Gravity is gone?

[McCoy]

Re: Gravity is gone?

 

(Cancer puts on his professor hat and hauls out his old ASTR 421 and 422 lecture notes.)

 

The Sun rotates very slowly (about a month rotation period), so let's forget rotation effects for now.

 

Suppose you have a bottle of gas. If you open the bottle, the gas will expand since the confinement is gone. The speed at which it expands is the speed of sound for the gas.

 

The sound speed turns out to be something you can compute fairly easily. The temperature at the solar photosphere is about 5800 Kelvin (there's a range, of course, but that's the standard representative temperature). The photospheric gas differs from air in that it's monatomic, so the ratio of specific heats is 5/3, and the ideal gas law applies. The mean molecular weight for solar gas is about 1.4 (remember, it's ~90% hydrogen by number, ~10% helium, ~<1% other stuff, and the level of ionization in the solar atmosphere is small). The sound speed is just

v = sqrt( gamma * R * T / M )

and that works out to be about 7600 m/second.

 

So in the one second after you let the solar atmosphere out of its bottle by turning gravity off, it will move outward by about 7.6 kilometers. Sounds big, but the solar radius is 700,000 kilometers. From our viewpoint 1.5e+8 km away, we would not even notice. From Earth that subtends about a tenth of an arcsecond. You'd see it using a space observatory, but you can't make out that level of detail from the ground.

 

The thermal timescale for the sun, a/k/a the Kelvin-Helmholtz timescale, is the time it takes to respond, thermally, to changes in its structure. "Thermally" means gas expansion or compression due to temperature changes. That timescale is on the order of ten million years. So messing with the Sun's structure by, say, turning gravity off for a second isn't going to make a lot of change. Turning it back on again has about the same effect. In terms of overall structural change, it isn't going to matter.

 

(If you want to compute the v = g * t impact speeds at the solar surface after your turn gravity back on, the sun's gravity at its photosphere is about 28 times the Earth's gravity at Earth's surface. The impacts are not going to raise the temperatures to the ten million Kelvin temperature regimes -- or cause high enough densities -- to get to the realm where you start fusion on the surface, which is what happens in a nova.)

 

One second is a piddly-*** short time for something as large and massive as a star.

"You must spread some Reputation around before giving it to Cancer again."

[Trained Chicken]

Re: Gravity is gone?

 

but we wouldn't see it until it crawled up from the layer of fusion to the surface. That's likely too long for a doomsday device to wait.

 

Commissioner Gordon: "Quick, Caped Crusader! The Joker's planted a time bomb under Gotham City's main nuclear plant!"

Batman: "How long have we got?"

Gordon: "Twenty years! God help us!"

Batman: "Say again?"

Gordon: "We have our best bomb squad men there right now. They say we've only got 20 years to disable the bomb!"

Batman: "..."

Batman: "I'm going back to bed."

[austenandrews]

Re: Gravity is gone?

 

A lot of stuff would hurl up into the air. But it would come crashing back down after 10 seconds. The moon is held into place by the gravitational pull of the Earth. The moon would spin off at a tangent, I'd have to seriously look into some math to see if it would go slingling off permanently.

 

There would be tidal waves, earthquakes, volcanos etc. LOTS of destruction.

I expect a lot of stored-up energy at tectonic boundaries could get released. Nasty quakes and tsunamis.

[Cancer]

Re: Gravity is gone?

 

Blanking out gravity on Earth for ten seconds is a different matter, because lots of the stuff that we care about is small and can change radically in 10 seconds.

 

Anyone in a device that depends on dynamic balance of forces is going to be in a world of hurt. The airplanes and helicopters go completely out of control as gravity vanishes while the aerodynamic lift, drag, & thrust don't. Driving in any ground vehicle with a suspension seems likely to be unpleasant also, since everything (that doesn't use a drag chute or retrorockets) brakes via contact with the ground, and as someone said, the compression on the springs will relax, giving everything a gentle push upwards, losing the ground contact that lets you decelerate. If you run into something at 60 mph, you will hurt a lot whether or not you do it in zero g.

 

About those springs ... 1000 kg car, assume the springs compress 10 cm under its weight, if the compression energy in the springs turns into kinetic energy, then the car will be moving upward at about 1 meter/sec. SO after 10 seconds the car is 10 meters above the ground, and then gravity turns on ... y'know, all those Wile E. Coyote cartoons come to mind here.

