xkcd: What If?

[neufer]

The amount of heat the Earth radiates is exquisitely sensitive to the details of our atmosphere. Our planet is heating up now because human activity is changing the atmosphere. But that same change can (and has) occurred naturally. And whether anthropogenic or natural, the heating totally swamps the heat actually produced by human activity. So the question comes down to how well the exact constituents of the atmosphere can be determined... and if you can do that, you have much more information about life than a simple thermal signature provides.

Greenhouse gases are NOT exothermic.
They do NOT increase the amount of outgoing infrared radiation.

The Earth absorbs about 121,000 terawatts of solar radiation
& radiates back out about 121,063 terawatts of infrared radiation

Code: Select all

Incoming  solar radiation:          173,000 terawatts
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Absorbed solar radiation:           121,000 terawatts
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Geothermal energy from nuclear decay:    46 terawatts
Waste heat from fossil fuel consumption: 13 terawatts
Tidal energy:                             3 terawatts
Waste heat from nuclear fuel consumption: 1 terawatts

[Chris Peterson]

Greenhouse gases are NOT exothermic.
They do NOT increase the amount of outgoing infrared radiation.

I know that the gases are not exothermic. But they do alter the infrared radiation signature of the Earth. These gases are transparent in parts of the IR spectrum that would presumably be monitored by the proposed telescope.

[neufer]

Greenhouse gases are NOT exothermic.
They do NOT increase the amount of outgoing infrared radiation.

I know that the gases are not exothermic. But they do alter the infrared radiation signature of the Earth. These gases are transparent in parts of the IR spectrum that would presumably be monitored by the proposed telescope.

Venus & Mars have lots of CO₂ in their atmospheres but no signs of life.

Mars has methane (which is at least encouraging).

What greenhouse gas did you have in mind

[Chris Peterson]

Venus & Mars have lots of CO₂ in their atmospheres but no signs of life.

Mars has methane (which is at least encouraging).

What greenhouse gas did you have in mind :?:

CO₂. CH₄. H₂O. You name it. They produce atmospheric heating, and that heating is detectable from outside the atmosphere by observing in IR.

I'm not talking about using the presence of the gases to detect life, I'm talking about the questionable value of using IR to measure heat balance as an indicator of life.

[bystander]

Sunset on the British Empire

• When (if ever) did the Sun finally set on the British Empire? — Kurt Amundson

[bystander]

Sunset on the British Empire

• Does an eclipse really count as the sun setting?

[neufer]

<<Fletcher Christian (25 September 1764 – 20 September 1793) was master's mate on board the Bounty during William Bligh's voyage to Tahiti for breadfruit plants. In the mutiny on the Bounty, Christian seized command of the ship from William Bligh on 28 April 1789.

Following the mutiny, Christian attempted to build a colony on Tubuai, but there the mutineers met with conflict with natives. Abandoning the island, he stopped briefly in Tahiti where he married Maimiti, the daughter of one of the local chiefs, on 16 June 1789. While on Tahiti, he dropped off sixteen crewmen. These sixteen included four Bligh loyalists who had been left behind on the Bounty and two who had neither participated in, nor resisted, the mutiny. The remaining nine mutineers, six Tahitian men and eleven Tahitian women then sailed eastward. In time, they landed on Pitcairn Island, where they stripped the Bounty of all that could be floated ashore before Matthew Quintal set it on fire, stranding them. The resulting sexual imbalance, combined with the effective enslavement of the Tahitian men by the mutineers, led to insurrection and the deaths of most of the men.

The American seal-hunting ship Topaz visited Pitcairn in 1808 and found only one mutineer, John Adams (who had used the alias Alexander Smith while on the Bounty), still alive along with nine Tahitian women. The mutineers who had perished had, however, already had children with their Tahitian wives. Most of these children were still living. Adams and Maimiti claimed Christian had been murdered during the conflict between the Tahitian men and the mutineers. According to an account by a Pitcairnian woman named Jenny who left the island in 1817, Christian was shot while working by a pond next to the home of his pregnant wife. Along with Christian, four other mutineers and all six of the Tahitian men who had come to the island were killed in the conflict. One of the four surviving mutineers fell off a cliff while intoxicated and was killed. Quintal was later killed by the remaining two mutineers after he attacked them.

Christian was survived by Maimiti and his son, Thursday October Christian (born 1790). Besides Thursday October, Fletcher Christian also had a younger son named Charles Christian (born 1792) and a daughter Mary Ann Christian (born 1793). Thursday and Charles are the ancestors of almost everybody with the surname Christian on Pitcairn and Norfolk Islands, as well as the many descendants who have moved to Australia, New Zealand and the United States.

Rumours have persisted for more than two hundred years that Christian's murder may have been faked, that he had left the island and that he made his way back to England. Many scholars believe that the rumours of Christian returning to England helped to inspire Samuel Taylor Coleridge's The Rime of the Ancient Mariner.>>

[bystander]

Sunless Earth

• What would happen to the Earth if the Sun suddenly switched off? — Many, many readers

[geckzilla]

Glad we got that one over with.

