Essay Abstract

Intended to offer a fresh look at a venerable chestnut of speculation, the essay describes why time travel of the variety portrayed in science fiction is not possible, and, in so doing, offers thoughts about time which are intended to shed light on its fundamental nature.

Author Bio

J. C. N. Smith is retired from the CIA's former Office of Scientific and Weapons Research. Reading, thinking, and occasionally writing about issues related to time have been his avocation and passion for more than 40 years, with specific aims being to gain a deeper understanding of the universe and its workings and to peel away misperceptions which may be impeding advances in modern physics. He has published several monographs on the nature of time.

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  • [deleted]

A very interesting and clear written essay and I like it very much (though i cannot agree to every conclusion within it).

The notion of time as changing configurations of matter is intuitive. The question for me is, can all the possible configurations exist "simultaneous" with the help of quantum mechanical superpositions? This would be the case if some concepts of the multiverse would be true. Nonetheless in such a multiverse - i only guess - the most universes would have their own "flow of time". So why need changes in the configuration of matter *time* at all? This is a somewhat stupid but interesting question for me, because the notion of "changes" implies "time" and the question is: Why can - or *must* - there exist such things like "changes" at all (and are "changes" a relative or an absolute property of all_that_exists)?

Thank you for your comments. Have you read Julian Barbour's book 'The End of Time'? If not, I highly recommend it. He addresses issues relevant to those you've raised. Definitely worth exploring.

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Thanks also for your book reference. I read Julian's essay on the nature of time contest and it's a really great pleasure to read, contemplate and enjoy his findings. I also saw the interview of him with Graig Callender at blogginheads.tv.

Barbours thoughts inspired my own thinking about certain issues that encircle my questions and possible answers to this questions. I will try to get *End of time* in the next weeks, i didn't read it yet, but the table of contents looks exciting and with high probability holds what his essay on fqxi promises. Though i also cannot agree to every conclusion Julian Barbour made in his video interview, i feel that it is a must to read his point of view properly.

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I have to say you have got a very nice essay and I agree with you that there is no time travel. However, your argument is incomplete in the following sense: in general relativity space-time can be bended back to itself, and while in every point the local causality still applies, globally it may not. Back to your argument, the calcium atom in the dinosaur can exist both in the dinosaur and in the time traveler's tooth at the same time. As long at the time traveler ultimately returns to its own time and there are no paradoxes created by this peculiar state of affairs, all is OK. For another argument against time travel and other references for and against time travel, please see my paper: http://arxiv.org/abs/0704.3074

In his book 'The Trouble With Physics,' Lee Smolin wrote, "More and more, I have the feeling that quantum theory and general relativity are both deeply wrong about the nature of time. It is not enough to combine them. There is a deeper problem, perhaps going back to the origin of physics." (page 256) This is precisely the point which I make, explicitly, in reference 4 to my current essay. See http://googlepages.com/time . This is why I believe we must question the validity of notions such as "space-time bending back on itself." That may be true IF you accept the conventional concept of time, but it is exactly this conventional concept which I believe we must re-examine and re-think.

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I am not convinced Smolin is entirely right in his assessment. For quantum mechanics, I do not think there is anything wrong or incomplete with it. For general relativity, in the classical domain the theory cannot answer the deep foundational questions because we need quantum gravity for that. I agree we need a better fundamental understanding of the concept of time and here Smolin is right: the answer seats at the foundation of physics. I think I have a good answer for that in my essay: "Heuristic rule for constructing physics axiomatization". Please feel free to comment on my interpretation.

About the time space bending back on itself, those are exact solutions of general relativity which are possible because the general relativity equations are local equations. Because they are so many of those solutions, they cannot be simply brushed aside. The (yet unsolved) quantum gravity challenge is to explain why those solutions will never occur in the real world. Hawking offered a partial answer to this, but the issue is still not decided.

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Mr. Smith, how can your assertion be falsified? If it cannot be falsified then can it be 'scientific' and, if not, physics? Thank you.

