What if everything was antimatter, EXCEPT Earth?

Sean Gallagher

This one doesn't end well for us. But—unlike most scenarios involving the word "antimatter"—the end is surprisingly slow and drawn-out.

The whole universe is matter, as far as we can tell. No one is sure why there's more antimatter than matter, since the laws of physics are pretty symmetrical, and so there's no reason to expect there to be more of one than the other.[1]Although when it comes down to it, there's no reason to expect anything at all.

It's possible that galaxies are made of antimatter, and we just haven't noticed because we haven't tried to touch them. This is a cool idea, but if there are zones of matter and zones of antimatter, we should see a telltale gamma-ray glow from the boundary between the zones. So far, we haven't seen that, although another telescope might help.

If the rest of the universe were swapped out for antimatter, we'd be in trouble. Outer space isn't really "space";[2]As far as I know, it really is "outer", for what that's worth. it's full of a thin gas.[3]Technically, plasma.​[4]Technically, there's also a substantial quantity of solid grains of dust.​[5]Look, there's a bunch of little bits that are hard to see, ok?.​[6]Ok, they're not always hard to see.

The Earth's magnetic field protects us from the solar wind, and would protect us from an anti-solar wind, too. A tiny fraction of the particles from the Sun do reach the Earth, funneled down by our magnetic field, and create the aurora. In this scenario, the aurora would get a lot brighter, but most of the time not bright enough to really cause problems.

Meteorites would be the real problem.

The Earth sweeps up space dust as it travels around its orbit.[7]Unfortunately for us, antimatter is probably attracted to matter by gravity. About 100 tons of dust per day enters the atmosphere in the form of tiny grains, most weighing about 10^-5 grams. An additional similar average per-day amount arrives in giant clumps all at once.

This inflow of antimatter dust would collide with the top of our atmosphere and be annihilated. The interactions between the nuclei and antinuclei and protons and antiprotons would be complex,[8]A lot of the energy would be carried away by neutrinos. but the end result would be a lot of gamma rays, which would turn into a lot of heat. This steady flow of material (which would be worst around dawn, when your house was facing in the direction of Earth's motion).

The heat and light added by the antimatter would most likely be enough to tip the Earth into a "runaway greenhouse" scenario, turning the Earth into something resembling Venus.

But the big asteroids would get us first. Even a relatively small object like the Chelyabinsk meteor would deliver as much energy as the meteor that killed the dinosaurs.[9]Although it would deliver it to the top of the atmosphere, so in some ways it wouldn't be as bad. Fairly large asteroids enter the atmosphere every few months—mostly unnoticed. If they were all antimatter, each one would trigger a tremendous pulse of energy in the sky and ignite a massive firestorm.[10]If an antimatter meteor is large enough, encountering a cloud could launch some of it backward without completely destroying it. However, it's hard to come up with a practical scenario in which a meteor would exhibit this effect in Earth's atmosphere—unless it were so large that it would have basically destroyed the planet anyway.

Right now, it's still an open question whether any significant percentage of the stuff in the sky is made of antimatter. It's probably not, but we'd need to build another orbiting gamma-ray telescope to really be sure.

However, it's easy to use a telescope to rule out one possibility: That everything in the sky is antimatter.

If you have a telescope, maybe you can get that result published.

What would happen if all the bodies of water on Earth magically disappeared?

—Joanna Xu

As is often the case with these questions, everyone would die.

The first people to notice would be swimmers and boaters, for obvious reasons.

To avoid a glass half empty scenario, we'll assume the water is replaced by air.

Most people swim in water which is relatively shallow, so most of them would survive the fall to the bottom, albeit with a few broken bones.[1]Those swimming in quarries and glacial lakes, on the other hand, could easily fall to their deaths a few feet from shore. People out on the ocean, on the other hand, would be in trouble.

The ones in shallow water would hit bottom first, since they wouldn't have as far to fall. Within the first second, a large fraction of the boats in lakes, rivers, and harbors would crash into the bottom, and many of those on board would survive.

