Mapping Gravity

overThruster writes: "No, you don't need to drink the water... Gravity is less strong in India--enough so that you weigh almost 1% less there. See BBC story about NASA's gravity map." Here's another story about the mission, and the GRACE home page (or NASA's less-informative page).

Comments

Re:okaaaaaay

[Anonymous DWord]

No, then it would be cheaper to ship things FROM there, since you get more than a ton per ton. And you could get on the plane with 70 lbs. of stuff, and when you arrive in (wherever) laugh uproariously at the ticket agent, dancing around and saying "ha HA! I have 71 pounds in my bag!"

Oh, it's not actually LAUNCHED yet

[Brento]

I was poking around in all of the sites for a few minutes before I found out that the satellites haven't been launched, and aren't scheduled to go up until Feb 2002. The BBC says it's going to be just a few weeks, but the official site says 97 days.

Interesting note from their site: A secondary experiment that GRACE will perform is to examine how the atmosphere affects signals from the Global Possioning Satellites (GPS). Ahhh, another Slashdot hotbutton! This project just keeps looking better and better the more you check it out.

More info and links

[Boiling_point_]

This was Astronomy Picture of the Day last week.

Plenty of depth/background available from there, as always!

This is so COOL!

[Freedryk]

Mapping the geoid is one of the most fundamental problems in oceanography. Ocean currents are all basically caused by water running downhill. The problem is that "downhill" in this case is relative to the geoid, which is a bumpy, not-nice surface. With this kind of map, we should be able to map surface currents from space; their velocity, their position, everything you want to know about how the surface currents are moving. This is important for climate studies of global warming, since the ocean currents are one of the main transporters of heat from the equator to the poles. This will allow us to get a much better idea of where the heat in the world is going, and how long it takes to get there, which in turn will give us a better handle on global warming.
Oceanographers have been trying to figure out a way to remove the geoid from their equations for a hundred years. Now we can just measure the damn thing. Crazy.

Re:This is so COOL!

[ralmeida]

Actually, when you have a slope in the ocean surface the water doesn't run downhill; it runs across the slope. If you have a "seamount", for example, water will circle it clockwise in the northern hemisphere.

Most of the large scale circulation is the result of the subtropical wind circulation, and small anomalies in the geoid will be insignificant. Also, part of the ocean circulation has a thermohaline nature, and is forced by the distribution of salt and temperature across the world.

(Yes, I'm an oceanographer)

Well, I already knew this.

[tlipcon]

Hell, in my physics classroom it's about 30% as strong as anywhere else. I proved it myself in a lab last week- it's about 3.2 m/s^2 in our corner of the room!

Strangely enough, it's just about 9.8 up front. I guess the earth is pretty aspherical.

-Toad

Launches...

[PRickard]

So if things weigh less in India, wouldn't launching rockets and shuttles from there be easier? A 500,000-pound rocket would only weigh 495,000 in India - not a huge savings overall, but you could reduce fuel consumption and save money or go a bit further on the same amount of fuel. And the location is about as far south as Florida, so that's enough planetary curve for them. Should we expect to see more US companies building launch facilities in SE Asia after this report has been out a while?

And what about...

[rice_burners_suck]

...the fact that moving at speeds approaching the speed of light will cause you to move faster through time, so that if you left Earth, travelled at near light speeds, and then came back shortly afterwards, 100 years might have elapsed on Earth in what you perceived as about 10 minutes.

I think that physical laws like this have a very significant effect on the lumpiness of the Earth, and therefore, on the variations in gravitational pull.

Imagine that you're running down a square field, from one side to the side parallel to it, and it takes you 10 minutes to run across this field. Ok, now imagine that you're running across the same field, but instead of running "straight," you're running at an angle, so that you're not perpendicular to the edges of the field that you're running from and to. It will take you a bit longer to get to the other side of the field, even though you're running at the same speed, because by going at an angle, you've increased the distance you have to go to get from one edge to the other.

Now suppose we call the field a 2-dimensional surface, like a piece of paper. You could say that the first time you ran across the field, you travelled along one axis, or dimension--let's say the X axis. But on the way back, you ran at an angle, which means that you've gone along two axes, the X and Y axes. But you went the same speed. This means that you have split the same speed across two dimensions.

