The physicist Tim Kowlowski listens to the discussion at the Time in Cosmology conference.

Philip Cheung for Quanta Magazine

The physicist Tim Koslowski listens to the discussion at the Time in Cosmology conference.

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Einstein once described his friend Michele Besso as “the best sounding board in Europe” for scientific ideas. They attended university together in Zurich; later they were colleagues at the patent office in Bern. When Besso died in the spring of 1955, Einstein — knowing that his own time was also running out — wrote a now-famous letter to Besso’s family. “Now he has departed this strange world a little ahead of me,” Einstein wrote of his friend’s passing. “That signifies nothing. For us believing physicists, the distinction between past, present and future is only a stubbornly persistent illusion.”

Einstein’s statement was not merely an attempt at consolation. Many physicists argue that Einstein’s position is implied by the two pillars of modern physics: Einstein’s masterpiece, the general theory of relativity, and the Standard Model of particle physics. The laws that underlie these theories are time-symmetric — that is, the physics they describe is the same, regardless of whether the variable called “time” increases or decreases. Moreover, they say nothing at all about the point we call “now” — a special moment (or so it appears) for us, but seemingly undefined when we talk about the universe at large. The resulting timeless cosmos is sometimes called a “block universe” — a static block of space-time in which any flow of time, or passage through it, must presumably be a mental construct or other illusion.

Many physicists have made peace with the idea of a block universe, arguing that the task of the physicist is to describe how the universe appears from the point of view of individual observers. To understand the distinction between past, present and future, you have to “plunge into this block universe and ask: ‘How is an observer perceiving time?’” said Andreas Albrecht, a physicist at the University of California, Davis, and one of the founders of the theory of cosmic inflation.

Others vehemently disagree, arguing that the task of physics is to explain not just how time appears to pass, but why. For them, the universe is not static. The passage of time is physical. “I’m sick and tired of this block universe,” said Avshalom Elitzur, a physicist and philosopher formerly of Bar-Ilan University. “I don’t think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it’s not there.”

Last month, about 60 physicists, along with a handful of philosophers and researchers from other branches of science, gathered at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, to debate this question at the Time in Cosmology conference. The conference was co-organized by the physicist Lee Smolin, an outspoken critic of the block-universe idea (among other topics). His position is spelled out for a lay audience in Time Reborn and in a more technical work, The Singular Universe and the Reality of Time, co-authored with the philosopher Roberto Mangabeira Unger, who was also a co-organizer of the conference. In the latter work, mirroring Elitzur’s sentiments about the future’s lack of concreteness, Smolin wrote: “The future is not now real and there can be no definite facts of the matter about the future.” What is real is “the process by which future events are generated out of present events,” he said at the conference.

Philip Cheung for Quanta Magazine

Andreas Albrecht, a physicist at the University of California, Davis, presents his work on the nature of time.

Those in attendance wrestled with several questions: the distinction between past, present and future; why time appears to move in only one direction; and whether time is fundamental or emergent. Most of those issues, not surprisingly, remained unresolved. But for four days, participants listened attentively to the latest proposals for tackling these questions — and, especially, to the ways in which we might reconcile our perception of time’s passage with a static, seemingly timeless universe.

Time Swept Under the Rug

There are a few things that everyone agrees on. The directionality that we observe in the macroscopic world is very real: Teacups shatter but do not spontaneously reassemble; eggs can be scrambled but not unscrambled. Entropy — a measure of the disorder in a system — always increases, a fact encoded in the second law of thermodynamics. As the Austrian physicist Ludwig Boltzmann understood in the 19th century, the second law explains why events are more likely to evolve in one direction rather than another. It accounts for the arrow of time.

But things get trickier when we step back and ask why we happen to live in a universe where such a law holds. “What Boltzmann truly explained is why the entropy of the universe will be larger tomorrow than it is today,” said Sean Carroll, a physicist at the California Institute of Technology, as we sat in a hotel bar after the second day of presentations. “But if that was all you knew, you’d also say that the entropy of the universe was probably larger yesterday than today — because all the underlying dynamics are completely symmetric with respect to time.” That is, if entropy is ultimately based on the underlying laws of the universe, and those laws are the same going forward and backward, then entropy is just as likely to increase going backward in time. But no one believes that entropy actually works that way. Scrambled eggs always come after whole eggs, never the other way around.

To make sense of this, physicists have proposed that the universe began in a very special low-entropy state. In this view, which the Columbia University philosopher of physics David Albert named the “past hypothesis,” entropy increases because the Big Bang happened to produce an exceptionally low-entropy universe. There was nowhere to go but up. The past hypothesis implies that every time we cook an egg, we’re taking advantage of events that happened nearly 14 billion years ago. “What you need the Big Bang to explain is: ‘Why were there ever unbroken eggs?’” Carroll said.

Some physicists are more troubled than others by the past hypothesis. Taking things we don’t understand about the physics of today’s universe and saying the answer can be found in the Big Bang could be seen, perhaps, as passing the buck — or as sweeping our problems under the carpet. Every time we invoke initial conditions, “the pile of things under the rug gets bigger,” said Marina Cortes, a cosmologist at the Royal Observatory in Edinburgh and a co-organizer of the conference.

To Smolin, the past hypothesis feels more like an admission of failure than a useful step forward. As he puts it in The Singular Universe: “The fact to be explained is why the universe, even 13.8 billion years after the Big Bang, has not reached equilibrium, which is by definition the most probable state, and it hardly suffices to explain this by asserting that the universe started in an even less probable state than the present one.”

Other physicists, however, point out that it’s normal to develop theories that can describe a system given certain initial conditions. A theory needn’t strive to explain those conditions.

Another set of physicists think that the past hypothesis, while better than nothing, is more likely to be a placeholder than a final answer. Perhaps, if we’re lucky, it will point the way to something deeper. “Many people say that the past hypothesis is just a fact, and there isn’t any underlying way to explain it. I don’t rule out that possibility,” Carroll said. “To me, the past hypothesis is a clue to help us develop a more comprehensive view of the universe.”

Philip Cheung for Quanta Magazine

Jenann Ismael, a philosopher at the University of Arizona, argues that the block universe holds within it the explanation for our experience of time’s apparent passage.

