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Comment by sneilan1 | Hacker News Reader

Not to sound like a crazy person, but, does taking energy from tidal waves mean taking energy from the momentum of the earth itself? I read a long time ago somewhere that if we extract enough tidal energy, the earth's rotation could slow down somehow. Obviously as a layperson on this matter I'm not that well-informed but just curious of the possibilities if anyone knows.

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The momentum of the Earth-moon system is 3.61x10^34 kg.m^2/s, of which 80% is the moon orbiting and 20% is the Earth (https://space.stackexchange.com/questions/50502/how-much-of-...).

KE = p^2 / 2m

Energy in the Moon's orbit: 5.7*10^45J

Energy in the Earth's orbit: 4.4*10^42J

So the lower momentum of the Earth (with a square term) and its (much) higher mass (Moon is 1.2% the mass of Earth) make Earth over 1000 times less energetic. So it's just the Moon that matters here.

Assume every joule extracted is coming directly from that budget and the moving water wasn't going to hit Scotland and turn some into heat anyway. 15.9 TW is average human energy usage.

5.7*10^45J / (15.9 TW * 1 year) = 1.14 * 10^25

So if we generated ALL human power from this method and every joule was taken from the Moon's orbital energy that would otherwise not be taken, we can spin the system down in just over a ten million billion billion years.

This is actually a bit more than I expected, though I knew it would be a lot from basic common sense of 80 billion billion tons moving at 1km/s. So maybe I've flubbed a few (tens of) orders of magnitude? In particular, the 1000:1 Moon:Earth energy ratio sounds plausible when I think about it, but it still was a bit of a surprise.

In any case, I think it's OK.

Edit, OK, so that was bunk, the orbital energy is 3.8×10^28J, so we can unbind the moon and donate it to Jupiter in only 65 million years.

> "So maybe I've flubbed a few (tens of) orders of magnitude?"

Yeah, a few tens! Part of it is that you seem to have reinterpreted angular momentum as linear momentum. That's not dimensionally cromulent.

Feh, knew it. Maybe I should have just used ChatGPT.

The real question is much energy do we need to harvest to slow down the moon's orbit enough to get exactly 12 lunar months per year?

EDIT: and can we also simultaneously slow down the Earth's rotation to have exactly 360 days per year? Fix the calendar once and for all.

> The real question is much energy do we need to harvest to slow down the moon's orbit enough to get exactly 12 lunar months per year?

Oh my god! That's an amazing idea!

Technically it's the momentum of the earth-moon system; tides are a continuous input of energy into the oceans taken from the rotation of the earth relative to the moon. Tides lose energy to friction. I don't think that increasing tidal friction would have effects back on the planetary system, but it might reduce overall tidal amplitude. Very slightly.

Friction makes the tidal bulges lag slightly, which means they're a little bit ahead of where the moon is. That produces a net acceleration on the moon that raises its orbit. Increasing tidal friction should increase the lag which should increase the speed at which the moon's orbit raises. Completely insignificant at human scale, of course, but technically it should be doing something.

The sci-fi writes itself. Having averted climate catastrophe by switching to renewables 23rd century humans face the imminent catastrophe of disrupting the entire solar system dynamics after wrecking Earth-Moon orbital stability with their reckless energy extraction.

This one's free, KSR, sir.

It’s raising the moons orbit and slowing the earths rotation. But changing either by even 1cm/second would take a long time even if you’re extracting 1 TW 24/7.

Those are rookie numbers. Exponential growth is real and lvl 1.2 in the Kardashian scale is the kpi.

edit: Kardashev scale

/s if not obvious

You can’t extract more energy from ocean tides than are actually in the ocean tides.

> Exponential growth is real

It’s really not. 99% of the energy used by humanity is still sunlight used to grow crops the same it’s been for thousands of years, and crop land use hasn’t been increasing exponentially. The majority of growth has been from efficiency gains not utilizing more energy.

Our ancestors realized irrigation and selective breeding allowed for more production from the same land. Fertilizer, better breeds, insecticides, meant more yield and automation meant less labor but the underlying energy input is unchanged barring increases in land under cultivation or the mouthes of livestock.

Instead many historic curves look exponential when you ignore the underlying population growth driving the whole thing and the recent reductions in fertility.

