Right now, this second, an old man is exhaling his last breath.  Elsewhere, two young lovers exchange their first kiss.  Farther afield, two asteroids silently collide. Sunrise comes to a planet orbiting a neighboring star. This very second, a supernova detonates in a faraway galaxy.

And yet ‘this very second’ across the universe apparently does not really exist! Our best fundamental theory of space-time, Einstein’s Relativity, expressly precludes a single, objective definition of simultaneity.  Events occurring ‘now’ by one observer’s estimation can — with equal validity — be said to occur at different times according to another observer who is far away and/or in motion relative to the first.  We don’t notice this issue much here on Earth, but it becomes very obvious for example in cosmology, where how one defines ‘now’ can determine whether the universe looks uniform or not, and even if it is finite or infinite!

But wait.  Of course ‘now’ is real: Why aren’t there any dinosaurs? Why has the Roman Empire fallen, but the planet around 51 Pegasi not yet been colonized?  Why has the iPhone been invented but not yet the iPortal?  Why have you read the last paragraph, but not yet the next? Whether or not these events have already happened may be ambiguous to some being in a faraway galaxy, but that being can’t observe these events.  For those of us who can, the demarcation is clear.

Then again, is it?  Einstein came to believe that passage of time is fundamentally a fiction: like frames of a cosmic movie, events are all laid out throughout spacetime, and it is only our limited nature that requires us to perceive them sequentially.  The relativity of simultaneity formed part of that conviction, but it went beyond that. General relativity shares with Newtonian mechanics, Maxwell’s electrodynamics and many other physical theories the feature of mathematical determinism, or ‘unitarity’: given a complete description of a system at one time (however defined), the system is uniquely specified at all earlier and later times. It’s as if one could take any frame of the cosmic movie and by choosing a particular chemical to add, reveal any other chosen frame of the movie. If all the frames are hiding in each one, how can we say that the past or future is any less real than the present?  As Einstein put it to a friend who had recently lost a loved one, “People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion”.  Why mourn someone who lives?

That seems crazy, though, even if Einstein said it.  We remember the past, but can’t affect it; we can try to predict and affect the future, but can’t recall it.  Physical laws distinguish past from future.  In particular, the Second Law of Thermodynamics, one of the most basic, says that entropy — a measure of how ‘generic’ the state of a system appears —  increases toward the future, not the past.  The universe thus has a clear ‘arrow of time’ built in that points in the direction of time increase.

And yet this arrow of time might be explainable using unitary laws of physics brooking no such distinction, given suitable boundary conditions for the universe. If these conditions specify that there is a time at which the world has much lower entropy than it might, then it is arguably natural for entropy to increase away from this time, which can then be called ‘the past’.  This thermodynamic arrow of time might then be used to explain others, such as why we remember the past but not the future.

We can question, though, whether ‘unitary’ really describes all the laws of physics. Imagine a physical system with a property P — ‘color’ for example — with two distinguishable alternative possible values A and A’. Quantum Mechanics, our core theory of how matter behaves, holds that the system can be put in a combined ‘superposed quantum state’ that has both values A and A’.  What if we measure P with some apparatus?  Theoretically, the apparatus itself can be ascribed a quantum state, and when it ‘measures’ the system, the system+apparatus combined state unitarily evolves into superposition of apparatuses, one having measured A and one having measured A’. A phenomenon known as quantum ‘decoherence’, however, ensures that this apparatus-level superposition behaves as two distinct classical systems, showing no indication of quantum behavior connecting them. It also indicates that the reverse process — two macroscopically distinct systems combining into one — essentially never happens, for reasons closely connected to the fact that entropy essentially never decreases.

While the mathematics points to A and A’, though, what we observe is A or A’ — that’s part and parcel of the apparatus behaving classically. How to reconcile the ‘and’ with the ‘or’ is a fascinating question over which much ink has been spilled. But in relation to the question of time, many proposed reconciliations have a similar feature: while there may be description in which the World evolves unitarily, at the level of observed reality it contained multitudinous forks and branches.

When we step back, we thus seem to have two rather different and contrary views of time’s nature.  In one, the ‘Unitary Block’, spacetime and quantum states are laid out ‘all at once’, specified once and for all by some set of boundary conditions. Everything at any time is uniquely determined by — and thus implicitly contained in — any other time, and the world exhibits no distinction between past and future.  At the same time, the ‘Experienced World’ we actually inhabit and observe has a very clear distinction between past, present, and future, produces entropy, and allows branching between a single present reality and several possible future realities.

Among knowledgeable and thoughtful people, there seem to be three basic views of this paradox:

  1. The Unitary Block is the fundamental, and by implication more true description; things such as the arrow of time, definite experimental outcomes, etc., are emergent phenomena that, if we only could make precise enough computations, could be reduced to ‘nothing but’ the fundamental description.
  2. The Unitary Block is wrong in some essential way; a more correct view would be much more like — and much more readily reconciled with — the Experienced World.
  3. The Experienced World is more fundamental than the Unitary Block, which is just the correct description of regularities in the Experienced World in very particular regimes.

View 1 is by far the most common amongst my theoretical physicist colleagues, but I’ll make three arguments as to why we should think carefully before embracing it.

The first is that future or past events in Experienced Reality cannot, even in principle, be exactly predicted using any amount of knowledge and precision of the evolution laws and state that comprise the Unitary Block.  The ‘branching’ behavior of quantum mechanics is one clear reason, but it goes beyond this.  One might argue that we can precisely compute probabilities for events in Experienced Reality. But we can’t. It is a theorem of quantum mechanics that even a hypothetical super-being in a lab cannot determine the lab’s exact quantum state, without dramatically affecting the lab. Even allowing for such alterations, I suspect that it would also be impossible for the super-being to determine its own quantum state precisely. Moreover, even leaving aside quantum mechanics, there are rigorous arguments in Relativity showing that an observer at some spacetime point cannot gather enough information from its past to exactly predict any event in its future. Finally, it has been shown using the theory of computations that if a physical system contains a decision-making apparatus (or being), there are decisions that cannot in principle be simulated or predicted better than letting the system actually work out the decision itself.

