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Thread: Earth's Gravity

  1. #1
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    Earth's Gravity

    If the Earth was impacted by a massive object, would this have changed the Earth's gravity? During the age of dinosaurs could Earth's gravity have been less that it is today?
    Last edited by kryton; 2007-Apr-12 at 01:57 AM.

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    It would have to be an incredibly massive object. During the time of the dinosaurs, the gravity was, for all practical purposes, identical to the current gravity.

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    Quote Originally Posted by kryton View Post
    If the Earth was impacted by a massive object, would this have changed the Earth's gravity?
    Several billion years ago, the Earth's net mass probably was increased with a roughly Mars mass object that struck the Earth, resulting in the formation of the moon. Of course, life could not survive such an impact.

    During the age of dinosours could Earth's gravity have been less that it is today?
    No. The Earth's mass has not changed significantly for billions of years. If there had been a massive impact so recently, this would be a dead world. Even the mass of a several mile diameter asteroid, like the one thought to have wiped out the dinosaurs, is irrelevant compared to the mass of the Earth.
    Last edited by Van Rijn; 2007-Apr-11 at 05:38 AM. Reason: typo

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  4. #4
    Several billion years ago, the Earth's net mass probably was increased with a roughly Mars mass object that struck the Earth, resulting in the formation of the moon. Of course, life could not survive such an impact.
    If a really large object(close to Earth's size) with a great mass(equal to, or greater than Earth) does hit Earth, would it be possible for earth to just "break" into half or whatever number of pieces?

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    Quote Originally Posted by unknownspiritx View Post
    If a really large object(close to Earth's size) with a great mass(equal to, or greater than Earth) does hit Earth, would it be possible for earth to just "break" into half or whatever number of pieces?
    It would need to overcome Earth's self-gravity, but if it's massive enough and fast enough, it's possible the Earth could be disrupted. The planet wouldn't literally crack in half, of course.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." Abraham Lincoln

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    Any impact that could break the Earth into separate large pieces
    would scatter vast numbers of smaller pieces all around. Everything
    would be enveloped in a cloud of hot vapor comparable in mass to
    the total mass of the pieces. It would cool rapidly, with some vapor
    condensing onto the solid pieces, but some of it would be blown
    away by solar wind. The impactor would have to be moving very
    fast to knock any material into an orbit far from Earth's orbit.

    The total increase in mass of the Earth from infalling meteoroids
    and asteroid impacts over the last two billion years is miniscule
    compared to the mass of the Earth. Earth is pretty big.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

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  7. #7
    Quote Originally Posted by Van Rijn View Post
    Several billion years ago, the Earth's net mass probably was increased with a roughly Mars mass object that struck the Earth, resulting in the formation of the moon. Of course, life could not survive such an impact.
    I was going to object to this, but I think it depends on what one means by "several". I always thought "several" implied two, or at the most three. But maybe it can be more. Apparently the impact was something like 4.5 billion years ago, and the earliest recorded life is from about 3.5 billion. It seems likely that life didn't even exist on earth at the time of the impact.
    As above, so below

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    Quote Originally Posted by Jens View Post
    I was going to object to this, but I think it depends on what one means by "several". I always thought "several" implied two, or at the most three.
    When I was a child, I was told, "You've been naughty several times today." I asked what several meant, and was told, "Any number except one and two."

    I don't think that's quite right - I don't think fifty counts as several - but I think the word means "a fair few".

  9. 2007-Apr-11, 11:06 AM
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  10. 2007-Apr-11, 11:06 AM
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    Dup. I'll never post at back up time again. Ugh.

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    Quote Originally Posted by Jens View Post
    I was going to object to this, but I think it depends on what one means by "several". I always thought "several" implied two, or at the most three. But maybe it can be more. Apparently the impact was something like 4.5 billion years ago, and the earliest recorded life is from about 3.5 billion. It seems likely that life didn't even exist on earth at the time of the impact.
    From http://dictionary.reference.com/browse/several

    1. being more than two but fewer than many in number or kind: several ways of doing it.


