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Space Science

Mapping Gravity 194

Posted by michael from the slim-fast dept.
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).
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Mapping Gravity More

Mapping Gravity

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  • okaaaaaay (Score:1, Redundant)

    by man_ls ( 248470 )
    So does that mean that it's cheaper to ship things to India, since when they get there they weigh less?

    If I shipped a ton of something over there, does that mean that when it gets there, it's only .99 tons of a thing?

    Earth is cheating UPS/Fedex/whatever shipping agency out of their fees...

    • Re:okaaaaaay (Score:5, Funny)

      by Anonymous DWord ( 466154 ) on Thursday November 22, 2001 @01:31AM (#2599478) Homepage
      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!"

      • Re:okaaaaaay (Score:1, Insightful)

        by Anonymous Coward
        OK, so the standard kilo mass is in france. If you bring a mass there to match it up to the kilo mass, and it matches, you've got your own standard kilo. Now, take it to india, and use it to calibrate machines, including the airport scales... The same standard kilo is used back in the US. So, that won't actually work -- the bag will weigh less in India, but the kilo will too!
        • That's because Kilograms denote mass, not weight. So the kilos will stay the same, but pounds will change.
          • Guess what the common English unit for mass is.

            Pounds!

            If you knew what you were talking about, you..well, wouldn't talk. There is lbm (pounds mass) and lbf (pounds force). So, if the scale were calibrated with a known mass (whether a kilo or a lbm) *at* its location of use, the scale would correctly report mass. In pounds.

            So, the force of gravity doesn't matter if the scale is correctly calibrated.
            • I thought the English/Imperial unit of mass was the slug, not the pound.
              • > I thought the English/Imperial unit of mass was the slug, not the pound.

                The slug is a unit of mass, with the pound as the corresponding unit of force AND the pound is a unit of mass, with a poundal as the corresponding unit of force. Yes, the Imperial system does suck for science and engineering compared with the metric system.

                In practice the pound gets used for both force and mass without too much confusion, just as people talk about their weight in kilograms.

      • http://www.bipm.fr/enus/3_SI/base_units.html

        It looks very curious (to me) that the kilogram is the last unit which is still defined by a prototype; wouldn't it make sense to define it based on an intrinsic matter constant ? (like, 6.022e23 times the mass of a hydrogen atom in this and that state blah blah ?)
        • Same site [www.bipm.fr].
          The original idea of the king's commission (which included such notables as Lavoisier) was to create a unit of mass that would be known as the "grave". By definition it would be the mass of a litre of water at the ice point (i.e. essentially 1 kg). The definition was to be embodied in an artefact mass standard.
          • It's not after the word "kilogram" I'm after, nor about the choice which has been made to move from the MGS to the MKS systems.

            What I was wondering about, is the fact that at the beginning, all units were defined as being this or that physical property of given artifacts (g, kg, grave) or Earth (m, s), with the accuracy available at the time of the definition or revision.

            Since the mid-XXth century, there has been a trend towards removing the dependency on specific objects (like the metre yardstick) and using absolute physical constants instead (see the modern definitions for the second or the metre). What puzzles me is that we still have the 1898 definition for the kg, and a kilogram is still defined as a comparison towards a specific artifact stored in a specific vault (which, despite all its vaultiness, is still a nukable place).

            Is there a physical reason why this is still so, or is it just that no one has come up with a practical enough definition based on measurable constant properties of readily available entities ?
            • Well, one litre is 0.001 cubic meters, water is water and freezing point is freezing point. I'll leave the rest to somebody with too much time - or somebody with students ;-)
              • Yeeeeah, right. What about the atmospheric conditions ?

                Anyway, whatever the definition could have been in 1793 is irrelevant. Since 1898, the definition is "the same mass as this $ARTIFACT", not "the mass of that artifact you can reproduce by doing $FOO, in conditions $BAR". This is very different, isn't it ?
                • Yeeeeah, right. What about the atmospheric conditions ?