Second, a theory F is generally described as more fundamental than an ‘emergent’ theory E if E could not have been any different without F being different, whilst E might have in principle emerged from several different theories F, F’, F’’. That is, there is a many-to-one mapping between Fs and Es. But that seems an inaccurate way to describe the one-to-many relation between the Unitary Block and the Experienced World that exists in terms of, for example, time-direction or quantum experimental outcomes.   The laws of physics may even have this relation: string theory appears to comprise a single high-energy theory that has many, many very different low-energy effective theories identified with it. This one-to-many relation means that Experienced Reality contains information that is lacking from the Unitary Block. Consider the ‘now’ with which we began this essay. Consider using a Relativistic Block description of the universe to predict tomorrow’s weather.  Without something like ‘now’ or ‘here’ you simply can’t get anywhere.  Elements of reality like ‘here’ and ‘now’ and (perhaps) ‘in this branch of the quantum state’ may be called ‘indexical information’ and are central to doing science, but have no place in a Relativistic Block.

A third claim takes this further: in addition to indexical information, there are important and meaningful things that exist in Experienced Reality but are essentially meaningless in the Unitary Block, making the latter too impoverished to be considered a true depiction of reality by any reasonable definition. Consider circles. They exist as defined mathematically, as imagined by people, as approximated by round objects, and all in all sorts of other ways.  But there are no circles in a quantum state or a unitary evolution rule.

Imagine, for example, presenting our hypothetical super-being a large box containing a computer, plugged into which is a quantum random number generation card.  At time T, on the basis of a random number so generated, a complex algorithm in the computer draws a circle or square on the screen.  Contemplating the circle or square, a person decides whether to have Indian or Chinese food for lunch.  Can the super-being predict the cuisine chosen? No way!  It can’t get the quantum state; even if it had it, it can’t predict a particular number from the card. Nor can it think both in terms of circles or squares, let alone transistors, microchips, programs, etc. and also in terms of the quantum state; nor, even thinking in these terms, can it necessarily simulate the prediction.  There seems no meaningful sense, then, in which the decision ‘pre-exists’ in the state of the universe at time T. They simply don’t exist on the same plane of reality.

What does this tell us? To me, our understanding of time is a particularly vivid demonstration of how we can usefully describe the same World in multiple ways that are rather starkly different in character and in import for such questions as free will and agency, universe or multiverse, epistemology, and others.  The mode of theoretical physics has tended toward elevating certain descriptions as more ‘true’ and ‘fundamental’, and the other descriptions as ‘in principle derivable’ from these.  I think it is worth contemplating a different approach: that just as the Experienced Reality description is far more useful than the ‘Unitary Block’ in some applications (and far less in others), that we take Experienced Reality as an equally true description that is complementary to, but not derivative of, the Unitary Block. Perhaps the nature of time is telling us something about the nature of truth.

Discussion Questions:

  1. If the speed of light were much slower, so we could really experience the subjectivity of simultaneity implied by Relativity, how to you think it would change our experience of time, space, and the world?
  2. If you truly believed that the past, future, and present all exist in just the same way, as in the Unitary Block view, would it change your attitude toward life — or death, or decisions?
  3. If there are aspects of the world that are unpredictable in principle, is there still a sense in which we can say that they are determined?  Or do those two ideas go hand-in-hand?
  4. I’ve suggested that part of the controversy over how to think about quantum mechanics is a controversy over whether the classical, macroscopic world emerges from the quantum world, or whether the quantum world is a particular, stripped-down limit of an essentially classical world.  How do you see it? What theoretical or experimental findings might lead you to accept one view as more viable than the other?

Discussion Summary


I’m again grateful to BQO for the invitation to contribute an essay and participate in the discussion.  I enjoyed and learned from both.    The online discussion brought up a number of issues.  Some were essentially technical scientific questions, which I attempt to address — except that I declined to get into a debate about the validity of Einstein’s Special Relativity.  Others raised a number of interesting quasi-philosophical issues.

A key point I tried to make, which many discussants appeared sympathetic to, is that the ‘Unitary Block’ description is not just difficult-to-work with when trying to describe all of reality, but actually incomplete.  It must be augmented in order to function, and we (by which I really mean theoretical physicists) often neglect or minimize the important of what we must add.  When you read, for example, that a person is ‘nothing but’ an arrangement of atoms, this gives the unfortunate (and perhaps unintended) impression that the arrangement has less ‘reality’ to it than the atoms themselves.  But the ‘arrangement’ is everything!  And we know from quantum mechanics that the atoms are far less material and ‘real’ than our intuitions like to make them.

Physics is, unfortunately, rather ill-equipped to describe this information in a useful way.  The concept of ‘Shannon Information’ is wonderful in many contexts, but in the Unitary Block it is neither created nor destroyed.  This is useful (just as the conservation of energy is useful) but in mechanics or quantum mechanics the conservation of energy is related to the general time evolution of a system, so energy conservation tells you a lot about how the system evolves.  Information theory is not presently able to do something similar in terms of capturing the crucial difference between, say, the arrangement of atoms that is a person, versus another special but nonliving arrangement.

