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Time and the Block Universe

This page considers the nature of time itself. Using a purely logical argument - involving no physics or mathematics whatsoever - we shall see that time must behave in a way which is completely at odds with our usual mental conception of the passage of time. As Einstein himself said, the flow of time is only a "stubbornly persistent illusion".

This page might very well shake the way you view the nature of time.

"There is nothing outside the universe"

On the previous page, Reality Is Relative, we were introduced to Lee Smolin's simple maxim: "There is nothing outside the universe" which he described as the "first principle of cosmology". This means there can be no absolute coordinate system for space or time outside the universe by which object positions and times can be defined. Instead, the position of every object in the universe must be defined solely in terms of the position of other objects in the universe.

This is an incredibly profound and important result - it is the principle underyling Einstein's theory of relativity - and, as we will see, it has staggering implications for our conception of how time operates in our universe.

Two Theories of Time

There are two dominant - and incompatible - theories of time: the tensed theory, and the tenseless theory. The tensed theory of time most resembles the popularly-held view of time. The tensed theory requires there to be a present moment (the "now"), and a distinction between an event in the past, present, and future (an event in the past was real, an event in the present is real, and an event in the future will be real). Notice that the "now" moves. This apparent movement of the "now" is an essential feature of the tensed theory of time.

The tensed theory of time

However, there is a philosophical (and logical) problem to this idea of a moving "now". Put simply, it raises the question which has puzzled philosophers: "How fast does time flow?". If the "now" moves then it must move with respect to some time reference. So is it moving with respect to itself? Surely not. To say "Time moves at the rate of one second per second" is meaningless. Rather, the rate of time flow would have to be measured with respect to some secondary, external time reference. However, in our earlier discussion on this page it was stressed that there was no clock outside the universe, so there could not be any such external time reference. It is simply logically impossible for there to be a moving "now". Time does not "flow"!

So what is the alternative? The alternative is to consider a universe in which all of time is laid-out (just as the space dimension is laid-out), and there is no moving "now". All times are equally real: as there is no special "now", there is no distinction between past and future. This forms the tenseless theory of time.

Most physicists would favour the tenseless theory as the most accurate representation of time. It is also called a block universe because all of spacetime can be viewed as being laid-out as an unchanging four-dimensional block: The tenseless theory of time

For a clear explanation of the block universe, see this excellent Scientific American article by Paul Davies.

But we all feel a "flow" of time in which an unknown and unfixed future becomes our present moment before being relegated to the past. How can we reconcile this feeling with the block universe in which all of time is laid-out, and there is no moving "now"? It emerges that the feeling we have of the passing of time is nothing more than an illusion of human perception due to the asymmetry of the time axis: we can remember the past, but we cannot remember the future. This then gives the illusion of a flow of time with the unknown future becoming the fixed past. For more details on this, see the Arrow of Time page.

Eternal Life

It might come as a surprise that this orthodox "block universe" view of time in fact leads us to conclude that we possess a form of eternal life! This is a consequence of the principle that in the block time model all periods of time are equally real. If a loved one dies, you might take some comfort from the knowledge that this period of time in which your loved one is dead has, in fact, no greater reality than the time when your loved one was alive. According to physics, it is just as valid to consider your loved one as alive as it is to consider them dead!

Einstein took comfort from this knowledge when his lifelong friend Michele Besso died. He wrote a letter consoling Besso's family: "Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion."

Of course, the flip-side is that you're already dead!

The Astounding Implications of the Block Universe

I do not believe the implications of the orthodox block universe model are widely realised - even among physicists! I regularly read phrases in published papers (even from highly-reputable authors) which make no sense at all from the point of view of the block universe. As Lee Smolin says in his book Three Roads to Quantum Gravity: "There are unfortunately not a few good professional physicists who still think about the world as if space and time had an absolute meaning". The conclusions presented here relating to the block universe model follow directly from Einstein's theory of general relativity and so should be considered to be orthodox physics.

According to the block universe model, every moment in time is equally real, so the whole of space and time must be laid-out in one unchanging spacetime block:

The universe structure is one unchanging spacetime block. Essentially, this means that the whole spacetime "cone" shown above exists as an unchanging structure.

It is true that there is a time dimension defined within the universe. And for an observer within the universe, objects appear to change with respect to this time axis. However, this apparent flow of time is just an illusion of human perception due to the asymmetry of the time dimension. As there is no clock outside the universe, there is no "external" time axis, and the external view of the entire universe structure can therefore never change with respect to that non-existent external time axis. This lack of temporal change in the entire universe structure has the following implications:

  1. The "Big Bang" does not represent the "start" of the universe. Remember, all times are equally real in the block universe - there is nothing special about time at the "Big Bang". As all times are equally real, the final state of the universe is just as real as the initial state. So the so-called "initial" Big Bang tells us nothing more about the existence of the universe than the "final" state does. While it is true that to an observer within the universe the Big Bang might appear like the start of the universe this is revealed to be an illusion of human perception caused by the psychological arrow of time (for more details on this, see the Arrow of Time page).

    The structure of the universe at the Big Bang does seem unusual because of its peculiar spatial geometry. But that does not make it the "start" of the universe. All we can say about the entire universe structure at the Big Bang is a comment about that unusual spatial geometry: "Along one of its dimensional axes (the backward time dimension), we find the spatial dimension decreasing in size until it reaches a point" (this is essentially describing the "cone" structure in the diagram above).

  2. The universe did not "emerge from nothing". It is meaningless to talk of the "start" of the universe, or the "emergence of the universe from nothing", or any other term which implies change of the entire block universe structure over time. The entire spacetime block is laid out as one unchanging structure. Here's a quote from Stephen Hawking's book "A Brief History of Time": "If the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be."

    This means that any theory which attempts to explain the existence of the universe solely in terms of events which happened at the Big Bang would appear to be plain wrong. This includes any theory which suggests the reason for the existence of the universe is because the universe "emerged from nothing" (so-called ex nihilo solutions). This includes the theories of Tryon and Vilenkin (considered at the top of this page) which suggest that the reason the universe exists is because it quantum tunnelled into existence from nothing.

    Ex nihilo explanations for the existence of the universe are a red herring.