                  Why, at STP (Standard Temperature and Pressure), of course... That's 1 atm and 25C for you non-scientists...
                  • Please note that while I've already seen ice melt when left at 1atm and 25C, I'm "not exactly" sure that 25C is really the temperature of melting ice at 1013 mbar in an atmosphere composed at 80% of diazote and 20% dioxygen (ignoring the rare stuff).

                    There is definitely a discrepancy here. The 1793 definition didn't mention the STP, it didn't even mention pressure. The 1898 definition (now in force) compares to a single piece of matter which could easily be destroyed (while I wouldn't be very happy to see an ICBM land 10km from my birthplace, this is alas a possibility), with no backup "intrinsic physical" definition like the metre or the second.
    • "The contents of this package are shipped by weight, not volume. Some settling may have occured."

      They're consistently defrauding India. Honeycomb's big (yeah yeah yeah) but it's not quite AS big in India? Sue sue sue! :)

    • Re:okaaaaaay (Score:2, Informative)

      by Soko ( 17987 )
      Weight != mass, dude. 1 tonne of mass takes just as much energy to move 1 kilometer across the earths surface, whether it exerts 1 "tonne of force" or 0.9 "tonnes of force" due to gravity. Basic physics. UPS hasn't ripped you off (not like they did to this guy [slashdot.org], anyway). It's only when you go vertical that you have to counter gravity - and that's when weight becomes significant.

      Oh, and the SI unit of force is a Newton (N), which is a kilogram-meter per second squared (k-m/s&#178). One tonne (1000 kilograms) of mass would exert 9.8 KN (KiloNewtons) of force at mean gravity on the earth. Weight apprears to be the same mass since we use gravity to comapre masses, but they are not the same. As well, in the US and Imperial systems, 1 lb of mass exerts 1 lb of force - just to be confusing.

      That concludes tonight's lecture. (My Physics teacher would be so proud. *snif*)

      Soko
      • Ummm... All this proves is that you remember some basic physics, but obviously failed your reading and comprehension. What the poster was saying is basically correct (although he got it backwards). The mass will remain the same, but the weight of shipped items would be less in India, therefore it would be cheaper to post something from India (assuming the rates per kg are the same).

      • Re:okaaaaaay (Score:2, Informative)

        by Chep ( 25806 )
        >>As well, in the US and Imperial systems, 1 lb of mass exerts 1 lb of force - just to be confusing.
        <<

        Actually, in metric land (precisely, in the land of the metre), the kilogram-force (kgf) has been in widespread use, about a century ago. It was more or less equal to the gravity force exerced by earth on a piece of matter with a mass of 1 kg. It took several decades to get rid of that unit (you can still sometimes see indications like "max 2000 kgf" on cranes in old workshops).
        Nowadays, low-level mechanics are taught to use the decanewton (daN) as their primary unit of force (be it weight or any other force)... no wonder why !

      • Re:okaaaaaay (Score:2, Informative)

        by rabidcow ( 209019 )

        As well, in the US and Imperial systems, 1 lb of mass exerts 1 lb of force

        The pound is never a measure of mass, the "imperial" mass unit is the slug.

      • My teacher would have deducted points for incorrect units, you forgot the 'g' (kg*m/s)
      • yes, but a ton is 2000 lbs, which makes his comment relavent and yours not.
  • and I notice a picture towards the bottom. A guy is throwing a ball up into the air and the caption reads "Nasa's Michael Watkins: A new map every month." What does that picture have to do with anything?
    • Well, if you had read the article, instead of just looking at the pictures, you would have noticed the paragraph that states:

      "Every month during Grace's five-year expected lifetime, we will get a map of the Earth's gravitational field," says Michael Watkins of the American space agency's Jet Propulsion Laboratory.
    • A guy is throwing a ball up into the air and the caption reads "Nasa's Michael Watkins: A new map every month." What does that picture have to do with anything?

