FOCUS THE HAUNTING QUESTION OF and science. Scientific discussion of the subject was prompted in the late 1940s by the logician Kurt Gödel, who was TIME able to show that time travel was theor etically possible. He demonstrated that closed, timelike curves could exist Time is a strange phenomenon. It is measurable, under certain conditions within the framework of Einstein’s general theory yet almost unfathomable. So does it even exist? of relativity. In this theory, time is And what does time travel have to do with physics? viewed as a geometrical dimension, meaning that – unlike classical phys- Three ETH researchers voice their opinions. ics – space and time are not independ- ent, but form a single entity called TEXT Florian Meyer space-time. This entity can be com- pared to a marble run: the planets orbit the sun along paths dictated by the cur- vature of space-time, much as marbles roll downwards along the grooves. The “Gödel Universe” contains elements whose geometries are so se- 25 verely curved that space-time actually bends around to form closed time-like curves, or loops. This allows space- time to take on the form of a rounded o you remember Marty McFly? sequently, it is impossible for them to doughnut. Space travellers accelerat- D Or maybe those memories no kill their grandfather, hence the para- ing at tremendous speed along these longer – or don’t yet – exist? Marty dox! loops in a rocket would eventually re- McFly was the main character in the For philosopher and physicist turn to the point where they “had al- Back to the Future science fiction films Norman Sieroka, who wrote a book ready been” and which “existed be- that wowed cinema audiences in the about the philosophy of time while fore”. There, they could theoretically 1980s. He uses a time machine to trav- working as a Privatdozent at ETH, any meet themselves or repeat their own el from 1985 back to 1955, the year his description of time travel implies par- past. parents fell in love. Marty’s appearance ticular views about time and about how Nobody is suggesting that we on the scene alters the course of events, it can manifest itself: “If someone is could be building these kinds of time and he suddenly faces the risk that his convinced that, by definition, the past machines in the near future. The parents will no longer fall in love – and no longer exists and the future does not rocket would have to reach a maximum he will never be born. But how is it pos- yet exist, then this will very likely influ- speed very close to the speed of light – sible for him to travel back into the past ence what that person thinks is physic and Renato Renner, ETH Professor for and change what happened if he wasn’t ally possible,” he says. “After all, any Theoretical Physics, argues that this is even born? straightforward possibility of time technically impossible: “General rela- The film derives many of its laughs travel would be excluded if the travel- tivity does allow for closed time-like from this curious sequence of events. ler’s destinations in the past and the curves, but calculations show that Essentially, Back to the Future is a play future do not even exist.” many of these loops are actually im- on the thought experiment known as the passable. Astronauts would need to grandfather paradox, in which some- The time machine endure an acceleration much greater one travels to the past and kills their Time travel has been a popular theme than the Earth’s gravitational pull over own grandfather before the conception ever since science fiction emerged as a several years. That’s simply inconceiv- of their father or mother, thus prevent- genre in the 19th century in parallel able, but science fiction tends to down- ing the time traveller’s own birth. Con- with the rise of industry, technology play this issue to some degree.”
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Gödel’s solution was not intend- ed as a work of science fiction. The purpose of thought experi- ments is to help us shed light on unexplained and elusive theo- retical concepts and open them up for debate. In this sense, his thoughts on time travel repre- sent a kind of “useful fiction”. Gödel’s model makes it clear that we should not confuse time in general relativity with sub- jectively experienced time, nor with time in classical physics. In both those latter cases, the world has an objective form of
time as a superordinate factor dimension of life A phenomenon – Time is such a fundamental ways. mysterious, often in many, itself that it manifests that is the same everywhere and independent of humans and objects. In that is external to the system. The two eral relativity – is a distinct concept. Gödel’s Uni-verse, on the other hand, great theories of modern physics – Quantum mechanics describes how
26 there is no all-encompassing, absolute general relativity and quantum mech states of matter, for example molecules time – and his description of travelling anics – have each shaken this concept and atoms, change over time. Time is back into the past was his way of high- of time in their own particular way. Yet, therefore a fixed background in which lighting this fact. He didn’t believe the in some aspects, they fundamentally things change, yet its theoretical status past actually exists in a form we could contradict each other. Renner there- is unclear. visit. fore argues that time is a key factor in Renner sees the measurement of The question is whether the uni- unifying them – and his approach to time as a key issue. “If we model a clock verse constitutes a closed whole and this challenge is to seek out the prop in the two theories, it can lead us to a time is an external variable in which erties of time that are independent