About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Wednesday, January 19, 2011

New 5-minute Philosopher video: What about metaphysics?

Simplicio: Hello, Hypatia. I would like to talk to you about something we left unexamined last time

Hypatia: Hi Simplicio. You mean when we were talking about how philosophy makes progress?

S: Exactly. You said that perhaps even metaphysics makes progress. But you did not sound very sure.

H: Yes, you picked up on something interesting there, Simplicio. You know, I think metaphysics is a difficult field of philosophy, and even philosophers themselves have strong opinions about its worth.

S: But isn't metaphysics one of the classic areas of inquiry in philosophy? Perhaps one of the most fundamental, from which much else philosophy stems?

H: Yes, historically that is certainly the case. The word itself comes from the fact that Aristotle wrote the books on what today we call metaphysics after he wrote those on physics. And as you know, "meta" simply means beyond. This is somewhat appropriate, because much of the discussion in modern times hinges on what exactly science has to say about metaphysics, and if the latter can be reduced to physics.

S: Okay, but before we get into that, can you remind me of some of the typical issues studied by metaphysicians?

H: Well, metaphysicians are concerned of course with the existence of god, and indeed with the very concept of existence. They are also interested in the difference between universals and particulars, in the idea of causation, and in the concept of time. They discuss free will, personal identity, and the difference between realism and anti-realism.

S: Wow, that is quite a lot. I seriously doubt we will be able into all of that today

H: Indeed, but perhaps we can pick on a few examples that show how metaphysics has interesting things to say, and where it can't do without a strong input from science.

S: Yes, that sounds like a reasonable approach. What do you think is the question in metaphysics where scientists have the most to contribute?

H: That would be the concept of time. It seems that these days one simply cannot seriously talk about time without getting into deep discussions of general relativity and perhaps even of quantum mechanics. Clearly, that is an area where physicists have a lot to say.

S: True, but don't you think that philosophers can also contribute? For instance, we can ask whether time travel is physically possible, which is again a question for physicists. But we can also investigate the logical puzzles that arise from time traveling, and perhaps even think about the coherence of the very idea of traveling in time.

H: That's right, Simplicio. For example, philosopher David Lewis published some interesting discussions about backward causation and causal loops.

S: Ah, yes, I remember! Backward causation happens for instance if I should punch a time traveler before he gets into his time machine and goes backward in time. The bruise from my punch would form before I actually punched him!

H: Yes, while an example of causal loop would be a situation where someone goes back in time to tell his younger self how to build a time machine, so that he can go back in time to tell his younger self how to build a time machine, and so on.

S: Wow, my head spins!

H: Exactly. Anyway, these are interesting discussions about the logic of time travel, and physicists and philosophers can get together for a better understanding of the underlying concept of time itself.

S: Okay, what about an example of metaphysics where science has relatively little to say?

H: Well, there are several, actually. My favorite candidates are the concept of causation, that of free will, and that of personal identity. Causation, for instance, is something that science takes for granted, but philosophers have come up with different theories of what it means.

S: Right, beginning with David Hume's analysis, we have the regularity theory as well as the counterfactual theory. My understanding is that these are accounts of causation, not theories in the scientific sense, right?

H: Yes, they are meant to investigate what we mean and how we think when we talk about causality. The same goes for theories of personal identity or free will.

S: This is interesting, because it gets to the root of the difference in the use of theory in science and philosophy, right?

H: Good point, Simplicio. In science a theory is an empirically verifiable set of statements about how the world works. In philosophy perhaps we should use the word "account" instead, to indicate that we are interested in how to think about certain concepts, as well as in the implications of certain ways of thinking about those concepts.

S: Well, it would be interesting to hang around and discuss free will or personal identity, but it's getting late. Until next time, Hypatia.

H: Always glad to see you, Simplicio.


  1. "Aristotle wrote the books on what today we call metaphysics after he wrote those on physics."

    And in doing so, Aristotle proposed that the Universe as a whole has a purpose and that we exist as part of such a goal-directed scheme of things.
    Philosophers of science who now reject this view see our existence as the result of a blind process of evolution. As more than one of them has written, "strictly from reason we can not conclude that human beings are made with any particular purpose in mind. Their nature may be the result of random forces of natural selection and thus cannot, without further moral premises, determine how they ought to live."
    So has metaphysics ceased to be concerned with purpose since science finds no further reason to deal with Aristotle's thoughts in that regard?

  2. I've always thought that science supports the idea that one can travel forward in time (by traveling in space much faster than others) but that science doesn't support the idea of going back in time. The 'past' is an idea only - an impression left on the present. There is no 'past' with any physical existence to go back to. All these efforts to work out the paradoxes of traveling back in time are too divorced from science for me.

  3. Hume is good, but don't forget Kant. What really underlies our conscious experience could be entirely incomprehensible to us. This idea was introduced by Kant in "A Critique of Pure Reason" (1783). His position was that observation plus reason aren’t enough to determine what really exists, because we can’t pierce the veil of perception to see what lies beneath.

    Lee Braver discusses this in "A Thing of This World":

    "The mind actively processes or organizes experience in constructing knowledge, rather than passively reflecting an independent reality. To speak metaphorically, the mind is more like a factory than a mirror or soft wax.

    Truth, it is said, consists in the agreement of cognition with its object. In consequence of this mere nominal explanation, my cognition, to count as true, is supposed to agree with its object. Now I can compare the object with my cognition, however, only by cognizing it...Since the object is outside me, the cognition in me, all I can ever pass judgment on is whether my cognition of the object agrees with my cognition of the object."

