stuff there that helps to put right a lot of my misunderstandings about pressure waves and the speed of sound. I find it very interesting to discover that the speed of sound is basically the speed of pressure propagation in a given media. It makes perfect sense, of course, but it never occured to me.

[JKDS]

How come bicycles stay upright while in motion?

[MadMojoMonkey]

How come bicycles stay upright while in motion?

The short answer is that when the top of the bike falls to the right, it turns the front wheel to the right. If the bike is moving fast enough, then it will pull the bottom of the bike to the right such that it's under the top of the bike. Now the bike is upright, but it has rotational inertia which pulls it to the left. So it falls to the left, and that makes the front wheel turn to the left, and yada yada yada.

[Keith]

is that why if the rider is drunk they exagerate the natural left/right wobble and end up going all over the road?

[MadMojoMonkey]

is that why if the rider is drunk they exagerate the natural left/right wobble and end up going all over the road?

TL;DR
Probably. It is well known that alcohol messes with your balance.

***
There are many more factors which help to balance the bike than the description I gave. What I gave is a kind of minimum necessary circumstance for it to maintain balance.

The gyroscopic effect of the wheels helps to maintain balance, too.
The design of the front axle often accentuates the front wheel's turning into the fall.
The fact that the mass is higher at the front of the bike than the back also accentuates the turning.

I'm sure there's more that I'm not mentioning. It's a much debated topic among physicists as to what are the vital factors, but it has been shown that these above factors are non-essential for riderless stability.

Of course, I've been talking about a riderless bike.

Throw a human on top and their own sense of balance and coordination will effect the balance of the bike.

[CoccoBill]

More geometry than physics I guess, but if you were looking up at space from earth and had vision good enough to see to the edge of the observable universe, what percentage of your field of view would be covered by matter (stars, planets, comets, what have you)?

[MadMojoMonkey]

More geometry than physics I guess, but

Well, it's about what is/can be observed... which sounds a lot like physics.
If not physics, then chemistry or biology.

and had vision good enough to see to the edge of the observable universe

It's hard to imagine what kind of vision I have if it can't be used to observe the universe. I'm assuming you're talking about visible light, yeah? If a telescope can absorb a photon and that is an observation, then if my eye was where the telescope is, my eye would have absorbed the photon and observed it.

The advantage of a telescope is that it is like a very big eye. I mean... it gathers photons from a much bigger area than a pupil. Also, the camera or CCD doing the actual "observing" has the benefit of using long exposures for a single frame. Whereas my brain keeps telling my eyes to forget the past and focus on the present.

I'm saying that you CAN see to the edge of the observable universe, although the stuff that's very far away is also quite dim. So much so that it doesn't register as more than black to our fast-paced, tiny eyes. Still, it has been shown that at least some people's eyes are sensitive enough to detect a single photon, so even though you may not consciously observe it, that photon has traveled from the edge of the observable universe to your eye.

if you were looking up at space from earth, what percentage of your field of view would be covered by matter (stars, planets, comets, what have you)?

OK, so I need to do a bit of digging for this to be more than a hand-waving guess.

I will answer this question soon.

First tactic will be to examine the Hubble Deep Field Survey and see what percentage of that shot was filled with visible stuff (galaxies).

EDIT: It's harder than I thought. My rough estimate is ~30% ish... but you could guess the same thing if you look at any of the various deep field surveys. I do suggest it. Here's one:

Kind of in the middle of the lower-right quarter of the main image, there is an 8-pointed star. That is a star in the Milky Way. I don't see any other 8-pointers, but they would also be foreground objects. The 8 points are diffraction caused by the optics in the HST. Everything else is a galaxy.

[MadMojoMonkey]

Ugh. The more I think about it, the more it becomes a problem of resolution and exposure time.
Resolution, because you really want to zoom in and know if you're looking at 2 things close together, or 1 thing.
Exposure time because the things you're looking at are up to 13.8 billion light years away and not too many of their photons happened to be pointed directly where you were eventually going to put your eye. You need to look for a long, long time to get an image, and you need to have a sophisticated understanding of "stillness" during that exposure.
(2,000 exposures / 500 hours of exposure time = 4 hours per exposure on average)

It's complicated by the fact that those deep field surveys are intentionally looking at "black" areas of the night sky. Whatever the percentage of those surveys, that's adding to the percentage of the night sky that is already blocked by something closer.

So.

What %-age of the night sky is visible stars and galaxies?
Should we really count those?
Is it in the nature of your question to assume we're not trapped on a planet in some random galaxy, but rather somehow observing from a (much more common, statistically speaking) empty region of intergalactic space?

[CoccoBill]

Thanks for entertaining the question, I in no way expected some precise measurement. I did think about your last point too, and sort of came to the conclusion that maybe the milky way's stars, being so much closer, would account for the vast majority of the visible matter, and at any given moment would block out a huge part of what's beyond. Then again I would think that the Hubble images show the halos of the stars, not the matter, and there would be waaaay less of matter than is apparent in the photos. Being in intergalactic space would probably change things a lot. I'd already be willing to accept the 30ish% for intragalactic. I thought about this first a long time ago as a kid watching star trek/galactica etc, how likely is it to hit something when moving at warp speed.

[JKDS]

Do individual pieces of tnt have gravity?

If you made a planet of tnt, would it have gravity?

If the planet then exploded, would the gravity (if any) change as the pieces exploded away from the center of the exploding tnt planet field?