Mr. Huffman,

Thank you for your comment! You raise a good point! And I have what I believe is a good reply to the point you raise. There is more to my proposed definition of a particular time than first meets the eye. Although I did not think it was appropriate to address all the ramifications of the proposed definition in this essay, I did address some of them in reference 4 to the essay. The proposed definition leads directly to logical conclusions which do lend themselves to falsification! Please see http://smithjcn.googlepages.com/time Thank you!

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So your essay here is incomplete or does not lead to logical conclusions?

How are we to evaluate the essay if it is incomplete? In the logical extreme we would have read each author's entire oeuvre.

It is not my intention to be argumentative. Thank you for addressing my peculiar concern with falsification.

Doug Huffman

Washington Island

Wisconsin

The theme of the current essay cmpetition concerns what is or is not possible in physics. The additional information to be found in my referenced essay does not bear directly on this theme, and hence I chose not to address it here. You could be correct to question the wisdom of that decision.

Of course another obvious way to falsify the assertion of my current essay would be to demonstrate that someone has in fact accomplished time travel of the variety portrayed in science fiction! Thanks again for your comments.

5 days later
  • [deleted]

I enjoyed the essay & basically agree with the premise that what we call "time" can only make sense in the context of relative configurations & motion. However, I think that for a complete alternative description, we need to specify what it is that allows or facilitates this motion/change. Unless we invoke Julian Barbour's "timeless" world, denying real motion altogether in favour of static configurations which transform under a "principal of least action" & described by a static/time independent wave function, then we must define the dimensional structure? in which motion takes place.

This is motivated also by the fact that it does not seem possible to fully describe the world in just 3 dimensions, that is, an "event" cannot be completely defined with just 3 (spacial) numbers. In your scenario the 4th "number" would relate to the particular unique configuration corresponding to some specified date/period. One reason why I see this as incomplete is due to Relativity, where there is no "special" or "preferred" frame of reference for the configuration to be specified in! (think of the "twin paradox"). This time dilation effect due to relative velocities for instance allows for the "future" to, in a sense, be observed/accessed without causal violation & taken to it's maximal conclusion (ie the speed of light), in principal allows for the whole future of the Universe to be observed.

Linking the dimensional structure with these Relativistic effects shows that any object has in a sense an "extended existence" potentially through all inertial frames of reference.

Quickly summing up, I feel that a more realistic description of what we perceive as time, might involve a kind of "velocity space" wherein all events exist.

Any thoughts on this please? Thanks again for the interesting essay.

Roy J.

Mr. Johnstone,

Thank you for your comments, which certainly are on target. I'm currently about three quarters through the process of very carefully re-reading Barbour's 'The End of Time,' and would like to withhold a direct reply to your points until I've completed this review. My views of time are something on the order of 98 percent aligned with Barbour's on most issues, but I am yet to be convinced of his claim that the universe is static. His description of what he calls the Machian distinguished simplifier in Chapter 7 seems to offer a way to explain dynamic phenomena without recourse to the notions of absolute space and time. This hope later appears to be dashed, however, in Chapter 17.

One very quick comment on your observations regarding relativity: Barbour correctly points out (on page 137 of The End of Time) that, "Relativity is not about an abstract concept of time at all: it is about physical devices called clocks. Once we grasp that, many difficulties fall away."

If you like the approach taken in my current essay, I'd urge you to read my reference 4, which offers a slightly different approach to the broader topic of time. As noted elsewhere in these posts, that essay does not address any implications for time travel, per se, so in the interest of staying on topic for this competition, I did not allude to some of the interesting (to me at any rate) ramifications of this approach to time which are addressed there.

I'll try to offer a better response to your comments after completing my review of The End of Time.

Mr. Johnstone,

Having now finally completed my re-reading of Julian Barbour's 'The End of Time,' (TEOT) I will do my best to comment on the excellent points you raised.

As noted in my previous post, my thinking is closely aligned with Barbour's on many points. Where I part company with his thinking is on the major and crucial subject of motion. He argues that the universe is static. I maintain that motion is real. Which brings us to the point you raised: how can we describe motion in a three-dimensional universe without recourse to some additional dimension? I believe that Barbour offers a path to accomplishing this in his Chapter 7, in which he describes his collaboration with Bruno Bertotti to lay out a genuine Machian theory of the universe.