    Kant distinguished between the distinct realms of phenomena and noumena. Phenomena are the appearances, which constitute our experience; noumena are the (presumed) things themselves, which constitute reality. In Kant’s view, the laws of physics are knowable precisely because they make no effort to describe the world as it really is but, rather only describe the structure of the world as we experience it.

    In this he anticipated functionalism/computationalism, right? If you are in a computer simulation, like The Matrix for example, there’s no scientific experiment you could perform that would allow you to discover this, UNLESS the simulation had flaws. Your experiments would only tell you about the rules of the simulation. The experiments would never reveal anything about the hardware the simulation ran on, OR the physical laws of the universe that contained the hardware.

  4. Derek, actually the possibility of time travel is scientifically sound, if speculative. And time is simply another dimension of space-time, according to the theory of relativity.

  5. "And time is simply another dimension of space-time, according to the theory of relativity."

    Simply a reversible dimension, I presume? Since it's simple to reverse causation and its constructions?

  6. From what I understand, there is no reason why the arrow of time cannot be reversed, hence the concept of time travel. But, obviously, nobody knows how to do it.

  7. From Eternity to Here: The Quest for the Ultimate Theory of Time
    Sean Carroll (Author) Physicist, California Institute of Technology

    'Time is a medium we move through and a way to sequence events. But the Arrow of Time' is also the only feature of the universe with one irreversible direction: time goes forward.'

  8. Baron, that's one physicist's opinion. Not particularly original, either, since he simply stating what we experience every day. Do you seriously believe that the science of time is settled?

  9. Sean Carroll on the other hand would not support my earlier commentary to the effect that Aristotle was on to something where some aspect of purpose served by our existence was concerned.

  10. True, he certainly wouldn't. ;-)

  11. But, Massimo, on the other hand, your comment I last responded to was not that the issue of time was settled, it was this one:
    "From what I understand, there is no reason why the arrow of time cannot be reversed."
    And Carroll (along with others such as David Albert, Professor of Philosophy, Columbia University) believes strongly that there is.

  12. Baron, as far as I know, nobody has figured out a physical reason why time can't be reversed. Carroll is simply stating a fact, not providing a reason.

  13. And my response to Sean Carroll on his blog, http://blogs.discovermagazine.com/cosmicvariance/ re our purposes went as follows:
    Baron P Says:
    January 19th, 2011 at 3:40 pm
    “The world keeps happening, in accordance with its rules; it’s up to us to make sense of it.”
    If there are universal rules, they serve at least one universal purpose.
    If not, such rules would be unenforceable, and their alleged existence undeducible by us humans. My sense making guess is that the universe serves not one but an endless number of acquired purposes, the ones we are most aware of being ours.
    42. Baron P Says:
    January 19th, 2011 at 5:15 pm
    “None of which is to say that life is devoid of purpose and meaning. Only that these are things we create, not things we discover out there in the fundamental architecture of the world.”
    So are we humans then presumably the first instance of the acquisition of purpose in our, or any, universe?

  14. "Carroll is simply stating a fact, not providing a reason."
    Oh come on, both have gone into detail as to their reasons.

  15. Baron, can you summarize his reasons? Also see this:


    Particularly the sections on general relativity and quantum mechanics.

  16. Massimo, I'm aware that there are those who feel that time travel is theoretically possible, and that the issue is not settled, at least with them.

    But you asked for reasons for the opposite position, right? So I pointed to a book about the subject, where someone in the know claims to have, as you put it, "figured out a physical reason why time can't be reversed." I'm not saying you should have been aware of it, only that you could have.

    If you now want a summary of the argument posted here, other than the brief note I made as to its content, I'll look for one that I'm fairly sure he's also written, and post the citation here.

  17. Aha, found it. http://discovermagazine.com/2010/mar/02-the-real-rules-for-time-travelers
    The Real Rules for Time Travelers
    by Sean Carrol

    And it ends with this:

    "On a closed curve, the entropy has to finish exactly where it started, but the arrow of time says that entropy tends to increase and never decrease. Something has to give.
    To emphasize this point, think about the hypothetical traveler who emerges from the gate, only to enter it from the other side one day later, so that his entire life story is a one-day loop repeated ad infinitum. Take a moment to contemplate the exquisite level of precision required to pull this off, if we think about the loop as “starting” at one point. The traveler would have to ensure that, one day later, every single atom in his body was in precisely the right place to join up smoothly with his past self. He would have to make sure, for example, that his clothes did not accumulate a single extra speck of dust that was not there one day earlier. This seems incompatible with our experience of how entropy increases. If we merely shook hands with our former selves, rather than joining up with them, the required precision doesn’t seem quite so dramatic. In either case, though, the insistence that we be in the right place at the right time puts a very stringent constraint on our possible future actions."

  18. If time travel into the past is possible then where are all the time travelers arriving here from the future? I think we might take their absence as evidence that either it is impossible or if possible it will never be discovered.

  19. What is the advantage of separating philosophy from metaphysics?

  20. @Massimo, @Derek: I think we do have some tools for time travel at our disposal. Memory is one of them, be it individual or collective. It's another question how to harness this thing.

    What happens with memory is that we go back in time to the desired event. And experience it. Just like we experience the events in the present.

    We may be seeing examples of time travel in, say a scatterbrained individual whose mind is probably a bit too far from the present for anyone's good.. As opposed to an athlete whose mind is much closer to the present than others, and is thus able to react to external events more quickly than others.

  21. I've always been a bit baffled by the concept of free will. It's one of those cases where I can understand all the features that go into it, but I can't completely connect what they have to do with each other, or relate this intuitively to any property I sense in my own mind (someone told me, half-jokingly, that this may be because "I" am a zombie and don't actually have any free will to sense).