One nice question in the discussion brought this to light: is Hamlet ‘implicit’ in the initial state of the universe — does it ‘exist’, in some sense, in the initial state.  A second question hit parallel note: do things like ‘stars’ exist in a universe in which there is no observer, or consciousness, to perceive them.  My answer to both questions was ‘Yes’…and ‘No.’  There is a strong case for ‘yes’ in both.  In the first, if the state of the Universe now can be directly 1-to-1 mapped to that at some early time, and Hamlet exists now, how can Hamlet not ‘implicitly’ exist at the beginning, albeit in perhaps scrambled or encoded form.  In the second, it seems absurd to think something ‘objective’ like a star could depend on being perceived.

Yet in both cases I assert that we bring a tremendous amount with us when we say that Hamlet or a star exist, that we tend to neglect what we have brought, and that it’s unclear that these objects really exist without it.  I think it would be very interesting to devote more thought to these questions.  Phrased a bit more generally:

New Big Questions

1) What theoretical tools could be developed that could allow us to recognize and distinguish  an object (such as a Piano) relevant to Experienced Reality and developed by a meaningful historical process, from a different meaningless arrangement of the same atoms, which may even contain more Shannon information (lower entropy).

2) What is the ontological status of ‘arrangements’?  How do we pick something out of a wavefunction, say?  Is it always in relation to an observer, and if so, what type of observer?

30 Responses

  1. George Gantz says:

    Excellent, thanks – you are right at the edge of the issue.  The determinism of the Unitary Block falls apart in light of the issues of complexity/computability and quantum indeterminacy.  Moreover, we cannot deny the evidence of our own experience – the Experienced World is where we live every day.

    Your comment that a being in a lab cannot determine the lab’s quantum state hints at the remarkable paradoxes that arise whenever self-reflective or recursive properties are invoked.  The very presence of consciousness brings with it a cascade of perplexities.

    The only answer that seems to make any sense to me is that mathematical laws provide the form into which physical laws (behaviors) emerge, driven by a fundamental cosmic creative propensity – some form of consciousness.  We can understand the forms (mathematicians do this) and observe the behaviors (scientists do this), but the nature of this fundamental propensity has not been adequately studied (although it is has been discussed by a few philosophers and many theologians).

    • Anthony Aguirre says:


      Thanks for your comment.  The self-reference issues involved here are indeed fascinating.  I think they’re also somewhat unavoidable when discussing issues of in-principle predictability, as anything making predictions is unavoidably part of a system that also contains whatever is being predicted.

      Your proposal of a sort of ‘cosmic propensity’ toward creativity is good foof for thought.  Creativity, indeed, is something that simply does not exist at the Unitary Block mode of description.

  2. Michael Carroll says:

    Thank you Anthony for such an excellent discussion of time.  Only here at Templeton’s “Big Questions” would such a topic be handled in such accessible terms. 

    Taking another angle on the topic of “time”, from a purely phenomenological vantage point, one could say that time is how consciousness describes its experience as being never the same at any one point but in an orderly pattern. Essentially, in any given instance the experience we are having is not the same and the way it is not the same has a pattern that we call “time”, “memory”  “history” etc..   So a “clock” to consciousness could be seen as simply a language for describing a pattern inherent in an infinite timeless variant.     

    Thanks again for bringing your insight to this strange human expereince we call “time”. 

    • Anthony Aguirre says:


      Thanks for your comment!  Indeed from the Unitary Block view a clock will tend to appear as a pattern — for example the pendulum of a grandfather clock forms a sinusoidal pattern in spacetime.  Our conscious perception of time passage must indeed contain some aspect of comparison between current mental processes/sensations/states and previous ones, though memory.  I suspect there are also many levels of internal periodic processes constituting mental clocks, which are combined with memory in the experience of time flow.  Indeed, we can experience the flow of time without any external simulus (though I expect it could get significantly distorted).  This is somewhat in contrast to space, where special relativity tells us that we cannot experience movement through space (at a constant rate) without some reference to an external system.



      • George Gantz says:

        On this point,  I’ve always wondered why we ended up with 24 hour days, 60 minute hours and 60 second minutes, and never found a good answer.  But it may be simple.  Day and night are roughly equally divided, and there is a natural break at midnight and midday – so the four quarters of a day would be natural.  The most convenient biological marker is the human heartbeat.  How many heartbeats (on average) do you have in one quarter of a day – 21,600!   How do you efficiently count them?  Well, if you use 6 large time blocks – 4 medium blocks of 15 time units each (or 6 x 10, or 12 x 5 if you prefer) – and another 4 smaller blocks of fifteen heartbeats each (or 6 x 10, or 12 x 5 if you prefer).   Perhaps the Babylonian method of counting in base sixty and the Egyptian use of base 12 evolved from human efforts to address the practical mathematical problem of dividing human heartbeats into days?  It is also interesting that so many tech saavy people I know continue to prefer analog display watches to digital displays (even when the mechanism is digital).  There is something so comfortable about the ticking of the second hand and the geometric display of quarters and twelfths.

  3. Meyer1953 says:

    If the speed of light were much slower, so we could really experience the subjectivity of simultaneity implied by Relativity, how to you think it would change our experience of time, space, and the world?

    A snail was getting patched up at the veterinarian’s after getting run over by a turtle. The vet asked for more details, but the snail said “I don’t know, it all happened so fast.”

    If the speed of light were slower, then all of subatomic physics would be slower and so on all the way down the line. We would still be standing around with our hands in our pockets asking questions. The point is, things happen in time that is relative to other things that happen. Anyone who has spent a weekend in a cozy winter retreat knows how time essentially shuts itself down to a fraction.

    I guess that there is a reason for the speed of light in a vacuum being what it is, something about the wave equation. But I think that time’s relativity is best compared to a weekend’s retreat.

