"Forget time" by Carlo Rovelli

I truly doubt that thermal time satisfies the properties that we usually expect from time. It might work for thermal equilibria, but what about states out of equilibrium? Especially those which are far out of equilibrium? If one place is hotter than another, does time run at different rates then? More disturbingly, let's say we have a plasma where the electron temperature is much much higher than the ion temperature. It will take quite some time before such a system will equilibriate, and so, it's possible to prepare such a system. Do the electrons experience a different clock rate from the ions then? At the same location? We know that the universe is clumpy; there are voids, superclusters, clusters, galaxies, stars, etc. So, the chemical potential varies from location to location (and so does the temperature). This means that the thermal time evolution has to be mixed with an approximately conserved baryon and lepton number symmetry which is "gauged" in the sense that it varies from point to point.

There's another huge problem with thermal time in a timeless universe with a thermodynamic arrow of time. If we choose to describe the ensemble at an earlier time, that would be different from describing the ensemble at a later time (based only upon the macroscopic degrees of freedom) because information is lost into microscopic correlations irreversibly. Which ensemble should we then use in a timeless universe? But if you say the earliest possible ensemble, then the flow of time would be way out of whack at "later times".

I think thermal time conflicts with the philosophy that time is what is measured by the correlation of partial observables with clock pointers. In a thermal equilibrium, clocks simply can't exist!

It seems to me that in terms of logical priority/primitivity, Quantum Theory (QT) should precede General Relativity (GR) as the more fundamental theory of Nature. Thus I believe we should take seriously what QT is telling us about Time and its peculiarities and try to derive GR as a macro approximation instead of doing it the other way round like the author is suggesting.

The Wheeler-DeWitt equation, upon which Rovelli's arguments (and very similar ones by e.g. J. Barbour) are based, is of questionable validity in reality due to the fact that we do not yet have any consistent theory of Quantum Gravity. Thus the "Timeless" world views inferred from it are of questionable validity as well, even if we don't consider the fact that these world views cannot be further from our physical intuitions...

I take back my comments about the ambiguity in choosing the ensemble when there's a thermodynamic arrow of time. It shows that I still haven't gotten the central concept of timelessness, namely that the only instant which exists is "now", and that statements like the future or past state of a given state make no sense in a timeless universe. Timelessness is a theory of instants.

"At the same time, the entry should differ substantially from any previously published piece by the author."

"If one place is hotter than another, does time run at different rates then?"

Yeah it would logically since the energy at that point would be greater, so spacetime would be "more curved" (according to classical general relativity) which results in time "running faster"...so there's a sort of "limit" to what we already know right there! It'd be interesting to relate this to the relativistic red shift, and so on.

Actually, this quoted criticism is a sign of a good theory of time in itself in my opinion.

Ming wrote: "The Wheeler-DeWitt equation, upon which Rovelli's arguments (and very similar ones by e.g. J. Barbour) are based, is of questionable validity in reality due to the fact that we do not yet have any consistent theory of Quantum Gravity."

Unless I am misinterpreting Dr Rovelli's paper, I think that he is calling the Hamiltonian constraint in general the "Wheeler-DeWitt equation" (as opposed to referring to the infamous one in the ADM forumalation of canonical general relativity).

In section V, where he discusses the "Wheeler DeWitt equation", it is merely some Hamiltonian constraint and nothing more. This is a constraint which always appears for relativistic systems, it seems to show up with relativistic particles and forces.

That's just my two cents though...

Hello Carlo,

Thank you very much for the paper, which I enjoyed.

However, instead of forgetting time, perhaps we should forget quantum gravity for a moment? For while time manifests itself throughout classical, relativistic, and quantum mechanical physics and our empirical reality, the graviton has never been seen.

