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Science

The Speed Of Gravity Revealed 935

Posted by timothy
from the blink-and-you'll-miss-it dept.
redwolfoz writes "New Scientist is reporting that the speed of gravity has been measured for the first time. 'The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.' Researchers made the measurement of the fundamental physical constant with the help of the planet Jupiter. One important consequence of the result is that it will help constrain the number of possible dimensions in the Universe."
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The Speed Of Gravity Revealed

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  • Wow. (Score:5, Funny)

    by Skyshadow (508) on Tuesday January 07, 2003 @08:29PM (#5036688) Homepage
    Wow, that's pretty cool. Now if we could only figure out why and how gravity works, we'd be in business.
    • Re:Wow. (Score:5, Funny)

      by 56 (527333) on Tuesday January 07, 2003 @08:32PM (#5036704)
      Gravity works because we believe in it. Just stop believing in it and it will stop working.

      Not working? You must not be trying hard enough.

    • Re:Wow. (Score:5, Informative)

      by GuyMannDude (574364) on Tuesday January 07, 2003 @08:47PM (#5036811) Journal

      Yeah, that's the real trick. For those who aren't aware, getting gravity to "play nice" with both general relativity and quantum mechanics is pretty tough. Relativity models gravity is a warping of space. But coming up with a quantum theory of gravity is mighty difficult. There are theories that gravity acts through particles (the so-called gravitons you always hear about on ST:TNG) but I don't believe this has been proven yet.

      GMD

      • Re:Wow. (Score:5, Informative)

        by PurpleFloyd (149812) <zeno20NO@SPAMattbi.com> on Tuesday January 07, 2003 @10:14PM (#5037401) Homepage
        Einstein's general relativity actually predicts the existence of gravity waves and gravitons (really the same thing, viewed two different ways). Trying to find gravity waves is one of the biggest scientific challenges of our time.

        It's accomplished via huge (4 ft. diameter, 2.5 mi. length) tubes in an L-shape. A laser is then bounced along the length of the tube, and measures its distance very accurately: to within 10^-16 (!) cm, or about one hundred millionth the diameter of a hydrogen atom. Any change in the distance is a possible indication of a gravity wave passing through from some distant, powerful source. The fact that gravity decreases exponentially with distance means that even gravitational waves from extremely powerful sources, like binary neutron-star systems, are very weak when they get to Earth.

        Of course, other vibrations can screw this up, so these observatories are really isolated (both geographically and mechanically) and data is compared from around the world. Lots of information is available at the LIGO [caltech.edu] (Laser Interferometer Gravitational-Wave Observatory) website, where I got most of the specs listed here.

        • Re:Wow. (Score:5, Informative)

          by kavau (554682) on Wednesday January 08, 2003 @12:40AM (#5038055) Homepage
          The fact that gravity decreases exponentially with distance means that even gravitational waves from extremely powerful sources, like binary neutron-star systems, are very weak when they get to Earth.

          Gravity is a long-distance force that decreases as inverse distance squared. This is Newton's famous 1/r^2 law, and it remains unaltered by the theory of general relativity (after all, Newton's laws are just a limiting case of General Relativity.)

          With a short-range gravitational force, decaying exponentially with distance, stable planetary orbits and galaxies, with their literally astronomical extent, could not exist.

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

          by FlemLion (572837)

          To increase the sensitivity ESA [esa.int] is building a flotilla of space craft that together form a network to measure gravitional waves.

          The mission is called LISA [esa.int] and will be supported by a pre-cursor mission SMART-2 [esa.int] to develop the necessary measuring and flotilla operations.

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

          by Alsee (515537)
          The fact that gravity decreases exponentially with distance

          They decrease with the square of the distance, not exponentially.

          very weak when they get to Earth.

          LIGO is hoping to detect gravity waves at a range of over 3000 light years. Even at that range the energy is 300 MILLION WATTS PER SQUARE METER. (About 28 million watts per square foot.)

          So while the measurable effects are tiny the energy is enormous.

          -
      • by hayden (9724) on Wednesday January 08, 2003 @12:16AM (#5037974)
        I think this quote sums it up pretty well.