    I could have been more specific, but I was writing quickly. Also, I did not state that there was life on the Earth at that time, but did note that life could not survive such an impact. That's one of the reasons we know there haven't been any more recent impacts on this scale. Objects with relatively insignificant mass can cause mass extinctions. After something like this, though, observing aliens would have to rewrite their equivalent of textbooks about the planet.

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    Innumerable might be a better choice; I certainly wouldn't like too try to count the particles that resulted from the Big Splash, as it is sometimes called.
    An image of the impact by Fahad Sulehria
    The first few minutes after impact
    Look at all those pretty particles.

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    Several., seems to contain the word seven. So more than a few wouldn't you think...
    No. planet Earth can not sustain its form as a half or any other recognizable part of the original sphere. The force of impact would completely destroy this planet. Some million years or so later the newly coalesce material would have settled into a spherical shape. Gravity would do this. But life would not return until the conditions became tolerant of it. This could take hundreds of millions of years. Or never.

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    "Several" and "a few" are synonyms, but used differently:

    I only took a few cookies.
    We only have a few miles to go.
    I got A's in several classes.
    You fouled up in several ways.

    I use "a couple" to mean "about two or three or so".

    I caught a couple fish.
    Life has existed on Earth for a couple billion years.

    eburacum, did you misunderstand what the "several" applied to?

    -- Jeff, in Minneapolis
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    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Oh yes; I thought it was referring to the bits flying off the Earth in the impact. mea culpa.

    I myself would use several to refer to at least three, and perhaps as many as twenty.

  16. #14
    Ah, so its all upto gravity to make it turn back into a sphere shaped object again? But I don't get the whole idea of gravity. Where does it even come from? I understand where gravitiy might come from from a planet or star (is it because of the core of the planet/star ?) But i'm a little confused about where the gravity comes from in just a vaccum area?

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    If it has mass it has gravity. The more mass the more gravity. Density is not important. The sum total of mass of a galaxy exerts its gravity as a whole.
    Yes if a number of particles are large enough gravity will pull it into a sphere. Density will increase as mass increases.

  18. #16
    Quote Originally Posted by astromark View Post
    If it has mass it has gravity. The more mass the more gravity. Density is not important. The sum total of mass of a galaxy exerts its gravity as a whole.
    Yes if a number of particles are large enough gravity will pull it into a sphere. Density will increase as mass increases.

    so space itself has no gravity? (or would have no gravity if there were no objects with mass in it?) its just all the objects that are in space that creates gravity between each other?


    Oh and one more question, is it always true that density will increase as mass increases?

    which means increase in mass= increase in gravity=increase in density


    Anyways, your explanation was simple but fantastic. I think i understand the mass-gravity relation well now. Thanks

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    For the second thing you need to know about gravity, do a web
    search for "Cavendish experiment".

    Different kinds of rock have different density, but typical is about
    three times the density of water, or a density of three. A rock with
    a density of three and a volume of one cubic meter has a mass of
    three thousand kilograms. A BIG rock with a density of three and a
    volume of one cubic kilometer has a mass of three trillion kilograms.
    A REALLY big rock of the same kind, comparable to the size of the
    Moon, with a volume of one trillion cubic kilometers, would have
    enough gravity to compress the rock in the interior slightly, so
    that it might have an overall density of 3.2, and its mass would
    be 3.2 x 10^24 kg.

    A larger body such as the planet Mercury or the Earth will be more
    compressed, and so will be even denser. Because Earth has a large
    iron core, and iron has a density of about seven, and the core and
    all the rest of the rock in the Earth is significantly compressed by
    the tremendous weight, Earth's overall density is about 5.5, the
    most dense of all the planets of the Solar System. Mercury is the
    next most dense at 5.4.

