Kurzgesagt – In a Nutshell 

Sources – What Is Time?


Thanks to our experts —

University of Salamanca

Aix-Marseille University

- Do your past, present and future all exist right now? Are you watching this video, being born and lying on your deathbed at this very moment? Surprisingly, the answer could be yes. But how can that be? And what does that even mean? How does time work? 


This video summarizes in a narrative format two well-known theories about time: the so-called "block universe" and the "growing block".


The block universe is an old theory of time which appears to be an unavoidable consequence of Einstein's theory of special relativity. In philosophical contexts, basically the same idea is known as "eternalism". Simplified, this theory posits that, although not apparent to our human perception, both the past and the future exist in the same way as the present does, and are therefore as real as the present is: The past still exists and the future exists already. As a consequence, time doesn't "flow" (even if it looks so to us) and things in the universe don't "happen" – the universe just "is", hence the name "block universe".


This idea has been explained many times in many places. It is often traced back to this seminal article by philosopher and mathematician Hilary Putnam:


#Putnam, H. (1967): Time and Physical Geometry. Journal of Philosophy, Vol. 64 (8)

https://www.jstor.org/stable/2024493


A good popular account is this article by theoretical physicist Paul Davies in Scientific American:


#Davies, P. (2002): That Mysterious Flow. Scientific American, Vol. 287 (3)

https://www.marcellodibello.com/PHI171/resources/Mysterious-Flow-Davies.pdf


The second idea, the "growing block", is a relatively new modification of the block universe which, very simplified, implies that the past still exists but the future doesn't yet. This idea has been proposed by cosmologist George Ellis in a series of many works over the past years. A good summary is:


#Ellis, G.F.R. (2006): Physics in the Real Universe: Time and Spacetime. General Relativity and Gravitation, Vol. 38

https://arxiv.org/abs/gr-qc/0605049 

https://link.springer.com/article/10.1007/s10714-006-0332-z

Quote: “The Block Universe idea, representing spacetime as a fixed whole, suggests the flow of time is an illusion: the entire universe just is, with no special meaning attached to the present time. This paper points out that this view, in essence represented by usual space time diagrams, is based on time- reversible microphysical laws, which fail to capture essential features of the time-irreversible macro-physical behaviour and the development of emergent complex systems, including life, which exist in the real universe. When these are taken into account, the unchanging block universe view of spacetime is best replaced by an evolving block universe which extends as time evolves, with the potential of the future continually becoming the certainty of the past; spacetime itself evolves, as do the entities within it. However, this time evolution is not related to any preferred surfaces in spacetime; rather it is associated with the evolution of proper time along families of world lines.”


And other works from him expanding or refining the idea are: 


#Ellis, G.F.R. & Rothman, T. (2010): Time and Spacetime: The Crystallizing Block Universe. International Journal of Theoretical Physics, Vol. 49

https://arxiv.org/abs/0912.0808

https://link.springer.com/article/10.1007/s10773-010-0278-5

Quote: “The nature of the future is completely different from the nature of the past. When quantum effects are significant, the future shows all the signs of quantum weirdness, including duality, uncertainty, and entanglement. With the passage of time, after the time-irreversible process of state-vector reduction has taken place, the past emerges, with the previous quantum uncertainty replaced by the classical certainty of definite particle identities and states. The present time is where this transition largely takes place, but the process does not take place uniformly: evidence from delayed choice and related experiments shows that isolated patches of quantum indeterminacy remain, and that their transition from probability to certainty only takes place later. Thus, when quantum effects are significant, the picture of a classical Evolving Block Universe (‘EBU’) cedes place to one of a Crystallizing Block Universe (‘CBU’), which reflects this quantum transition from indeterminacy to certainty, while nevertheless resembling the EBU on large enough scales.”


#Ellis, G.F.R. & Goswami, R. (2012): Space time and the passage of time

https://arxiv.org/abs/1208.2611 

Quote: “This paper examines the various arguments that have been put forward suggesting either that time does not exist, or that it exists but its flow is not real. I argue that (i) time both exists and flows; (ii) an Evolving Block Universe (`EBU') model of spacetime adequately captures this feature, emphasizing the key differences between the past, present, and future; (iii) the associated surfaces of constant time are uniquely geometrically and physically determined in any realistic spacetime model based in General Relativity Theory; (iv) such a model is needed in order to capture the essential aspects of what is happening in circumstances where initial data does not uniquely determine the evolution of spacetime structure because quantum uncertainty plays a key role in that development. Assuming that the functioning of the mind is based in the physical brain, evidence from the way that the mind apprehends the flow of time prefers this evolving time model over those where there is no flow of time.”