The Alternative Origins of Time

Can the arrow of time be understood without invoking the past hypothesis? Some physicists argue that gravity — not thermodynamics — aims time’s arrow. In this view, gravity causes matter to clump together, defining an arrow of time that aligns itself with growth of complexity, said Tim Koslowski, a physicist at the National Autonomous University of Mexico (he described the idea in a 2014 paper co-authored by the British physicist Julian Barbour and Flavio Mercati, a physicist at Perimeter). Koslowski and his colleagues developed simple models of universes made up of 1,000 pointlike particles, subject only to Newton’s law of gravitation, and found that there will always be a moment of maximum density and minimum complexity. As one moves away from that point, in either direction, complexity increases. Naturally, we — complex creatures capable of making observations — can only evolve at some distance from the minimum. Still, wherever we happen to find ourselves in the history of the universe, we can point to an era of less complexity and call it the past, Koslowski said. The models are globally time-symmetric, but every observer will experience a local arrow of time. It’s significant that the low-entropy starting point isn’t an add-on to the model. Rather, it emerges naturally from it. “Gravity essentially eliminates the need for a past hypothesis,” Koslowski said.

The idea that time moves in more than one direction, and that we just happen to inhabit a section of the cosmos with a single, locally defined arrow of time, isn’t new. Back in 2004, Carroll, along with his graduate student Jennifer Chen, put forward a similar proposal based on eternal inflation, a relatively well-known model of the beginning of the universe. Carroll sees the work of Koslowski and his colleagues as a useful step, especially since they worked out the mathematical details of their model (he and Chen did not). Still, he has some concerns. For example, he said it’s not clear that gravity plays as important a role as their paper claims. “If you just had particles in empty space, you’d get exactly the same qualitative behavior,” he said.

Increasing complexity, Koslowski said, has one crucial side effect: It leads to the formation of certain arrangements of matter that maintain their structure over time. These structures can store information; Koslowski calls them “records.” Gravity is the first and primary force that makes record formation possible; other processes then give rise to everything from fossils and tree rings to written documents. What all of these entities have in common is that they contain information about some earlier state of the universe. I asked Koslowski if memories stored in brains are another kind of record. Yes, he said. “Ideally we would be able to build ever more complex models, and come eventually to the memory in my phone, the memory in my brain, in history books.” A more complex universe contains more records than a less complex universe, and this, Koslowski said, is why we remember the past but not the future.

But perhaps time is even more fundamental than this. For George Ellis, a cosmologist at the University of Cape Town in South Africa, time is a more basic entity, one that can be understood by picturing the block universe as itself evolving. In his “evolving block universe”model, the universe is a growing volume of space-time. The surface of this volume can be thought of as the present moment. The surface represents the instant where “the indefiniteness of the future changes to the definiteness of the past,” as he described it. “Space-time itself is growing as time passes.” One can discern the direction of time by looking at which part of the universe is fixed (the past) and which is changing (the future). Although some colleagues disagree, Ellis stresses that the model is a modification, not a radical overhaul, of the standard view. “This is a block universe with dynamics covered by the general-relativity field equations — absolutely standard — but with a future boundary that is the ever-changing present,” he said. In this view, while the past is fixed and unchangeable, the future is open. The model “obviously represents the passing of time in a more satisfactory way than the usual block universe,” he said.

Philip Cheung for Quanta Magazine

George Ellis, a cosmologist at the University of Cape Town, listens to a presentation.

Unlike the traditional block view, Ellis’s picture appears to describe a universe with an open future — seemingly in conflict with a law-governed universe in which past physical states dictate future states. (Although quantum uncertainty, as Ellis pointed out, may be enough to sink such a deterministic view.) At the conference, someone asked Ellis if, given enough information about the physics of a sphere of a certain radius centered on the British Midlands in early June, one could have predicted the result of the Brexit vote. “Not using physics,” Ellis replied. For that, he said, we’d need a better understanding of how minds work.

Another approach that aims to reconcile the apparent passage of time with the block universe goes by the name of causal set theory. First developed in the 1980s as an approach to quantum gravity by the physicist Rafael Sorkin — who was also at the conference — the theory is based on the idea that space-time is discrete rather than continuous. In this view, although the universe appears continuous at the macroscopic level, if we could peer down to the so-called Planck scale (distances of about 10–35 meters) we’d discover that the universe is made up of elementary units or “atoms” of space-time. The atoms form what mathematicians call a “partially ordered set” — an array in which each element is linked to an adjacent element in a particular sequence. The number of these atoms (estimated to be a whopping 10240 in the visible universe) gives rise to the volume of space-time, while their sequence gives rise to time. According to the theory, new space-time atoms are continuously coming into existence. Fay Dowker, a physicist at Imperial College London, referred to this at the conference as “accretive time.” She invited everyone to think of space-time as accreting new space-time atoms in way roughly analogous to a seabed depositing new layers of sediment over time. General relativity yields only a block, but causal sets seem to allow a “becoming,” she said. “The block universe is a static thing — a static picture of the world — whereas this process of becoming is dynamical.” In this view, the passage of time is a fundamental rather than an emergent feature of the cosmos. (Causal set theory has made at least one successful prediction about the universe, Dowker pointed out, having been used to estimate the value of the cosmological constant based only on the space-time volume of the universe.)

The Problem With the Future

In the face of these competing models, many thinkers seem to have stopped worrying and learned to love (or at least tolerate) the block universe.

Perhaps the strongest statement made at the conference in favor of the block universe’s compatibility with everyday experience came from the philosopher Jenann Ismael of the University of Arizona. The way Ismael sees it, the block universe, properly understood, holds within it the explanation for our experience of time’s apparent passage. A careful look at conventional physics, supplemented by what we’ve learned in recent decades from cognitive science and psychology, can recover “the flow, the whoosh, of experience,” she said. In this view, time is not an illusion — in fact, we experience it directly. She cited studies that show that each moment we experience represents a finite interval of time. In other words, we don’t infer the flow of time; it’s part of the experience itself. The challenge, she said, is to frame this first-person experience within the static block offered by physics — to examine “how the world looks from the evolving frame of reference of an embedded perceiver” whose history is represented by a curve within the space-time of the block universe.

Ismael’s presentation drew a mixed response. Carroll said he agreed with everything she had said; Elitzur said he “wanted to scream” during her talk. (He later clarified: “If I bang my head against the wall, it’s because I hate the future.”) An objection voiced many times during the conference was that the block universe seems to imply, in some important way, that the future already exists, yet statements about, say, next Thursday’s weather are neither true nor false. For some, this seems like an insurmountable problem with the block-universe view. Ismael had heard these objections many times before. Future events exist, she said, they just don’t exist now. “The block universe is not a changing picture,” she said.“It’s a picture of change.” Things happen when they happen. “This is a moment — and I know everybody here is going to hate this — but physics could do with some philosophy,” she said. “There’s a long history of discussion about the truth-values of future contingent statements — and it really has nothing to do with the experience of time.” And for those who wanted to read more? “I recommend Aristotle,” she said.