  3. The universe is not expanding. Again, there is no temporal change in the entire universe structure, so it is meaningless to talk of a universe which is expanding with time. After all, expansion means an increase in size with respect to some time reference. With no external time reference axes, there is no absolute directional reference axis for time for you to say "the universe is expanding" rather than "the universe is contracting" - one is obviously just the reverse of the other, and with no external time reference axis how could you possibly prefer one statement over the other? (Also see Julian Barbour's article The Non-Expanding Universe).

    It seems to my mind (and to John Cruickshank who made this suggestion in a comment posted on the Arrow of Time page) that we are relying far to heavily on the psychological arrow of time to determine our time directionality, and hence decide whether the universe is expanding or contracting. We "perceive" the universe to be expanding because our brains determine our feeling of directional time flow in the forward time direction. But that psychological arrow of time is always going to align itself from a low entropy universe state to a high entropy universe state. That is no basis to say "the universe is expanding" - that just says something about the distribution of entropy in the entire universe structure. It is more accurate to say the universe is neither expanding or contracting. It just has a structure. It just is.

  4. The Grandfather Paradox is solved. If you've seen the movie Back to the Future (or virtually any episode of Star Trek) then you are aware of the so-called grandfather paradox. The paradox poses the question: "What happens if you were to travel back in time to kill your own grandfather?" If you do kill your own grandfather, then you are never born. But if you are never born, then you cannot go back in time to kill your own grandfather. So it's a real puzzle: your grandfather appears to be in an oscillatory state of being dead, then alive, then dead, then alive again, etc.

    But the block universe model provides a solution to the grandfather paradox. And, as the block universe model has been derived by a solid, logical approach, we can say that this is a definitive solution. According to the block universe model, all of space and time is laid-out in an unchanging spacetime block. There can be no place for an oscillatory grandfather: the grandfather must be defined as being in an unchanging state of either dead or alive. It can never be possible to change that state. The only possible time loops would be consistent time loops.

The Wheeler-DeWitt Equation

A great puzzle in physics has been how to reconcile Einstein's theory of general relativity with quantum mechanics. General relativity remains our main theory for describing gravity, and is extremely accurate for with large objects (stars and planets, etc.). Quantum mechanics, on the other hand, is our main theory for dealing with microscopic objects, and the other three fundamental forces which act at the atomic scale. General relativity describes space as being a smooth surface, but quantum mechanics reveals a discontinuous microscopic world with constant fluctuations and activity. So, each of these theories is accurate in its own right but they describe the nature of space and matter so differently that it has proven highly problematic to combine the theories into a single unified theory.

As part of the effort to reconcile quantum mechanics and gravity, many physicists are seeking to find a quantum theory of gravity. As part of this quest, we are going to introduce the Hamiltonian. The Hamiltonian is an incredibly useful property of a system, which represents the total energy of a system - the sum of the kinetic and potential energy. To be precise, the Hamiltonian is the sum of the kinetic and potential energy of a closed system expressed in terms of momentum, position, and time (see here).

But if we want to consider the Hamiltonian (total energy) of the universe we come up against a problem. As we have just seen, there are no axes of reference outside the universe. Hence, it is impossible to define a position for our "universe object" (so we cannot say it has a potential energy) and it is impossible to define a speed (so we cannot say it has kinetic energy). As Misner, Thorne, and Wheeler say on page 457 of their book "Gravitation": "There is no such thing as the energy (or angular momentum, or charge) of a closed universe, according to general relativity, and this for a simple reason. To weigh something one needs a platform on which to stand to do the weighing" (see here). So in this case, the Hamiltonian (the total energy) of the universe is zero:

This is called the Hamiltonian constraint (it is actually true that the Hamiltonian is zero for any system which has general covariance - for a derivation of this, see here).

You may find it hard to accept this idea that the total energy of the universe is zero. This is only possible if we consider gravity to provide "negative energy". This is described well in this article by Filippenko and Pasachoff: "You can easily see that gravity is associated with negative energy: If you drop a ball from rest (defined to be a state of zero energy), it gains energy of motion (kinetic energy) as it falls. But this gain is exactly balanced by a larger negative gravitational energy as it comes closer to Earth's center, so the sum of the two energies remains zero."

Stephen Hawking also explains this principle clearly in this extract from his book "A Brief History of Time": "The total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity. Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together. Thus in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero." Also see this New Scientist article by Lawrence Krauss.

In order to convert the Hamiltonian to its quantum mechanical form, we have to consider the Hamiltonian as representing the total energy. We can then use the quantum mechanical formula for energy which we derived previously on the Quantum Casino page:

As this is an energy operator, we need something for it to operate on. So we have had to again introduce this strange concept of a wavefunction, , extending through space. So another new concept must now be introduced: the wavefunction of the universe. The principle of the "wavefunction of the universe" imagines the entire universe as a single object, a quantum object. Michio Kaku explains it well: "When the universe was born, it was smaller than an electron, which is a quantum object that can exist simultaneously in many states. So the universe must also be a quantum object and exist in many states." (see here). So we can apply our Hamiltonian operator to our "wavefunction of the universe":

This is the Wheeler-DeWitt equation - a sort of Schrödinger equation for the gravitational field. It is the most famous equation in quantum gravity.

(A variation on this canonical quantization of gravity eventually leads to the recent, cutting-edge theory of loop quantum gravity - see this Physics World article by Carlo Rovelli, or Lee Smolin's Scientific American article Atoms of Space and Time).

Time and the Wheeler-DeWitt Equation

There's something remarkable about the Wheeler-DeWitt equation, and it can be seen if we expand the Hamiltonian operator:

Or, expressed in words, the rate of change of the state of the universe with respect to time is zero. The universe isn't changing with time! But we look around us and we see things changing all the time: people are walking, birds are flying. So is the equation wrong? Well, no. What the equation is once again telling us is that there is no external time reference by which we can measure the progress of time within the universe: there is no clock outside the universe! As Andrei Linde explains: "The notion of evolution is not applicable to the universe as a whole since there is no external observer with respect to the universe, and there is no external clock that does not belong to the universe" (see page 25 of Andre Linde's paper Inflation, Quantum Cosmology and the Anthropic Principle).

