Summer School 2010.4: Quantum Physics

Heisenberg’s work is a prominent feature of summer school lesson four, Don’t Underestimate the Power of Virtual Particle Exchange.

7 replies on “Summer School 2010.4: Quantum Physics”

  1. Chad Cloman says:

    [NOTE: In this post I use the caret symbol in place of the delta symbol.]

    Blaine,

    This one was a bit harder to understand, I think in part because I would do a great job of following the discussion, but then you’d make a conclusion that seemed to come from nowhere. So here are my comments and questions:

    1) In section 4, regarding the sentence that begins with “Our forces are mediated by virtual particles who are limited by…”.

    Since particles aren’t people, I think it would be better to use “that” or “which” instead of “who”.

    2) Still in section 4, you first state this: “Our forces are mediated by virtual particles who are limited by a maximum value of ^E^t.” This statement tells me that the ^E^t value of the virtual particle cannot exceed 6.

    But then, in the 4th bullet and its footnote, you switch things and make 6 the lower limit — saying that the particle’s ^E^t must be higher than 6. That seems contradictory.

    3) Still in section 4, regarding this sentence: “There is no reason to expect a minimum speed for virtual particles, or a minimum kinetic energy, so we are led to another seemingly inevitable conclusion: the particles mediating these two forces have mass.”

    I don’t see the connection for this conclusion. Is it because the particle has more energy than it’s allowed to, and it therefore converts the excess energy into mass?

    4) You do a very good job of explaining how energy is transferred between particles via the mechanism of the virtual particles. But I don’t understand how that becomes an attraction or repulsion. I think that part is glossed over — here’s what you have to say about it: “Therefore, we can safely conclude that energy is transmitted in the form of short lived particles, and these particles mediate interactions between other particles, including all forces.” The “including all forces” portion of that sentence is the first mention that forces are just the transfer of energy, and it’s presented as a fait accompli without explanation.

    5) For most of the article, only the transfer of energy is discussed. But in section 6, you mention that momentum is also transferred. Is this an important distinction? Is it the loss of momentum that causes the “attraction” of a force?

    6) In section 5, here’s how I understand it. The strong nuclear force increases with distance until the virtual particles in the middle get enough energy to put them beyond Heisenberg’s limit, and they become real particles, which then interrupt/intercept the attraction of the strong nuclear force between the original particles. So there should be a constant stream of new particles being created between any two points in the universe that are moving apart. Do I have that right? If so, and since the universe is expanding, why isn’t there a line of new particles between the Earth every star in the universe? Or is there?

    • Chad: Just to be clear, I’m not ignoring you. My day job got suddenly and insanely busy this week. You’ll get a proper response over the weekend.

    • 1) True, noted. Edits will appear in the future.

      2) Yeah, I botched that up in a midnight edit. (You can see the artifact of this edit with “an lower” where I only half changed “an upper.”) We have Et<c for virtual particles. They have a maximum lifetime range, so something is enforcing a maximum lifetime. If there’s a minimum energy, then d<E for some value of d. Thus, dt<Et<c, so dt<c and t<c/d, giving the maximum lifetime we see. Thus, a minimum energy would explain the behaviour, which is exactly how we explain the range of the weak force.

      3) This is likely more clear with the above correction with point 2. The minimum energy can’t be on the kinetic side, as there’s no lower speed limit, so it must be on the mass side of the particle’s total energy.

      4) Yeah, it is glossed over, and it’s glossed over virtually everywhere I look. The math works, but it works because we have coupling constants in place that we have jammed into the formulae to fit experiment. It’s all momentum exchange, but I haven’t seen or figured out a clear explanation of why situation A is attractive and situation B is repulsive.

      5) Yes, it is, but again, it’s because we forced the math to fit experiment as far as I can tell, which means we don’t yet have a purely conceptual means to explain why things happen this way.

      6) There are virtual particles all over the place becoming real particles, but that’s not quite why we don’t get the stream here. Once we have a set of quarks in close proximity that form a charge neutral collection, the gluons they exchange interact with each other so much that they end up in a closed and bound state. I should revise that to clarify.

  2. dark grep says:

    Fantastically good reading. Thanks for posting it. Really looking forward to the rest.

  3. ray says:

    Hope the following doesn’t earn me a black mark because I really do love math, science, physics, and the natural order of nature – but don’t know about all of this. All these particles that we have never seen or detected, with properties that we define so that if they exist they behave the way we need them to fit the model we have created reminds me of the type of conjecture that prevailed in the middle ages regarding what existed beyond the end of the (flat) world. An elaborate conceptualization.

    I love reading this stuff, it certainly makes one think, and I also really appreciate the effort many have put into trying to understand what we cannot see or precisely measure, but I cannot help but believe that we have grossly overcomplicated the model because we lack some basic understanding of the elementary properties of space, time, and the various manifestations of fields.

    Most of the scientific community seems to be in lockstep with all of this – trapped by the need to conform to existing theory. Result: more elaborate particle creations with unique properties – no size, no mass, doesn’t feel gravity, particles that return to the source unless they are intercepted by another particle…what is all of this?. If the LHC fails to bring something new into view what are the odds that science will just want a bigger collider rather than rethink the possibility that we have reached the end of the road with our “model” via trying to find verification through particle collision experiments?

    There are many complex relationships in nature that with the limits of intellect we have as humans we could never solve with 4 function math. Enter Newton and Calculus. This gave us a completely new set of tools allowing new insight to the natural world around us. I just have the feeling that something similar needs to happen again so we can grasp what currently seems to be transgressing the threshold of reality.

    In the meantime, I promise to keep reading and will try to keep an open mind, as I concede many many people much smarter than me have spent lifetimes thinking about all of this, and to that end it deserves no less than a tremendous amount of respect. But I cannot help but wonder…..

    • There are professionals who say the same. Others ask why the correct theory must be simple enough for humans to understand at all. It really comes down to gut feelings and aesthetics.

      Also, keep in mind that there are good reasons some of these are still around. If we were to rate scientific theories based on their ability to accurately predict experimental results, then quantum electrodynamics (describing the electromagnetic force interactions at the quantum scale) is the single best scientific theory mankind has ever developed. Nothing fits experiment this well. Similarly, quantum chromodynamics (related to the strong force) is in the top five. We may be misinterpreting the math, but the math is almost undoubtedly right.

      Of course, there was a time we could have said the same about Newtonian mechanics, so I wouldn’t stake my life on it.

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