Correction: A photo caption was revised on July 25, 2016, to correct the spelling of Jenann Ismael’s name.

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  • I thought the distinction between the past and present is due the collapse of the wave function. The wave function never un-collapses itself. I would like to know what other readers think on my views.

    P.S: Only an undergraduate at the moment

  • Before the Big Bang, there was no time. Since the Big Bang stuff happened. Time is not the measure of events. Events are the measure of time. <a href="">Source</a>.

  • The statement "The future already exists" is a consequence of Einstein's 1905 constant-speed-of-light postulate:
    Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this striking example of time dilation proves that time travel (towards the future) is possible. As a gedanken experiment (if we neglect practicalities such as the technology needed for reaching velocities comparable to the velocity of light, the cost of the fuel and the capacity of the traveller to sustain high accelerations), it shows that a sentient being can jump, "within a minute" (of his experienced time) arbitrarily far in the future, say sixty million years ahead, and see, and be part of, what (will) happen then on Earth. This is a clear way of realizing that the future "already exists" (as we can experience it "in a minute")."

    So the statement "The future does not exist" entails that time travel into the future is impossible and Einstein's 1905 constant-speed-of-light postulate is false.

    Pentcho Valev

  • From my point of view there is ONLY one time and that is NOW. Time is our perception of the state of things, whether it be a previous state (backwards in time) or an expected future state (forward in time). If you wanted to travel back in time, it would require changing the state of the universe to what it was at a previous time (based on your perceptions). But that would only mean that you had somehow created the next state of the universe to be identical to a previous state. Not really traveling backwards in time. Still going forward, as far as the next state of the universe is concerned. Time, in my opinion is our perception of change. Everything is always changing so time marches on with each change in state. And due to random change at the quantum level the future can not be reliable predicted. But we can make good guesses at the macro level.

  • Our perception of time leads us to think that either (i) all moments of time including present, past and future are equally real (because physical processes that make it up are deterministic, and by knowing current state you could work out what future state would be if you were clever enough) and we just go through it, i.e Block Universe model which follows a deterministic view of physical world; or (ii) “now" is the only thing that exists, with future state being non-existent because choices which will shape it having not been made yet.

    These two very simplistic views are missing an important detail which has been known to scientists studying non-linear systems (such as fluid dynamics) for some time now. Our world is so complex that very few processes that occur naturally in it are linear in nature – in fact, nature is full of processes that are non-linear and they constantly interact causing multiple feedback loops and non-precise periodic behaviour. Any non-linear process of sufficient complexity will exhibit behaviour which is chaotic making it mathematically impossible to determine what state it will take no matter how well you know the starting conditions. In my view, this illustrates why both views are mistaken – deterministic block universe view is incorrect as it is mathematically impossible to work out a sequence of outcomes in a process of sufficient complexity; and the future state is not completely unknown because possible outcomes in any non-linear process is constrained to a clearly determined range of possibilities (which is only dependant on the properties of interacting parts).

    Anant, does this answer your question regarding wave functions, which is clearly describing the range of possible outcomes given the properties of particles involved? Collapsing wave-function is a whole different explanation however.

  • Is every one mixing up illusion with allusion

    Illusion is done with fiscal props (ie) smoke and mirrors . Is the unaverse using fiscal props?

    I think the word allusion is proper in this case and has. Quite a different meaning.

  • Here we measure time by the tick of a clock or the loops of the earth around the sun. But is all relative. This is something humans made up on their minds to count events and occasions which is really the only thing that matters.
    Like always with this magazine start by reading the last part which always comes to the same conclusion stating nothing no matter the topic if you do thisyou will save a lot of "time" by not reading the rest.

  • There is a way to reconcile the A-theory and the B-theory of time, and that is the idea that both are valid *partial* explanations of reality.

    That is to say, the idea is that there is no one universal view-point of reality, but several partial viewpoints that are equally valid.

    In this view, one could believe both that time is fundamental *and* that the block-universe picture is also valid. Both viewpoints could be complementary explanations of reality (epistemological pluralism).

  • I am proposing that time is a natural consequence of nonlinear dynamic equilibrium involving underlying fundamental particles, perhaps at a Planck scale. This may involve dynamic equilibrium in multidimensional space. At some point, effect of several variables reaching criticality led the system to overcome the confinement threshold and create a run-away scenario such as Big Bang. There are plenty of examples at macroscale following these basic principles. So using fractal analog, similar situation may be occurring at Planck scale.

    Just a thought!

  • I'm just a layperson, but to me, it seems like sometimes physicists and philosophers make things too hard without reason. For instance, just because one can mentally change the sign of the time variable in an equation to a negative sign, this doesn't mean that the physical process this variable describes also has to go in reverse (e.g., that that time itself should be able to go backwards). It seems very important to distinguish between one's mental or mathematical description of something and the thing itself, in this case time.

    Also, for me, time is not fundamental. It's just a function of physical things happening (e.g., physical change). 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 doesn'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 the physical events happen to be going backwards because of some physical phenomenon. But, because events are still happening, time is still moving forward.

    Also, the idea that there would be very low entropy (disorder) at the beginning of the universe makes sense to me because if the universe started from a single existent state, whether this be a singularity or a quantum fluctuation or whatever, which then proliferated to produce the much higher number of existent entities in our current universe, this initial existent state suggests that there would have been initially a very low amount of disorder or entropy.

  • Anant Saxena, the first commenter, asked about the reduction of the wave-function. I was also wondering about this as I read the article. I tend to think of that in terms of entanglement. That when systems interact, they become entangled and their properties become correlated, an ultimately irreversible process since decoherence is, as I understand it, does not actually disentangle anything. Anyway, so from that perspective, the present moment is a transition from fewer to more possible states in the wavefunction and increasing entanglement between interacting systems, right? I'm asking. I'm not an expert. Trying to understand. I was wondering how this is incorporated into the 'block universe' model? It seems like every point in the block should be the source of an expanding sphere of possibilities.

  • In conjunction with a story I'm writing, I have had to consider the definition of time. I have concluded that time doesn't actually exist in the universe, but that we have invented time in order to explain why/how thing happen in an irreversible sequence. I have also defined "now" as the zero length "time" period in which the future becomes the past. We have defined time with reference to cosmological events that we observe: rotation of the earth around the sun, the rotation of the moon around the earth, and the rotation of the earth itself. I we lived on Mars, for instance our time scale would be completely different, which means that time is an arbitrary vector that we have devised to suit our own observations.