Therefore, the Wheeler-DeWitt equation agrees with our earlier analysis of the nature of time because it suggests a block universe model in which all of time is laid-out (just as the space dimension is laid-out), and all times are equally real: there is no special "now", no distinction between past and future. In fact, "past" and "present" do not exist - the movement of time is considered to be just an illusion of human perception.

Free Will in a Block Universe

Some people have suggested that the block universe model is incompatible with any notion of free will. This, they would say, is because the future appears to be set-in-stone in the block universe model, so we are never at liberty to change it by our choices. I would disagree with this reasoning as I believe it shows a fundamental misunderstanding of what the block universe implies. We will see that the block universe is completely compatible with the notion of free will.

The misunderstanding arises because the notion of free will is so poorly defined. We all think we know what "free will" is, we have a feeling, but it is very hard to write down what the phrase actually means and implies. In the absence of a satisfactory definition, I am going to define "free will" in what I believe is the best and most accurate description:

Free will is defined as the ability to make decisions.

(This appears to be the same definition of free will used by the Compatibilism school of philosophy which states that you have free will if you feel free to make a conscious decision without, for example, someone forcing you to make a particular decision by pointing a gun at your head. Hence, the Compatibilists believe you can have free will even in a deterministic universe. But I am taking the Compatibilist view further by saying that not only can you have free will in a deterministic universe, but you can also have free will in a deterministic block universe).

So what do I mean by "making a decision"? It means the ability to consider a range of many possible courses of action, and to select only one course of action from that range of possibilities. To all intents and purposes, I think that is a reasonable definition of free will.

This definition of free will is completely compatible with the block universe model. The key thing is that only one course of action results when we make a decision. There is only one outcome. There is only ever one stream of events. For example, the sequence of events when we come to a fork in the road might be:

EVENT 1) You walk along the road and come to a fork in the road.

EVENT 2) You decide to turn to the left.

EVENT 3) You continue your journey along the left road.

This is just a sequence of three events, and that's all the block universe is: a sequence of successive events. So these three events can easily be incorporated into the block universe model.

In the block universe model, events are unchanging and "frozen-in-time". But that does not mean that those events do not represent the expression of free will. For example, when we look back into the past we consider those past events to be "frozen", and nothing could change those events. However, we might also remember some of those past events as representing moments when we made decisions, i.e., expressed our free will. So the notion of free will is in no way incompatible with the block universe "frozen-in-time" representation of unalterable events.

In his book The Fabric of Reality David Deutsch suggests that some sort of "branching" multiverse universe is required to account for free will and the human decision process (see the New Scientist article Taming the Multiverse). In Deutsch's model, when the human comes to a fork in the road, the universe (and the person) splits into two different universes so the person is capable of travelling down both roads. Deutsch appears to think that this is the only way that the human can have free will. But Deutsch is ignoring the fact that a decision can only ever have one outcome, so only one road is travelled after the decision is made (i.e., after the human expresses his free will). This is therefore completely compatible with a single block universe: no branching "multiverses" are necessary.

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Comments are now closed on this page.

I've been trying to wrap my mind around all these concepts, QM, relativity, the theory of everything, etc, for a while now, reading everything that I could find (which is just a small fraction of what's out there), and everytime I think I come close to some understanding, I stumble upon something else that throws me right back to square one. Tonight it is the block universe concept that did it again.
So I have a few questions, and I ask you to go easy on me :) because I got to the point when I walk around the apartment talking to myself :):):)
First of all I think that all the physics of the 20th century, beginning with Einstein's special and general relativity, then quantum mechanics and all the thories that try to unite them are absolutely counterintuitive.(Although they amaze and fascinate me). But that happened throughout the history after all, there were many "crazy" concepts that we ended up accepting as real. So hopefully I will end up understanding all this :)I need some help though...
1. It is stated here that: " It is true that the universe appears to change for an observer within the universe, as there is a time dimension defined within the universe. So it is valid to consider objects within the universe changing with time. But there is no clock outside the universe, so the "external" view of the entire universe structure can never change with "time".
For me, this statement contains a contradiction: since we are all whithin the Universe, how can we tell what is or what is not outside the universe? Since we are whithin the universe and it is valid to consider objects changing with time, isn't this block universe actually a view from outside the universe?
2. How can we leave all human bagagge and sensations at the door, while we are still human? And since the time arrow is a result of a trick that our mind plays on us, how do we know that for instance the block universe is not actually another trick of our mind? What makes one concept more real than the other?
3. From the perspective of an unchanging block universe, are our actions of any importance and consequence? (OK, this is more a philosophical question, but then again, isn't all this very much like philosophy?)
4. Going back to QM, I was trying to understand the effects of it (lets assume now there are causes and effects in the univers) on us, human beings, since we are all built out of atoms and therefore particles that are in superpositions. Can QM have a role in our very way of looking at the universe and everything within it, including time? Maybe our answers will come when we eventually understand our brains, and the illusions or realities these brains object us to?
Not long ago I came across Edgar Cayce and Akashic records, which actually sustain the idea of a block universe, where there is no past, no future. But since this man gave all the "predictions" under hypnosis, could it be possible that everything is encapsulated in our brains?
Uff, it's a wild world:) - Monica Garliceanu, Winnipeg, Canada, 24th March 2009
Hi Monica, that's a lot of very interesting questions. I'm glad it has got you thinking. Firstly, did you read Paul Davies's excellent article on the Block Universe: I really can't explain it better than that, but I will try to answer your questions.

1) Yes, that extract was not very clear and I have modified the text - thanks. The passing of time is an illusion for objects (such as humans) within the universe. The block universe is the true picture in which all times are equally real.

2) I think from our very earliest years we accumulate human "baggage". For example, we are told in school that "times passes". I think maybe we can get rid of our human baggage by pretending we are children again, with no preconceptions of how the world works.

3) I have just added a section to the end of the main article called "Free Will in a Block Universe" which hopefully explains how we can still make decisions (i.e., express free will) in a block universe. So our actions and decisions are definitely of importance and consequence.

4) Yes, quantum mechanical effects definitely play a role in making it appear as though time is passing: this is the "quantum mechanical arrow of time". Again, see my "Arrow of Time" page for a discussion of this.

