In what sense there is no time

Is the passage of time an illusion?

Astronomers at the European Southern Observatory had searched in vain for their telescopes for 26 years. On May 19 of this year, the time had finally come: They observed the star S2 as it flew towards the center of the Milky Way at 25 million kilometers per hour. When it got into the clutches of the black hole sitting there, it suddenly glowed reddish, its light was stretched by the effect of the gravitational field.

This effect, which has now been proven for the first time, is called "Gravitational redshift". It was predicted by Albert Einstein's general theory of relativity. Just like many other strange phenomena in the cosmos, the gravitational waves, the existence of black holes, the deflection of light - et cetera. The chief engineer of the universe was right. Once again.

The general theory of relativity has withstood every test in the more than 100 years of its existence. Her equations describe the relationships in the universe so accurately that one cannot help but get the impression that Einstein looked a little over the shoulder of God (the "old man", as he used to say) in a moment of inspiration.

As if this wasn't just a successful model, but a description of the world as it really is. So it is not surprising that our everyday sense of time was removed from the worldview without great resistance and replaced by the message of the reformer Einstein.

The end of the absolute

"The now has been killed by physics," says Italian gravitational physicist Carlo Rovelli. And by that he means the absolute time, as it was once implanted by Isaac Newton in the cosmos: in its place comes the four-dimensional universe, in which the mass bends space and fast-moving clocks tick more slowly. The present in the global sense no longer exists. The seemingly unproblematic now, Einstein taught the astonished world public, is not a meaningful term without specifying the movement of the observer. Time flows - but it does so relatively.

Now there are physicists and philosophers who even deny that. This model is called the block universe, in which the difference between yesterday and tomorrow dissolves and time, the epitome of change, becomes a rigid and static thing. "In the general theory of relativity, it is natural to look at space-time like a canvas. Like a painter who stands outside of space and time," says Renato Renner from the Swiss Federal Institute of Technology (ETH) in Zurich. "Seen from the outside, every event is already drawn on this canvas. Everything is already there."

Future and past

Indeed, when physicists solve Einstein's equations, it makes no difference whether they calculate the future from the past or, conversely, the past from the future. And because in this completely determined universe, as it is shown in the space-time diagrams, everything is already drawn in, there also seems to be no becoming and no passing away. At least that's what Brad Skow claims. "Time doesn't flow," says the philosopher from the Massachusetts Institute of Technology (MIT).

"Of course, if you say that at a cocktail party, everyone is going to say you're completely crazy," says Skow. In academic circles, however, he doesn't mince his words. The widespread view that we "move through time like a ship through the sea" no longer has a place in the modern worldview. Skow only accepts time when it is frozen. And physicists like Vesselin Petkov from Concordia University in Montreal agree with him.

So is the passing of time just an illusion? That goes too far for some in the scientific community. Renato Renner admits that Einstein's canvas does not define the here and now - nor does it make any statements about the flow of time. "If a theory has nothing to say about it, that does not mean that there is no such thing. I think it is dangerous to call our perception an illusion."

Rescue attempts

Carlo Rovelli sees it similarly. He draws the conclusion from the debate that time is much more complicated than we previously thought. "The difference between the future and the past may not be fundamental. But locally, for me, there is one. Like the cat in my room: it is not fundamental either and still exists."

Physicists have already made various attempts to breathe something like history back into the block universe that has frozen in eternity. The theorist Tim Koslowksi believes, for example, that gravity alone is capable of reanimating the flow of time. Rovelli wants to achieve something similar with the help of thermodynamics and is working on relativistic thermodynamics. The South African George Ellis is using quantum theory to work on an "evolutionary" variant of the block universe.

Renner also thinks this is a good approach. Nevertheless, the physicist from ETH Zurich came across an effect in his research that complicates the already difficult to understand debate even more.

Past crumbles

Two years ago, Renner demonstrated that reconstructing the past with the help of quantum theory - at least in thought experiments - can lead to fundamental contradictions. "If you ask two quantum physicists about a past coin toss, it can happen that one says 'heads' and the other 'tails', even though neither has made any mistakes.

Nowadays we would say: These are alternative facts. "Is the past crumbling in the attempt to save the arrow of time? Renner does not believe this - and sees such contradictions as a signpost for himself and his colleagues. For the development of a theory of quantum gravity, the finally speaks the last word about time.

At first glance, it may seem surprising that no agreement has been reached on such a fundamental problem. The undeniable polyphony also has something to do with the nature of the question, emphasizes the Viennese gravitational physicist Peter Christian Aichelburg. "Physics is not concerned with things in themselves. It is only concerned with how things relate to one another. Does it make a difference whether I believe in the block universe? In everyday research it doesn't."

Think over the edge

You could also put it this way: Treating time as a quantity in models is one thing. To ask "What is time?" but it is something completely different, namely a philosophical problem. Violetta Waibel, an expert on the philosophy of time at the University of Vienna, relies on Immanuel Kant in this matter. The Königsberg enlightener proved in 1781 that time is a prerequisite for thinking. As a "form of intuition", as a category of order that we ourselves put into the world.

So it is perhaps no surprise that physicists are laboring with intellectual contortions when they pretend to be able to look at the universe from outside - beyond time. This exercise reminds Waibel of a religious imagination, the all-seeing God eye: "For us, perception is always structured in time. We cannot think over the edge."

Aichelburg takes this finding with some composure. And follows in the footsteps of Augustine: "If I knew what time is, then I would be famous." (Robert Czepel, September 15, 2018)

Books on the subject:

  • Carlo Rovelli: "The Order of Time", € 20/190 times, Rowohlt Verlag, Hamburg 2018
  • Richard A. Muller: "Now: The Physics of Time", € 25/480 pages, Fischer Verlag, Frankfurt am Main 2018
  • Peter Aichelburg: "Time through the ages", € 70/260 pages, Vieweg / Springer, Heidelberg 1988

    On the subject
    How do humans shape the earth? What can artificial intelligence do? What is space and time? These questions are at the heart of the current issue of the science magazine FORSCHUNG. in the STANDARD online shop and the magazine is available from specialist retailers at a price of 5.90 euros.