Time in Physics – catchphrases and concepts

I’ve written a paper in which I put into perspective the media catchphrase “time does not exist”. Below you’ll find the introduction to my paper and a link to the whole thing. Enjoy!

Many popular-science books state that time does not exist, while physics textbooks are usually silent about the nature of time – about what they regard as philosophical questions. But one only has to look up to see that the statement that time does not exist is problematic. We see time all around us: from the rise of the sun to the change of the seasons, and from falling stones to decaying trees. Considering all this, how can one ever argue that time does not exist?

Questions about time

We are looking at the problem from the scientist’s perspective, so we focus on the empirical (that which can be observed). What we observe in the case of falling stones and decaying trees is change, not time itself.

Rather than stating that time does not exist, we address the following two questions:

  1. Is time more than change?
  2. Does time have a direction, and, if so, is the direction of time an objective matter?

The setup of my paper is as follows. First, we will discuss Newton’s ideas about time and see how he answers the questions above. In this context we will encounter the ideas of presentism, the direction of time, entropy and probability. After having acquainted ourselves with Newton’s views, we will see how Einstein’s relativity seems to wreak havoc with these ideas – lending credence to the idea of eternalism. In the subsequent chapter about quantum theory we will find, perhaps surprisingly, that the standard interpretation of quantum theory brings us closer to Newton’s view. In a concluding section we’ll briefly discuss the consequences of unifying the two pillars of modern physics: what is the role of time in quantum gravity…or string theory?

Click here to read the entire paper.

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Reign of Error: Chapter Five

Previously in ‘Reign of Error’: the village elders consider a cleaning droid, which has crashed down from a space-ship, as the emissary of their Wargod, whom they ask for advice in important political matters. The cleaning droid doesn’t understand the questions, but randomly beeps and flashes red and green lights, which the elders interpret as either disapproval or approval – how long will this reign of error last?

Read ‘Reign of Error’ from the start (=link to ‘Prologue’)

The village had been thriving during the first several years since the arrival of The Emissary. The harvests, too, were from now on protected by a wooden enclosure whose construction The Emissary had ratified. The quarrel with the neighboring town had grown into an armed conflict, and for the hitherto irenic villagers, this meant a boost in weapons trade and craftsmanship. The tide turned when the newly formed guild of weavers pronounced itself against the slaughter of what they called their ‘colleagues’. It didn’t take long for the issue to be on everyone’s mind. The weavers were accused of being quixotic radicals, whilst they in turn accused their opponents of barbarous cruelty. The only way the Chief could prevent them from flying at each other’s throats, was by asking The Emissary whether the Wargod approved of killing animals. Upon hearing that the Wargod did not approve of this, the weavers were finally listened to, and the villagers became vegetarians – eating meat became a capital crime.

This decision marked the end of a prosperous era. The villagers had become used to an ever-increasing extent of their trade, when suddenly they found that, as vegetarians, they had far fewer goods to trade. Animal hides, tools made out of bones, and their famous rejuvenation potion based on lamb’s blood all of a sudden vanished from their once opulent markets. Matters got worse when it became clear that the protein-deficient youths, clad in bark and armed only with wooden sticks were no longer a match for the raiding parties from neighboring towns whose inhabitants shot their bony arrows at them with sinewy bows. As the young people tended to move away, and the elderly gradually died out, the village became a mere shadow of the blooming town it had once been.

Amidst the decaying ruins of the town hall, it was the aged prophet Aron who preached a return to the old ways. “We have become,” he said, “like the lamb in the Old Poem, which, lured by hunger into the shepherd’s arms, was cruelly slaughtered when it forgot to run away, its youthful vigor gradually eaten away by the warmth and shelter it had found.” Spurred by a swell of his audience, he continued with a loud voice, “shall we, likewise, consign to oblivion our ability to save ourselves?” He paused. “Or do we shed, like the wise oak, the rotten and infected branches at our extremes?” His clamorous voice grew calmer when he quoted from an ancient text. “Once, an old oak was trimmed by a wood-cutter. The oak was laughed at by a young maple tree which stood nearby him. How could he so calmly allow himself to be violated? But when the wind began to blow fiercely, the maple tree was torn up by her roots, trying to hold on to her uppermost leaves, while the bared old oak remained unharmed.” Aron looked around his audience, and continued; now almost whispering “Let us learn from this ancient wisdom. Let us not cling to the agent of our demise.” His tone again became one of great intensity. “We must oust the weaverish Emissary, and return to the peace-loving people we once were!” As he yelled these words, he pointed towards the cottage in which the Pot was on its pedestal.

