# Uncertanity Principle **URL:** \ **Category:** Factual Questions\ **Created:** [July 4, 2006, 8:45pm UTC](https://boards.straightdope.com/t/uncertanity-principle/363203 "2006-07-04T20:45:55Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [July 4, 2006, 10:38pm UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/21 "2006-07-04T22:38:51Z")
> [@yelimS](#): > > tim314: That made perfect sense! I’ve heard that particles are explained as waves, but somehow I’ve still thought of them as points. But it seems to me now that QM is the result of giving up the point explanation and just plain hypothesizing, that the bricks of the universe are waves, not points, without any proof for this, except that the results deducted from this postulate add up with reality. Am I getting this right? Glad to be of help. Although it’s true that many of the at first counter-intuitive predictions of quantum mechanics (including the uncertainty principle) follow from the assumption that quantum mechanical particles behave like waves or wave-packets, it’s not the case that physicsts simply guessed that particles were waves and saw what happened. Rather, they were led to a wave equation by attempts to explain the existing physical data. To elaborate: Equations of a certain form are called [wave equations](http://en.wikipedia.org/wiki/Wave_equation). If you can show that a certain physical phenomenon is described by such an equation, then it must be a wave. For instance, in the 19th century James Clerck Maxwell showed that the equations of electromagnetism give rise to a wave equation predicting electromagnetic waves travelling at the speed of light. He concluded that light itself is an electromagnetic wave. Similarly, physicists in the early 20th century wanted to understand the spectrum of hydrogen (i.e. the frequencies of light that it can absorb and emit). Niels Bohr created a model of the atom which explained this spectrum by assuming that electrons could only orbit the atom at certain discrete energy levels, but didn’t really give a satisfactory explanation of _why_ only these levels are allowed. Erwin Schrödinger later found an equation which predicted these energy levels, and that equation (the Schrödinger equation) had the form of a wave-equation. A couple years earlier Louis De Broglie had also proposed that particles had wave-like properties. In that case, the prediction was based on the discoveries by Max Planck and Albert Einstein that the blackbody spectrum and the photoelectric effect (respectively) could be explained by assuming that light (which was previously thought to be a wave) had _particle-like_ properties. So it’s not quite right to think that physicists guessed there was a wave-particle duality and then found this resulted in predictions that agreed with experiments. What actually happed was something more like this: physicists found certain perplexing experimental results, discovered that _those_ results could be explained by postulating a wave-particle duality, and then found that this assumption led to even more predictions which were themselves confirmed by experiment. ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [July 4, 2006, 10:57pm UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/22 "2006-07-04T22:57:13Z")
> [@yelimS](#): > > There is probably an organism that couldn’t grasp the simplest of concepts no matter how hard you tried to teach it, right? > There is probably an organism that evolved from the last one, that could grasp some concepts. > Who’s to say there are limits to this evolution? Bang, we’ve evolved to humans, and we now have the potential to truly and intuitively grasp EVERYTHING, if we, or someone else teaches us? > Until the day someone explains infinity to me in detail, I’ll believe we have a limited potential. Don’t get me wrong. There may very well be aspects of our universe which are beyond our comprehension. My point was just that I don’t think the counter-intuitiveness of _quantum mechanics_ is due to it being beyond our comprehension, so much as the fact that we just don’t have much experience with it in our daily lives. If you’d never seen a ball thrown through the air, the fact that it moves in a parabola might also be counter-intuitive to you. But most people have seen this many times since a young age, so an intuition for such things is formed. At any rate, physicists certainly _hope_ that in the end our universe will prove to operate according to laws that are simple enough for us to understand. Maybe this is a futile hope, but so far they are encouraged by the fact that many seemingly complex phenomena have turned out to be described by relatively simple laws. There’s no particular reason that the interactions of subatomic particles or the formation of stars had to be comprehensible to us as human beings, and yet they are. This perhaps-surprising simplicity is in fact one of the main reasons I personally have chosen to study physics. ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [July 4, 2006, 11:12pm UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/23 "2006-07-04T23:12:58Z")
> [@Exapno Mapcase](#): > > The bricks of the universe can be expressed in many ways. They can be vibrating strings, for example, which is the basis for string theory. And these strings can have two ends or have their ends connected. Expressing the bricks as waves is another approach. > > The virtue of these approaches is that they work. They predict the types of “particles” that we encounter. String theory even predicts their masses, which QM can’t do. > > But there isn’t any theory which works when you think of all the particles as being points. And there is no experimental evidence that particles are points. > > What makes you think that particles are - or rather should be - points? It’s not physics that says this should be true as much as human prejudices about everyday macroworld experiences. But everything we know says that the macroworld is nothing at all like the microworld. (And, going the other way, the makeup of the universe as a whole is nothing like the makeup of the earth.) It may be non-intuitive for humans, but