Thanks a lot, Monica. Keep thinking! - Andrew Thomas, 24th March 2009
These models do not explain why the Second Law of Thermodynamics is not consistent - both life and gravity cause entropy to decrease. Further - they assume that the BB and the expanding universe are correct. This is a house of cards as they are interdependent - if one fails then so does the other. For a simpler explanation for the mechanism of time go to: - Unknown, 31st March 2009
Hi, thanks for your comment. Sorry my anti-spam software filtered-out your name. If you want to post your name again, I can correct that.

But life does not cause entropy to decrease - I'm afraid we all go wrinkly as we get older. As far as gravity is concerned, gravitational entropy seems to work in the opposite way to normal entropy. I will quote Roger Penrose from his book "The Road to Reality": "Gravitation is somewhat confusing, in relation to entropy, because of its universally attractive nature. We are used to thinking about entropy in terms of an ordinary gas, where having the gas concentrated in small regions represents low entropy, and where in the the high entropy state of thermal equilibrium the gas is spread uniformly. But with gravity, things tend to be the other way about. A uniformly spread system of gravitating bodies would represent relatively *low* entropy, whereas *high* entropy is achieved when the gravitating bodies clump together". See - Andrew Thomas, 31st March 2009
But the question reamains that there is a 50-50% chance of everything existing than nothing existing, so some factor must exist that swayed the balance to existence. This is where i believe in God. Maybe not as religeon sees it, but omnipotence must be that factor. Plese submit your opinions on this. Very thought provoking site, though. Thanks Andrew Thomas!
- Shaun Goold, 2nd April 2009
One more thing. If time is equaly real, then using the other three dimensions in some way, surely a beam of light has no particular speed. (if time does not progress or regress, then S=D/t does not apply) - Shaun Goold, 2nd April 2009
Hi Shaun, thanks for your comments.

Regarding existence, and your 50/50 idea, we simply don't understand existence and "Why is there something rather than nothing?" Still, that doesn't stop us thinking about it! I don't really see we can assign a 50/50% probability to existence as you suggest: it might be that there is a 100% chance of existence (so we HAD to exist), or a 0.00001% chance of existence and we just got lucky! We just can't tell.

As far as the speed of light is concerned, the movement of light is no different from the movement of any other object as regards this discussion about the block universe. I think you are thinking about the fact that time does not pass for a photon due to relativity: - Andrew Thomas, 3rd April 2009
Hi, Andrew:
Let me involve myself in this your wonderful web site. I'll try to be concise. It's curious that in our world there only one universal movement exists: oscillatory movement. Earth, Moon, Sun and galaxies all them have a movement that can be treated as oscillatory, as we know since College. Rectilinear movement is an exception even in Earth. In the microworld it's a common transformation as well. This is just what the clocks count. And it's curious in our universe exists another general phenomenon also: the increase of entropy, that tell us about the Arrow of Time. In my mind for evolution both are conditions needed. Let's see the simple oscillatory movement (harmonic)
–simple, but the must essential-. From school we know that along a semicycle it passes from a free forces point to an intermediate tendency of centralization point, then to a maximum centralization of forces point in the extreme and then to another decentralization of forces point. The first point is completely the opposite of the third and so do the second point and the third. It's the natural way of transformation. But besides there is the increment of entropy, that finally stops the pendulum. If there is not an increment or decrement of entropy it will be a transformation but not evolution, everything will returns to its origin. This increment is not only a bad thing, because of it exists evaporation, mixtures, amalgams; you can't recombine an egg if it hits the floor but thanks to this you can crack the egg to eat it. I can suppose a world where along its oscillations entropy decrease and there it will be evolution as well, but if entropy is constant along the oscillations then there is no evolution. It's the case of the electromagnetic waves in vacuum and for that its velocity is absolute, doesn't evolve. A particular kind of *oscillation plus entropy* (what maybe is bad called negentropy) occurs in biological systems, where a whole unique is constructed from the cells aggregations –dividing, uniting like oscillations-. Also in thinking, who goes from analysis to synthesis to analysis, etc. And, finally, in the successions of civilizations throughout the History as we have described in The tiny Leonardo´s smile, mentioned in a commentary I'd done some steps up in this same page. It also explains the deep condition of beauty in the art masterpieces –could be done for literary works too- throughout the History. It's curious. Alberto Pérez-Delgado. Havana. Cuba.
- Alberto Pérez-Delgado, 23rd April 2009
Hi,Andrew. A few explanations. In my haste I wrote that the second point is the opposite of the third, it´s wrong, is the opposite of the fourth. Also,I misprinted: bad named, I should wrote: it´s wrong -or incorrectly- named negentropy. It´s necessary to explain that in the context of my note when I say that the increment of entropy finally stops the pendulum it´s not necessary refers to friction because heat is not like mechanical energy, an energy we can transform totally in other forms of energy, when the pendulum becomes warm we can´t use it in restore the movement, precisely caused by the Second Law of Thermodynamics and heat is the only form of energy that does this kind of thing. It´s curious. - Alberto Pérez-Delgado, 28th April 2009
I studied Experimental Psychology at Cambridge in the early 'fifties with Richard Gregory as my tutor; the subject had only just been moved from the 'woolly' arena of association with politics and economics to become a part of the Natural Science Tripos. We were thus some of the first students to treat psychology with the rigour of a scientific subject rather than as the somewhat disreputable basis for 'psychoanalysis'.

This fact, I think, affected my thinking about science as a whole over the rest of my life (I was actually studying medicine at the time).

In particular, I now see, it has made me impatient with physics, which pretends -- I use the world in its old-fashioned sense -- to describe the universe to us. I could never understand how this could be so, since physics almost studiously avoided (and still avoids) the mind as an entity for study.

I do understand, of course, why this should be the case; the 'scientific method' -- which I find admirable in itself -- is largely unsuited to framing hypotheses (which need to be falsifiable by way of observation) about the nature and attributes of the mind. If I may put it light-heartedly: the consequence of this has been that physics despises psychology as being the province of artists who would like to fancy themselves as scientists, while the humanities despise physicists as mere mechanics who should stick to designing better mouse-traps and leave the greater questions of 'How?' and 'Why' to philosophers who (being gentlemen and ladies) are better educated for the task.

