The Science of Interstellar

The atmosphere’s O₂ is destroyed by breathing, burning, and decay. When we breathe in O₂ our bodies combine it with carbon to form carbon dioxide, CO₂, releasing lots of energy that our bodies use. When wood is burned, the flames rapidly combine the atmosphere’s O₂ with the wood’s carbon to form CO₂, which generates the heat that keeps the burning going. When dead plants decay on the forest floor, their carbon is slowly combined with the atmosphere’s O₂ to form CO₂ and heat.

The atmosphere’s O₂ is created primarily by photosynthesis: chloroplasts in plants[24] (Chapter 11) use energy from sunlight to split CO₂ into C and O₂. The O₂ is liberated into the Earth’s atmosphere, while the plants combine the carbon with hydrogen and oxygen from water to form the carbohydrates that they need for growth.

O 2 Destruction and CO 2 Poisoning

Suppose evolution creates a pathogen that destroys chloroplasts, as speculated by Elliot Meyerowitz at the end of the last chapter. Photosynthesis ends, not all at once, but gradually as plants die out. O₂ is no longer being created, but it is still being destroyed by breathing, burning, and decay—primarily decay, it turns out. Fortunately for the remaining humans, there is not enough decaying plant life on the Earth’s surface to swallow up all the O₂.

Most of the decay will be finished after thirty years, and only about 1 percent of the O₂ will be used up. There is still plenty for Cooper’s children and grandchildren to breathe, if they can find anything to eat.

But that 1 percent of the atmospheric O₂ will have been converted into carbon dioxide, which means 0.2 percent of the atmosphere will then be CO₂ (since most of the atmosphere is nitrogen). That’s enough CO₂ to make breathing unpleasant for highly sensitive people and perhaps drive the Earth’s temperature up (via the greenhouse effect) by 10 degrees Celsius (18 degrees Fahrenheit)—unpleasant for everyone, to put it mildly!

To make everyone’s breathing uncomfortable and induce drowsiness, ten times more atmospheric O₂ would have to be converted into CO₂; and to kill most everyone by CO₂ poisoning, an additional five times more would have to be converted, a factor of fifty in all. I have not found a plausible mechanism for this.

So is Professor Brand wrong? (Even theoretical physicists can make mistakes. Especially theoretical physicists. I know; I am one.) Probably yes, he is wrong, but conceivably no. The Professor could be right, but it would require geophysicists’ understanding of ocean bottoms to be severely flawed.

There is undecayed organic material on the ocean bottoms as well as on land. Geophysicists estimate that the amount on ocean bottoms is about one-twentieth that on land. If they are wrong and there is fifty times more on the ocean bottoms than on land, and if there is a mechanism to quickly dredge it up, then its decay to produce CO₂ could leave everyone gasping for oxygen and dying of CO₂ poisoning.

Now, once every many thousand years, an instability triggers the ocean to turn over. Water from the surface sinks to the bottom and drives bottom water to the surface. It is conceivable that in Cooper’s era there is such an overturn so vigorous that the upwelling bottom water brings with itself most of the ocean bottoms’ organic material. Suddenly exposed to the atmosphere, this material could decay, converting atmospheric O₂ into lethal amounts of CO₂.

Conceivable, yes. But highly improbable on two counts: highly unlikely that there is 1000 times more undecayed ocean-bottom organic material than geophysicists think, and highly unlikely that a sufficiently vigorous oceanic overturn will occur.[25]

Nevertheless, in Interstellar the Earth is surely dying and humanity must find a new home. The solar system, aside from Earth, is inhospitable, so the search is on, beyond our solar system.

13 Interstellar Travel

Professor Brand tells Cooper, in their first meeting, that the Lazarus missions have been sent out to search for new homes for humanity. Cooper responds, “There’s no planet in our solar system that can support life, and it’d take a thousand years to reach the nearest star. That doesn’t even qualify as futile. Where did you send them, Professor?”

The worse-than-futile challenge, if you don’t have a wormhole, is obvious when you realize just how far it is to the nearest stars (Figure 13.1).

Distances to Nearest Stars

The nearest star (other than our Sun) thought to have a habitable planet is Tau Ceti, 11.9 light-years from Earth, so traveling at light speed you would need 11.9 years to reach it. If there are any habitable planets closer than that, they can’t be much closer.

To get some sense of just how far Tau Ceti is compared to more familiar things, let’s scale its distance down enormously. Imagine it as the distance from New York City to Perth, Australia, about halfway around the world.

^{Fig. 13.1. All the stars within 12 light-years of Earth. The Sun, Proxima Centauri, and Tau Ceti are circled in yellow, purple, and red. [I adapted this map from Richard Powell’s www.atlasoftheuniverse.com.]}

The very nearest star other than the Sun is Proxima Centauri, 4.24 light-years from Earth, but there is no evidence it has habitable planets. With Tau Ceti’s distance imagined as New York to Perth, then Proxima Centauri’s is like New York to Berlin. It’s not a lot closer than Tau Ceti!

For comparison, the most distant unmanned spacecraft that humans have sent into interstellar space is Voyager 1, now about 18 light-hours from Earth. It has been traveling for thirty-seven years to get there. With Tau Ceti’s distance imagined as New York to Perth, then Earth to Voyager 1 is about 3 kilometers (2 miles): the distance from the Empire State Building to the southern end of Greenwich Village. That’s hugely less than New York to Perth.

The Earth to Saturn is even smaller: 200 meters, two east-west blocks in New York City, from the Empire State Building to Park Avenue. The Earth to Mars is just 20 meters; and the Earth to the Moon (the greatest distance humans have ever yet traveled) is just 7 centimeters—about two and a half inches!

Compare what we have achieved in going to the Moon, two and a half inches, with the challenge of going halfway around the world. That’s the leap of technology required to take humans to habitable planets outside our solar system!

Travel Times with Twenty-First-Century Technology

Voyager 1 is traveling out of the solar system at 17 kilometers per second, having been boosted by gravitational slingshots around Jupiter and Saturn. In Interstellar, the Endurance travels from Earth to Saturn in two years, at an average speed of about 20 kilometers per second. The fastest speed I think rocket technology plus solar system slingshots are likely to achieve in this, the twenty-first century, is about 300 kilometers per second.

24

Chloroplasts and photosynthesis also occur in algae, and in cyanobacteria in the ocean, both of which I treat as plant life in my simplified description. (In some sense, cyanobacteria are a form of chloroplast.)

25

For some quantitative details and explanations of the huge uncertainties in the geophysical estimates, see Some Technical Notes at the end of the book.