    I guess I'm therefore ambivalent regarding the question of what science can tell us about free will. There are some empirical questions regarding whether the universe is deterministic (or "deterministic enough" to control human actions) and whether or not there seem to be breaches in the laws of physics, or special laws of physics, in human brain matter. I'm not opposed to investigating this, but I don't think there's anything there to find. There's a bunch of interesting scientific questions in psychology regarding nature vs. nurture, and the root causes of various behaviors, and there are questions in applied ethics about sanity and responsibility for one's actions about which science would appear to have a great deal to say.

    There seems to be a lot more to various discussions regarding free will, but any time I can't substitute in a more specific concept (non-deterministic decision-making, ethical responsibility for actions, the feeling of having agency), the discussion starts to lose me.

    Long story short: the only aspects of the idea of free will that I really care about are the ones where science does seem to have something to say. So even though your stance makes sense, it was rather a surprise to me because I always think of free will in heavily scientific terms.

  22. Sean, there are philosophers like Dan Dennett who actually argue for a meaningful concept of free will within a deterministic universe, not to mention of course that we know our universe is not deterministic. Science has plenty to say about specific issues within the free will debate, but one also needs to clarify what one exactly means by the concept and what any particular meaning of it implies, before we can even decide which science has anything relevant to say about the issue.

    Dave, thinking of memory as time traveling is cheating. Clearly the discussion is about the possibility of actual physical time travel, nobody doubts that we can remember past events. If you wish to experience the difference, the next time you are about to go on vacation, just think about being there instead of taking the plane... ;-)

    Thameron, that's an old objection to the concept, but how would you actually know if time travelers are or are not around? Plenty of good scifi imagines situations where time travel agencies enforce strict rules on their customers, or where time travel scientists adhere to equivalents of the Prime Directive.

    Baron, the entropy argument is an old one, but again it seems to me that it largely restates the facts. The "arrow of time" explanation isn't really an explanation, because it essentially makes an equivalence between time passing and entropy increasing, which begs the question of why the two should be that deeply connected. Carrol may be right - I', entirely agnostic on the actual possibility of time travel (except that I really like Dr. Who) - but the literature I referred you to seems to argue otherwise, and I've learned that when experts disagree the best bet is agnosticism...

  23. Brian, what makes you think I was trying to separate philosophy from metaphysics?

  24. Free will isn't really about determinism vs. indeterminism - quantum randomness doesn't help.

    "Physicists often fire the opening salvo against free will. In the classical Newtonian scheme, the universe is a gigantic clockwork mechanism, slavishly unfolding according to deterministic laws. How then does a free agent act? There is simply no room in this causally closed system for an immaterial mind to bend the paths of atoms without coming into conflict with physical law. Nor does the famed indeterminacy of quantum mechanics help minds to gain purchase on the material world. Quantum uncertainty cannot create freedom. Genuine freedom requires that our wills determine our actions reliably." -- Paul Davies

    It seems to me that either there is a reason for what I choose to do, or there isn't.

    If there is a reason, then the reason determined the choice. No free will.

    If there is no reason, then the choice was random. No free will.

    I don't see a third option.

    As for Dennett's compatibilism, I think it basically boils down to just redefining a bunch of terms and then falsely claiming that the result is what people meant by "free will" all along.

    Which is basically like unilaterally changing the definition of "unicorn" to mean "a horse with a horn glued to it's forehead" and then displaying such an animal and thereby claiming, "Unicorn's exist! This is what you meant by 'unicorn' all along."

    I agree with the critics of compatibilism in this passage:

    "Critics of compatibilism often focus on the definition of free will: Incompatibilists may agree that the compatibilists are showing something to be compatible with determinism, but they think that something ought not to be called 'free will'.

    Compatibilists are sometimes accused (by Incompatibilists) of actually being Hard Determinists who are motivated by a lack of a coherent, consonant moral belief system."

  25. Now that Massimo has called out my inner Doctor Who geek:

    "People assume that time is a strict progression of cause to effect, but actually, from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly...timey-wimey...stuff."

    - The Doctor, "Blink"

  26. Well, if we are playing Dr. Who quotes, here is one of my favorite ones (from the 9th Doctor):

    “The thing is, Adam, time travel is like visiting Paris. You can't just read the guidebook, you've got to throw yourself in. Eat the food, use the wrong verbs, get charged double and end up kissing complete strangers. [beat] Or is that just me?”

  27. Time travel must be a lot like a trip to heaven. No-one that's ever gone on either journey seems to have come back to tell us all about it. Alien abductees excepted.

  28. If 'time' is, as some argue, nothing more than a metaphor for the measurement of change, then if you can reverse the process of causation, which I'd agree is hardly linear, but comes at you from all possible directions, then you could in theory measure a reverse of those directions. But, as Carrol would infer, whatever underwent the changes could not replicate the sequential order in reverse, or worse, project that order back to the future.

  29. Massimo -

    That sounds very much like the excuses that conspiracy theorists give in that there can be no disconfirming evidence because you can always give reasons for a lack of evidence. The people who assert that our planet is regularly visited by aliens could make exactly the same argument. How much credence would you give them?

    Let me then use a common retort. So these proposed time travelers have never once made a mistake so large that they got found out? Then they wouldn't be very human because humans are fallible creatures and make mistakes. There is quite a lot of future out there. You'd think someone would screw up once every billion years or so, so much that we'd notice.

    And you can't really fault me for using an old argument considering you are constantly bringing up arguments put forth by a man who died twenty three centuries ago. Clearly you don't believe old is equal to invalid.