We say that time is a fourth dimension. Now picture this: No matter what's happening, you're ALWAYS moving through the 4 axes (the three "space" dimensions and the one "time" dimension) at exactly the speed of light. It's just that you're splitting that speed (the speed of light) across some combination of the 4 dimensions. You're doing one of the following:

• Standing perfectly still in the 3 space dimensions and moving only through time. (I know that motion is relative, but imagine for a moment that your motion is relative to the universe itself and that you can guarentee that you're really not moving through space at all but only through time). Therefore, you're moving through time at the speed of light.
• You're moving through space and time, which means you're splitting your motion across at least one of the space dimensions and the fourth time dimension, which means that you're moving somewhat more slowly through time. If you're going through space really really fast, whatever speed is left over for time will be much smaller. So if you're moving through space at speeds approaching the speed of light, what might be 10 minutes for you might be a much longer time for everybody else. Because you're moving through time much more slowly, since you're using up all that speed in the other dimensions.
• You're only moving through space itself and are therefore not moving through time at all. Photons, which are light particles, do this. Since they're light, they move through space at the speed of light. (Yeah, that makes sense, right?) This means that there is NO speed left over for moving through time. As a result, if a photon travels in a straight line, it is EVERYWHERE along that line at the same time. We think it takes 8 minutes for a photon leaving the sun to arrive at Earth, because we're the outside world. For the photon, the trip was instantaneous, but for us, it took 8 minutes. Just like if you're travelling through space really really fast (almost the speed of light), you'll think it was 10 minutes but for us it was 100 years.

I think all of these physical laws have a very significant effect on the lumpiness of the Earth, and therefore, on the variations in gravitational pull.

And, of course, the obligatory OH WELL.

Re:And what about...

[Dyolf Knip]

The effect of velocity on perception of elapsed time is not linear as far as i know

Correct. As I recall, you have to ramp up to .85c just to age half as slowly (or mass twice as much or be half as long). The equation is pretty simple; I don't happen to remember it at the moment and am to lazy to Google it.

actually, "moving through time" at all is pretty meaningless, unless you have another time axis to measure against

Why? If I'm moving at all (though the effects only become noticable relativisticly), I'm 'moving through time' at a different rate than someone in an different inertial frame. You don't need a y and z axis to describe differences in motion along x. I get headaches thinking about 4 dimensional geometry.

so moving through time "at the speed of light" is meaningless

Very true. If you move at the speed of light, your perception of the passage of time drops to zero and the life of the universe passes by you in no time. Literally. But since accelerating a body to that speed would require an infinite amount of energy (which I had once, but misplaced), it's not something I feel I need to worry about.

I've always been fascinated by the potential loophole here. You can go slower than light (everything we see) or you can go faster (tachyons?). The only thing actually forbidden is attaining that exact velocity. So figure out a way to jump from one speed to another without going through the intervening velocities (an easy task, right?) and you're golden.

Re:And what about...

[dragons_flight]

You make some elementary mistakes, but I'm only going to deal with two of them.

First off relativity has nothing to do with variations in the earth gravitational field. This is entirely related to the fact that the mass density of the materials making up the earth are not uniformly distributed. Some rocks are denser than others, and moisture and magma move around. Relativistic mass scales as 1/Sqrt[1-v^2/c^2], where v is an objects velocity and c is is the speed of light. Thus for a 1% increase in mass you would have to identify objects moving at > 14% of c as measured by a stationary observer on the Earth's surface. Besides which this deals with inertial mass (F=ma), but gravitational fields (F=G*m1*m2/r^2) are more complicated in a relativistic framework.

Standing perfectly still in the 3 space dimensions and moving only through time. (I know that motion is relative, but imagine for a moment that your motion is relative to the universe itself and that you can guarentee that you're really not moving through space at all but only through time). Therefore, you're moving through time at the speed of light.

There is NO UNIVERSAL FRAME OF REFERENCE. When not accelerating everyone experiences time as moving at the same constant rate, and ALL are equally justified in saying they are moving solely in the time direction. One person observering another having a nonzero relatively velocity will interpret their motion as having decreased temporal component and appropriately increased spatial component(s). Sometimes it is useful for someone to interpret their own motion in terms of another person's perspective (such as saying the car is moving along the ground as opposed to saying the ground is moving under me), but this makes no difference to the objective or subjective experience.

Widespread applications

[crisco]

One way this is used is in high precision GPS land surveys. Since the GPS satellites orbit the center of earth's mass, the basic measurements don't reflect these changes in the earth's gravity field. But the traditional instruments used in surveying that were used to build most everything out there right now do reflect these variations. So they use something called a Geoid Model, a mathematical model that approximates the undulations in the gravity field. Previous geoid models were done with pretty sparse datapoints, leaving various small error and lots of confusion. With this, GPS will be even more useful for the land surveyor and related professoins.