  • 7.20

    What if the block universe is a riverbed and time is the water that flows through.

  • "Teacups shatter but do not spontaneously reassemble; eggs can be scrambled but not unscrambled. Entropy — a measure of the disorder in a system — always increases"

    But this is a simplistic view and one that hampers thinking on the subject. The teacup was once clay, the egg was chicken feed. This is how order arises out of chaos and it is a fundamental problem in our understanding of entropy; it doesn't always increase. Something makes the teacup and the egg.

  • The block universe is four dimensional, (x,y,z,t). We are moving at a constant velocity in the t direction. If we have a velocity in x,y, or z we move slower in the t direction and time passes slower. Is there a force that can accelerate us to change velocity in the t direction?

  • I asked my daughter whether time was real and if there was a past, present, future and she replied certainly there is a past, just look at the stars. Their light took "X" amount of time to travel here with the current definition of the speed of light. So if light travels a certain distance in a certain amount of time, then time certainly exists and is accompanied by a past, a present and a future. (Or do we now redefine the speed of light if time is only an abstract phenomenon)?

    As an aside, Life does not follow the trajectory of the 2nd Law of Thermodynamics. As life has evolved, there is found increasing complexity within life's systems. Perhaps Life follows the Law on a chemical/energetic basis, but certainly not on an Informational level. Life uses the accumulation of Information to be disentropic in an entropic Universe.

  • @Donald Henderson – No, we mean illusion (a thing that is or is likely to be wrongly perceived or interpreted by the senses), not allusion (an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference).

    I think that you are mixing up fiscal (of or relating to government revenue, especially taxes) with physical. Yes, the Universe can be said to be using physical props … every subatomic particle in the entire Universe.

  • Philosophers who talk about the block universe vs. a "tensed" universe (was, is, will be…) use the terms "A-theory" and "B-theory of time" to specify whether they see time as reversible or not.

  • "Causal Set Theory" postulates space-time atoms continuously coming into existence.

    How big are they? An atom is the smallest irreducible unit of its kind – an individual creature is an atom of its specie. If size doesn't matter, then our specific universe could have begun as a space-time atom which popped into existence with a commonplace Bang which only we consider unique.

    (Inspired by the ebullition in my kettle while making tea.)

  • Comments are strange. One guy asks "Is every one mixing up illusion with allusion? ? The answer is NO. The article is talking about A theory and B theory of time. It is a very well know debate brought about the special relativity of simultaneity and space time diagrams that show that a now slice of time here is different that a now slice in a distant location. If you believe your future has already happened and in fact there is someplace in the universe that your future time in there "now" time then it's the B theory , it is tenseless. I haven't read the article yet, it will be fun , just noticed the comments are off the wall. Another comment thinks past and present are due to wave function collapse. The answer is no. The wave function in QM has nothing to do with this. The people who think the block time is the correct theory lack proper grounding in physics. They assume the graphs and numbers are the actual reality and fail to understand the symbols and mathematical expressions that use a block universe are just that , symbols , and do not show the passage of time. If you just follow the numbers you can get all sorts of nutty results like going back in time…Einsteins field equations can be solved to prove this. Which is nuts , it is forbidden by the law of causality.

  • Donald: "Illusion" refers to a mistaken impression about something. There is no requirement for props. "Allusion" is when you reference something indirectly; without being explicit – like a hint.

  • Perhaps at the Planck scale the atoms of space-time , mentioned by Rafael Working , move about and bounce off each other with the collision contact manifesting energy/mass and the free resonations between collisions as chronons of time .
    More reading about this in Einstein's Intuition by Thad Roberts .

  • According to this article "…about 60 physicists, along with a handful of philosophers and researchers from other branches of science…" gathered to discuss the question of whether time is real or an illusion. They were unable, it seems, to arrive at a satisfactory resolution. I would suggest that this is because they only discussed mathematical models and philosophy, not science.

    Science, if they had bothered to consider the question on its terms could have provided a succinct answer: time does not exist except as a human concept derived from the observation of matter-energy systems. In that way it is similar to temperature which is also a human concept derived, in this case, from the observation of molecular behavior under the influence of energy. Nobody wastes time debating whether temperature exists as an independent causal entity that is responsible for things getting hot or cold.

    Put another way, there is absolutely no empirical evidence for the existence of a physical entity called time. No one, scientist or not has ever observed or measured such an entity or demonstrated its existence. Therefore scientifically speaking discussing the nature of time, independent of the matter-energy systems by which we conceive it, is like discussing the nature of angels. It is not a proper scientific topic. The human concept of time, like that of temperature, has no correlate in physical reality.

  • Space is easy to comprehend while time presents some problems. In fact both space and time need frame of Reference. That is these are measured only in a relative manner. There is no absoluteness Big Bang is a mere theory of how the universe came about and so is the Steady State Theory. There is no way to confirm absolutely such conjectures as experiments become difficult to conduct for distant universe that existed close to the start of the universe. Both accuracy and precision limit their validity. Time to me is related to birth and death of any event we observe. Thus it is connected to life itself. If there is no such duality time is not needed. Time reversibility expts have all shown observations are independent, except some processes like radioactive decay where the continuity lies in their random nature!

  • As a poor black kid out of nowhere who is now working as a mechanic in a soda factory I can't help but think about how good it is to get paid to wrestle with such issues as time, matter, energy and entropy. It isn't like you're up at 3:15 am trying to solve problems before 5:00 and you don't get your hands dirty.

  • The ending there felt oddly abrupt. Ismael stands up and defends the block theory, but, as an economist would say, "expands the curve of possibility" by pointing out that philosophy as a whole has long struggled with Time. Ismael implies that we are digging in the wrong place!

  • Pentcho Valev- Well worded- It is staring us right in the face- The future certainly exists even if we are not able to precisely calculate all possible outcomes for a variety of reasons. Incredibly- We are constant time travellers.

  • It seems to me that physics has confounded itself with a metaphysical question in this instance. The notion of "time" is a philosophical one and this is born out by the fact that physics has entirely failed to come to grips with it within the context of the "physical" world. The concept of time is entirely unnecessary in understanding motion, although to date physics refuses to separate the two concepts. Motion can be explained by nothing more than space and mass. The desire to integrate the philosophical concept of time into the the science of physics has so far led to absurdities like the "block universe" theory and, most perplexing of all, that time is an illusion. If it is an illusion, I would suggest leaving off the grandiose attempts to integrate it into knowledge.