  30. Thameron, there are serious differences between the two cases. First of all, plenty of physicists consider the possibility of time travel. Second, just because it is erotically possible it doesn't mean it is technologically so. Third, even if possible, we may never be able to send a macroscopic object back in time. Lastly, I really would like to know how can you tell that there are no time travelers among us. I mean, aliens are supposed to be green and have pointed ears, but what if time travelers are found only among history professors?

  31. Hey, if the present looks forward to that "erotically possible" has the future already made its magnetic presence felt?

  32. Now who is putting too much trust in science? Please bear in mind that these same physicists considering traveling back in time have also failed to make fusion a reliable source of electricity even though they have a rather prominent example of fusion power and have worked on it for decades.

    To my knowledge there have been no observations of things traveling backwards in time so physicists have no example of that phenomenon to work from. And while I am certainly no expert on the subject I think that particles moving backward in time and particles moving forward might have an observable out of the ordinary interaction, perhaps along the lines of particles and anti-particles meeting in which case all the successful time travelers have exploded spectacularly on arrival. Tunguska anyone?

    And then there is the small matter of space. Someone gives you a time machine which allows you to go back one year in time to give yourself advice about the stock market and advantageous mating opportunities. You pull the lever and suddenly find yourself in the depths of space because the earth was not in the same position a year ago since it, the entire solar system, and the galaxy are always in motion. One hopes you brought a year's supply of air, your asbestos skivvies and a parachute to wait for it to get where you are. Who knows. Perhaps space (and the Earth's mantle) is littered with successful time travelers. To avoid that unpleasantness any time machine will need to double as a teleporter. Maybe you can get a bargain twofer rate on improbable machinery at Bubba's Tech Shack.

    As for the aliens they listened to our radio and television broadcasts on the way in to comprehend our languages and culture and since they are technical and genetic geniuses they have been able to blend seamlessly into the biosphere and many philosophy departments where they seek to convince us that we can't prove negatives no matter how preposterous they are or how much evidence there isn't.

    Furthermore time travel is way overused as an all-too-easy get out of jail free plot device in science fiction movies. Just sayin'

  33. "Sean, there are philosophers like Dan Dennett who actually argue for a meaningful concept of free will within a deterministic universe, not to mention of course that we know our universe is not deterministic."

    I'm aware of that, but I'm skeptical that the feature that Dan Dennett is talking about is sufficiently closely related to what other people are talking about with regard to free will. I think he's making a good case against defeatism, but at the expense of co-opting a term which seems to assume a naive view of what minds are. What does it mean to say that "I" did something, you know? If one part of the brain tries to do something and another inhibits it (or fails to do so), what heuristic do we use to decide which one is the part with free will (or is it a property of the brain as a whole)? To decide whether an action is freely chosen, does it matter whether the reason why you think you did something is more or less right, or an after-the-fact rationalization of an unconscious decision? To me it makes more sense to simply dump the phrase "free will" and find out how in practical terms what factors have the most influence over particular types of behavior or inclination, but again, that's because I don't really understand the value of the idea in the first place. Or rather, I think it probably has aesthetic value without having clear relevance beyond that.

    I'd also say (though I think it's not very relevant to free will) that it's flatly wrong to say that we know that the universe is non-deterministic. I think there are four possibilities at the current moment:

    a) Quantum mechanics appears non-deterministic because it is (Copenhagen and objective collapse theories, requires some explanation for what causes "collapse").

    b) Quantum mechanics appears non-deterministic because all outcomes occur but we only perceive one (many worlds, relational interpretation, requires some explanation for what it means for one world to be "more likely" if both actually exist).

    c) Quantum mechanics appears non-deterministic because we can never get all the information necessary to make predictions (non-local hidden variable theories, requiring some justification for accepting something apparently untestable even in theory).

    d) Some new fundamental physics will swoop in as a deus ex machina and make quantum phenomena deterministic again (doesn't seem likely).

  34. As for time travel: The "Where are all the time travelers?" conundrum has no import in general relativity. GR-based time machines can never take you back before they were created, so if one was invented in the future it would have no impact before the day of its invention.

    And for entropy, that's an argument that seems not to apply so well to digital messages. The amount of entropy in a text message can be compensated for with a negligible amount of heat (otherwise it would be pretty hard to copy and send them). Objects which cannot be exactly duplicated pose a bigger problem, but not an obviously insurmountable one, not least because there's no object actually being exactly duplicated (which would, admittedly, be a bizarrely unlikely feat). If one abandons the idea that causation only works forward, there's no element of chance the object going into the time machine just happening to be identical to the object coming out. In fact, it is rather difficult to form a blanket argument against all time travel that isn't fundamentally question-begging; the most common arguments against it rely implicitly on our intuition that causality works forwards.

  35. Baron, clearly, I meant "theoretically," but I guess I had something else on my mind...

    Thameron, it isn't a matter of "trusting science too much" (it should be obvious that I don't), it's that your two examples are simply not comparable at all, for the reasons I put forth.

    Sean, the issue of indeterminism in q.m. is still unsettled, though my reading of the relevant scientific community is that option (a) is the current near-consensus. Of course, as pointed out above, quantum phenomena don't get you free will in any meaningful sense of the term. (Wait, is there a meaningful sense of the term??)

  36. Okay so when you said "... plenty of physicists consider the possibility of time travel. "

    You weren't trying to imply that since physicists are in the hardest of the hard sciences (and therefore have the most science cred) that therefore must imply that what they are thinking about enjoys a larger probability of being both possible theoretically and doable practically?

    Because if it wasn't meant to imply that then I am not at all sure why you said it. My point was simply that it has (to my knowledge) never ever been observed in particles or people here or out there, so there is no reason to think that it is possible or if it was that we could make a machine (out of matter going forward in time) that could make it happen.

