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I have a few questions

  1. Apr 11, 2010 #1
    hello, registered just to ask this questions and "if possible" i want philosophical, descriptive, or, "made of words" answers since i don't really know much about physics.

    1. heisenberg and his uncertainty principle; i see a problem there. he says, to see where is the electron, you have to use photons, right? and that gives "unnatural" results, as photon affects electrons (tell me if i'm wrong) BUT, then, if i'm right, we are under constant photon bombardment... so, even if we can measure where this electron is without photons, it will be still unnatural it seems to me... or, there's no natural at all to begin with... so, what do you think about this?

    2. vacuum energy. can't we put this in the very beginning instead of little big bang thing better? or, can we say, big bang was "in" vacuum energy? since this vacuum energy thing present even in devoid of matter? and then, can't we say, there was never nothingness, as in, big bang and whatever "surrounds" it? cause, if there are quantum fluctations then i say there's something. simply, putting something in the beginning doesn't help. they still ask you what was there before it, who put it there and stuff like that. i need an omnipresent thing like god too. or maybe i don't, why then?
  2. jcsd
  3. Apr 11, 2010 #2
    no that is not what heisenberg uncertainty principle tells us, it is a statistical property of the outcome of measurment of position and momentum. It is quantum resolution, even if we could in principle determine the position and momentum of particles with infinite resolution, the stastical behaviour of many such measurments will lead to the heisenberg uncertainty relation, that the standard deviation of x times standard deviation of p is larger than h-bar/2

    so the answer is really physical, the statement of the uncertainty relation you wrote is false, hence all the derivations from that premise will be false as well.
  4. Apr 11, 2010 #3
    i don't understand, how can you say that, starting with an "even if" and ending it like "will lead to"... i mean from where this "standart deviation" comes from? if not from our methods of determining position and momentum. don't we use light to measure those things? isn't this formula of yours what we have derived from using light and failing? and if we don't use light to measure those thing, again, how can you say "still".
  5. Apr 11, 2010 #4


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    What he means is this. Prepare a large number of particles (thousands or millions) in the same quantum state. At the same time after preparation, under the same conditions, measure the position of each particle. In general, you get a different result each time, no matter how precise your measuring apparatus is. From these measurements, calculate the
    mean and standard deviation. Call the standard deviation [itex]\Delta x[/itex].

    Prepare another collection of particles the same way, and measure their momenta under identical conditions, at the same time after preparation as the first batch. Again, in general you get a different result each time, no matter how precise your measuring apparatus is. From these measurements, calculate the mean and standard deviation. Call the standard deviation [itex]\Delta p[/itex].

    The Heisenberg Uncertainty Principle says that [itex]\Delta x \Delta p \ge \hbar / 2[/itex] for the ensemble of measurements, regardless of the precision of each individual measurement.

    If we know the QM wavefunction for the particles, we can calculate, in advance, [itex]\Delta x[/itex] and [itex]\Delta p[/itex], and they will always satisfy the HUP; but we cannot calculate in advance the precise position and momentum that we will measure for any single particle.
  6. Apr 11, 2010 #5
    prepare thousand of particles... you don't need particles, even if they are ants, you will get different results, if you measure their position and momentum less than .5 seconds each time... we are talking about ultra fast electrons here, of course "each time" their position will change. (let alone momentum)

    let's say we use a camera, you take a shot, and you pictured electron in this x position. now, even if you take the second shot .00000001 seconds later, i bet electron will spin around protons and stuff 928374 times. so what this "each time" means, i really don't understand.

    then, i don't know what's the meaning of putting this as if it's so mysterious, calling it uncertain. tell me our way of measuring electrons simply sucks and i'll understand it. but the principle and you guys talk like as if we have the best apparatus and it's just electron that is acting weird or uncertain or random or fuzzy. and from this "each time" example of yours, i doubt it.

    so what i mean is, even if i'm right and even if uncertainity principle says "we just can never take two shots, so close to each other, so we can see electron moving 2 or 3 steps... we will never, reach that technological level".... then, again, i can't see how we are deriving formulas and physics equations, having such neanderthal tools, if you know what i mean.
  7. Apr 11, 2010 #6
    One of the beauties of higher education is understanding the foundation of these formulas.
    Not that it's necessary, but, if you desire to challenge them it most definitely is.
  8. Apr 11, 2010 #7
    i don't really challenge any formulas, i only challenge jtbell's "each time" concept. tell me one thing, can we even measure where's the electron and how we are doing that? i was reading since half an hour, found this; http://en.wikipedia.org/wiki/Heisenberg's_microscope

    simply, he says...

    If the photon has a long wavelength and low momentum, the collision doesn't disturb the electron's momentum very much, but the scattering will reveal its position only vaguely.


    If the photon has a short wavelength, and therefore a large momentum, the position can be measured accurately. But the photon scatters in a random direction, transferring a large and uncertain amount of momentum to the electron.

    so, as far as i can see, what we use, or "thought" to use to measure electron's position and momentum is a photon just like i said. and i say, just because photon has these effects on electron, it doesn't mean, electron is actually acting weird or has any uncertainty in its nature. it's just our tools that makes the uncertainty.
  9. Apr 11, 2010 #8


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    It is important to note that this in not a "measurement problem"; and it is not an "inadequate technology" problem.

    The nature of the universe is such that there is no way, even in principle, of measuring both the position and momentum of a particle simultaneously beyond a certain degree of accuracy. The electron does not have both.