Do we have to quantize gravity? Could it be that nature is as it is, and that God or the Prime Mover/Creator came up with both QM and GR, which seem to coexist perfectly well in their current forms? For instance, this laptop computer is powered by quantum phenonema, and too, it is held on my lap by gravity. Each one has a role, and each seems perfectly content to play it. Perhaps both mathematical predictions and the experimental search for gravitons has fallen short because gravitons do not exist. Now this is no reason to stop looking, but too, it is not exactly a reason to keep looking, and it is certainly not a reason to get rid of time, which does seem to exist, as my laptop's clock tells me I am running late, yet again. :)

A book you would enjoy is Freeman Dyson's THE SCIENTIST AS REBEL. On page 219 Freeman Dyson writes,

"(Brian) Greene takes it for granted, and here the great majority of physicists agree with him, that the division of physics into seperate theories for large and small objects is unacceptable. General relativity is based on the idea that space-time is a flexible structure pulled and pushed by material objects. Quantum mechanics is based on the idea that space-time is a rigid framework within which observations are made. Greene believes there is an urgent need to find a theory of quantum gravity that works for large and small objects alike. . . As a conservative, I do not agree that a division of physics into separate theories for large and small is unacceptable. I am happy with the situation in which we have lived for the last eighty years . . . The question I am asking is if there is conceivable way we could detect the existence of individual gravitons. I propose as an hypothesis that it is impossible in principle to observe the existence of individual gravitons." --Freeman Dyson, THE SCIENTIST AS REBEL, pp 219-220

And yet, it is possible to observe, contemplate, experience, and witness time in multiple manners and ways.

I vote that we keep time. :) All I know is that we never have enough of it.

My simple theory--Moving Dimensions Theory--views time as a phenomenon that naturally emerges because the fourth dimension is expanding relative to the three spatial dimenions. Change is fundamentally woven into the fabric of spacetime via dx4/dt = ic, which makes sense, because change is fundamentally woven into our everyday existence, empirical observations, and all branches of physics! Indeed--it would not be possible to make a measurement without change! A great thing about MDT is that it allows us to keep all of relativity while unfreezing time and liberating us from the block universe, which is yet a meaningful artefact that arises from certain interpretations of relativity. And who knows, perhaps MDT will tell us something about quantum time, which will tell us something about quantum gravity. For MDT also provides a framework for quantum entanglemnt and nonlocality, and thus it provides a *physical* model underlying qm's inherent nonlocal, probabilistic nature.

Think about MDT as a simple *physical* unification of relativity and QM--both entanglement and nonlocality can be accounted for via the same principle that ensures a photon does not age, no matter how far it travels. A photon's timelessness, implied by relativity, represents a nonlocality in time. Both quantum entanglement and the agelessness of a photon descend from a common principle: dx4/dt = ic. A photon is matter that "surfs" the fourth expanding dimension, and thus it remains in one place in it, while traveling through the three spatial dimensions at c. Perhaps this is MDT's simplest proof: The only way to remain stationary in the fourth dimension is to move at c through the three spatial dimensions: egro, the fourth dimension is expanding relative to the three spatial dimensions.

And a great thing about MDT is that it also presents a *physical* model for entropy, as briefly elaborated on in my paper:

http://fqxi.org/community/forum/topic/238

MDT represents the kind of tehory we have not seen for awhile--a simple postulate and a simple equation that present a novel aspect of the universe--the fourth dimension is expanding relative to the three spatial dimension: dx4/dt=ic. This fundamental invariance underlies the invariance of the speed of light--both the constant velocity of c meausred by all inertial observers and, the constancy of c that is independent of the source. MDT also underlies relativity's two postulates, and all of relativity may be derived from its simple principle of a fourth expanding dimension.

In your conclusion your write, "It is possible to formulate classical mechanics in a way in which the time variable is treated on equal footings with the other physical variables, and not singled out as the special independent variable. I have argued that this is the natural formalism for describing general relativistic systems."

But when we look at Einstein's 1912 Manuscript, we see that time plays a different role from position. x1, x2, x3 represent the three spatial dimensions, which we generally use to demarcate position. And then along comes x4, which Einstein equates with ict. So as t progresses on our watches, x4 must progress. Time is very, very different from the three spatial dimensions! Perhaps it is not a dimension after all, but a parameter emerges because the fourth dimension is expanding relative to the three spatial dimensions, as suggested by x4=ict.

In your conclusion you also write, "The peculiar properties of the time variable are of thermodynamical origin, and can be captured by the thermal

time hypothesis."

But what underlies the "thermodynamical origin?"

MDT and dx4/dt=ic underlies time's thermodynamic arrow, and in my paper I account for and unify all of time's arrows and assymetries with MDT's simple postulate and equation. And in addition to this, all of relativity may be derived from MDT, while qm's entanglement and nonlocality are explained with a *physical* model, along with entropy.

Thanks for the paper Carlo!