    Science can enable us to do all that can be done (with all that should be done being an argued subset), but I don't think traveling back in time is in that set and the number of physicists thinking about it won't change that at all since physicists are there to discover the law not dictate it.

    Subject as always to updates with changing evidence.

  37. We have evolved to be the ultimate participants in the determination of our futures.
    Free to do so because we have been freed to do so. Predetermined to be freed or predestined to be freed, or accidentally impacted, we are not free to do otherwise than self determine to survive.

    Or not.

  38. Since the future is determined by too many unknown (and perhaps unknowable) factors we have the illusion of free will, and that will have to be enough since we are not likely to receive more, but I do admit that 'unknowably deterministic' just doesn't have the emotional appeal of 'free will'.

  39. "option (a) is the current near-consensus"

    I'm not at all convinced of this. Or rather, I think that option (a) is popular because it sits well with a "just do the math" attitude, where one is not particularly concerned with whether wavefunction collapse really occurs or what causes it, as long as it looks like it happens (after all, most physicists do not find that the various interpretations of quantum mechanics routinely impinge upon their work). I don't see that it's a clear consensus among physicists with a particular interest in measurement and decoherence; very many think that some kind of Everett interpretation (option b) is true, including a lot of the big names. Part of the reason is that postulating an objectively real collapse isn't clearly parsimonious with respect to the fundamental laws of physics; collapse is an extra feature which, in our current understanding, has to be tacked on to quantum phenomena.

    If a bare majority of physicists who use quantum mechanics is sufficient to produce a consensus, then you may be correct that such a consensus exists. But I don't think that there's a consensus in the sense of there being a clear agreement based on specific evidence; in fact, I'm not even confident that there's a majority in favor of any particular view, if you focus on the specific physicists who are most interested in the problem.

  40. Thameron, it's got nothing to do with physics being a hard science. It's got to do with legitimate discussions within a community of legitimate experts. Surely you are not going to suggest that string theory has the same status as alien abductions, right? And yet, we have precisely zero empirical evidence that string theory is true.

  41. Well Massimo there is a motto I try to go by in my beliefs and that motto is: 'What does the evidence support?' To my knowledge the evidence does not support alien visitation, abduction and subsequent digestive system exploration but the evidence of human psychology does support an alternate conclusion of why someone might claim such a thing. Alien abduction claims do not make pretense of adding substantially to the human understanding of the universe. These things pretty much make it a dismissible sideshow in the human story.

    String theory on the other hand makes a range of extravagant promises about the expansion of human understanding of our universe and, let's say it all together now, "Extraordinary claims require extraordinary evidence" and what evidence or method for obtaining evidence has string theory given us? None at all as far as I know. And here is a part I confess I just don't get. They say that the ramifications of String Theory are not evident at our low energies which is all well and good, but there are some extremely energetic events happening out there in space all the time. Why can't the implications of String Theory be seen there? If a black hole feeding and spitting out particles at nearly the speed of light or an enormous star exploding in a fraction of a second aren't energetic enough for them then what the hell would be? String Theory is nearly as old as I am and has yet to produce one test and I am way too practical not to be leery about any theory for which that is true. The City of Science has had more than one dead end street. I don't think one testable result is really too much to ask of a theory that purports to explain everything. As of right now it reminds me more of the early explorers chasing rumors of El Dorado or the Fountain of Youth than science.

    Both alien abductions and String Theory may be the result of flawed cognition although certainly not the same flaw so perhaps they are cousins if not siblings.

    As of right now there can be no legitimate discussion of time travel among experts because there isn't any proven time travel and hence no experts in it. What there is is wild speculation about a phenomenon no one has ever observed or produced and one nobody has any idea of how to produce if it is even possible. Such discussions might bear the disclaimer 'for entertainment purposes only'. If only more time were spent on the actually achievable. Some examples of the achievable include things you might be able to create by building objects one atom at a time, terraforming and celestial engineering. Those would be enough to keep our species busy for the next million years or so. Fanciful implausibilities like time travel don't yield much in the way of results. Except ironically wasted time.

  42. Thameron, but in fact there is a legitimate discussion among experts, the expertise being the theoretical physics and philosophy of space-time. And your insistence in bringing up alien abductions as if they were even remotely in the same category seems quite strange to me.

  43. We don't see time travelers or compelling evidence for their existence.

    We don't see alien visitors or compelling evidence for their existence.

    When pressed for why we don't see evidence for time travelers those defending the possibility of the idea have a list of excuses. The time travelers have a code of conduct, are too clever for us, have never made a recognizable mistake etc.

    When pressed for why we don't see evidence for alien visitors those defending the possibility of the idea have a list of excuses. The aliens have a code of conduct, are too clever for us, have never made a recognizable mistake etc.

    That is the similarity. Of the two we know that interstellar travel is at least possible (if difficult). You are the one who brought up String Theory not I. I don't actually know if time travel is a necessary part of that theory (or theories) or not.

    Theories are all very nice, but if they never explain anything about the material world we live in then they are not terribly useful except as art and source of income for theoreticians. I think this is a shame since there are so many things which we have every reason to believe are possible (because they have been observed) that these people could be working on. Regrowing limbs for instance.

    I stand by my contention that there are no experts in non-existent fields of endeavor unless you want to give similar credit to theologians for the complexity of their ideas.

    It occurs to me though that if a time traveler beat the odds and actually managed to arrive on the surface of the Earth the space that they would arrive in would not be empty. It would contain several liters of air and (should they survive the few scattered fusions) that would need to be dealt with. Perhaps we should be on the lookout for people who appear suddenly and who are burping and farting uncontrollably. Mere flatulence or time travel? Who's to say?

