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The Physics Quote that changed my life

  1. Nov 19, 2009 #1
    This quote that i found in the book Super Force by Paul Davies, once read, litterally summed up all the thoughts that I have had since I could remember about how the universe works. tell me if you also think its true

    "An intelligence knowing, at any given instant of time, all forces acting in nature, as well as momentary positions of all things of which the univers consists, would be able to comprehend the motions of the largest bodies of the world and those of the smalles atoms in one single formula, provided it were sufficently powerful to subject all data to analysis; to it, nothing would be uncertain, both future and past would be present before its eyes." -Pierre Laplace
  2. jcsd
  3. Nov 19, 2009 #2


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    It is now usually thought not to be true. One way to think of the distinction between classical physics and non-classical modern physics is whether or not this statement is true.

    This is approaching deep waters; but a simple example can show of a specific case in which the statement is thought to be false.

    The decay of an unstable atom is not predictable, not even in principle. In modern quantum physics, this is thought to be an event which is undetermined. All that you can obtain, given hypothetically complete information about the prior state, is a probability that the atom will decay. Many other similar examples exist: photon emission from an excited atom, identifying where an individual photon will end up in the double slit experiment, and so on.

    In other words, the universe is inherently unpredictable, or non-deterministic.

    There are some interesting proposals for ways to recover some level of determinism in physics; treating the universe as continually branching into alternate histories, all equally real (Everett's many worlds model for quantum mechanics) or some kind of "hidden variable" extension of physics (usually thought to be disproved; but some physicists are still looking for ways to manage it).

    But pragmatically, it does seem that the statement is false; given the radioactive decay example. It just is not predictable; existing quantum mechanical physics suggests that there is no information, no hidden variable, which could be used to tell in advance when a decay will occur.

    Cheers -- sylas

    PS. There's another thread currently giving more detail on this: [thread=355056]Wave function collapse and the statistical nature of quantum states [/thread].
    Last edited: Nov 19, 2009
  4. Nov 19, 2009 #3
    Thanks, and yes I guess when I think about this quote I forget that quantum is never predictable no matter the amount of info. But I remembered distinctly thinking about this since I was very little and it just struck me as so amazing that someone put it into words so beautifully.
    Thanks again for your insights and i will definately check out that thread
    Sincerely FC
  5. Nov 19, 2009 #4
    The Heisenberg uncertainty principal: Biggest buzz-kill in the history of man.
  6. Nov 20, 2009 #5
    true that man, true that
  7. Nov 20, 2009 #6
    I always compare this idea with that of life. We just don't know how an individual will act in a particular situation. But as a whole, the society will behave in a deterministic way. Is that a sensible analogy? Of course the difficulty in predicting the individual's behavior is perhaps due to the complexity of the interactions while in the quantum world, uncertainty is inherent.
  8. Nov 20, 2009 #7
    Yes I always thought that, but then thats saying like if you were to go out and study people. In the quote it is talking about someone who knows EVERYTHING, like God. This person would know all the things that affect the person and the postion of all their chemicals in their brain, so knowing that couldnt you then predict what would occur? since it is all chemical reactions and such. But then I wouldnt know how big of a differenct it would make once Quantum levels are also put into play, since those are as you said inherently uncertain
  9. Nov 20, 2009 #8
    Personally i don't like the claim of chemical interactions explaining all the beautiful intricacies of life. :) Ok. Maybe i'm violating some PF rules here. *Shut up Ganesh*
  10. Nov 20, 2009 #9
    pssssttt...... me either :), just dont tell anyone and we wont get in trouble ha ha
  11. Nov 20, 2009 #10
    Do you come from the 19° century?:smile:
    Things changed a lot since then...
  12. Nov 20, 2009 #11
    I like to think we're beyond harassment here.

    Besides, everyone who understands why the quote isn't true thought that is was at some point. You. Me. Hell, Einstein.
  13. Nov 20, 2009 #12
    yes as i said, I have had this idea in my mind for a while. But Quantum kinda ruined it for me...... BOOO QUANTUM!!!

    But dont you sometimes wish it were true? Like archosaur said, everyone did at one point

    Dont know thought you guys would have some cool insights, Thanks everyone :D
  14. Nov 23, 2009 #13


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    I'm trying to remember something I read about 25 years ago. The idea of the universe being deterministic was used in a Mark Twain short story The Mysterious Stranger. Written in between the LaPlace quote and the development of quantum mechanics, it's about a stranger who arrives in town and knows what will happen in the future.

    This post brings to mind Asimov's novel Foundation (or was it a trilogy?). The premise was that we had learned to predict the future evolution of sociey hundreds of years in advance, without knowing what actual individuals will be doing, much as the weather is predicted today. (Vastly different time scale though.)
  15. Nov 23, 2009 #14
    I disagree.. the mechanisms behind social interaction may be chaotic. If one person decides to buy a newspaper, that might mean a manager reaches his sales target and buys a Porsche. If you have a particularly fun physics teacher at school, you might do a PhD and discover something new!