As I write this, the DOW is down by 777! I have to run, so I apologize for any typos, as I only had time for one quick proofread. And once posted, these posts remain forever frozen in the past. :) There is no way to go back and change them--another proof that time travel into the past is impossible, which MDT agrees with.

Best,

Dr. E :)

Do a search at NSF on "time quantum gravity."

http://nsf.gov/awardsearch/piSearch.do;jsessionid=7F7E1A09E64B392775A7855BA8211E51?SearchType=piSearch&page=1&QueryText=time+quantum+gravity&PIFirstName=&PILastName=&PIInstitution=&PIState=&PIZip=&PICountry=&Search=Search#results

You will see that using quantum gravity to understand time is nothing new.

The research has received millions upon millions of dollars, and yet has produced abosolutely nothing but for aging quantum gravity regimes. NSF is just the tip of the iceberg.

All these millions upon millions, and yet, there is no:

1) graviton

2) consistent theory of quantum gravity, nor anything even close

3) any reason to go on

And yet, as the purpose of fqxi is generally to

1) fund well-funded, institutionalized crackpottery &

2) recreate physics in old physicists' image,

I imagine any essay that mentions time and quantum gravity will receive an award or two from the ruling pseudo-physicists.

Quantum Gravity Conspiracy Theorist,

Your poorly copied URL link returns the following:

"Nothing found to display."

I doubt this is your fault; instead these multi-millionaire physicists deleted all records to throw us off their trail. Congratulations you've earned a scooby-snack for all of your detective work. It is a shame that earlier generations fell for the "ether" hoax and wasted all that money on the Michelson-Morley experiment. Everyone should write their congressmen and tell them not to support null experiments before we spend any money on them.

I try to reply to the various posts.

A certain number of posts raise a question that in my opinion is a very good and a very important question. The question, that some of the post present as a strong objection, is that the hypothesis of thermal time is not good, because it only deals with thermal equilibrium, while we need non-equilibrium states to have non-trivial time phenomena. I think that this point is well taken, but also that I have an answer, which is the following. The thermal time hypothesis does NOT state that the only relevant quantities in the description of a system are thermodynamical. If it was so, I would agree that in an equilibrium state there would be no way of seeing time flow. Nothing happens to thermodynamical quantities, in equilibrium. However, all the usual dynamical variables exist and have their dynamics, even if immersed in a thermal state. To make this precise, consider for instance quantum field theory at a given temperature. This is a theory about a situation where there is a temperature, namely a state of equilibrium. But quantum field theory at a given temperature allows us to compute scattering amplitudes, propagation, et cetera, namely the same quantities as quantum field theory at zero temperature. Why? because it describe dynamical phenomena (say scattering) when there is an overall thermal bath "around" these phenomena. This is the context in which the thermal time hypothesis make sense. I am not saying that we only measure equilibrium thermal quantities. This would be manifestly in contradiction with everything we measure. I am saying that we measure dynamical phenomena (that, correlations between observables quantities) in a context in which there exists a thermal bath. (Concretely, this bath exists for real, given by the cosmic background radiation at 3K.) More technically, the thermal time hypothesis regards KMS states. Once a KMS state is given we can nevertheless measure quantities such as the correlations that are in fact in the very definition of the KMS states. These are explicitly time dependent. It is like observing "departures" from equilibrium, and study the way the behave.

A different version of the same question is formulated by the posts that ask whether bodies at different temperature define different times. I am not sure I have an answer to this point. But it important to recall that the thermal time hypothesis does not REPLACE dynamics. My entire point is that dynamics can be expressed as correlations between variables, and does not NEED a time to be specified. The thermal time is only the one needed to make sense of our sense of flowing time, it is not a time needed to compute how a simple physical system behaves. The last can be expressed in terms of correlations between a variable and a clock hand, without having to say which one is the time variable. Therefore the question about the flow of time defined by bodies at different temperature is a question about thermodynamics out of equilibrium. Unfortunately, like much of today's physics, I have not much to say on this. In any case, I am aware that the thermal time hypothesis is highly speculative. I would like the readers to keep it separate from the main idea defended in the essay, which is that mechanics can be formulated without having to say which variable is the time variable.

