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    (Original post by Davetherave)
    What about the energy the Earth receives from the Sun? Doesn't energy have mass according to a certain famous equation? As a practical example, when plants convert sunlight into nutrients, isn't this a net increase in the mass of the Earth? Or is it all eventually radiated back out to space?
    The earth and sun are in radiative equilibrium. There is an equal energy in as out. If there wasn't the earth would be getting hotter.
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    Conservation of mass
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    (Original post by F1 fanatic)
    The earth and sun are in radiative equilibrium. There is an equal energy in as out. If there wasn't the earth would be getting hotter.
    Ah!
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    light has mass, but the point of it is the energy is so high it is irrelevent compared to the energy ( hense allthough light can be taken in by a black hole, it is often just bent in its path)

    the earth will be losing mass. within the earths possible environment of use (everything from the core to the atmosphere) is recycles, but some is lost as pointed out before.
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    (Original post by alex p)
    light has mass, but the point of it is the energy is so high it is irrelevent compared to the energy ( hense allthough light can be taken in by a black hole, it is often just bent in its path)

    the earth will be losing mass. within the earths possible environment of use (everything from the core to the atmosphere) is recycles, but some is lost as pointed out before.
    wha? I was referring to energy being converted into mass, not the gravitational pull on a beam of light.
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    (Original post by alex p)
    light has mass, but the point of it is the energy is so high it is irrelevent compared to the energy ( hense allthough light can be taken in by a black hole, it is often just bent in its path)

    the earth will be losing mass. within the earths possible environment of use (everything from the core to the atmosphere) is recycles, but some is lost as pointed out before.
    the energy of photons is not completely insignificant, but as I said, the same amount goes out as goes in, so it doesn't contribute. The relative energies don't come into it. True or not, it's not important.

    Also, Avocado, conservation of mass (technically energy) only applies to the universe, not necessarily the earth system.
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    Since the temperature of the earth is relatively constant, the radiation from it is roughly the same as that absorbed (evaporation being a good example of this), hence the mass is over a sufficient period of time the same in relation to this.
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    (Original post by alex p)
    light has mass
    No, it doesn't. Fact.
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    the average aldedo of the earth is about 40%, that is the ratio of reflected to incident radiation. So the Earth does absorb quite a lot of radiation. (is this then emitted? or does our CO2 layer stop a lot of it?)

    Another factor is that the heat energy inside the earth (why it has a molten core) comes from radioactive decay in which mass is converted into energy. Some of this radiation convects/conducts to the surface and is then , no doubt, emitted into space.

    I dont know if these factors are particularly significant but....:cool:
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    (Original post by F1 fanatic)
    The earth and sun are in radiative equilibrium. There is an equal energy in as out. If there wasn't the earth would be getting hotter.
    Is this just a fluke? Or is there a reason for it than can be summarised in the space offered by a TSR post?
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    (Original post by Davetherave)
    Is this just a fluke? Or is there a reason for it than can be summarised in the space offered by a TSR post?
    Yes, there's a reason. Everything will eventually tend to a state of equilibrium if it's unvarying for long enough. Consider if it was not in equilibrium and there was more energy emitted by the sun. Now, if the Earth receives more energy, then it will heat up, and by virtue of heating up it will emit more radiation. This will continue until the energy in matches the energy out and there is a stable situation of radiative equilibrium. It would take a change in the output of either to alter this balance.

    Drummy --> The Earth's albedo is closer to 30%. But eventually, all radiation that is absorbed will be re-emitted. It's a principle of a black body (Kirchhoff's law) that the absorptivity and emissivity of a black body are the same. You may know this in the form "Good absorbers are good emitters". Anything that is absorbed will eventually be re-emitted. The effect of CO2 is more to alter the form of the black body as observed from space. It behaves like a blanket so that from space the Earth appears cooler as it is emitting more from the upper atmosphere. This allows the air close to the surface to be warmer, but the important point is that the overall amount of energy emitted by the Earth is the same.
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    (Original post by Worzo)
    No, it doesn't. Fact.
    Depends what kind of mass one is talking about, but in the normal meaning of the term you are correct.
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    E=MC2

    light has heat and speed as energy. to make the equation work this figure has to be divided by the speed of light2, so allthough it gives a very small figure, it is still a mass, as the equation shows, for something to have energy it must have a mass, no matter how small.

    if you say it has none then E = 0 x c2 so E=0 which doesnt work
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    (Original post by alex p)
    E=MC2

    light has heat and speed as energy. to make the equation work this figure has to be divided by the speed of light2, so allthough it gives a very small figure, it is still a mass, as the equation shows, for something to have energy it must have a mass, no matter how small.

    if you say it has none then E = 0 x c2 so E=0 which doesnt work
    light has heat energy? Isn't E just the change in energy, or increase in detectable electromagnetic energy due to a small loss in mass. So therefore you cant apply e=mc2 to a light wave coz its already a light wave. mass and energy are inconvertable so light can have 0 mass.
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    E=MC2
    ...
    if you say it has none then E = 0 x c2 so E=0 which doesnt work
    That's only true for things with zero momentum, more generally E^2=m^2c^4 + p^2c^2
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    photons dont have mass.
    Anything with mass cannot travel at the speed of light.
    A hot object, however, has a slightly bigger mass than an identical cold object and the difference in mass can be calculated from [latek]E=mc^2[/latex].
    A hot object can lose energy by emitting photons therefore one can imagine that these photons have a mass "associated with them" even though they have no mass.
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    photons dont have mass.
    Anything with mass cannot travel at the speed of light.
    A hot object, however, has a slightly bigger mass than an identical cold object and the difference in mass can be calculated from
    E=m c^2
    A hot object can lose energy by emitting photons therefore one can imagine that these photons have a mass "associated with them" even though they have no mass.

    (thats better)
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    Remember we're talking about a "rest mass" when using that equation. As plmokn says, the more general equation involves momentum. The thing is, light does not have a rest frame, therefore it has no rest mass. If it had an inertial mass then it could never attain the speed of light. (inertial mass being the mass in F=ma, required to accelerate a body).

    However, light does have a gravitational mass, in that it can warp spacetime. Light feels gravitational fields. In Newtonian classical mechanics, inertial mass and gravitational mass are the same, but this is not necessarily true at relativistic speeds.

    Edit: you know you can edit posts right?
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    (Original post by F1 fanatic)
    . In Newtonian classical mechanics, inertial mass and gravitational mass are the same, but this is not true at relativistic speeds.

    This seems to be where the confusion lies.

    Edit: you know you can edit posts right?
    I thought the whole point of general relativity is that these things CAN be thought of as the same thing? (equivalence principle)
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    (Original post by Davetherave)
    I thought the whole point of general relativity is that these things CAN be thought of as the same thing? (equivalence principle)
    I'll hold my hand up and say, I think you have me on that one. I think I meant to say "inertial rest mass" ... either that or I've generally just lost the plot which wouldn't surprise me.

    You're right, the equivalence principle says that gravitational and inertial mass are the same thing as viewed in different frames of reference, the typical one being man in lift scenario.
 
 
 
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