    I think these sorts of multipliers mean than an individual is much more predictable than a country- look at how disastrous economic management is compared to psychological treatment.
  16. Nov 23, 2009 #15
    A lot of time i have behaved like Forrest Gump in my life.
    "I just felt like running"
    "So... you just ran?"
    "well... yeah!"
  17. Nov 23, 2009 #16
    You remember correctly. Asimov's psychohsitory.

    http://en.wikipedia.org/wiki/Psychohistory_(fictional)" [Broken]

    I must have read it more than 30 years ago.
    Last edited by a moderator: May 4, 2017
  18. Nov 23, 2009 #17
    Every interpretation of quantum mechanics, that I know of, attempts to find it true.

    To date, there must be hundreds of threads on PF involved in arguments to establish, or insist on some particular way over another way, that determinism is a property of quantum mechanics.
    Last edited: Nov 23, 2009
  19. Nov 23, 2009 #18
    Ah but now you must see at what caused this person to buy the magazine. If you were to somehow view this situation in a movie like scene where you could not interact with it, you will see that the simulation will run along the same path because the factors havent changed. Say i have a ball and i put it on a certain spot on a hill, at the bottom of a hill is some dominos, I can put it in the same spot over and over again and it will still hit the dominos the same way everytime. Therefore since everything has a cause you could take all the information you could possibly imagine and put it in a computer and run simulation after simulation and as long as you dont change anything and all your information is correct you could in fact possibly predict the future. Quantum how ever is uncertain so you could either be slightly off or disasterously wrong. Who knows but Does quantum being of such unimaginably small proportions be that much of a factor?
  20. Nov 24, 2009 #19
    Well this is the essence of the situation, is it not? The basic question is whether causality is a natural law in the earth? Does every action have a cause? Generally speaking causality is a natural law in the earth. Note that the quantum mechanical arguments of radioactive decay really prove nothing. One cannot substitute ignorance for action.

    Case in point. Are coin flips truly random? No they are not. There is only our ignorance of the detailed forces, inertia, moments of inertia and aerodynamic actions that prevent our predicting the outcome of each and every coin toss HAD we the information needed to calculate it. Similarly, while we know we can't predict when an atom will decay and eject a particle, we certainly have no way of knowing (quantum mechanical claims to the contrary not withstanding) just what determines such an ejection. And we do not know that IF we understood the exact mechanisms that we might not be able to exactly predict particle ejection. Ignorance is not a substitute for knowledge.

    All this leads us right back to our fundamental question. Is the universe causal or random? In other words does all physical phenomena have a cause. Or are certain things acausal? Do radioactive particles simply fly out of atoms at random without any cause initiating that? We do know that in classical physics causality is considered a natural law. One can define causality by saying that no two actions can happen at the same time. This means that IF one is going to look for acausal actions in quantum mechanics the proof is going to be in the old 19th century concept of "action at a distance" which is now termed "non-local" to cover up what it really is.

    And surprisingly such non-local actions have apparently been observed. The whole area needs much further study and even worse quickly progresses into the fringe as such actions quickly lead to transluminal communications and other "forbidden" areas. So in terms of physics this is clearly an area in need of serious investigation, but at the same time, quickly leading AWAY from physics and into philosophy. All of which makes this, like cosmology, a very speculative and difficult area of exploration.
  21. Nov 24, 2009 #20


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    At this point I usually like to remind people what they actually mean by determinism, and to point out that the concept isn't so easy.

    Determinism is usually understood to be the principle by which "if we know everything about the current state of the universe, and we know the correct laws of physics, then we can determine uniquely what will be the future state of the universe".

    In fact, of a given theory, it is pretty simple to find out whether it is deterministic or not: does the theory in question allow you to calculate what is the future state of a universe that follows exactly the theory, if you know whatever is to be (according to the theory) the current state, and the laws the theory stands for ?

    Typically, Newton's mechanics is considered to be deterministic, up to one caveat: the fact that the "current state of the universe" in Newtonian mechanics is given by real numbers (positions and momenta), and that we don't know how to even write down the vast majority of real numbers (let alone "measure" it). This caveat is what gives rise to the field of "deterministic chaos". But apart from that, it is deterministic in that knowing current positions and momenta of all matter points and the correct force laws, allows you to know in principle the positions and momenta of those matter points in the future.
    So there is already a caveat in Newton's mechanics itself (deterministic chaos), but moreover, the universe described by Newton's laws looks only approximately to the "real" universe.

    The same can be said about classical electromagnetism (Maxwell's laws) and about classical relativity.