    But I'm not sure I understood your question.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    What if the Earth was impacted by a dead star? Neutron or pulsar etc. would this affect the gravity. Is this even realistic?

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    Quote Originally Posted by kryton View Post
    What if the Earth was impacted by a dead star? Neutron or pulsar etc. would this affect the gravity. Is this even realistic?
    The earth would add a very thin layer to the neutron star. The mass of the earth is tiny compared to that of a neutron star, which would have roughly half a million to a million times as much mass. So the neutron star's gravity would be very slightly affected.

    But, while it is theoretically possible a neutron star could come rolling through the solar system, it is very, very unlikely.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." Abraham Lincoln

    I say there is an invisible elf in my backyard. How do you prove that I am wrong?

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  22. #20
    Quote Originally Posted by Van Rijn View Post
    The earth would add a very thin layer to the neutron star. The mass of the earth is tiny compared to that of a neutron star, which would have roughly half a million to a million times as much mass. So the neutron star's gravity would be very slightly affected.

    But, while it is theoretically possible a neutron star could come rolling through the solar system, it is very, very unlikely.
    Would a neutron star be of any danger to planet Earth IF it did approach our solar system?

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    Quote Originally Posted by unknownspiritx View Post
    Would a neutron star be of any danger to planet Earth IF it did approach our solar system?
    If it got close enough, definitely.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." Abraham Lincoln

    I say there is an invisible elf in my backyard. How do you prove that I am wrong?

    The Leif Ericson Cruiser

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    About Gravity

    Isaac Newton was the first to describe how gravity "works".

    He defined what "force" is in his second law of motion, which is
    commonly expressed as "force equals mass times acceleration".
    That is, a force is a push or pull which can cause a mass to
    accelerate. A given amount of force will cause a given mass
    to accelerate a specific amount.

    By analyzing the speed of the Moon in its orbit around the Earth,
    and using an estimate of the mass of the Earth, he realized that
    there is a force of attraction (a "pull") between the Earth and
    Moon of

    force = constant * mass of Earth * mass of Moon / distance^2

    where the constant is the Universal Gravitational Constant "G"
    that Henry Cavendish later measured directly for the first time,
    and the distance is the distance between the centers of mass
    of the Earth and the Moon. All the mass of Earth pulls on all the
    mass of the Moon, and all the mass of the Moon pulls on all the
    mass of Earth in such a way that you can pretend that the mass
    of each body is located at a point at the center of the body.
    That works very well for spheres which are some distance apart.
    It works less well if the bodies are not spheres and are close
    together. In that case, the analysis to calculate the force is
    much more complex.

    But in general, any two masses attract each other by gravity.
    The attraction is in a straight line between those two masses.
    And if the two masses are far apart, or are spheres, then you
    can use the distance between their centers to calculate the
    force.

    There is a force between the mass of my body and the mass of
    the filing cabinet ten feet away from me. Since my mass is small
    and the mass of the filing cabinet is small and gravity in general is
    an extremely weak force (the weakest of the fundamental forces),
    the force between the filing cabinet and myself is very, very weak.

    Similarly, there is a force between the mass of my body and the
    hill to the east of me. The hill is much farther away from me than
    the filing cabinet, but it is also far, far more massive, so I expect
    that the force of attraction between my body and the hill is quite
    a bit stronger than the force between me and the cabinet.

    Likewise, there is a force between the mass of my body and the
    mass of the Earth as a whole. The mass of the Earth is an average
    of 6,400 kilometers away from me. Half of it is closer than that,
    and half of it is farther away. So to calculate the force between
    my body and the Earth, I use that distance of 6,400 km, which is
    the distance between my center of mass and the center of mass
    of the Earth.

    The total mass of the Earth is so enormous that even though it is
    on average so much farther away than the hill, that I can easily
    feel the force of gravity between it and my body. In fact, it is
    making me somewhat uncomfortable right now, despite the fact
    that my body is not very massive for an adult American male.