I always saw that this was nonsense, but could never persuade either side to take much of an interest in the other's area of expertise -- an area which in the case of the mind is, as a matter of sheer fact, poorly examined, perhaps because it requires little in the way of expensive particle acccelerators and so forth. In any case, I have always been more saddened by the refusal of physics to study the mind than by the inability of psychologists (including myself) to get a fair grip on the mathematics of physics.

This has been the state of affairs for several centuries. It cannot, though, continue thus for much longer. Descartes was right; the only way we know we, and the universe, exist is by way of looking at what we experience with our minds, there being nothing else we can directly examine. All of physics, therefore, is carefully and expertly constructed (but wholly by inference) from what we can observe, namely, our mental experiences; and, now that we think of computers as able to reason and in some sense to 'observe', the interface between the immaterial mind -- of which physics is justifiably leery -- and the material world has become vital to our study if we are to progress in making sense of the Way Things Are as a whole.

We cannot treat matter and mind as conveniently separate entities any more; something physical is happening where they meet, and we need to be able to describe what it is. - Martin Woodhouse, 23rd May 2009
Thanks a lot for that, Martin. A super contribution. Personally, I just stay clear from ALL questions about consciousness and the mind. For a start, we seem to have such a poor understanding of it. But secondly, my gut feeling is that it consciousness as such does not play a fundamental role in the universe (sorry!). I just feel it's an emergent property when billions of neurons are firing at the same time.

I've always had an interest in John Searle's Chinese Room: . I think it's the best thought experiment for revealing insights into intelligence and consciousness. I think it shows that if any AI system is to be considered "conscious" then it has to be sufficiently complex to contain internal models of itself ("self-awareness"). Simple pattern matching of inputs and outputs is not enough - it has to contain that internal model of itself. I think when it achieves that self-awareness, it is conscious. That's just my gut feeling on the subject. Thanks again. - Andrew Thomas, 23rd May 2009
Hi Andrew, greetings from the US; I am loving your articles and the comments as well.

Ever since I was about 19 y.o. (that's 34 years ago, I think) I've had a couple of problems with 'block universe'-type views of reality.

First, when we apply the logic of 'block universe' as a logical formalism at every level, then, because all logic is time-directional/evolving, we MUST wind up producing such statements as 'what I say has no meaning,' 'rationality is, rationally speaking, a delusion,' 'the idea of rational discourse is a red herring', 'the idea of red herrings is a red herring' and so on--versions of the liar's paradox. If time is an illusion, so is logic. In a block universe where evolution does not exist there can be no valid development of an argument, no experimentation, no meaningful statements about errors, etc. In a REAL block universe all my endeavors are absurd, all my goals red herrings, and so on. But when we are so obviously attached to all of our time-directional, purposive, meaning-manipulating endeavors, including science and experiments and Internet comments and so on, it seems a little hasty to throw a logic bomb like that into the mix--and self-defeating in a potentially serious way. Since we obviously CAN'T and WON'T adopt such a philosophy for our everyday lives, why make ourselves so absurd, even in the abstract? In other words, no one REALLY believes that, in the sense of acting on it; they just SAY it.

A second problem I have is the blitheness with which some people seem to make such statements as 'there is nothing outside the universe', 'there is no clock outside the universe', 'there is no external observer', and so on. While for certain purposes (oops...that's certain illusory purposes) it makes sense to think of the universe we observe as a closed 'eternal' system, where is there ANY evidence that it is actually like that and not, for example, one of a pair of 'entangled' 'quantum' systems [to use an analogy], or an S3T bubble inside a universe of different dimensions, or who knows what?

So again thanks Andrew, and if anyone can show me the errors in my logic (see shorter statements below) I would be grateful...oops, I would think I was, I would have the illusion of thinking I was grateful...ah, shoot!

Love ya.

Contention i. Assumption of the absence of time-directionality in the 'real' universe reduces ALL linguistic, rational and scientific statements, (including 'time-directionality is an illusion') to absurdities, or 'red herrings'.

Contention ii. There is no evidence AT ALL for such statements as 'there is nothing/no clock/no observer outside the universe', and it is weak thinking to base 'scientific' descriptions of reality on such unfounded, scientifically untestable 'maxims'.
- deWitt, 16th October 2009
Hi deWitt, thanks for a great comment (I'm lucky that people do post great comments on this site). I'm pleased to tell you that the block universe model is completely compatible with the way we live our lives and changes nothing about our "logic" or our "goals" (of course, if the block universe model was not compatible with our experiences then it would have been rejected as an inaccurate model of the world).

Just as an example, let's consider your statement "all my endeavors are absurd, all my goals red herrings". I can reassure you that the block universe model is completely compatible with a universe in which people make decisions and those decisions have outcomes. This is because each decision only has ONE outcome. I have just added a new section to the end of the main article called "Free Will in a Block Universe" to explain how free will is compatible with the block universe model precisely because any decision only has one outcome.

And I'm pleased to tell you that "logic" can also be incorporated into the block universe model in a similar way. You say "time is an illusion". No, time exists, it is only the FLOW of time which is an illusion. The block universe changes nothing about how we experience logic and time. The block universe model is not absurd in any way, in fact it is absurd to suggest that time flows at a certain rate (as then we would have to ask "How fast does time flow? One second per second?" - a clearly absurd statement).

I totally believe the block universe model is true, as the alternatives are absurd. However, that does not affect my daily life, or the way I approach life, in any way. It is just the logical way of looking at time. There is no logical alternative.