#Ellis, G.F.R. (2013): The arrow of time and the nature of spacetime

https://arxiv.org/abs/1302.7291

Quote: “This paper extends the work of a previous paper [arXiv:1208.2611] on the flow of time, to consider the origin of the arrow of time. It proposes that a `past condition' cascades down from cosmological to micro scales, being realized in many microstructures and setting the arrow of time at the quantum level by top-down causation. This physics arrow of time then propagates up, through underlying emergence of higher level structures, to geology, astronomy, engineering, and biology. The appropriate space-time picture to view all this is an emergent block universe (`EBU'), that recognizes the way the present is different from both the past and the future. This essential difference is the ultimate reason the arrow of time has to be the way it is.”


#Ellis, G.F.R. (2014): Time really exists! The evolving block universe. Euresis Journal, Vol. 7

https://euresis.org/wp-content/uploads/2019/06/EJv7id2_Ellis.pdf

Quote: “My starting point is the question, did it make sense for the Planck team to announce the present age of the universe? This talk will propose that it did, because spacetime is an evolving block universe, with the present being the future boundary of a spacetime which steadily extends into the future as

time progresses. The present separates the past (which already exists) from the future (which does

not yet exist, and is indeterminate because of foundational quantum uncertainty). There are some

technical aspects to this namely (1) simultaneity has no physical import, it is a purely psychological

construct, (2) one can define unique surfaces of constant time in a non-local geometric way (and

show how this relates to the standard ADM formalism), (3) this proposal solves the chronology

protection problem (it prevents existence of closed timelike lines). In this context, (4) the arrow of

time is distinguished from the direction of time, which is non-locally defined in the evolving block

universe context.”


#Ellis, G.F.R. (2014): The Evolving Block Universe and the Meshing Together of Times. Annals of the New York Academy of Science, Vol. 1326 (1)

https://arxiv.org/abs/1407.7243 

https://nyaspubs.onlinelibrary.wiley.com/doi/10.1111/nyas.12559

Quote: “It has been proposed that spacetime should be regarded as an evolving block universe, bounded to the future by the present time, which continually extends to the future. This future boundary is defined at each time by measuring proper time along Ricci eigenlines from the start of the universe. A key point, then, is that physical reality can be represented at many different scales: hence, the passage of time may be seen as different at different scales, with quantum gravity determining the evolution of spacetime itself at the Planck scale, but quantum field theory and classical physics determining the evolution of events within spacetime at larger scales. The fundamental issue then arises as to how the effective times at different scales mesh together, leading to the concepts of global and local times.”


#Ellis, G.F.R.& Drossel, B. (2020): Emergence of time. Foundations of Physics, Vol. 50

https://link.springer.com/article/10.1007/s10701-020-00331-x

Quote: “Microphysical laws are time reversible, but macrophysics, chemistry and biology are not. This paper explores how this asymmetry (a classic example of a broken symmetry) arises due to the cosmological context, where a non-local Direction of Time is imposed by the expansion of the universe. This situation is best represented by an Evolving Block Universe, where local arrows of time (thermodynamic, electrodynamic, gravitational, wave, quantum, biological) emerge in concordance with the Direction of Time because a global Past Condition results in the Second Law of Thermodynamics pointing to the future. At the quantum level, the indefinite future changes to the definite past due to quantum wave function collapse events.”


A good popular account of the growing block is:


#Meerali, Z. (2015): Is the Future Already Written? Discover Magazine

https://www.discovermagazine.com/the-sciences/is-the-future-already-written



The Block Universe


- Imagine the universe like a child painting pictures on paper. Each picture shows everything that is happening in the universe in a single moment. With each new moment, all kinds of things occur everywhere – people are born and die, galactic civilizations expand, you miss the bus – and our universe-kid makes a new picture that replaces the old one.

In this way you get something like a movie – only the moment we are in right now is real. The past is what happened before, now it’s gone. The future is still to come and has not been drawn yet. This is kind of how time feels, right? Each moment being replaced by the next one. The past is far behind us, the future doesn’t exist.


The drawings/movie metaphor symbolizes the intuitive idea we all have about time: the universe is extended in space, all places in the universe share a common instant of time that we call "present", and only things existing or happening in the present are real: The past ist gone and doesn't exist anymore and the future is yet to happen, so it doesn't exist yet either.


#Ingram, D. & Tallant, J. (2023): Presentism, The Stanford Encyclopedia of Philosophy

https://plato.stanford.edu/entries/presentism/ 

Quote: “Presentism is the view that only present things exist. So understood, presentism is primarily an ontological doctrine; it’s a view about what exists, absolutely and unrestrictedly.”



But what if time is something else? 


- What if the universe-kid has already finished all its drawings and stacks them on top of each other? This way we get a block – a block of time that contains the whole history of the universe. All moments that have ever existed or will ever exist. But in this block, in this stack of moments, the past, the present and the future are equally real and exist at the same time. 