    There is also a force between the mass of my body and the mass
    of the Moon. The center of the Moon is 60 times farther away
    from me than Earth's center, and the mass of the Moon is only
    1/80th Earth's mass, so the force between my body and the Moon
    is only about as strong as the force between my body and the hill.
    Both could be measured with a sensitive device, but I could never
    detect either with just my own senses.

    Even if the force between my body and the hill were so huge that
    it caused an easy-to-measure deflection of a plumb bob, it would
    not necessarily feel odd to move around near the hill. The horizon
    would just appear to be tipped slightly, and I might feel a couple
    of kilograms heavier. That's all.

    Albert Einstein developed a theory of gravity called "the general
    theory of relativity". It is called "general" because it describes
    situations which the "special theory of relativity" does not try to
    describe. The general theory of relativity, or GR, is that energy
    (including mass) causes spacetime to be warped, or curved. This
    curvature in turn causes matter to move. John Wheeler described
    the relationship this way: Matter tells space how to curve; Space
    tells matter how to move.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  25. #23
    Quote Originally Posted by Jeff Root View Post
    Isaac Newton was the first to describe how gravity "works".

    He defined what "force" is in his second law of motion, which is
    commonly expressed as "force equals mass times acceleration".
    That is, a force is a push or pull which can cause a mass to
    accelerate. A given amount of force will cause a given mass
    to accelerate a specific amount.

    By analyzing the speed of the Moon in its orbit around the Earth,
    and using an estimate of the mass of the Earth, he realized that
    there is a force of attraction (a "pull") between the Earth and
    Moon of

    force = constant * mass of Earth * mass of Moon / distance^2

    where the constant is the Universal Gravitational Constant "G"
    that Henry Cavendish later measured directly for the first time,
    and the distance is the distance between the centers of mass
    of the Earth and the Moon. All the mass of Earth pulls on all the
    mass of the Moon, and all the mass of the Moon pulls on all the
    mass of Earth in such a way that you can pretend that the mass
    of each body is located at a point at the center of the body.
    That works very well for spheres which are some distance apart.
    It works less well if the bodies are not spheres and are close
    together. In that case, the analysis to calculate the force is
    much more complex.

    But in general, any two masses attract each other by gravity.
    The attraction is in a straight line between those two masses.
    And if the two masses are far apart, or are spheres, then you
    can use the distance between their centers to calculate the
    force.

    There is a force between the mass of my body and the mass of
    the filing cabinet ten feet away from me. Since my mass is small
    and the mass of the filing cabinet is small and gravity in general is
    an extremely weak force (the weakest of the fundamental forces),
    the force between the filing cabinet and myself is very, very weak.

    Similarly, there is a force between the mass of my body and the
    hill to the east of me. The hill is much farther away from me than
    the filing cabinet, but it is also far, far more massive, so I expect
    that the force of attraction between my body and the hill is quite
    a bit stronger than the force between me and the cabinet.

    Likewise, there is a force between the mass of my body and the
    mass of the Earth as a whole. The mass of the Earth is an average
    of 6,400 kilometers away from me. Half of it is closer than that,
    and half of it is farther away. So to calculate the force between
    my body and the Earth, I use that distance of 6,400 km, which is
    the distance between my center of mass and the center of mass
    of the Earth.

    The total mass of the Earth is so enormous that even though it is
    on average so much farther away than the hill, that I can easily
    feel the force of gravity between it and my body. In fact, it is
    making me somewhat uncomfortable right now, despite the fact
    that my body is not very massive for an adult American male.

    There is also a force between the mass of my body and the mass
    of the Moon. The center of the Moon is 60 times farther away
    from me than Earth's center, and the mass of the Moon is only
    1/80th Earth's mass, so the force between my body and the Moon
    is only about as strong as the force between my body and the hill.
    Both could be measured with a sensitive device, but I could never
    detect either with just my own senses.