Did you see this question coming, and are you therefore psychic?

[a500lbgorilla]

Do individual pieces of tnt have gravity?

Do they have mass?

If you made a planet of tnt, would it have gravity?

Does it have mass?

If the planet then exploded, would the gravity (if any) change as the pieces exploded away from the center of the exploding tnt planet field?

Lets say that you were on the middle of an infinite frozen pond. For any direction you looked, there was ice to the horizon. As you moved one foot, it effortlessly slides and you fall down. It take's some coordinated effort to even stand. But you, having an idea, take off your clothes and bundle them up and throw them Thataway, and for your work, you move Thisaway. You see, the center of mass of you and your clothes has not moved, yet you and your clothes have.

You know how the Moon moves across the sky? Well, the Moon is bound to the Earth because of Earth's big fat gravity(mass). The Moon is caught falling to the Earth, yet it manages to never quite get there as it has found a balance between renegade movements and degenerative movement. And the Earth is falling into the Sun, it just has the spunk to dodge it for every moment.

About twice the distance of the Moon from the Earth, any object that might find themselves there is more under the influence of the Sun than it is the Earth. You see that for all her mass, the Earth just isn't as big as the Sun.

If the Moon moved faster, you see, it would be able to break from the Earth and fly free around the Sun. And if it moved faster still, it could break from the Sun and wander across the greater black plains of space.

Now, if the Moon simultaneously exploded because moon dust was basically TNT and the ignition was basically you shrugging your shoulders, some bits would move quickly away propelled by the explosion, propelled by the explosions beneath them, themselves propelled by their explosion and those below them, which are also propelled by the explosions beneath them... and it goes on.

What if some of those bits gained enough velocity to escape the Earth?

What if they were fast enough to escape the Sun?

[MadMojoMonkey]

Lets say that you were on the middle of an infinite frozen pond. For any direction you looked, there was ice to the horizon. As you moved one foot, it effortlessly slides and you fall down. It take's some coordinated effort to even stand. But you, having an idea, take off your clothes and bundle them up and throw them Thataway, and for your work, you move Thisaway. You see, the center of mass of you and your clothes has not moved, yet you and your clothes have.

I'd suggest keeping your clothes on and simply inhaling over your left shoulder and exhaling over your right.

...but...

If you want to imagine JKDS with his clothes off, or trick him into imagining you with your clothes off, that's cool, too.

[MadMojoMonkey]

Do individual pieces of tnt have gravity?

Gravity isn't really a property that stuff has. Mass is a property that stuff has. Gravity is a consequence of what mass does to space-time.

To answer you, though, I guess a "yes" is in order. TNT is made of atoms, which are indeed stuff, and therefore have mass.

If you made a planet of tnt, would it have gravity?

Yes. See above.

If the planet then exploded, would the gravity (if any) change as the pieces exploded away from the center of the exploding tnt planet field?

As the mass density changes, the curvature of spacetime changes, and therefore the gravity changes.

The same total gravitational acceleration is expressed outside of the planet (sploded or not), since the total mass density inside a sphere of that radius was mostly empty space and one solid TNT body, and is still mostly empty space and the same amount of atoms, just in a wider spacing.

If you were in a place which was outside the planet before it sploded and inside the planet after it sploded (or inside and further inside), then you would experience a significant decrease in the acceleration you feel due to gravity. However, you would still need the same escape velocity to get "away" from the planet, because whatever lesser gravity you now feel, you have further "up" to go to get out of the planet. As you go up, you put more distance between you and the center, and therefore more stuff which was pulling you in all directions is now pulling you back to the center.

If the Earth were a hollow shell, there would be no gravity inside the shell, since there is no mass within that radius, and all of the gravitational pulls from the various bits of the shell cancel out.

In the same way, the Earth-moon system expresses it's gravity as though it's a single gravitational body, with all of its mass located at the gravitational center of the Earth-moon system. When we are close to the system, we begin to see and feel that it is not a single gravitational source. When we are "very close" to one of them, it's as though the other isn't even affecting us and the only gravitation we can discern is that of the closest source.

Did you see this question coming

Well, I was hoping, with a strong expectation.

and are you therefore psychic?

Nope. Physicist. Haven't you heard? We predict the future with math, rather than wild speculation.

In this case... people can be rather predictable when motivated in certain ways.

[OngBonga]

Psychics don't predict the future with wild speculation, they know the future. That's not speculation.

[JKDS]

But, I don't appear to have a very strong (if any) gravitational field. Neither does a piece of tnt to my knowledge. Large bendy things (palm trees for example) don't seem to bend towards each other either, so it's not just tiny things that seem to not have a field.

Perhaps you are saying that, like a piece of tnt, I am merely a piece of a giant ball that has gravity. Like the tnt, myself and the rest of the planet pieces are all acting as one thing, and our masses combine in such a way as to have noticable gravity. Such would explain why less dense smaller planets have less gravity.

But, if this is the case, do objects in space tend to clump together? For example, on a trip to the moon (or mars), would free floating objects tend to group together?

If they were already grouped together, would an astronaut be drawn to them? So like, given time...We'd just have a big clump of space toys bundled loosely together?

Also, what are the lotto numbers.

[MadMojoMonkey]

But, I don't appear to have a very strong (if any) gravitational field. Neither does a piece of tnt to my knowledge. Large bendy things (palm trees for example) don't seem to bend towards each other either, so it's not just tiny things that seem to not have a field.