This is a summary of the main conclusion of the block universe theory, which will be explained in more detail below. The past and the future are as real as the present:


#Davies, P. (2002): That Mysterious Flow. Scientific American, Vol. 287 (3)

https://www.marcellodibello.com/PHI171/resources/Mysterious-Flow-Davies.pdf

- This feels wrong – the only things that we perceive as real are those things happening now. How can the past and future be real right now? The problem is that according to the theory of relativity, they kind of have to be.


Heavily simplified, relativity says that time and space are not separated, but one connected spacetime. When you move through space, you are also moving through the block. This means time passes differently for different people, depending on how they move through space relative to each other. And this also means that what someone perceives as “now” is a certain cut along the block – a cut that will depend on how fast they are moving.


So what you think is “now” is really only your now – there are many different “nows” in the universe and all of them are equally real. This also means there is no universal past or future. 


One of the most important consequences of Einstein's relativity is the idea of relativity of simultaneity. Intuitively, we all think of the present as "all events happening now, simultaneously, in the universe". However, Einstein showed that two events A and B that are simultaneous for one observer (i.e. two events that happen in what that observer might call "the present") won't be simultaneous for another observer that is moving with respect to the first one: For the second observer, A might happen in their past and B might happen in their future, for example. As explained in physics textbooks, this is one of the first and most basic features of relativity, and it's related to the well-known fact that time is not an absolute concept, but passes differently for different observers. 


#Encyclopedia Britannica (retrieved 2023): Special relativity 

https://www.britannica.com/science/relativity/Special-relativity

Quote: “Einstein concluded that simultaneity is relative; events that are simultaneous for one observer may not be for another. This led him to the counterintuitive idea that time flows differently according to the state of motion and to the conclusion that distance is also relative."



Ok. This is a lot – how does this work?


- Imagine three alien spaceships a million light years away. The first one just hovers in space, not moving relative to you. You both experience the same “now”, the same present. If you had a magical instantaneous internet connection, you could do a video call right now and chat about alien things.


We have to make a big disclaimer here: It is crucial to understand that the "instantaneous internet connection" mentioned in the video is just a metaphor to quickly and intuitively convey the idea of what we mean by simultaneous events. An instantaneous internet connection would mean the ability to transfer information between two places at infinite speed. But one of the consequences of relativity (and probably the best known one, together with E=mc2) is also that it's physically impossible to transfer information faster than light – so an instantaneous internet connection simply can't exist.


Here we are using it only a metaphorical way to convey quickly and intuitively that, in the case mentioned in the video (a distant alien spaceship that is not moving with respect to you), you and the alien would agree on which events in the universe are simultaneous, and as a consequence you would share the same notion of "present". How this works in a general case is explained in more detail below.



- The second spaceship is flying away from us at 30 km/s, about 3 times faster than a human rocket. It is moving differently through the block of time than you are, which means its “now” is different from yours. 

With the magical internet, the aliens can talk to your ancestors in 1924, when humanity was discovering the first galaxies outside the Milky Way. 


When an observer O' (in this case, the alien) is moving at speed v with respect to an observer O, the positions x' and times t' as measured by O' are related to the positions x and times t as measured by O by a set of equations called "Lorentz transformations", which again can be found in any physics textbook and are given below.


Geometrically, the relation between the (x, t) and (x', t') respective axes (i.e. reference frames for each observer can be understood as a "squeezing" of (x', t') with respect to (x, t):


#Ling, S.J. et al. (retrieved 2023): The Lorentz Transformation. University Physics Volume 3

https://pressbooks.online.ucf.edu/osuniversityphysics3/chapter/the-lorentz-transformation/

(In the diagram and equations above, movement between O' and O is assumed to be along the x direction of space.)


The alien measures time t', so the "now-slices" for them are given by the surfaces of constant time t', i.e. the (oblique) lines t' = constant in the diagram above.


Let's say that the Earth is located at x = 0 and that the Earth-time t = 0 corresponds to 2024, i.e. our present. This means that, from our point of view, now the alien is located at the spacetime point t = 0 and x = 1 Mly (1 megalight year = 1,000,000 light years).


Plugging these values in the top-right formula of the 8 equations highlighted above gives us the value of the time t' as measured by the alien. Let's call this (fixed) value t'_A. The (oblique) line t' = t'_A gives us the "now-slice" of the alien, i.e. all events in the universe that, for the alien, are happening "now".


To find which time t on Earth is happening "now" for the alien, we just have to take the same equation, substitute t' = t'_A and solve for t at x = 0 (since the Earth is located at x = 0). With simple algebra we find:


t = – v·(1 Mly)/c2


where v is the speed of the alien with respect to us (v = +30 km/s). This means that, in years, t is given by:


t = – (v/c)·106 years


Since v/c = 10(–4), we get t = –100 years. And since t = 0 was chosen to be 2024, we find that, for the alien, the Earth is "now" in 1924. 