If there is motion, as I maintain, then how do we describe the 'trajectory' of the universe from one configuration to another without recourse to an absolute time and space? On page 120 of TEOT, in describing what he has called the "Machian distinguished simplifier," Barbour writes, "When this distinguished simplifier is used as 'time', it turns out that each object in the universe moves in the Machian framework described above exactly as Newton's laws prescribe. Newton's laws and his framework both arise from a single law of the universe that does not presuppose them. In such a universe, the ultimate standard of time that determines which curve is traced by Galileo's ball when it falls off his table in Padua is unambiguous. It is the average of all the changes in the universe that defines the Machian distinguished simplifier. Time is change, nothing more, nothing less." The goal, as I see it, is to have a dynamics which describes how the three-dimensional universe moves from one configuration to another without requiring the a separate dimension for time.

Barbour writes (p. 69), "Of course, to say that time has passed, we must have some evidence for that. Something must move. That is the most primitive fact of all." Of course, I fully concur with that view. But I do not take the point for granted. Another "primitive" fact which misled us for centuries was the apparent rising and setting of the sun; this, like the concept of motion, was something we could literally see with our own eyes. And yet it was fooling us into holding a faulty perception of the universe. Could our perception of motion be simply another trick of nature? Perhaps, but I do not think so. We corrected our faulty perception of what it meant to observe the sun apparently rising and setting when we obtained more information and a better perspective in the form of empirical, astronomical observations. I do not foresee that happening with our perception of the reality of motion.

As to the point you raise about the "twins paradox," I would simply reiterate my previous comment, echoing Barbour, that relativity deals with the behavior of mechanical devices which we call clocks. I concur totally with his comment (page 107) that ". . . there is only one ultimate clock: the universe." The twins must be put into this larger perspective to see what happens with them and with their clocks, relative to the bigger clock of the universe.

Now, I must admit that I was somewhat shocked, for lack of a better term, to re-discover a brief section in the epilogue of TEOT which deals specifically with the possibility of time travel! How could I have forgotten it? The answer to this question is, I regret to say, that his discussion of the topic is, sadly, in my opinion, quite forgettable. It is almost as though Barbour does not believe in the reality of his own theory of time! He states (p. 68), "What is the reality of the universe? It is that in any instant the objects in it have some relative arrangement." This is precisely my view! Barbour's "instants" or "Nows" clearly correspond with what I call "particular times." But whereas I see these relative arrangements of all the objects in the universe in a literal, real, naive, primitive sense, Barbour seems to see them as being abstract, mathematical concepts, just one of many possible hypothetical configurations in his relative configuration space of Platonia.

I sometimes have the impression that Barbour and I are looking at the universe and at the concept of time as though we were looking at a Necker Cube, an optical illusion in which it is possible to look at the same physical picture and arrive at two different interpretations of reality.

I hope this at least partly addresses the points you raised.

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Mr Smith

Thank you for your excellent & prompt response! I can see I must also revisit TEOT, having read it quite a few years ago. By the way, I was remiss in my previous post in not stating that I agree with you that time travel,in the Sci-Fi sense, ie possible causal violation, is not possible.

Have you read any of Barbour's papers or seen his mathematical formalism for deriving the "best matched" configurations? As I recall from the book, he uses a time-independent Schrodinger equation to achieve a static wave function which would "encompass" the possible configurations represented by points in his "Platonia". Then I assume he must remove the kinetic term from the Hamiltonian & work with just the potential. This would give rise to all the "changes" that define his "Machian distinguished simplifier". I recall having trouble reconciling this mechanism, which would give rise to zero energy states only, with the fact that, as he himself I think stated, locally or in any arbitrary sub-system of the Universe, there can be positive total energy, even though the Universe globally could have (must?) zero total energy!

Also, what does it mean to say that time is "the average of all the changes in the Universe" as defined by his "MDS"? I can understand him wanting to do away with local dynamics in favor of global configurations.....but an average?