    • Anthony Aguirre says:


      You’ve put your finger on the fact that you have to be pretty careful about how to talk about ‘changing the speed of light’.  Just asserting, for example, that “light travels at 10 meters/second” doesn’t really say much: a thoughtful physicist would just respond: “well, that’s an odd definition of either meter, or second, that you’re using”.  To be unambiguous, physical measurements must be phrased in terms of dimensionless quantities.  We usually do this in secret, by defining our units in terms of some agreed-upon length.  Thus when we say “the tree is 10 meters away”, we really mean “the ratio of the tree’s distance to some bar of platinum we call one meter, is 10.0” (we don’t define the meter this way now, but we used to).  Now in terms of the laws of physics, we can do the same thing.  Planck’s constant, the speed of light, Newton’s constant, etc., all have units, but if we like we can choose a different set of dimensionless constants — for example the fine structure constant combines the electric charge, speed of light C, and planck’s constant into a dimensionless quantity ~ 1/137.  A full list of dimensionless constants defining physics and cosmology can be found, for example, in the paper http://arxiv.org/pdf/astro-ph/0511774v3.pdf.  To imagine what it would be like to have a different speed of light — but without changing anything else — we could dial all of the dimensionless constants that contain C in the same way.  We could then see exactly what effect changing C would have on the physical world.  There are some elegant papers back in the 70’s exploring how these constants govern everyday physics — for example you can express the size of stars, the maximal height of mountains, and all sorts of other things (even an attempt at why toast falls butter-side down) in terms of them.  But I don’t think anyone has really used this technique to investigate what a C=10 m/s world would be like!

  4. Richard says:


    In your view had there never been conscious minds in the universe would time still exist? Is the question even meaningful? Could there be events like stars birthing and dying without there being time?

    • Anthony Aguirre says:


      An excellent question, with a lot of subtleties.  One issue of course is how to define consciousness — is it a property only of brains like ours, or of information-processing physical systems, or something fundamental (i.e. as fundamental as ‘stuff’)?  It’s also tricky to know quite how to think of the question. Stars, for example, would seem to exist without any conscious observers around — how can a hot ball of gas’s existence depend on observers?  But without observers, do ‘ball’, ‘gas’ and ‘hot’ really exist? I think we — even as theoretical physicists — tend to neglect or ignore how much of reality we have constructed through eons of physical and biological evolution, thought, and social creation.  Yet it doesn’t seem quite right either to say that things are not hot unless minds are around.  As a further example, the second law of thermodynamics really can’t be posed without some sort of input we decide on — technically (and depending upon the type of entropy), how we partition phase space into macrostates, or assign probabilities to different microstates, etc.  And the second law is a pretty central part of time.  Yet all physical systems, all the way down to viruses, proteins, and even complex molecules *obey* the second law.  How do they obey a law that does not exist to them?  

      Lots of fun questions!


  5. wondering14 says:

    If a Martian interpreter came to earth with a Universe phone, he could facilitate talk between Mars and earth. A Universe call could be arranged once the two time zones was aligned. There would be voice and sight delay, of say, 10 minutes between earth and Mars, more or less depending on orbiting, interference, etc. A comment and its response would take at least 20 minutes. Is this delay real (time)? Does the delay exist for Einsteinian reasons? After the phone call, is the call in the past both on Mars and the earth? Or is it the present or the future?

    • Anthony Aguirre says:


      I don’t think we need to go so far as consider Martians and interpreters — the very experiment is does all the time with space probes controlled by NASA or other agencies.  The delay is both real and unavoidable, and after the conversation the whole conversation would lie in the past of both Earth and Mars.  These sorts of problems can be nicely analyzed using ‘spacetime diagrams’, a core tool for thinking about special relativity, using which you can take a spacetime and understand exactly which events can affect (or be affected by) which others, given that effects must propagate slower than the speed of light.  

  6. ntadepalli says:

    Thanks for the good article . The following is true for our reference frame :
    First we have the world of experience which is subjective and later the
    objective world of science & mathematics.The subjective worlds are many, one
    for each individual and the objective world is being seen as at classical level and
    as at quantum level.
    Out of these worlds the classical world is important to us as
    1. It is the cause for its effect of our immediate experience,though their
    relationship is not one to one.It is one to many even in single
    individual,depending on individual`s moods and attitudes.These can be the sources of
    unpredictability in this deterministic world.
    2. It is referred as real in our immediate experience.
    3. The quantum world has to explain it.

    Reference frame will defintely influence the subjective and objective
    But if determinism is true in objective world related to one frame then we can
    expect that to be true in all frames
    Observational differences from different frames may not be there within the unitary block..

  7. abed.peerally says:

    Nice comprehensive article. Difficult subject because there are so many different perspectives to what is time. I find the Unitary Block block more philosophical while the Experienced World nearer to reality. Time is something that was part of the mindset  which created the universe. We have to be different from the Mind and from Angels in a manner of speaking. We are humans in a massive universe where galaxies, planets and other bodies are dynamical and because of these dynamics we have a law of proportionality between SR and GR as shown in my paper if SAJS:104. Time is a fundamental constituent of entropy and both of them were behind the master minding of our universe. Whoever did us, ourselves as difficult to explain as God himself, must have been incredibly artful and scientific. We were a calculated and deliberate creation and I will explain in scientific details how time comes in. Time exists unlike the view of Barbour just as the indeterminacy of quantum reality whch is just as indispensable for the entropy’s progress.

  8. jsg says:

    Many thanks for this fascinating article. The subject of time and predictability of events always seems to come down to Laplace’s assertion that a complete knowledge of the present state of the universe would lead to complete knowledge of its future. This in turn seems to reduce to the assertion that every syllable of “Hamlet” was implicit in the Big Bang, if only someone had been around to foresee the script.

    However, I think this description of the block-time view from the intriguing essay essentially gives the game away: “Everything at any time is uniquely determined by — and thus implicitly contained in — any other time, and the world exhibits no distinction between past and future.”