      He studies gravity, making gravity maps for NASA. Get it? Throwing a ball up, the ball comes down, forces at work.... Ringing any bells?

      I thought it was brilliant. One of the funniest publicity photos I've seen in a while, better than the dot-com ones.

  • Oh, it's not actually LAUNCHED yet (Score:4, Interesting)

    by Brento ( 26177 ) <brento@@@brentozar...com> on Thursday November 22, 2001 @01:33AM (#2599486) Homepage
    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 [utexas.edu] 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.

    • Re:Oh, it's not actually LAUNCHED yet (Score:1, Informative)

      by Anonymous Coward
      One satellite that was launched and is busy producing data, is CHAMP [gfz-potsdam.de]. It too is mapping the Earth's gravity field, by virtue of GPS tracking and a three-axes accelerometer. And it is sounding the atmosphere by GPS limb sounding, just like GRACE will do and METSAT has been doing some time ago already.

      For those not aware of the importance of GPS limb sounding, it is a method to determine indirectly the scale height and thus temperature around the tropopause, a useful indicator for global warming.

      Actually one thing nobody here has pointed out is that GRACE aims at studying changes in the gravity field, mostly due to movements of ocean water, ground water, ice, air etc. etc. (in short, the 'blue film' we see in space photographs of Earth!). The sensitivity of the GRACE mission, which will consist of a satellite pair tracking each other, to such changes is quite unbelievable.

  • More info and links (Score:4, Informative)

    by Boiling_point_ ( 443831 ) on Thursday November 22, 2001 @01:33AM (#2599487) Homepage
    This was Astronomy Picture of the Day [nasa.gov] last week.

    Plenty of depth/background available from there, as always!
  • Does this explain why I lost 2 centimetres after moving to Australia five years ago? Went to a medical the other day and the shrinkage was quite unexpected...

  • This is so COOL! (Score:4, Informative)

    by Freedryk ( 117435 ) on Thursday November 22, 2001 @01:35AM (#2599494)
    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! (Score:5, Informative)

      by ralmeida ( 106461 ) on Thursday November 22, 2001 @07:26AM (#2600012) Homepage
      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)
  • If one believes the theory of an asteroid destroying the dinosaurs, wouldn't we see a detectable gravity differential wherever such supposed asteroid hit? Anyone know if that particular theory about the dinosaurs has pointed the supposed impact in India?
    • Search google for "sandwell chicxulub cretaceous." It'll point you to a gravity map of the crater. I'd give you the link but slashcode keeps mangling it.

    • Re:Soo... (Score:3, Informative)

      by Dyolf Knip ( 165446 )
      As I recall, the debate is between an asteroid/comet impact in the Yucatan vs a violent and prolonged period of volcanic activity in India causing the mass extinctions 65 MYears ago. Both would produce huge amounts of dust and ash and lay waste to whole continents. Problem is, geology can't quite pin down which one caused it. Hell, it could be both that pushed them over the edge, though the timing for that would be rather amazing.
    • I don't know what differences an asteroid impact would leave, but I do know there is at least one theory for a mass extinction asteroid event on the Indian subcontinent. According to theory though this was a biggie, and the asteroid impact punched through the crust of the earth and released large quantities of magma from the mantle to fill in the crater. The main evidence for the theory anyway, is the observation of massive granite deposits (which forms from cooled lava).

      My memory is a bit fuzzy as to which time period and event they were trying to associate it to, but I think it was much before the extinction of the dinosaurs. I'm tempted to say they wanted to connect it with the mass extinction immediately preceeding the age of dinosaurs, but I'm not sure now. Anyway, usually people claim a Yucatan impact site as being the most likely location for the event that may have killed the dinosaurs.