You may be interested in looking at the Cavendish Balance experiment.
I have performed this experiment within the last 6 months.
Here's a video link showing some college student doing it
I spent an hour finding a video link that wouldn't bore the pants off of you, and where this one lacks explanation, it has a much needed time lapse to show the experiment being performed. Do a YouTube search for Cavendish experiment and you'll find plenty more info, including autobiographical excerpts of the weirdo that was Cavendish.

anyway...

What's in that video is a Cavendish balance. Find an image to help visualize it. It's 2 masses on a balanced rod. That rod is suspended by a thin wire. Mounted onto the rod/wire is a small mirror. Near, but not touching those masses are 2 much larger masses. The 2 large masses are arranged one on either side of the suspended rod, such that they are "leading" the smaller masses around. The 2 larger masses are on a separate balanced rod which the experimenter is free to rotate.

In the video, the experimenter starts with the experiment set up as described. He has left it in this position for a while... probably hours. It's probably the middle of the night to reduce foot traffic in the building and vehicular traffic outside. This experiment is that sensitive. Anyway. He remarks that the laser is pointed at a specific measure by his meter stick. The laser is pointed at that mirror I mentioned earlier. The mirror is fixed to the balance rod with the 2 small masses, and is used to amplify the angle of position of the rod. The laser is pointed so that the mirror is directing the beam perpendicular to the meter stick.

Now the experiment is primed and ready to go. He moves the large masses so that they are now opposite the other balls, but on the other side. In the starting position the large masses were leading the small masses clockwise. Then they are moved so that they are leading the smaller masses counterclockwise. Or vise versa. Symmetry makes the change irrelevant.

Then the video speeds up and he shows that the mirror undergoes a damped oscillation before coming to rest at a new position on the meter stick. The movement of the mirror is directly the related to the rotation of the balance rod with the small masses. The balanced rod has moved and come to rest at a new position, and the only change was the position of the large masses. Note again that the large masses are on a fully independent support from the smaller masses.

Gravity is real and measurable, even on the scale of a 2" lead ball for the large masses, which is what I used. The difference in angle measure is on the order of a few milliradians... it's subtle... but a tiny bit of trig and a measure of the distance from the mirror to the meter stick and you can solve it.


The actual math is a headache, but the proof is only 2 to 3 pages. Not too bad, considering the topic at hand. The reason it's as long as it is is due to the fact that you have to account for the fact that each of the large masses interacts with each of the small masses. Also, the rotating support for the large masses has its own mass that must be accounted for in order to get a good measurement.

Perhaps you are saying that, like a piece of tnt, I am merely a piece of a giant ball that has gravity. Like the tnt, myself and the rest of the planet pieces are all acting as one thing, and our masses combine in such a way as to have noticable gravity. Such would explain why less dense smaller planets have less gravity.

The only reason the giant ball has gravity is because of the accumulated sum of all the atoms from which it is made having massive nucleons (protons and neutrons). The reason the smaller planets have lower mass is because they contain fewer nucleons (to an amazingly high precision).

But, if this is the case, do objects in space tend to clump together? For example, on a trip to the moon (or mars), would free floating objects tend to group together?

How do you think stars and planets are formed? Hint: 'cause gravity pulls the particles in a nebula toward the local gravitational center, which is itself pulled to the next local gravitational center and vise versa and all around.

Yes, all objects tend to clump together... that's what gravity means.

Fun fact: There are asteroids that are loosely bound balls of gravel. I imagine they're all squirly and haven't really pulled themselves into a packed position because the forces expressed between each individual rock are so minuscule. Even still... when there's no other force around, that F = ma will become the dominant and overriding force... and given a few billion years... yeah... everything tends to clump together.

If they were already grouped together, would an astronaut be drawn to them?

Yes. We're drawn to and drawing to us everything which we have the ability to see. Gravity travels at the speed of light. If its light can reach us, then our light can reach it and we are influencing each other via gravitation.

So like, given time...We'd just have a big clump of space toys bundled loosely together?

That's a great way to describe the universe!

Also, what are the lotto numbers.

https://www.arizonalottery.com/

[OngBonga]

But, I don't appear to have a very strong (if any) gravitational field.

You were right until you said "if any". You have a ridiculously tiny gravitational field. Like, let's pull some numbers out of my arse and say you're a trillionth of the mass of the earth. One trillion of you = one earth. We'll also assume one trillion of you is exactly as dense as one earth to stop mojo nitpicking my dumb analogy. So, your personal gravity is one trillionth that of earth.

Large bendy things (palm trees for example) don't seem to bend towards each other either,

That's because local influences, such as wind, play a much more significant role. And by much, I mean a shit load of zeroes.

But, if this is the case, do objects in space tend to clump together? For example, on a trip to the moon (or mars), would free floating objects tend to group together?

I reckon I can answer this too. Where there's no other influences at work (a huge ask), sure, two items freely floating in space will slowly be drawn to one another, not necessarily in direct collision, more like stable orbits. But we're probably talking astronomical amounts of time for tiny objects, and even slight influences like a comet a million miles away would probably disrupt the gravitational harmony.

Also, what are the lotto numbers.

They are a randomly selected group of numbers that could win you millions of dollars.

[JKDS]

EXCUSE ME, SIR.

I was SPEAKING to the monkey with the calculator tyvm

[MadMojoMonkey]

EXCUSE ME, SIR.

I was SPEAKING to the monkey with the calculator tyvm

[OngBonga]

Fuck you, it's the internet, I can be rude and answer questions not directed at me if I want.