A qualitative diagram of the now-slices on Earth and on the alien spaceship is given below:

For the "human rocket" mentioned above, we took the escape velocity from Earth, which is 11 km/s (so 30 km/s is about 3x that).



At the beginning of the 20th century (in the year 1924, note the “1924” in the equation above), people already suspected that there was more beyond the Milky Way, but there was no evidence. 


Only Edwin Hubble was able to provide the proof: He found a star in the Andromeda constellation that could be used to measure distances. He discovered that the star was much too far away to be within our galaxy.


#NASA (2023): Edwin Hubble

https://science.nasa.gov/people/edwin-hubble/ 

Quote: “Up until the early 20th century, our perception of the cosmos fell within the bounds of the Milky Way. Although astronomers speculated about the existence of other galaxies in our universe, they had no observable evidence of them. It wasn’t until Hubble pointed the Hooker Telescope at the constellation Andromeda that our perspective shifted.


Hubble studied what was then known as the Andromeda Nebula, an object that for centuries appeared as an elongated cloud of light. In 1923, he resolved individual stars in this “nebula.”


Hubble’s continued observations of Andromeda resulted in one of the most transformative discoveries in cosmology. He uncovered his first Cepheid variable star, a type of star used to measure distances in space by how it changes brightness. A Cepheid variable star’s intrinsic brightness is directly related to its cycle from bright to dim and back to bright again. By charting the changes in these stars, Hubble discovered that Cepheid variable stars in Andromeda were much farther away than those in the Milky Way. This contrast in distance led Hubble to believe the Andromeda Nebula was a galaxy in its own right. Hubble used this technique to study other so called “nebulae” in the universe, and concluded that millions of galaxies existed beyond our own.”



- The third spaceship wants to visit Earth and is flying towards you at 30 km/s, moving at the opposite angle of the second ship through the block of time. It experiences yet another “now” – with the magical internet, the aliens can talk to your descendants in the year 2124, when humanity has already built cities on Mars and Venus.


Now we need to apply the same formula that we derived above:


t = – (v/c)·106 years


only taking into account that, since the alien is moving towards the Earth, their speed is negative, v = – 30 km/s. This gives us:


t = + 100 years


And since t = 0 was chosen to be the year 2024, we find that the "now-slice" for this alien crosses the Earth in our year 2124. 


A qualitative diagram of the now-slices on Earth and on the alien spaceship is also given below:

- OK so we have three different “nows” – so which one is correct? Well, that’s the problem. Relativity is based on one powerful principle – cosmic democracy: the fact that the point of view of all observers in the universe is equally valid. All those “nows” have to be equally real. But if this is the case, your past, present and your future all have to exist at the same time, right now! Because for the different aliens, they all happen in their present.


"Cosmic democracy" here refers to the "principle of relativity", the postulate on which Einstein's relativity is based. This principle states that the speed of light and the laws of physics have to be the same for all observes regardless of how they are moving with respect to each other.


#Encyclopedia Britannica (2023): relativity

https://www.britannica.com/science/relativity 

Quote: “The constancy of the speed of light and the universality of the laws of physics for all observers are cornerstones of special relativity.”



- This means that the distinction between the past, the present and the future is an illusion. The universe is not a bunch of things evolving through time, like in a movie – but a static block in which the past, the present and the future all coexist and are real.


How can that be? Well, think about a galaxy outside the observable universe, too far away to ever visit or see. But even if you can’t get there and don’t see it, it is still real. The future might be the same!


But if the past is not far behind us and the future actually exists, then…  there is no “movie”. Things don’t happen in the universe. The universe just “is” – like a frozen block of dead, cosmic ice, with everything that will ever happen already written and decided.

Here we're just reinstating the main claim about the block universe that we made at the beginning.


#Ellis, G.F.R. (2006): Physics in the Real Universe: Time and Spacetime. General Relativity and Gravitation, Vol. 38

https://arxiv.org/abs/gr-qc/0605049 

https://link.springer.com/article/10.1007/s10714-006-0332-z

Quote: “This view can be formalised in the idea of a block universe (Mellor 1998, Savitt 2001, Davies 2002)2: space and time are represented as merged into an unchanging spacetime entity, with no particular space sections identified as the present and no evolution of spacetime taking place. The universe just is: a fixed spacetime block. In effect this representation embodies the idea that time is an illusion: it does not `roll on' in this picture. All past and future times are equally present, and there is nothing special about the present (`now’). There are Newtonian, Special Relativity, and General Relativity versions of this view (see Figures 1-4), the latter being most realistic as it is both relativistic and includes gravity."



Is the Future Already Written?

