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Evidence for String Theory?

Posted by CowboyNeal on from the quarks-and-stuff dept.

Izeickl writes "PhysOrg.com is reporting that scientists working at a neutrino detector nicknamed AMANDA at the South Pole report that evidence for string theory may soon be coming. Extra dimensions predicted by string theory may affect observed numbers of certain neutrinos and this is what the scientists will be looking for. The article further states 'No more than a dozen high-energy neutrinos have been detected so far. However, the current detection rate and energy range indicate that AMANDA's larger successor, called IceCube, now under construction, could provide the first evidence for string theory and other theories that attempt to build upon our current understanding of the universe.'"

4 of 258 comments (clear)

  1. Don't get your hopes up by Anonymous Coward · · Score: 5, Informative

    This is not the only experiment which could probe large extra dimensions; the Large Hadron Collider at CERN is another notable experiment. However, this article is not implying that AMANDA (or any other experiment) has found evidence for string theory, or even that they are likely to.

    Normally, string physics is thought to appear at the Planck scale (far beyond what we will ever be able to probe directly), because that is thought to be the size of the "curled up" extra dimensions. However, it's possible that the dimensions aren't actually that small, that they could be much larger — possibly not much smaller than a millimeter. (They could even be infinitely large, not curled up at all, and we could be living on a 4-dimensional "brane" close to another one.) In those cases, stringy behavior is brought down from the Planck scale to as low as 1 TeV (tera-electron volt), which is the energy that corresponds to a distance somewhat below a millimeter. (By the Uncertainty Principle, higher energies correspond to shorter distances that can be probed.)

    The problem is, there isn't a lot of reason to believe that these scenarios ought to be true; they are highly speculative (even relative to string theory as a whole!). To a large extent, they are just hopeful thinking — that stringy physics might occur at in an energy regime we can probe. They could be helpful in understanding the hierarchy problem (the question of whether and why there is an absence of new particles between the electroweak and Planck scales), but when you get down to it, most high energy physicists are not betting on large extra dimensions. So these experiments might very well not show up any evidence of string theory (even if string theory is true).

    1. Re:Don't get your hopes up by Roger+W+Moore · · Score: 4, Informative
      The problem is, there isn't a lot of reason to believe that these scenarios ought to be true; they are highly speculative (even relative to string theory as a whole!).

      Actually there is a good, theoretical, reason to think that these "Large Extra Dimension" (LED) scenarios might be correct (though I'll only believe it if we get data to back it up). If LEDs do exist they can solve the problem that the Standard Model of particle physics has explaining the huge difference in energy scales between the Planck scale (10^16 GeV) and the electroweak scale (10^2 GeV).

      If LEDs exist then gravity might become a lot stronger above the ~TeV energy scale i.e. the Planck scale is actually ~10^3-4 GeV and not 10^16 GeV and a lower energy scales we are fooled into thinking gravity is a lot weaker simply because we can't see these extra dimensions where it spends a lot of its time.

      The problem that LEDs have is in explaining proton decay. It is very likely that protons do in fact decay (this is linked to the fact that we only see protons and no anti-protons in the Universe) but with an incredibly long lifetime caused by the very high energy of the Planck scale. If you lower this energy to a few TeV you either end up with rapidly decaying protons (bad!) or having to put a conserved symmetry in which prevents all proton decay (also bad!).

      So LEDs are an interesting theory which could solve some real problems with existing theory at the cost of introducing some new problems of their own. As a result I think Supersymmetry (which solves the problem which LEDs answer as well as the missing dark matter problem) is a better bet but I'll only believe it if we see it! Unfortunately from an experimentalists point of view LEDs would be a far more interesting discovery since it would mean we could start doing quantum gravity experimentally before the theorists have figured it all out....but not knowing exactly what you'll find is part of the fun of physics!

  2. Re:Neutrino Detector at the South Pole? by dragons_flight · · Score: 5, Informative

    Construction on AMANDA began in 1994, and South Pole was chosen because you need high transparency ice. That means you need an ice sheet substantially thicker than 1400 meters (the bubble conversion zone) in a region with few dust or volcanic impurities. South Pole satisfies both these properties very well.

  3. Some comments from an Amandroid... by kievit · · Score: 5, Informative

    I work for AMANDA/IceCube. It's nice to see that our supercool experiment gets media attention, but there are a few statements in that article which need a comment or two. User davidoff404 already commented on the theoretical aspects of the article, so I will mostly limit myself to the experimental aspects.

    "No more than a dozen high-energy neutrinos have been detected so far."

    Actually, we see about 900 neutrino events per year. Their directions are homogeneously distributed over the sky and the energy spectrum is (still) compatible with the assumption that all these neutrinos were produced in interactions of high energy cosmic rays (protons, nuclei) with the Earth atmosphere (all around the globe). It might be that there are neutrinos among them from extraterrestrial sources, but individual events cannot be identified as such. We continue taking data until neutrino events from single extraterrestrial sources (or with higher energy than expected from atmospheric neutrinos) pile up enough such that they stick out over the atmospheric neutrino background.

    Note: we do not detect those neutrinos directly; they interact with the ice, and may convert into a "muon" (which is like an electron, only about 200 times heavier, and it decays after a little while). That muon still carries most of the neutrino's energy with it, so it flies practically with the speed of light through the ice, sending out Cherenkov light (the electromagnetic equivalent of a sonic boom) along the way. The tracks can be kilometers long. We only see the part of the track in or near our detector, so we can only estimate a lower limit of the energy of an individual muon. When the neutrino does not convert into a muon, then the energy is dissipated in a relatively small volume; which makes it much harder to estimate the direction, but easier to estimate the energy.

    (And of course those atmospheric neutrinos are not only background. We are happy to see them, as they prove that our detector is not blind. And we can use them to test the models of cosmic ray spectra and to study properties of neutrinos themselves.)

    AMANDA, funded by the National Science Foundation, attempts to detect neutrinos raining down from above but also coming "up" through the Earth. Neutrinos are so weakly interacting that some can pass through the entire Earth unscathed. The total number of "down" and "up" neutrinos is uncertain; however, barring exotic effects, the relative detection rates are well known.

    Actually, neutrinos are so weakly interacting that the vast majority of them just flies right through the Earth. It is really tiny fraction of them which happens to bump into an terrestrial atom. And an even tinier fraction which bumps into an ice molecule near our machine. So they come from all directions, up and down, the Earth is not shielding them. However, like everywhere on Earth there is a lot of cosmic rays thundering down on the atmosphere above the South Pole, and some of it results in high energy muons which make it all the way down to our detector. Their rate is about a million times higher than that of the muons originating from the neutrinos. Only when we see a muon track going upwards, or when it has an energy much higher than expected from the cosmic ray spectrum, then we call it a neutrino event.

    When we start talking about really very high energy neutrinos (PeV and more) then the picture gets a little bit different: at those energies the probability that a neutrino interacts with atoms gets so high that the Earth is indeed opaque for neutrinos. If there are such high energy neutrinos flying through the universe, then we expect to see them from above and horizontally. This is already expected with standard model physics, without assumptions about microscopic black holes; so I am curious as to what Goldberg and Feng are after.