(As far as your argument about the "clock outside the universe", though, you might have a point as I suggest you look at my section "But ... surely there's nothing outside the universe?" on the "Living in the Matrix" page of this website). - Andrew Thomas, 16th October 2009
I have a simple question that should have an obvious answer, but I can't quite grasp the concept. How can information processing occur in a static block universe? It seems if time is truly frozen the particles must be as well. Is it the intrinsic relationships between the structures and the information that cause the appearence of change? Are the particles actually moving in time, as it described in some theories, while the structure remains unchanged? Any help would be appreciated. - J, 16th October 2009
Hi, J, that's a very good question. If there is no moving "now" then we do we see the appearance of change? I think you provide the answer yourself: "Is it the intrinsic relationships between the structures and the information that cause the appearence of change?" For example, the relationship between the hands of a clock and the position of a pendulum would be interpreted by us as "The pendulum is moving", but really there is no moving "now", there is only the relationship. You could consider it like frames of a movie on a reel of film. In each frame you see the relationships between the objects in the movie, but it's only when we play it back through a projector do we see things actually moving. I've found the best description of this on page 11 of - Andrew Thomas, 16th October 2009
Hi, Andrew.
First, I'd just like to say that I appreciate the incredible amount of effort you spent putting this site together. It's an excellent resource of modern physics made easy to understand for laypersons like me.

You stated "There is no actual 'flow' of time, no movement of a 'now' point." I'm very surprised that nobody has brought this up, but surely there IS a "now" point and surely that "now" point is constantly changing. Even if all points in time exist simultaneously along a sort of time dimension then why is there a constantly changing location along that very time dimension, an apparently arbitrary location, at which your memories of the past are delineated from your lack of memories of the future?

In other words, why am I experiencing THIS particular point along the time axis (typing out a post on this website) and not another (like playing with baby toys or being diagnosed with arthritis)?

As long as there is a "now" you would of course "feel" a flow of time even if time is not really flowing. There is not only "now", there is "yet" and "until"; more ways we describe the passage of "now". You might know what you will do on 23rd March 2011 (not a special day, just thought up at random) if you were able to remember the future and have no freewill to change what you will do that day, but you will also know that 23rd March 201 is not "now" and that you are not experiencing that point on the time axis "yet".

So there MUST be a "now" moment; even if the points in time in which I have finished typing this post already exist, I can at least say that I have not experienced it yet because "now" I am still typing this post.

The question is, why is 23rd March 2011 NOT "now"? The fact that it is not makes it apparent that there IS a now, even if "now" just a perception of movement through a dimension in which all moments "before" and "after" the moment called "now" already exist.

I'd like to add that the reality of a changing "now" moment is apparent in the fact that a "past" and a "future" can ONLY be defined in relation to "now".

In other words, "now" could seem to be defined as the point in time before which you are able to remember and afterwards you are not.

This seems no different than defining the points along a road ahead of you and behind you; all the locations along that road exist simultaneously...but you can only define what's "ahead" of you and "behind" you along the road in reference to "here", or your location on the road.

That you are NOT able to remember tomorrow may not necessarily mean that tomorrow hasn't happened yet, but it DOES mean that tomorrow is not "now". - Taurus, 16th October 2009
Hi Taurus,
You make some very good points, and you're right: it is surprising that nobody has brought this up.

I'm going to start by emphasising that there is no special "now" point in time, unlike your suggestion. All times are equally real and there is nothing to identify any particular point as being "special".

But surely, you might protest, we know that the time "now" is special because it is the point when all the past is in our memory and all the future is unknown to us. Surely there is a clear division, with time being divided into "past" and "future" by the "now". That is the point you are making.

Well, the answer is that *every* point on the timeline is exactly the same. No matter which point on the timeline you might select, you would always feel like that is the "now". No matter which point you might select, you would always be able to remember the past and unable to know the future.

So the point is, no matter which point you pick, you would always feel the same – you would always feel like that is the "now" point. So (I look at my clock) right now it is 3pm on Friday the 23rd October 2009. And it feels like "now" to me. So if I could see the whole timeline I would see myself on Friday the 23rd October 2009 feeling like it is now. But I could also see myself again at a later point on that timeline (all points in time are equally real) a year from now looking at my watch and feeling like it is "now" as well.

The timeline is just a long line of dates and times and I exist at each date and time feeling like it is the "now". So to answer your question "Why am I experiencing THIS particular point along the time axis (typing out a post on this website) and not another" is that no matter what point you might pick in time, you would always find yourself feeling like it is "now". There is nothing special about this particular point – you would feel that same way whichever point you picked.

Thanks a lot for an excellent question. - Andrew Thomas, 16th October 2009
Andrew, in your post where you state that to measure how fast time passes "we would need some sort of second measuring clock *outside* the universe, outside of time itself, and such a clock could never exist."

You're forgetting here that Einstein solved this puzzle a bit ago. Both the speed of light and alternate frames of reference are perfect yardsticks for measuring the flow of time.

Scientists have measured differing flows of time for astronauts (slower aging) with atomic clocks, and for our entire GPS system to work, compensations in the calculations must consider the changes in the flow of time as different sats whirl at different relative speeds to our locations.

Now is there a master flow of time that all of these "slower speeds" of time are relative to? Perhaps... and maybe (maybe!) THAT can only be known by having a measuring stick outside of our dimensions. However quantum theory and gravity are not well understood and these may in fact contain such a measuring stick.

From the perspective of earth, a person traveling by at the speed of light would be practically motionless because time would be passing so slowly. Of course from that speedy guy's perspective, the time on earth would be flying by incredibly fast. This is one reason among many why achieving the speed of light itself (within our dimensional universe) is probably impossible for a physical being. - MarkL, 17th October 2009
Hi Mark, good question. Yes, if we put an astronaut into space then we find that he ages at a slower rate that someone on earth. So you could possibly say that "time passes at a slower rate *relative* to someone on earth". So you make a good point that there can be *relative* differences in an apparent flow of time.

However, this does not answer the question "How *fast* does time pass?" As we all appear to feel a flow of time, precisely how fast is that time moving? (This is the "master flow of time" to which you refer). If we imagine the "now" point to be a pointer sliding on a scale of events, a "moving now", how fast is that pointer moving?

Quantum theory and gravity doesn't come into it. A simple philosophical argument (it is absurd to say that "time flows at one second per second") shows that *time itself* does not move, and it is illogical to suggest that it does. It's just a feeling.

You mention Einstein, so let's finish with a quote from Einstein about the illusion of the "moving now" (which is included on the "Cosmic Universe" page of this website): "People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion." - Andrew Thomas, 17th October 2009
Everything is always measured in terms of something else. So measuring time in reference to a baseline makes perfect sense.