    As others pointed out, quantum theory killed the idea. Now, there are interpretations of quantum theory which would make nature appear again as "deterministic", but where there is some property of nature which is part of the theory itself which doesn't allow any being of KNOWING the precise state of the universe at a certain moment (I'm thinking of Bohmian mechanics here). This means that even though nature might "be" deterministic, we cannot "exploit" that property because of some principle, which will forbid us to "know" the current state beyond a probabilistic description, and this probabilistic description will then propagate to the end result, only allowing for probabilistic predictions - as does standard quantum theory.

    But I always like to throw in the following. When we talk about "laws of nature", usually we think about relatively succinct mathematical formulations: relatively "simple" mathematical structures that generalize easily to the situation at hand. But this doesn't need to be so. You could think of another kind of law of nature, one in which all past and future events are simply summed up in some kind of table. Call it "the Book". Now, we don't have "initial conditions" and a rather simple calculation rule on how to deduce future events, we simply have the full list of all events in the universe, as a "law of nature". There's of course no way to DISCOVER this "law of nature", but it is conceivable as law of nature. In fact, it is rather more amazing that extreme simplifications are possible, which reduce to just a few simple mathematical structures, applicable to a vast array of conditions.

    So, in Newton's world, instead of giving you a simple differential equation from which we can DEDUCE all positions of matter points as a function of time, where this differential equation is the "law of nature", the law of nature in "the Book" would then simply be the functions of time which give us the positions of the matter points themselves. These functions are of course way more involved than just the differential equation to which they appear to be a solution, but that's just a matter of complexity of the laws of nature. What is amazing in physics, is that there are such immense "short cuts", such as a differential equation.

    But let us for a moment imagine that the "true" laws of nature are just the "list of all events" (the Book). Now, is that "deterministic" or not ? On one hand, of course it is! If ever you had the Book, you could just READ what's going to happen, the future events are specified without any ambiguity. So it is of course deterministic. But there doesn't need to be a "law" that allows you from just the slice of "now" events to deduce the next slice of events. There may be only a statistical correlation if you insist on a "simple" mathematical law (a kind of correlation that's given by a theory like quantum theory for instance).

    Or we could have a totally "random" universe, with no correlation between different slices of events. No causality at all, not even a probabilistic causality. This would be a very weird place to live in and certainly it is not our universe. But it is conceivable and it is even conceivable to have such a totally random universe, described by a very precise "Book", one in which every event is listed, but in which there are no obvious "simple" correlations between events in different time slices.
  22. Nov 24, 2009 #21
    Quite simply, quantum has not been around very long, the fact that it can't explain everything due to uncertainty principles et al may just be due to the lack of understanding of the field. Think about how long after Newton that Einstein formulated relativity.

    A more precise statement should be, present understanding of quantum mechanics cannot predict all events. Consider also that we have not necessarily (and i really don't think we have) discovered all the forces in nature. In fact, the forces we are missing may simply be occurring on such a massive scale that we don't even realise they are independent forces at all. It's like the statement, a fish cannot fully grasp the concept of water, because he has never experienced anything else.
  23. Nov 24, 2009 #22
    It sounds like a case of hard determinism vs a libertarian position (include compatibilism if you want to). Of course a libertarian position can either take the route of a supernatural soul (something which I believe), or of course, quantum mechanics.
  24. Nov 24, 2009 #23
    Wow, these are some good responces.... Thanks everyone!!

    They have all helped me understand a little bit more. I guess this quote "changed" my life because it made me realize that everything can be explained using the laws of physics, Why we see color or a rainbow, why the earth goes round the sun and the moon around the earth, why i feel pain, why i hear. And all this got me thinking. If we could explain it that means we know what causes it. I think that eventually once we understand Quantum, maybe there will be something else to explain, or maybe Quantum will be the last straw. but we will be that much closer to being able to comprehend it all, and even predict the future.
    So i guess what im askin now is what else other than Quantum is still left to be explained fully?
  25. Nov 25, 2009 #24


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    Might be, but quantum mechanics is now with us for more than about 80 years...

    It is hard to imagine that one would "understand better" quantum mechanics and "find out" how to go beyond the uncertainty principle (except by some improved version of Bohmian mechanics), because in quantum mechanics, this uncertainty is not an "not knowing" but rather a "not making sense". It is not that there is something that tells you "thou shall not know", but rather that for instance a single momentum state *is made up* of different position states, so there's no meaning attached in quantum mechanics to "a precise momentum AND a precise position", not any more as there would be a precise frequency attached to a step function (which has a spectral decomposition in several frequencies).

    So if "precise position and precise momentum" would make sense again, quantum mechanics as we know it would be largely out of the window - not just "better understood".
  26. Nov 25, 2009 #25
    Yes, determinism with a set of known initial conditions leads to a predictable future, but the chaotic element comes from a slight deviation from these initial conditions. So I am suggesting if the person does not buy a newspaper, then the manager does not buy a Porsche, and we have arbitrarily small changes at t=0 leading to large changes in the world after sufficiently long times. I do think life itself is chaotic, even without the need to invoke probability of QM.

    vanesch I think your post above (#20) was very good!
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