How long does it take for our universe to collapse?

May 2011: Can time end?

Some physicists speculate that the arrow of time could someday be reversed and that the universe would slowly become tidier again. But for mortal beings, whose existence depends on a forward arrow of time, such a reversal would mean an end of time, as would heat death.

Recent research suggests that direction is not the only quality that time loses in its slow death. Something similar could happen to the duration. Time as we know it can be measured in units: seconds, days, years. If this were not the case, we could still say which of two events preceded the other, but no longer how long they lasted. Such a scenario is presented by Roger Penrose, physicist at the University of Oxford, in his new book "Cycles of Time".

In his entire scientific life, Penrose has repeatedly dealt with the subject of "time". Together with Stephen Hawking from the University of Cambridge, he showed in the 1960s that singularities should not only occur under special conditions, but almost everywhere. He also claimed that matter dropped into a black hole no longer exists and that time has no place in a truly fundamental theory of physics.

Penrose's most recent attack on time begins with an elementary observation of the early universe. He compares the situation to a box full of Lego bricks that have been dumped on the floor and have not yet been put together into anything - a mishmash of quarks, electrons and other elementary particles. From this, structures such as atoms, molecules, stars and galaxies had to be formed step by step and completely by themselves. The first step was the creation of protons and neutrons, each consisting of three quarks and about a femtometer (10–15 Meters) are tall. These particles formed about ten microseconds after the big bang (or the big rebound or whatever big event).

Before this point in time there was nothing that was composed of several particles, and thus nothing that could have functioned as a clock. Every clock, i.e. every physical system that divides time into equal sections, needs a spatial distance to function: the length of a pendulum, the distance between two mirrors or at least the size of an orbital in an atom. There was nothing like that. Particles could temporarily clump together, but they couldn't measure time because they weren't a fixed size. Quarks and electrons alone are also unsuitable as a basis for clocks, since they have no size at all: They always appear point-like, no matter how close they are to the particle physicists. Their only magnitude-like property is their so-called Compton wavelength. This value, which is essential for quantum effects, is inversely proportional to the mass, and even it was not defined until about ten picoseconds after the Big Bang, because only then did the process that gave the particles mass begin.

If there can be no clocks, there is no time

"If there are no clocks," says Penrose, "things don't notice how time passes." Without any structure that could in principle make the passing of time noticeable, the particles in the "primordial cosmic soup" could make no difference whether an attosecond or a femtosecond (10–18 or 10–15 Seconds) had passed.

Penrose claims that this situation not only describes the beginning but also the end of time. Long after all the stars have passed, the universe becomes a gloomy stew of black holes and individual particles. But even the black holes disintegrate at some point, leaving only particles. Most of them will be massless, for example photons, and thus again unsuitable for building clocks. Things also look bad for watches in alternative doom scenarios like the Big Crunch.

One could object that a term like duration remains meaningful even when there is no longer any means of measuring this quantity. But physicists have the habit of fundamentally questioning the existence of a quantity that cannot even be measured in principle - this is what happened with absolute space, which could not withstand the theory of relativity due to a lack of measurability. Accordingly, the impossibility of building a clock would be a sign for the researchers that time has lost one of its defining properties. "If time is what a clock measures, and there are no clocks, then there is also no time," says the physics philosopher Henrik Zinkernagel from the Universidad de Granada (Spain), who also deals with the lack of time in the early cosmos has dealt with.

Despite its elegance, the Penrose scenario has weaknesses. Not all particles in the distant future are massless. At least some electrons will survive, and that's enough for a watch. Penrose speculates that the electrons will somehow lose their mass, but he himself admits that he is moving on unsafe ground with them: "That is one of the more uncomfortable sides of this theory." And besides, if the early universe had no measure of length, what then does it mean that it expanded, thinned and cooled?

However, if Penrose were to be on the right track, it would have remarkable consequences. Although the densely packed early universe and the emptying cosmos of the distant future seem as contradicting as possible, they suffer in the same way from the absence of clocks and other measures. "The Big Bang is similar to the distant future," says Penrose. He boldly suggests that both could actually be one and the same phase of a great cosmic cycle - hence the title of his new book »Cycles of Time«. When time ends, it kind of runs back to the beginning and there is a new big bang. So Penrose, who has spent a large part of his scientific life studying the singularities that put an end to time, may have found a way to make time go on anyway. The executioner of time is also its savior.

Time becomes space

Even if the notion of duration becomes meaningless and the distinction between femto- and attoseconds becomes blurred, time is not completely dead. It still determines the order of cause and effect. In this respect it differs from the room, which hardly imposes any restrictions on the arrangement of objects in it. Two events that are adjacent in time - I type on my keyboard and a letter appears on the screen - are inextricably linked. Two neighboring objects in the room - the keyboard and a sticky note on it - do not have to have anything to do with each other. Spatial relationships just don't have the same inevitability as temporal ones.