        "Remember the Unified Field Theory? Well, forget it. Physicists have pretty much thrown in the towel on unifying gravity with the other elemental forces, so now we have the Standard Model, which says that everything works together in intricate harmony except gravity, which is on holiday in Tasmania and need not concern us further."
        - Jon Carroll on the Higgs Boson

      • Re:Wow. (Score:5, Interesting)

        by JebusIsLord (566856) on Wednesday January 08, 2003 @01:48AM (#5038276) Homepage
        I have a question:

        If they have confirmed that gravity travels at the speed of light, how does gravity escape a black hole? obviously it does because the only energy that escapes a black hole is in the form of gravitational waves, but if the escape velocity is higher than than the speed of light, how can it get out?
    • Re:Wow. (Score:3, Informative)

      by silvaran (214334)
      Stephen Hawking's A Brief History of Time describes it as a series of virtual particles emitted by all matter. They're spin 1 particles (I think - whatever that means). Something about how many times you have to rotate the particle to get back to the same perspective as when you started. Like spin 2 particles, you turn them around 1/2 revolution and you're back to where you started. Spin 1 particles need to be rotated all the way around once. Spin 0 particles look the same in every direction. Spin 1/2 particles (which matter is made up of) have to be rotated twice to get back to the same position. They're called virtual particles because they can't be directly seen.

      Anyways, since gravity is made up of particles, it follows the same rules as all other particles, and thus can't go any faster than the speed of light.
    • by angry old man (211217) on Tuesday January 07, 2003 @09:07PM (#5036966)
      You kids and all your fancy-schmancy gravity experiments. Back in my day, we showed a little more respect for our elders... and for gravity because it tells you up's up and down's down. Once you start foolin' around measuring this gravity speeds this and that gravitron's that and the next thing you know you'll be flung up into the air and there won't be a down to go to. Bagh. That will serve you right for messing around with things that you should have a little respect for.

      I had to walk up hill to school both ways and because of gravity, I liked it!

    • Re:Wow. (Score:5, Interesting)

      by refactored (260886) <cyent@NoSpAM.xnet.co.nz> on Tuesday January 07, 2003 @09:46PM (#5037240) Homepage Journal
      We're in business then.

      Mass warps space-time.

      Simple proof. Take an object. eg. Pen.

      Lob it gently across the room.

      Observe how it makes a graceful parabola before walloping your cow-orker on the nut.

      Now remember I said Mass warps space-Time. Time is just another dimension. We live in a 4 dimensional space.

      We measure time in this silly "seconds" unit but thats stupid. We already have a useful unit for spatial dimensions and that is meters. The conversion from meters to seconds is simply the speed of light. 1second=3e8m

      So that graceful parabola is actually very stretched out along the time dimension. It is quite a flat curve.

      Now retrieve your pen from your irate colleague and hurl it hard at him.

      Note that the curvature seems a lot less, but it took less time. Thus you will find in 4 space the curvature was the same.

      Now retrieve pen from the poor sods ex-eyeball and lob it very slowly and gently. Note the curvature seems less, but it takes more time. So in 4 space it is the same curvature.

      Carefully, carefully retrieve your pen from your ex-friend and use it to calculate the radius of curvature for all three throws. Wow! Its the same!

      And it is the radius of the earth.

      In other words, Mass warps space time. There is no gravity, only bent space time.

      As they say, Gravity is a myth, the Earth sucks.

      • Re:Wow. (Score:5, Insightful)

        by kavau (554682) on Wednesday January 08, 2003 @12:52AM (#5038089) Homepage
        Mass warps space-time.

        This is exactly the picture introduced by Einstein's general theory of relativity. An object's trajectory is simply the path of least action in warped spacetime. Spacetime is warped because of the presence of masses.

        And it is the radius of the earth.

        Now I don't know where you got this statement from. First of all, the radius of the earth and the curvature of spacetime near its surface can't be the same, because those two quantities have different units. It would also be very surprising if they turn out to be numerically the same, since the amount by which an object distorts spacetime is not determined by its size, but by its mass, which is size multiplied by density.

        There is no gravity, only bent space time.

        It seems to have eluded you that Gravity is the name we attribute to the phenomenon that mass bends spacetime. Hence your statement really says: "There is no gravity, only gravity."

        Other than that, I really like your illustrative description of how gravity works!

    • Re:Wow. (Score:4, Interesting)

      by deblau (68023) <slashdot.25.flickboy@spamgourmet.com> on Tuesday January 07, 2003 @10:06PM (#5037353) Journal
      Now if we could only figure out why and how gravity works, we'd be in business.

      There is a theory that explains why gravity is directly proportional to mass, and why it's always attractive: that gravity is anti-mass. This is not the same as antimatter, which has the same mass as normal matter but reversed charge.

      When matter is created from energy, that energy goes into deforming the rubber-sheet of space-time. The main idea of the theory is that space-time doesn't like being deformed (in precise, conservation law terms), and so creates a restorative energy called gravity. Since matter always has positive mass, the energy of the gravity field must always be negative, i.e. always attractive, and always precisely proportional to the mass.

      In effect, when you create matter you are borrowing energy from somewhere to deform space-time. That energy could be kinetic from atoms in atom-smashers, or from energy-to-matter conversion in stars, or any number of other sources.