    • Yucatan not India (Score:2, Informative)

      by ZigMonty ( 524212 )
      IIRC the asteroid theory only gained popularity when they found that huge son of a bitch hole in the Yucatan peninsula and managed to date it to 65 million years ago, which matched the time of the Cretaceous extinctions.

      It's huge. It's only hidden because it's under water. Check here [akamaitech.net] for pictures of said hole in the ground.

  • Well, I already knew this. (Score:5, Funny)

    by tlipcon ( 304220 ) on Thursday November 22, 2001 @01:42AM (#2599511)
    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... (Score:4, Insightful)

    by PRickard ( 16563 ) <[pr] [at] [ms-bc.com]> on Thursday November 22, 2001 @01:44AM (#2599513) Homepage
    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?
    • Keep in mind India's jolly neighbour, Pakistan.

      Relations between the two countries are tenuous at best. However, both sides are currently working towards some form of temporary ceasefire over Kashmir. The possibilty of the Indian government permitting foreign launch stations on their soil would be counterproductive, and therefore out of the question.

      • The possibilty of the Indian government permitting foreign launch stations on their soil would be counterproductive, and therefore out of the question.

        Huh? Probably they aren't going to let Pakistan launch there, but launch fees from the US, Japan, perhaps China and a few other space-faring nations could certainly provide a welcome source of revenue. How would it be counter-productive? If anything I would think the other nations would be reluctant since the Indians are likely to require inspection of the rocket and payload.

        • The revenue is irrelevant WRT to the *peace talks*.

          It would be counterproductive due to the high level of paranoia on both sides [in this example, Pakistan]. Are the installations well guarded? Does the host country have secret access to blast the opposition? And so on...

    • Re:Launches... (Score:1, Interesting)

      by Anonymous Coward
      Well, I know Ariane launches rockets from the equator because the force of the spin on the earth is greatest there. I wonder if this gravity map is corrected for actual and measured gravity?
    • OK, first let's ship everything we need to launch to India... then we've already spent any money we would have saved from that 1% difference in gravity :)

      -Toad

      • Re:Launches... (Score:1, Insightful)

        by mgv ( 198488 )
        Possibly not.

        Shipping things at ground level is alot cheaper than launching things.

        There are alot of issues in getting a maximum usable payload.

        But other things are probably more important, such as the rotational forces from being near the equator, etc.

        Michael

    • Re:Launches... (Score:3, Informative)

      by shaunak ( 304231 )
      "Should we expect to see more US companies building launch facilities in SE Asia after this report has been out a while?"

      Not the least bit likely. India has it's own launching agency, Indian Space Research Organisation [isro.org]. They currently launch satellites using Polar Satellite Launch Vehicles, and have actually launched a Geosynchronous Satellite using an indigenously designed Launch Vehicle. They're close to breakthrough on indigenous cryogenic engines for the GSLV which they had to develope from scratch because of the US of A feeling threatened by the transfer of technology from Russia. (The launch used Russian engines).

      The fact is, the Indian government is a launching agency, and getting permission to set up a private launch facility is not possible.

      Besides, ISRO are said to provide the cheapest (money wise) launches compaired to Airaine and NASA.
  • enough so that you weigh almost 1% less there

    Here's a little food for thought though:

    "Even a fat bastard on the moon still looks like a fat bastard"

    :)
  • ...but you'd weigh more when you got back! It's a proven fact that, among other things, the metabolism slows down in low-gravity environments.
    --
    erik

  • And what about... (Score:5, Informative)

    by rice_burners_suck ( 243660 ) on Thursday November 22, 2001 @02:14AM (#2599584)

    ...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... (Score:2, Informative)

      by mgv ( 198488 )
      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:

      Yes, yes, its all true. We all move through space time at a constant speed (Except when I'm waking up in the morning).

      But these effects are relavistic. You have to be travelling at near light speed to have an appreciable effect on mass or gravity. They do apply to high speed subatomic particles, but these are pretty few and far between on the planet in terms of total mass.