    Even if this were  true, which it well might be, pace quantum indeterminancy, and “Hamlet” really was implicit in the Big Bang (where else could that play have come from?), and an all-knowing someting-or-other could have read “To be, or not to be,” in the primal explosion, what does this really tell us? Only, I would argue, that one state evolves into another — the implicit becomes explicit — and that very evolution of states is what we call “time.” 

    Thought itself is a product of time brought about by evolving neurological states. So without time, we could not even be having this discussion!


    • Anthony Aguirre says:

      jsg — 

      Thanks for this comment.  Indeed, I think that if there is a Unitary Block description in which the whole universe can be specified by some set of initial data, then I think ‘Hamlet’ is implicit in that data. However, I only think it’s implicit in more or less the same way that it’s implcit in PI: creating some mapping from digits to letters, all possible letter sequences exist in PI, including Hamlet.  But you won’t find it unless you go looking for Hamlet (and have a lot of patience).  The point is that you bring Hamlet with you.  Looking at a pure Unitary Block description of the Universe (if we could) would I think be similar — we’d have to bring a lot of things with us to find anything of interest.  And we only have those things to bring because they have been created through a historical process that is probably better described at the ‘Experienced Reality’ (or other) level rather than the Unitary Block. ‘Hamlet’, I think, is ‘made out of’ sorts of information that simply are not present in the Unitary Block, as I argued in the essay.



      • jsg says:

        Thank you for the thoughtful and thought-provoking reply to my comment. If “Hamlet” is implicit in the Big Bang, but not predictable from the initial state of the system, as you point out, is “Hamlet” nonetheless predetermined in the sense of being a link in a chain of causal antecedents? And if “Hamlet” is the product of deterministic processes whose outcome cannot be foreseen in advance — as in the case of deterministic chaos — can there be any meaningful distinction between determinism and “free will?” 

        • Anthony Aguirre says:


          I don’t think ‘Hamlet’ is predetermined, in that quantum indeterminacy will lead to *either* a mismatch between the Unitary Block description and the Experienced Reality (in that there are for example potential experimental outcomes that persist in the UB description but were ‘not seen’ in the ER description) OR there are various bifurcations so that there is not a single causal chain, but a bifurcating one as pertains to questions like that of the existence of Hamlet.

          Whether and how to distinguish between ‘determined’ and ‘in principle predictable’ is a subtle question.  I like the latter because it is a bit more amenable to physical argument in particular theories, whereas ‘determined’ is a sort of all-or-nothing, highly philosophical concept. 

          I think the question of ‘Free Will’ can be posed on a variety of levels.  Personally, I am inclined to think that we have more freedom than we think in terms of our decisions being ‘determined’ by the unitary block, and a lot less freedom than we think in terms of our decisions being determined by all sorts of unconscious habits, factors, and processes.

  9. peter jackson says:

    Nice essay Anthony. A reflection that we’ve made little progress since the fqxi ‘Nature of Time’ essays. I’ve noticed few attempts to answer the questions so if I may I’ll offer a short one for your comments with some ‘thinking out loud’.; Q1.

    If we slow time down to the speed of sound I hypothesise that we may find it behaves exactly the ‘same’ as sound; i.e always propagates with a certain speed Mach1 in and with respect to  the local dielectric medium bulk CofM ‘rest frame’ (i.e. ‘datum’ for local d/t).

    Of course we well know it does so.  Applying the same ‘change of speed’ we use for sound between moving media (the sound in the train is a ‘different ‘Mach 1 to the sound Mach1 along the embankment.  Because it  CHANGES speed and Doppler shifts on interaction with the new medium (wrt it’s ‘new’ rest frame). Now that all works fine, because our eyes and nerve ‘channels’ are equivalent to dielectric media (also add local index n).

    But there is still the problem in the vacuum of space.  But is there?  We learned the vacuum is quite ‘full’ of the same particles that make dielectrics decades ago, very diffuse of course, and well know now that ionospheres, far more dense than deep space, ARE quite effective dielectric media!

    So why are we still using the old false assumptions from before space travel on which to base our theries? I suggest it is that failure to ‘review’ that has led to all the confusion and anomalies.  The Postulates & Prnciple of SR (the onle ‘proven’ parts of it) are well served by a diffuse sielectric model using J D Jacksons established ‘extinction distances (the ‘old’ signal takes time to be re-scattered to become the new one, so refraction is the gentle curve we know from GRIN lenses).

    So should we perhaps not go ‘back’ in time a little and review the foundations in the new light to see if the confusion lifts? If we co-join ‘the observer -as part of the system – dependence’ (our eyes and brain create the ‘picture’ of the moon, yet it’s there reflecting the light) with the Local Reality Einstein was seeking, then we can find convergence of SR and QM. Coherent answers for the next 3 questions also emerge.

    Time itself is ‘absolute’ but what we will see when it’s slowed down is that the emissions of clocks are the SIGNALS, not ‘time’ itself – just like sound, so are also dilated and contracted by Doppler shifts. Dielectrics (pure plasma or electron based) then come in local ‘discrete fields’ based on and at rest around around matter at all scales (don’t confuse with Stokes ether). Now for me, all the problems then melt away. I’ve searched very hard for any that don’t. I still am but it’s a forlorn task.

    Can you think of any Anthony? Or can anybody else help by doing so?

    • Anthony Aguirre says:

      Peter Jackson and Pencho Valev,

      I’m sorry, but I just can’t agree — I think that as a physical theory, Special (and General) Relativity are simply beyond reproach in terms of their description of the dynamics of objects/fields in spacetime, and symmetries obeyed by laws of physics governing them.  These theories (especially SR) have been tested so thoroughly that it’s hard to conceive of them being ‘wrong’ in some way that could manifest in observable physics.  And if some alternative does not differ in observable physics, then I don’t think I’d have much interest, as Relativity is so elegant a theory that few would be willing to abandon it unless forced to by Nature.  But so far Nature has not even mildly suggested that, at least to me.