- If all times coexist and are equally real, then the future has to be already written. 


But that’s not how you experience things. It feels like you can mold your future with your decisions. It really feels like you are free to choose to stop watching YouTube to not miss the bus. But if the future is set in stone, you can’t “decide” anything. So are your choices an illusion? Well… maybe. Maybe your free will is a mirage. And maybe you missing the bus was already predetermined at the Big Bang, so feel free to continue watching. 


The block universe implies that the past and the future are "already there" and, as such, they should be fixed, or determined. Therefore, the idea of a block universe is intimately linked to the notion of a fully deterministic universe: A universe in which full knowledge of the state of all particles at a given instant of time would enable us to perfectly reconstruct the past and fully predict the future. This is certainly the case in classical physics, in which all processes are governed by differential equations that, under given "initial conditions", imply a unique solution (i.e. "state of the universe") both towards the past and towards the present.


#The Feynman Lectures on Physics (retrieved 2023): The Relation of Wave and Particle Viewpoints

https://www.feynmanlectures.caltech.edu/III_02.html 

Quote: “It is true classically that if we knew the position and the velocity of every particle in the world, or in a box of gas, we could predict exactly what would happen. And therefore the classical world is deterministic.”


The idea was famously put forward two centuries ago by French mathematician, physicist and philosopher Pierre-Simon Laplace.


#Stanford Encyclopedia of Philosophy (2023): Causal Determinism

https://plato.stanford.edu/entries/determinism-causal/ 

Quote: “We ought to regard the present state of the universe as the effect of its antecedent state and as the cause of the state that is to follow. An intelligence knowing all the forces acting in nature at a given instant, as well as the momentary positions of all things in the universe, would be able to comprehend in one single formula the motions of the largest bodies as well as the lightest atoms in the world, provided that its intellect were sufficiently powerful to subject all data to analysis; to it nothing would be uncertain, the future as well as the past would be present to its eyes.”



Except. Quantum stuff is ruining everything again.


- Quantum processes can’t be predicted, not even in principle. Not because we are silly and don’t know how to do it – according to quantum physics, quantum particles are intrinsically random. 


For example, if you have a radioactive atom, it could decay at any moment, in the next second or in the next million years. We can calculate the probability that it will decay tomorrow, but no oracle in the universe will ever be able to tell you with absolute certainty if it will do so or not.


Quantum mechanical processes are known to be intrinsically random. That is: we can repeat an experiment twice under the exact same conditions, and get one result the same time and a different result the second one – meaning that, in quantum mechanics, the future is not determined by the past. The most we can predict are the probabilities of the different results, but not the result itself with 100% accuracy.


#Encyclopedia Britannica (2023): Quantum mechanics

https://www.britannica.com/science/quantum-mechanics-physics

Quote: “A fundamental concept in quantum mechanics is that of randomness, or indeterminacy. In general, the theory predicts only the probability of a certain result. Consider the case of radioactivity. Imagine a box of atoms with identical nuclei that can undergo decay with the emission of an alpha particle. In a given time interval, a certain fraction will decay. The theory may tell precisely what that fraction will be, but it cannot predict which particular nuclei will decay. The theory asserts that, at the beginning of the time interval, all the nuclei are in an identical state and that the decay is a completely random process.”


A key issue is that quantum randomness is fundamental, and therefore very different from the randomness found in classical physics, which is only apparent and due to a lack of knowledge of the initial conditions.


##Encyclopedia Britannica (2023): Quantum mechanics

https://www.britannica.com/science/quantum-mechanics-physics

Quote: “Even in classical physics, many processes appear random. For example, one says that, when a roulette wheel is spun, the ball will drop at random into one of the numbered compartments in the wheel. Based on this belief, the casino owner and the players give and accept identical odds against each number for each throw. However, the fact is that the winning number could be predicted if one noted the exact location of the wheel when the croupier released the ball, the initial speed of the wheel, and various other physical parameters. It is only ignorance of the initial conditions and the difficulty of doing the calculations that makes the outcome appear to be random. In quantum mechanics, on the other hand, the randomness is asserted to be absolutely fundamental. The theory says that, though one nucleus decayed and the other did not, they were previously in the identical state.”



- But quantum particles can change the world. 


Imagine a radioactive element randomly decays and causes a genetic mutation in a nearby mammal. And then many generations later that mutation has led to a weird mix of duck and mammal that makes no sense. Or the atom decays a day later and the weird creature never will exist.


There are many instances in which we can conceive of a microscopic quantum event having consequences in the macroscopic world – probably the best known example of this is the thought experiment known as "Schrödinger's cat". A real-world example is biological evolution, since many mutations are known to be caused by radiation, which is an intrinsic (and hence random) quantum process.