[MadMojoMonkey]

Fuck you, it's the internet, I can be rude and answer questions not directed at me if I want.

[JKDS]

This question was both informative and entertaining. Was well worth it. Ty gentlemen

[a500lbgorilla]

It struck me as a much easier question to ask simply about a very, very small ball of tnt in empty space. Would any of the outer most bits move to infinity after detonation?

I didn't do the work to figure out how TNT actually works, but it seems to be the most sensible angle of attack.

[MadMojoMonkey]

It struck me as a much easier question to ask simply about a very, very small ball of tnt in empty space. Would any of the outer most bits move to infinity after detonation?

I didn't do the work to figure out how TNT actually works, but it seems to be the most sensible angle of attack.

This was fun.

escape velocity = sqrt(2*G*M/r)

where G = 6.67(10)^-11 m^3/(kg s^2), M is the mass from which escape is happening, and r is the distance from that center of mass.

The mass of a sphere is M = 4/3*pi*r^3*rho, where rho is the density.

Substituting into the first equation,

escape velocity = sqrt(2*G*(4/3*pi*r^3*rho)/r)
escape velocity = sqrt(8/3*G*pi*r^2*rho)
escape velocity = 2*r*sqrt(2/3*G*pi*rho)

Solve for r.

r = {escape velocity}/(2*sqrt(2/3*G*pi*rho))

Plugging in the detonation velocity of TNT (6,900 m/s) as the escape velocity, and the density of TNT for rho (1654 kg/m^3), we can solve for the critical radius of TNT which would have escape velocity equal to the detonation velocity.

r = 7.18 million meters

So a ball of TNT which is 14,355 km (~9,000 miles) in diameter would be just big enough to keep itself mostly not escaping to infinity when it blows up. Any smaller and the surface bits would be escaping to infinity. The bits deeper inside have lower escape velocity, but further up the gravity well to go. Since they start with the same 6,900 m/s, and they would lose velocity as they move upward in the gravitational field, they would decelerate and therefore would not have enough velocity to escape.

[a500lbgorilla]

woot

[Eric]

Daily Mail says the earth's core is two and a half years younger than its crust. In the 1960s they estimated the difference was just a day or two. What are the cliff notes on the way they figured out the difference is 2.5 years?

[MadMojoMonkey]

Daily Mail says the earth's core is two and a half years younger than its crust. In the 1960s they estimated the difference was just a day or two. What are the cliff notes on the way they figured out the difference is 2.5 years?

:/
Tectonic plate movement means the crust of the Earth is constantly (albeit slowly) being dragged down into the core. Volcanic activity moves the molten core back to the crust where it cools and becomes new crust.

The Earth is ~3.5 billion years old and someone is saying there's a way to tell that parts of it are a couple years older or younger than the rest? I'm going to have to find the article you read and look for sources. Maybe there's some exciting geology that I am about to learn.

[MadMojoMonkey]

OK, so it's about gravitational time dilation.

I'm soft on the specifics of General Relativity.

A plot of (Newtonian) gravitational acceleration (g) by distance from the center of mass (r) looks like this:


Where the planet has radius R, and g is the acceleration of gravity at a given distance from the center.

So I'd assume that the surface of the Earth has the highest gravity (most dramatic time dilation), since it is decreasing as you move either toward or away from the center.

[MadMojoMonkey]

Ah, so there's a problem with my picture. That's the acceleration of gravity, which means it's a graph of the curvature, not the depth of the gravity well. The well is deepest in the center.

So it's certainly observable that the deeper you go under ground, the more time dilation you would experience.

The problem is that the interior of the Earth is a complicated bunch of stuff that we barely understand. There are convection currents which move material out from the core and back into the core, at least for much of the interior (or so we believe). I should clarify that we know things like the Pacific plate is sliding to the West and being dragged down by its own weight into the Mariana Trench. The crust gets deposited back into the molten mantle, and volcanic activity pushes molten mantle back to form new crust. So we know there are convection currents near the surface of the Earth, albeit slow moving currents. We know less about what's going on in the deep interior near the core. We still don't have good models for what creates the Earth's magnetic field, but it is widely suspected that vast underground currents of molten Iron are the root. If these exist, then there is significant mixing of the molten material inside the Earth, and it is not a rigid body.

The problem with making an actual calculation is that it will depend on what factors you count and which ones you deem insignificant in order to come up with a number. So the notion that the core of the Earth is 2.5 years younger than the surface is dubious without knowing the error bars on that calculation and what factors were taken into account.

That said, it is entirely plausible that the interior of any massive body is some finite positive number of years younger than the surface of that body, provided the body is rigid.

So it's plausible that the interior of the Earth is younger than the surface. If there is a solid (rigid) core at the center, then it's more than plausible, it's a given.

[Eric]

Right, there is more gravity in the core area so it is younger than the crust area. What I don't understand is how early estimates had the difference at around a day while more recent estimates have the difference at around 2 or 3 years.

[MadMojoMonkey]

Again, I'd need to know what they deemed significant and what they discarded to come up with their numbers in either calculation.
Which is a bit of a lie, because I don't know how to do a full GR calculation, but that is the most likely source of discrepancy between 2 calculations made decades apart.

***
The statement of "more gravity" is ambiguous since it's not clear whether you're talking about mass density, the small g acceleration, or the depth in the spacetime field.