    Even if the force between my body and the hill were so huge that
    it caused an easy-to-measure deflection of a plumb bob, it would
    not necessarily feel odd to move around near the hill. The horizon
    would just appear to be tipped slightly, and I might feel a couple
    of kilograms heavier. That's all.

    Albert Einstein developed a theory of gravity called "the general
    theory of relativity". It is called "general" because it describes
    situations which the "special theory of relativity" does not try to
    describe. The general theory of relativity, or GR, is that energy
    (including mass) causes spacetime to be warped, or curved. This
    curvature in turn causes matter to move. John Wheeler described
    the relationship this way: Matter tells space how to curve; Space
    tells matter how to move.

    -- Jeff, in Minneapolis
    some good examples there. Also, this formula you used where f= K*mass1*m2/distance^2...i've realized its practically the same as when finding the force between two charges, where f=k*q1*q2/r^2. I just learned that a few days ago in physics class. I am excited now ^_^

  26. #24
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    Quote Originally Posted by cjl View Post
    It would have to be an incredibly massive object. During the time of the dinosaurs, the gravity was, for all practical purposes, identical to the current gravity.
    Or quadrillions of small objects...

    A fair reasonable estimate can be obtained by computing the total mass of all micrometeorite collisions with the Earth each day, on average (a fairly well-known quantity), and comparing it to the mass of the Earth's atmosphere we lose to outerspace each day (also a fairly well-known quantity).

    The earlier years of our planet's existance saw both higher rates of impact from larger particles, as well as a higher pressure atmosphere with greater rate losses than today. Since I know neither, I'm not qualified to comment as to the net effect over time.

    I would hazard a guess that it was not materially different than it is today, and possibly the Earth is more massive today, as the Earth's atmosphere is an exceedingly small component of it's overall mass.

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    I once got my hands on the values and equations that that calculation would need, and figured out that, in order for increasing gravity of the Earth to cause a change in an object's weight of just one ounce in billions of years at the current rate of infalling dust accumulation (because I lacked the information to have it change with time), the object would have had to weigh tons to start with (and end up weighing tons and an ounce today). I just don't remember whether it was more like 4 tons or 400 tons... but when the change is a whopping ounce, that's about the same anyway.

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    Wink somewhere over the rainbow....sings Judy G.

    Quote Originally Posted by Van Rijn View Post
    If it got close enough, definitely.
    Van Rijn. Somewhere in my readings of the last decade or so...I came across an article about the prospect of a neutron star colliding with the Earth. I recall thinking...this is going to be a big splat with annihilation of the Earth as a solar system object. Not so. Computer simulations done by the authors showed that the pulsar would penetrate the Earth, and because of the enormous momentum...pass through. The transfer of kinetic energy did melt the crust and boil the oceans and annihilate all lifeforms...but the Earth survived to coalesce another day. I was quite surprised...but that's not the first time. (I think it was the Los Alamos Journal Spring Edition...about 1990-93.(It's semi-annual .OK 14 years ago.) I'll hunt for it.
    I also remember thinking afterwards. If you shoot a jellyfish near the surface of the ocean with a 22...it goes right through, and the jellyfish can repair that. Pete.
    Last edited by trinitree88; 2007-Apr-13 at 08:30 PM. Reason: error in journal name.

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    Cool symmetry rules

    Quote Originally Posted by unknownspiritx View Post
    some good examples there. Also, this formula you used where f= K*mass1*m2/distance^2...i've realized its practically the same as when finding the force between two charges, where f=k*q1*q2/r^2. I just learned that a few days ago in physics class. I am excited now ^_^
    unknownspiritx. Symmetry rules. When you notice the similarity in the two equations, that's due to the inverse square law that applies here, and elsewhere in physics...illumination, radioactivity.
    Mathematician Emmy Noether found that symmetry is intrinsically related to conservation laws. see;
    http://www.google.com/search?hl=en&q...=Google+Search

    pete.
    Last edited by trinitree88; 2007-Apr-14 at 06:37 PM. Reason: punctuation..what me? lol

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    Quote Originally Posted by Jens View Post
    Apparently the impact was something like 4.5 billion years ago, and the earliest recorded life is from about 3.5 billion. It seems likely that life didn't even exist on earth at the time of the impact.
    Right. Astrobiology Magazine clarifies....