#Ellis, G.F.R. (2006): Physics in the Real Universe: Time and Spacetime. General Relativity and Gravitation, Vol. 38

https://arxiv.org/abs/gr-qc/0605049 

Quote: “The fact that such events happen at the quantum level does not prevent them from having macro-level effects. Many systems can act to amplify them to macro levels, including photomultipliers3 (whose output can be used in computers or electronic control systems). Quantum fluctuations can change the genetic inheritance of animals (Percival 1991) and so influence the course of evolutionary history on Earth. Indeed that is in effect what occurred when cosmic rays4 – whose emission processes are subject to quantum uncertainty - caused genetic damage in the distant past:


‘The near universality of specialized mechanisms for DNA repair, including repair of specifically radiation induced damage, from prokaryotes to humans, suggests that the earth has always been subject to damage/repair events above the rate of intrinsic replication errors … radiation may

have been the dominant generator of genetic diversity in the terrestrial past” [45].5’


Consequently the specific evolutionary outcomes on life on Earth (the existence of dinosaurs, giraffes, humans) cannot even in principle be uniquely determined by causal evolution from conditions in the early universe, or from detailed data at the start of life on Earth. Quantum uncertainty prevents this, because it significantly affected the occurrence of radiation-induced mutations in this evolutionary history. The specific outcome that actually occurred was determined as it happened, when quantum emission of the relevant photons took place: the prior uncertainty in their trajectories was resolved by the historical occurrence of the emission event, resulting in a specific photon emission time and trajectory that was not determined beforehand, with consequent damage to a specific gene in a particular cell at a particular time and place that cannot be predicted even in principle."



- If quantum stuff is really uncertain, the future can’t be set in stone. But if the future is an untold story, it can’t be real in the same way as the past is.


The fact that quantum events are intrinsically unpredictable and hence uncertain until they happen makes the idea of an "already existing future" much more difficult. At least, it makes difficult to accept that the future is "as real as" (or "in the same sense as") the past is, since once is fully determined and the other generally isn't. This caveat has been noted even by proponents of the block universe idea.


#Davies, P. (2002): That Mysterious Flow. Scientific American, Vol. 287 (3)

https://www.marcellodibello.com/PHI171/resources/Mysterious-Flow-Davies.pdf 

Quote: “A second possibility is that our perception of the flow of time is linked in some way to quantum mechanics. It was appreciated from the earliest days of the formulation of quantum mechanics that time enters into the theory in a unique manner, quite unlike space. The special role of time is one reason it is proving so difficult to merge quantum mechanics with general relativity. Heisenberg’s uncertainty principle, according to which nature is inherently indeterministic, implies an open future (and, for that matter, an open past). This indeterminism manifests itself most conspicuously on an atomic scale of size and dictates that the observable properties that characterize a physical system are generally undecided from one moment to the next.


For example, an electron hitting an atom may bounce off in one of many directions, and it is normally impossible to predict in advance what the outcome in any given case will be. Quantum indeterminism implies that for a particular quantum state there are many (possibly infinite) alternative futures or potential realities. Quantum mechanics supplies the relative probabilities for each observable outcome, although it won’t say which potential future is destined for reality.”


And it's one of the starting points of George Ellis' proposal of the "growing block universe" that will be explained below.


#Ellis, G.F.R. (2006): Physics in the Real Universe: Time and Spacetime. General Relativity and Gravitation, Vol. 38

https://arxiv.org/abs/gr-qc/0605049 

Quote: “The past has happened and is fixed, so the nature of its existence is quite different than that of the indeterminate future. However we cannot causally retrodict uniquely to the past from present day initial data by using the appropriate evolution equations for the matter, because of friction and other dissipative effects (see Section 2.2) and the quantum measurement process (`collapse of the wave function’, see Section 2.3)."


It should be noted that Ellis' arguments to reject the block universe idea go beyond quantum uncertainty and include other well-known physical phenomena such as complex emergent behavior, chaotic dynamics and the appearance of so-called "catastrophes", or tipping points, in the evolution of classical systems. For the sake of clarity and concision these arguments have not been mentioned in this video, but can be found in the papers by George Ellis listed in this document.”



- So what happens when uncertain things, like the decay of our atom, become real? Is that moment the present? Is this “now”? But before we saw that cosmic democracy makes it impossible to define an absolute “now”. What’s going on here?


So we have two conflicting arguments. On the one hand, Einstein's relativity implies that "presentism", the idea of an absolute present, is physically untenable. But on the other, quantum uncertainty seems to single out quite unique "now moments" – the instants when unpredictable quantum events happen. Can these two ideas be reconciled?



- It turns out that for every individual object – you, an alien, an atom – the past, the present and the future are always well defined. Your death will always happen after your birth – never before, and never at the same time. Now you are clearly between your birth and your death. So for you at least, "now" makes perfect sense.