      If you read Einstein, you know that there are some forms of electromagnetism that create gravity (most don't, including plane waves of light). According to general relativity, the EM field is more fundamental than the gravity field, and so in theory it should be possible to create gravity waves using electromagnetism. For more info, see page XX of the preface to The Geometry of Einstein's Unified Field Theory [amazon.com].

  • by EuroChild (523969) on Tuesday January 07, 2003 @08:31PM (#5036701)
    But of course, travelling at the speed of light, all the flying colours just appeared red due to the red-shift.
    • Event Horizon (Score:3, Interesting)

      by Jetson (176002)
      Well, if gravity travels at the speed of light, wouldn't the gravitational pull of black holes be confined by the event horizon as is the case with light?
      • Re:Event Horizon (Score:3, Interesting)

        by adpowers (153922)
        I don't think so. Light has mass which allows it to be pulled into the black hole while gravity doesn't have mass.
        • Re:Event Horizon (Score:5, Informative)

          by DrMegaVolt (560884) on Tuesday January 07, 2003 @08:40PM (#5036772)
          Light has mass? no it does not.. the energy of a photon has a mass equivalence, but it does not have mass.
          • by skaffen42 (579313) on Tuesday January 07, 2003 @09:28PM (#5037117)
            the energy of a photon has a mass equivalence, but it does not have mass

            Bastard. I'm going to have sleepless nights trying to figure out what the hell you mean by that. This is not the kind of concept mere mortals like me should have to deal with while sober.
            • Re:Event Horizon (Score:5, Informative)

              by Pxtl (151020) on Tuesday January 07, 2003 @09:53PM (#5037278) Homepage
              Okay, I'll bite.

              A photon delivers an impulse when it is fired or when it is destroyed on impact with matter - but when it is in transit in space it has no mass.

              Imagine a giant cluster of light, like fired by a superlarge pulse laser. It will transfer momentum to whatever it hits, but it does not actually have mass, so when its in transit this massive ball of light will not suck in anything with its gravity.
              • Re:Event Horizon (Score:4, Informative)

                by Fulcrum of Evil (560260) on Tuesday January 07, 2003 @10:01PM (#5037326)

                How about this: a photon has zero rest mass. However, it is never at rest, but travelling at C. It does have energy, which translates to a very little mass and does warp space time, but when it hits something, that energy goes away, and so does the photon.

                I wonder if a sufficient density of photons would collapse into a black hole.

                • by CrystalFalcon (233559) on Wednesday January 08, 2003 @03:17AM (#5038520) Homepage
                  The mass of photons is very real. Try this experiment, which a professor did at one of my Engineering Physics classes:

                  Take a relatively large gong. Make sure it is reasonably well polished.

                  Next, take a professional-class camera flash and set the intensity to "fry".

                  Third, fire the flash at the gong. As the photons bounce off the (polished) gong, it will resound as if having been struck with a solid object.

                  This was a very awakening demonstration to me...
                  • by alispguru (72689) <bane@@@gst...com> on Wednesday January 08, 2003 @09:54AM (#5039707) Journal
                    It's much more likely the ringing comes from the air right next to the polished gong surface suddenly heating up.

                    There's a similar confusion about what drives those "solar radiometer" things - you know, a little black-and-white paddlewheel inside an evacuated glass ball that spins when you shine a light on it? People often say the reason they run is photon momentum, when the actual explanation is that the black sides of the paddles are hotter than the white sides, so when the few gas molecules left inside the ball hit the paddles, they leave the black sides going faster than the white sides.

                    The proof of this is the direction the paddlewheel turns - it turns white-side-first, and a photon-mass explanation would have the paddle turning black-side-first. If you put a paddlewheel inside a REAL hard vacuum, with a REAL low friction bearing, and REALLY isplate it from outside vibration, it turns the right way. See here [ucr.edu] for a more coherent and complete explanation.
                    • There was another poster that also claimed this would have been due to the air heating up near the gong.

                      However, this professor continued his demonstration with sooting the gong heavily (taking it from polished to near-black), and then firing the flash again. The sound was significantly softer, noticable by all attendees (around 120), and he explained this by the photon package having been absorbed instead of bouncing (the gong only got half the impulse from before).

                      In a scenario where heat was the cause of the sound, sooting the gong would have caused a significantly louder sound as the light was absorbed, instead of (as happened) as softer one.

                    • In a scenario where heat was the cause of the sound, sooting the gong would have caused a significantly louder sound as the light was absorbed, instead of (as happened) as softer one.

                      If the gong is reflective, the air near it gets heated both by the incoming light and by the reflected light. If the gong is sooted, only the incoming light heats the air.