  • Kant was right that time must be taken a-priori, since it is an integral part of our perception which can not be removed. We have no conception of a place in the universe where time is not measured with a clock. In space time theory we change the time coordinate to space by multiplying by the speed of light. The theory tells us what we will measure with local clocks. I know of no physical theories where the coordinates can not be decomposed into what a clock measures. We have no definitions to say anything else about time.

  • Science has agonised long enough over the nature of Time. And is tying itself into knots in the process, each tangle making science more bizarre than inspiring to most people, and at the same time leaving an open goal for mystical opportunism on a grand scale.
    The Laws of Thermodynamics (which all scientists agree must be obeyed) reduce all existence to the level of energy exchange; which creates everything we know, and us, and how we perceive everything which this exchange of energy creates. But since perpetual motion is not possible, and nothing lasts forever, everything is gradually running out of steam. The prediction is that eventually nothing will exist anywhere at almost absolute zero, at which point Time will cease.
    Time is the bi-product of the progress of the Laws of Thermodynamics. The perpetual, universal conversion of energy which gives us life but which also means that when we break a glass, it can never be unbroken. Which is presumably why we say 'Remember that Time when you broke the glass'.
    A scientist might say 'Remember when you initiated that irreversible transference of energy.' A moment in time is merely a unit of transference, which, because it is irreversible, appears to us to instantly happen in an unrecoverable place called the past, which indeed it is. It seems we are surfing the wave of Entropy. Our buzz of energy-transference (life) interacting with and reacting to the maelstrom all around us.
    So that is the proposition. What we call Time is how Entropy feels from the inside. In other words, Time = The Laws of Thermodynamics in action. No magic or mystery. Just the fact you can never unbreak the glass. What we see as an invisible river of inaudible ticks and tocks is really a series of energy-exchanges that light us all the way to dusty Heat-Death.

  • I am neither physicist nor philosopher, but really enjoyed the article AND comments. What I find "funny" is the last sentence, that implies (at least to me) that Aristotle maybe knew as much centuries ago! I realize that is an exaggeration, but seems to me science will have a very very difficult time actually answering the questions they are debating: The "nature" of time. I humbly think the answers will come from the philosophers, not the mathematicians. Why? Because we cannot unscramble the egg or un-break the tea cup nor see the future. It hasn't happened. That is logical, correct. I guess I believe in the block universe theory. I do have a question that I would love to hear answered. I think the answer is yes, but would like to see what others have to say. Is every atom in the universe, or say, on earth still in existence since the beginning of (I guess I have to put this in quotes) "time"?

  • We experience reality as flashes of cognition and so think of time as the point of the present moving from prior to succeeding events. Physics codifies this by treating it as measures of duration, from one event to the next.
    The reality is that action is forming and dissolving these events, such that they go future to past. To wit, tomorrow becomes yesterday because the earth turns.
    This makes time an effect of action, like temperature. With time, we measure frequency, while temperature is an effect of frequency and amplitude.
    Duration is the state of the present, as events form and dissolve. It is not a dimension external to the present.
    Every clock is its own action and every action is its own clock. That is why different clocks, from cesium atoms at different altitudes, to organisms with different metabolic rates, can co-exist in the same present.
    The past does not dictate the present, as events are first in the present, then in the past. They have to occur, in order to be determined. The input into any event cannot be fully known before it occurs.
    Simultaneity is dismissed because observed events appear in different order from different points of view, but this is as elementary as seeing the moon as it was a moment ago, while seeing stars as they were years ago. It is the light striking your eye that is simultaneous, not the events being observed. Which no longer exist, because they radiated away that energy for you to observe.
    Blocktime is contrary to the conservation of energy, as every state of events would require a universe worth of energy to manifest. Yet it is this energy, manifest as the present state, which creates the effect of time, as it changes form.
    Time is asymmetric because it is a measurement of action and action is inertial. The earth turns one direction, not any other.
    There was a time we spent a great deal of effort trying to explain how the sun and stars swung across the heavens from east to west, before realizing it was the earth turning west to east. We still see it that way, but we understand why it is. Similarly we will always experience time as a sequence of perceptions, but eventually we will understand the narrative effect is not actually physically manifest and we will never travel through wormholes in the fabric of spacetime.

  • Could it be that time as we perceive it, is the fracture of symmetry. The past and future are not cause and effect, but trying to reach a state of equilibrium. The future doesn't proceed from the past but a check and balance is maintained.

  • Richard Wallace (a fine Scottish name) writes: "As a poor black kid out of nowhere who is now working as a mechanic in a soda factory…"

    I was instantly reminded of Steve Martin's "The Jerk"; "I was born a poor black child…" I actually started laughing when I read your intro 🙂

  • JOHN EREMITA asks: "Is every atom in the universe, or say, on earth still in existence since the beginning of (I guess I have to put this in quotes) "time"?"

    Well John, the standard answer from contemporary physics goes something like this:

    – "Energy can neither be created or destroyed" — 1st Law of Thermodynamics, Conservation of Energy.

    – E = MC<sup>2</sup> — Einstein's General Theory of Relativity.

    So there you have it. Every atom in the universe still exists. There may be no more and no less.

  • "A more complex universe contains more records than a less complex universe, and this, Koslowski said, is why we remember the past but not the future."

    …I would ask this guy to find a job as farmer somewhere on the countryside… fantastic that he gets paid by a university !!

  • Once again y'all are confusing the map for the terrain. The fact that "the laws that underlie these theories are time-symmetric" doesn't necessarily mean anything. Have you ever noticed that maps are flat, even in mountainous country. I wonder what that means? It means that the maps are imperfect. No map, other than a perfect simulacrum, could convey all of the information the terrain it describes does. So, the fact the math describing the quantitative aspects of physical laws is time-symmetric doesn't necessarily mean nature is.

  • I'm just a dumb old PhD in biology, but it seems to me that all that exists is the present moment. The past is memories, and the future is expectations. The journey from the present to the future is real only in the sense that things change, like the hands of a clock or your body rhythms. Our sense of being alive relies on these perceived changes.

  • How can we accept cosmological inflation of space without admitting the possibility of an inflation of time, which would create a boundary that is the present. Even a stationary object is moving along the time dimension at a good clip. One can't move in the time dimension faster than the rate of time inflation, because there's no there there yet. And one can't move in that dimension slower than the rate of time inflation because there would be two things in the same place at the same time, your past would catch up with you.

  • I assume that most scientists work under the assumption that consciousness is an emergent property of space time, rather than the other way around. However, if some universal awareness is fundamental and space time emerges from it, then perhaps time could be viewed as the collapse of Schrodinger's wave function due to observance. A self aware universe of infinite potential "observes" itself, causing a collapse of what we call the "wave function".

