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

How small can the quantum world get?

  1. Oct 29, 2003 #1
    Hey all,

    I just watched a show on string theory on PBS and I thought it was so fascinating!
    It got me to thinking....

    First we discover molecules, then atoms, then protons and electrons, then quarks, then these strings....

    Maybe, since the universe has so many instances of symmetry in the natural world... both the normal world we inhabit and the quantum world are as big. Meaning, however large the outside world is, the quantum world will be symmetrically just as small.

    Say, the end of the universe is 10^47m in every direction (just a guess)... why wouldn't the quantum world be also 10^-47m?

    I don't know if that has any merit, but I just wanted to see what you guys think. =)
  2. jcsd
  3. Oct 29, 2003 #2
    Well, I'm not sure off of the top of my head, but I'm pretty sure the quantum world breaks down at the Planck level. As you saw in the Popularization of SST on NOVA, at the Planck level, spacetime becomes distorted.
    Paden Roder
  4. Oct 29, 2003 #3


    User Avatar
    Science Advisor

    Strings have not been discovered. I'm not sure we can convincingly say that quarks have been discovered, just that some experiments have been carried out consistent with theory, and no experiments have contradicted the theory. When all aspects of particle behaviour that could rationally disprove quarks have been probed, then we can say quarks are discovered. For strings, there has been no experimental verification of previously unobserved phenomena, nor is there likely to be for a very long time.

    Observation raises questions; questions inspire theories, theories provoke experimentation, experimentation yields observation.

    Assuming string theory is correct, and assuming experiments can be done to verify it someday, we might well observe more in those experiments than we set out to see. Those additional observations tend to spawn the next generation of new ideas.

  5. Oct 29, 2003 #4
    Symmetrically about what?

    (We don't actually know whether the universe has a finite size.)

    According to your reasoning, there must be something special about a distance of 1 meter. Why should 1 meter be the exact middle of the scale? A distance of 1 meter has no fundamental physical significance: it was (originally) defined to be 1/10,000,000 of the distance from the Earth's equator to the north pole. The laws of physics cannot be based on such an artificial, man-made distinction.

    (What if we had chosen, say, the kilometer as our conventional unit of length? The universe would then be 10^43 km, so your "law" would say that the quantum world would then extend down to 10^-43 km, or 10^-40 meters, instead of the 10^-47 meters you'd obtain if you measured the size of the universe in meters. It's clear with this "law", you can make the quantum world whatever size you like, by choosing your units appropriately.)

    That being said, theories of quantum gravity do usually have a fundamental length scale, which does not depend on our choice of units, usually somewhere near the Planck length √(G hbar/c3) ≈ 10-35 meters.
  6. Oct 29, 2003 #5
    I want to make a small comment here, to avoid confusion: atoms make up molecules, protons and electrons make up atoms, and quarks make up protons, but strings do not make up quarks, they are quarks (and leptons, such as electrons).
  7. Oct 29, 2003 #6
    Quark to Quark Measure

    Quark to Quark Measure

    Dickt said: The reason I cn't comment on all this is that you have collected good stuff from different areas, and the point is how they link together to you. You had an illustration showing what they mean by a spacelike slice of simultaneous points in spacetime, and another I think showing Randall-Sundrum gravity spreading in the bulk (but I'm not sure because I couldn't read the fine print). And some statements that I couldn't interpret.<p>The spacelike slice, which is much used by the LQG people, survives, you don't have to adopt Einstein's view if it's too difficult. Same with R-S brane and bulk, if it suits you to think of them with substance go ahead. Just remember that there are people with a more abstract view.

    Well most certainly there will be generalizations, but they will be based on the mathematics. Yet if people cannot see what it is they are doing mathematically, it won't make much sense?

    All one has to do is think of Susskind when he was at the blackboard
    working the equation. He saw this wriggling form in place of the math, much like kaluza saw the cylinder. The math must be able to describ what he was seeing?

    Last edited by a moderator: Oct 29, 2003
  8. Oct 29, 2003 #7
    This question Re: How small can the quantum world get?

    Should actually be rephrased and equivelent in scale to this question;How accurate do you have to be in explaining the Precision needed to Understand Stringtheory?..namely:

    5)No accuracy needed at all
    6)Within Mathematical accepted numbers
    Last edited: Oct 29, 2003
  9. Nov 19, 2007 #8

    Personally, I think you are on to something. I don't know if anyone else will agree. But just consider the quantum foam, and then consider the large scale structure of clouds of galaxies. If you could freeze an instant of quantum foam, wouldn't it look very similar in structure to the large scale structure, which is described as a kind of foam in which the galaxies occur in walls with huge empty voids between them? But that's just motivation.

    Consider that the Planck mass has to be some fraction of the mass of the universe. Consider that the Planck length has to be some fraction of the distance across the universe. Consider that the Planck time has to be some fraction of the age of the universe. Since there has to be a relation between Planck time, space, and mass, why should we be surprised to find that there is a similar relationship between the mass, size, and time of the Universe?

    I know there is no fixed size to the universe, even if you define universe. Consider DSR, in which the constants c, Pl and Pt are not necessarily constants at all. Consider the idea of a curled dimension. Is size scale a dimension? I should think so. Is it surprizing that the largest dimensions of our universe are curled? You start at the smallest scale with quantum foam and end at the largest scale with galactic foam. Should it be surprizing that we human observers are half way from largest to smallest, half way from the beginning to the end of the universe, halfway from one side of the universe to the other? Infinite curl in expansions that are taking place in every dimension.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?