    One of the ways of reconciling this is to realize that an electron is not a shiny, rigid ball such that we could determine its exact position; it is smeared out across space (as all matter is). The more precisely we try to pinpoint a specific spot, the less of the "smearing" we are including in our measurement. Conversely, to try to capture all the "smearing" that constitutes the electron, we cannot talk about a pinpoint. Loose and oversimplified, but it might help.
    Last edited: Apr 11, 2010
  10. Apr 11, 2010 #9
    the point is, even if this photon throwing thing works like heisenberg thought, aren't we under constant photon bombardment and if we are, how can you say electron is not affected?

    edit since mr. dave added four more lines: then, did heisenberg simply tried to do something impossible, pinpointing something can not be pinpointed and just because he failed this impossible mission, saying uncertain and stuff ? what was that microscobe he thought and all, if a smearing thing was the case?

    besides, how can you tell something is smearing, has a glow, or it's the glow itself, while you can't even measure it's position and momentum. its dimensions or shape should be even harder to determine. like, can't we be wrong about it, making out smearing things, to rationalize (wrongly) uncertain (in theory) results?

    because from what i read at that heisenberg's microscope page, all i can say is electron is just too fast, photon is too big to throw at it (if you throw strongly it changes electron's momentum)
    Last edited: Apr 11, 2010
  11. Apr 11, 2010 #10


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    You cannot learn enough about a subject in so short a time and be able to draw conclusions from it, let alone challenge it. If you're interested in learning why it is the way it is, grab a book and read.

    Alternately, ask your questions one or two at a time, and we'll try to answer them. Stream of consciounsess writing is not very constructive.
  12. Apr 12, 2010 #11
    Last edited: Apr 12, 2010
  13. Apr 12, 2010 #12
    okay then, one simple question each time: how do we measure electron's position? (only position) do we still "have" only heisenberg's microscobe? do we still throw photons at it or did scientists found a better way since heisenberg?

    simple enough i guess.
  14. Apr 12, 2010 #13
    But that measurments distrub the system also occur in the "classical" world, it is nothing special with quantum physics in that sense. So why one should be intersted in that issue when it comes to QM is not clear to me.
  15. Apr 12, 2010 #14
    i can tell you the same thing now, you just seem nervous. how can you even compare thermometer's effect on my body to photon's effect on electron! do i change the momentum of this car when i take two pictures of it?

    if electron is not a rigid ball, how to say, even after throwing a photon at it and calculating its deviation from its original route, that electron was here or there?

    like, if you throw a billiards ball to another, you both know their weight, what they are made of and stuff... but throwing a photon to a pretty much dreamy glowing/smearing/energy packet...

    i'm just trying to understand, i guess you people think i'm attacking quantum mechanics though, no.
  16. Apr 12, 2010 #15

    I am not the one who is nervous, I am PHD in theoretical particle physics..

    Yes photons change the momentum of the car when they hit it... but very very very small. You are also too quick to go to systems which you think you can handle. What if I gave you a tiny ball with mass = electron mass and radius 1 fm, and put it on a surface with no friction. Strike this ball with photons and it will move due linear momentum conservation. But let us now switch on quantum physics, then one additional source of uncertainty appears, due to the INTRINSIC wave nature of this "ball". THAT is where Heisenberg uncertainty relation comes in, it gives the INTRINSIC QUANTUM uncertainty of measurements. Thus even if one IN PRINCIPLE could measure position and momentum with infinite good precision, one will still have this quantum uncertainty - THIS is what you must understand and accept.

    The thing is that you are approaching quantum mechanics from an intuitive everyday life way, this is not what one should do - QM is contraintuitive, one must "do the math" to understand why and how quantum phenomomena occurs.

    Of course the effect of the photon striking the electron will have greater effect, but still the effect on the car is there (put the car in vacuum and on a surface with no friction and you will see this)

    And in physics, it is perfectly fine to have collisions/scattering of objects with no finite extension, since the logical language of physics is MATH not "plain words" as in philosophy.
  17. Apr 12, 2010 #16
    well, it was you who said what i'm talking about also happens in classical world. yet, now you are removing friction out of that world. even though i understand what you mean, you have to spend rest of your life typing "very" and then when you die, i can add "small". and still, that won't be close to describe how very small is photon's effect on car. you went compared that to quantum particles and now saying i have phd shut up, or so it seems to me...

    consider this dart i have is a photon. and consider electron is the dart board (in fact, it's only the bullseye of dart board, but it moves or waves so fast it kind of makes the rest of the dart board) also, i have a robot to throw the dart and it's throwing it always at this constant speed. and finally, dart has no needle.

    i crack open this robot's skull and i enter in what angle it should throw the dart. dart goes and hits bullseye then bounces 2 meters away with x angle and y momentum and whatever... this is my first data. then i open its skull again and entering new angle. dart hits the board again, but not bullseye, bounces back, gives different results.

    is it like this?
  18. Apr 12, 2010 #17
    If you remove friction of the car, then you would notice the effect much easier, the issue here is "in principle"
  19. Apr 12, 2010 #18
    "So why one should be intersted in that issue when it comes to QM is not clear to me."

    because there's no friction in qm, because stuff "in principle" doesn't really affect classical world and because of all this, photon's effect on electron is an issue, unlike photon's effect on car, because there's friction and removing it saying in principle doesn't change the facts.
  20. Apr 12, 2010 #19
    so the car put on a frictionless surface is NOT a classical situation?
  21. Apr 12, 2010 #20


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    Yes, it is true that you measure an electron with photons. But the uncertainty imparted by the photon is not the cause of the Heisenberg relations. You can measure the position of an electron as precisely as you like. So where is the issue, you may ask?

    The answer is that the electron behaves as a point particle when you consider its position. But it behaves as something more akin to a wave when you consider its momentum. For example, it can exhibit interference effects that do not appear when you consider its position.

    You may find it easier to understand if you consider entangled particle pairs. Entangled electrons obey the HUP too, even when separated. Research the exploits of Alice and Bob and I think that may help put you in the ballpark.
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