    Our sense of time could be the universe "observing" a particular collapse to at least 4 dimensions, from inside our 3 dimensional brain. Apparent causality in a particular direction could be due to how our limited 3 dimensional brain collects, processes and remembers information – one Plank moment at a time from simple to more complex states.

    Since the universal awareness is the same in each of us, we all experience the same "flow of time".

  • Sharing an article about one of my favorite topics which I believe will have lasting impact both in research and applications. I believe it even relates to discussion of time.
    Quantum drag
    Physicist says current in one iron magnetic sheet can create quantized spin waves in another, separate sheet
    July 20, 2016
    University of Iowa
    A physicist proposes that a current in one iron magnetic sheet creates a current in a separate sheet. The study's finding could be important in the emerging field of spintronics, which seeks to channel energy from spin waves generated by electrons to create smaller, more energy-efficient electronic devices.

  • My model is the best model. Why are these people pretending like it either doesn't exist or that I am not the authority on it?

  • Dimensions are like a scaffolding erected to contain a building that is being built, only in their case dimensions are the supporting framework for the universe. As such they must have appeared in their completed form at the beginning of the universe (or the universe would collapse) – in particular, a dimension we might call time, or the dimension that contains time, was created in its entirety at the beginning and so contains past, present and future from the beginning to the end of the universe. I suppose this is a block view of the universe. From the moment time appeared things were able to happen with events taking place throughout space (defined by the spatial dimensions) as a wave that self-propagated through the containing vessel of the time dimension. If this is correct then "now" is this wave, the past is all events it has already created and the future is as yet an empty nothing marked only by the unfilled part of the time dimension. Time travel into the future would be uninteresting and there would be no room left for travel into the past so we should enjoy the present.

  • I would just like to compliment Dan on a very nice piece of writing. I thoroughly enjoyed it.

  • We cannot abandon the concept of "time" in our daily life, even in experiments of Physics.

    Without time, we cannot measure speed. Without time, we cannot compare the duration of events. Without time, we cannot calculate the rate of return of investments.

    What makes time an illusion is the lack of foundation of a universal time, as both Special and General Relativity reveals that due to constant speed of light appear to every observer, time and motion are inter-dependent and thus time itself can no longer be well defined as a fixed background like Newton did.

    In my point of view, when all particles and matters have no motion, we cannot define time at all. As we cannot compare the different coordinate of Spacetime, as no events occured and no Worldline named by Relativity.

    The feature of Quantum Mechanics always guarantee a quantum fluctuation to prevents silence of Universe. So no need to worry about non-define of time.

    Anyway, the concept of time help to explain and measure many phenomena, both scientific and daily experience, and its impact is quantifiable.

    If we apply the Occam's Razor, we cannot eliminate the concept of "time" from our system of science and philosophy, as time is a crutial tool in explaining our World.

    Even if it is an illusion, one can say it is the greatest invention of human beings in understanding the World.

  • For the illusion of Past, Present and Future, one can say so if he has read the Universe as a story book and he knows the beginning and ending and every single events along the Worldline(s) included in the book. (Oh, that is God)

    According to SR and GR, due to changing in Spacetime, we cannot use a single Worldline to coordinate all events in Universe. Present on Earth maybe just a future in Mars. But a lack of universal Worldline does not conclude that time is an illusion.

    However, we are not only an observer, but also a participant in the Universe. One may say the Story Book (Worldlines of Universe) is interactive and can be never ending, depends on the evolution of things within it. What makes it meaningful is the record of events (past), reflection based on history and take action of the best choice (present) and unknown events to be changed (future).

    Time is not an illusion but symmetry of events on timeline is. Due to break of CP Symmetry of particles, no one can guarantee events can be evolved backwards in time exactly equal to that has been occurred and recorded.

    The break of rule of symmetry of time may even help to explain why matters are much more than anti-matters and thus the existence of Universe.

    Passed is passed. Sometimes we mixed up the past events, imagination and future events, because we dream and we believe nothing is impossible. The sequence of actual/imaginery events may got mixed up and thus we makes no different between these events.

    Past is a "history", "his" "story" that we can learn from (to know the actual consequences). Same as fiction and futurism, as we may find directions and insights from the stories created from our brain.

  • They are all wrong. I am sure entropy is the mechanism that determines time. All things progress from less entropy to more entropy, then to whatever happens next. Even black holes are subject to entropy…Maybe I am crazy but it seems so simple.

  • I am sorry to make this comment.
    But all things can be reduced to a simple term. What that is I am not sure, but some of the very complicated, though well written comments, are so complicated they make little sense to me.
    Think simple, basic, reduce things to their smallest components…just a thought.

  • Time does not exist. What exists is change. Time was invented to measure the rate of change. Always there is change. We cannot perceive the present. It is an infinitely small interface between the future and the past. Our perception of the present is actually a perception of a quantity of change. Our perception of reality,is facilitated by our short term memory. It was that, now it is this. Without change there is no perception of reality.

  • The whole debate appears to be another version of the old parable like 7 wise but blind men describing an elephant. Whatever is the final conclusion in course of "time", will the scholars help the laymen decide how to live his life and come to terms with the statement that his future already exists which he cannot see but somebody light years away can. Has the answer any implications for "free will"?
    I am new to this debate as I am educationally not qualified to really contribute anything but only interested in the implications.

  • I think simpler:

    Time is the prime supposition that underlies dimensional physics. Being a supposition of nature/ physics, it does not belong to nature, nor is dimensional. As a prime supposition it cannot be observed or thought about in any meaningful sense without already presupposing it:

    1) If time is a process of any kind (as authors mentioned in the article propose), the concept of "proces" presupposes time.
    2) If time is related to change or flow, "flow" and "change" presuppose time.
    3) The "symmetry of time line of argument" presupposes concepts like "flow", "process", "before", "after", "now".

    So even when time permeates everything physics does, running through it's equations, you have to deny it's existence (Einstein is a consequential man). So the only reasonable definition of time would be:

    Time is the gift that physics/ nature cannot accept. Being over Nothing is the unaccepted present.

    Can you think that?

  • If time has no significance other than describing human events and such, then how will you describe the dilation in time according to special relatively? If time is slow or fast relative to an observer, is it not enough to explain at least it has a physical significance and can be treated as a variable or measurable quantity such as entropy.

  • Any time a scientist invokes philosophy, or psychology, to explain physics, then they are admitting defeat.