If we don’t play tricks like going to the other side of the universe and using aliens in funny ways to find out what “now” means, things again start to look ordered and nice, and individual “nows” seem to exist. Can we do something with them?


One possible way out of the "paradox" just mentioned is to realize that maybe the problem arose because we were trying to define "now-surfaces" across the whole universe, i.e. "extended nows" to define a present simultaneously at all points of space.


However, no observer (or a quantum particle) ever experiences an "extended present". Instead, it follows a one-dimensional trajectory along the four dimensional spacetime. As also explained in physics textbooks, such a trajectory (called "world-line") is made by joining events experienced by the observer. And the fact that it is one-dimensional means that those events always have a well-defined order. In terms of the local time experienced by the observer (i.e. the time as indicated by their clock), this means that, at any single spacetime location, the observer always experiences a uniquely defined future and a uniquely defined past. These labels are conventions since they depend on the observer and on the particular spacetime point ("present") chosen, but are always well-defined with respect to them.


An attempt to relate this well-known fact to a more physical notion of "present" was emphasized in 2004 in this article by James Hartle:


#Hartle, J.B. (2005): The Physics of 'Now'. American Journal of Physics, Vol. 73 (2)

https://arxiv.org/abs/gr-qc/0403001 

Quote: "The world is four-dimensional according to fundamental physics, governed by basic laws that operate in a spacetime that has no unique division into space and time. Yet our subjective experience of this world is divided into present, past, and future. What is the origin of this division? What is its four-dimensional description? Is this the only way experience can be organized consistently with the basic laws of physics? This paper reviews such questions through simple models of information gathering and utilizing systems (IGUSes) such as ourselves.


Past, present, and future are not properties of four-dimensional spacetime but notions describing how individual IGUSes process information. Their origin is to be found in how these IGUSes evolved or were constructed. The past, present, and future of an IGUS is consistent with the four-dimensional laws of physics and can be described in four-dimensional terms. The present, for instance, is not a moment of time in the sense of a spacelike surface in spacetime. Rather there is a localized notion of present at each point along an IGUS’ world line. The common present of many localized IGUSes is an approximate notion appropriate when they are sufficiently close to each other and have relative velocities much less than that of light."


which already advanced some of the ideas that would be later developed by George Ellis in his "growing block" theory.

- Let’s return to our block universe. Maybe the block does not contain the future – and maybe we just imagined it wrong. Maybe the block is just the past, and a thin layer on the surface is the present. That surface is not smooth, but bumpy and uneven. It’s been made by joining countless individual “nows” – each experienced by someone or something in the universe, each equally real and valid. And all observers do their bit, so cosmic democracy is still true

As new things happen and uncertain things become certain –radioactive atoms decay, new species of mammals arise, people miss the bus– the border moves upward, creating new time in the universe. Instead of a frozen block of time with a future that has already been written, the block is growing and things happen.


The fact that the transition from the past to the future shouldn't be considered globally but locally along the individual trajectories of particular observers ("worldlines") is the technical starting point in the idea of the growing block:


#Ellis, G.F.R. (2006): Physics in the Real Universe: Time and Spacetime. General Relativity and Gravitation, Vol. 38

https://arxiv.org/abs/gr-qc/0605049 

Quote: "One can suggest that in this case, the transition from present to past does not take place on specific spacelike surfaces; rather it takes place pointwise at each spacetime event. The implicit “now” of figures 8 and 9 (or of any “flowing time” concept) is replaced by a “here-now” (space-time point), and for both the “now” and the “here-now” the past is determined (exists relative to the [here]-now), the future is undetermined, and the [here]-now is a moment of passage from one state to the other." 


whose main achievement is arguably to point out how all those "individual nows" could be woven into an extended "now-surface" in a way that is still consistent with relativity ("cosmic democracy"):


#Ellis, G.F.R. (2006): Physics in the Real Universe: Time and Spacetime. General Relativity and Gravitation, Vol. 38

https://arxiv.org/abs/gr-qc/0605049 

Quote: "However the constraints on what future can emerge at a given here-now are not pointwise constraints but (in relation to any local coordinates) constraints involving spatial derivatives, or, roughly speaking, neighbouring points. So if evolution takes place pointwise, it still involves a degree of spatial coordination between neighbouring points, even though the neighbouring point might not “yet exist” relative to a different here-now until it lies in the past. It is convenient to introduce local coordinates in order to determine how this works, involving a splitting of spacetime into space and time as in the ADM formalism [1, 2, 38, pp. 520–528] and evolution along the coordinate lines introduced as determined by the shift function. But then it is physically sensible to focus on this feature: that is, while it may in a sense take place pointwise, it is more convenient to consider the evolution as taking place along timelike world lines,16 rather than being determined by any universal time defined by spacelike surfaces. Indeed this is strongly suggested both by the way that time is determined in General Relativity as a path integral along timelike world lines, and also by the nature of the examples discussed in the previous section, where physical effects that determine what happens are focused on changes that take place along histories of matter, represented by timelike worldlines.17"

What Is Real, Then?


