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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 some

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.

nbf_1995 1y ago
https://cs.stanford.edu/people/zjl/pdf/tide.pdf https://cs.stanford.edu/people/zjl/pdf/tide.pdf
brian_spiering 1y ago
Given the current rate of development, it probably won't be an issue for a while.
- aeve890 1y ago
  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.
HPsquared 1y ago
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.
- bobmcnamara 1y ago
  There are conversions between each.
nachexnachex 1y ago
That's correct, while clean it'd be a mistake to classify it as "renewable". https://cs.stanford.edu/people/zjl/pdf/tide.pdf https://cs.stanford.edu/people/zjl/pdf/tide.pdf is a pretty accessible entry.
- MostlyStable 1y ago
  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.
  - nick238 1y ago
    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.
- theptip 1y ago
  On the timescales involved here, there is no such thing as renewable energy.
- Symmetry 1y ago
  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.
pjc50 1y ago
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.
- wat10000 1y ago
  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.
  - aeve890 1y ago
    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.
    - Retric 1y ago
      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.
      - aeve890 1y ago
        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
        Retric 1y ago
flohofwoe 1y ago
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 https://en.wikipedia.org/wiki/Earth%27s_rotation
kryogen1c 1y ago
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.
- BurningFrog 1y ago
  Strictly speaking nothing, including posting HN comments, is guaranteed to never have catastrophic impact. We all need to live with some calculated risks.
- FirmwareBurner 1y ago
  >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.
  - kryogen1c 1y ago
    During transfer, the source experiences removal. Covering the ground in non transparent panels removes heat from the ground.
    - FirmwareBurner 1y ago
      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.
    - nico_h 1y ago
      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.
grues-dinner 1y ago
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-the-total-angular-momentum-of-the-earth-moon-system-is-due-to-the-mo 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.
- perihelions 1y ago
  > "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.
  - grues-dinner 1y ago
    Feh, knew it. Maybe I should have just used ChatGPT.
- meatmanek 1y ago
  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.
jillesvangurp 1y ago
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.
elihu 1y ago
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.

nbf_1995 1y ago

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

brian_spiering 1y ago

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

aeve890 1y ago
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.

HPsquared 1y ago

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.

bobmcnamara 1y ago
There are conversions between each.

nachexnachex 1y ago

That's correct, while clean it'd be a mistake to classify it as "renewable". https://cs.stanford.edu/people/zjl/pdf/tide.pdf https://cs.stanford.edu/people/zjl/pdf/tide.pdf is a pretty accessible entry.

MostlyStable 1y ago
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.
- nick238 1y ago
  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.
theptip 1y ago
On the timescales involved here, there is no such thing as renewable energy.
Symmetry 1y ago
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.

pjc50 1y ago

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.

wat10000 1y ago
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.
- aeve890 1y ago
  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.
  - Retric 1y ago
    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.
    - aeve890 1y ago
      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
      - Retric 1y ago

flohofwoe 1y ago

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

kryogen1c 1y ago

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.

BurningFrog 1y ago
Strictly speaking nothing, including posting HN comments, is guaranteed to never have catastrophic impact. We all need to live with some calculated risks.
FirmwareBurner 1y ago
>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.
- kryogen1c 1y ago
  During transfer, the source experiences removal. Covering the ground in non transparent panels removes heat from the ground.
  - FirmwareBurner 1y ago
    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.
  - nico_h 1y ago
    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.

grues-dinner 1y ago

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-the-total-angular-momentum-of-the-earth-moon-system-is-due-to-the-mo 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.

perihelions 1y ago
> "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.
- grues-dinner 1y ago
  Feh, knew it. Maybe I should have just used ChatGPT.
meatmanek 1y ago
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.

jillesvangurp 1y ago

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.

elihu 1y ago

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.