What physicists know is impossible

The physics of the impossible

This text is taken from the print edition 05/2009 of Technologie Review. Just like the current issue, the magazine can be ordered online here free of charge.

For centuries, scientists have declared impossible technologies that will later become self-evident. In order to understand what the future might bring, we must not think one-dimensionally, says Technology Review essayist Michio Kaku.

Michio Kaku studied physics at Harvard University and received his PhD from the Lawrence Berkeley National Laboratory of the University of California, Berkeley, in 1972. As a scientist, he is mainly concerned with string theory. In addition, he has now written numerous popular science books on theoretical and physical topics.

Ask the world's brightest people, the best scientists and technicians of our time: What is impossible? At first glance, the question is quite simple, and the answer will always be similar: Anything that violates the laws of nature.

But what does that mean in concrete terms? Unfortunately, the pages of the history books are full of damning judgments by famous scientists about technologies that are taken for granted today: William Thomson, for example, better known to posterity as Lord Kelvin, perhaps the most famous physicist of the Victorian era, kept flying machines heavier than air, simply impossible - just like X-rays. And he was convinced that under no circumstances could the earth be older than a few million years. Or take Lord Rutherford, the discoverer of the atomic nucleus. He declared the idea of ​​the atom bomb to be utter nonsense. The energy contained in an atomic nucleus is simply too small for a weapon to be built, he said.

Of course there are also counterexamples. The most imaginative scientists often represented ideas that even science fiction authors would have considered too bizarre, freely following the motto formulated by Albert Einstein: "If an idea does not appear absurd at first glance, it is no good." In 1914, the British writer H. G. Wells, one of the great pioneers of science fiction alongside Jules Verne, described in his novel "Liberated World" how a scientist discovered the secret of the atomic bomb in 1933. The physicist Leó Szilárd stumbled upon the story in 1932 that inspired him to the idea of ​​the chain reaction. This marked the beginning of the development of the first atomic bomb in the following year - just as Wells predicted. The chain reaction was able to multiply the energy that starts with the splitting of a single atomic nucleus so much that Rutherford's objection became obsolete.

Robert Goddard, the father of modern rocket research, stuck to his development of a theory of rocket propulsion for space travel despite fierce criticism, even though his opponents objected to him that due to the lack of air on which the rocket could repel, it was quite impossible to advance in space get. The editors of the "New York Times" even fumed in 1921: "Professor Goddard does not know the relationship between action and reaction and also does not know that one needs something better than a vacuum in which to react. He seems to lack the basic knowledge which is taught daily in our high schools. " Almost 50 years later, a rocket brought the first people to the moon.

So why shouldn't other, seemingly absurd ideas from science fiction also be implemented? The "Spaceship Enterprise" series, for example, inspired the Mexican physicist Miguel Alcubierre to think about drives that are faster than light. Albert Einstein had already introduced the idea that space and time are linked to one another in 1915 (see box). Only Alcubierre found an exact solution for Einstein's equations, which enabled exactly the properties known from the television series. Unlike in "Star Trek", however, no "dilicon crystals" are required for this, but a completely new fuel - the so-called negative matter, which has not yet been discovered in nature.

Antimatter, however, also known from the same SF series, was not invented by Gene Roddenberry, the creator of "Star Trek". This honor goes to the British physicist Paul Dirac, whose relativistic description of quantum mechanics predicted a new, peculiar type of matter as early as 1931, which is completely identical to the matter known to us, except for the crucial difference that all particles have exactly the opposite electrical charge as usual: An anti-electron is positively charged, an anti-proton is negatively charged. Today, with particle accelerators like the Large Hadron Collider, we can actually generate rays of antimatter and use them for medical purposes, for example.

Science fiction continues to stimulate new physical developments to this day: I used to tell about invisibility, which the ancient Greeks already discussed, in my optics courses that it was impossible. To do this, you would have to wrap light around an object, so to speak, and reform it on the back of the object, similar to how a stream flows around stones. That is not possible, at least that is what one thought.

In 2006, researchers from Duke University in North Carolina and Imperial College London demonstrated that microwaves, using materials such as water, can be used to circulate around an object and recombine behind it, making the object invisible. In August 2007, scientists from the University of California at Berkeley, the University of Karlsruhe and the Ames Laboratory in Iowa showed that laser light in the visible range can be folded around objects in a similar way at the microscopic level. Even if it will take many decades before the technology is perfected - the principle has been proven, and Harry Potter's invisibility cloak is no longer out of the question. There are only minor obstacles to circumnavigate: For example, the wizard student would not be able to see through if he were put into a cylinder of this metamaterial. So you would have to punch two peepholes - but then outsiders would see two eyes floating in the air. In addition, there would have to be a different metamaterial for each frequency.