                      At least, this seems logical to me. A way to test it would be to put a vibration sensor on the gong, and try it both in air, and in a vacuum. If you're right, the sensor should read the same, if I'm right the impact in vacuum should be much less.
        • 100% wrong. (Score:3, Informative)

          by rebelcool (247749)
          Here's a good explanation [nasa.gov]

          Photons are not particles in the sense of neutrons, electrons et. al which are massy particles.

          Photons are better described as 'packets of energy'. Gravity doesn't just affect mass - it affects energy as well. Light doesnt get 'pulled into' a black hole, it just gets redshifted so much (by the gravity sucking the energy out of it), that its wavelength becomes infinite, and thus immeasureable.

          Photons can exert a pressure though because they have MOMENTUM. Thus they have a 'mass equivalent', but they do not have mass, and that is not why they cannot escape black holes.

        • Re:Event Horizon (Score:5, Informative)

          by mcc (14761) <amcclure@purdue.edu> on Tuesday January 07, 2003 @08:58PM (#5036892) Homepage
          Um, no. This is one of the main points of the theory of relativity.

          Light does not have mass. This is why it is capable of travelling at the speed of light-- it is impossible for anything with mass to travel at the speed of light. The reason light can get pulled into a black hole is becuase an object with gravity warps the space around it-- the more mass you have, the more that you warp space. When light gets "pulled into" a black hole, the light is in fact moving in a straight line-- it's just that space is curved in the vicinity of the black hole, so if you travel straight forward you will get pulled into the black hole.

          Think of it like you've got a matress, and you have something very heavy sitting on the matress, and the matress is kind of indented in the area of the heavy object. Now imagine if you take a marble, and roll it in a straight line toward the dent the heavy object makes. It will go in a straight line, then when it enters the dent it will start kind of curving around the inside of the dent. The standard metaphor is that gravity works like that, light is like the marble moving in a straight line, but its path is being "bent" by the curvature of space. A "black hole" is an object so heavy it's managed to tear through the fabric of the matress, meaning it's impossible for anything that's fallen into its area to roll back out.

          I will let my rudimentary explanation stand until someone who's actually studied relativity can fill in whatever gaps i've left.
          • Re:Event Horizon (Score:5, Interesting)

            by m1a1 (622864) on Tuesday January 07, 2003 @10:49PM (#5037585)
            it is impossible for anything with mass to travel at the speed of light.

            This is wrong. It is impossible for anything with mass to accelerate to the speed of light. If it is already travelling that speed it can continue indefinitely.

            the light is in fact moving in a straight line-- it's just that space is curved in the vicinity of the black hole,

            This is semantics. If I throw a softball straight ahead the ball actually moves in a straight line. It is just that space is curved in the vicinity of the earth. Gravity works the same way regardless of the density of the body creating the gravity, so long as the mass is the same.
  • Uh-oh (Score:4, Funny)

    by Saint Aardvark (159009) on Tuesday January 07, 2003 @08:33PM (#5036712) Homepage Journal
    I've got a 3-year old goddaughter who is just gonna cry when I explain to her why, in real life, the coyote would get the road runner.
  • by handy_vandal (606174) on Tuesday January 07, 2003 @08:33PM (#5036713) Homepage Journal
    Sure, this experiment will "help constrain the number of possible dimensions in the Universe" ... but will it lead to new weapons?
  • by Anonymous Coward on Tuesday January 07, 2003 @08:33PM (#5036714)
    I'm sorry, I don't mean to ask the stupidest question ever, but how does gravity have speed? The last I was taught on the subject (and believe me, it was a while ago) was that gravity was a force, but didn't have mass. Doesn't something need to have mass in order to have speed?
    • by GuyMannDude (574364) on Tuesday January 07, 2003 @08:41PM (#5036777) Journal

      You're confusion arises because you were taught elementary Newtonian physics. In general relativity, one learns that any "information" cannot travel faster than light. Gravity is considered information because if you feel a gravitational force on you, you know that there is a body out there acting on you. That is, you have information about it (you could even estimate its mass by measuring the tug it exerts on you).

      In Newtonian physics, lots of things are assumed to happen instantaneously (like gravity) so they don't have a speed per se. But in general relativity, everything has a speed -- and that speed is no greater than the speed of light.

