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Imagine we drop a 1000 kg object from 5 km height. When it reaches the ground, we obtain potential energy. Then we convert the object (mass) into energy, we transfer the energy back to 5 km height (assuming no energy is lost) then we convert back the energy into mass, then we repeat the process. Drop the object, obtain potential energy, convert to energy, transfer it to 5 km height, convert it back to matter. I understand this is hypothetical since no technology we have today allow us to perform this. How conservation of mass-energy get along with this?

*Energy can actually be converted into matter https://www.forbes.com/sites/paulrodgers/2014/05/19/einstein-was-right-you-can-turn-energy-into-matter/

Kenneth Kho
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If you can turn 1000 kg of something into energy that easily, what about just using that energy right away, instead of zapping it up to 5 km height?

Daniel
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I don't think conservation of mass and energy would have a problem with this, I believe you are missing a step, or misinterpreting how mass-energy equivalence manifests.

We start with a mass, drop it, and get potential energy from that. No problems here, that makes sense.

Then, you take that mass and turn it into energy; you acknowledge that it's theoretical; so I don't necessarily have a problem with this step.

It's the next step where things get a little sticky I believe. You say:

we transfer the energy back to 5 km height (assuming no energy is lost)

Now, I can even get behind the fact that you don't lose any of the energy (the equivalent amount as the $1000 \ kg$ mass); but what I can't get behind is how you move that energy up there.

What type of energy is this; and what are you using to move it against gravity?

Basically, you would not be able to freely move all that energy to the $5 \ km$ point without putting the energy needed in to raise the equivalent mass to that location. Consider, in theory that you had a device that could contain all this energy in some non-mass form. It wouldn't matter. The result would be the container weighing the same as if that were a mass itself (assuming a perfect container; which is necessary for the no energy loss condition).

JMac
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