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Quantum over Classical?

  1. Jul 16, 2003 #1
    I must start by saying I have very limited knowledge of quantum physics. I have only just started to delve into what it’s all about.

    My question is how often are the various quantum physics put to practical use, such as in engineering? What I understand of quantum is that its real achievement is being able to explain so many phenomena because it does so at a fundamental level. However it seems like it would be quite impractical to use it in place of classical for most engineering purposes (more complication for not much more accuracy).

    Perhaps because I’m so new to it I’m speaking too soon. What do you think?
  2. jcsd
  3. Jul 16, 2003 #2

    Tom Mattson

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    PET Scan
    CAT Scan
    Nuclear Reactors
    Photoelectric Cells
    Scanning Tunneling Microscopes

    I'm sure others will add more.
  4. Jul 16, 2003 #3
    I dont think it is a wise idea to judge a science
    on what its current accomplishments or uses as it can
    be very difficult to predict what a certain science may accomplish
    in the future. If we did that then science would not be where it
    is today because the government would never give funding.

    Thats just my opinion
  5. Jul 17, 2003 #4

    Ivan Seeking

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    Newton did his work roundabout 1650. The industrial revolution resulted from this, but it didn't reach a full head of steam until 200 years later. Far more complex, and orders of magnitude greater in it's potential, QM is only about 70 years old. We have not even begun to feel the full impact of this revolution. The revolution in electronics is but a birth pang of what's to come. Thanks to QM, anyone alive today has a chance of living 400 - 600 years. We seek to build machines that act as artificial viruses to cure disease. People are planning machines, barely bigger than a typical molecule but that can construct matter one atom at a time, and in whatever form desired. We look to build computers that will do in one second what my laptop could only do in 10,000 years. It's going to be real interesting!

    In two hundred years the students will be asking: Obviously the primitive concepts from QM have practical applications, but what good is Farn's Great Grand Theory of Everything?
  6. Jul 17, 2003 #5
    Alright, not a bad list. I would imagine that there are probably many more applications that fall into that category of technology. A lot of those probably came about because of the quantums.

    I'll narrow my question... Are there any engineering fields that once only used classical that now use QM? Also, what are the definite limitations of classical mechanics?

    I'll level with you... it's not that good. In fact don’t even bother with it.:wink:

    But thanks for the encouragement! I'll have to get started on it now.
  7. Jul 17, 2003 #6
    Good question.
    I think that the greatest contribution of QM to the engineering has been the electronic devices.For example,earlier we had mechanical governers in the automobiles while now we have electronic sensors and actuators doing the same job at comparibly very high accuracy and with low power consumption.This also partially answers your second question

    the electronic devices have very low voltage requirements as compared to mechanical energy-conversion based electrical systems and also causes low pollution.

    The present nano-tech research is more and so QM-based and if we are successful in manipulating atoms like we manipulate bricks the all-quantum engineering day is not far away.
    Advantages? The nanotubes and like have far high elastic strength and if we make things like structures(civil engineering),cloths(fabric industry) we will attain higher reliability.Already,there have been successes in bionanotechnology which is QM based while the earlier('old') bio-technology was not QM based.
  8. Jul 18, 2003 #7
    quantum over classical

    I think you do have a point. You might find the writings of Professor Edwin T. Jaynes to be of interest. I don't recall the exact papers but in several of his papers discussing quantum mechanics, which he was quite competent in as a research physicist, he noted that often working physicists relied on classical approximations to complicated quantum mechanical equations.
    Professor Jaynes was also an advocate for a neo-classical theory of electrodynamics.
  9. Jul 19, 2003 #8
    QM...just a few more digits?

    Ivan Seeking:

    "Thanks to QM, anyone alive today has a chance of living 400 - 600 years"...???

    I like where you were going up until then...I think you are streching things a bit.


    About the uses of QM in engineering, I have a unique perspective to answer this question. I have an undergrad. physics degree and have recently joined an elect. engineering dept.

    P.S. I love the list TOM gave, I want to elaborate on one or two of these who aren't familiar with the role QM plays in the device applications.

    PET (positron emission tomography):
    The PET scanner, used in medical imaging, fundamentally relies on the positron. It was theoretically predicted by Dirac when he devised a relativistically correct "Schroedinger Equation", called the Dirac equation. A natural consequence of the theory showed that particles resembling electrons, only with an opposite charge, must exist. It was decades later when Anderson experimentally observed the positron.

    Quantum heterostructure devices, such as a MQW (multi-quantum-well) laser, fundamentally rely on the quantization of energy levels due to quantum confinement and the always fascinating phenomenon known as quantum tunneling.

    Atomic physics:
    Something as simple as looking at the discrete spectrum (the different colours of light) coming off of an arc-lamp (something similar to a fluorescent light), can't be explained without quantum mechanics.

    So, you see, although one can look at QM and observe that it produces more accurate results for, say, the dynamics of macroscopic objects, the NEW, PREVIOUSLY-UNPREDICTED results is what makes it ground-breaking and (without exageration) revolutionary.

    Another comment I have:
    Semi-classical electrodynamics is OK depending on what you want the theory to do for you, and some may be satisfied with it, but it has limited scope. When people use semi-classical theories to explain phenomena, it is implied that quantum effects are insignificant or (remarkably) coincide with quantum results for DEEPER reasons. When this is the case, why use QM? For example, I don't use Schrodinger's equation to solve the trajectory of a baseball, do I (it would be much harder, if not impossible analytically)? I am pretty sure Prof. Jaynes would have agreed with me.

    For example, atomic lifetimes:
    The classical lifetime of an atomic excited state has a classical expression, but it is simply wrong. One can't do laser spectroscopy and predict lifetimes (or atomic linewidths) using a semi-classical theory.