This relates directly to my "twin paradox" point which you have responded to by quoting Barbour's statement of relativity dealing with physical clocks. This is certainly true to an extent. However, I raised this point directly in response to your suggestion that unique configurations represent specific times & I believe that you are still left with the problem of specifying which clock? Our observation of relativity is derived from our use of clocks, but I see that as secondary to the primary effect due to the absolute & reference frame invariant speed of light. I don't think you can simply say that relativity is about physical clocks.

I share your desire to find a model of reality which can be described purely in terms of relative configurations which still possess kinematics, true motion. I seem to recall Barbour using the example of a Kingfisher in flight to explain firstly, the difficulty of accepting that it isn't really moving and secondly, the way our minds give the illusion of motion by the use of "snapshots" of previous configurations which best match the current postion of the Kingfisher. This is something like Jim Hartle's IGUS hypothesis. But I can't help wondering, why does the Kingfisher have wings in the first place? Are Darwinian laws compatible with a static Universe? Can natural selection operate in the absence of classical kinematics?

Your Barbour quote "Time is change, nothing more, nothing less" prompted a thought experiment to pop into my head. What if the Universe underwent a "global boost", that is a uniform change of scale. As sub-systems we could have no way of knowing this was occurring. In Barbour's scenario, would this constitute a change in configuration? If yes, how would a notion of time arise & how could he derive an "MDS"? If no, how does the re-scaling occur in the absence of time or in his alternative framework?

I think, as you do, that we need a model which includes relative motion but can be formulated in a way Richard Feynman would like. Where the time components & time operators of our theories are replaced with relative displacements.

Thanks again for a thought provoking essay & good luck with the contest!!

Roy J.

Mr. Johnstone,

Thank you for your thought-provoking comments, questions, and good wishes. I hardly know where to begin a reply, but perhaps a broad comment on my own background would be helpful to put things into perspective. First, I am not a professional physicist; I am a layman who likes to imagine himself as following in the footsteps of a long and honorable, but perhaps vanishing, line of what once were known as "citizen scientists." I mention this by way of responding to your question about whether I've read Barbour's papers or seen his mathematical formalism for deriving the "best matched" configurations. Let me say that I have looked at the papers which he has kindly made available on his website, but I will not pretend that I have been able to follow all the mathematics presented in the papers. This might explain why citizen scientists are a vanishing breed; science is becoming so specialized and complicated that few amateurs can truly master its intricacies.

My perspective on these topics is more qualitative than quantitative, a perspective which I hope may include a few advantages along with its obvious disadvantages. There is no risk of my being led astray by pursuing the siren song of elegant mathematics purely for its own sake, for example. I rely heavily, but not exclusively, on popularizations such as 'The End of Time,' 'The Trouble With Physics,' etc., to stay abreast of current thinking. Which reminds me that TEOT is now already a decade old. Perhaps we're about due for an update?

As to your question about why the Kingfisher has wings if there is no motion, that is indeed an excellent question. And why do we have legs, arms, and opposable thumbs? The argument for a static universe apparently is summarized thusly, "A classical theory that treats time in a Machian manner can allow the universe only one value of its energy. But then its quantum theory is singular - it can only have one energy eigenvalue. Since quantum dynamics of necessity has more than one energy eigenvalue, quantum dynamics of the universe is impossible. There can only be quantum statics. It's as simple as that!" (p. 253, TEOT) This is a case in which I fear we perhaps are being led astray by either the theory or the math or an unholy combination of both. On balance, I believe the evidence in favor of motion is stronger than the evidence against it.

Regarding your thought experiment involving a "global boost" in the form of a uniform change of scale, it appears that Barbour is toying with just such an idea even now. See his 'Dynamics of pure shape' under the heading Ideas at his website. Not having given it a great deal of thought, my first question would deal with gravity. Assuming the masses of objects do not change, and if we can consider them as point masses, then if we double all the distances between them wouldn't the attractive force of gravity be reduced by a factor of four? Wouldn't that be measurable? Or perhaps I'm missing your real point?