  44. I actually agree with Thameron that the time traveler analolgy is a pretty good one when compared to the alien analogy. If time travel (human or alien) were indeed possible we not only should be seing time travelers, but we should have always seen them. The strict adherence of a "prime directive" seems obsurd when we can't even get people to follow traffic laws or even to not murder each other. This isn't really evidence that it is not possible, but indicates to me that it is highly unlikely.

    I consider this a different question however than whether time travel is theoretically possible, particularly for small particles. Theoretically possible and practical/possible on an entirely different scale are 2 different things.

  45. I would still like to know how on earth would you guys be able to tell a time traveler from your neighbor...

  46. "And your insistence in bringing up alien abductions as if they were even remotely in the same category seems quite strange to me."

    Sticking to the ideas that we have always had time travelers among us, in comparison to the idea that there are regular visitations from aliens, this analogy does not seem that strange to me. Both require simlar types of arguments about the behavior of these visitors that make them invisible to our observations.

    Now this has nothing to do with the broader questions of the theoretical possibility of time travel, or the possibilities of life outside of earth

  47. "I would still like to know how on earth would you guys be able to tell a time traveler from your neighbor... "

    Well (s)he could tell me, or show me some advanced technolgy. He certainly would have some strange accent (to me) even if he spoke the American variety of English. I think even 50 years from now then language would be very different. I think it is very strange that only people who stuck to a very rigid rule of conduct would be the only ones with access to time travel... not to identify themselves as time travelers. Not to mention that they would have to obey certain rules about not profiting from the knowledge of the future in any way because this would identify themselves... whether that be winning the lottery or a nobel prize for settling our issues in quantum mechanics and relativity.

  48. Right, and all of that is impossible because...?

  49. ... he could pose as a "real" psychic and be fought over by the governments of the world for advice/secrets. All it would take is a series of "predictions" of unforseeable events that actually come true.

    It is not impossible but highly unlikely because, with simple planing, a person of above average intelligence could easily become one of the most powerful people in the world for a very long time. Since we have no evidence of any of this it appears unlikely. The world we observe does not match a world in which time travel by humans is possible. If it is, then it is a very rare event.

  50. "Right, and all of that is impossible because...?"

    Its not that it is impossible, but improbable it no less probable than the alien visitation.

    Unless you are trying to tell us something Massimo... what time are you from? =)

  51. Darn, I knew I shouldn't have been that explicit... ;-)

  52. If you travel back in time and still remember the future, then you will have violated the time-symmetric boundary conditions.

  53. Guys, Guys (and especially Thameron)

    General relativity, which is an extremely successful and elegant theory, does predict scenarios that we would call time travel. That fact alone makes it plausible. Time is a physical entity and has properties which are testable and tightly woven into practically all physical laws. No meaningful discussion on time travel can be made without a profound understanding of these laws and properties. I am a physicist myself but my understanding of theoretical physics is limited and theres actually little more that i could bring to the table that laymen couldn't. There are experts on time travel and these are the people who understand the (established) science of what time is and how it works. There is no established science of time travel so these people have to "mentally" extrapolate from what they know. Theres no consensus on matters like that since there are many ways to extrapolate but that doesn't make their conclusions "guesses" nor does it make them as plausible as yours and mine.

  54. Ah Massimo, so we have strayed from the realm of what can reasonably be proven given the available evidence and into the much wider realm of that which we cannot disprove. That's where God lives you know, out there at 0.000000001% give or take. Maybe the imperceptible dragon in your garage is an alien time traveler.

    There is a great deal that I cannot disprove about my neighbors or about myself for that matter. I can't disprove that they are all spies or time travelers, or aliens in disguise or cleverly constructed androids or particularly vivid hallucinations etc. etc. To paraphrase your fellow philosopher I cannot disprove the assertion that I am a butterfly dreaming I am a man, but is there a compelling reason to accept any of these implausible things? I don't think so. No one gets certainty. All we get are best guesses and mine is that time travel is not possible. When it is observed or demonstrated then I will bow to the evidence and change my mind.

    The physical universe is under no obligation to conform to human models of it. Quite the opposite really. Just because a bunch of highly educated people have some equations that say it can be so doesn't mean that the universe doesn't have other as yet undiscovered laws which say it can't and which will force the very educated people to formulate some new equations.

    I understand though that there would be resistance from these people to the idea that they are engaged in what amounts to idle speculation rather than pushing the frontiers of discovery, but as Randi might tell you the smart people are easier to fool than the dumb ones. Perhaps they are more adept at fooling themselves as well and come to love the mathematical beauty of their models too much rather like Kepler and his perfect geometric solids.

    So how long should we wait without evidence of a phenomenon before we abandon the idea that it is possible? Forever?

  55. Thameron, did you not read Kostas' post at all? It's right above your comment.

  56. Kostas' post? The one that says "Time is a physical entity and has properties which are testable and tightly woven into practically all physical laws."

    That statement is ridiculous on its face.

  57. Really? Why, exactly? It accords to everything I've read about the physics of time. Or you don't agree that time is another dimension of space-time, just like the three standard ones?

  58. Baron

    To which part of this sentence are you objecting exactly? Are you saying that time is metaphysical or supernatural or that its just a parameters in our equations or maybe that physics could do just fine without the concept of time ?

  59. Time is not an entity, a thing with distinct and independent existence. “Time has no independent existence apart from the order of events by which we measure it.” —Albert Einstein

    Dimensions are not entities, no more than a ruler is a rule, or every yard contains a yardstick.