- Let’s recap. We started with time as a movie – one “now” after another, where only the current “now” was real. Then we found out that because of relativity there are multiple “nows”, all of them real somehow – which could mean that we are living in a frozen block universe where things don’t happen and you don’t really have free will. And we ended up with a kind of growing block universe, where time passes and the future is open.


So which is correct? What is real? The present? The past? Are the dinosaurs as real as you are right now? What do the aliens on the other corner of the universe think about all this?

To be honest, no one knows. What we’ve learned are two possibilities to describe time, but they are not the only ones.


In this video we've described three ideas of time:


1) Presentism: The intuitive and naive notion we all share in which the present is a well-defined instant that exists throughout the whole universe, and whose events are the only ones that are real (the past is already gone and the future hasn't happened yet, and therefore both are non-existent).


2) Eternalism, or Block Universe: The idea, which appears to be an unavoidable consequence of Einstein's relativity, that both the past and the future are as real as the present.


3) The Evolving/Growing Block: A relatively new alternative to the classical block universe theory, which asserts that the past may still exist but the present doesn't yet, and all that in a way that is still compatible with Einstein's relativity. 


The most important takeaways of all this are maybe two:


A) On physical grounds, presentism can NOT be true, even if it looks the most natural option for us, since it would contradict Einstein's theory of relativity. 


B) But apart from that, physicists haven't reached any consensus about which theory should replace presentism. The block universe is very popular among physicists, but it is by no mean the only existent theory about time. A good summary at the popular-science level can be found e.g. here:


Falk, D. (2016): A Debate Over the Physics of Time. Quanta Magazine

https://www.quantamagazine.org/a-debate-over-the-physics-of-time-20160719# 


And also: 


Odenwald, S. (2022): The struggle to find the origins of time. Astronomy

https://www.astronomy.com/science/the-struggle-to-find-the-origins-of-time/ 

- Some scientists think that the idea of “now” only makes sense near you, but not in the universe as a whole. 


This idea has been very beautifully explained in this short paper by theoretical physicist Carlo Rovelli.


#Rovelli, C. (2019): Neither Presentism nor Eternalism. Foundations of Physics, Vol 49

https://arxiv.org/abs/1910.02474

Quote: “Is reality three-dimensional and becoming real (Presentism), or is reality four-dimensional and becoming illusory (Eternalism)? Both options raise difficulties. I argue that we do not need to be

trapped by this dilemma. There is a third possibility: reality has a more complex temporal structure

than either of these two naive options. Fundamental becoming is real, but local and unoriented. A

notion of present is well defined, but only locally and in the context of approximations.”



- Others think that time itself doesn’t even exist – that the whole concept is an illusion of our human mind. 


This idea has several proponents, and it is often based on the argument that time might be a useful "auxiliary parameter" to label events with no actual physical counterpart. On of the most famous proponents of this idea is the independent physicist Julian Barbour. A very good and accessible summary is:


#Barbour, J. (2008): The Nature of Time. Essay written for the FQXi contest on the Nature of Time

https://arxiv.org/abs/0903.3489

Quote: “A review of some basic facts of classical dynamics shows that time, or precisely

duration, is redundant as a fundamental concept. Duration and the behaviour of clocks emerge

from a timeless law that governs change.”


For another example of similar ideas, see:


#Rovelli, C. (2008): “Forget time”. Essay written for the FQXi contest on the Nature of Time

https://arxiv.org/abs/0903.3832

Quote: “Following a line of research that I have developed for several years, I argue that the best strategy

for understanding quantum gravity is to build a picture of the physical world where the notion

of time plays no role at all. I summarize here this point of view, explaining why I think that

in a fundamental description of nature we must “forget time”, and how this can be done in the

classical and in the quantum theory. The idea is to develop a formalism that treats dependent and

independent variables on the same footing. In short, I propose to interpret mechanics as a theory

of relations between variables, rather than the theory of the evolution of variables in time.”



- And others think that time does exist, but that it is not a fundamental feature of the universe – rather, time may be something that emerges from a deeper level of reality, just like heat emerges from the motion of individual molecules or life emerges from the interactions of lifeless proteins.


The idea that time might be real but not fundamental has been proposed several times in different contexts and it has gained some traction in recent years due to theoretical advancements inspired by (although not necessarily based on) string theory. A recent good popular summary is:


#Becker, A. (2022): What Is Spacetime Really Made Of? Scientific American, Vol. 326 (2)

https://www.scientificamerican.com/article/what-is-spacetime-really-made-of/