There is not more gravity in the core. The core is deeper in the curved spacetime well.
(which is what I think you meant to say)

The acceleration due to gravity at the center of the core is 0 m/s^2.

The density of the material may be higher, which might be a meaning for the phrase "there is more gravity there." (I doubt this is your intended meaning, but it is the most pedantically attached to the semantics of your statement.)

[OngBonga]

We still don't have good models for what creates the Earth's magnetic field, but it is widely suspected that vast underground currents of molten Iron are the root.

I figured it was obviously due to molten iron moving around.

Electric fields come from charges. So do magnetic fields, but from moving charges, or currents, which are simply a whole bunch of moving charges. In a permanent magnet, the magnetic field comes from the motion of the electrons inside the material, or, more precisely, from something called the electron spin. The electron spin is a bit like the Earth spinning on its axis.

The earth contains a large amount of molten iron. It is rotating on its axis, while drifitng through space at breakneck velocity. Other local moving objects such as the Moon, Jupiter, et al will cause further fluctuations in the electric currents moving through the core.

A large magnetic field seems inevitable, considering these aspects.

[MadMojoMonkey]

I figured it was obviously due to molten iron moving around.



The earth contains a large amount of molten iron. It is rotating on its axis, while drifitng through space at breakneck velocity. Other local moving objects such as the Moon, Jupiter, et al will cause further fluctuations in the electric currents moving through the core.

A large magnetic field seems inevitable, considering these aspects.

But why, after 3.5 billion years, is the Iron still moving around?
(insert Nobel Prize for correct answer)

The thing is that molten Iron is sticky stuff. It's viscous. It drags on the surrounding material unless that material is moving with the same velocity. The fact that this shear friction has not completely locked any molten currents into a single rate of rotation (i.e. 1 revolution per day) is mind-boggling with any known models of the Earth's core.

However, the electric current can flow even if the Iron isn't flowing, so there's that. BUT there's a similar effect called Eddy Currents that are produced by any current in a conductor. Basically, if you have a current in a conductor, it will induce other currents nearby to the main current. Kinda like if there's a strong ocean current, the surrounding waters may actually flow in the opposite direction nearby because of the generation of vortices.

So even if it's just the electric current flowing through the Iron, given 3.5 billion years of Energy being dissipated by Eddy Currents and Joule heating (stuff gets hot when you pass a current through it)... given all that it's hard to figure why there's any current that is left undissipated. Many younger planets and moons are completely tidally locked and have no magnetic fields.

***
The sun, moon, Jupiter, etc. are pretty much neutral of charge and have next to no bearing on the electromagnetic fields on and quite near Earth. That said, Jupiter's and Saturn's magnetic fields are pretty immense, IIRC, and could maybe be measured on or near Earth with sensitive equipment.

[OngBonga]

Anyway I cam here to ask... why don't they make vacuum walls in cold, hot, or particularly noisy places? Is it simply because of the implosion risk? Have we not got strong enough and cheap enough materials to indefinitely withstand a difference of merely one atmosphere of pressure?

[MadMojoMonkey]

Anyway I cam here to ask... why don't they make vacuum walls in cold, hot, or particularly noisy places? Is it simply because of the implosion risk? Have we not got strong enough and cheap enough materials to indefinitely withstand a difference of merely one atmosphere of pressure?

I can only guess, but I guess the main reason is cost.

Also, vacuum is a bit misleading, since there is no known place in the universe which is a perfect vacuum. There's always a particle per cubic km or something, even in deep intergalactic space.

Outgassing is a thing and it probably doesn't mean what you want it to mean. If you create a pressure vessel and remove all the air in it, it will slowly regain some air pressure inside due to some of the atoms which are in the container walls escaping to the low pressure interior. Not to even mention the logistical difficulties of extracting the final few atoms from the chamber when they refuse to kindly just happen to be in your pump's chamber as it closes and not be there when it opens. Statistics plays hell in getting the pressure to keep dropping.

But I digress...

Double paned windows are a thing which accomplish much of what you're asking for. Note that windows are more expensive than walls.

[OngBonga]

The thing is that molten Iron is sticky stuff. It's viscous.

Yeah, molten iron at the surface is for sure. Would something not become significantly less viscous the hotter and more pressurised it becomes?

But why, after 3.5 billion years, is the Iron still moving around?

Because it's still immensely hot and under intense pressure? Obviously I'm guessing here if the correct answer equals a Nobel prize. But it seems logical to me that so long as there's molten iron at the core, it will move with the rotation of the earth, and fluctuate in line with other influences such as gravity. It will keep moving for as long as it's molten, regarless of how viscous it is.

It drags on the surrounding material unless that material is moving with the same velocity.

What surrounding material? The rest of the molten iron and whatever else is mixed in with it? I don't expect we're talking about a liquid moving against a solid. We're talking about an iron rich mixture here of molten whatever. It's all liquid, so the viscosity is kinda irrelevant. What happens if water comes into contact with mercury? Does the water (more viscous) experience significant friction as it comes into contact with the mercury (less viscous)? Can they mix at very high temperatures and pressures? What happens when they mix? Does the mixture share a common viscosity?

The sun, moon, Jupiter, etc. are pretty much neutral of charge and have next to no bearing on the electromagnetic fields on and quite near Earth.

It's the gravity which I suspect has influence, not the electrical charge. The liquid interior will be subject to tides, just like the oceans. It's this movement that causes the constant fluctuation that results in a magnetic field, I believe.

Where's my Nobel prize?

