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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/

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  • $\begingroup$ How do you think this would create energh $\endgroup$ Feb 14, 2018 at 17:45

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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).

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  • $\begingroup$ Wait, energy is massless, how could you say that the container would weight as if it has mass? $\endgroup$ Feb 13, 2018 at 19:46
  • $\begingroup$ "energy is massless" That's not true. Mass-energy equivalence is saying that mass and energy are equivalent. Energy cannot be massless; mass and energy are the same. A $1000 \ kg$ mass is energy; just in a form that we cannot easily convert into other types of energy. $\endgroup$
    – JMac
    Feb 13, 2018 at 19:54
  • $\begingroup$ But, photon has energy and no mass. $\endgroup$ Feb 13, 2018 at 20:21
  • $\begingroup$ @KennethKho Photon has no rest mass, but if you contained $1000 \ kg$ equivalent of photons into a container; it would weigh $1000 \ kg$ (plus the extremely high mass that would probably be required to contain all that energy). Some more articulate thoughts on that can be found in the answers here :physics.stackexchange.com/questions/10612/… $\endgroup$
    – JMac
    Feb 13, 2018 at 20:33
  • $\begingroup$ I've read the explanation and it really cleared up my confusion, thanks!! $\endgroup$ Feb 13, 2018 at 20:54
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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?

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  • $\begingroup$ That's why it's hypothetical, if such thing is possible, would it break the conservation of mass-energy (which i'm pretty sure it doesn't), but I can't tell the reasoning behind it $\endgroup$ Feb 13, 2018 at 19:05

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