Earth Mass Increasing? Photons and Photosynthesis making more matter on earth?

  • #26
Drakkith
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Photosynthesis does not convert sunlight into mass. It converts sunlight into chemical energy (sugars). If one wants to argue the Mass/Energy equivalence, OK, but any increase in the total mass of the earth, if true, would be miniscule compared to the gain in mass from impacts of meteorites and cosmic dust, as several members here have already mentioned.

"The mass of the earth is 5 983 000 zettagrams, and it gains another 40 Gg (gigagrams) every year from captured meteorites and cosmic dust."

http://www.essex1.com/people/speer/large.html [Broken]
Plus this mass would be lost when the sugar is used in cells, eventually ending up as thermal radiation.
 
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  • #27
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It's reradiated, but with a lower frequency, meaning less energy and so the Earth gained this energy difference as mass.
No, it is re radiated as a larger number of lower frequency photons, so the energy is the same but the entropy is much higher.
 
  • #28
davenn
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Originally Posted by Sakha
It's reradiated, but with a lower frequency, meaning less energy and so the Earth gained this energy difference as mass.
No, it is re radiated as a larger number of lower frequency photons, so the energy is the same but the entropy is much higher.
and further to that ... aren't photons supposed to be massless ?
so isnt it irrelevent to discuss their coming to and going from the earth causing an addition or loss of mass of the earth ?

Dave
 
  • #29
Drakkith
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and further to that ... aren't photons supposed to be massless ?
so isnt it irrelevent to discuss their coming to and going from the earth causing an addition or loss of mass of the earth ?

Dave
I don't believe so. Before the photons are emitted the increased temperature of the Earth should result in it being more massive than after the photons are emitted.
 
  • #30
D H
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and further to that ... aren't photons supposed to be massless ?
so isnt it irrelevent to discuss their coming to and going from the earth causing an addition or loss of mass of the earth ?
No, it's not irrelevant. You have to remember that mass is bound energy. There's two things going on here. Most of those incoming solar photons go into warming things up a bit. A few of those incoming solar photons fuel photosynthesis in plants.

A photon absorbed by a rock makes the rock a bit warmer, and this does make the rock a tiny bit heavier. The same rock is more massive (but only immeasurably) when it is hot compared to when it is cold. Thermal energy is bound energy. There's a problem with this point of view: It's only looking at one tiny part of the picture. Another part of the picture is that the rock cools at night. The rock's temperature exhibits periodic variations, but there is little if any secular drift. The Earth's surface is more or less in thermal equilibrium. This is still but a part of the picture. To look at the whole picture, you need to look at the thermal energy of the Earth as a whole. The Earth as a whole has been and continues to cool down from its very high temperature of 4.56 billion years. In this sense, the Earth isn't gaining mass; it's losing it. Imperceptibly so.

The other route sunlight could be viewed as increasing the mass of the Earth is photosynthesis. The mass of a sugar molecule is larger (imperceptibly so) than the masses of the compounds that went into the making of that sugar. Chemical potential energy is a kind of bound energy. So does this change the Earth's mass? Yes, but this too isn't looking at the whole picture. The plant itself uses most of that bound energy to make starches, cellulose, and other compounds. Animals and fungi eat plants, further reducing the mass gain. A tiny bit isn't recycled by life and will eventually become coal or oil. There is a mass gain here. This still isn't looking at the entire picture. The Earth went through a couple of episodes of "life gone wild!" in which life was very prolific, much more prolific than it is now. We humans have been burning the accumulated bound energy from those episodes at a prolific rate. That imperceptibly small increase in the Earth's mass due to photosynthesis today is more than offset by our consumption of fossil fuels. In this sense, the Earth once again isn't gaining mass; it's losing it. Imperceptibly so.
 
  • #31
davenn
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thanks for that DH

this isnt my particular field of physics so always willing to learn :)

so, I assume, we aren't even talking about rest mass of a photon, its purely the heating effect caused by the transfer of energy by the absorption of the photon ?

Dave
 
  • #32
Drakkith
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thanks for that DH

this isnt my particular field of physics so always willing to learn :)

so, I assume, we aren't even talking about rest mass of a photon, its purely the heating effect caused by the transfer of energy by the absorption of the photon ?

Dave
Exactly.
 
  • #33
D H
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so, I assume, we aren't even talking about rest mass of a photon
Correct.

its purely the heating effect caused by the transfer of energy by the absorption of the photon ?
Incorrect. There's also chemistry. Photosynthesis is an endothermic reaction. The sugars created by photosynthesis are more massive than the masses of the compounds that went into forming those sugars; it's a consequence of an endothermic reaction.
 
  • #34
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and further to that ... aren't photons supposed to be massless ?
so isnt it irrelevent to discuss their coming to and going from the earth causing an addition or loss of mass of the earth ?
The statement that a photon is massless means that its energy equals its momentum in units where c=1. I.e. The formula for mass in such units is m^2=E^2-p^2. So, if you have an object with m=1 at rest (p=0 and E=1) and it absorbs a photon with E=1 (m=0 and p=1) then by conservation of momentum and energy after the absorption it has E=2 and p=1, which gives m=1.7 So even though a photon is massless another object can still gain mass when it absorbs one.
 

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