      • Pentcho Valev says:

        Relativity is so elegant a theory that few would be willing to abandon it unless forced to by Nature.  But so far Nature has not even mildly suggested that, at least to me.

        John Norton and Banesh Hoffmann did notice the suggestion:

        John Norton: “In addition to his work as editor of the Einstein papers in finding source material, Stachel assembled the many small clues that reveal Einstein’s serious consideration of an emission theory of light; and he gave us the crucial insight that Einstein regarded the Michelson-Morley experiment as evidence for the principle of relativity, whereas later writers almost universally use it as support for the light postulate of special relativity. Even today, this point needs emphasis. The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE.”

        “Relativity and Its Roots” By Banesh Hoffmann, p.92: “Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton’s laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether.”

        Pentcho Valev

      • peter jackson says:


        You grouped Pentcho’s description with mine yet they propose quite different cases. Your comments referred to Pentcho’s proposition that SR is false but you didn’t address any points specifically, which is whats needed for validity (rather than relying on ‘greater authority’). The whole value in asking the ‘Big Questions’ is surely to avoid just doing that and analyse from the foundations up, Yes?

        Contrary to Pentcho I agree entirely that SR’s postulates are correct, and point out that it’s only the postulates that ALL the actual falsified evidence supports (do point out any I may have missed). I assume you agree that all unproven ‘assumptions’ should be dug out and re-examined? So we may still find other interpretive aspects may not be so cast in stone (however ‘elegant’). Indeed Einstein himself said exaclty what I’m saying many times! (quotes by request) due to the incessant anomalies (Astronomy is riddled with them) and persistent ‘apparent’ paradoxes.

        Let me ask you a couple of question to answer specifically. SR employs both ‘Proper Time’ and ‘Co-ordinate Time’ , which SciAm defined well 2yrs ago. Let’s ignore space for the moment and just consider dielectric media, in which we know light propagates at c/n wrt the medium frame (just taking some a BEC on a train proves that) so speed is always found at d/t (as M&M and ALL other experiments) then let’s consider what happens if we consistently consider both d/t_p (Proper time) and d/t_c (co-ordinate time).  

        It takes some thought, but in the study of quasar jets I’ve found a resolution of the superluminal quasar pulse problem (an ever more persistant apparent violation of ‘special relativity’) now confirmed at <46c, emerges from electron shear plane collimations and Compton/Raman scatering at c. What is more, this ‘special (dual) case’ solution recovers the SR postulates and propagation at <c. More interestingly, SR and QM do then seem able to converge.

        I’ve found that no findings contradict this model, and many other anomalies are resolved. Our currrent ‘understanding’ lumps the assumptions in with the ‘proven’ parts of SR, mainly CSL) and now seems now so deeply ingrained that it’s accepted with no thought or discussion. So it’s become ‘taboo’.

        So; a) Are we too fearful of what others may think to anywhere ask intelligent questions and examine details of what have become our ‘sacred cows’?

        b) If you suggest there’s no contrary evidence to parts of current understanding, what suggestion do you have that no astronomer has thought of to explain the ubiquitous apparent FTL quasar jet pulse speeds; ever faster and more common?

        NASA recently confirmed a pulse in our neighbour M87 at 4.5c from angular change rate, well outside all possible error bars and corrections. Should we really just dismiss possible vaid solutions?

  10. Pentcho Valev says:

    2. The Unitary Block is wrong in some essential way…

    Yes it is wrong, and the reason is that the speed of light (relative to the observer) varies with the speed of the observer:

    A light source emits a series of pulses the distance between which is d (e.g. d=300000km). A stationary observer/receiver measures the frequency of the light pulses to be f=c/d:


    An observer/receiver moving with speed v (let v be small so that the relativistic corrections can be ignored) towards the light source measures the frequency of the light pulses to be f’=(c+v)/d:  


    From the formula f=c/d one infers that the speed of the light pulses relative to the stationary observer/receiver is c. From the formula f’=(c+v)/d one infers that the speed of the light pulses relative to the moving observer/receiver is c’=c+v, in violation of special relativity.

    In other words: As the observer starts moving towards the light source with speed v, the speed of the light pulses relative to him shifts from c to c’=c+v (in violation of special relativity) and, as a result, the frequency the observer measures shifts from f=c/d to f’=(c+v)/d:  

     “Doppler effect – when an observer moves towards a stationary source. …the velocity of the wave relative to the observer is faster than that when it is still.”

    As the observer starts moving away from the light source with speed v, the speed of the light pulses relative to him shifts from c to c’=c-v (in violation of special relativity) and, as a result, the frequency the observer measures shifts from f=c/d to f’=(c-v)/d:

     “Doppler effect – when an observer moves away from a stationary source. …the velocity of the wave relative to the observer is slower than that when it is still.”

     Albert Einstein Institute: “The frequency of a wave-like signal – such as sound or light – depends on the movement of the sender and of the receiver. This is known as the Doppler effect. (…) Here is an animation of the receiver moving towards the source: (…) By observing the two indicator lights, you can see for yourself that, once more, there is a blue-shift – the pulse frequency measured at the receiver is somewhat higher than the frequency with which the pulses are sent out. This time, the distances between subsequent pulses are not affected, but still there is a frequency shift: As the receiver moves towards each pulse, the time until pulse and receiver meet up is shortened. In this particular animation, which has the receiver moving towards the source at one third the speed of the pulses themselves, four pulses are received in the time it takes the source to emit three pulses.”