Big deal, you say? Think of the existing physical infrastructure in a city. Now think of a new development that has to tie into the existing water, sewer, storm drainage and roadway systems. If you use GPS and don't take these things into account, you're going to take a chance on sewers that don't drain, storm drainage forming lakes and a general mess (not to mention lawsuits).

Not the typical /. fare but great stuff for those that measure land, play with math and lots of other physical sciences.

Physics of it all

[Simm0]

You probably hear the 9.8 m/s^2 acceleration due to gravity touted but this is just the net affect across the whole of the globe which is actually very inaccurate when used at specific locations.

Did you know that its actually easier to break the force of gravity ontop of mount everest. I'll show it using the formula:

g = G*(m/r^2)
= ((6.67*10^-11)*(5.98*10^24))/(6.389*10^6)
= 9.77 m/s^2

The value of g also can vary locally on the surface because of the presence of irregularities and rocks of different densities. Such variations in g also known as 'gravity anomilies'. Mineral deposits, for example, have a greater density than surrounding material; because of the greater mass in a given volume g can have a greater value on top of such a deposit then at its sides.

Overall altitude, underground minerals and distance from the equator all play apart in changing the acceleration due to gravity across the globe.

Gravity increasing over time due to space dust

[Dag Maggot]

Is it possible that gravity can increase over the lifespan of a planet? I read recently
that 50,000 tons of space dust fall on the earth every day.

Maybe in the time of dinosaurs the earth actually had lighter gravity. Let's see-
50,000 tons of dust X 50 million years = 2,500,000,000,000 (that's 2 trillion tons of dust) that would be enough to effect gravity wouldn't it.

I'm sure my math is off, and that the earth must also lose a fair amount of matter via outgassing etc- But it would explain why such impossible beasts like the brontosaurus were
able to stand under their own weight.

Re:Gravity increasing over time due to space dust

[Dyolf Knip]

2.5 Tera-tons might seem like a lot to you and me, but it's still less than a millionth of Earth's total mass. Assuming that it remains constant at that rate and losing none of the gains to outgassing (or it's offset by periodic large impacts), to accumulate a 1% increase in mass would take a half trillion years. Don't hold your breath.

Arthur C. Clarke: "The View fro Serendip"

[Mad Man]

Back in 1978, Arthur C. Clarke ended his book The View from Serendip by writing about a gravitational anomaly which was found off the coast of Sri Lanka (formerly Ceylon) -- the small island near India where he lives.

I am able to visit my favorite spot (Chapter 13) for only a few days a year. But now, quite unexpectedly -- and literally since I wrote the preceding paragraph! -- Serendipity has struck again. While researching a totally different subject, I've discovered a good reason for spending more time on the south coast.

It concerns the greak Sanskrit epic, the Ramayana. In this 2,200-year-old poem, the demon-king Ravanna kidnaps Sita, wife of Rama, and takes her to his island stronghold of Ceylon. Needless to say, she is ultimately released, after aerial battles involving what look suspiciously like atomic weapons and laser beams.

To heal the wounded, the heroic monkey-general Hanuman is later sent back to India to fetch a medicinal herb found only in the Himalayas. Unfortunately, when he gets to the right mountain he is unable to identify the herb. No problem; he brings the whole mountain back! However, one piece drops off, on the southern tip of Ceylon. The locals believe this fragment is in fact my favourite bay, for its name in Sinhalese means "there it fell down" (onna watuna).

There it fell down. Place names usually have a meaning, though it is often lost in the mists of time. Did something really fall down, centuries or millennia ago, at Unawatuna Bay? A meteorite would be the obvious explanation; it must have been a big one for the legend to have lasted down the ages.

And here's another weird coincidence. Little Unawatuna, believe it or not, is the closest point on dry land to the world's greatest gravitational anomaly, a few hundred kilometres out in the Indian Ocean. On the Goddard Space Flight Center's 3-D map of the Earth's Gravimetric Geoid, that strange phenomenon looks liek a deep pit [1] into which the whole island of Sri Lanka is about to slide.

Let's put two and two together. A few thousand years ago, a huge object of peculiar density plunged into the Indian Ocean, creating a tradition that is remembered to this day. And it's still there, distorting the earth's gravitational field -- Terran Gravitational Anomaly I.

That might make an opening for a pretty good science-fiction movie . . . and an even better ending for this book.

Ayu Bowan.

1. One hundred and ten metres below zero reference on the Goddard model (March & Vincent, 1974).