For example we could postulate "Earth-Time-Flow" as our baseline. With that agreement, relativistic effects would enable us to measure both faster and slower time-flows.

The fact that time may be an illusion doesn't preclude the measurement of it. Most interpretations of Quantum mechanics indicate that mass itself is an illusion... yet my ruler, speedometer, and GPS work quite effectively, because at the illusion level there's an obvious baseline and shared agreement.

What's more, we know what both the minimum and maximum timeflows are (time stops as you approach the speed of light which has a known max, using calculus limits), giving us a gradient scale or "time speedometer."

I disagree that the flow of time isn't measurable. You're looking for an external yardstick, but one isn't necessary just as one isn't necessary to measure distance. When we say a mile is a mile, how do we know that a mile on the other side of the earth isn't "stretched" by some extra dimensional force? We don't! In fact many physicists believe that gravity is the effect of the stretching of space-time.

The point is, that you don't need an external reference to compare or measure things. You just need to agree on a baseline. An inch is about...*holding fingers up* that long.

We also don't need an extra-dimensional reference to measure how fast time-flows. A second is about "thaaaat" long. Now we can look at others moving at relativistic speeds and clearly measure that a second is longer or shorter for them.

Thus, even today, we can (and have) measured the flow of time. - MarkL, 20th October 2009
Hi Mark, yes, we can absolutely measure time using a clock, for example, and we can say things like "This time is longer than that time". Time is just another dimension, like length, so yes, we can measure it. But the question we are considering here is the question of the moving "now". Do we have a "now" point which is actually moving, or are all times equally real?

Measuring lengths of time is a very different thing from saying "How fast is the 'now' point moving? How fast is time itself moving?" We can measure lengths of time without needing to have a moving 'now' point at all.

You suggest we could send an astronaut close to the speed of light, and then we would find the clock of the astronaut has measured less time than our clock, yes, absolutely. But that is all possible without needing a moving "now". That is possible by just having fixed events ("We read our clock", "the astronaut reads his clock") in a fixed spacetime structure. For a great explanation of this, see the section "How Time Doesn't Fly" in Paul Davies's great article:

So, yes, by using a clock we can measure a time duration, a *length* of time. But what we are not necessarily measuring is the *speed* that something (the "now" point) is moving.

You could attempt to convert that time duration into some sort of measure of speed of a moving "now" point, and calling it a measure of the speed of the "flow of time" (as you suggest), but there is really no justification in physics for doing that (other than the emotional feeling we have of the "now" moving).

(You make a good point that if there could ever be an *absolute* baseline "time flow" then we could consider different rates of time passing relative to that baseline. I actually cover this in the section "Spacetime in a Simulation" on the Living in the Matrix page: In that example, a "clock outside the universe" was provided by the external simulating universe. However, we don't have access to such an absolute baseline).

It's interesting you mention length, and the problem of finding an absolute yardstick. You might enjoy Julian Barbour's article "The Non-Expanding Universe": which considers eliminating absolute size and the universal clock.

Thanks a lot, Mark. - Andrew Thomas, 20th October 2009
Well, I guess we'll disagree on this one. To me if I can see that an atomic clock on an astronaut's space ship is going slower than mine, and that his now is progressing at a measurable, slower rate, I can indeed measure the flow of time. I can state that his flow, or his time rate, or the speed with which his now is moving forward is different than mine... and can measure the difference in our flows quite precisely. - MarkL, 24th October 2009
Hi Mark, let's consider this "flow" of time thing:

Your proposal is that someone on earth can measure the "flow" of time and use that as a standard so that everyone else in the universe can measure a "flow" of time relative to the earth standard. But in order to achieve this - in order to set a standard "flow" of time - it has to be possible for *someone somewhere* to measure the "flow" of time, i.e., it has to be possible for someone somewhere to measure the speed at which *time itself* moves.

So ask yourself this, Mark: how would *you* measure the speed at which *time* moves? Yes, it is possible to get a stopwatch and measure the speed at which light moves between two points. But that's not measuring the speed at which *time* moves, that's measuring the speed at which *light* moves.

Using a stopwatch it is possible to measure the speed at which an object moves with respect to time, but it is not possible to use a stopwatch to measure the speed at which *time itself* moves with respect to ... what?

So it's not possible for the person on earth to set the standard speed for the "flow" of time because it is impossible for *anyone* to measure the flow of time using a stopwatch.

I can strongly recommend getting hold of a copy of Craig Callender's book "Introducing Time" which explains this all much better than I can. Here is a four-page extract:

(In that extract, Callender refers to the block universe model as the "tenseless" theory, with your proposed model being the "tensed" theory). The book is well worth a few quid of anyone's money.

I suspect we're going to have to agree to disagree on this one, Mark. Thanks for all your comments. - Andrew Thomas, 25th October 2009
Andrew, thank you for your thoughtful reply....

...but I still can't wrap my head around this concept.

You're right to point out that every single point on the timeline would be percieved as a "now" moment....but I don't see how this in and of itself is particularly convincing as an argument against a "now" moment because *the same would STILL be true if there WAS a moving "now".

If there was a moving now moment, you would recall each point along the timeline as "now" the same exact way you recall your location along a road you are moving along as "here". It would seem fallacious to conclude that no "here" location exists merely because you always perceived yourself as "here" along every point on the road.

From an external reference point, you could look back on every moment in the past (or future) as "now" the same way you could look at every location of your vehicle along a road as "here", but that doesn't make "now" or "here" not special either; "here" is the point along the road before which you have already traveled and after which you have yet to travel. If you are actually moving along the road from point A to B, then "here" necessarily changes.

"Here" and "now" can also be quantified as reference points; If you were moving from point A to B, "here" could be ~70km from point A. "Now" could be ~13.7 billion years from point A, could it not?

....and I ask again, if the "flow" of time is an illusion caused by our inability to remember the future, then how can you define the future without a "now" moment? How can you delineate between the past and future without the "present"? As I said, even your picture frame example shows "The Present" clearly marked.