As to whether it would constitute a change of configuration in Barbour's scenario, you are raising some tough questions! These are questions probably best posed to Mr. Barbour himself. My first thought is that it would depend on whether or not any measurable effect resulted from the change. If we truly had no way of knowing a change of scale had occurred, then according to Leibnitz's principle of the identity of indiscernibles, if two things are identical in all their attributes, then they are actually one. If a change has no measurable effect, how can we call it a change? But again, I may be missing your point.

I hope some day we will have good answers to all these questions, but it probably will not happen tonight; the hour is late, and I fear that my brain is rapidly turning to mush.

Cheers!

.

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Mr Smith

Thanks again for your response. These brief discussions have prompted me to already begin reviewing TEOT following which I will look at the papers on Barbour's website. Then, as you suggest, I will pose any questions I still have (I think there will be many!) to him. If you like I will let you know what, if any, response I get.

Your comments regarding the "citizen scientist" & the way science has become so highly specialised & complicated are right on the mark. It has resulted in, for example, the mother of all "elegant maths for its own sake" "theories" - "String Theory" which I think, as a description of reality, at best barely qualifies as a model because no-one knows what it really is!

I too am a layman & it is refreshing to discuss these deep issues with another non-professional, non-mathematical, non-specialist, particularly one who shares the same basic view of the "problem of time"!! It would actually be good to keep the lines of communication going on this issue, in a different forum of course (email perhaps).

But for now, I have a lot of reading to do!!!

Cheers

Mr. Johnstone,

I welcome the idea of keeping the lines of communication open. I don't know off hand whether FQXi offers a more private "back channel" for communicating email addresses or other personal information. If not, it might be a feature they should consider adding; other forums to which I've posted have offered this convenience.

In the meantime, this forum will serve well enough, so long as we stay more or less on topic. Like you, I am immersing myself in reading (actually mostly re-reading) several popular books which address our topic. I'm currently enjoying a re-visiting of Smolin's 'The Life of the Cosmos,' and will follow that with a re-read of his 'The Trouble With Physics.'

The latter book includes the following extremely enticing sentence: "More and more, I have the feeling that quantum theory and general relativity are both deeply wrong about the nature of time. It is not enough to combine them. There is a deeper problem, perhaps going back to the origin of physics." (p.256) This just happens to be precisely the point I have tried to make, very explicitly, in reference 4 to my current essay. And I proceed there to offer what I hope may be a first baby step toward correcting that early misdirection.

Another book which I would highly recommend, in case you've not already read it, is David Deutsch's 'The Fabric of Reality,' a key idea from which is, " . . . scientific discovery need not begin with observational evidence. But it does always begin with a problem. By a 'problem' I do not necessarily mean a practical emergency, or a source of anxiety. I just mean a set of ideas that seems inadequate and worth trying to improve." (p. 62) This, to my way of thinking, is an apt description of "the problem of time."

Elsewhere, Deutsch continues, "Given a shred of a theory, or rather, shreds of several rival theories, the evidence is available out there to enable us to distinguish between them. Anyone can search for it, find it, and improve upon it if they take the trouble. They do not need authorization, or initiation, or holy texts. They need only be looking in the right way - with fertile problems and promising theories in mind." (p. 94)

Now, from my own experience, I would add one caveat to Deutsch's comment: should a "better explanation" be discovered and proposed by someone from outside the mainstream of science, it is not always the easiest thing in the world for such a person to find a forum in which to present his or her ideas. That, in a nutshell, is the beauty of an open competition such as these made possible by FQXi. The competition offers a level playing field for ideas in much the same way as those offered in "open" competitions in sports such as tennis or golf.

Back on topic, when Copernicus and Kepler offered a profoundly new way of explaining our perception that the sun moves around the earth from east to west (an "obvious fact" which even a child can observe), their ideas gradually replaced the former geocentric picture because their ideas did a better job of explaining what we observe about the universe. The key is to observe well, and then find the best explanation for what we observe. At some point (probably not initially) there should be an "Aha!" moment in which the new way of seeing things suddenly becomes "obvious," and self-evident.