     Let “the distance between subsequent pulses” be 300000 km. Then the frequency measured by the stationary receiver is f = 1 s^(-1) and that measured by the moving receiver is f’ = 4/3 s^(-1). Accordingly, the speed of the pulses relative to the moving receiver is:

     c’ = (4/3)c = 400000 km/s

    in violation of special relativity.

     The relativistic corrections change essentially nothing. The speed of the receiver is (1/3)c so gamma is 1.05. Accordingly, the corrected f’ is (1.05)*(4/3) s^(-1) and the corrected c’ is (1.05)*(400000) km/s. Special relativity remains violated.

     Pentcho Valev

  11. John Merryman says:


     The point I keep raising about the issue of time is that we experience it as a sequence of events and so think of it as this point of the present moving along a narrative vector from past to future, but the actual physical process is the changing configuration of what exists is constantly creating and dissolving these personal reference points. Thus it is actually the future becoming past. Ask yourself, does the earth travel/exist along some vector from yesterday to tomorrow, or does tomorrow become yesterday because the earth rotates? What is more real, the physical presence, or the fleeting forms it manifests? What relativity does is to correlate measures of distance and duration, to create a four dimensional geometry, but measuring from one event to the next in a particular sequence only enforces that impression of the narrative vector, not actually looking beyond it. Yes, it can be used to create quite impressive and accurate patterns, but should we then make those patterns the agent of force, in this case, gravity? Epicycles were also quite impressive and accurate patterns for their day, but assigning agency directly to them, in the form of giant cosmic clockworks, was an error. I think trying to argue the basis of time is this essentially static topography, when time is such a fundamental measure of dynamic activity, will prove to be short sighted. 

     Consider how it contradicts the premise of the conservation of energy, given that it is the transmission of energy which is the basis of this changing configuration and so prior events cannot physically exist, since their constituent energy is manifesting current events. 

     There is also the issue of multiworlds; If we think of time as a vector from a determined past into a probabilistic future, it does naturally branch out, as whatever has occurred is necessarily determined, but the total input into any event only occurs with the happening of that event, so the future is inherently probabilistic, from any singular point of view and multiple points of view will detect different interpretations of the same event. Yet if we think of it as future probability coalescing into current actuality, it makes much more sense. Probability precedes actuality.

     The reason clocks run at different rates is because they are different actions. While a faster clock might evolve quicker, it also burns/ages quicker as well, so its physical manifestation doesn’t move further along the vector into the future more rapidly, but falls into the past faster.

     Time as an effect of action/measure of change makes it much more like temperature, than space. Time is to temperature what frequency is to amplitude. Since our cognitive functions are based on the sequence of time, from narrative to causal logic, we tend to be less intellectually objective about it, while temperature, which regulates our metabolism, tends to be dismissed as a cumulative average. We tend to think of time in terms of the particular and temperature in terms of the aggregate, but that is our perspective. We measure time in terms of particular events, such as the spinning of the planet, or cycles of a cesium atom, yet the total effect of change is due to lots of such rates and while we assume there must be some universal clock rate, measuring out that pace from past to future, it ultimately is just that cumulative effect of many such actions. With temperature, we instead measure the cumulative, which is based on the energy levels of lots of different actions and the amplitudes of energy they express. These individual actions do exchange energy and so are physically seeking out that thermal equilibrium of an average temperature, otherwise known as entropy. So both time and temperature are lots of particular actions creating a larger effect.

     We could as well use ideal gas laws to construct a temperaturevolume, similar to the distance and duration of spacetime, but we have a better perspective of the relation of temperature and volume, so while they may be physically relatied, we still understand the distinction that seems to be lost with space and time. When we measure space, be it distance, area, or volume, we are measuring space, but when we measure time, we are measuring action and its rate of change.

     1)If the speed of light were slower, it is likely all actions would be correspondingly slower, given the essential nature of light, so we wouldn’t notice it.

     2) That’s a big if. Personally I live a fairly physical and thus present oriented life, so it’s more about the immediacy of particular actions and their infinite layers of structure and texture, etc, so I feel quickly overwhelmed trying to view larger scenarios with any amount of real life detail. As I see it, knowledge is a function of selection. Much as we distill signal from the noise, there is no ‘God’s eye view’ because the absolute is basis, all energy and thus noise, knowlege and selection cancelled out, ie. white noise. Like leaving the shutter on a camera open too long might give you more energy, information is lost.

    Combined with my point about time, this means there can be no such thing as a dimensionless point in time, as that would freeze the very action creating the events. Like a temperature of absolute zero, not only would there be no amplitude, there would be no frequency.

     3) Yes, As I stated above, what has occured is necessarily determined, but it has been determined by the actual physical processes creating it and prior to the occurrence of these events, they are probabilistic, even if predictable.

     4)I think that in many ways the quantum view is bottom up and the classical view is top down, but there are large gaps in our conceptual ability to fuse these two halves. The parts add up to a larger whole, as the parts of our lives and bodies add up to us, but our minds are more object/frame oriented, rather than process oriented, so we tend to think of the classical world as big objects/events and the quantum world as small objects/events. 


     John M

  12. Akinbo says:


    Thanks for your essay and I note your perspective to the problems. I have two questions for you, although I would have asked more but I take note of your reply to Peter Jackson and Pentcho Valev so no need for repetition.

    The first question is, given the evidence for the presence of non-baryonic dark matter from the higher than escape velocities of outlying stars in our galaxy, its abundance, its transparency, its ability to cause refraction or bending of light traversing galaxies, its capacity to gravitationally interact and the acknowledgement from some peer-reviewed and non-peer reviewed papers that there is likely to be an earth-bound (ref. below) component, would such dark matter have any influence on fundamentally important earth-based surface experiments on light’s dynamical behavior?