    Further, Einstein wrote (in 1952):
    "Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence."

    So I certainly do agree that time is another dimension of space-time. A measurement of the evolution of change.
    And note that he said evolution.

    He also wrote that "for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one."

    None of this or anything else Einstein has said supports time as a physical entity or travel by such entities outside of their dimensional boundaries.

  60. But if time is just another dimension of space-time, why should it be inconceivable that we can travel it? We certainly travel the other dimensions. I'm not saying it is possible, or that anyone will ever do it. But inconceivability is a pretty high level of impossibility.

  61. Massimo:
    As regards dimension, I am unable to conceive how you would "travel it." You could conceivably change time as a dimension, but would you retain your own dimensions in the process? As I said before, I'm not proposing anything to a certainty, but I have my serious doubts.

  62. Nothing has a distinct and independent existence. Can you measure velocity without time? Can you measure mass without force? Your standards are too high. Entities are measured in units that are always a function of time, mass, energy and length. Thats one of the reasons why time is so difficult to grasp.Its an elementary magnitude so it cannot be defined on terms of other magnitudes just like energy which happens to be the conjugate magnitude of time(http://en.wikipedia.org/wiki/Noether's_theorem#Example_1:_Conservation_of_energy)(And thats why energy is also a term so commonly abused by non experts.It has this magic aura that all elementary magnitudes do).Even though they are elementary they dont make sense without each other and even though each others dimensions are different any description of those magnitudes is bound to refer to at least one of the others.Einstein was spot on because thats exactly what time is.

    Also: Dimensions and magnitudes are one thing and units are another. A yard is arbitrary but distance is not.

    Now, then, before, after, past and present are humans terms that describe our perceptions. Time on the other hand has a certain meaning in physics and a certain meaning when it comes to language. The two are clearly related and the perceptual definition is clearly reflected in physics but they are not necessarily the same thing.

    Bottom line: If time is not a physical entity then neither is space, energy and mass and certainly not the magnitudes defined by those like speed, angular momentum, spin, temperature... you name it

  63. "If time is not a physical entity then neither is space, energy and mass and certainly not the magnitudes defined by those like speed, angular momentum, spin, temperature... you name it"

    Tell that to Einstein.

  64. Baron, I think you misunderstand Einstein.

  65. Well if time is just an illusion then Einstein is no more dead than alive and i should be able to tell him that. Somehow i cant...

    Just kidding, but i think we wouldnt disagree on this

    You seem to dismiss time as "a measurement of the evolution of change". You shouldn't because thats part of what time is and thats very important and its part of the very hard core of existence. It doesn't necessarily make it any less real than anything else. Look into the relationship between time and energy and you ll see how one cannot exist without the other and how these two are inextricably connected

  66. “Time has no independent existence apart from the order of events by which we measure it.” —Albert Einstein

    What part of that did I misunderstand?

    Or even better, what part did Carroll or Albert misunderstand?

    But I suppose, by your reckoning, we could someday travel back in time and ask him.

  67. And Massimo, if my agreement with Einstein is due to a misunderstanding of that quotation, what is the understanding that you have that is not at the same time a disagreement with it?

  68. Baron, I'm beginning to sound like a broken record. See Kostas' (latest) post above.

  69. In special relativity, there is "mixing" between space and time. You can imagine that if two people are standing in a plane, and one rotates to face a different direction, that person will now have a different (and equally valid) conception of which direction is "left" or "forward". Similarly, if there are two observers, and one receives a Lorentz boost (i.e. changes velocity), that person will have a different (and equally valid) conception of which direction in space-time is "pastward".

    The size of these effects is limited by certain constraints, such as that the speed of light must appear the same in all reference frames. So people can't have more than a certain degree of disagreement about which direction is "pastward". Nonetheless the conflict is there.

    In general relativity, the non-Euclidean geometry of space-time gives rise to new effects, such as gravity. One can view an object in a gravitational field as having their "futureward" direction pointing towards nearby massive objects. That is, in GR there is not a force pulling things downward, but rather spacetime is warped in such a way that the local "up-down" direction and the "past-future" direction have become mixed. This is a slight effect on the Earth; one can still exert energy to move an object upwards. However, there's a detectable difference between time elapsed on Earth and time elapsed in space because of the Earth's gravitational field.

  70. Yes neither of you wants to be in disagreement with Einstein, so you'd rather argue that I, if not others, misunderstand him.

    Kostas tells me, "You seem to dismiss time as "a measurement of the evolution of change".

    Yet here is what I actually wrote:
    "So I certainly do agree that time is another dimension of space-time. A measurement of the evolution of change. And note that he said evolution."
    So I'm the first here to point that factor out as significant, and you two are those who have dismissed the implications of that statement.

    Time takes the measure of an entity's evolvement. You would have it measure that entity's devolvement with the same ruler.

  71. To say time travel is possible in general relativity is simply to note that, in some space-time geometries, there are trajectories that return to one's own past. One of the most well known cases (but by no means the only one, nor the simplest) is the wormhole. One goes in one mouth and comes out another, possibly in the distant past. There is no local violation of causality (each observer perceives himself as simply moving normally and forward space-time), nor is there any process of "moving through time" as in H.G. Wells time machine. In any sufficiently small local area, everyone agrees about which direction is "the past" to the degree required by special relativity. It's simply that two space-time regions are connected, as if you could walk through a door and come out the other side in another location 100 years earlier. Of course this requires that one end of the wormhole must have been been present 100 years earlier and roughly where you want to be; one can't return to any arbitrary past point, but only one particular one which has been set up in advance.

