3 ms·

There's no way that a tidal plant running for 100 years is going to change the day by 1 second. You're many orders of magnitudes off. That means that if you bu

by avar 10y ago

There's no way that a tidal plant running for 100 years is going to change the day by 1 second. You're many orders of magnitudes off. That means that if you built 100 of those plants that in 1 year you'd change the length of the day by 1 second. The 2011 Tōhoku earthquake and tsunami released 3.9×10^22 joules of energy[1] and changed the day by just over a microsecond[2]. A single Tsar Bomba releases 2.1×10^17 joules, so you'd need the energy of around 200 thousand of those to produce the same amount of energy as the earthquake, and then assuming you can direct that energy you need 1000 times more to shift the day by a second instead of a microsecond. That means that each one of your proposed tidal plants would need to generate the energy of 20,000,000 Tsar Bombas per year. That's one Tsar Bomba's worth of energy being released every 1.5 seconds at each one of those 100 plants. Buy stock in sunscreen companies now! Each one of your 100 plants would satisfy all of current yearly world energy production in just 30 minutes[3][4]. I'm too lazy to calculate what that would do to the surrounding area, but I'd wager that they're going to severely depress local property prices due much of the surrounding beach-side property area now being turned into permanent molten lava. The good news is you could probably build really good wind power plants now, due to constant hurricane winds not seen on Earth since the K–T impact, but I digress. I didn't check your molten rock calculation but given how off your tidal plant calculation is I wouldn't be surprised if it's way off as well. 1. https://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami#Energy https://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_an... 2. https://www.nasa.gov/topics/earth/features/japanquake/earth20110314.html https://www.nasa.gov/topics/earth/features/japanquake/earth2... 3. https://en.wikipedia.org/wiki/World_energy_consumption https://en.wikipedia.org/wiki/World_energy_consumption 4. ((5.67x10^20)/(2.1x10^17))/1.5/60 = 30

skrause 10y ago
> and then assuming you can direct that energy you need 1000 times more to shift the day by a second instead of a microsecond. Still off by a factor of 1000, a microsecond is one millionth of a second.
- avar 10y ago
  Yes you're right. I mixed up micro & milli there. So: * [...] 20,000,000,000 Tsar Bombas per year * One Tsar Bomba going off every 1.5ms * World energy production satisfied in 1.8 seconds We're also going to need more sunscreen.
adekok 10y ago
> There's no way that a tidal plant running for 100 years is going to change the day by 1 second. You're many orders of magnitudes off. It was a recollection of a problem from 25 years ago for crying out loud. Thinking about it some more, it's probably slowing down the rotation of the earth by 1 second over the course of 100 years. Not 1 second per day. Which makes about 4-5 orders of magnitude difference. > I didn't check your molten rock calculation but given how off your tidal plant calculation is I wouldn't be surprised if it's way off as well. The point was to get a "back of the envelope" calculation. Not to get a precise answer to 4 decimal places. What's with the bike shedding? Is the point here to have a discussion, or to prove that you're smarter than everyone else?
eric_h 10y ago
> given how off your tidal plant calculation is I wouldn't be surprised if it's way off as well. he showed his work for his first calculation. Looked reasonable to me.
- avar 10y ago
  His numbers look fine, but I think the assumptions are false. > And these furnaces are maybe 50 feet across. [...] > So the Earth has a heat stored which is > (4K miles / 50 feet)^3 This seems to assume that the earth is also cube shaped like a hypothetical 50ft^3 blast furnace, instead of a sphere (or oblate spheroid before anyone out-pedants me). It also seems to assume that a 50ft^3 blast furnace contains energy equivalent to 50ft^3 of molten mantle. If you look at diagrams of blast furnaces you can see that a relatively tiny part of them is actually molten material. Not being able to find numbers on the stored joules of industrial blast furnaces, I'm just going to just state that 50ft^3 of mantle has a crapload more potential energy than 50ft^3 of blast furnace, most of which is air, equipment & empty space, not molten material. He is right that there's at least as much energy stored in the earth as he's saying, but it's going to be a crapload more than that. The insides of blast furnaces are also significantly colder than the average temperature of the mantle.
  - adekok 10y ago
    The point is that these are "back of the envelope" calculations. Fermi was known for asking questions like "how many gold balls git into a suitcase". The point isn't to get a perfect answer. The point is to get a reasonable answer. In this case, the point is to answer the OP question in laymans terms. Using not much more than reasonable assumptions, and grade 5 math. Or, you can bike shed over details.

by avar 10y ago

skrause 10y ago

> and then assuming you can direct that energy you need 1000 times more to shift the day by a second instead of a microsecond. Still off by a factor of 1000, a microsecond is one millionth of a second.

avar 10y ago
Yes you're right. I mixed up micro & milli there. So: * [...] 20,000,000,000 Tsar Bombas per year * One Tsar Bomba going off every 1.5ms * World energy production satisfied in 1.8 seconds We're also going to need more sunscreen.

adekok 10y ago

> There's no way that a tidal plant running for 100 years is going to change the day by 1 second. You're many orders of magnitudes off. It was a recollection of a problem from 25 years ago for crying out loud. Thinking about it some more, it's probably slowing down the rotation of the earth by 1 second over the course of 100 years. Not 1 second per day. Which makes about 4-5 orders of magnitude difference. > I didn't check your molten rock calculation but given how off your tidal plant calculation is I wouldn't be surprised if it's way off as well. The point was to get a "back of the envelope" calculation. Not to get a precise answer to 4 decimal places. What's with the bike shedding? Is the point here to have a discussion, or to prove that you're smarter than everyone else?

eric_h 10y ago

> given how off your tidal plant calculation is I wouldn't be surprised if it's way off as well. he showed his work for his first calculation. Looked reasonable to me.

avar 10y ago
His numbers look fine, but I think the assumptions are false. > And these furnaces are maybe 50 feet across. [...] > So the Earth has a heat stored which is > (4K miles / 50 feet)^3 This seems to assume that the earth is also cube shaped like a hypothetical 50ft^3 blast furnace, instead of a sphere (or oblate spheroid before anyone out-pedants me). It also seems to assume that a 50ft^3 blast furnace contains energy equivalent to 50ft^3 of molten mantle. If you look at diagrams of blast furnaces you can see that a relatively tiny part of them is actually molten material. Not being able to find numbers on the stored joules of industrial blast furnaces, I'm just going to just state that 50ft^3 of mantle has a crapload more potential energy than 50ft^3 of blast furnace, most of which is air, equipment & empty space, not molten material. He is right that there's at least as much energy stored in the earth as he's saying, but it's going to be a crapload more than that. The insides of blast furnaces are also significantly colder than the average temperature of the mantle.
- adekok 10y ago
  The point is that these are "back of the envelope" calculations. Fermi was known for asking questions like "how many gold balls git into a suitcase". The point isn't to get a perfect answer. The point is to get a reasonable answer. In this case, the point is to answer the OP question in laymans terms. Using not much more than reasonable assumptions, and grade 5 math. Or, you can bike shed over details.