    My second question is on the arrow of time and the Second law of thermodynamics you discussed, viz. ” If these conditions specify that there is a time at which the world has much lower entropy than it might, then it is arguably natural for entropy to increase away from this time, which can then be called ‘the past’”. It follows from this statement, that the beginning which can have no past must be of zero entropy. What are the physical characteristics and attributes of a state or system of zero entropy and how would that state/ system react if it is physically disturbed, that is if it is capable of being disturbed in the first place?



    http://arxiv.org/abs/0805.2895,&nbsp;http://www.dapla.org/earth_dark_matter_halo.htm,&nbsp;http://iopscience.iop.org/1751-8121/41/41/412002/,  http://arxiv.org/abs/0806.3767, http://scienceblogs.com/startswithabang/2013/07/03/does-dark-matter-affect-the-motion-of-the-solar-system/,&nbsp;http://arxiv.org/abs/0902.3790

    • Anthony Aguirre says:


      Thanks for your questions.  On dark matter, the most widely-investigated dark matter candidates, which are supersymmetric particles and axions, can have terrestrial effects, but those are extremely hard to detect because the interact so weakly with ordinary matter.  Still, there is hope that they will be detected in this way, perhaps quite soon.  Other dark matter candidates could have other effects, but it is hard to have one with a *significant* terrestrial effect, because in general the same sort of interactiont that would give such an effect would imply quite large effects astrophysically.

      In terms of the second law, the time of minimum entropy need not be zero entropy, though it could be. What exactly ‘zero entropy’ means gets technical, and depends on exactly what sort of entropy you are talking about — there are several interrelated notions that are useful in different contexts.  Technically, zero ‘Boltzmann entropy’ would mean that the Universe was in a particular macrostate associated with only one microstate in it.  Zero ‘Von Neumann entropy’ would just mean that the universe is in a particular known microstate.  In fact in the Unitary Block description, Von Neumann entropy is constant, so would just stay at zero, and in that sense there is no second law unless you add some additional ingredients such as splitting the universe into subsystems, or coarse-graining into macrostates.  Boltzmann entropy does (generally) increase even in the Unitary Block, but is defined in terms of a partition of microstates into macrostates that is ‘added’ to the Unitary block in a way that (I would say) makes it useful for describing Experienced Reality.

  13. Roger says:

        I believe I lean towards the Unitary Block view with the following comments:

    1. To me, time is not fundamental. It’s just a function of physical things happening in the universe (e.g., physical change in the shapes and interactions of the existent entities that make up our universe).   If there were absolutely no physical change in the universe, there would be no time. I think this explains why time is moving irreversibly from past to future: because things keep happening. To go from future to past, there would have to be a reduction in the number of things that have already happened (e.g., the number of physical changes that have occurred) in the universe. This can’t happen. Even if the events of a process happen in reverse like if a broken cup spontaneously reassembled, this doesn’t mean that time is going backwards; it just means that physical events are still happening that lead to the reformation of the cup.  But because things are still happening, time is still moving forward.  This reasoning seems to fit with the Unitary Block view in the essay/ Given this, I don’t think the Unitary Block view says that the past and the future are all right here with us now.  The past is still in the past because it was equivalent to some lower number of physical changes/events having already occurred.  Now is equal to a higher number of physical changes/events than the past and a lower number of physical changes/events than will occur in the future.  Each region of space may have different rates of time based on how fast the physical changes/events are happening there which gravity, etc. might affect. 

    2. I think you can combine the Unitary Block view with the quantum idea by saying that even if you know the exact properties of all the physical entities of “now”, these exactly known entities can seem to specify a finite set of possible futures, any one of which could happen.  

    3. In regards to Discussion Question 4, I have two comments: 

        A. I think it’s possible to have a combination of classical and quantum views as a more fundamental level as described in my point 2, above, and in point B, below.

        B. Suppose one has a physically existent spherical object, A, and it can specify a set of spheres all around its surface.  But until those spheres appear, there is no space and no locations around A.  Looking back after the fact, we could say that those spheres could have been in any of an infinite number of locations around A.   But this is incorrect because there was no space and there were no locations around A until after the spheres appeared.  Our after-the-fact bias towards thinking space and possibility is infinitely divisible and continuous could lead us into thinking that there were an infinite set of possibilities for where the spheres could have occured (that is a more quantum view) when in fact there were no possibilities at all until after the spheres actually appeared.  This might be one way of combining a classical and seemingly quantum way of looking at  the universe.

        Thank you for listening!

  14. Pentcho Valev says:

    “The mode of theoretical physics has tended toward elevating certain descriptions as more ‘true’ and ‘fundamental’, and the other descriptions as ‘in principle derivable’ from these.  I think it is worth contemplating a different approach: that just as the Experienced Reality description is far more useful than the ‘Unitary Block’ in some applications (and far less in others), that we take Experienced Reality as an equally true description that is complementary to, but not derivative of, the Unitary Block.”

    That is absurd. The “Experienced reality” is consistent with the assumption that the speed of light (relative to the observer) does depend on the speed of the light source while the “Unitary Block” is consistent with the antithesis (Einstein’s 1905 constant-speed-of-light postulate). To take them as equally true is tantamount to believing that the speed of light both depends and does not depend on the speed of the source.

    Pentcho Valev

    • Anthony Aguirre says:

      Pantcho Valev – 

      I did not mean to suggest that the “Experienced Reality” description is something like the “inbuilt physical intuition” description.  The latter is often fiarly useful, but generally incorrect when applied to phenomena outside the range of our everyday experience and evolutionary heritage.  Special Relativity (including the constancy of light’s speed) holds very well for macroscopic objects, even if it violates our intuitions. 

      • Anthony Aguirre says:

        Hi All,

        Thanks for all you comments, which have been thought-provoking for me and I’m sure others.  Apologies to those for whom I did not have a chance to write a detailed reply.  And thanks to BQO for setting up this great venue!