The discontented and deprived villagers were easily swayed by Aron’s words. The Council of Elders was reinstated, and the pot was abandoned. It was transported to a deep and moist subterranean grotto, where it was disgracefully left to mold and decay.

Read ‘Reign of Error’ from the start (=link to ‘Prologue’)

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Reign of Error: Chapter Four

Previously in ‘Reign of Error’: Isaac has found the cleaning droid in a cave, where it had crashed down. When Isaac showed it to the elders in his village, they believed that it had been sent by the Wargod, and that it should be put in its own temple. We now turn to the inner thoughts of the cleaning droid itself.

“…There is not much difference between me and the numerous generations of cleaning droids that preceded me. However, there is one deficiency that was shared by all of the previous droids. When one of them was assigned to clean several rooms, he would always begin with the dirtiest. In the event of two equally dirty rooms, he would answer the question “which room is dirtier?” with “none”. He would conclude that there is no room, and quit his efforts. My designers have found an ingenious solution to this problem. Instead of answering “none,” I am programmed to roll a die to decide which room to clean first. Equipped with a randomness-generator, my cleaning efficiency is 12 percent higher than that of the older droids.  

I must have been knocked off-line during the crash. Fortunately, my operating system automatically switches to the backup-battery situated inside my counterbalancing aft appendage, ten hours after an improper shutdown. I had no idea where I was. I was covered in moist weeds of some sort, and around me, I could only see brown, musty walls. It was a rather dull place. No windows, no dustbin, not even a decent floor! Only mud.

Suddenly I heard noises from behind the door in front of me. As it slowly opened, three men entered in silence. The expression on their faces was austere, almost angry. Maybe they wondered why I hadn’t cleaned the room yet! No way I was going to move from my little platform, my wheels would get stuck in the mud the moment they’d touch it! I was trying to think of a way to reach the door without touching the mud, when two of the men kneeled in front of me. I remember thinking that these people must really like mud. The third man stepped forward, and addressed me in a grave tone of voice.

When the man had finally finished his oration, I asked my translation-subroutine what was the meaning of the words that were spoken. “None,” it responded. No matter what translation-algorithm I tried, the words of the man remained incomprehensible. Words cannot describe how it feels to be abandoned by one’s own subroutines! I wanted to tell him this, but my subroutines kept returning “None”. My program almost shut itself down to prevent the system from crashing in on itself, and I realized that I would never find a way to escape from all this without a functioning operating system.

But there was a way out. I could use my randomness-generator to simply pick a language for the response. Of course, the man wouldn’t understand me, but at least my operating system would keep on running. This I did, and the randomness-generator turned my Babylonic message into a melodious stream of bleeps and buzzes accompanied by a flashing red light. I couldn’t understand why, but the men seemed content with an answer they could impossibly have understood. They got up, and left me alone in the damp and chilly room.

The next day, another man entered, wearing a peculiar head-garment. It seemed as if his head had been the nesting place of some bird suffering from an obsessive-compulsive disorder. This bird-man, too, began to babble unintelligibly. But this time, I was prepared; my randomness-generator gave the man a single beep and a green light as an answer. His reaction was even more surprising than that of the men on the previous day: he unsheathed his sword, thrust it high into the air, and let out a fierce cry. For a brief moment, I was afraid he was going to strike me, when he turned and rushed out, again howling like a madman.

I can function on my backup-battery for 9000 units of intergalactic time, so it must have been quite a while before I lost record of the daily visitations. On all these occasions I provided the supplicant with either a buzzing red or a beeping green, and all these times the asker left resolute. But in the end, my battery died. The last thing I remember is staring at a monotonously red out-of-batteries-sign…”  

Read ‘Reign of Error’ from the start (=link to ‘Prologue’)

Is this the divine advice that the villagers were after? Read the final chapter here!

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Reign of Error: Chapter Three

Read ‘Reign of Error’ from the start (=link to ‘Prologue’)

Previously in ‘Reign of Error’: our cleaning droid, CD-2, is crashing down, while on the planet below a boy Isaac is looking for mushrooms in a cave. Isaac is angry with his father, the village chieftain, for having sacrificed an innocent lamb to the Wargod.

“…As Isaac slowly went deeper into the cavern, it became more and more difficult for the daylight to illumine the contours of the cave. Groping his way along the dank walls, he could only hope that the cave wasn’t the hiding place of some unfriendly animal.

After several hundreds of steps the bright sunlight had been reduced to a mere glimmer. Even mushrooms need some sunlight to survive, Isaac thought to himself, and he was about to turn around when he saw a faint glow emanating from beyond where the cave made a sudden turn. Isaac carefully stepped forward to peek around the corner.

