Where does the gravity go?

*blinks*

Creolla Deville, Creolla Deville,
Prove to him it ain't so yet he believes it still.Did anyone see the Disney movie? I don't understand how the heat from the sun radiates
through space yet space is so cold!

quote:

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For instance a hot brick has a little more gravitational pull than a cold brick since its total mass + energy has increased from when it was cold.

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Does a moving body have more gravity than a body at rest? My logic is that its kinetic energy would increase, and hence its mass. If so, wouldn't this imply that a photon has mass?Also, pertaining to Rrhains original question, what happens to the gravity when mass is converted to energy? Does the space-time distortion just get dissipated since the mass/energy is less concentrated?This message has been edited by JustinC, 02-28-2005 10:07 AM

Temperature is a property of matter, and space is not matter. So, space itself isn't cold, things in space are cold. Or warm, as the case may be. How the radiation from the sun affects you depends on how far away from it you are. If you orbit the sun in the same orbit as Mercury, you'll be crisp in no time.

They are proportional, therefore pure energy has mass. The amount of pure energy multiplied by the constant of proportionality (c²) is the amount of mass.

Same way the light from the sun radiates through space. It's all just photons.

Yes. A photon has mass, but not for that reason. It has energy and therefore mass. It has no rest mass (a term that most physicists dislike); if a photon were not moving (although a non-miving photon is fundamentally meaningless) it would be massless.Only things with rest mass experience the increase of mass with increasing speed. Pertaining to my original reply, assuming that "mass is converted to energy" means something (which it may not), the gravity continues to exist. The "shape" of the gravitational field may change if the energy is distributed differently in space than the original mass was.

Huh?? I'm not sure I understand. Does that mean 'space' doesn't get warmed by the heat/photons because it's a void and therefore there is nothing to warm? And space seems black because there is nothing in the void to reflect the light/photons?Really confused.

Is that because space itself is a void? Then why does something in space, say liquid water, freeze once exposed to space? What is conducting the heat out of the water? Where does the heat go?This message has been edited by custard, 02-28-2005 19:41 AM

Conduction only occurs when there's something close to the object (water) to take the heat away. If nothing's nearby (like in the middle of space), then the heat will radiate away from the source―like light from a torch―until it comes into contact with something else.

Yes. When you expose liquid water to space two things happen. It starts evaporating like crazy and it radiates heat (photons at wavelengths we don't see with our eyes).All objects in all environments radiate heat, and things that are hot enough radiate a noticable amount in the narrow frequency range that our eyes see ... "red hot" and "white hot" come to mind. Hotter objects radiate more energy. All objects in all environment also receive radiant heat (photons) from the environment. The net effect depends on which way the most energy is moving.On Earth heat is also carried by conduction (heat moving through solids) and convection (liquids or plastic solids moving and carrying heat along with it). This doesn't happen in space (but see below).So the heat transfer equation for our water in space is:total heat loss = radiation loss - radiation gain - 0 (conduction) - 0 (convection).Since radiation loss is much bigger than radiation gain, the total heat loss is big. Between the evaporation and the radiation, you quickly get small pieces of ice. Which continue to evaporate and radiate, but the evaporation is slower and the effects of the radiation aren't as noticable (and the amount of radiation is smaller 'cause it's colder).There's another interesting effect that speeds the process up. Evaporation is just molecules flying out of a solid because they happened to have enough kinetic energy to escape and are at or near the surface. Temperature is the measure of the average kinetic energy of the atoms ... therefore the ones that escape are hotter than the ones which remain, on the average. So, the evaporation produces significant cooling (as evaporation does on Earth) which could be called convection.

Very nicely put. I actually understood it. I'll go pat myself on the back now. Seriously though, thanks for taking the tame to post such a detailed answer.

Space is black because the Universe is expanding.But anyway - there aren't many particles between Earth and the Sun, and thus, not much to warm. When not in something's shadow they can get quite hot (the Sun's corona is, technically, some two million degrees).The photons only excite something when they hit it. Ie, Earth or the Moon. You will still cook if you went flying off along Earth's orbit without protection (Earth's shadow is good) for long enough - even at this distance, the Sun still provides a lot of energy.

JonF answered this much better than I could have. Thanks, JonF.

JonF, that was the concept I think I was trying to wrap my head around: The mass becomes energy (whatever that may mean...it's a convenient description for the conversion of "particles of matter" to "photons" and such) that radiates away, essentially "taking the gravity with it."I found an article pointing out that the sun is losing 9.13e-14 of its mass per year. As I suspected, this is an incredibly small amount. If I wander through my calculations on the mass of the sun and thus this means it would lose 1% of its mass on the order of 10^11 years. Considering it's only on the order of 10^9 years old and will only last about 10^10 years total, it would seem that the sun is only going to lose about one-tenth of one percent of its mass over its regular lifetime. As I suspected, you'd never notice this with any sort of typical equipment.Mass has a lot of energy and fusion certainly liberates a lot, but not much of that mass will get converted into energy in the process.

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Rrhain
WWJD? JWRTFM!