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So in my world, there is something called magic energy: a form of energy that can be control and, you know, do magic stuff. Like normal energy, they can not be created or destroyed, but only change form. At somepoint in history, my human learned how to "harvest" energy in the surrounding and transform them into magic. These energy input can be heat, wind, sound, light or biological etc.

And here the question. (Disclaimer: it may sound basic or even stupid, but i just can't wrap my head around) "Can you darken the sky by (purely) harvesting too much light energy?"

This mage harvest energy in a 30-meter radius, and light energy will immediately transform when they enter the zone. Would it result in a 30-meter dome of shade in the midnoon?

I understand that it would mean a tremendous amount of energy, but this mage can do that. it could be equal to hundreds of the Hiroshima bomb and still be okay: the scale is not a problem.

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Taking a different point of view that @Michael I tried to more answer how this darkening would be possible not violating laws of physics.

The harvesting of all radiation energy that enters the 30-meter radius half sphere could be explained scientifically that the magician disperses some kind of magical material that it can control. The material would move as the magician wishes (perhaps possible with magnetism and/or nanobots) to fill evenly the space and it would also have the ability to absorb all photons that scatter from the surrounding air or straight from the sun. This would require massive numbers of this mysterious material and it should also be able the store that energy. After collecting some energy the magician would command the bots to return with the energy.

By capturing all the photons for someone inside the sphere it would seem as just getting dark.

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    $\begingroup$ Oops that was a brainfart. You are right, I'll edit. $\endgroup$ – Communisty Aug 29 '16 at 11:07
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You cause mass extinction

If you harvest all energy in a 30 meter sphere... then you instantly harvest so much energy that you cause mass extinction, unless you can keep it contained.

How does this happen? Because mass is energy as well, according to Einstein's famous equation: $E = mc^2$

If you are — for simplicity's sake — sitting in a boat when you do this, so the mass you collect is water, then you gain in the order of $5\cdot10^{24} Joule$, which is approximately 10 times the energy of the impactor that created the Chicxulub_crater, and caused mass extinction.

If you can keep all that energy contained however, well that means that now you are lugging a weight that is the same as a 30 meter half-sphere of matter... because energy is mass; this works both ways. In the best case that is water, which means all your collected energy weighs 57 000 tons. :D

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One thing I think is being ignored (though E=MC^2 has already been addressed) is that light (radiation/energy) is constantly being rained down on this construct you wish to be without it.

Now there are two ways (I can think of off the top of my head) to get around this.

  1. You absorb the energy and use that to power something else. Think of it like a fridge. The chemicals are absorbing the heat and transporting it outside. So you transport that energy to power up something you don't need or want anyone else to know about (your own personal McGuffin (tm))
  2. You bend light around the area you want darkened. This is going to cost energy, but it gets the effect you want.

The reason why I say these are your only two options, is becuase you do not only face the E=MC^2 problem (mass and energy within this sphere), but you need to remember this isn't happening in a vacuum. Meaning that air (pressure/currents) will still try to balance itself out, and if you are converting the air in a sphere into energy, it will suck in more air for you to convert as well. The same with water, the same with light raining down on you. It's an ever increasing problem that the laws of nature will try to correct and therefore frustrate your machinations.

Seeing as you cannot destroy the energy or lose it, you need to find an equilibrium with your surroundings, otherwise you have a ticking timebomb with you are the centre of it.

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