 

10 seconds is enough time for Earth-rotation effects to be potentially nasty, though it's going to sound small (well, to a dingbat astronomer like me it sounds small). Something just rest on the surface will move in a straight line tangent to Earth's surface at the instant gravity vanished. At the equator, it moves at about 0.463 km/sec. Ten seconds later, when gravity turns back on, it will have traveled in its straight line to a point about half a meter above the point on Earth's surface it would be on if it'd stayed on the circular path it would follow if it had stayed glued to the surface. (That half meter is from radius of Earth = 6378 km * 10 seconds / 86400 seconds ... strictly speaking there should be a sine in there, but the angle is so small that the angle and its sine are close to each other.) The effect is smaller as you go away from the equator (I think multiply by cosine(latitude) so in temperate latitudes you have about 70% of that rough half-meter). This would happen to approx anything that was just sitting on the ground stationary at the moment the gravity switched off, and didn't have springs or anything that would make it jump up more.

 

EDIT: Thinking about this on the bus ride home, I'm off by a factor of pi. Triple that half-meter rough number.

 

I'd have to look into the compressibility of water to see what the ocean might do. Water's pretty incompressible, but the release of gravity might make it expand vertically a bit, and when gravity turns back on that might give you tsunamis.

 

Austen's comment about tectonics ... I have no clue offhand how to guess at those effects. I look up, not down .

[egyptian]

Re: Gravity is gone?

 

It starting to sound like you could have more luck tampering with one of the other fundamental forces. Ten seconds without the strong force would probably eliminate all evidence of your terrible crime.

[Jaxom]

Re: Gravity is gone?

 

Ok, so I was gone but this sounds too fun. The tangential velocity sounds so fun but I think it and rigid motion loses. 10 seconds is only .04 degrees rotation or about 0.0007 radians. At that kind of angle, tan(theta) can be approximated by theta. Your verticle height is going to be roughly 0.0007 of your horizontal motion (correctly about 4.3 kms as mentioned before) or about 3 meters at the equator (ouch, but not OUCH). You'd also drift about 2 centimeters to the west as the earth fell away under you (rotational effects are so fun!).

 

You're going to get a lot more fun from expansive systems.

 

If you can up your device to 2-3 minutes you get more meaningful scales.

 

I still say that you can have more fun using real physics in a rotating space station. Coriolis effects are huge on that scale and can be oh so much fun...

 

"What do you mean I missed by 20 feet? I shot straight at him!" Oh, and all those poor jump-throughs.

[J.J.]

Re: Gravity is gone?

 

Keith "Favorite doomsday weapon: Ice-9" Curtis

 

Mayhap a relation!

 

http://www.newbabynews.net/hospitals/stf33/public/stf33birthannouncement.pl?babyID=h33-440

[BoneDaddy]

Re: Gravity is gone?

 

I guess that's why they call it the weak force. Turn off the EM force for ten seconds and significantly more catastrophic results occur.

[McCoy]

Re: Gravity is gone?

 

Blanking out gravity on Earth for ten seconds is a different matter, because lots of the stuff that we care about is small and can change radically in 10 seconds.

 

Anyone in a device that depends on dynamic balance of forces is going to be in a world of hurt. The airplanes and helicopters go completely out of control as gravity vanishes while the aerodynamic lift, drag, & thrust don't. Driving in any ground vehicle with a suspension seems likely to be unpleasant also, since everything (that doesn't use a drag chute or retrorockets) brakes via contact with the ground, and as someone said, the compression on the springs will relax, giving everything a gentle push upwards, losing the ground contact that lets you decelerate. If you run into something at 60 mph, you will hurt a lot whether or not you do it in zero g.

 

About those springs ... 1000 kg car, assume the springs compress 10 cm under its weight, if the compression energy in the springs turns into kinetic energy, then the car will be moving upward at about 1 meter/sec. SO after 10 seconds the car is 10 meters above the ground, and then gravity turns on ... y'know, all those Wile E. Coyote cartoons come to mind here.