It's not difficult for me to grasp that the flow of time would be an illusion if time were a dimension that we merely experience in a particular way. But it *IS* difficult for me to grasp that my perception of that dimension is not akin to traveling along the rest of the three dimensional world; a span of time may be laid out the same way a span of road is, but I delineate between the "past" and "future" using "now" the same way I delineate between "behind" and "ahead" using "here". "Here" is just as real and useful as "now" in this sense, is it not? - Taurus, 28th October 2009
Hi Taurus, thanks, this isn't an easy thing to "get one's head round" at all. And I don't think the majority of general amateur physics readers are even aware of this fundamental principle, which is surprising considering how crucial the concept of time is.

Firstly, you're absolutely right when you say "the same would STILL be true if there WAS a moving now" - the end result would be exactly the same as the block universe model. In the example you provide about moving along a road, yes, you're right, the end result is exactly the same as if you are existing at every point in the road.

So, you might ask, why should the block universe model be favoured? Well, there's actually a few reasons.

Firstly the laws of physics we have discovered (such as Newton's laws) do not include any mention of any "now" moment - no reason why, say, the current year 2009 should be preferred over 1975, for example. Even though the current year feels very special to you, as far as the equations are concerned it is not special at all and is treated just the same as 1975. The "now" just doesn't figure in the equations anywhere. In fact, the only reason why we seem to feel the need for a "now" is because of the way we "feel" - we "feel" like the time now is special, and that that now is moving. So apart from an emotional response, there is no need for a "now" in known physics.

Secondly, there are some strong philosophical arguments against the moving "now", such as MacTaggarts's "Unreality of Time" argument (which I find quite hard to understand):

and D.C. Williams's argument "How fast does time flow?" which is much easier to understand:

And then, most seriously, Einstein's theory of relativity appears incompatible with the moving now according to the Rietdijk-Putnam argument (see the idea of Hilary Putnam in that previous link). It is simply not possible to cling to the idea of the moving "now" once we introduce relativity.

As to your objection that we cannot identify "past" and "future" without having a "now" point I would just like to quote Einstein again: "People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion."

(Regarding my diagrams which appear to show a "now" moment, yes, you're right, that's not great - that's inaccurate. But it would be extremely difficult to illustrate the idea of the lightcone without using the conventional idea of time with a "now" point - I'd basically have to draw a long series of light cones with lots of different "nows", and the diagram would look a mess. So as you say a the end - it is useful to talk about a "now" sometimes! But in strict physics terms it is very dubious.)

Thanks a lot. - Andrew Thomas, 29th October 2009
One more thing Andrew!

Doesn't the Heisenberg Uncertainty Principle imply that the "Block Universe" must not be an accurate description of time if the future is indeterminate?

You also didn't mention anything about time being quantized in Quantum Gravity, Chronons, or the general implications of the idea that time may not be continuous and how that would relate to the "Block Time" concept. - Taurus, 29th October 2009
Hi Taurus, yes, you're right that the block universe model ignores any quantum mechanical effects - it merely deals with the implications of relativity. For example, the argument of Hilary Putnam shows that the tensed theory is incompatible with special relativity - he doesn't refer to quantum mechanics at all.

However, the argument of D.C. Williams ("How fast does time flow?") does not refer to either relativity OR quantum mechanics. In fact, it doesn't depend on any physics at all! Instead, it seems to reveal a fundamental flaw in the human intuitive model of time. (Basically it says that "time itself" cannot move because it cannot move with respect to time. How fast is time moving? One second per second? That's a nonsense).

So there is a logical flaw in the tensed theory of time which does not involve relativity or quantum mechanics at all. The human model - which is so widespread - has a simple logical flaw. I don't see how it can be correct. No QM or relativity required. - Andrew Thomas, 29th October 2009
Thanks again for your answer, Andrew.

I have *NO PROBLEM* with a universe in which the "future" (and all of my actions therein) is essentially already written, indeed this was how I understood the implications of special relativity and how I explained it to friends for decades.

My apparent mistake was that I understood that time is laid out like the spatial dimensions of a road along which a "now moment" progresses. Like a moving roller coaster car bound to a track that's already built, no part of the track comes into existence as you move forward; the track, the distance it traverses, and the directions in which it is laid out are already there, but the roller coaster is MOVING along the track.

As I tried to explain, I thought that "now" was akin to "here" in the above analogy.

I thought that the rest of my life was laid out along the time dimension *but that I am MOVING along that dimension* just as I move along the spatial dimensions.

As Andrew has elucidated (and as my own further research has confirmed), this appears not to be to the case (at least as far our current understanding suggests). It appears that there is no "now" moment and that I am NOT moving through a time dimension that is akin to the other spatial dimensions. If I understanding Andrew correctly, I am not moving through time at all!

Since I had understood time to be like a spatial dimension for many years, I'd assumed I was moving through it the same way I was moving through the other dimensions. I'm very surprised, even startled, to find that this is apparently not the case... In fact, I'm having some difficulty understanding exactly how it can be the case and it is my difficulty that troubles me, not the concept itself.

....and I'm not *entirely* convinced that the block time concept is unassailable... Just dug up something while looking for more (layperson-oriented) information on the topic: Physicists Brian Cox and Fay Dowker seem fairly certain that Einstein's view of spacetime is fundamentally wrong... - Taurus, 30th October 2009
Hi Taurus, that's a great video you found there - thanks. I have just added a trimmed-down version of that video to the main article. I remember when I first saw that Brian Cox film last year I thought that it was the first time the block universe model had been explained on TV.

However, Brian Cox's explanation is rather misleading when he shows himself travelling down the road "into the future". If all times are equally real then he is *already* in the future - there is no need for a moving "now".

In fact, it's not good that Brian Cox says in the video: "According to Einstein, the past, present and future all exist", whereas in fact what Einstein said was: "The distinction between past, present, and future is only a stubbornly persistent illusion." Einstein certainly did not believe in the distinction between past, present, and future, so that was a mistake by Brian Cox.

Then later in the video, those quantum gravity theories must be considered speculative, whereas the block universe model derives directly from Einstein's theory of general relativity which has been thoroughly experimentally tested. So the block universe model should be considered to be the orthodox model, whereas Dowker's model should be considered to be speculative. Thanks again for your contribution, Taurus. - Andrew Thomas, 31st October 2009