of common concept of time – because the things and events can be arranged as if these two theories. clock is the same.” Renner focuses on they were in a box. This question arises For example, Renner says that we the theoretical foundations of clocks in because physicists regard objects as could resolve the grandfather paradox, quantum mechanics, a task that has led systems that have an environment – which illustrates a principle of general him to work closely with Sieroka. One whether the objects are solar systems, relativity, by linking it to Schröding- obstacle that has to be overcome is the individual atoms or even clocks. The er’s cat, a well-known paradox of quan- fundamental uncertainty of quantum universe is also a physical system, but tum mechanics in which the cat is mechanics, which means it is impos does it have an environment? simultaneously both dead and alive. sible to observe a quantum system Viewed in this way, the grandfather without altering it. The key to new physics would be both dead and alive. This This also applies to the measure- Just like space and gravity, time is one could work if the timelike loop through ment of time, as Wolfgang Pauli – win- of the key concepts that must be in- which the time traveller passes forms a ner of the 1945 Nobel Prize in Physics cluded in any physical theory that figure eight, whose surface – like a Mö- and former ETH professor – was the seeks to explain the real world. Typic bius strip – has two sides that merge first to discover. “In Pauli’s day, the ally, it is physical time – as opposed to smoothly into each other, so that they clock was something external. When it experienced time – that is the form of would sometimes be “on top” and came to observing a quantum system, time measured and displayed by a sometimes “underneath”. the classical clock was outside the sys- clock. In classical physics, however, This attempt to unify the theories tem. But that’s all changed,” says the time on a clock is part of the envir is far from simple, because time in Renner. “Nowadays, we also treat the onment, in other words a form of time quantum mechanics – unlike in gen clock as a quantum system, so obvious-
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ly the same thing applies: we can’t read ecule with the state of the laser field. the time without altering the clock.” “For a closed system – like the uni- Renner therefore argues that the small- verse – there is no time because it has er the quantum system used as a clock, no relation to its environment,” says the less and less accurate time meas- Schild. “In fact, time really only exists urement becomes. He is encouraged if the chosen clock shows the time as by the technological advances of the clearly as the hand of a classical clock.” past ten years: “At ETH Zurich, we Even the accuracy of this process now have a whole series of highly so- is a matter of choice: since we can de- phisticated quantum technologies that fine different clocks in the closed sys- help us understand how time works at tem, the measured time depends on an atomic and subatomic level. the selected clock. Different clocks may show different times. Thus, the The realm of attoseconds measurement result only corresponds Electrons are archetypal quantum ob- to the idea of a clock-independent jects. The movement of electrons in “real” time to a limited degree – which molecules and atoms takes place on a brings us back to philosophical ques- time scale of around 100 attoseconds, tions. “Time is not just one philo- which in other words is equivalent to sophical concept among many,” says just 0.0000000000000001 seconds. In Sieroka. “Time is a fundamental di- 27 comparison, it takes one whole second mension of human existence that to wink! Today’s scientists use highly manifests itself in various ways.” intense laser pulses to measure these ultra-short reactions. “Laser pulses can be very good clocks,” says Axel Schild, FIND OUT MORE an Ambizione fellow who works in the Ultrafast Spectroscopy and Attosec- ond Science group at ETH. He is devel- Renato Renner is Professor for Theoretical Physics oping a new computing method for and head of the research group for Quantum Informa- simulating these kinds of dynamics in tion Theory. His research interests include quantum ultrashort laser fields. information science, quantum thermodynamics and the So how does it work? It all starts foundations of quantum physics. with a closed quantum system – for → www.qit.ethz.ch example consisting of a molecule and a laser field. This system has next to Axel Schild works in the Ultrafast Spectroscopy and no interaction with its environment. Attosecond Science group. Supported by an Ambizione Within this closed system, the re- grant from the Swiss National Science Foundation, he searcher arbitrarily defines one el is developing a new method of simulating the dynamics ement as a clock and another element of electrons in molecules that interact with strong, as the actual quantum system. Time is ultrashort laser pulses. defined by comparing the changes of → www.atto.ethz.ch/AmbizioneAxelSchild the clock and the quantum system. The interesting part is that, at first, Norman Sieroka developed his own “philosophy of time doesn’t exist at all in the closed time” as a Privatdozent, the managing director of the system. Time is only introduced Turing Centre and a member of the Critical Thinking through the different ways in which we initiative at ETH Zurich. He has been Professor for treat the quantum system and the Theoretical Philosophy at the University of Bremen clock – for example by comparing the since April 2019. release of an electron from the mol
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