    Either the universe can be described by physical laws and the mathematics that can represent them, or we are left with ghosts, souls, and ultimately, unknowable 'magic'.

    It is one thing to wring hands and admit ignorance, it is another to suggest that answers could even hope to lie within arbitrary and capricious notions.

    The bottom line is, still, that nobody knows a single thing about time, that time remains a total mystery at best described in vague terms and weak concepts, and that is okay. For now. Better, I think, to simply admit that. Time just hasn't had a breakthrough… yet.

  • Perhaps this is overly simplistic, but it seems that time is only a measurement of motion and/or action. One cannot speak of time without relating it to some other event, such as the completion of an orbit, radioactive decay, light moving a certain distance, etc. — all of which involve motion.

  • In a recent documentary by Stephen Hawking, he used atomic clocks to demonstrate that time is related to the power of gravity. He showed that time passed more slowly on a mountaintop than at sea level. It was a very interesting show and this is a very interesting article.

  • This rock I hold is not present in the past, it is present right now, even though it has the imprint of activities from a past era – the forces that formed it and shaped it. Similarly I can make predictions about this rock, how I expect it to be, but always when I observe the rock, the rock is present now – there is no rock embedded in the past, nor will there ever be a rock in the future – its future or past existence is a convenient illusion.

    The illusion is a product of our human memories and information processing apparatus – our brains. It stands to reason that we would extend this illusion to our logic, our mathematics, our models and our ideas.

    Just because it is convenient (in fact mandatory) to create and think about our reality with a time dimension, doesn't make time part of that reality – it just makes that reality understandable to us.

  • 1.Everything exists in the moment "now". The now – a period of transformation of the Future into the Past. Future decreases the Past is increasing.
    2. The matter moves in the moment "now". Photon time has stopped. The energy of a photon is proportional to 1/second. Reduction of the Future creates an increase in the size of 1/second. This creates the effect of "red shift" in the moment "now" relative time of emergence of the photon.
    3. Math: 2% "redshift from the expansion of the Universe. 98% of the movement time (the transformation of the Future into the Past).
    Stop "the darkness".

  • What if our Galaxy is a bubble in the space-time ether that flows through the river bed of the block Universe propelled like the accelerating ball Galileo/Newton first described.

  • The three body problem says that under some conditions the exact future cannot be predicted. The quantum theory had randomness built into it. As a result there is no already existing future. Unless a block universe is a singularity it will not be the same every place and any difference at all gives rise to time. The physicist who explores a block universe brings time into the block universe with him. If a block universe cannot be explored it is unknowable and once it is known it has time.

  • I vehemently disagree with the concept that we remember the past but we do not remember the future. we know that our brain literally simulates near future events extremely accurately. if time is an infinitely complex picture that has always had a definite past present and future (in this universe), then perhaps re-cognizing these future simulations (like imagining your hand in a certain position, knowing it's possible, and then moving it there) is just as valid as re-membering past events. it just appears that the veracity of this self-controlled event is trapped within the possibilities of our own cognition (the observation principle on the macro-quantum scale? I think therefore I am–rather, I plan therefore I know the future)

    perhaps our perceptions of the very words in our language are distorted by the illusion of chronology built into our intelligence. we have a finite duration, we remember past events, we can also know future events. in fact, there are even brain studies proving that the slow chemical reactions taking place in our brain begin secreting neurotransmitters before we even experience stimuli. so we definitely have memories of future events.

    something tells me that if we achieved biomechanical immortality and our duration seemed infinite, we stopped aging, or could take on whatever appearance we wished, and if we enhanced our post-human intelligence to remember the past with 100% fully sensory clarity, and predict the future (perhaps not with 100% accuracy, but a much much more accurate simulating ability than we currently have), then we might begin to acquire a more symmetrical concept of the fourth dimension instead of arrows pointing in directions, and then stop believing the illusion of time even more. seems like observation or the exotic physics within our skulls could be the key to solving at least part of this question. and mass.

  • Time is an uncertain figment of our imagination. Truth bye and bye, is absolute. They should have invited me. =

  • I am one of those curmudgeons that doesn't believe as much as I would like to. I do believe that matter cannot be created or destroyed. The reason I don't think we will ever have time travel is that the paradox would not be how you might affect history – it is that your atoms are arranged very differently and two copies of them cannot exist. As much as I would like to believe in bending space and FTL travel, we have not observed anything that requires it to be explained. I think time is very real and completely beyond our control. You can plug in whatever values you want for it in formulas because the rules are the same at every point in time. I don't think time slows down when you travel at incredibly high speeds; it is just our perception. At every point, all the same matter exists; some of it just has slower activity – the electrons' orbits take longer and chemical reactions take longer so aging and other biological processes occur slower relative to matter that isn't moving as quickly. Passengers on a space ship don't move through time faster; everything from the atomic level up is just happening slower.

  • Replace "block" universe with "hyper-cone universe" and the apparent issues w/ thermodaynamics goes away.

    Think about it. Our universe is a 4D CONE (actually, a parabolic cone since its 3D spatial volume grows w/ an acceleration term due to dark energy). The tip point is the Big Bang, the 'base' is the enormous- if not infinite- empty 3D spatial volume of the far distant future. Plain old geometry then gives you the 2 possible 'arrows' of time, the fact that a cone is not symmetric along this dimension gives you the special asymmetry time alone has among the 4 dimensions, and an explanation for why spatial horizons, time, & entropy share this deep connection. If we take Feynman diagrams seriously, antimatter is simply matter whose arrow of time is opposite ours, so you might even be able to explain the matter-antimatter asymmetry this way. The antimatter went into a parallel universe that in our reference frames is 'before' the big bang, connected to our universe (or half of the universe) only by the moment of the big bang. CP-violation in our cosmos is slight but observable because the global structure of space-time is asymmetric, but is always locally symmetric (think tiny spherical bubble in a giant cone).

    Not saying any of this is actually true, but the "block" universe implies a cosmos that in 4D looks like a loaf of bread, and we ALREADY know that isn't so.

  • Justifying time is moving forward in terms of scrambled eggs could be inappropriate. Becuase we are looking at only a small part of the universe. If you think egg made up of a number of particles, they dissociate based upon force acting on it, just as gravity which pulls it downward. At a particular relative position, the eggs will scrambles e.g. in the earth where gravity is there. But if you take it to microgravity, it will behave differently based on the forces acting there (again a relative position). So, if you want to make scrambled eggs into an egg, you need to find a position, where all the forces act in a way to make it an egg. But finding that position is very tough, because, of the number of possible arrangements of the particles of the egg. We need to find a position in the universe, where all forces act together in a way that scrambled egg gets assembled.

