Web special: 10 impossibilities that weren’t
IMPOSSIBLE! Preposterous! These words are often thrown about when people declare certain things to be scientifically ridiculous. Aliens cannot reach the Earth in spaceships, they proclaim, because the distance between stars is too great. Telepathy is impossible since the brain does not emit or receive messages. And it’s impossible to instantaneously transport an object from A to B because you cannot know the location and momentum of all its atoms – teleportation would violate the Heisenberg uncertainty principle.
Yet if you carefully analyse these examples, you realise that they are merely impossible today or in the near future. The real question is, are they impossible with technologies that lie decades, centuries or even millennia beyond ours? Perhaps these “impossibilities” are merely very difficult engineering problems. The late Arthur C. Clarke once said, “Any sufficiently advanced technology is indistinguishable from magic.” So a better question would be: do these impossibilities violate the known laws of physics?
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History has shown that it is always dangerous to declare something impossible. The celebrated Victorian physicist Lord Kelvin was known not only for his pioneering work on thermodynamics, but also for making a number of incorrect predictions. He said flatly that “heavier-than-air” craft were impossible. He also thought X-rays were a hoax, and that radio had no future. For good measure, he declared that the Earth could be no older than a few million years.
Not to be outdone, Ernest Rutherford, who in 1911 discovered the nucleus of the atom, was once asked if an atomic bomb could release the energy stored in the nucleus. He quickly dismissed the idea. Indeed, the road to progress is littered with erstwhile impossibilities. Having seen so many of these become realities, physicists today paraphrase T. H. White in The Once and Future King: anything that is not forbidden is mandatory. Unless there is a law of physics forbidding a technology, then it is not only possible, it is sure to be built someday.
“Unless there is a law of physics forbidding a technology, then it’s not only possible, it is sure to be built”
A more systematic approach to understanding the physics of the impossible is to establish a hierarchy. I divide impossibilities into three categories. Class I are those that might become possible within a few decades to perhaps a century. Class II are those that may take centuries, millennia or more to perfect. And Class III are those that violate the known laws of physics, which I argue makes them truly impossible.
What Class I impossibilities have in common is that they could be achieved in the foreseeable future using the known laws of physics, but may require sophisticated engineering. These include invisibility, force fields, ray guns, psychokinesis, starships, antimatter engines and even certain forms of teleportation and telepathy.
For instance, I used to teach students in my optics class that invisibility was impossible. For an object to become invisible, light would have to wrap around it, like water flowing past a boulder. Downstream the presence of the boulder has been totally washed out. However, using Snell’s law, which describes how uniform materials refract or bend radiation, you can show that light would have to travel faster than the speed of light in order to bend around an object in that way. This would seem to be impossible.
However, two years ago physicists at Duke University in North Carolina and Imperial College London showed that a “metamaterial” can render an object inside it invisible to microwave radiation. By placing tiny impurities within a material, they could force it to bend microwaves in novel ways. And just last year two groups, from the California Institute of Technology in Pasadena and the University of Karlsruhe in Germany, succeeded in creating metamaterials capable of bending and laser light in the same manner. This is a significant breakthrough, since for the first time we can bend visible light in a way that could eventually produce invisibility devices.
Given the astonishing pace of scientific discovery, it is conceivable that within a decade or two, physicists may be able to render an object totally invisible, at least to a single colour of light. Who knows, an invisibility cloak somewhat like Harry Potter’s could be possible within this century. This is just one example.
Another is telepathy, long considered the province of mystics, cranks and magicians. Yet these days MRI machines are becoming so sensitive that scientists at the University of Pennsylvania in Philadelphia claim they can spot brain activity that indicates someone is telling a lie. In the future, researchers may be able to compile a “dictionary of thought” – a one-to-one correspondence between brain signals and specific thoughts. Within a decade we might be able to catalogue the thoughts behind a score of different MRI patterns, for instance.
This mental information could then in principle be transmitted between people. Neuroscientists at Brown University in Rhode Island have implanted an array of electrodes directly into a paralysed patient’s brain, with the array wired up to a computer. The patient has learned to move the cursor on a laptop screen just by thinking – and can do it well enough to play simple video games and answer email. As neural prostheses improve, stroke patients and others afflicted by brain injuries may be able to communicate in ways that were once inconceivable.
Likewise, the idea of teleportation was once considered impossible. Yet today physicists regularly teleport single photons over distances of , and can also teleport whole caesium and beryllium atoms. More precisely, they can teleport the quantum information contained within a photon or atom onto a distant photon or atom. Within a decade, the first molecule may be teleported in this way, and within a few decades researchers could teleport more complex organic molecules and perhaps even the first virus or strand of DNA.
To achieve this, physicists exploit an exotic property called quantum entanglement. If two particles are brought together in such a way that their quantum wave functions vibrate in unison, then they form a bond like an invisible umbilical cord that connects them even if they are separated by vast distances. If you later disturb one particle, then the information you impart onto it is transmitted instantaneously to its partner – so the entangled partner forms a ready-and-waiting template for whatever information is to be teleported. Demonstrations of this phenomenon mean quantum teleportation is in theory only a Class I impossibility.
Entanglement is a very delicate phenomenon, however. The two particles have to be vibrating in precise unison, so the slightest disturbance can break the ephemeral bond. This is the main reason why it is hard to progress beyond a few atoms. In principle, entangling large objects is just an engineering problem, but in practice it is extremely difficult to get more than a handful of atoms to stay in precise synchronisation. Teleporting a person made of trillions of atoms may take several centuries to perfect – and that takes us into the next class of impossibility.