      GMD

    • by dracken (453199) on Tuesday January 07, 2003 @09:00PM (#5036910) Homepage
      Something doesnt have to have mass to have speed. Plonk a stone on a pond, you create waves. The waves have speed but dont have "mass" per se. Gravity in general relativity is similar. The entire universe is immersed in Space-time (kinda like water in a pond). Objects inside spacetime tend to "warp" spacetime. Like poking a finger into a stretched rubber sheet, creating a depression. So objects near them tend to tumble into the depression - bingo! this is gravity. This is what general relativity says. Now if this is true, warping in spacetime cannot occur instantaneously (no wave travels faster than light) as assumed in newtonian mechanics. This is what has been proved now. "Distortions in space-time does not propagate faster than light" or in other words "gravity does not travel faster than light". So no "mass" or "moving object" is involved per se.
  • I'd expect... (Score:5, Interesting)

    by Masami Eiri (617825) <brain...wav@@@gmail...com> on Tuesday January 07, 2003 @08:34PM (#5036722) Journal
    ...a topic like this to be a bit more precise in the summary. There's a signifigant difference between .95 times the speed of light, and the speed of light. Not to mention the large .25 margin of error. Which theoretically shouldn't be able to get to +.25 anyhow.
  • by grub (11606) <slashdot@grub.net> on Tuesday January 07, 2003 @08:36PM (#5036741) Homepage Journal

    When I fall down drunk I never thought I was moving that fast.
  • Deriving e=mc^2 (Score:4, Informative)

    by sQuEeDeN (565589) on Tuesday January 07, 2003 @08:36PM (#5036743)
    It turns out you can derive e = mc^2 with very little effort. All it takes is a spot of calculus and a bit of relativity. All you need to know is that Work = delta KE and that mass is really m0 / (1-v^2/c^2)^.5, something previous physicists had derived.

    Since work is the integral of dMomentum / dt with respect to dx, you can mux that around to get, eventually, Ke = (the mass formula)*c^2 - m0 * c^2.
    What einstein said that was amazingly brilliant is that the equation fits Total E = KE + PE. He called that bit with the mass formula Total E, so therefore m0*c^2 = PE!! HA!, you are a smart as Einstein now!! (just kidding)
  • by petsounds (593538) on Tuesday January 07, 2003 @08:37PM (#5036752)
    If the speed of light is slowing down [slashdot.org], then is the speed of gravity also slowing?
    • by 3seas (184403)
      The speed of Light is not slowing down but rather we are moving faster and our method of measure is relative to us, not to light.

      In other words.... when we finally manage to reach the speed of light we will also, as a by-product, figured out antigravity...

      History? A matter of how fast you are moving relative to the speed of light.

      Time is relative to Speed.
      • by Guppy06 (410832) on Tuesday January 07, 2003 @10:12PM (#5037387)
        "The speed of Light is not slowing down but rather we are moving faster and our method of measure is relative to us, not to light."

        Welcome to the wonderful world of Einsteinian physics, where the speed of light is always measured as c no matter what the observer is doing.

        (Seriously, either I'm missing something painfully obvious in special relativity, or there are some really dense mderators out there).

        "In other words.... when we finally manage to reach the speed of light we will also,"

        How can you "reach" the speed of light when, no matter what you do, you'll always be 3E8 m/s slower than light?

        "History? A matter of how fast you are moving relative to the speed of light."

        I don't know about you, but I'm always moving at (c - 3E8 m/s).

        "Time is relative to Speed."

        No, time and space are relative to each other. They rearrange themselves so that light always moves at 3E8 meters per second relative to the observer.

        And, on top of that, space-time coordinates are always constant. While observers may not be able to agree on space and time individually, they can always agree on space-time.
        • I don't know about you, but I'm always moving at (c - 3E8 m/s).

          Technically c is just under 3E8 m/s. So you're constantly moving backward?

  • by beej (82035) on Tuesday January 07, 2003 @08:40PM (#5036764) Homepage Journal
    "One of the problems has to do with the speed of light and the difficulties involved in trying to exceed it. You can't. Nothing travels faster than the speed of light with the possible exception of bad news, which obeys its own special laws. The Hingefreel people of Arkintoofle Minor did try to build spaceships that were powered by bad news but they didn't work particularly well and were so extremely unwelcome whenever they arrived anywhere that there wasn't really any point in being there."

    --Douglas Adams, Mostly Harmless

  • by DeComposer (551766) on Tuesday January 07, 2003 @08:45PM (#5036807) Journal
    Jack Daniels makes the speed of gravity faster.
  • by j3110 (193209) <samterrell@gHORS ... minus herbivore> on Tuesday January 07, 2003 @08:50PM (#5036839) Homepage
    "Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity."

    The theory of relativity was appearantly used to detect the speed of gravity. This would be fine if the theory of relativity didn't assume a speed of gravity. Basically, all he did was prove his given. So, if eggs are green, then eggs are green!
    • by BabyDave (575083) on Tuesday January 07, 2003 @09:21PM (#5037066)
      The theory of relativity was appearantly used to detect the speed of gravity. This would be fine if the theory of relativity didn't assume a speed of gravity. Basically, all he did was prove his given. So, if eggs are green, then eggs are green!
      sigh

      You can't prove a physical theory - you can either show that it fits experimental evidence (in which case it might be right), or that it doesn't (in which case you've disproved it).