[bystander]

Extreme Boating

• What would it be like to navigate a rowboat through a lake of mercury? What about bromine?
  Liquid gallium? Liquid tungsten? Liquid nitrogen? Liquid helium? — Nicholas Aron

[neufer]

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Extreme Boating

<<Tom McClean is a veteran of both the Parachute Regiment and the SAS and is a survival expert who lived on the island of Rockall from 26 May to 4 July 1985 to affirm Britain's claim to it. In 1990 McClean sailed the Atlantic again, this time in a 37 ft boat shaped like a sauce bottle which he had constructed himself. His boat, the Typhoo Atlantic Challenger, sailed from New York to Falmouth, England. McClean's most recent feat was the construction, in 1996, of a boat shaped like a giant whale. The boat, 'Moby' Prince of Whales, stands 25 ft high and 65 ft long. It has a spout which can launch water as high as 6 metres in the air.>>

[bystander]

Free Fall

• What place on Earth would allow you to freefall the longest by jumping off it?
  What about using a squirrel suit? — Dhash Shrivathsa

[neufer]

[b][color=#0000FF][size=150]The western slopes of Maxwell Montes are very steep[/size] [where] it rises about 6.4 kilometres above the Lakshmi Planum.[/color][/b]

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Free Fall

• What place on Earth would allow you to freefall the longest by jumping off it? What about using a squirrel suit? — Dhash Shrivathsa

Terminal velocities on Venus are only about an eighth of those on Earth such that all times would increase by a factor of 8.

Terminal velocity :

• ~ 6.9 m/s in free fall
  ~ 2.2 m/s in a wingsuit

Venus:

• surface gravity = 0.904 that of Earth
  buoyancy mass = 0.935 that of Earth
  air density = 54.7 that of Earth

[henry654]

what if there was no gravity? Coul we survive?

[geckzilla]

I don't think we would ever come to exist without gravity. Matter wouldn't clump together. If gravity were to suddenly disappear Earth would stop orbiting and fly away on a tangent and then probably fall apart pretty quickly. I guess the core would explode outward using all of the energy that gravity previously balanced which would then burn and destroy everything and everyone. It makes the ending of The Neverending Story seem nice.

[bystander]

what if there was no gravity? Coul we survive?

Answering your hypothetical questions with physics, every Tuesday.

From the creators of:


Submit a Question

[bystander]

Bouncy Balls

• What if one were to drop 3,000 bouncy balls from a seven story parking structure onto a person walking on the sidewalk below? Should the person survive, what would be the number of bouncy balls needed to kill them? What injuries would occur and what would the associated crimes be? — Ginger Bread

[geckzilla]

I sent a question in yesterday morning. Too bad it's probably not physics-y enough to get answered. You'll never know what I asked!

[neufer]

Bouncy Balls

• What if one were to drop 3,000 bouncy balls from a seven story parking structure onto a person walking on the sidewalk below? Should the person survive, what would be the number of bouncy balls needed to kill them? What injuries would occur and what would the associated crimes be? — Ginger Bread

They may charge Ginger Bread with murder but they will never be able to catch him.

[bystander]

Drain the Oceans

• How quickly would the ocean's drain if a circular portal 10 meters in radius leading into space was created at the bottom
  of Challenger Deep, the deepest spot in the ocean? How would the Earth change as the water is being drained? — Ted M.

[geckzilla]

Wouldn't teleporting all of that ocean to Mars cause the orbits of both planets to change dramatically?

[rstevenson]

Wouldn't teleporting all of that ocean to Mars cause the orbits of both planets to change dramatically?

The mass of all the oceans is only about 0.02% of the total mass of the Earth. So in this scenario, where not all of the water is moved, there would be minimal effect on the orbits of both planets. I'll try pushing aside some piles of stuff to find my notes from last year so I can crunch the numbers.

Edit:
Even without the numbers, we can deduce an effect...

The simple form of the equation we need is P² = (4pi²a³)/G(M_S+M_E)

You can see that the mass of the Earth (M_E) is in the denominator, so if that mass is reduced, the denominator is less and the value of the right side of the equation is therefore greater, so P (the orbital period) is greater. In other words, the Earth will take a little longer to go once around the Sun, implying its orbit will be a little larger. This might be a good way to control global warming! Except that the amount of reduction in M_E is just 0.02% or even less, and M_E is already tiny compared to M_S (the mass of the Sun), so the effect on P is very small.

Rob

[geckzilla]

Right, I forget how thin our biosphere is.

[geckzilla]

What if you increased the mass of the earth to the mass of the sun? Actually, I got this Universe Sandbox program last night and it's able to answer just that and more. Pretty fun. Picked it up for a couple bucks on Steam sale last night. Apparently schools can get it for free. The rest of us have to pay after the free trial is up. http://universesandbox.com/

[neufer]

What if you increased the mass of the earth to the mass of the sun?

We would all be living on a white dwarf and I would be somewhat more degenerate than usual.