Boats out on the ocean would take longer to fall. Over the next five seconds, a wave of crashes would spread outward from the continents, as boats struck the continental shelf farther and farther from shore. These boats would be smashed to tiny fragments, killing everyone on board.

After the first six or seven seconds, there would be a brief lull in the ship destruction rate. Continental shelves drop off steeply, and most of the ships out over the deep sea would take a little longer to fall.

The Titanic sank in about two miles of water. After it disappeared beneath the surface, the two halves of the ship took between 5 and 15 minutes to reach the bottom.[2]When the Titanic bow hit the sea floor, it was moving at almost exactly the same speed as when it struck the iceberg three hours earlier. (This is not quite a coincidence.) Without the ocean there, it would have reached the bottom in about 30 seconds, striking it at airliner cruising speed.[3]Although no one has ever dropped a cruise ship from a high altitude,^{[citation needed]} their terminal velocity at the surface is probably a little below the speed of sound. Because the air in the ocean basins would be compressed, the terminal velocity of ships near the bottom would be lower than at the surface. This compression also means that to magically replace the water, you'd need more air than you'd expect from the ocean's volume alone, since it would need a varying density profile. In other words, your water-replacement spells will need to have some calculations behind them.

Sufficiently advanced magic is indistinguishable from technology.

Within the first minute, just about every large ship would be on the bottom. The final boat to reach the bottom would probably be a small sailboat or life raft that was crossing an ocean trench when the water vanished. Thanks to low weight and/or drag from the sails, one of these vessels could take many minutes to reach the bottom.

If there were a seaplane floating on the deep ocean, it could conceivably survive, although it would take some luck and quick thinking by the pilot. The plane would initially drop, but as it gained speed it would tend to pull into a glide. After the initial shock, the pilot would have a reasonable amount of time to try to start the engine. Thanks in part to the thicker air, it's possible a seaplane could successfully land on a smooth patch of seabed. If the engine got started, the pilot could also try to fly to shore and land on a runway.

Fish, whales, and dolphins, and nearly all marine life would die immediately. Those near the bottom would suffocate or dessicate, while those near the surface in deeper water would suffer the same fate as boats.

Then the really weird stuff starts.

Without evaporation from lakes and oceans feeding the water cycle, it would stop raining. Without pools of water to drink from, people and most animals would dehydrate and die in a matter of days. Within a few weeks, plants would start withering in the ever-drier air. Within months, mass forest die-offs would begin.[4]Some drought-resistant trees could survive for years, but others wouldn't.

Huge amounts of dry, dead vegetation lead inevitably to fire, and within a few years, most of the world's forests would have burned. Forests store huge amounts of CO₂, and this burning would roughly double the amount of greenhouse gas in the atmosphere, accelerating global warming.

All in all, Joanna's scenario would result in virtually all life dying out pretty fast. But then things would get even worse.

Without a water cycle to weather rocks, the carbon-silicate feedback system which acts as a long-term thermostat to stabilize climate[6]CO₂ is added to the atmosphere by volcanoes (although at the moment, it's being added about ten times faster by people.) When water flows over certain rocks, chemical reactions suck CO₂ from the air and eventually bury it in seafloor sediments. With less CO₂, the planet gets colder. A colder planet means less evaporation, which means less weathering, which means CO₂ removal slows down. This feedback loop—which operates over much longer timescales than human-caused climate change—is probably what's kept the Earth's temperature relatively stable over the last few billion years[5]Give or take a few snowball Earths. even though the Sun has gotten hotter. would shut down. Without this feedback, volcanic CO₂ would build up in our atmosphere, leading—in the long term—to scorching temperatures similar to what's happened on Venus.[7]Interestingly, because of Venus's lighter color (and thus higher reflectivity), it only absorbs about half the solar radiation that Earth does despite being substantially closer to the Sun. The thick blanket of CO₂ in its atmosphere is what keeps it hot.

We were going to lose our oceans anyway. As the Sun gets hotter, eventually water will start escaping through evaporation, and—one way or another—the planet will dry out and heat up. However, the loss of the oceans never seemed like something worth worrying too much about, since it's a billion years in the future. The oceans will be here long after our species is gone.

Unless Joanna ruins everything.