Evidence for String Theory?

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.'"

Re:Now we know..

[rufty_tufty]

For those who aren't aware:
http://en.wikipedia.org/wiki/Heim_theory [wikipedia.org]
Predictions of faster tan light travel (amoung other things)

Don't get your hopes up

[Anonymous Coward]

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).

Re:Neutrino Detector at the South Pole?

[dragons_flight]

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.

Re:Neutrino Detector at the South Pole?

[imsabbel]

The really cool thing about the very thick ice on the south pole is that beneath the few douzend meters of surface layer, its incredibly pure and transparent.

Now if a neutrino causes a shower of cherenkow radiations, it can be detected many many meters away.
So instead of building huge watertanks in deep mines, one can use the deeper ice layers as a large detector.

You just melt holes into it and put photodetectors in a grid pattern, and get billions of tons of detector mass (which you need because low chance of neutrino interaction with matter)

Re:wake me when this matters to us 3d people

[musonica]

You may think we are only operating in 3+1 dimensions, perhaps our terms of understanding other dimensions are limited. These other dimensions are said to make matter what it is, for instance a certain multi-dimensional vibratory resonance pattern is what makes a hydrogen atom different from an atom of gold. (IAACST (I am a crackpot string theorist)). A dimension "is a parameter or measurement required to define the characteristics of an object" (wikipedia) so i tend to look at things like color, taste or emotion as other "dimensions", though perhaps this is sematics to some, or difficult comparing mathematics to the real world.

Re:well is it

[ceoyoyo]

"String theory" isn't a theory, it's a collection or class of competing theories. Kind of like "celestial mechanics." You can't falsify celestial mechanics, but you CAN falsify the geocentric model, thereby realizing that the heliocentric model is better.

Re:wake me when this matters to us 3d people

[Gromius]

I completely agree, at the time most fundamental physics research seems completely pointless at the time but often in 60-80 years time its extremely important. Take the example of quantum mechanics, in the early 1900s, researching into being able to explain the precise movements and behaviour of subattomic particles, effects so small they had no practical application in everyday life may have seemed a bit pointless. 60 years later the understanding this lead to the invention of the transistor, which some people might argue is of some importance in todays world.

Anyway as an aside, evidence for extra dimensions != evidence for string theory. String theory isnt the only model which predicts extra dimensions. Evidence for no extra dimensions is evidence that string theory doesnt exist. However we'ld probably have to go to the planck scale to be sure which is probably impossible for the time being. Anyway we're far more likely to pick up string theory by the breaking of the E6 symetry group which produces extra massive neutral gauge bosons (Z').

Just your friendly neighbourhood extra dimensional researcher (CDF expt, Fermilab)

Re:Communicating with other dimensions.

[Anonymous Coward]

"TO ME"? WTF?

It's "TO US". Dumbshit.

See "Not Even Wrong" Blog

[pmjbf]

This is discussed in this blog entry:

http://www.math.columbia.edu/~woit/wordpress/?p=33 5 [columbia.edu]

A snippet of which is:

The half a dozen references to string theory in the short press release might lead the gullible to think that we're about to be provided with evidence for the "exotic predictions of string theory", but that has little relationship to the reality here, one aspect of which of course is that there are no "predictions of string theory" about any of this.

...and which might be worth reading if this interests you.

Re:Don't get your hopes up

[Roger W Moore]

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!

Some comments from an Amandroid...

[kievit]

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 [slashdot.org] already commented [slashdot.org] 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.