Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

If everything is random how can we have laws?

  1. Nov 28, 2012 #1
    I haven't studied quantum mechanics yet but I know that at the lowest level everything is random.
    But if everything is random how can we have definite laws like newtons laws, electromagnetic laws. I mean if everything is random at the lower level these laws will also vary right, they can't be fixed!
     
  2. jcsd
  3. Nov 28, 2012 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Things APPEAR random if you make one measurement of one thing at a time. However, this is extremely unusual. Our normal world deals with us making "measurements" (or interactions) a gazillion times each picosecond! When that occurs, then quantum mechanics make very accurate and uncanny predictions, and classical mechanics appear to not be random, resulting in our theories being quite accurate.

    Zz.
     
  4. Nov 28, 2012 #3
    Quantum mechanics isn't exactly random. There are probabilities, and certain events have a much higher probability. When you multiply that high probability among largescale systems (keep in mind, one inch is around 1033 plank lengths. So if you have a probability of 1/10 of something wierd happening to one electron, multiply that out over a huge number of times, that chance becomes virtually zero. So on the every day scale, the more probable configurations become virtually the only configurations.

    In theory, you could run into a wall and the spaces between your molecules and the wall's molecules could line up exactly right so that you would pass right through it (it's called quantum tunnelling). But the chance is so low, you could run against the wall 100 times a second til the Earth is engulfed by the sun transitioning to a red giant and it still probably wouldn't happen.

    On the other hand, we exploit quantum tunnelling on a small scale all the time in modern computing. We manipulate the chances so it is more likely an electron will go through the "wall".
     
  5. Nov 28, 2012 #4
    "Random" in physics means that, of a finite number of available options, it is not possible to predict which will manifest.
    Think of flipping a coin. It will come up either 'heads' or 'tails', randomly. It will NOT come up 'red', or 'negative', or 'charmed' - ever. Those are not options available for a coin toss.
    The laws come up in defining the options, not the outcome.
     
  6. Nov 28, 2012 #5
    The outcomes, and the relative probabilities for those different outcomes. Hence the predictive power of quantum mechanics.
     
  7. Nov 28, 2012 #6
    So are the laws just an approximation of the quantum phenomena?
    I would take the example of newton's laws since they are very easy: -
    First law states that a body would continue to move in the same velocity as long as not acted upon any force. But the individual atoms or molecules could change their energy and suddenly speed up or slow down right? But that has a very low probability and as an approximation we have the first law.
    Is my interpretation right?
     
  8. Nov 29, 2012 #7
    You should be more careful with saying words like "everything". On the quantum scale, there are experiments whos outcome cannot be predicted due to randomness, but that does not mean "everything" is random.

    For example, a particle in its eigenstate will remain in its eigenstate and will may be easily predicted. Also, even though it's not possible to predict when exactly an excited state will decay, it's perfectly possible to predict the average time it will the decay, given many particles. So the varible called "average decay time" is not random at all.

    Furthermore, as has been pointed out by others above, what is random in any case is only the particular outcome of an event/interaction. The laws governing that event is not random, it's just that the laws act on "probabilities". How those probabilties evolve under the laws can be very well defined, it's just that they are still probabilties in the end, which is why the outcome may not be predictable.
     
  9. Nov 29, 2012 #8
    I want you to give my an example of a law.
    Everyone says that newtons laws are not entirely correct. They are just an approximation...I want to know if this is true
    Is the above interpretation right?
     
  10. Nov 29, 2012 #9

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    This makes very little sense because we have not detected what you quoted. So you are essentially asking us to explain something that has never happened.

    Note that even for elementary particles, we haven't seen such a change in trajectory that you are describing. So what you claim is unverified and highly speculative.

    Zz.
     
  11. Nov 29, 2012 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    When the word 'Law' was first used in order to 'explain' the world, it really was thought of as a Law of God. i.e. God commanded things to work that way. Laws in Physics are not regarded like that any more. A Law is more of a description of what has been observed consistently for some while and so it can be used to predict what will happen in a new circumstance or to explain why something occurred the way it did.
    Take most laws and you will find apparent exceptions to them. That doesn't make them any less valid or useful even if it may be personally upsetting.
     
  12. Nov 29, 2012 #11
    I haven't heard it said this way, but I think a law is basically a mathematical model of something physical.

    -Dave K
     
  13. Nov 29, 2012 #12
    No. Random doesn't mean particles can defy the laws of physics. It just means that, on some level, we can't predict the exact values of a tied pair of variables ( such as momentum and position) within a certainty of plank's constant. So any given particle has a wavelength. And we can't predict exactly what it's momentum (speed) or position will be at some given moment in the future. But that doesn't mean it can be anywhere or do anything. It is still confined by the wave function.

    But wavelengths can cancel and amplify each other. So when you add particles together to scales above the quantum level, they behave in an extremely predictable fashion.

    When people say Newton's Laws are an approximation, they are often refering more to Einstein's relativity, which expands Newton's laws to situations of extreme velocity and acceleration.
     
  14. Nov 29, 2012 #13
    Oh okay that made me understand a bit. I had the wrong notion of random. I need to take a course on quantum mechanics as soon as possible!

    Thanks for correcting me.

    So can the macro-laws be derived by adding all the wave-functions of particles in an object
     
  15. Nov 29, 2012 #14
    Yes, to high accuracy. Although very soon the processing power required becomes the limiting factor.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook