How to Destroy the Universe (22 page)

In 1998, a group of astronomers led by American Saul Perlmutter were making measurements of a particular type of supernova explosion—outbursts marking the deaths of massive stars—in distant galaxies. These explosions have a very well-defined brightness,
meaning that, as with the Cepheid variable stars, they can be used to gauge distance. The data gathered by Perlmutter's team seemed to show that not only is the Universe expanding, but that the expansion is accelerating. But what was causing this acceleration? The only thing known to have this kind of anti-gravitational effect is the energy locked away in empty space. It was Albert Einstein's cosmological constant back from the grave. US cosmologist Michael Turner dubbed this material “dark energy,” and the name has stuck. Now all the astronomers had to do was to calculate the exact amounts of ordinary matter, dark matter and dark energy that our Universe is made from.

Speculation was growing over what this all might mean for the fate of the Universe. There were two principal scenarios. If the total density of matter in the Universe is greater than a number called the “critical density” (an average of about five hydrogen atoms per cubic meter), the cosmic expansion will eventually come to a halt and gradually reverse. Slowly galaxy redshifts will start to become blueshifts as gravity hauls the Universe back in on itself (as galaxies begin rushing together, the frequency of their light is shifted toward the blue end of the electromagnetic spectrum). The cosmic history that astrophysicists slaved away to piece together over the course of the 20th century will now
be reversed. Galaxies will crash together and merge. Space gets hotter until the superheated conditions of the Big Bang are recreated. The particle processes of the early Universe are undone, as the four major forces of nature merge back into one. And then in an instant the Universe winks out, crushed into a superdense state called a “gravitational singularity” from where it's gone just as quickly as it first appeared. Scientists have dubbed this the Big Crunch.

The other possibility is that, if the density of space is greater than or equal to the critical density, or if the acceleration caused by dark energy is large enough, the Universe will continue to expand forever. Rather than burning itself out in a cataclysmic Big Crunch, it fades away gradually. Slowly the last stars burn out, using up the last of the hydrogen and helium nuclear fuel, making it impossible for a new generation to form. With the stars gone, all that's left are neutron stars, white dwarfs—and black holes, which gradually swallow the other objects up, along with any last smatterings of gas and dust. After a near-eternal 10
¹⁰⁰
(1 with 100 zeroes after it) billion years, these black holes themselves have evaporated away by Hawking radiation, all the protons and neutrons that make up ordinary matter have decayed and the dregs of particles and radiation that remain have been stretched apart and diluted by the cosmic expansion to virtually nothing. At this point the
Universe is truly dead. This bleak scenario is known as the Heat Death.

Detailed observations made by spacecraft in recent years suggest that our Universe is made up of about 4 percent ordinary matter, 22 percent dark matter and 74 percent dark energy. And oddly enough, this all adds up to pretty much bang on the critical density. With the extra outward shove provided by the huge dark energy component it seems there can be little doubt we are heading for a Heat Death. Or are we? In 2003, US cosmologist Robert Caldwell put forward a third alternative for cosmic Armageddon—known ominously as the Big Rip. Caldwell wondered what might happen if the dark energy filling the Universe took a particularly extreme form—known as “phantom energy.” He calculated that in a phantom-energy-dominated Universe, the rate of cosmic expansion will accelerate to become so large it will tear apart galaxies, stars, planets, people and eventually subatomic particles as well. Cosmological observations aren't yet good enough to say whether phantom energy rules, though even if it does things aren't set to get ugly for another 22 billion years. Plenty of time for some bright physicist to come up with a plan.

• Space speed record

• Ion engines

• Solar sails

• Ultimate speed limit

• Warp drive

• Fantasy fuel?

To say light is rather quick on its feet is perhaps the biggest understatement in physics. At a speed of 300 million m/s, it could run the hundred meter race in about three millionths of Usain Bolt's world record time of 9.578 seconds. Could human beings ever hope to move that quickly, or maybe even quicker? And, if so, how?

The
Apollo 10
spacecraft fell back to Earth in May 1969 having just completed the dress rehearsal for the Moon landings two months later. The re-entry capsule accelerated to 11,107 m/s. That's about 40,000 km/h (25,000 mph). It is the greatest speed ever achieved by a manned spacecraft, and the crew—Thomas Stafford,
John Young and Eugene Cernan—remain the fastest men in history. Yet
Apollo 10
's speed was just 0.004 percent of light speed. Light moves unbelievably quickly, covering the distance from London to New York in just 0.02 of a second.

It is certainly possible to travel faster than
Apollo 10
using a rocket, but the amount of fuel needed quickly becomes colossal. Early 20th-century Russian space flight visionary Konstantin Tsiolkovsky worked out an equation for the fuel requirements of rocket journeys into space. It showed that the mass of fuel the rocket has to carry grows exponentially with the speed you want it to reach. You might have expected the relationship between fuel and speed to be “linear”—in other words, if accelerating the rocket by 100 m/s requires 1,000 kg of fuel then a linear relationship would mean that to reach 200 m/s will require just another 1,000 kg, so a total of 2,000 kg. But exponential growth means that you need many times this amount of fuel.

One way to get around the enormous fuel demands is to run your spacecraft on something with a little more oomph. Tsiolkovsky's equations revealed that the maximum speed attainable by a rocket is proportional to the speed at which it spits out its exhaust. Use a new fuel that spits out exhaust gases five times faster and
your spacecraft can go five times as quickly as it would have. It turns out that ordinary rockets, which burn liquid oxygen and liquid hydrogen to release the chemical energy stored in these fuels, produce a relatively slow exhaust. For a Space Shuttle main engine—one of the best rockets that engineers have designed to date—the hot gases move at around 4,400 m/s (14,400 ft/s). But that's nothing compared to the speeds that can be achieved using a new kind of spacecraft engine, known as an ion drive. The latest experimental ion drives can generate exhaust speeds of over 200,000 m/s (670,000 ft/s)—nearly 50 times better than the Space Shuttle.

Standard rocket engines burn fuel in a confined space: the engine's combustion chamber. As the fuel burns it expands, raising the pressure in the combustion chamber and causing the hot gases to rush out at high speed. Ion drives work very differently. Rather than burning the fuel, they accelerate each particle using an electric field. This is possible because the fuel particles are electrically charged ions. Normally fuel is made of atoms that are electrically neutral. An atom consists of a nucleus within which are proton particles, which carry positive electrical charge. Around the nucleus orbit negatively charged electrons. Normally there are an equal number of electrons and protons, giving the atom a net charge of zero. An ion, however, has slightly more or slightly fewer electrons, making its
overall charge non-zero. This means it can be accelerated by an electric field, in much the same way that electric current is made to flow through a wire by a battery.

First put forward in 1906 by US rocketry pioneer Robert Goddard, ion drives are now a tried and tested technology. In 1998, NASA launched its
Deep Space 1
robotic probe, which was powered by an ion drive using xenon gas as fuel. Although a relatively low-powered design, with an exhaust speed of slightly over 30,000 m/s (100,000 ft/s), the mission was a resounding success. Many of the world's other space agencies have now flown ion drives of their own—and the power of these devices has been steadily increasing. In 2006, the European Space Agency (ESA) carried out a test of a new ion thruster with an exhaust speed of 210,000 m/s (690,000 ft/s). The drawback of ion drives is that they can only accelerate a small mass of fuel at a time, making the acceleration they deliver very gradual. This low thrust means ion engines are little use for launching spacecraft from Earth's surface, where a high impetus over a short space of time is needed.

However, the slow rate of fuel consumption is balanced by the fact that ion drive engines can run continuously for a very long time: days, weeks, even months. Once in the zero-gravity environment of outer space, a
spacecraft equipped with ESA's new ion drive and carrying a fuel load making up 90 percent of its total mass (the typical proportion for a chemical rocket) could reach a speed of nearly 700,000 m/s (2,300,000 ft/s). That's a vast improvement on
Apollo 10
, but still just 0.2 percent light speed.