      I think that the variations in gravity relate to variations in mass density of the earth more than anything else.

      Michael
    • 3 things:

      The effect of velocity on perception of elapsed time is not linear as far as i know. (I could be wrong)

      Time is not measured with distance units, so moving through time "at the speed of light" is meaningless. (actually, "moving through time" at all is pretty meaningless, unless you have another time axis to measure against)

      What does this have to do with the lumpiness of the earth or variations in gravity along its surface? Force (say, gravity) does not require motion.

      • Re:And what about... (Score:4, Interesting)

        by Dyolf Knip ( 165446 ) on Thursday November 22, 2001 @03:51AM (#2599730) Homepage
        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.

        • 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.

          No, Special Relativity says that anything moving at a speed less than c (speed of light) can never reach a speed of c or greater. Anything moving at a speed greater than c can never reach c or less. And finally, anything moving at exactly c (eg photons) can never move at any other speed than c.

          To be precise, "moving" refers to movements which could be used for information transmission. It is easy to devise experiments in which something "moves" at arbitrary speeds but cannot be used for information transmission: Think of a light spot moving over the surface of the moon produced by a fast rotating laser on earth.

          • I think the idea is that if you can find a phenomenon which converts ordinary matter into tachyon matter, you wind up going faster than the speed of light without ever having traveled _at_ the speed of light (which is excluded for particles having rest mass).

            What I've always found interesting is that according to relativity, going backwards in time is equivalent to traveling faster than light, but according to quantum mechanics, going backwards in time is equivalent to being made of antimatter. Weird, eh?
          • Interesting. I have a question though.. If a photon can never go faster or slower than c, then how do they "slow down" when pumped through a medium other than a vacuum? The only thing I could think of was that the photons would have to bounce around, thus travelling a longer distance but still going the same speed. That seems highly implausible though. This has always confused me. Someone once tried to explain it in terms of phonons and hand-waving, but I didn't really buy it (even though most of it was over my head ;). Any insights? Thanks.

      • Time is not measured with distance units, so moving through time "at the speed of light" is meaningless.

        You can make it have distance units by multiplying it with c (the speed of light). This is possible because c is a fundamental constant, ie it is the same in every reference frame.

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

        In non-relativistic mechanics, you describe movements by stating your position in space as a function of time. In relativistic mechanics, you describe movements by stating your position in space and time as a function of some new parameter, which has to be independent of the movement. It is in fact possible to come up with such a parameter. One popular choice is called the proper time. So, yes, in a sense you measure movement through time against "another time axis".

    • ...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.


      The problem is, and of course the word 'relativity' is supposed to clue you in to this, is that the Earth is also moving away from you at near light speeds. So, 100 years might elapse for you while on Earth they only perceive 10 minutes.


      Tricky shit.

      • This is generally known as the Twin's Paradox, and is a popular question in elementary special relativity classes. At first, it would seem to be self-contradictory. After all, if 1 of a pair of twins goes off on the rocket and the other one stays at home then there must be a definite answer as to which one is younger when they meet back up again - it can't be dependent on whose perspective you take, since at this point their perspectives are identical.

        The key is that Special Relativity is only valid in inertial (ie. non-accelerating) frames. We can consider the Earth to be in an approximately inertial frame, so special relativity is valid. However, whilst the rocket spends most of it's time in an inertial frame, there are 3 points at which it is most definitely not: when it's leaving the Earth (and accelerating up to high speed), when it's turning round, and when it's slowing down to a halt back on Earth. At these points special relativity no longer applies, and we must resort to general relativity.

        Hence the symmetry between the 2 views is broken, and our solution becomes clear. The Earth is the only place where special relativity applies for the duration of the journey, and since from it's point of view time in the rocket must pass more slowly, then the rocket twin must be younger.