 

10 seconds is enough time for Earth-rotation effects to be potentially nasty, though it's going to sound small (well, to a dingbat astronomer like me it sounds small). Something just rest on the surface will move in a straight line tangent to Earth's surface at the instant gravity vanished. At the equator, it moves at about 0.463 km/sec. Ten seconds later, when gravity turns back on, it will have traveled in its straight line to a point about half a meter above the point on Earth's surface it would be on if it'd stayed on the circular path it would follow if it had stayed glued to the surface. (That half meter is from radius of Earth = 6378 km * 10 seconds / 86400 seconds ... strictly speaking there should be a sine in there, but the angle is so small that the angle and its sine are close to each other.) The effect is smaller as you go away from the equator (I think multiply by cosine(latitude) so in temperate latitudes you have about 70% of that rough half-meter). This would happen to approx anything that was just sitting on the ground stationary at the moment the gravity switched off, and didn't have springs or anything that would make it jump up more.

 

EDIT: Thinking about this on the bus ride home, I'm off by a factor of pi. Triple that half-meter rough number.

 

I'd have to look into the compressibility of water to see what the ocean might do. Water's pretty incompressible, but the release of gravity might make it expand vertically a bit, and when gravity turns back on that might give you tsunamis.

 

Austen's comment about tectonics ... I have no clue offhand how to guess at those effects. I look up, not down .

"You must spread some Reputation around before giving it to Cancer again."

[Cancer]

Re: Gravity is gone?

 

It starting to sound like you could have more luck tampering with one of the other fundamental forces. Ten seconds without the strong force would probably eliminate all evidence of your terrible crime.

 

Gah. Ten seconds with no strong force and everything is gone in a dispersing-at-almost-lightspeed cloud (so it's ten light-seconds in radius, about 6 million kilometers in diameter) of photons, leptons, and bare protons and neutrons.

[austenandrews]

Re: Gravity is gone?

 

"Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light."

[Basil]

Re: Gravity is gone?

 

At the equator, with a radius of 6,378,000 meters (and ~4,004,7156 circumference), your tangential velocity is 463.8 m/s*. In 10 seconds an object will travel 4638 m tangentially.

 

This means the object will be 1.69 m above "ground level". The object will be circa .8 mm "behind" where it would've been if it hadn't left the ground.

 

All in all, pretty unimpressive. Though the effect of objects hitting the ground when that ground is, effectively, travelling faster than the objects could get interesting, at least when you consider the "degravitied" objects would have been slowed by air resistance.

 

 

 

*Important note: the air will be taking off tangentially, so solid objects will not "break the sound barrier".

[TaxiMan]

Re: Gravity is gone?

 

I think being 1.69m higher than normal could be a very big deal, if you imagine that the ocean is doing that. When gravity returns, there will be an enormous "slosh", bigger than anything we've experienced. Of course, the oceans might not rise that much at the equator, it's very complicated. What will fill the 'void' left by the uprising water? I don't know.

[keithcurtis]

Re: Gravity is gone?

 

What would happen to very tall skyscrapers? They are under a lot of compression. I think the biggest problems would arise from all those vehicles losing control. Every car becomes a missle. With that many crashes, firefighting companies will be ineffective, particularly since their trucks will be unusable (After they crashback into the ground, ruining suspensions if not breaking axles, and certainly popping tires). We could be seeing firestorms in major cities.

 

Keith "People below Niagara falls aren't going to be very happy either" Curtis

[Guest HeroPink!]

Re: Gravity is gone?

 

I say bugger the science, do whatever you think is fun, and will not over-extend your players "suspension of disbelief." If you want everything hurtling up & angled at 100 MPH, go for it!

[austenandrews]

Re: Gravity is gone?

 

I think being 1.69m higher than normal could be a very big deal' date=' if you imagine that the ocean is doing that. When gravity returns, there will be an enormous "slosh", bigger than anything we've experienced. Of course, the oceans might not rise that much at the equator, it's very complicated. What will fill the 'void' left by the uprising water? I don't know.[/quote']

Good point about the oceans. The 2004 tsunami was caused by a shift in the entire water column along a limited line of displacement. With all of the oceans displacing simultaneously... that would be extremely ugly.

 

I don't think there'd be a void at the bottom. Rather the gravity-fed water pressure would, for ten seconds, no longer be maintained. So the deeper waters would push outward. On the underside, it would quickly push into the "void" caused by its own displacement. On the top side, it would push upward on the more shallow waters. I expect the surface of the oceans would be displaced more than just the tangential movement. A pressure wave would fling upward. Come to think of it, the wave would push into the coasts at nearly the same rate. Good bye coastal cities, I'm thinking.