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

In what sense is the standard model enormously successful

  1. Jul 4, 2012 #1
    i've read a lot of authors claim that the SM is enormously successful, that it has passed every experiment with flying colors, etc etc, i won't bother with quotes because i think everyone agrees on that. yet i've also read that the amount of dark energy that the SM predicts is off by anywhere from 116 to 123 orders of magnitude (different authors give different numbers). so if the SM is so amazingly successful then why does it fail in the face of dark energy?
     
  2. jcsd
  3. Jul 4, 2012 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It is successful to predict particle physics experiments. It does not include dark energy and it does not contain gravity, which "feels" this energy, either. There is a hand-wavy way to get some number which could be interpreted as dark energy, but as this is off by >100 orders of magnitude it is not the correct way.
     
  4. Jul 4, 2012 #3
    That big density is the Planck density, the density that one would expect from quantum gravity.

    So if one decides that the Standard Model does not include gravity, then that's not a problem for the Standard Model.
     
  5. Jul 4, 2012 #4

    DaveC426913

    User Avatar
    Gold Member

    Because dark energy is fairly new. The best theories require refinement when new data comes in.
     
  6. Jul 4, 2012 #5

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Also, dark energy is a really bad name. All that is really observed is accelerated expansion. The best mathematical model simply introduces a constant of integration formerly set to zero to a specific nonzero value to match the data. Dark energy is simply one guess as to why the constant needs to be set to nonzero. If you assume this constant has some other source (e.g. initial conditions, or the nature of quantum gravity - which is simply outside the SM), then it has no bearing at all on the SM.

    I think the 'recent' finding that is much more direct evidence that SM is incomplete is dark matter. The evidence is pretty strong that this must be a very weakly interacting particle with properties not matching any SM particle. Thus, the SM is incomplete as theory of fundamental particles, unless there is a flaw in the analysis of dark matter.
     
  7. Jul 9, 2012 #6
    as far as dark energy being a bad name, i agree. roger penrose said that energy is something available for work and dark energy doesn't do any work. i like to call it a repulsive force, the opposite of gravity. it's what happens to space when there is no matter in it, it repels or expands. that's why galaxies don't expand, they have matter in them.

    i have another question about the SM. just exactly when does a theory get incorporated into the orthodox SM and who decides? clearly what belongs to the SM and what does not must be a bit fuzzy. surely some scientists would love to have their theory incorporated into the SM but others would disagree.
    I heard sean carroll talking on scifri that maybe the data suggests there are five different Higgs. Peter Higgs did not predict that. I've also read that glashow's theory that protons would decay in 10^32 years that experiment proved false. So that theory never got incorporated into the SM.
    Maybe it's just whatever theories are successful and confirmed get included into the SM. Then the fact that the SM is successful just become tautologous since the SM is defined as successful physics.
    There's a big philosophical debate called science realism. Does science get the real world right is the basic question. The debate is a bit of a red herring because science is virtually defined as successful knowledge. if a theory doesn't get the world right, like astrology or the steady state theory, then it doesn't get included into science.
     
  8. Jul 9, 2012 #7

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    @Georges Lemaître , I also agree with Pallen and Penrose here, "energy" is a REALLY BAD NAME for the curvature constant Lambda (Λ) that appears naturally in the Einstein GR equation.

    I guess I would call Lambda the "Einstein constant" by analogy with calling G the "Newton constant". The 1915 GR equation happens to be our best available law of gravity and two constants appear naturally in it, usually denoted G and Λ (capital Lambda).

    Two years after GR was proposed, in 1917, it was realized by Willem De Sitter that one family of solutions to this law of gravity (also a law of geometry) has geometry expand and calls for the expansion process to speed up in a certain sense (in accordance with an inherent curvature or "vacuum curvature" constant. De Sitter's 1917 title refers to the constant correctly as a curvature, not as a fictitious "energy". It has already taken some years for people to accept the idea that, in fact largescale geometry is slowly expanding. And it seems to be taking even longer for them to assimilate the idea that the constant Lambda is nonzero and that this expansion is very very gradually speeding up.

