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Does Time Exist? DIE ZEIT EIN GASTBEITRAG Does Time Exist? ABOUT THE POSSIBILITY OF A TIMELESS UNIVERSE From Antiquity time is anything, it derives Quantum Mechanics to Newton and Einstein from what happens in the uni- The nature of time and motion verse. To understand the issues that The dawn of philosophy wit- arise when one attempts to has been a mystery since serious nessed the first great debate Newton had a very different combine quantum mechanics about the nature of time and idea. He claimed «Absolute, and general relativity as a sin- philosophy and science began motion. Heraclitus, born in the true, and mathematical time … gle theory, the first thing to sixth century BCE, argued that flows uniformly without rela- note is that quantum mechan- over 2500 years ago. In the last everything is in flux, but his tion to anything external.» He ics left Newton’s concept of contemporary Parmenides also conceived space as a fixed time essentially unchanged. It half century, theoretical argued «Existence is timeless, arena in which every object in flows independently of every- change is impossible.» Parme- the universe has a definite po- thing in the universe. The rad- physicists have found evidence nides had a big influence on sition at each instant of time. ical changes that quantum me- Plato and, through him, on the chanics introduced concerned that the quantum universe whole of Western philosophy Newton’s ideas were criticized the behaviour of matter and and science. by Leibniz and above all Ernst what we can say about it. might best be thought of as Mach, who in 1883 said: «It is Plato argued that only perfect utterly impossible to measure In Newtonian mechanics, one static. Nothing happens in it all. unchanging forms exist and the changes of things by time. can always say where, at a that motion is an illusion. Quite the contrary, time is an given instant, a body is and A physicist from Oxford Uni- Thus, being is real, becoming abstraction at which we arrive what its momentum is (the is an illusion. One can see the from the changes of things.» momentum is the velocity of versity gives an historical survey strength of Plato’s argument Like Leibniz, he also insisted the body multiplied by its if one considers the problem of that motion is relative, the mass). Position and momen- of notions of time and motion describing mathematically the position of any one body in tum are simultaneously meas- motion of a galloping horse. the universe being defined urable and their values can in as they have developed It can only be done in terms relative to all the other bodies. principle be predicted with of the changing shapes of the perfect accuracy. However, in since antiquity and explains horse. This suggests that Mach’s ideas had a profound the atomic domain, where shapes, existing as mathemati- influence on Einstein and be- quantum mechanics holds why we are now confronted cal ideals in a timeless Platonic came the single greatest stimu- sway, things are very different. realm, are primary and that lus to the creation in 1915 of First, it is in general impos- with the possibility motion is a reflection of differ- his theory of gravity, the sible to make definite predic- ences between them. This is wonderful general theory of tions. Quantum mechanics that time does not exist supported by the fact that a relativity. However, Einstein gives only probabilities for movie consists of stills. Time implemented Mach’s ideas what can be observed. It also and motion is an illusion and motion are eternally elu- indirectly, and their full impli- says that one cannot measure sive. cations for the nature of time position and momentum si- created by consciousness. only started to become appar- multaneously. St Augustine had a deep an- ent over 50 years later when swer to the question why God people attempted to create a What is very remarkable in had not created the world theory that combined the in- quantum mechanics is when sooner than he actually did. sights of general relativity and one considers a system of par- He responded «God did not quantum mechanics. ticles and wishes to measure create the universe in time but their positions. One does not with time.» To the extent that get separate probabilities for 60 60-63__barbour__#05.indd 60 19.11.12 13:39 DIE ZEIT EIN GASTBEITRAG each individual particle but ever, some remarkable indi- quantum theory of the uni- probabilities for complete con- cations of what might emerge verse can be expressed in figurations of the particles. For came to light in the 1960s. terms of the triangles consid- example, if we consider a sys- Ultimately they are conse- ered earlier. Let us suppose a tem of three particles, they will quences of the fact that, in an toy model of the universe con- at any instant form a triangle. indirect way, Mach’s criticisms sisting of just three particles. Quantum mechanics gives of New tonian dynamics are Ordinary quantum mechanics separate probabilities, which taken into account in general says that the particles can form change with time, for all pos- re lativity. a triangle of any shape and sible triangle shapes and sizes size that has any position and and for positions and orienta- Let us first deal with the rela- orientation in space. It gives tions of the triangle in space. tivity of position. In Newtoni- probabilities for all these pos- This is the situation in ordi- an theory and in quantum me- sibilities. However, position nary quantum mechanics, chanics, the positions of ob- and orientation in space of the which was created using the jects are defined relative to complete universe have no meaning in general relativity. Accordingly, quantum gravity can only give probabilities for the shape and size of the uni- verse, not ‹where it is›. In fact, the overall size of the universe is almost certainly a meaningless concept, and this suggests that quantum gravity should only give probabilities for possible shapes of the uni- verse, not sizes as well. This does not affect the issue of time, to which we now turn. What Is an Instant of Time? Let us shed all ideas that the Newtonian way of thinking in- duced in us. Why do we say time passes? Without distort- ing anything, we can consider only what we see; what comes to us through the other senses Figure 1 adds richness to life but noth- Cover of «The End of Time», 1 ing to the scientific aspect. We Phoenix Paperbacks see a succession of views of the world. We could imagine essentially the framework that space (strictly an inertial frame taking snapshots of them like Newton had proposed. Now of reference, but that is an in- stills of a movie. We do not see we must consider what hap- essential difference). However, them embedded in a river of pens when we try to combine it is meaningless to say that time. As Leibniz said, time is quantum mechanics with gen- the universe itself has a posi- merely the succession of co- eral relativity. tion in space. As already Leib- existing things, which we can niz pointed out, if you imagine identify with the different ob- the whole universe moved by jects that we see simultaneous- Quantum Gravity a certain amount in invisible ly: trees, rocks, flowers, etc. space, nothing in anything you There are many difficulties could actually observe would This suggests that an instant encountered in the attempts to be changed. of time is nothing to do with create a theory that combines something outside the uni- general relativity with quan- When Einstein’s theory is re- verse, say a line on which each tum mechanics. It should lead presented in the appropriate point represents an instant to a theory of quantum gravi- way, this is indeed what it (this is the most common sci- ty, but none yet exists. How- says. The implication for a entific way of representing an 61 60-63__barbour__#05.indd 61 19.11.12 13:39 die Zeit EIN GaSTBEITRag 10 instant of time) but is just a Anticipations of this remark- Now consider 100 black dots StartUp-Impuls possible arrangement of things able picture of a timeless uni- on a white sheet of paper. within the universe that occur verse, in which there are sim- There is neither motion nor Zehn Jahre erfolgreiche together. It is like a movie still. ply different probabilities for life in them. But with those 100 different possible configura- dots a skilled cartoonist can Gründungen This way of thinking leads to a tions, can be found in the con- conjure up an evocative story, much richer notion of instants victions of Plato, St Augustine, say of a girl making eyes at a of time. They are not mere fea- Leibniz and Mach. They have young man. More directly tureless points on a mathemat- become acute with the advent ­relevant, a photograph of a ical line. They are complete of general relativity and quan- speedboat creating waves on configurations of the universe. tum mechanics. They force us the smooth surface of a lake In the simplest case of the toy to ask insistently: What is tells us that it represents the Dr. Julian Barbour universe consisting of three time? In what sense does it boat in motion. Shown the born in 1937, is a British particles, the configurations exist? I have already given my photo, a physicist, knowing ­physicist with research ­interests are just the possible triangles definition of an instant of time.
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