      This experiment shows that a key assumption of GR is consistent with real life. That's it. That's all we can do, and that's all that is being claimed - observations of Jupiter give (roughly) the results we'd expect if gravity travels at c.

  • by edashofy (265252) on Tuesday January 07, 2003 @09:04PM (#5036942)
    Well, now we have the speed of light AND the speed of gravity! If we can find out the Speed of Darkness, we'll be all set!
  • Wild ramblings... (Score:5, Interesting)

    by Restil (31903) on Tuesday January 07, 2003 @09:04PM (#5036943) Homepage
    The sun couldn't suddenly disappear, although that scenario works for the purpose of explaining the speed of gravity. Consider this alternative.

    Take the sun and instantly accellerate it to almost the speed of light, toward a collision course with Earth. For most of the 8 minutes between acceleration and collision, nobody would notice anything, as light, all other energy, and gravity would all present the sun as occupying its original location.

    However, brief moments before the collision, the sun's change of accelleration toward earth will be noticed. Of course, you're noticing the change that happened 93 million miles away, even though the sun is about to impact. However, one second later, the sun will appear to be almost 186000 miles closer, and it will FEEL like it's 186000 miles closer. Suddenly the gravitational accelleration has increased to reflect the new position of the sun. But within that second, you get all the accumulated influences of gravity over a much larger stretch of space than just the 186000 miles it travelled in that time. Since the sun is moving at almost the speed of light, let's say 99% of it, after 99 seconds, the influence of the sun's gravity will only be 1 second ahead of the sun. However, within that one second between the position of the sun and the gravitational influence of the sun is contained the gravitational influence of the sun over the last 99 seconds. You get the combined force in 1 second that you normally would have gotten in 99. So when the Sun's influence is finally felt by Earth, you will not get a force that implies a steady rise in gravitational force of a sun massed object until impact, you'll get a very quick rise in force of an object that is, generally, about 99 times as large as the sun.

    And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.

    Ok, this had no purpose at all, but it was interesting to think about. Go on with your business... nothing to see here. Rant over.

    -Restil
    • in short, no (Score:3, Informative)

      by lingqi (577227)
      If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon.

      the slightly longer answer is "because in the sun's inertial reference frame (i am going to leave gen.rel out of this) the sun still has the same mass."

      if you don't understand what I just said, read more about special relativity, kay?

    • Re:Wild ramblings... (Score:4, Informative)

      by barawn (25691) on Wednesday January 08, 2003 @12:30AM (#5038020) Homepage
      And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.


      point of note: a "nova" is what happens when fresh yummy hydrogen falls on a white dwarf. Boom! A "supernova" is what you were talking about. Confusing the two is a little dangerous, because they're two completely different processes.

      Depends on the mechanism of acceleration, really. If it's merely "moving" at a Lorentz factor of 100, then no, of course not, because all you did was Lorentz boost the system, which you can always do. In the Sun's rest frame, it's fine still, of course. In the boosted frame, it's also incredibly flattened (like a pancake - by a factor of 100, no less) but amazingly enough, you can still work out hydrostatic equilibrium for it, and determine that yes, it is still in equilibrium, and not going to blow up. Beauty of relativity - laws of physics are Lorentz boost invariant.

      However, if you're actually accelerating the thing, now that's a different story. You (still) won't make it go supernova, because you're NOT actually increasing the number of particles inside it, and that's what breaks hydrostatic equilibrium - pressure generated versus gravity, and BOTH of those change in the boosted frame - but you WILL screw it up really badly by sending pressure bubbles through the whole thing. Since the Sun isn't a rigid body, you'll probably strip the chromosphere right off of it, and leave the core bare. This, however, won't due much except really really confuse distant astronomers.
  • by certron (57841) on Tuesday January 07, 2003 @09:04PM (#5036947)
    While this is very interesting, is the speed of the propogation of gravity constant or can it be affected by certain conditions? This brings to mind the experiments at slowing down light in a special supercooled gel (is this an Einstein-Bose condensate?).

    I don't think I like the idea of light being the fastest anything can travel, though. Perhaps it is for many things, but what happens if some forces travel at speeds faster (or multiples), or perhaps simple fractions, and we discount those readings instead of seeing if the old model can be adapted or remade? Well, many questions, few answers from me.

    Does anyone remember the 'gravity shielding' story a while back, where a spinning superconductor was supposedly responsible for changes in weight? Podkletnov comes up in a google search for 'superconductor gravity shield' but I haven't heard anything further about it.

    Also, what about magnetic forces? How do those work, and at what speed do they 'travel' ?
  • by saforrest (184929) on Tuesday January 07, 2003 @09:11PM (#5036992) Homepage Journal
    From that they worked out that gravity does move at the same speed as light. Their actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25.

