Макс Тегмарк - Life 3.0 - Being Human in the Age of Artificial Intelligence

But hold on: didn’t Einstein’s special relativity theory say that nothing can travel faster than light? So how can galaxies outrace something traveling at the speed of light? The answer is that special relativity is superseded by Einstein’s general relativity theory, where the speed limit is more liberal: nothing can travel faster than the speed of light through space, but space is free to expand as fast as it wants. Einstein also gave us a nice way of visualizing these speed limits by viewing time as the fourth dimension in spacetime (see figure 6.7, where I’ve kept things three-dimensional by omitting one of the three space dimensions). If space weren’t expanding, light rays would form slanted 45-degree lines through spacetime, so that the regions we can see and reach from here and now are cones. Whereas our past light cone would be truncated by our Big Bang 13.8 billion years ago, our future light cone would expand forever, giving us access to an unlimited cosmic endowment. In contrast, the middle panel of the figure shows that an expanding universe with dark energy (which appears to be the Universe we inhabit) deforms our light cones into a champagne-glass shape, forever limiting the number of galaxies we can settle to about 10 billion.

If this limit makes you feel cosmic claustrophobia, let me cheer you up with a possible loophole: my calculation assumes that dark energy remains constant over time, consistent with what the latest measurements suggest. However, we still have no clue what dark energy really is, which leaves a glimmer of hope that dark energy will eventually decay away (much like the similar dark-energy-like substance postulated to explain cosmic inflation), and if this happens, the acceleration will give way to deceleration, potentially enabling future life forms to keep settling new galaxies for as long as they last.

Figure 6.7: In a spacetime diagram, an event is a point whose horizontal and vertical positions encode where and when it occurs, respectively. If space isn’t expanding (left panel), then two cones delimit the parts of spacetime that we on Earth (at apex) can be affected by (bottom cone) and can have an effect on (top cone), because causal effects cannot travel faster than light, which travels a distance of one light-year per year. Things get more interesting when space expands (right panels). According to the standard model of cosmology, we can only see and reach a finite part of spacetime even if space is infinite. In the middle image, reminiscent of a champagne glass, we use coordinates that hide the expansion of space so that the motions of distant galaxies over time correspond to vertical lines. From our current vantage point, 13.8 billion years after our Big Bang, light rays have had time to reach us only from the base of the champagne glass, and even if we travel at the speed of light, we can never reach regions outside the upper part of the glass, which contains about 10 billion galaxies. In the right image, reminiscent of a water droplet beneath a flower, we use the familiar coordinates where space is seen to expand. This deforms the glass base to a droplet shape because regions at the edges of what we can see were all very close together early on.

How Fast Can You Go?

Above we explored how many galaxies a civilization could settle if it expanded in all directions at the speed of light. General relativity says that it’s impossible to send rockets through space at the speed of light, because this would require infinite energy, so how fast can rockets go in practice? *8

NASA’s New Horizons rocket broke the speed record when it blasted off toward Pluto in 2006 at a speed of about 100,000 miles per hour (45 kilometers per second), and NASA’s 2018 Solar Probe Plus aims to go over four times faster by falling very close to the Sun, but even that’s less than a puny 0.1% of the speed of light. The quest for faster and better rockets has captivated some of the brightest minds of the past century, and there’s a rich and fascinating literature on the topic. Why is it so hard to go faster? The two key problems are that conventional rockets spend most of their fuel simply to accelerate the fuel they carry with them, and that today’s rocket fuel is hopelessly inefficient—the fraction of its mass turned into energy isn’t much better than the 0 . 00000005% for gasoline that we saw in table 6.1. One obvious improvement is to switch to more efficient fuel. For example, Freeman Dyson and others worked on NASA’s Project Orion, which aimed to explode about 300,000 nuclear bombs during 10 days to reach about 3% of the speed of light with a spaceship large enough to carry humans to another solar system during a century-long journey.5 Others have explored using antimatter as fuel, since combining it with ordinary matter releases energy with nearly 100% efficiency.

Another popular idea is to build a rocket that need not carry its own fuel. For example, interstellar space isn’t a perfect vacuum, but contains the occasional hydrogen ion (a lone proton: a hydrogen atom that’s lost its electron). In 1960, this gave physicist Robert Bussard the idea behind what’s now known as a Bussard ramjet: to scoop up such ions en route and use them as rocket fuel in an onboard fusion reactor. Although recent work has cast doubts on whether this can be made to work in practice, there’s another carry-no-fuel idea that does appear feasible for a high-tech spacefaring civilization: laser sailing.

Figure 6.8 illustrates a clever laser-sail rocket design pioneered in 1984 by Robert Forward, the same physicist who invented the statites we explored for Dyson sphere construction. Just as air molecules bouncing off a sailboat sail will push it forward, light particles (photons) bouncing off a mirror will push it forward. By beaming a huge solar-powered laser at a vast ultralight sail attached to a spacecraft, we can use the energy of our own Sun to accelerate the rocket to great speeds. But how do you stop? This is the question that eluded me until I read Forward’s brilliant paper: as figure 6.8 shows, the outer ring of the laser sail detaches and moves in front of the spacecraft, reflecting our laser beam back to decelerate the craft and its smaller sail.6 Forward calculated that this could let humans make the four-light-year journey to the α Centauri solar system in merely forty years. Once there, you could imagine building a new giant laser system and continuing star-hopping throughout the Milky Way Galaxy.

Figure 6.8: Robert Forward’s design for a laser sailing mission to the α Centauri star system four light-years away. Initially, a powerful laser in our Solar System accelerates the spacecraft by applying radiation pressure to its laser sail. To brake before reaching the destination, the outer part of the sail detaches and reflects laser light back at the spacecraft.

But why stop there? In 1964, the Soviet astronomer Nikolai Kardashev proposed grading civilizations by how much energy they could put to use. Harnessing the energy of a planet, a star (with a Dyson sphere, say) and a galaxy correspond to civilizations of Type I, Type II and Type III on the Kardashev scale, respectively. Subsequent thinkers have suggested that Type IV should correspond to harnessing our entire accessible Universe. Since then, there’s been good news and bad news for ambitious life forms. The bad news is that dark energy exists, which, as we saw, appears to limit our reach. The good news is the dramatic progress of artificial intelligence. Even optimistic visionaries such as Carl Sagan used to view the prospects of humans reaching other galaxies as rather hopeless, given our propensity to die within the first century of a journey that would take millions of years even if traveling at near light speed. Refusing to give up, they considered freezing astronauts to extend their life, slowing their aging by traveling very close to light speed, or sending a community that would travel for tens of thousands of generations—longer than the human race has existed thus far.