in the scope of the universe, we’re the odd exception to normality and we just need to acknowledge that. Actually, unlike quantum mechanics string theory hasn’t yet been experimentally confirmed. In fact, it hasn’t yet been developed to the point where it can make a definitive experimental prediction which we could use to test the theory. (Some would dispute this claim, but all the proposed “predictions of string theory” I’ve ever seen don’t really “test the theory” in the usual sense.) For something to be considered a “scientific theory” it needs to be testable, and for this reason string theory is sometimes called a proto-theory rather than a full-fledged theory. That said, there are some reasons to hope that the theory can eventually be developed to the point where it makes testable predictions – at least enough so to keep a lot of smart people interested in working on string theory. (Plus, even if string theory turns out to be wrong as an an ultimate theory of everything, it is still a mathematically rich structure which may lead to interesting physical insights. In particular, certain types of quantum field theory are actually mathematically equivalent to string theories.) String theory certainly doesn’t predict the masses of the elementary particles. Some hope that it _will_ eventually explain why they have the masses they do, and this hope has been much touted. However, there are some recent results which suggests there may be 10^500 possible solutions to string theory, in which case such a prediction may prove unobtainable. Time will tell . . . . ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [July 5, 2006, 12:01am UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/24 "2006-07-05T00:01:51Z")
> [@yelimS](#): > > Very well then, explain the end of the universe to me. Oh, it’s somehow circular. Explain how something can excist without being created. We can accept a lot of these things, and even to some extent explain them, but I doubt we’ll understand them intuitively, as I would say we do with a lot of other things that we have learned rather than know instinctively. > Better yet: I don’t think anyone here will ever be able to truly grasp more than four dimentions. > I could be wrong, of course, but my main objection is that it wouldn’t make sense for an evolution of awareness to have an end. You don’t have to travel very far before reality/nature goes beyond intuitive sense. Can you understand how your body works, how the chemical/electrical pathway operate, how the enzymes work, how the cells function, what the brain does? Can you explain even how a plant works? Can you explain weather and the interrelatedness of wind/sun/clouds/water? Can you explain basic electricity? The earth isn’t even intuitively round. My point is just that wanting an intuitive understanding of the basic composition of the universe is really asking a lot. You should look at it the other way round and marvel that we have come up with explanations of so much. **tim314** , Oh, granted on string theory. I was just using that as an example that we can explain the underpinnings in a variety of ways not limited to cutting things smaller and smaller until they are point-sized. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 5, 2006, 1:03am UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/25 "2006-07-05T01:03:44Z")
It’s worth mentioning, by the way, that the Uncertainty Principle (or something analogous to it) shows up in _all_ wave theories, not just quantum mechanics. There’s also a version of it which would apply to a vibrating guitar string, or to waves on the ocean, or amber waves of grain. Quantum mechanics, it turns out, happens to be a wave theory, and so it has an Uncertainty Principle. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [July 5, 2006, 11:21pm UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/26 "2006-07-05T23:21:54Z")
> [@yelimS](#): > > I’d love to hear as complex an explanation as anyone would care to give, by the way. I’m just a bit worried I might not make much sense out of it. But hell, I’m sure I’ve had worse, so if anyone has any time to waste, enlighten us! It’s not complicated at all. It’s simply that operators on a rigged Hilbert space fail to commute with their Fourier transforms. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [July 5, 2006, 11:23pm UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/27 "2006-07-05T23:23:58Z")
> [@Exapno Mapcase](#): > > There are infinities in math, but there are no infinities in nature. We live in a finite universe. BIG, but finite. Oh yeah? Prove it. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [July 6, 2006, 1:07am UTC](https://boards.straightdope.com/t/uncertanity-principle/363203/28 "2006-07-06T01:07:04Z")
> [@Mathochist](#): > > It’s not complicated at all. It’s simply that operators on a rigged Hilbert space fail to commute with their Fourier transforms. Heh… I was waiting for someone to bring that up. 🙂 The best way to understand the Uncertainty Principle that I’ve heard is to look at the difficulty in determining the fequency components in a classical wave You don’t need to understand tht math of Fourier Series and Transforms to see the problem if you look at it this way: Suppose there is a radio wave that you are trying to measure and someone asks you to determine the frequencies in the wave at an exact point in time-- IOW, you are not allowed to sample the wave, you have to measure it at only one, infinitely small, point in time. Well, you can’t do it. A wave simply doesn’t have a frequency at a point in time. In order to determine the frequencies, you need to sample the wave over a length of time. And the shorter the time that you sample the wave, the less accurate your measurement of the frequency components is going to be. Uncertainty! So, just as we say it makes no sense to ask what the exact position and momentum of a particle\* is, it makes no sense to ask what the frequency of a wave is at a specific time. \*A particle whose position is defined by a wave function, that is. [Previous page](https://boards.straightdope.com/t/uncertanity-principle/363203.md?page=1)