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The physics of the "present"

This is a discussion on The physics of the "present" within the General Discussions forums, part of the Community Boards category; Originally Posted by manasij7479 Maybe I'm missing your point, but matter is quite commonly considered as waves, in duality with ...

  1. #16
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    Quote Originally Posted by manasij7479 View Post
    Maybe I'm missing your point, but matter is quite commonly considered as waves, in duality with particles.
    Matter behaves in ways that can be predicted using equations that involve waves. This just means it can be predicted like that, it doesn't mean that particles are waves. Whether a particle "is" a wave is maybe not even a meaningful question.

    The wave function of any particle is, in general, complex-valued and thus has no direct physical meaning. What the wave is "waving" is measured in units of probability amplitude, it is not a real thing which exists. It is just a mechanism to predict what happens.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    Quote Originally Posted by brewbuck View Post
    Matter behaves in ways that can be predicted using equations that involve waves. This just means it can be predicted like that, it doesn't mean that particles are waves. Whether a particle "is" a wave is maybe not even a meaningful question.
    Similarly, "Matter behaves in ways that can be predicted using equations that involve" particles.
    I didn't say that a particle is a wave, but 'matter' and 'particle' are definitely not the same thing, afaik.

    The wave function of any particle is, in general, complex-valued and thus has no direct physical meaning. What the wave is "waving" is measured in units of probability amplitude, it is not a real thing which exists. It is just a mechanism to predict what happens.
    Everything in physics is a mechanism to predict what happens, or abstractions of what is happening; physical significances are totally a human invention.
    The probability function wave does not predict the probability of a 'particle' existing at a point in space but the magnitude of the behaviour exerted , for example chemistry uses this to determine interaction between atoms quite effectively.

    N.B: This just my understanding of how things are, and I'd be happy if you point out where I'm wrong.
    Last edited by manasij7479; 05-30-2012 at 06:41 PM.
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
    Slow and Steady wins the race... if and only if :
    1.None of the other participants are fast and steady.
    2.The fast and unsteady suddenly falls asleep while running !



  3. #18
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    Quote Originally Posted by manasij7479 View Post
    Everything in physics is a mechanism to predict what happens, or abstractions of what is happening; physical significances are totally a human invention.
    The probability function wave does not predict the probability of a 'particle' existing at a point in space but the magnitude of the behaviour exerted , for example chemistry uses this to determine interaction between atoms quite effectively.
    If you are saying that the wavefunction is a statistical property of a whole ensemble of particles, that isn't the case -- it works even for a single particle, and really does denote probability, not collective tendencies. But wave/particle dualities occur in many contexts and not all of these are the same. The equations are similar because many physical relationships can be described as proportionalities between some quantity and its various derivatives. Mathematically, those sorts of systems have wave-like solutions. But the fact that waves arise everywhere doesn't signify that they are all of the same nature.

    I base my own interpretation strongly on Feynman's, and by no means claim that it is the only valid one.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    This link will take you to a video that is not safe for work!

    ^_^;

    I finally dug up the link.

    I was going to post this the other day.

    This is "off topic", but I immediately thought of this when reading one of the posts.

    Soma

    "Dimension X" NINJA TURTLES/10th DIMENSION RAP by Lil Deuce Deuce - YouTube

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    The wave nature of particles (e.g. electrons) can be seen in the double slit experiment.
    Electrons will produce interference patterns, much like light. These interference patterns
    of alternating light and dark bands are characteristic of waves.
    In addition, there is a probability wave function which predicts where the electrons will
    strike the target and therefore where the bands will appear.

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    Quote Originally Posted by megafiddle View Post
    The wave nature of particles (e.g. electrons) can be seen in the double slit experiment.
    Electrons will produce interference patterns, much like light. These interference patterns
    of alternating light and dark bands are characteristic of waves. In addition, there is a probability wave function which predicts where the electrons will
    strike the target and therefore where the bands will appear.
    It's not "in addition," it's the whole reason it happens. The wavefunction is not the electron. For just one particle you might convince yourself that the wavefunction is the electron, but for more than one particle it all breaks down. For instance, a system of two electrons has ONE wavefunction. You cannot write this wavefunction in terms of the sum of two wavefunctions belonging to each electron individually, it doesn't come out right. electron != wave
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    Quote Originally Posted by brewbuck View Post
    It's not "in addition," it's the whole reason it happens. The wavefunction is not the electron. For just one particle you might convince yourself that the wavefunction is the electron, but for more than one particle it all breaks down. For instance, a system of two electrons has ONE wavefunction. You cannot write this wavefunction in terms of the sum of two wavefunctions belonging to each electron individually, it doesn't come out right. electron != wave
    I didn't mean that the wave function was the electron. I would say that the wave function
    describes the electron, in this case it describes it's location. However the electron does
    behave as if it were a wave; cover one slit and the interference pattern disappears.
    Isn't that what is meant by the "wave nature" of a particle?

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    Quote Originally Posted by megafiddle View Post
    I didn't mean that the wave function was the electron. I would say that the wave function
    describes the electron, in this case it describes it's location. However the electron does
    behave as if it were a wave; cover one slit and the interference pattern disappears.
    Isn't that what is meant by the "wave nature" of a particle?
    You are confusing de Brouglie waves with probabillity waves .
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
    Slow and Steady wins the race... if and only if :
    1.None of the other participants are fast and steady.
    2.The fast and unsteady suddenly falls asleep while running !