This is a long-winded route by which to return to Barbour's notion of a static universe. One of the principal features we observe about the universe is motion. So the question becomes whether Barbour's concept of blue mists shining at instants containing time capsules will ever achieve an "Aha! moment in which we suddenly recognize that this is a better way of thinking about the observed phenomenon of motion. In all honesty, it is difficult for me to envision such an outcome. In this case, I think that if theory contradicts observation we need to re-think theory, very carefully.

In my own scheme, we are allowed to retain the idea motion while still dispensing with time, at least as being something separate from configurations of the universe. Perhaps not surprisingly, I find this a much more palatable way to proceed. As a way of looking at things, however, it represents nothing other than a beginning, a baby step. Nevertheless, I am encouraged by the fact that it leads directly and logically to the falsifiable "prediction" of the equivalence of mass and energy, as explained in my reference 4.

With apologies for being so verbose, good luck with your reading. It sounds as though we both will be immersed in it for a while. I plan to follow 'The Life of the Cosmos' with 'The Trouble With Physics,' and then move on to re-readings of Brian Greene, David Deutsch, etc.

Let's do figure out a way to stay in communication.

Cheers!

At the risk of violating forum "etiquette" by submitting back-to-back posts (this ideally should be a dialogue or a multi-logue rather than a monologue) I would like to mention one other interesting (to me at any rate) ramification to the definition of a particular time which I have proposed in my essay (a particular time is identically equivalent to, and is completely defined by, and only by, a particular configuration of the universe).

This definition provides a clear and direct link between entropy and the so-called "arrow of time." According to the above definition, any observable change whatsoever in the configuration of the universe would constitute a change from one particular time to another. If the configuration of the universe were changing randomly, however, as it would in a state of maximum entropy, then time, too, would change randomly rather than "advancing" in the manner to which we have become accustomed to thinking of it.

A god-like observer of a universe having a configuration which was changing purely randomly (as it would at maximum entropy) would not perceive that the universe was "aging" in any meaningful sense of the term. Changing, yes; aging, no.

In order for time to change in a manner which an observer would interpret as "advancing" in some meaningful sense, the configuration of the universe must change in a manner which is (or which at least gives the appearance of being) non-random. For the universe to be truly and accurately characterized by non-random motion, however, it must be in a state of less than maximum entropy.

The universe in which we live is (fortunately for us!) at a state of less than maximum entropy, and its configuration is changing (evolving) in a more or less predictable manner according to some rule or rules which we strive to understand. It is this predictability of the universe's evolution which allows us to perceive what we have termed "the advance of time" or "the flow if time." Hence the correlation between increasing entropy and the arrow of time.

5 days later
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There best argument I have for not being able to access a distant past_time, is if I could travel back to one nano second "ago", there would be hardly any change in the local configurations of nearby local matter, the T-REX or Tooth-Time concept?..but if I could lean out a bit farther back say to 10 nano seconds ago, still little alteration to local"structure" configurations, or say ten ordinary seconds ago, there would start to occur an entropy "creep" , which would inflict subtle little atomic changes.

Now go back to the diosuars tooth, a tiny alteration to the "then", lets say I scrapped a few atoms of enamel away, then a nano-second later I would not see any change locally (the tooth would still effectivly be total/complete)..but as the nano-seconds started to pass by, leading to one second then 1 hour..one day..1 millenium, then I would most definately alter many paths, or configurations?

Thus the farther one goes back in time, the more alterations would creep into our present time at an ever increasing rate,to the point of a present time being totally unstable?.. and things would start to disappear quite rapidly.

Its the butterfly effect, but on an evolution scale, a small shaving of a dinosaurs tooth "then" ,would have grave consequences for the large scale matter configuration "now"!

It must also operate as a two way process?.. the local alterations made whilst trying to pierce and probe backwards in time, would themselves have a "cause" effect locally, thus you could never really expect to fing those pesky "T-REX" monsters in an "ago", think about the configurations of matter, and the specific configuration of the space_time it occupies, the eponential changes that occur would really mean your historical past T-REX, would have to vanish agian in the "ago" era also? well thats what I believe to be so.