Even faster speeds could be made possible using a novel kind of spacecraft propulsion that does away with fuel entirely. Known as a solar sail, it uses a vast sheet of silvered material to literally hitch a ride on the light streaming outward from the Sun. In our everyday experience light behaves like a wave, but it can also be thought of as a hail of tiny solid particles, known as photons. Just as the particles of air in a strong sea breeze impart some of their momentum to the sails of a yacht, a solar sail gets a steady push from the photons that make up sunlight, causing it to gather speed.

As with ion drives, the acceleration generated by a solar sail is gradual but continuous, and able to produce very high velocities over long enough time scales. According to some estimates by scientists at ESA, the top end could be as quick as 25 percent light speed: 75 million m/s (250 million ft/s).

So how do we go faster still? This is where the laws of physics start to make life difficult. Albert Einstein's special theory of relativity describes the dynamics of rapidly moving objects. He formulated the theory in 1905 in response to weird discrepancies that had arisen between the existing laws of motion and the seminal theory of electricity and magnetism developed by Scottish physicist James Clerk Maxwell in the late 19th century.

The trouble seemed to lie in the way the standard laws of dynamics handle the relative motion of objects traveling at, or close to, the speed of light. The relative motion of two bodies moving toward one another is normally given by just adding their speeds together. So if two cars are driving toward each other at 80 km/h (50 mph) then the speed of one car relative to the other is 160 km/h (100 mph). Applying the same rationale to two oncoming beams of light would mean they were converging with a relative velocity of twice light speed. But this contradicted Maxwell's theory, which seemed to be saying that light—and all other electromagnetic phenomena—should look the same to an observer no matter how fast they are moving.

Einstein constructed a theory of relative motion where the speed of light stays the same for all observers. This is the key postulate of special relativity. However,
keeping the speed of light constant in all frames of reference leads to distortions of space and time that bring about some very weird consequences indeed. The first is called length contraction. As a moving body approaches light speed its length in the direction parallel to the motion, as measured by a stationary observer, shrinks. This isn't an illusion—the actual physical length gets shorter.

Stranger still, time slows down for the moving observer, an effect called time dilation. For example, each second ticked off on the watch of someone on a spacecraft moving at 0.99 light speed takes just over 7 seconds as measured on the watch of a stationary observer. You can get an idea of why time dilation happens by remembering Einstein's postulate that the speed of light must stay constant no matter how fast you move. Stretching out the length of each second means that a light beam, as seen from the spacecraft, seems to cover more ground each second, so that its speed relative to the accelerating craft always remains the same.

But there was a third revelation, one with massive consequences for spacecraft engineers on a quest to go faster. And it was this: the faster a spacecraft travels, the more energy it takes to make it go any faster. Like length contraction and time dilation, the effect is unnoticeable at everyday speeds. But as a rocketship approaches the
speed of light, the energy required to accelerate it further grows and grows, becoming infinite at the light barrier itself. The physical interpretation of this was plain and simple: according to special relativity traveling at light speed or faster is impossible.

As far as high-speed travel is concerned, the special theory of relativity seemed to be nature's way of saying “thus far and no further.” Then Einstein came up with a new theory and everything changed again. He realized that special relativity was not compatible with gravity. The best theory of gravity at the time was the universal theory of gravitation, formulated by Isaac Newton in 1687. This had gravity as a force that travels at infinite speed so it is felt by all objects simultaneously, which is manifestly at odds with Einstein's special relativity, where nothing can travel faster than light.

Einstein's solution was ingenious. He imagined space and time as a stage on which physics is played out. In special relativity the stage is flat. Einstein built gravity into the theory by allowing it to be curved by the matter it contains. Announced to the world in 1915, the new theory was called general relativity and it was soon verified to high accuracy by astronomical measurements of the Solar System. While special relativity
said that nothing can move across the space–time stage at faster than light, general relativity placed no such constraints on the movement of the stage itself.