  9. #24
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    Quote Originally Posted by megafiddle View Post
    I didn't mean that the wave function was the electron. I would say that the wave function
    describes the electron, in this case it describes it's location. However the electron does
    behave as if it were a wave; cover one slit and the interference pattern disappears.
    Isn't that what is meant by the "wave nature" of a particle?
    When you cover one slit the interference disappears because the wave function isn't interfering anymore, it still doesn't mean the electron is a wave. This is in contradiction to what you read in introductory quantum texts, but not contradictory with what you read in the more advanced texts. It becomes clearer when you start considering systems of more than one particle.

    There is a great lecture by Feynman which was videotaped, and it's on Youtube. He addresses this exact question at the end of the lecture. When I get home I can post a link. For now, it basically goes like this (paraphrased):

    Student: "You just spent all this time telling me that things are particle, but then you show me all these equations that look like the equations of waves. So, aren't you just being sneaky and trying to hide the fact that your theory of matter is really just a wave theory?"

    Feynman: "It's true that for a single particle the equations are wave equations. A wave is a function of position and time. But things change when you consider systems of more than one particle. There is a wavefunction for the system, and it is a function of multiple positions and time. That is not a wave. Further, if a single wavefunction describes more than one particle, how can it be said that this wavefunction represents this particle or that particle? You can't."
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    Quote Originally Posted by manasij7479 View Post
    You are confusing de Brouglie waves with probabillity waves .
    Yes, it is de Broglie waves that I'm thinking about. Doesn't that apply to the wave nature of electrons
    and the interference patterns in this case (the double slit experiment)?

    Quote Originally Posted by brewbuck View Post
    When you cover one slit the interference disappears because the wave function isn't interfering anymore, it still doesn't mean the electron is a wave. This is in contradiction to what you read in introductory quantum texts, but not contradictory with what you read in the more advanced texts. It becomes clearer when you start considering systems of more than one particle.

    There is a great lecture by Feynman which was videotaped, and it's on Youtube. He addresses this exact question at the end of the lecture. When I get home I can post a link. For now, it basically goes like this (paraphrased):

    Student: "You just spent all this time telling me that things are particle, but then you show me all these equations that look like the equations of waves. So, aren't you just being sneaky and trying to hide the fact that your theory of matter is really just a wave theory?"

    Feynman: "It's true that for a single particle the equations are wave equations. A wave is a function of position and time. But things change when you consider systems of more than one particle. There is a wavefunction for the system, and it is a function of multiple positions and time. That is not a wave. Further, if a single wavefunction describes more than one particle, how can it be said that this wavefunction represents this particle or that particle? You can't."
    Don't you get the same results though, whether there are many electrons or just one? When the beam intensity
    was reduced to the point where only a single electron at a time was passing through one of the slits, the interference
    pattern persisted. Same as a dense beam.

    Feynman's lectures were great, I've watched all that I could find. He has a way of explaining a most
    difficult subject.

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    Quote Originally Posted by megafiddle View Post
    Don't you get the same results though, whether there are many electrons or just one? When the beam intensity
    was reduced to the point where only a single electron at a time was passing through one of the slits, the interference
    pattern persisted. Same as a dense beam.
    With many electrons the pattern builds up quicker, but other than that it's not different than a single electron. The electrons in the beam for the most part do not interact with each other and so they can be described by independent wavefunctions. In other circumstances (like the two-electrons-in-a-box problem) there is an interaction and therefore only a single wavefunction for the system.

    "Real waves" can interfere with other waves. Wavefunctions are not like that, they can only interfere with themselves.

    I'm not arguing at all that something wave-like is happening, I just refuse to identify "electron" with "wave," a la Feynman. This isn't a settled issue!
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  12. #27
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    Here's the video I was thinking of:

    Feynman on Wave Particle Duality (QED Lecture in New Zealand) - YouTube

    You've probably already watched it if you've watched/listened to Feynman, but I toss it out there just in case.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    Quote Originally Posted by brewbuck View Post
    With many electrons the pattern builds up quicker, but other than that it's not different than a single electron. The electrons in the beam for the most part do not interact with each other and so they can be described by independent wavefunctions. In other circumstances (like the two-electrons-in-a-box problem) there is an interaction and therefore only a single wavefunction for the system.

    "Real waves" can interfere with other waves. Wavefunctions are not like that, they can only interfere with themselves.

    I'm not arguing at all that something wave-like is happening, I just refuse to identify "electron" with "wave," a la Feynman. This isn't a settled issue!
    In the case of electrons then, the wave function of a single electron is interfering with the same wave function through the other slot?
    As opposed to electromagnetic waves, for example?

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    Quote Originally Posted by brewbuck View Post
    Here's the video I was thinking of:

    Feynman on Wave Particle Duality (QED Lecture in New Zealand) - YouTube

    You've probably already watched it if you've watched/listened to Feynman, but I toss it out there just in case.
    Thanks for the link. I had seen it before, but it makes somewhat more sense now.

    So for a single particle, the probability wave function is mathematically the same as the the amplitude wave function for an electromagnetic wave?
    Last edited by megafiddle; 05-31-2012 at 08:45 PM.

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    Quote Originally Posted by megafiddle View Post
    Thanks for the link. I had seen it before, but it makes somewhat more sense now.

    So for a single particle, the probability wave function is mathematically the same as the the amplitude wave function for an electromagnetic wave?
    Yes, which is why the wave theory of light works so well.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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