    So, really, they're triumphantly announcing that the speed of the light is somewhere between 0.7 c and 1.2 c, and just supposing it has to be c for everything to make sense.

    Physicists have been accused of being loose with rigour, but this is really stretching it.
    • by Idarubicin (579475) <allsquiet@hotm a i l .com> on Wednesday January 08, 2003 @12:22AM (#5037995) Journal
      So, really, they're triumphantly announcing that the speed of the light is somewhere between 0.7 c and 1.2 c, and just supposing it has to be c for everything to make sense.

      Physicists have been accused of being loose with rigour, but this is really stretching it.

      That's an excellent measurement for astrophysics. Recall, there was a recent announcement that astronomers are 95% certain that the age of the universe is between 11 and 20 billion (thousand million in the UK) years old. That's 15.5 plus or minus 29%.

      If you read the original paper [arxiv.org] proposing the measurements back in July, the technique requires interferometric measurements timed to within picoseconds (1e-12 seconds) to give an accuracy of at best plus or minus 10%. That translates to pegging the apparent position of a little speck of light (and radio waves) in the sky to within five millionths of a second of arc. (Roughly speaking, that's the apparent width of a bacterium at twenty miles.) I think that they did a pretty good job to be able to call the number to within 25%, especially given that nobody has ever attempted this sort of measurement before.

      No doubt it will be refined in the future; meanwhile, it's another piece of evidence which supports a subtle result general relativity. GR is a really neat theory, in that it made predictions and had consequences that we are still only beginning to be able to test nearly a century later. Even more interesting, it has yet to be contradicted by a reproducible experimental result. Hats off to Einstein, yet again.

  • by flacco (324089) on Tuesday January 07, 2003 @09:24PM (#5037085)
    One important consequence of the result is that it will help constrain the number of possible dimensions in the Universe.

    YESSS! In your FACE, Infinite-dimension Universe theorists! You SUCK!

    Yeaaaaah! Gimme one up top, bro!

  • by EvilBastard (77954) on Tuesday January 07, 2003 @09:34PM (#5037153) Homepage
    Gravity waves have been used in many stories as a FTL communication system, now that's all out of date.

    Venus is a big swampy planet, eh guys ?
  • Officer... (Score:5, Funny)

    by batobin (10158) on Tuesday January 07, 2003 @10:11PM (#5037383) Homepage
    ...you see, it's all component forces. If you look at the free body diagram of my car travelling on the road, you'll see the normal force, force of gravity, and my velocity in the x direction. As I mentioned, one of these component forces is gravity, labelled FsubG. It was recently discovered, and posted on slashdot, that the speed of said force is 3x10^7m/s.

    And THAT'S why, officer, your radar reported that I was going 60 in a 40 zone!
    • It was recently discovered, and posted on slashdot, that the speed of said force is 3x10^7m/s.

      And THAT'S why, officer, your radar reported that I was going 60 in a 40 zone!

      Well, no wonder you had a problem. The speed of light is ten times slower in your universe that in that of the officer's. When his radar beam slowed passing into your frame of reference, your apparent speed increased proportionally.

      But just try explaining that to a jury.

  • Dammit! (Score:4, Funny)

    by digitalsushi (137809) <slashdot@digitalsushi.com> on Tuesday January 07, 2003 @10:22PM (#5037457) Journal
    I asked my physics teacher the very same exact question- "if the sun disappeared, would the earth fly off from its orbit instantly, or would it take about 8 minutes?" He goes "it would be like snipping a cord- instantaneous". Discouraged, I went into the slashdot-posting, linux compiling netadmin that I am today, never knowing the true path of lab coats, leather gauntlets, and welding glasses that is physics- How dare you Stockwell! You stole my life with an assumption and I want my five years back!!!
  • Jupiter, again... (Score:3, Informative)

    by burris (122191) on Tuesday January 07, 2003 @10:57PM (#5037637)
    Ole Roemer measured the speed of light back in 1676 by measuring the time difference between predicted and observed eclipses of Io by Jupiter. It's amazing that Jupiter was once again utilized to provide the first measurement of the speed of gravity.

    http://www.what-is-the-speed-of-light.com/roemer-s peed-of-light.html [what-is-th...-light.com]

    burris

  • 25% explained (Score:4, Informative)

    by mark-t (151149) <markt@lynx. b c .ca> on Tuesday January 07, 2003 @11:03PM (#5037660) Journal
    Wondering how on earth the can explain +.25 from .95c when, according to general relativity, nothing goes faster than c? Listen up. Although I wasn't privy to how they performed this particular experiement, I've participated in other studies in the past, and have a good handle on how they are performed. What follows is my understanding of how they obtained the results that they did.