    Excerpt: Does the first evidence of life date to 3.85 billion years ago (Ga), or 3.65 Ga? A 200-million-years discrepancy may seem trivial almost 4 billion years after the fact. And yet scientists continue to debate whether some of the oldest rocks ever found date to 3.85 Ga, or "just" 3.65 Ga.

    The discrepancy matters because the rocks, however old they are, indicate that life already existed at the time they formed. The dispute is not just a matter of how early life began, however, but under what conditions: The earlier date was during the tail end of an asteroid storm called the "late heavy bombardment," while the later date was after the bombardment ceased.
    Everyone is entitled to his own opinion, but not his own facts.

  31. #29
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    I, personally, use "several" to mean anywhere between two and a dozen. (Okay, sometimes, I use it to mean more than that, but I always catch myself and promptly feel dumb.)

    I can't help much with gravity, but definitions? Oh, yeah.
    _____________________________________________
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    "Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"

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    Cool Jack-in-the-Box energy has gravity, too

    Quote Originally Posted by astromark View Post
    If it has mass it has gravity. The more mass the more gravity. Density is not important. The sum total of mass of a galaxy exerts its gravity as a whole.
    Yes if a number of particles are large enough gravity will pull it into a sphere. Density will increase as mass increases.
    astromark. Energy has gravity, too. E=mc2. So, m=E/c2
    So if you have energy in the form of photons or neutrinos...that too has an equivalent mass. Suppose you have in your hand a box with some gas inside, and a few slightly greater than ~1.022 Mev photons....just enough to create electron/positron pairs. We magically give the box perfect internal wall mirrors, as suggested in another thread elsewhere, and add the additional condition that the atoms of the internal gas can facilitate the creation of the particle/antiparticle pairs, but not absorb any of the photon/neutrino energy as they do so. (We get to be control freaks here..but this is the sort of stuff theorists do).
    Now, we set the gears in motion. The traveling neutrinos/photons, chugging along at c, hit an atom and create a pair of particles with rest mass each of 511 kev, or 0.511 Mev....the electron, and the positron. Photon/neutrino energy of E=hv has "disappeared", simultaneously two particles with non-zero rest mass have"appeared". Is the box "heavier"? No.
    The two non-zero rest mass particles travel a bit and annihilate, back into the 1.022 photon/neutrino. Is the box lighter? No. The total mass/energy is constant. So the total gravitational field is constant. The strength of the gravitational force felt by you and the box in your hand is still Fg= G (mbox)(massyou) / (separation of centers of mass)2......we'll put you in space to eliminate Earth here.
    To think otherwise, whenever the pair forms, Fg goes up....and when they annihilate, Fg goes down. Now if we made our conditions more exotic with a much more energetic photon, or neutrino say 10Google ev....you'd have a wildly oscillating Fg. not going to happen. (Epstein, Thinking Physics...a great little book). As you sit with your hot meal...a hard boiled 212 F egg...it has a little more field than it does when it cools off by radiation, conduction and convection...no magic mirrors there.
    I need also to stipulate that the neutrinos in the box never mirror, when they form, they pair form as electron/positron and then annihilate back to photon before reaching the edge of the box....otherwise I'd be suggesting lepton number conservation failure, something never seen in a particle physicist's world. pete.
    Last edited by trinitree88; 2007-Apr-16 at 10:16 PM. Reason: subscript boxed wrong..my fatigue is showing, apostophe

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