  • There will be no "equilibrium" on the entropy scale due to breaking eggs because other eggs continue to be self-replicating. All other measures of entropy have the same let-out.
    This is one of many "little-bangs" that serve to sustain our universe.

    Also, we perceive time to be one-dimensional but time itself could be a multi-dimensional experience if only our perceptions were widened.

  • @ Pentcho Valev:

    Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this striking example of time dilation proves that time travel (towards the future) is possible. As a gedanken experiment (if we neglect practicalities such as the technology needed for reaching velocities comparable to the velocity of light, the cost of the fuel and the capacity of the traveller to sustain high accelerations), it shows that a sentient being can jump, "within a minute" (of his experienced time) arbitrarily far in the future, say sixty million years ahead, and see, and be part of, what (will) happen then on Earth. This is a clear way of realizing that the future "already exists" (as we can experience it "in a minute")."

    So the statement "The future does not exist" entails that time travel into the future is impossible and Einstein's 1905 constant-speed-of-light postulate is false."

    No, the statement that "The future does not exist" is correct.

    The space traveler, with an Earth-bound twin, who returns to Earth "say sixty million years ahead" returns to Earth sixty million years after the death of his Earth-bound twin when the Earth of 60 million years ago is long gone — sixty million Earth years ago, in fact. The travelling twin has not time traveled, he has simply aged well. In fact he has hardly aged at all, but he cannot return to the Earth of his youth because that Earth ceased to exist 60 million years ago.

    So no, the Earth of today does not endure for ever, neither has it existed for ever. It exists only for a day. It is possible, however, under the right conditions of gravitation or acceleration for an individual to live very slowly relative to an Earth-bound twin.

    This refutes the Newton-Barbour concept of time "flowing uniformly". Time has no reality apart from the events by which it is measured. Thus, in different places, the evolution of like systems, for example the aging of twins, are not necessarily correlated.

  • I'm sorry, but this was one of the least enlightening articles I have read on time and the block universe in particular that I have read in the last several years. The descriptions of competing models are so vague they are virtually meaningless. If Natalie Wolchover had written this article, I'm sure I would have learned more.

  • The word "dimension" should mean a quality required for existence. If an object does not have width height and depth it can not exist, if its behavior does not have a past future and present it can not occur.

    This suggests that just as an object must have 3 spacial dimensions to exist an event must have 3 temporal dimensions to exist.

    Objects are such to the degree they do not change but stay the same, so they are defined by mass, events are such to the degree they do not stay the same but change, so they are defined by energy

    The arrow of time points away from the past toward the future while the arrow of space points away from vertical (up) and toward the horizontal (down)

    dividing the past from the future is the present, dividing the vertical from the horizontal is depth

    The past and the future of time require memory and imagination because they hide information outside information but the present requires only the senses

    The vertical and the horizontal of space require only the senses but depth requires memory and imagination because it hides information behind information

    The past and future of time as well as the depth of space hide information so 3 of 6 dimensions require memory and imagination, 2 of time and 1 of space

    the present of time as well as the vertical and horizontal of space do not hide information so 3 of 6 dimensions require only the senses, 2 of space and 1 of time

    since space and mass have 2 dimensions that require only the senses and only 1 dimensions that require memory and imagination space and mass are considered
    more "tangible"

    since time and energy have two dimensions that require memory and imagination and only 1 dimension that requires only the senses time and energy are considered less "tangible"

    It seems to me theory should proceed with an acknowledgment of these symmetries in mind

  • Karen Barad (2007) posits "matter does time and space". – vital or energetic matter performs time and space in her theoretical particle physics. She extends the work of Neils Bohr in intriguingly ethical ways.

  • In a physical sense time is a by-product of space, and doesn't really exist except as a perception of space. If all of creation existed as a singularity, there would be no time as there is no space. And if you compress or expand space, then you also compress or expand time (as General Relativity tells us, and time dilation has been proven experimentally on several occasions).

    In a metaphysical sense, all time exists simultaneously and that we as spiritual beings choose to perceive it chronologically because it's more fun that way…

    This is also the Schrdinger's Cat problem with time, does it exist when the box is closed, when it's not being observed?

    So you can only get to both time travel and a particle theory explanation of time thru the route of the metaphysical.

    But as you can't hold a conversation with an echo, once space has moved on, that's it, the moment has passed…

  • I liked the article. I have a few comments.

    1. The sense of now for vision is about 200 ms. Now has a rolling duration.
    2. We can't think about a block universe without introducing time.
    3. If gravity cannot be reversed then it will always measure time in the same way but what makes light radiate outward.
    4. If we experience it then it is real by definition. Reason is secondary to sensation.

  • 1) Atoms isnt "forever". For istance the heavy atoms come from explosion of supernova!
    2) If we intend TIME as OUR time, then S. Augustin wrote the truth.
    3) If we intend TIME as the "t" of the rules/fisical laws:
    The laws are only our facility forms, our tools to understand the reality: But they arent the reality!! Lee Smolin says also the rules are evolving with the relative conditions of the universe…
    4) One example of 3): the arrow of time is unidirectional due to the complexity of reality which is dissipative, no linear laws:
    PS excuse my bad english!

  • I believe gravity does give rise to time, it is well known through general relativity that gravity/acceleration directly alters the speed of times arrow, taking it a step further maybe the arrow of time in a sea of dark energy runs backwards as well as information travelling faster than the speed of light through quantum entanglement processes.

  • Space is real. Motion is real. Things that move in space are real. Time is not real.
    The notion of time is not required if there are no mental activities that require an analysis of things moving in space. Only life forms require this kind of analysis. A universe that has no life has no need for a notion of time because it has no need for an understanding
    of motion. We life forms use the concept of relative motion to understand motion and that is a concept that automatically injects the notion of time. Regards.

  • Entropy always increases according to the 7th paragraph, but Chandrasekhar has given an example where this is not strictly so. In a vessel filled with fluid having a layer of particles at the bottom, the particles will rise up from the bottom, the work being done at the expense of the internal energy of the fluid,
    i.e., it will cool off. Chandra gives the decrease in entropy per particle and discusses the fact that this process cannot be used to make a useful engine. The second law needs a more careful statement, found in many places, than the usual one mentioned in the opening sentence above. The entropy in question is given by Eq. 439 in Stochastic Problems in Physics and Astronomy, Phys. Rev. Vol. 15, No 1. This superb article was my bible for Stochastic Processes. It is reproduced in Wax's Noise and Stochastic Processes, Dover.

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