    In mathematical sciences you look for the SIMPLEST BEST FIT MODEL. The simplest equation that fits the data and makes the most precise reliable predictions. Then you get used to it. GR has these two constants G and Λ. It is amazingly precise, by far the most reliable model of evolving geometry (time and gravity) that we have. It also has strange unexpected consequences that we have trouble getting used to. That's life :biggrin:
     
    Last edited: Jul 9, 2012
  9. Jul 9, 2012 #8

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    One way to put it is that the SM has been extremely successful for astronomers, but a perpetual enigma for physicists. It is largely an empirical model -- that is, we have parametrized the dynamical equations that govern the expansion of the universe with time. This means that the SM predicts all sorts of astronomical observations concerning the distribution of galaxies, perturbations in the microwave background, the intergalactic medium, etc. However, our attempts to connect these parameters to fundamental physics have been woefully unsatisfying. We still don't know what particles make up the majority of the dark matter, nor do we know the origin of the "dark energy" component. A great deal of astronomical research in the next decade will be geared towards constraining the time evolution of this dark energy term -- that is, determining whether it is truly an "Einstein constant" or something more interesting.
     
  10. Jul 9, 2012 #9

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I think the term SM is about as old as the current SM. If something new comes up, it might be "added" to the SM, like the neutrino mixing does at the moment. There is no authority doing this, of course, which makes it hard to predict how scientists will use a word in the future.

    "We call successful physics SM" and "the SM is extremely successful" are not the same. The second statement requires a lot of successful physics :).
     
  11. Jul 9, 2012 #10

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Perhaps the OP should clear up a confusion: sometimes SM is used to refer to the Lambda CDM cosmological model, most of the time to electro-weak+QCD. I believe SpaceTiger is using the former definition and the OP is asking about the latter. I could be wrong ...
     
  12. Jul 9, 2012 #11

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi SpaceTiger, good to see you back!
    I think it is not EITHER OR. My hunch is that the "vacuum curvature" or Einstein constant has an underlying cause that will be revealed in whatever quantum theory of gravity/geometry succeeds.
    The quantum relativists will have to explain to us why and how space has this very slight intrinsic curvature bias.

    Quite a few papers recently suggest that the Einstein equation (with its two constants) is emergent, say as a thermodynamic equation of state, from underlying degrees of freedom which we have yet to discover. It is these QG degrees of freedom which would then, presumably, explain the G and Lambda constants for us. Hopefully :biggrin:

    So I think of it not either or but as BOTH a constant curvature and as "something more interesting."
     
  13. Jul 9, 2012 #12

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Rereading his posts, I think you're probably right. In that context, this question might be better posed to the General or High Energy Physics forum, since they probably have more to say about the limitations of the SM. It's still worth noting, however, that the standard model of cosmology does not require that the "dark energy" be a ZPE, though a huge value of the latter is certainly inconsistent with cosmological observations.
     
  14. Jul 9, 2012 #13

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, you have to remember that I look at this from an astronomer's point of view. I would be far more interested in a time-variable dark energy since it would lead all sorts of interesting observational follow-ups and would allow us astronomers to make another exciting discovery concerning fundamental physics. Your point is well taken, however. :wink:
     
  15. Jul 9, 2012 #14

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I can certainly understand that PoV! If it were found that the cosmological constant was actually varying in time that would be terrific! The standard cosmic model LambdaCDM with its constant Lambda would be out, all kinds of exciting possibilities would open up, with possible futures for the universe very different from the one we get from LCDM.

    However my impression as an interested lay bystander is that over the past 5 or 10 years the evidence has tended increasingly to favor a constant Lambda. So I am prepared to see it as a small constant curvature bias in the classical GR equation.

    My point, as you observed I think, was that EVEN IF it turns out to simply be a curvature constant in the classical equation, quantum relativists might (by probing deeper into the degrees of freedom underlying geometry) discover why there is this constant and come to understand it. IMHO astronomers win either way :biggrin:
     
  16. Jul 9, 2012 #15

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, technically the observations are consistent with w=-1 and w_a=0 (i.e., a cosmological constant), but our constraints aren't that great. The latest WMAP results (7-year) measure w to be consistent with -1 at the ~15% level, but the time variation of the equation of state is still wildly uncertain (w_a = 1 or w_a = -1 are still consistent with the data). Many of the next generation of astronomy experiments are going to try to measure these parameters with increasing precision (e.g., LSST).

    Honestly, I share your suspicion that we'll end up with an Einstein constant, at least to the precision that we'll be able to measure anytime soon, but I think we need to keep pushing anyway. For a long time, many thought it was safe to assume that we lived in a decelerating universe and look how that turned out. :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: In what sense is the standard model enormously successful
Loading...