        The calculations in general relativity are pretty horrible, if I remember correctly, but if you work through them it turns out (as it should) that the corrections on the 3 GR legs of the voyage are exactly what is required to ensure consistency between the two perspectives.

        Finally, for any sceptics out there, all this has been experimentally proven with a pair of atomic clocks, one on the ground and one orbiting in a satellite.

      • Re:Except.. (Score:3, Insightful)

        by smaughster ( 227985 )
        No, what you are referring to is something called the twin paradox, but it is not a paradox since the cases aren't exactly the same. Why? If you move at high speed away from earth and return, then at a certain point in time, energy had to be used to make you change your velocity and head back to earth. Either you used the brakes, turned and accelerated again, or a giant with a large bat gave you a smack, but whatever way you look at it: work is done to make the change happen. This is not the case with the earth. So the earth might appear to move away at near light speeds, but time will not pass slower on earth with respect to your frame :)

    • Re:And what about... (Score:5, Informative)

      by dragons_flight ( 515217 ) on Thursday November 22, 2001 @05:12AM (#2599866) Homepage
      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.

      • There is NO UNIVERSAL FRAME OF REFERENCE.

        The frame of reference in which the microwave background radiation of the universe is stationary.

    • Actually, if you travel close enough to to speed of light to make Lorentz contractions an issue (time dialation), then time would slow down for you.

      (Time in new coord) = (gamma)*(time - (distance*velocity)/c^2)

      Where Gamma is 1/sqrt(1-velocity^2/c^2)

      However, you could not travel at 99% the speed of light for *your* 10 years, then return, expecting to be younger than everyone else.

      Once you start heading back, you reverse your velocity, and all time effects reverse. You would be the same age as everyone else... it's called the Twins Paradox.

  • Strong man. (Score:3, Funny)

    by thetechweenie ( 60363 ) <jsatrape@gmail . c om> on Thursday November 22, 2001 @02:23AM (#2599604) Homepage
    Is that why those people stand on their heads over there? You see all of these Hindu guys doing handstands for days and the like... I knew there was I reason I can't do a handstand. Damn that unfair gravity!
  • Thats how they faked the moon landing. They just went over to India and set up shop. They didnt need any wires pulling up the astronauts up, its was all 'real'. btw, I guess all the stuff i learned in physics 101 needs to be recalculated since the gravity isnt 9.81 anymore.

  • Widespread applications (Score:4, Interesting)

    by crisco ( 4669 ) on Thursday November 22, 2001 @02:38AM (#2599633) Homepage
    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 (Score:5, Interesting)

    by Simm0 ( 236060 ) on Thursday November 22, 2001 @02:38AM (#2599634) Homepage
    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.
    • Why do you say that 9.8 m/s^2 is an inaccurate figure? Granted, it may be 9.77 m/s^2 on top of Mt. Everest, but that's a difference of .03, which is less than 1% error.
    • Hmm. Your calculations assume that the mass of the Earth is a point mass at the centre - or that you're hovering about 9km above a perfectly spherical earth.

      In fact, of course, the whole of Nepal/Tibet is several km above sea level, so you're standing with more mass 'underneath' you than at any other point on the globe - implying there should be more gravitational pull on you in Nepal than e.g. below sea level in the Netherlands.

      So if you're on the top of Everest, the gravitational pull will be less, but probably not as little as you calculate.

      Incidentally, you've only got about 1/3rd of an atmosphere air pressure pushing down on you, does this also affect your overall preceived weight?

  • Gravity increasing over time due to space dust (Score:4, Interesting)

    by Dag Maggot ( 139855 ) on Thursday November 22, 2001 @02:47AM (#2599650) Homepage
    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 (Score:4, Insightful)

      by Dyolf Knip ( 165446 ) on Thursday November 22, 2001 @04:00AM (#2599744) Homepage
      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.
    • Maybe in the time of dinosaurs the earth actually had lighter gravity.