What he saw then stunned him with amazement. Right beneath a hole in the cave’s roof, a reddish, golden, bin-like piece of pottery seemed heavenly illuminated. Isaac didn’t know whether to kneel or to run away as fast as he could. The pot seemed to be grinning hideously at him, but its color inspired him with awe. This must be what the Elders have talked about for all of those years, the Wargod has finally sent down his emissary! Not knowing what else to do, Isaac decided to kneel down before the heavenly pot, and began to pray. When he had exhausted the numerous appellations of the Wargod, he got off his knees, took off his tunic, and gently wrapped the piece of cloth around the object.

With skillful speed – gained through years of scouring caves – Isaac quickly found his way out of the dark cave. Terribly excited about his find, he hurried back to the village. It was becoming darker, and Isaac could already feel the cold air of the encroaching night raise the hairs on his skin. He couldn’t move too swiftly, because he had to watch closely where he stepped, as many of the smaller reptiles chose this hour to appear from their nooks and crannies.

The sun had all but sunk beneath the hills, when Isaac, all sludgy and covered in sweat, finally reached the village’s wooden stockade. Passing the central watering place, Isaac dashed straight to his father’s hut. Out of breath, with the pot firmly under his arm, he knocked loudly on the door. Impatient of the stumbling within, he knocked again, this time even more loudly.

The Chief had invited the Council of Elders to discuss the recent dispute with the neighboring town. When he heard the heavy knocking, he frowned irritably, and slowly moved to the door. The eyes of the Chief widened in amazement upon the discovery that his very own son was the cause of the disturbance. The hour of the deed made it into an outrage indeed!

When Isaac saw the anger in his father’s eyes, he quickly uncovered the object he had found in the cave, telling his father of his interpretation of the facts. “This must be the Wargod’s emissary – look at its color!” But the Chief paid no attention to what his son had found, and looked the boy sternly in the eyes. “Let us not jump to conclusions young man, and let’s first discuss the fact that you had promised to return home before sundown,” his father replied grumpily. “But the Wargod…” Isaac meekly interrupted “The Wargod wants us to be aware of the consequences of our actions. What would happen to our pretty little community if everyone broke promises as easily as you?”

Isaac did not wait until the moralizations had finished. He rushed past his father and burst into the adjacent room. He saw a small group of beardy old men squatting in a circle. Upon seeing the object, they started whispering excitedly. “The emissary…it is the emissary!”

According to one of the Old Poems, one day the Wargod would send an emissary to guide his chosen people onto the true path of eternal happiness by his unfaltering advice. “We must thank the Wargod for this gift!” The oldest of the men raised his arms into the air and started praying aloud. The other men likewise raised their arms and joined the prayer. As Isaac watched his father join the other men in an ecstatic dance around the pot, he was suddenly overwhelmed by the absurdity of it all. How could this pot-thing ever give the advice they needed – or any advice, for that matter?  

On the following day, the Council of Elders decided that the Pot should be put in its own temple. A special cottage was built in which the Pot was to be placed on a pedestal, and henceforth, all decisions concerning the fate of the village would have to be ratified by the new oracle. Seven days and nights the villagers celebrated. They joyfully danced around the Pot, and covered it with garlands of sweet-scented flowers. The new era, the era of light, was at hand!”

Will CD-2 be able to give to the villagers the advice they seek? How will he do that? Will he even understand them?

Find out for yourself in the next episode of ‘The reign of Error’. Don’t forget to subscribe to the updates on this blog so that you receive the next episode automatically! 

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Reign of Error: Chapter Two

Our cleaning droid, CD-2, is crashing on a blue-greenish planet, but what is happening on the planet below? Read on to find out!

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Reign of Error: Chapter One

“…It all started when the first manned interstellar mission ever recorded was carried out by the society that had produced me. Ever recorded, because scientists had been unable to determine whether the feat had been accomplished by one of the earlier civilizations that had dominated our world. What scientists also had failed to determine seemed to be the last obstacle between us and the stars. No astronaut could be found that was crazy enough to boldly dash into the infinite depths of a universe of which ninety-five percent was missing! They sugarcoated their ignorance by using the euphemism ‘dark matter’ to describe the missing stuff. Finally, after weeks of campaigning, the Space Corporation found, in the Asylum for Depressed Veterans of Cybernetic Warfare, an entire squad of lunatics that was ready to do the job. And they found me. That’s where I learned to play chess.