Fair enough.

But I have no doubt the energy gained by gifting the Kardashians escape velocity would be scientifically significant.

Funnily enough, this looks like the same surname with a Persian root for “stone carver” and Armenian and Russian surname suffixes. Given Persian origins, one can speculate that it was russified rather than armenified. So Kardashev’s ancestors might be Kardashians!

Less "23rd century" and more like "23,000,000th century".

I've read a lot of science fiction involving macroscale engineering on such levels, but I think even the most misanthropic science fiction writers have a hard time imagining a species that can start meaningfully affecting the orbital dynamics of their solar system but are clueless about possible negative side effects. By the time you're postulating such things, all the negative side effects that may leap to your mind involve energies many, many orders of magnitude smaller than the disruptions themselves, e.g., "oh no our satellite orbits", well, divert .000000000001% (I just hit some zeros, that's not calculated carefully) of the energy to fixing the satellite orbits. You're going to anyhow.

They don't have to be ignorant about the negative side effects, just unwilling to acknowledge or mitigate them. Could make for a good allegory about climate change.

> but are clueless about possible negative side effects

When has that ever stopped greed?

Misplaced misanthropy. The largest and most destructive atmospheric composition change in the history of the planet, against which our slight modification of CO2 levels is just a blip, was performed by completely unconscious species with zero capability for reflection or prediction of the results. Compared to the Great Oxygenation Catastrophe, we've done nothing.

On a more amusing note, if you're interested in a counterexample to your direct claim, which involves another catastrophe that makes the worst predictions of climate change look like human paradise, I would recommend to you "The Nitrogen Fix" by Hal Clement. I won't spoil what the catastrophe is in case you might be interested in reading it, but Google will spoil it readily if you prefer.

But that species didn't call itself Homo Sapiens, and didn't flatter itself that it knew better.

If you're going to move the goalposts that far to stretch for your misanthropic points there's no point in conversation.

That's not moving the goalposts, that's making explicit the obvious context that they were referring to intelligent, technological species.

> The sci-fi writes itself. Having averted climate catastrophe by switching to renewables 23rd century humans face the imminent catastrophe of disrupting the entire solar system dynamics after wrecking Earth-Moon orbital stability with their reckless energy extraction.

In the interest of avoiding spoilers, I will merely advise anyone interested in a story with similar themes and content to read Signal to Noise and its sequel A Signal Shattered by Eric S. Nylund. You may know him from his work on the Halo franchise or other popular games. I don’t really know his other work, since I first discovered him via his original works.

https://en.wikipedia.org/wiki/Eric_Nylund

Fact: The Moon orbit moves away 3.8 centimeters per year.

We have reduced tidal friction much more significantly by removing mangroves and killing coral reefs, general coastal buildup.

Earth's rotation is already slowing down because of 'tidal drag' by about 1..2ms per century (but surprisingly, also speeding up since recently):

https://en.wikipedia.org/wiki/Earth%27s_rotation

Given the current rate of development, it probably won't be an issue for a while.

Why the downvotes?

The 2004 Indian Ocean earthquake and following tsunami shortened Earth's day by 2.68 microseconds. The energy of the tsunami alone was around 4.2E15 joules. Considering Earth's mass, radius and moment of inertia it would take ~2.9E26 joules to shorten the day by 1 minute.

It seems it really won't be an issue for a while.

Not in a way that matters (is at all noticeable) until long after the sun expands and wipes out all life on earth in a few billion years. Of course by then, the moon will be quite a bit further from earth than it is today and might become tidally locked with the earth as the earth rotation slows down and eventually matches the speed at which the moon rotates. So there is that.

In the same way, we're not running out of geothermal energy (a tiny part of the heat actually comes from the moon pulling magma around, the rest from radioactive decay and residual hit from when our planet was created). Technically more heat radiates out via our crust naturally than we'll ever need.

So, technically yes but not in a way that actually matters on the time scales we have left on earth, which technically will become a lot more hostile over time anyway. A billion years from now, things will be very much changed here. Minuscule loss of momentum in the moon's orbital movement will be the least of our concerns there.

Conservation of angular momentum says no. Energy, momentum and angular momentum each have their own conservation laws.

Edit: although maybe it could facilitate transfer of angular momentum between Earth rotation and the moon's orbital motion.