      If I am recalling this information correctly, the Earth also had shorter days in the time of the dinosaurs (about 18 hours). (I think that a lot of this extra energy was spent in putting the moon into a higher orbit, which is a consequence of tidal forces.) This means that the Earth was spinning faster making things weigh a little less because of centrifugal force (which, as we all know, isn't actually a force of its own since it is only inertia).
    • All very neat, but I was just down at the Grand Canyon and brought back a lot of books and posters and a stuffed toy coyote, so deem myself an expert on the subject at hand. Seriously, from the top of the South Rim, near the village to the Colorado River below are many layers of various limestones and an unaccounted for "great unconformity" (980 million years of missing deposit, possibly due to an earlier erosion?) down to Vishnu Schist and Zoroaster Granite (~1.8 billion years ago.) Total drop in elevation about 5,000 feet. So, assume the great unconformity accounts for another 5,000 feet and you've only increased the diameter of the earth (~25,000 miles) by 4 miles. Granted this is all speculative, but the material which makes up these layers came from somewhere, possibly the aformentioned space dust which fed life, was processed by life, or just filled in gaps between life forms, 5,000 feet of rock came from somewhere.
  • ...when you think about it. But that's another topic. You want a demonstration of force, try the weak nuclear force. When you drop a ball of off a building, it accelerates (~9.8M/s/s) but when it encounters a weak nuclear force (the atoms in the 'ground' where it 'hits') it effectively 'stops'

    In other words, it's not the fall that kills you, it's that sudden stop at the end ;)

    Gravity smavity... let's investigate something interesting

    (in all fairness, my buddy's father is a nuclear scientist who holds the current best measurement for Big G, but I still can't believe it's a 'force' per se)
    • Use the term "Sudden Deceleration Trauma" to describe someone who fell off a building and you'll get dirty looks. I know, I've tried. Using the phrase, I mean, not jumping off a building.

      Oh, and it's electromagnetism that binds a rubber ball together and keeps it from merging with the ground, not the weak nuclear force. That one's responsible for atomic decay. It and the strong force have very little direct influence outside the nucleus.
    • I'd crashed on my bike (again - don't ask), and was being checked by a nurse. A kid is lying in a bed a few meters away, with both arms and legs in casts. He had a conversation with a passing nurse. It went something like this:

      Her: "What happened to you?"
      Him: "I fell out a window."
      Her: "How far up were you?"
      Him: "2nd floor."
      Her: "It must have hurt a lot falling out the window."
      Him: "No - didn't hurt at all."
      Her: "Oh come now. You've broken your arms and legs. It must've hurt a lot."
      Him: "No, it didn't hurt falling out the windows. The landing was a bit tricky though."

      I couldn't help but laughing out really loud, cause the kid couldn't have been more than 10 or 11, and he showed both a very good sence of humour and a fairly precise knowledge of how to use the language.

      The nurses of course couldn't see the humour in his joke, and didn't think I was very polite by laughing at the kid. Grown-ups.

      When I grow up, I want to be a child.
  • I was in India once. It was hot and humid. Shouldn't it have been cooler, given the reduced energy requirement in moving from point to point?

    Quoth Dr. Whitehouse: [...] should add a new dimension to our understanding of our planet.

    Aha! I knew there was a catch.

  • earth doesnt have any gravity (Score:1, Informative)

    by Anonymous Coward
    gravity
    n 1: the force of attraction between all masses in the universe;
    especially the attraction of the earth's mass for bodies
    near its surface; "gravitation cannot be held
    responsible for people falling in love"--Albert Einstein
    [syn: {gravitation}, {gravitational attraction}, {gravitational
    force}]
  • Well that explains the bed of nails.