[OngBonga]

Also, vacuum is a bit misleading, since there is no known place in the universe which is a perfect vacuum. There's always a particle per cubic km or something, even in deep intergalactic space.

Well yeah, but vacumm flasks work because we create a near vacuum, which greatly increases its insultion properties.

Whenever I say vacuum, please think the word "near" before it.

Ok so vacuums leak and slowly regain neutral pressure, that's a heavy flaw. Why not fill the cavity with argon then? That's still a much better insulator than a single wall, right?

[OngBonga]

So while I was out walking I was thinking about vacuum panels to make walls out of, because thinking shit is good when walking. If you take two identical sized sheets of plastic, and one slightly smaller sheet of metal, placed the metal between the two sheets of plastic so there is a complete border around it, heat it up, then cool it again... the metal will expand as it heats, then as it cools and contracts it will create a (near) vacuum. The plastic would need to melt to seal the edges, but solidify with full strength before the metal has fully contracted. Find me the right plastic and metal please.

[MadMojoMonkey]

Using data from this link, I found the viscosity of water is ~0.001 Pa s and the viscosity of castor oil is ~0.7 Pa s. I know that castor oil is thicker than water, so higher value means less runny. Which is good since viscosity is, roughly, resistance to flow.

From this link, I found that the viscosity of the Earth's lower mantle is ~10^22 Pa s.

So to answer your question as to whether the pressure and temperature make it runny: No. I mean, well, it's runny compared to solid Iron.

Crap. Then I found this link, which is a scholarly 2 page paper and not for the feint of heart in the physics zone, yadda yadda.
"Our overall conclusion is that the shear viscosity of liquid iron in the Earth's core is ~0.015 Pa s, with an uncertainty factor of 3."

So yeah... The outer core is about as runny as ethelene glycol (IDK, it's from the link), and about 10 times thicker than mercury.

***
If we're talking currents in the outer core, then the inner core and the lower mantle are the surrounding material. Yes, it's liquid moving against those solid surfaces (which there is no reason to assume are rotating in sync with each other if the layer in between isn't).

But why aren't they in sync? What source of energy is keeping that stuff flowing after 3.5 billion years? Why hasn't the drag against the mantle bled off all currents which weren't in sync?

***
If not charge, but gravity, then the effects are just too weak to account for the lack of energy dissipation. Tidal effects from the moon are significant to ocean currents, but can't account for the numbers involved. (Sorry. A bit hand-wavey, there.)

Tidal effects from the sun are difficult to detect. Tidal effects from Jupiter are likely too small to detect. While Jupiter makes up 3/4 of the mass of the solar system which is NOT the sun. The sun still makes up ~99.8% of the mass of the solar system, Jupiter included. Also, Jupiter is further away than the sun.

[OngBonga]

But why aren't they in sync? What source of energy is keeping that stuff flowing after 3.5 billion years? Why hasn't the drag against the mantle bled off all currents which weren't in sync?

I dunno about all this, I mean I'm imagining the inner earth to be pretty much molten below the crust, at least until we get to the core where the pressue is so great that the concept of liquid is kinda redundant. There's surely more liquid within the earth than on it.

If not charge, but gravity, then the effects are just too weak to account for the lack of energy dissipation. Tidal effects from the moon are significant to ocean currents, but can't account for the numbers involved. (Sorry. A bit hand-wavey, there.)

I'm still leaning tidal. I mean when you think about it, the Earth's magnetic field isn't strong, relatively to other forces we're subject to. Even gravity utterly dominates natural magnetism on Earth. So perhaps it's simply the result of a very large amount of molten iron moving very slowly in fluctuation with, predominatly, the moon. That slow motion is why it's a weak field. The continued motion... gravity is a beast because it's relentless and unyielding. It might be weak, but it always wins. I'm thinking that it's keeping things moving at a very slow rate, the same rate as the drifting of the poles, and that very slow motion is enough to give us a weak magnetic field.

Still want that nobel prize, hand wave all you like!

[OngBonga]

In fact it's probably the Earth's own gravity that dominates. There's a large amount of water moving in rhythm to the moon, which means the Earth's own centre of gravity will fluctuate with the tides. This will mean that the molten iron's flow will be constantly shifting its course, albeit only slightly. But again, I suspect we're only in need of a small amount of motion for us to have a significant magnetic field, thanks to the volume.

[OngBonga]

I'm obviously no electromagnetism buff, but what happens if you take a large amount of volts and it flows at a low ampage? Does that still create a large magnetic field?

I guess I want to know if flow rate and volume are both important factors when it comes to the stength of a magnetic field.

[OngBonga]

If we're talking currents in the outer core, then the inner core and the lower mantle are the surrounding material. Yes, it's liquid moving against those solid surfaces (which there is no reason to assume are rotating in sync with each other if the layer in between isn't).

I'm thinking more about this. I just can't see a sudden change from solid to liquid and back to solid within the Earth. I'm thinking it's more likely that it's a gradual change, that the viscosity decreases with temperature the deeper you go, until you reach a point where the pressure is so great that it begins to increase again until it's in effect a solid fluid. I think if you had a graph that shows the viscosity of the earth as you get deeper, you'd have a bell curve. Where that curve is at its lowest is where the flow is happening at its greatest, under the influence of gravity and the Earth's own rotation. The rotation keeps things flowing, while the gravity keeps things fluctuating.

I'm so glad that I haven't got a scientific background, otherwise I'd know how much shit I'm talking.

