
Ask HN: Is gravity a renewable energy source - aspiretoreality
Is gravity an infinitely renewable resource (source of energy) or, if a company harnessed it, would it be catastrophic over the long term, affecting Earth&#x27;s rotation or orbit.  Another way to put this: can gravity be <i>used up</i> ?
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airbreather
The conversion of kinetic to potential and back to kinetic energy is a form of
energy storage, but not creation.

Harnessing gravity could occur in the form of the "slingshot" orbits, where a
spacecraft steals a small amount of the stored orbital energy of the moon or
planet from it's parent plant or sun, but due to the relative masses and
speeds the impact is real, but practically negligible - steal too much energy
from the orbiting body and and there will be a noticeable impact to the orbit.

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aspiretoreality
Hmm... for example, the scheme of solar or tidal energy to move water _uphill_
, then grab hydroelectricity? is an example of storing gravity? is that a
valid characterization?

And yes, you immediately got the worry; that accessing gravity would at scale
over a few hundred years, be a danger.

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airbreather
In the uphill concept, consider gravity as the spring or rubber band that is
stretched.

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ramtatatam
Not being an expert in physics there are a few base concepts you should take
under account before asking this question.

a) What gravity really is - you need to ask this question before you can
really move forward and start asking other questions (like questions you
asked). And what the gravity really is? Well, this is what nowadays physics is
trying to find out - we do not yet know. This is connected a bit with an
attempt to come up with "theory of everything" \- a supreme set of laws that
connect everythig with everything else (i.e. electromagnetism with gravity) -
such theory does not exist (yet?) and therefore phenomenons like gravity, or
light for that matter, are not yet bound by definite set of theorems (of
course we all heard about black holes that can lock light within event
horizon).

b) What energy is - again a concept of energy is present in physics and I'm
sure you was learning about it in your primary school physics course; in short
energy is a way to represent a potential to do work - right? In such way we
already know what kind of energy can you harness gravity to store a potential
to do work for you - i.e. you can bring huge mass somewhere high enough so you
can drop it, when object hits the ground you can transform that original work
you have done into different kinds of energy - i.e. energy of impact (maybe to
destroy an object, or to have another object lifted?) or heat (which is
actually energy loss); I can imagine when you say "energy" you really think
"electricity" and as such I have not heard people being able to use a space
attribute which (according to my current knowledge) gravity is to transform
potential energy you can store with it to electricity.

So - is your question possible to answer? In my humble opinion, with current
state of knowledge, not to your satisfaction.

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aspiretoreality
Thanks for your sharing. Perhaps in the future once gravity is better
understood, we will know if it can be depleted.

I imagine 200 years ago nobody worried overly much about burning coal or
later, kerosene etc. _The atmosphere is a pretty huge volume and besides, the
rain cleans the air._ ... one can imagine sentiments like that being directed
at anyone complaining about air pollution.

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ramtatatam
All pleasure is mine :-) I was hoping somebody would revise my post as it's
almost 10 years since my uni physics course. I always enjoyed reading physics
books though could mess up with details.

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wwalser
Gravity isn't an energy or a source of energy. It's more like a spring or
magnet than sunlight or heat. Gravitational forces can be used to store
energy, similar to compressing a spring. Bill Gates talks about this principle
toward the end of an interview about his energy investments[1].

1\.
[https://www.youtube.com/watch?v=RH5Lgv-4te8](https://www.youtube.com/watch?v=RH5Lgv-4te8)

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czbond
I'll take a shot, as a non-scientist:The only way I can think of, is if you
could destroy or convert gravitons into another form of energy.

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mazeway
I think you need to learn some basic physics

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aspiretoreality
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