So, in the end, is anything possible if the scientists only show enough imagination? In order to bring some order to the answer to this question, I have divided "impossibility" into three categories: For me, invisibility is one of the "impossibilities of the first degree". These are developments that only appear to violate the known physical laws, but could nevertheless become possible in the coming decades or centuries. The things that are popular in classic science fiction, such as beam weapons, lightsabers, intelligent robots, spaceships and antimatter drives, do not violate the laws of physics and are therefore mainly an engineering problem.

"Second degree impossibilities", on the other hand, are techniques that are impossible today, but could become possible in a few millennia. The most famous example of this is time travel, which has sparked the imagination of science fiction authors for centuries: the astrophysicist Stephen Hawking tried to prove that time travel is impossible: it violates a fundamental law of physics that he defined chronology - called the protective hypothesis that would "make history safe for historians". After some effort, however, he finally had to admit that he could not prove his hypothesis.

If time travel is in principle possible, physics must also find a solution to the question of the time paradox that often occurs in science fiction works. It would be possible, for example - assuming time travel - that one becomes one's own father or son or one's own mother or daughter. For example, a woman might decide to have sex reassignment, then travel back in time as a man, and start fooling around with herself as a teenage girl. If the girl became pregnant and had a daughter, with whom the man then traveled further into the past, the baby could grow into the original teenager and later into the woman from the beginning of the story.

The solution to this paradox can be found in science fiction stories with parallel universes - a concept that can also be found in quantum theory. If time is a river, the new trick would be that the time flow has vortices and can even split into two rivers. This is exactly a solution to this sticky paradox of time, which, for example, also allows you to kill your parents before you are born - the splitting of the flow of time into two rivers. A time travel in this case would be simply switching from one quantum time stream to the other. As a result, you have simply changed someone else's past into their time stream, but your own time stream remains the same. So you can only change the past of other people.

Unfortunately, at the very moment you step into the time machine, another problem arises: quantum effects lead to the creation of radiation, possibly so much that it kills the time traveler or closes the time portal. It follows that to solve the time travel question we need a "theory of everything", an all-encompassing, universal theory that links the theory of spacetime and gravity with quantum theory. That's how I earn my bread: I'm working on what is known as string theory, the leading and so far only candidate for such a "world formula".

My personal science fiction inspiration came from reruns of the old "Flash Gordon" films that I watched as a child. I was completely fascinated by how Buster Crabbe conquered the universe with his rickety spaceship and beam weapon. After a while I realized that the real hero of the series wasn't Flash Gordon, but the physicist Dr. Zarkov: It was he who built Gordon's spaceship, the city in the clouds and the invisibility ray. It was then that I realized: without science, there is no science fiction. Flash was the womanizer, Zarkov was the real engine of the show.

I later read the Foundation Trilogy by Isaac Asimov with great enthusiasm. It opened up a whole new universe of Category II impossibilities for me: Asimov seduced his readers to imagine a civilization that is 5000 years in the future. On this timescale there could of course be technologies that would take a very, very long time to develop - so I had to learn to differentiate between technologies that would be very complicated to produce, but ultimately "only" involve solving engineering problems, and technologies that violate the fundamental laws of physics. Such technologies are what I call category III impossibilities: machines with the help of which one can see into the future, for example, or the famous perpetual motion machine - a drive that runs for an infinite amount of time without any external energy supply.

Simply ignoring the laws of physics is, of course, an easy exercise for science fiction or fantasy writers. But we physicists cannot take something like this lightly. As a young PhD student, I thought a lot about Category III impossibilities. What is the origin of these fundamental laws of physics that to our knowledge cannot be violated, such as the law of conservation of energy? I literally almost fell off my chair when I heard about the so-called Noether theorem: Emmy Noether, a brilliant German mathematician who had taught in Göttingen at the beginning of the 20th century until she fled the Nazis to the USA, had the following very basic idea : If a physical law has a symmetry, there is a corresponding conservation law in physics. The conservation of energy, for example, follows from the fact that physical laws are always the same regardless of the selected starting time.