[MadMojoMonkey]

I'm so glad that I haven't got a scientific background, otherwise I'd know how much shit I'm talking.

Oh, but you're such a cute little fella, aren't ya!

[OngBonga]

Until I shit on the carpet.

[OngBonga]

Dynamo theory has it covered imo, and it's not a million miles away from what I've been saying. I think where I'm wrong is that I thought gravity is a dominant factor, that it is causing the fluctuations that give us magnetism. But dynamo theory suggests it's convection currents, which makes a whole lot of sense. Columns of hotter, rising molten iron, surrounded by sinking cooler iron. Much like air around a fire. This motion will be constant so long as there's a constant heat source.

Gravity plays a role though in that it provides one means for the interior of the planet to heat up... tidal heating.

I would imagine that in terms of flow, convection is going to utterly dominate gravity.

According to this theory, for a magnetic field we need three things...

planetry rotation,
an electrically conductive fluid,
an internal energy source to drive convective currents.

Mercury is small, but has a magnetic field, probably because it meets these criteria.
Io doesn't have a magnetic field, despite an abundance of tidal heating, it is lacking a conductive fluid.

I think they're close to solving this without my help. There goes that Nobel prize.

[MadMojoMonkey]

I think they're close to solving this without my help. There goes that Nobel prize.

Don't be convinced by clever sounding words which describe how it could maybe happen without talking actual numbers and time frames.

There's a lot of plausible sounding ideas out but no one has a working model that supports what data we have.


EDIT:

an internal energy source to drive convective currents.

Q: What is the source of energy for the Earth's outer core?
A: The inner core
Q: and what is the source of energy for the Earth's inner core?
A: ...

[OngBonga]

Q: and what is the source of energy for the Earth's inner core?

Um, don't we know that already? I thought there was a shit load of radioactive decay going on down there?

And tidal heating. The interior does indeed have a tide, and when it moves it experiences friction.

Plus the friction caused by the convection currents.

Plus we're floating in a vacuum, so the planet is well insulated.

It's no surprise to me that the planet isn't cooling down in any hurry.

[MadMojoMonkey]

Um, don't we know that already? I thought there was a shit load of radioactive decay going on down there?

No, we don't know. Whatever it is, it's been down there for 3.5 billion years, so it'll need a quite long half-life to not have already decayed. Which implies that there must be a heck of a lot of it to keep it going.

Sources cite the solid inner core as being composed of iron-nickel composites. Not radioactive.

And tidal heating. The interior does indeed have a tide, and when it moves it experiences friction.

Sounds plausible that it contributes. How much energy is added to the Earth's core by tidal heating over time?

Plus the friction caused by the convection currents.

Which dissipates energy from the system, 'cause 2nd law of thermo says entropy never decreases and friction is not a conservative force, so therefore cannot yield a 0 entropy change.

Plus we're floating in a vacuum, so the planet is well insulated.

The law of blackbody radiation says that everything radiates energy in the form of photons, merely due to the fact that it is not at a temperature of absolute 0.

It's no surprise to me that the planet isn't cooling down in any hurry.

Don't be convinced by clever sounding words which describe plausible answers without talking actual numbers and time frames.

[OngBonga]

Don't be convinced by clever sounding words which describe plausible answers without talking actual numbers and time frames.

Can't I leave all that to you while I sit here and wildly speculate?

[MadMojoMonkey]

Can't I leave all that to you while I sit here and wildly speculate?

I'm impressed at how you're digging into this one.

I haven't looked at this in over 5 years, and I don't remember all the details, just the conclusion that the Earth's inner workings are poorly understood and we don't yet have a (complete) working model of the Earth's magnetic field.

[OngBonga]

I'm impressed at how you're digging into this one.

I actually provided links to show I'm researching! It's interesting stuff, I like thinking about this kind of shit. This is easily my favoutire thread here.

[OngBonga]

Sources cite the solid inner core as being composed of iron-nickel composites. Not radioactive.

Isn't everything radioactive when it's hot enough or under enough pressure?

Also, isn't pressure basically heat? Isn't the constant force of gravity going to cause the core to be hotter than its surroundings? It's under the most pressure.

The law of blackbody radiation says that everything radiates energy in the form of photons, merely due to the fact that it is not at a temperature of absolute 0.

Based on some quick googling (in particular https://en.wikipedia.org/wiki/Earth's_energy_budget), the planet is cooling down in terms of radioactive heat loss at a very similar rate to the warming thanks to sunlight. We're in almost perfect balance.

Considering we're in a vacuum, radioactive heat loss is the only way the planet loses heat. Sure there will be microscopic amounts of conduction thanks to the fact space isn't a perfect vacuum, but surely this is comletely negligible considering we're constantly bathed in sunlight.

Convection and conduction will obviously be significant means for the interior of the planet dissipating heat to the atmosphere, but the atmosphere is merely acting as a buffer to limit the radioactive heat loss. A great amount of thermal radiation reflected at the suface will be absorbed by water vapour before it gets to leave the planet.

Is there any way for the atmopshere to return heat to the interior? Surely this is impossible thanks to the fact the interior is hotter and heat always flows from a hot place to a cold place?

Sounds plausible that it contributes. How much energy is added to the Earth's core by tidal heating over time?

I dug around for a while and this was the best I could find...

In addition to the effect of the ocean tides, there is also a tidal acceleration due to flexing of Earth's crust, but this accounts for only about 4% of the total effect when expressed in terms of heat dissipation.