There are conversions between each.

https://cs.stanford.edu/people/zjl/pdf/tide.pdf

Yes, but tides are already sucking a huge amount of rotational inertia out of the Earth all the time even if we aren't actually using it for any practical purpose. The only reason its not a problem is that the Earth has so much rotational inertia to begin with that it will take a very long time to run out.

I have always had this concern about wind and solar. Removing energy from one part of the earth always sounded very haphazard and untestable.

It's very unclear to me that removing heat from the ground, reducing wind speed/pressure, and lowering tidal forces is guaranteed to never have catastrophic impact.

>removing heat from the ground

You're not removing anything, you're just transforming kinetic energy into electrical energy. Energy transforms, everything transforms on earth, as per the laws of physics. When you die, your body doesn't get "removed", it gets transformed into worm food. It's the cycle of matter and energy. "Yeah science Mr. White!"

I doubt human devices that capture wind and water wave energy are enough to negatively impact the climate in a meaningful way, considering how powerful nature is.

During transfer, the source experiences removal.

Covering the ground in non transparent panels removes heat from the ground.

That's why you install solar panels where covering the ground has little to no impact, like on top of urban buildings or deserts where nothing lives or grows.

Yes.

The side effects of solar panels is indeed a cooler ground underneath. Plants have difficulty growing in the shade.

Panels have a darker shade than most ground they are covering, so they might actually absorb more heat than the typical ground they are covering. They are distorting the local albedo.

I think for geocooling by solar panels shade, the effect is completely local and only surface deep. After all stone/ground is an insulator, and geothermal energy is considered renewable.

If anything on average solar panels will warm the earth, because they are on average darker than whatever they're covering. (this effect is much less than due to CO2)

It prevents some heat from reaching the ground there (solar panels are ~20% efficient: most of the energy still reaches the ground). The energy (energy + heat is generally more than would normally be absorbed by heat in the ground) gets used and then turns into... heat. Which either makes it into the air or the ground, which is where it was going to wind up anyway.

And to take this to its ultimate conclusion, that electrical energy eventually gets turned back into heat one way or another.

If you drive an Nvidia 5090 and an Intel i9-14900KS then yes, it gets converted back to heat but that can also be reused like in some dorm rooms in Finland that are heated by the waste heat of the Nokia networking equipment.

"In a closed system entropy always goes up

That's the second law, now you know what's up

You can't win, you can't break even, you can't leave the game

Cause entropy will take it all 'though it seems a shame"

-MC Hawking

> I doubt human devices that capture wind and water wave energy are enough to negatively impact the climate in a meaningful way

Exactly what we used to say about industrial gas emissions.

At this point i think removing any heat from ground is a net win.

Solar is just transforming light into electricity, i don’t think it’s removing any heat from the system unfortunately, unlike radiative cooling paint.

Now that we should be painting any tropical building we can with.

I would expect large solar farms increase heat, as the panels are dark and absorb a lot of energy (by design). Normal groundcover is not so dark and reflects a higher percentage of sunlight.

Considering how little of the landmass needs to be covered, any effects are absolutely trivial compared to CO2 gains.

Wind turbines can have local effects, meaning less wind in areas behind the turbines, which can mean smog stays longer in a city.

But like the other comment said, with solar you are not taking anything.

But solar panels do heat up the air around them more than vegetation would do.

They do. But they also keep that heat from reaching the ground, which is a good thing for many agricultural applications.

See also, "Agrivoltaics."

Do you have this same concern about literally every structure man has ever constructed?

They do the same exact thing in terms of 'slowing wind down' and 'preventing the sun's energy from reaching the ground'.

This idea is understandable, but it falls apart for the same reason the wind turbine bird death concern does (the number of birds that have died due to humans liking windows is 1,000,000x the number that have died in turbines).

> Do you have this same concern about literally every structure man has ever constructed?

To a lesser extent, yes. However, power generating facilities are different in that they are intended to remove as much energy as possible, whereas sky scrapers etc are not.

This is exactly why science education is so important.

But if it makes you feel better, all man made structures combined cover a small fraction of the earths surface people tend to be in areas with other people and thus it looks like we’re doing more than we are. NYC for example has 291x the average population density of the rest of the US and that’s including over a square mile devoted to Central Park.