    Here is an example of when a classical approach is fine, Rutherford scattering:
    Classical mechanics (and electrostatics) predict a certain differential scattering cross-section to be observed when alpha particles strike a thin metallic foil, and the results agree with observations. Why? The theory got lucky.

    When one uses QM to solve the problem given the nuclear potential making the Born approximation (among others), calculating the scattering amplitude the same result is obtained. So why do the two agree, sheer luck (my intuition tells me there may be a deeper reason, but I don't know what it is).

    In this case, however, the math involving both solutions are comparable in difficulty, so it is the physicists choice how he wants to model his results.
  10. Jul 19, 2003 #9
    Physics Coincidences

    I have become excited thinking of other "coincidences" in physics. There are a few, such as the agreement b/w the classical and QM results for Rutherford Scattering. Here is another (who can tell me what the fundamental reason is and why this result is NOT a coincidence):

    When using Schrodinger's equation (w/o higher order corrections) on, say, any atom other than hydrogen (or hydrogenic systems), all states with the same principal quantum number, n, are found in general NOT to be degenerate. With hydrogen, however, we find all states sharing the same n values are degenerate. Why?

    In other words, the eigenenergy of any stationary state is found to depend only on n for the hydrogenic potential (their is no mention of l or m). Why?
  11. Jul 19, 2003 #10

    Ivan Seeking

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    Re: QM...just a few more digits?

    I take this quote directly from a leader in genetics research...
    If you don't trust me on this I can dig it up. But I assure you that a person of significant status and in an appropriate position to do so has made this claim. He said: "anyone alive today has a chance of living 400 - 600 years." I wouldn't make such a statement as a personal opinion.

    The fact is that several researchers have one or several species of worms living 4 to 6 times their normal lifespan right now. Many mechanisms for aging are being identified. Recent findings about the limitations of cell replication due to a failure of [I'm not sure of the technical name…I think it was telomeres…has been very promising. These telomeres act basically likes caps that hold the cell together. These fail after about 25 replications [in some cases] and apparently this failure can be prevented. It also seems that a number of genetic clocks are built in; not really like time bombs...but more as simple limitations that start creating failures. Nano technology, genetic engineering, gene therapy and others all offer potential solutions to these problems. I know the claim is extreme but I'm just the messenger.
  12. Jul 19, 2003 #11

    OK Ivan Seeking, I will take your word on it. I am sure you understand my hesitation, it does sound bold and outrageous.

    Where is the connection to QM, though?

    I don't know much on the current state of genetic engineering - not my specialty.

    Sounds promising, nevertheless!
  13. Jul 19, 2003 #12
    I'm doing graduate studies in the department of electrical engineering at my university. I am researching the application of quantum dots for quantum computing. Although most of the people in my field consider themselves physicists and applied physicists, I consider myself an engineer because I apply the principles of science for the sake of making machines (or advancing technology). I do not study nature for the sake of studying it which is what true scientists (including physicists) do.

  14. Jul 19, 2003 #13
    Physicist - Engineer ... we are all on the same side!


    WOW! We are almost colleagues. I am also doing grad. work (master's) on quantum dots in an elect. eng. dept, however my group's motivations are telecom based.

    Aside question: Do you guys make your own QDs or do you collaborate with another dept. or purchase from a supplier (i.e. Evident). Are they collidal or epitaxially grown? What kind of QDs do you guys use (CdSe, PbS, PbSe)?

    Although I am now in an elec. eng. program, my background (undergrad.) is in physics. So, why am I studying QDs?

    I have always loved the fundamental physics, and have perhaps even more loved the applications of physics in the context of technology. While it is almost ultruistic to study science just for the sake of it (as some physicists do), when it comes to applying for funding ALMOST ALL PHYSICISTS cite the applications, whether short-term or long-term. This is something new profs. learn quickly when applying to grant agencies...

    To summarize, I love science, I love physics, and consider the scientific method priceless (I can't imagine a world without it). I get an even greater thrill when I can make or optimize a device using the powerful tools created by science.

    What do I consider myself? Personally, I don't care for labels. I don't have an engineering designation, so I am not an engineer. I won't have a grad. degree in physics, so I am not a physicist. But at the end of the day, I am some type of hybrid that will make devices (like an engineer) yet help address fundamental questions (like a physicist).

    This world needs all sorts! Even those studying string theories, gravitational waves, and pure mathematics!
  15. Jul 19, 2003 #14
    Re: Physicist - Engineer ... we are all on the same side!

    We use gate defined quantum dots made of a two dimensional electron gas (2DEG, for those of you reading who are not in the know) suspended in a GaAs/AlGaAs heterostructure. We make the quantum dot structures ourselves using standard optical and e-beam lithography methods, but we get the material from various collaborating material science research groups. They use molecular beam epitaxy to grow such a material.

  16. Jul 19, 2003 #15

    Ivan Seeking

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    Re: ...aging

    Absolutely! That's why I make a point to remember such statements.
    It shocked the heck out of me also, but I couldn't deny the credentials of the person making it. Of course you understand that on a forum like this, I dare not leave such claims questioned without a response. I have made a career of learning about the amazing and the hard-to-believe; which includes Quantum physics of course...and, if I'm anywhere but here, hardly anyone believes these claims either.

    Ultimately all of these technologies and the tools used to implement them depend on an understanding of QM. Even the electron microscope is a child of QM.
    Last edited: Jul 19, 2003
  17. Jul 21, 2003 #16

    Ivan Seeking

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    Re: QM...just a few more digits?

    Look what just popped up as one of my daily news items:

    "I think we are knocking at the door of immortality," said Michael Zey, a Montclair State University business professor and author of two books on the future. "I think by 2075 we will see it and that's a conservative estimate."

    180 years old? Experts debate limit of aging: CNN
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