I don't really know how to interpret this though.

Certainly tidal heating is significant though. The slow process of tidal locking that has claimed the moon and will eventually claim the earth is creating heat wherever it causes friction... in the interior, in the oceans. This alone will maintain a certain level of thermal activity within the planet for as long as the process continues.

As for the 3.5billion years of radiocative decay... I haven't got a clue. I have no idea what can decay for so long. Is it not possible that under the immense pressure within the core, nuclear fusion is happening at a very low rate, just enough to keep producing radiation-emitting matter?

We know nuclear fission is happening down there.

http://blogs.scientificamerican.com/...f-earths-heat/

This claims over half our interior heat is the result of nuclear fission.

And Earth is chock full of such radioactive elements—primarily uranium, thorium and potassium. Over the billions of years of Earth's existence, the radioactive isotopes have been splitting, releasing energy as well as these antineutrinos...

They don't seem to have a problem with these elements taking so long to decay.

There seems an abundance of heat sources to keep things active down there, and a distinct lack of means to lose the heat to space.

I'm only really questioning why the atmosphere doesn't get hotter and hotter as it gains energy from the both interior and the sun, while only radiating the equivilent of the solar energy.

[MadMojoMonkey]

Did you get to the final paragraph of that article?

The new measurements suggest radioactive decay provides more than half of Earth's total heat, estimated at roughly 44 terawatts based on temperatures found at the bottom of deep boreholes into the planet's crust. The rest is leftover from Earth's formation or other causes yet unknown, according to the scientists involved. Some of that heat may have been trapped in Earth's molten iron core since the planet's formation, while the nuclear decay happens primarily in the crust and mantle. But with fission still pumping out so much heat, Earth is unlikely to cool—and thereby halt the collisions of continents—for hundreds of millions of years thanks to the long half-lives of some of these elements. And that means there's a lot of geothermal energy—or natural nuclear energy—to be harvested.

more than half - significantly less than all
other causes yet unknown - we're like, all, duh, and stuff
may have been - see above
primarily in the crust and mantle - not the core
hundreds of millions of years - significantly less than billions of years

[OngBonga]

I didn't get that far down, no.

I mean I could see that it's full of uncertainty, I did hear you when you said there's a Nobel prize up for grabs for solving this.

The fact of the matter is though, the heat is there. I'm not going to speculate why it shouldn't be there, because it is there, and it's there because it should be. I'm going to try and speculate why it is there.

I just did a google search for "half life of uranium". We're good for another billion years if uranium 238 is plentiful, and it seems that it's the most abundant form of uranium in the planet. Plutonium is realtive short, potassium can get over a billion years.

So the plutonium and potassium should have decayed by now. If it hasn't, well then the planet must be producing the stuff.

[OngBonga]

Is it possible that within the core, gravity suppresses radiation? If there's plutonium in the core that can't radiate until it has enough gravitational freedom, then the core could be slowly seeping out a constant source of radioactive fuel.

[OngBonga]

Surely that's nonsense. Surely we'd need to be a black hole to suppress radiation.

[OngBonga]

So I was thinking about this in the bath.

Yes I bathe.

The uranium has it covered. We have an abundance of nuclear energy. So, I'm still wondering... why isn't the atmosphere getting hotter? How does it remain stable?

[OngBonga]

I found my answer.

Temperature doesn’t infinitely rise, however, because atoms and molecules on Earth are not just absorbing sunlight, they are also radiating thermal infrared energy (heat). The amount of heat a surface radiates is proportional to the fourth power of its temperature. If temperature doubles, radiated energy increases by a factor of 16 (2 to the 4th power). If the temperature of the Earth rises, the planet rapidly emits an increasing amount of heat to space. This large increase in heat loss in response to a relatively smaller increase in temperature—referred to as radiative cooling—is the primary mechanism that prevents runaway heating on Earth.

The fourth power? That's one hell of a relation! So if the atmosphere warms up, it radiates a significantly greater amount of heat into space. In this way, thermal balance is maintained.

[OngBonga]

I'm currently reading about half lifes.

The thing with half lifes, things don't stop decaying when their half life is up. They continue to decay. If something has a half life of a million years, then after 7 million years it retains 1/128 of its original energy value and continues to decay.

So the plutonium and the potassium have *mostly* decayed, while the uranium hasn't even lost half its energy yet.

[boost]

Not a physics question, but I'm going to ask it here anyways, so... yeah, sorry...

How do I figure out the length of the curved segment of a semi circle when all I have is the length of the flat side and the circumference/diameter of the whole circle and the flat side is not the diameter? The best I could come up with is that if I start from either of points at the end of the flat side and draw a line of the same length as our flat side that also terminates on the circumference of the circle, then continue to do this until I make it around to the other point of the original line, I can divide the circumference by the number of segments it takes me to make an orbit. The problem with this is that it would require me to draw it, and while it would be accurate enough for my purposes, I know there's a way to do it with math that's better.

[boost]

Oh, I think writing it out may have got me thinking about it enough to figure it out, but I'm not sure how I test it mathematically.

I'm thinking that (x/diameter)circumference=y

edit: nope, doesn't work, because if we draw the diameter then draw a radius at a right angle from the center of the circle, we know that a^2+b^2=c^2, and with a right angle c!=a yet that's what would be necessary for my equation above to work. =-\

[OngBonga]

I tried solving this but I'm doing something wrong. My methods were suspiciously lacking in pi.

Mmmmm pi.