    To perform the experiment, numerous (probably several thousand) measurements are taken, but due to imprecision in the process of taking the measurements (imperfect measuring equipment, human error, etc) you get a variety of results. These answers could vary from well below c to well above it. If Einstein was right and nothing propogates faster than c, the higher results could only be attributed to imprecise measuremements, but you can't throw those measurements out if you are trying to be objective.

    At the end of the process, you have something vaguely resembling a normal bell curve, where the height of the curve at a point along the x axis (velocity) is a measurement of the relative frequency with which that speed of gravity was obtained as a measurement. The total area under the curve will be exactly 1. In many cases, the curve may not be symmetric, but for an experiment such as this, you are unlikely to obtain an assymetric curve (Central limit theorem of statistics, or some such thing). A line right down the middle of the curve shows the measured average result (.95c).

    A confidence interval is then picked (it is a shame that this interval is not mentioned in the article, but it is almost assuredly at least 95%, probably even 99%, or 99.9%). This percentage is converted to decimal (95%=.95, 99%=.99, etc), and a symmetric region around the average score with that area is blocked off. This blocked off area has a minimum X component of .7125c, and a maximum X component of 1.1875c, the difference between each of these and the average measured velocity being .2375c, which is 25% of .95c.

    And that's where the 25% margin of error comes from -- for their desired level of confidence, the variance in measured results was off by no more than 25% of the value that was actually obtained as the mean.

    Since the value of 'c' lies WELL within the bounds of the margin of error of the experiment, and pre-existing theories support the speed of gravity being c, this experiment supports those theories. It is important to note that this experiment did not prove anything, it only failed to disprove that the speed of gravity is anything other than something very close to c.

  • 1882 and 1915 (Score:4, Informative)

    by iggymanz (596061) on Wednesday January 08, 2003 @12:03AM (#5037928)
    Strange, 20 years ago I was taught other people had experimental evidence agreeing with a prediction that the effects of gravity move at light speed:

    In 1882 Simon Newcomb observed an excessive perturbation in precession of the orbit of mercury, to the tune of 43 seconds of arc per century. In 1915, Albert Einstein showed this could be explained by the propogation of gravitic wave effects at the speed of light...

    But thanks for playing.....
  • "You can't go faster than the speed of light."

    "Of course not. That's why scientists changed the speed of light in 2208."
  • by NetGyver (201322) on Wednesday January 08, 2003 @01:38AM (#5038242) Journal
    If Gravity's speed is equal to that of the speed of Light, then how do you explain the pull descrepencies between blackholes and low graivty environments? Go to the moon, you'll notice that the gravitational pull there is much lower than that of Earth's. And Earth's is far far less then a blackhole's gravitational pull.

    So how can one say that Gravity's pull is as fast as the speed of Light when Gravity itself doesn't stay constant in different environments? I never heard light not traveling the "speed of light" so it's a bit confusing.

    Ao, from what I gather, blackholes have so much gravitational pull that even light can't escape. Which suggests to me that Gravtiy is stronger than light. It would also suggest to me that gravity is is faster than light because of this. I don't have any sources to back this up, all of this is just my train of thought in words here.

    I'd appreciate a simple-as-possible answer as to why my train of thinking is wrong, as i said, i'm no scientist, but this topic is interesting none the less :)

  • Case not Closed (Score:3, Informative)

    by Washizu (220337) <{bengarvey} {at} {comcast.net}> on Wednesday January 08, 2003 @09:24AM (#5039524) Homepage
    I wouldn't take their word for it.

    This paper [ldolphin.org] gives a good case for gravity traveling faster than light and I'm pretty sure all the working Newtonian gravity calculations assume instantaneous gravity:

    "Standard experimental techniques exist to determine the propagation speed of forces. When we apply these techniques to gravity, they all yield propagation speeds too great to measure, substantially faster than lightspeed. This is because gravity, in contrast to light, has no detectable aberration or propagation delay for its action, even for cases (such as binary pulsars) where sources of gravity accelerate significantly during the light time from source to target"
  • What if..... (Score:3, Insightful)

    by cardshark2001 (444650) on Wednesday January 08, 2003 @12:21PM (#5040713)
    What if gravity has different properties from a long way away, such as intergalactic distances?

    I've often wondered lately if perhaps gravity is both a repulsive and an attractive force. For local (i.e. interstellar) distances, the attractive force prevails. But for really vast (intergalactic) distances, it might act as a repulsive force. This could partly explain why the galaxies are accelerating away from each other.

    Physicists don't have much of an idea what dark energy is... maybe it's just gravity, and Newton's law needs an amendment.

    I've never heard this idea proposed, but it would make a certain kind of sense to me if it turned out to be the case.

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