Another series of posts questions the validity of the Wheeler-DeWitt equation. I agree that at present we cannot rely firmly on a well defined version of this quantum gravity equation. But this is not what I mean when I talk about the hamiltonian constraint that define the dynamics. I refer much more simply, to the fact that ALL dynamical systems (classical) can be simply reformulated in a way that puts time on the same ground as the other variables, an din this case the dynamics is expressed by a "Wheeler-DeWitt-like" constraint. This idea is much older than me. I have only developed it and I have tried to use it as a general way to reformulate dynamics, without the need of singling out time.

One posts quote the phrase "At the same time, the entry should differ substantially from any previously published piece by the author." I suppose that this is a criticisms to the point that what I have written reflects things I have already published. I fully accept this criticism. In my understanding, the FQXi context is not the place for totally novel ideas. I suppose an scientist with a totally new idea would publish it on PRL, rather. The beautiful aspect of the FWXi context is that it opens a forum for focusing and expressing clearly the ideas that are around, articulating them and discussing them. I had never previously written a compact exposition of my ideas on time, which are either scattered in papers, or that require reading all my book, which technically heavy and the ideas on time are submerged by technicalities that have to do with quantum gravity and not just with time. In this sense, the essay for FQXi is, among my writing, strongly original. Of course parts of it do nothing but repeating points I have published elsewhere.

Dr.E. ha posted a friendly note, arguing that perhaps gravity needs not to be quantized, quoting Dyson on this, and pointing out his "Moving dimansion theory". I thank him for the indication. I'll look at it (I suppose this is one of the purposes of the Forum). I have also immense admiration for Freeman Dyson, but I think Freeman is wrong here. The need for quantum gravity does not come from the fact that we are disturbed by two different theories, and has little to do with seeing or not the graviton. It comes for our concrete interest in certain real phenomena, which we would like to study, and we have no theory for them. The main one is the early universe. If we take our gravitational theory seriously, as Freeman demands, we must conclude that the universe was in a state of over Planckian curvature sometimes in the past. On the one hand, we know for sure that generally relativity goes wrong in this regime. On the other, we have a simple explanation for this going wrong: quantum effects on the gravitational field are disregarded. Therefore the happy coexistence of general relativity and quantum mechanics, which was perhaps possible when Freeman was a younger man, and we knew less about the universe, is very, very hard to defend today. Either we find something else, or we are in the dark.

Then there is a post on the waste of public money on research about time in quantum gravity. I take this seriously. Often at conferences I listen to talk after talk, and I wonder "is public money wasted here"? Maybe yes. But was it wasted public money the money that the Ptolemy's Kings put in Ptolemy's astronomy? Or that the Church put in supporting Copernicus completely useless searches? Or that supported Maxwell and Faraday, Shcroedinger or Einstein? No, it clearly was not. Is there a way to chose a priori who will be next Dirac? No, there is not. Research needs courage, wasted time and money, false directions. The history of our civilization is the proof that all this money is not wasted, in my opinion.

Carlo Rovelli

Dr. Rovelli,

I would agree time is a measure of motion, similar to temperature, but that doesn't clarify how the effect emerges. Temperature is like a parallel processor, with lots of activity and interacting elements. Time is a serial processor, with the point of reference/observer/hand of the clock proceeding though a series of intervals/events. Observer and observed co-exist, as all elements in the thermal medium co-exist, so there is no dimensional projection out from this state. As a serial processor consists of innumerable serial processors, innumerable clocks exist in a thermal medium, as every point of reference moving in this thermal state constitutes the hand of its own clock, while all other points are face to that clock and hand of their own clock. So while all these points move from past events to future ones, the events, once created, are replaced by the next and so go from being future potential to past circumstance. Just as tomorrow becomes yesterday.

Correction: fifth sentence; As a parallel processor consists of innumerable serial processors....

This essay contest is wonderfully explaining how individuals have widely varying interpretations for a given phenomenon. This is parallel to a common man's observation when he listens to different eyewitnesses who happen to see the same event but their their descriptions vary substantially. Absolute truth appears a mysterious quantity and science can only describe relative truths about a process/phenomenon that change with time. There lies the mystery about time too.

Yoga is a technique to quieten the mind. It was evolved by a saint in India thousands of years back. That scripture has two relevant quotes about the search for truth. One states ' The cognizor, the process of cognition and the object of cognition must all merge in order to find the truth about the latter. Another states' there are mental distractions like ego or I-ness, ignorance, pleasure and pain which prevent the human thought processes from reaching the highest level required to get to the truth.Thus, without extreme humility and contemplation in 'silence', it is well-nigh impossible to know the 'truth'.