Class II impossibilities are much more difficult. They may require millennia or even millions of years to achieve, but crucially they remain within the realm of possibility. What makes them so difficult is that they generally require vast amounts of energy, and their underlying physics is not totally understood. This class includes time travel, faster-than-light travel via wormholes, and entering parallel universes.
Stephen Hawking tried in the 1990s to prove that there must be a law of physics preventing time travel, which he called the chronology protection conjecture. Yet after years of hard work he failed, and he now concedes that time travel is possible, though highly impractical. A careful study of Einstein’s equations of general relativity shows that if you could assemble huge amounts of energy, you could indeed open up a hole in space and time, perhaps connecting the present to the past. Anyone brave enough to enter this wormhole might find himself emerging before he left.
There are huge obstacles to building a time machine or wormhole, though, not least that you’d need to assemble astronomical amounts of energy, roughly equivalent to the mass of a black hole. In 1963, physicist Roy Kerr was able to show that the singularity of a spinning black hole might form a ring rather than a dot, and that anyone falling through the ring might not be crushed to death, but instead enter a parallel universe. Although astronomers have now identified hundreds of spinning black holes in outer space, there is a problem (beyond getting there in the first place): each singularity is surrounded by an event horizon, so that you can’t return after passing through.
However, in 1988 physicist Kip Thorne found a that was “traversable”, corresponding to a wormhole that in principle would let you go back and forth. A round trip through the wormhole could be as straightforward as a ride on a plane, but to open up such a wormhole you’d need the energy equivalent of a stellar black hole’s mass. What’s more, keeping the wormhole open and stable would require negative energy – an exotic phenomenon in which quantum fluctuations render the energy density in a region of space less than zero – equivalent to the mass of Jupiter. Physicists have been able to create minuscule amounts of negative energy in the lab, but this technology is only conceivable for a civilisation significantly more advanced than ours.
That hasn’t stopped physicists from proposing specific designs for time machines. My favourite envisions a battery of atom smashers – each about 10 light years long and capable of accelerating particles at 200 billion electronvolts per metre – arranged in a sphere where they all point inwards. These accelerators would fire converging beams of particles at the centre of the sphere until that point reaches something called the Planck energy, which is about 1019 billion electronvolts. This is the energy at which space and time become unstable and wormholes should appear.
All this is nothing compared with the granddaddy of them all: the Class III impossibilities, those that genuinely violate the known laws of physics. Either they are truly impossible, or we will have to discover new laws of physics. I once made a list of seemingly outlandish technologies found in science fiction, and realised to my surprise that most were Class I or II. After careful study, I found only two impossibilities that qualified as Class III: perpetual motion machines, and precognition.
No-go ideas
The latter is defined as foretelling the future. This is problematic because it violates causality, the fundamental ordering of cause and effect. Precognition is actually a problem within time travel, but physicists have devised ingenious ways to make time travel consistent with causality; if you tinker with the past, for example, perhaps you open up a parallel universe so there is still no foretelling the future. But if true precognition were shown to exist – say, a telephone line linking the present to the future – it would represent a collapse of the foundations of physics.
As for perpetual motion, it has been the subject of a string of hoaxes going as far back as the 8th century. Most are quite simple. They usually involve a spinning wheel or chain of some sort. After each cycle, a tiny amount of energy is produced apparently from nowhere, and so the inventor claims that over many cycles he can extract unlimited energy for free.
Perhaps the most celebrated perpetual motion machine was created in 1872 by John Keely, who swindled wealthy investors out of $5 million. It consisted of resonating tuning forks that he claimed could extract energy from the “ether”. Keely would regularly invite investors to his house and dazzle them with his hydro-pneumatic-pulsating-vacuo-engine, which whizzed about without any apparent power source. He spent some time in jail for his swindling, but died a wealthy man. After his death his house was torn down, revealing an elaborate network of concealed tubes that secretly supplied compressed air to his machines.
Even so, you might still be wondering: why don’t perpetual motion machines work? Why do we have the law of conservation of matter and energy in the first place? If we knew the answer, then perhaps we could find a clever way to evade it.
When I was a graduate student in physics, I learned the reason behind this hallowed principle, which is something called Noether’s theorem. It states that whenever a system has a conservation law, its origin lies in a “symmetry” of the system. For example, because the laws of Newton and Einstein have a symmetry – they don’t change with time – they automatically possess a conservation law. The fundamental laws appear to be immutable, no matter how long we wait, and according to Noether’s theorem this inevitably produces conservation of energy.
I suddenly realised its significance: if you analyse the light coming from galaxies billions of light years away, you find the very same spectral lines of hydrogen that we find in our laboratories. In other words, the laws of atomic physics have not changed for billions of years, going right back to the big bang, and so energy must have been conserved since the beginning of the universe. The conservation of energy has been valid for billions of years, and its violation by perpetual motion would signal a collapse in our known laws of physics.
We now see the difference between Class III and the other types of impossibilities. The essence is that Class I and II are compatible with the two dominant theories of modern physics, quantum mechanics and general relativity. So far, no one has found a single deviation to either within its respective realm. And in both quantum mechanics and general relativity, the fundamental laws remain the same over time – they conserve matter and energy.
Might the fundamental laws of physics themselves be incomplete, though? Perhaps. After all, relativity is thought to break down at the instant of the big bang or at the centre of a black hole, and quantum theory cannot explain gravity. At present, the leading contender to combine quantum mechanics and relativity into a single theory is superstring theory (which is what I do for a living), and its laws too remain constant over time.
In considering what the future may hold, then, we should keep an open mind to Class I and II impossibilities. What is unthinkable today might not be forbidden in a few decades or centuries. Yet we also have to draw the line somewhere and keep our feet firmly planted in the known laws of physics. Ultimately they are the best guide we’ve got.