  • Stupid yet again US-centric news site. (Score:1)

    by Anonymous Coward
    "Gravity is less strong in India". Less strong than where? I know it's too hard to become less US-centric, but you could at least mention "less strong than in the States".
  • How does the military deal with changes in the force of gravity due to altitude and location? A 1% change in gravity is a big deal if you are firing an artillery shell at a target over a long distance. I was watching a documentary on the ENIAC computer and it said the computer's primary task was to calculate gunnery tables for the military. Wouldn't all of those carefully calculated tables be useless if the force of gravity changed?
    • How does the military deal with changes in the force of gravity due to altitude and location? A 1% change in gravity is a big deal if you are firing an artillery shell at a target over a long distance. I was watching a documentary on the ENIAC computer and it said the computer's primary task was to calculate gunnery tables for the military. Wouldn't all of those carefully calculated tables be useless if the force of gravity changed

      The change in gravity would be rather small over the fairly short distances used by artillery, so the error from firing from one gravitational level to another would be almost non-existant. The real worry would be firing in an area of high gravity vs low gravity, and my guess is that the error produced by differences in air density, wind direction temperature etc.. Although the tables may be very accurate, there are things that are impossible to measure, A change in air temperature (and therefore density) would affect the course of a shell. An undetected difference in wind direction above the ship would affect the shell. Even a difference in the temperature of the powder would make a difference in the explosive force. Also When the ENIAC tables were in use, there was also no computer control to compensate for the rocking of the ship on the water. The tables would be to get you very close, but not always dead on.

      From everything I've seen, the military expects minor inaccuracies, and corrects by firing, correcting and firing again. I don't know about ships, but for ground based artillery the standard was to have people watch, tell them by how far they had missed, then fire again. Good response was to hit about a 5 foot area by the third shot.
  • A new gravity map of the Earth suggests that if you want to lose weight you should go to India, where the pull of gravity is slightly less than it is elsewhere on the planet.

    Since you weight less, wouldn't you be expending less energy when you move, and therefore get less excercise, and therefore get fatter?

    • That explains why cows in India are worshipped, instead of eaten. They don't have enough gravity to burn off the calories of beef.

      -

  • Aha! (Score:2, Funny)

    by cygnusx ( 193092 )
    ...Gravity is less strong in India...

    So *this* explains the Indian Rope Trick! [skepdic.com]
    :-)
  • Your country just sucks more than their country does.

    Sorry. Couldn't resist...

  • Well, that finally explains the magic carpet thing...

  • Anyone ever heard of the theory that gravity was a push and not a pull?

    Maybe india is just less pushy then the rest of
    the planet.

  • Arthur C. Clarke: "The View fro Serendip" (Score:4, Interesting)

    by Mad Man ( 166674 ) on Thursday November 22, 2001 @11:19AM (#2600403)
    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).

  • What about tides? (Score:2, Interesting)

    by jbuhler ( 489 )
    There's quite a large bulge of ocean that trails the moon around the earth, and a similar bulge diametrically opposite to it. There's a smaller bulge due to sun tides.

    The GRACE home page doesn't seem to mention the effects of tides. Doesn't all that moving mass of H2O change the planet's mass distribution enough to mess with gravimetric readings?

    (Disclaimer: I am not an earth scientist.)
  • There are few pseudo-sciences as well entrenched as Astrology. Every once in a while somebody tries to rationalize the effects of Astrology as an actual gravitational effect of the planetary alignments that has a slight but important effect on world affairs and on individual people's destiny. The problem with this is, that there are so many other variations in the Earth's gravitational field that no such effect could get through the background noise. As a geophysicist, I've used measurements of the variations in the local gravitational field to model underground structures, ranging in size from the Rio Grande Rift in New Mexico, to small landfills and service tunnels on the campus of UT Dallas. We never correct for planetary gravity. In fact, when doing gravity measurements in the field, you have to make sure to park the truck a few yards away from where you take your measurement, because an SUV has enough mass to mess up your reading. The mass of Mars, or even Jupiter is very large, but so far away that the SUV a few feet away has several orders of magnitude more influence.

    Astrology doesn't work through any physical medium.

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