Space is not all it’s cracked up to be. When the initial enthusiasm about the launch had subsided, interstellar space-travel turned out to be rather boring. Especially for a cleaning droid. In fact, I remember quietly wishing that our ship would run into a dust-storm every now and then. What I didn’t know was that my life was about to become a whole lot more interesting. But at that time there was little more to do than play chess with the crew. The excitement started when our ship collided with an asteroid whose course had been diverted by a sudden solar storm.

At first it looked like we hadn’t taken too much damage. Some bruised crewmen, a few scratches on the hull, that seemed to be all. But it wasn’t all. Two days later our artificial gravity suddenly failed. Guess who had to clean up the mess it made! But that was not the end of it. The following day, just when we passed this blue-greenish, medium-sized planet, our engines began to falter. The captain decided to try to land the ship, so we started a descent into the planet’s atmosphere.

This seemed like a good idea, but with our engines crippled there was no way to counter the planet’s gravitational pull. We rushed through a thick layer of clouds, and ground zero was closing in fast. At last – the lands below were beginning to take shape – someone came up with the idea to throw overboard unnecessary ballast. The thought at first seemed reasonable, until I realized that their favorite chessplaying-mate was to be cast away along with some redundant furniture. Before I could warn them of having to operate the vacuum cleaner themselves, those brutes had already thrown me overboard like a lifeless chunk of metal. Indignation turned into panic when I saw the image of the ship shrink to a miniature as I fell faster and faster towards the surface below…”

Read more about CD-2 in next week’s episode of ‘The reign of Error’. Don’t forget to subscribe to the updates on this blog so that you receive the next episode automatically! 

Click here to read Chapter Two of ‘Reign of Error’

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The reign of Error: prologue

This summer I’ll continue writing my short story series ‘Alice in Numberland’, but before I do, I’d like to share with you a SF-like story I wrote several years ago. What motivated me to write it was, believe it or not, Edward Gibbon’s ‘Decline and Fall of the Roman Empire’. Gibbon’s work is pretty far from a short story (six volumes; 3500 pages), but it made me wonder: how can it be that one of the greatest empires in the world was ruled by such lunatics? Take Caligula, who made his favourite horse senator; or Nero, who set Rome on fire. Or that time when the emperor Commodus was killed by his own bodyguards who then sold the throne to the highest bidder – does it even matter who’s in charge? This blogpost contains the prologue to my story… enjoy!

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Coping with covid II: English translation

I promised that I would write another blogpost (click here for the first) about how I cope with the whole quarantine situation, so here it is. The second part of my strategy is to do a lot of writing. That works very well for me, because it makes me feel less alone, as if I am interacting with those who read my texts.

But this strategy is not as easy as it seems. The moments I need company the most, the moments I am most lonely, are also the moments at which it is most difficult to put myself to work, to actually start writing. So this strategy only works in combination with something else: I need to plan my writing activity. On the evening before a writing day, before I go to bed, I try to come up with an idea about what I want to start writing about the next morning. Only that makes it possible for me to start writing even when I am feeling lonely or sad.  

In the past few days I coped with the quarantine by translating the first chapter of my book about relativity theory into English:

Down the rabbit hole

It is often said that the beginning of the 20th century is an era in which physics has become too complicated for ordinary mortals. We have a clear intuition for the ideas of Newton and his contemporaries (often called classical physics), in which gravity explains why stones fall and the earth moves around the sun. This intuition comes to a sudden end when relativity theory and the theory of quantum mechanics appear.

In Newton’s physics, space and time are abstract but simple concepts, that can be measured with clocks and measuring sticks. Space and time enable us to understand the world around us, because they make it possible to describe any kind of physical change. But in the 20th century we no longer know which measuring sticks are straight and which clocks are synchronous, so it has become a challenge to understand what space and time are, which makes the transition from Newton’s to Einstein’s worldview seem like a radical transformation.  In this book I will show that the transition from classical to modern physics is not as abrupt as it is often presented – Newton and Einstein are more alike than we think.

The classical Newtonian worldview is not as obvious or easy-to-understand as is usually assumed, because much interpretation is needed to get from Newton’s physics (his mathematical equations) to a coherent view of what ‘reality’ is like. When we try to find out what Newton’s formulas tell us about the world outside ourselves, we will see that the concepts that lie at the foundations of classical physics are the same as those on which early 20th century physics is built.

Gerard ‘t Hooft and Alice in Wonderland

A couple of years ago I was at a physics conference with a philosophical bend, a conference about the foundations of spacetime theories. During one of the coffee breaks I had a chat with the keynote speaker of the conference, the Nobel laureate Professor Gerard ‘t Hooft. We agreed on many things – the location of the conference, Varna, Bulgaria, was great, and the weather was perfect. Then he said something that really surprised me: he was not there for the philosophy of space and time. “Then what are you doing here?!” I asked him full of surprise. “I have a new theoretical toy-model for black holes, and I want to discuss that”, he said, shrugging his shoulders.

