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's rotation or orbit. Another way to put this: can gravity be used up ?

[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.

[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.

[airbreather]

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

[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.

[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.

[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.

[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

[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.

[mazeway]

I think you need to learn some basic physics

[aspiretoreality]

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