Agriculture has a bigger impact because it cover so much land, but that’s offset by it being relatively close to nature.

Cities probably have a significantly larger effect on the way that energy flows around the earth than renewable power generation does. It's relatively easy to change how much a very large amount of heat moves in comparison to how easy it is to harness energy into usable work. (See also why the greenhouse effect from CO2 emissions is such a big deal in comparison to basically any other thing that humans have done, as far as the energy balance of the earth is concerned. CO2 is responsible for about 20 times more energy being absorbed by the earth than humanity uses in total, from any source)

Sure, but again, wind turbines are on the scale of millions whereas there are tens of billions of structures swaying in the breeze.

I get your point, but a 1,000x+ difference in quantity cannot be ignored.

With solar humanity's total power consumption is just a fraction of a percent of the Sun's luminance. With wind in some areas you get towards a larger percentage. But either one pales in comparison to the impact of hydropower which has dammed some of the largest rivers and waterfalls in the world to offer only a small fraction of our existing electricity supply.

Its all a question of scale. All these systems can afford some extra energy loss just like the planet could cope with a certain amount of CO2 production because the plants and oceans would grab it and store it. Once we exceed those (currently unknown) limits however it can become a problem and the biggest solar farms are impacting the area around them as they change that energy balance.

> Removing energy from one part of the earth always sounded very haphazard and untestable.

.. compared to taking energy and carbon from the ground, and changing the atmospheric composition enough to significantly change the temperature? Because that's the alternative to not-renewables.

Yeah everything has an effect. I have been quietly strangling infants in their cots to prevent CO2 increase from their breathing resulting in runaway climate change. We need to take action and stop disrupting a system in homeostasis. We need to go back to the era of 10,000 humans and I volunteer everyone else to sacrifice themselves for my future.

Strictly speaking nothing, including posting HN comments, is guaranteed to never have catastrophic impact.

We all need to live with some calculated risks.

That's correct, while clean it'd be a mistake to classify it as "renewable".

https://cs.stanford.edu/people/zjl/pdf/tide.pdf is a pretty accessible entry.

I am extremely skeptical of that 1000 year estimate. It is almost entirely depending on the assumption of the continuous energy increase of 2% per year every year, for the next 1000 years, and that tidal energy remains 1% of that total the entire time.

I think that those assumptions are wrong in multiple ways and that reasonable estimates of the amount of tidal energy that could be extracted would lead to time scales where the risk no longer becomes relevant.

Yeah, the "2% growth forever" feels like a sneaky addition which is extremely controversial in economic theory: if endless growth is required. 1.02 ** 1000 ~= 400,000,000. So if the world population continued to grow at 2% in those same 1000 years, there'd be 2.8 quintillion people. Evenly distributed over the planet (water included), each person would get a square 1.35 centimeters on a side.

On the timescales involved here, there is no such thing as renewable energy.

I think if we're positing a world where our energy use increases 2% annually for a thousand years and that tidal power will remain a fixed fraction of that we're not dealing with a reasonable projection. In any event, at the end of those thousand years humanity won't be very far from Dyson Sphere territory and the tidal locking of Earth wouldn't be much of a problem for the civilization implied, but I don't think it's possible to extract tidal energy that fast.

If the world’s energy use increases by 2% annually for a thousand years and we’re generating it with anything other than wind/solar/tidal/geothermal, we will raise the equilibrium temperature of the Earth by tens of degrees just from thermodynamics.

Nah, to keep extracting the Earth's rotational energy that fast through tidal means we'll have had to import all the available liquid water from the rest of the solar system, rendering the climate change effects of the other energy use moot. ;)

If the world's energy usage increases by 2% for a thousand years we use 3.4 * 10^4 times more power than the solar radiation reaching the earth (1.02^1000 * 15/170000). Enough power to boil off the oceans in about a day (if I can believe Reddit and my math isn't off)

Yeah, I've done the calculations for 3% at 2%. I didn't feel like doing them again, so just went with a conservative bet.

It isn't a mistake to classify it as "renewable" because "renewable" doesn't literally mean until the end of time. Is solar not renewable because the sun will eventually explode? Ridiculous.

And as others have said, 1000 years is a hilariously wrong estimate.

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