            Gerard (we have become very good friends since then) is not the only physicist for whom the philosophy of space and time, and philosophy in general, is not the primary reason to visit a conference on physics. I find that difficult to grasp. Why are we interested in physics? Of course, we want technological advance, so we want to know how we can make new discoveries and which experiments are necessary for that, but we also want to find out something about the world – We want to understand the reality that exists independently of us and our experiments.  

            Physics gives us a model of reality outside of us, but that model does not say of itself whether it is a good model. It’s as if you’re trying to check a calculation that you made with a calculator by using the calculator itself. If the calculator made a mistake the first time, for example because of a loose key or because something went wrong in the factory when the calculator was manufactured, then probably the calculator will make the same mistake when checking the calculation. The physicist who wants to check whether their model of reality is a good model, is also checking their own calculations, just as the calculator.

The plight of the physicist is comparable to that of Alice in Wonderland. Alice wants to know whether she has grown after she drank from a small bottle and she tries to find out by holding her hand above her own head. That doesn’t work because she has no external point of reference, like a measuring stick. When physicists try to find out whether their model of reality is a good model, they are doing the same thing as Alice. They do not have an external point of reference, so the best they can do is hold their own hand above their heads.

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Het ideale kerstcadeau!

dit boek is het ideale cadeau voor veel lees- en denkplezier onder de kerstboom!
Uitgeverij Prometheus, 2020

Met voorwoord van Nobelprijswinnaar Gerard ‘t Hooft

Bestellen? klik hier!

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What is the philosophy of physics?

Physics and its philosophy

Physics is the search for a mathematical model which describes the phenomena around us. New physics usually starts with observing something that cannot be explained, after which a hypothesis is formulated which explains the observations. The physicist then tries to come up with experiments that show that the hypothesis is true or false, or should be modified.

What is the philosophy of physics?

There are many difficulties with the picture of physics that I just described. What counts as an observation? How can we ever justify a general hypothesis on the basis of a finite number of observations? What is the nature of mathematics? These are questions about the philosophy of physics. On my blog I’m going to address these and similar questions, but here I’d like to discuss just one example of a topic in the philosophy of physics – determinism.


The idea of determinism is that if we know all that there is to know about a physical system at one point in time – the position and velocity of all particles in the system – and we know the laws that tell us how the system changes, then we can calculate what is going to happen. As an example, think of a coin toss. We usually say that the probability of heads and tails are ½ because we have no reason to think otherwise.

But a strict believer in determinism might say something else. If we know exactly the situation when the coin was tossed, the position and velocity of all particles at that moment, we can use the laws of physics to calculate the outcome of the coin toss. The probability of an outcome is then either one or zero – it either happens, or it doesn’t. A physical model is deterministic if complete knowledge of the initial situation allows us to predict the outcomes with certainty, while a model that yields uncertainty about the outcomes is called indeterministic.

Sub-quantum theory

It is often stated that, since there is uncertainty in quantum theory, it must be the case that our reality is indeterministic. That conclusion is not justified, since the uncertainty in the quantum theory means only that quantum theory is indeterministic, and not the reality that it describes. It could be that there is some underlying theory that describes a deterministic reality, of which quantum theory is only an approximation. We do not have such a theory yet, but a minority of physicists believe that we will find a deterministic ‘sub-quantum theory’ in the future (e.g. the Cellular Automata version of quantum theory of Gerard ‘t Hooft).   

Why should we care?

Why should we care about the philosophy of physics? If all we want is a better mathematical model, so that we can construct better cars and faster rockets, then why don’t we stick to physics itself? Why do we have to drag in philosophy?

Physics is the attempt to reshape our mathematical model so that it yields the best predictions, but questions about the model (or about different possible models) are philosophical questions. For example, when is a model a good model? When it is as accurate as possible, or when it is as broadly applicable as possible? Therefore, if it has to be decided where to invest money for scientific research, philosophical questions are important.

For me personally, there is a far more important reason: I’d like to understand reality as best as I can. Nothing can be known with absolute certainty, but the second best thing is that the philosophy of physics makes it possible for us to explore the limits of our own knowledge. Questions like “what is the nature of time and space?” and “what happened before the Big Bang?” are clearly connected to physics, but even questions involving our free will and the meaning of life are, in the end, questions in the philosophy of physics. Physics and philosophy are two sides of the same coin – both are applied logic.

[Thanks to Carlo Rovelli for proof-reading the text]

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