    There is a very good chance that no such wormholes, nor any other geometry that could be created in this universe, could actually be produced. In fact, many physicists are pretty skeptical that it will ever happen, for a variety of reasons. However, this is not a closed question, and this sort of time travel emphatically is not something that has been shown to be physically impossible. And even if time travel never becomes possible across macroscopic distances/times, it may become physically observable (or falsifiable) in quantum gravity regimes; very small and short-lived knots in space-time would require fewer resources to generate than large stable ones.

  72. Kostas writes:
    "Well if time is just an illusion then Einstein is no more dead than alive."

    Actually, that shows the limits of your logic. If time was assumed to be an entity that was integral to his physical state, Einstein would be dead at present but be resuscitatable in the future past.

    And nobody has yet said here that time was just an illusion. It's "independent existence" is what Einstein denied. Was the inference being made that a belief to the contrary was illusory?
    It's one that I would draw, but perhaps you two are right and that's just me.

  73. Sean

    I am not sure what you mean by "futureward" direction but the only thing that acceleration/speed/gravity changes is the relative passage of time compared to an observer who is in a different frame of reference.The direction in which time moves cannot point to an object or in any direction in 3D space.


    I am not sure what we re arguing about any more. What are the implications of that statement? What would it mean for time to have an independent existence if all that stuff i said doesn't cut it? Does energy (or momentum or speed)have an independent existence? Maybe our definition of independent existence is not the same.

  74. Energy exists as a physical entity, or if you will, THE physical entity. Time is a metaphor for what we take to be one of its dimensions. A metaphor in turn is an analogy and I doubt if it has a physical existence except as a structural section of a neuron here and there. But as to that metaphor being independently existent, each aspect of that alleged existence would arguably be dissimilar.
    Also do you see speed as existing independently of the speeding object? I don't. Do you see momentum as independent to the entity involved or as measurable evidence of the application and/or intervention of the physical nature of energetic forces?
    There is no denying there is order in the universe. We take the measure of that order and call one aspect of that measure time. But as Einstein would in my imagination infer, time did not order energy to take its measure.

  75. "The direction in which time moves cannot point to an object or in any direction in 3D space."

    Not self-consistently within a single inertial reference frame. But from the point of view of one reference frame, another reference frame may have its conception of "futureward" in a different direction. That is, a vector that represents the spacetime distance between two events may look time-like in any reference frame, but there is only one set of reference frames in which the two events occur in the same place (and thus the vector representing the displacement from one event to another has a time component only). This is even true in non-relativistic mechanics, but has no interesting implications as time distortion effects disappear in that limit. However, it's possible to view time distortion in special relativity as a consequence of this rotation.

    Note that I'm only giving this as a useful way to picture the effects of relativity because I thought it would be simpler than getting into the math here. This mental model is substantially justified by the fact that a Lorentz transformation is described by rotation through a hyperbolic angle. Many (though not all) intuitive notions carry over between normal rotation and hyperbolic rotation, partly because they are linear transformations with the same form.

    I don't know how familiar you are with this. The wikipedia page here treats the subject more precisely: http://en.wikipedia.org/wiki/Lorentz_boost#Matrix_form

    Note particularly the .gif, which shows a combination translation (passage of time) and rotation (acceleration left and right). This is what I'm referring to; trajectories which are vertical in one frame in one frame are slanted in others, or, more banally, things can be still in one frame and moving in another. In nonrelativistic mechanics we view these only as trajectories, but in special relativity they also represent a skewed local "direction" of forward time compared to an observer in a different frame.

  76. "Energy exists as a physical entity, or if you will, THE physical entity."

    If you are referring to "everything is made of energy" claims, I disagree. Energy is not a substance, but a calculated quantity. Due to mass-energy equivalence, there is indeed a quantity of energy locked up in all substances (which is why there is so much pop-sci rhetoric and shorthand involving all things being composed of energy). But you are not "made out of" energy, nor are photons "made out of" a certain amount of energy or electrical arcs "made out of" energy. Rather, you possess a certain amount of energy as a property, much as you possess a weight and a net charge and occupy a certain amount of volume. It may be impossible for speed or charge to occur unless they are attached to some object, but it is equally impossible for energy to occur except as a property of field excitations (i.e. particles and configurations thereof).

    This metaphor by which all things are said to be composed of a substance called "energy" irks me, and it strikes me of an example of my fellow physicists (or, more usually, science educators) going too far, using an analogy to reifying an abstract property. They used to think that heat was a physical fluid as well, and a similar mistake with respect to electromagnetic fields led to the luminiferous ether. I don't think that the energy thing is likely to lead to the same degree of errors, but only because it's a matter of terminology that doesn't really make it into the theories, except in appropriate instances of mass-energy equivalence. Rather, it's a metaphorical way of speaking which has insinuated itself deeply into the way physicists talk about things, despite not being in any way a useful principle.

    But as far as "entities" go, I guess you have to decide what entity should be. Maybe energy is an entity as well as a property. Maybe time is an entity as well as a coordinate. Maybe not. Does thinking of time as an "entity" lead to any serious benefits or errors, or can we all just agree that when people call it that, they mean "phenomenon", or "type of coordinate", and get on with our lives?

  77. Sean writes knowledgably:
    "Energy is not a substance, but a calculated quantity." Ah, but there's the rub, a quantity of "what should we call it" if not energy? And what is a substance - other than as you put it, the stuff that has to have a "quantity of energy locked up" in all of the formations that make up substance.
    Which when you come full circle is not a substance but a calculated quantity of, for want of a better way we human blobs of substance have to put it, energy.
    Which if you take someone like Hawking's word for it, is not really energy but a spontaneity that owes its quantitative calculus to nothing.


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