This comment is posted here but is too general to be applicable to all the contributors including the self!

John Merryman's post says:

"I would agree time is a measure of motion, similar to temperature, but that doesn't clarify how the effect emerges." and then observes that temperature does not have the sequencial aspects that characterize time.

I thinnk that there is something vey interesting in this post, when it says "doesn't clarify how the effect emerges". My reply: "which effect?" What is the "effect" we are talking about here, and trying to understand?

The central point about all my ideas about time is to distinguish two different "effects". One is the observed fact that there are laws of nature that fix how variables are correlated. My claim is that this perfectly captured by our dynamical theory, but these theories do not truly distinguish "time varables" or "clock variables", from the other variables. This is effect one and point one.

But this is a bit unsatisfactory, because we have an idea of time as something "flowing", something very peculiar, very different from other variables. This peculiarity, this flowing, is effect two. So, the "effect" that remains to be understood is the peculiar impression we have about the flowing of time. The thermal time hypothesis states that this effect two is of thermodynamical origin. This is not my idea: it has long been remarked that all our impressions of "flowing" time are related to thermodynamics: only in a thermodynamical situation we may have irreversibility, for instance, and we may have memory. Eddintong has remarked that a periodic clock is not truly a clock unless we can count the cycles, namely unless we break periodicity. And we introduce some sort of irreversibility. Eddington noticed that the true prototype of a clock is not a pendulum: it is a burning candle (we ourselves are burning candles). A burning candle is a thermodynamical phenomenon. Going towards the future is mouving towards thermodynamical equilibrium.

So, summarizing: all temporal "effects" that are captured by ordinary mechanics have nothing to do with thermal time. They just have to do with the fact that there are laws that govern the relations among variables. The additional peculiar "flowing" of time is an "effect" which is not the same thing as temperature, but (if we believe the thermal time hypothesis) it emerges in a thermodynamca;/statistical situation only.

Carlo Rovelli

Dr. Rovelli,

My point is that time and temperature are both descriptions of motion. Temperature is the level of activity against a given scale. Time is the rate of change relative to a given reference frame, or point. If you change the level of activity, you affect the rate of change. The candle burns faster if it is hotter. As a person in space ages slower than a person in a stronger and more active gravity field. So there is the element of time in temperature and the element of temperature in time.

Time has this sense of "flowing" because we exist as individual points of reference in a larger context, just as individual atoms of water move about in their fluid context, the whole of which is measured as temperature. Just as economic statistics function as a temperature reading of masses of human activity. So to the extent we "move" or "flow" within our medium, it moves the opposite direction. To the hands of the clock, the face moves counterclockwise. This manifests in the larger scale by the events which all these points of reference collectively create go from being in the future to being in the past, as each is replaced by the next, while those references go on to future events. Tomorrow becomes yesterday.

Molecules of water.

Dr. Rivoli,

Your idea to discard time as a variable appears fine. However, the concept of time appears hard to discard. It seems to me that the quantum conjugation of time with energy provides a possible way out of the problem of time. Initial distortions of time may have resulted in the release of the energy content of the Universe. Similarly, the space distortions provide the momentum, a combination of mass with motion. Without any motion or vibrations, the Universe will become meaningless. Pure, vibration free consciousness can not give rise to either mass or energy.

Conceptually, it may be possible to replace the time with some other physical quantity and still retain the physical explanations for various observed phenomena. you have proposed some kind of thermodynamical variable to replace time. However, i am unable to comprehend the basis and depth of your argument. I am an experimentalist among most of the essay authors who are theory experts! However, science is neither theory or pure experiment. It starts with some precepts based on observation and experience of the scientist. These are then translated into some viable logical concepts. After this gets done come the question of choosing the variables. The nature of variables again depends on what we take as dependent or independent entity. There can be a subjective element here. As Einstein often expressed that in spite of proposing the quanta of energy, he was unhappy with the quantum mechanical explanations of the phenomena that is entirely probabilistic in nature.

The nature can not be purely governed on such a consideration as it exhibits order, logic and symmetries, besides elements of random nature o/c impossibility of measurement on single event to study a process! Thus, the need for measurement requires averaging over several independent events. Order contains disorder but not vice- versa. Also, silence contains noise but not vice-versa. How to work out this dilemma, appears to be challenging in order to work out a single theory to explain everything!