But under certain conditions, time itself could lose this elementary property of order and thereby become a further spatial dimension. This idea first came up in the 1980s when Hawking and his American colleague James B. Hartle tried to explain the Big Bang as the moment when space and time began to differ from one another. Three years ago, Marc Mars from the Universidad de Salamanca and José M.M. Senovilla and Raül Vera from the University of the Basque Country in Bilbao (both Spain) applied a similar idea to the end of time.

The three researchers were inspired by string theory, according to which our four-dimensional space-time is only a very “thin” part of a much higher-dimensional space - a “membrane” that drifts like a leaf in the wind in this very spacious environment. One after another, string theory has developed many variants of such membranes; since one of them was abbreviated as »MBrane«, all such membranes are only called »branes« (branes). We are trapped on our bran like a caterpillar clinging to the leaf. Usually we can move freely in our four-dimensional prison. But when the bran winds hard enough, all we can do is hold on, but no longer move. More precisely, we would have to be faster than the light to change our location on the Bran, but that is impossible. Since all processes require some kind of movement, everything comes to a standstill.

The world line of an object on the bran, that is the set of all points in space-time in which the object is located, does not end in this case - viewed from the outside. It is merely bent from a time-like line to a space-like line. The bran is still four dimensional, but all four dimensions are spatial. Mars describes it this way: “All objects are forced by the bran to move at speeds that are increasingly approaching the speed of light. Ultimately, their trajectories are deformed so much that they actually move faster than light and there is no longer any time for them. An important point is that you don't notice any of this yourself. "

Since all our clocks also slow down and eventually stop, we would have no way of knowing that time is turning into space. We would only notice that distant objects such as galaxies are moving faster and faster. It's downright scary: This is exactly what astronomers actually observe - and usually attribute it to an unknown type of dark energy. Could it rather be the swan song of the time?

Model hologram

But even if there is neither duration nor causality, the events can still be put in an order. In what way will time lose this last quality too? Several groups of string theorists have recently brought this question closer to an answer. Emil J. Martinec and Savdeep S. Sethi from the University of Chicago, Daniel Robbins from Texas A&M University, the aforementioned Gary Horowitz, Eva Silverstein from Stanford University, and Albion Lawrence from Brandeis University in Waltham, Massachusetts, have had the holographic principle, a powerful idea in string theory, examines what happens over time in the singularities of black holes.

A hologram is a simple picture that makes the viewer believe that a three-dimensional object is floating in front of the picture itself. The holographic principle says that our whole universe resembles a holographic projection. A complex system of interacting quantum particles can generate the perception of depth, i.e. a spatial dimension that does not even exist in this system.

However, not every two-dimensional structure is a hologram; it has to wear the right pattern. If you scratch a hologram, the spatial illusion is lost. Accordingly, not every particle system creates a universe like ours, rather it has to be equipped with a certain pattern. Let us now assume that the necessary regularities develop in an originally disordered system. Then at that moment the third dimension arises - and disappears again as soon as the associated order dissolves.

The melting of the third dimension

Now what happens when a star collapses into a black hole? We perceive it as three-dimensional, but "in reality" it is only a certain pattern in a two-dimensional particle system. As its mass increases, this two-dimensional system becomes increasingly violent in motion. Eventually its order collapses - this is the moment when the singularity arises. Just as the water molecules of an ice cube melt from a regular crystalline arrangement into the chaotic conditions of a liquid, the third dimension melts away, so to speak.

It is the same with time. If you fall into a black hole, the time shown by a falling clock depends on the distance to the center of the black hole. This distance, however, melts away together with the third dimension in which it is defined, and with it also time. It is not possible to determine the exact point in time at which an event occurs, nor the exact location at which an object is located. "The conventional geometric concept of space-time is being lost," says Martinec.

This means that space and time no longer give the world a structure. If you try to measure the position of a thing, you will find it in several places at the same time. Spatial distance no longer matters, things move from one place to another without crossing the space in between. This is exactly how the trail of an unfortunate astronaut who fell into the black hole can leave it again. "If there is neither space nor time near a singularity, the event horizon is no longer well-defined either," says Horowitz.

String theory is thus not just a kind of repair paste that can be used to smear the ugly singularities without significantly changing the rest of the universe. Rather, it undermines the concepts of space and time, with implications that extend far beyond any singularity. Of course, the theory still needs something like a time in the particle system whose holographic image is supposed to be the universe. The scientists are already working on a dynamic that does not require any time. But time is still so deeply anchored in physics that scientists can hardly imagine its total disappearance so far.

Science opens up the incomprehensible to us by breaking it down into manageable pieces, thus showing that even the greatest journey ultimately consists of small steps. It is the same with the end of time. And while we think about it, we also learn to reassess our own position in the cosmos.

The qualities that time gradually loses are all prerequisites for our existence. We need the direction of time to develop ourselves; we need duration and unit of measure in order to be able to form complex structures; we need a causal order so that cause and effect events can take place, and we need spatial spacing so that our bodies can create small areas of order in the world. If even one of these properties melts, it is incompatible with our survival. We may imagine an end of time, but we can never experience it ourselves - just as we cannot be fully conscious at the moment of our death.