[QUOTE=human_extinction]
I just saw this TED Talk: Brian Greene: Making sense of string theory | TED Talk explaining string theory and how its dimensions could be accounted for by use of a particle collider. These string theorists think that with a powerful enough collider we will see the energy of these other dimensions ‘spill out’ into our own, and that the difference in energy will be measurable with sensitive instruments. If the dependent variable of this experiment is the power of the collider as they seem to imply, then shouldn’t we have seen the effects of these other dimensions during the more powerful ‘experiments’ of atom and hydrogen bombs?
Another aspect that I do not understand is why these extra dimensions would contain extra energy - if they are contained within our 4 dimensions, then wouldn’t they already be contributing to the net energy? Are they proposing that these extra dimensions are somehow departmentalized from the rest (and wouldn’t this mean there is untapped energy in our very midst - breaking other laws of physics)?
Hopefully Stranger will come along and help me understand this. Thanks.
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Well, I’m not expert in string/superstring/M-theory despite spending a couple of years trying to make my way through Zweiback’s A First Course In String Theory, along with several texts on the Standard Model and gauge theories, and Penrose’s utter brilliance/jibberish, i.e. The Road to Reality : A Complete Guide to the Laws of the Universe, which espouses a loop quantum gravity alternative to supersymmetric string-based theories, so all I can tell you is what little I understand. The “string” part of string theories is based on the math; that is, the representation of fundamental particles and their composite structures and interactions as a series of periodic equations that are similar to vibrating strings (or waves). The amount of energy contained within and the way they can interact are based upon their vibrational characteristics, just as the interaction between wavefronts from two pebbles dropped in a pond depend upon the size and velocity of the pebbles. It is, as with the conceptual analogy of “particles” in quantum mechanics, that it is the image of a string that is presented to the non-technical public who then assumes that there are actual strings made of something that vibrate around; in fact, the “strings” are vibrations in some kind of extradimensional plenum analogous to Minkowski spacetime in General Relativity as the Greene presentation (which I sampled just briefly) states.
Now, the energies and temperatures that are seen in nuclear fusion are nowhere near enough to expose these extra dimensions; indeed, these reactions (which are in the MeV, or “mega electron-volt”) range are just barely enough to force one proton close enough to enough to allow normal nuclear interactions to cause them to stick together, resulting in an unstable nucleus that releases energy in the form of fast moving neutrons, charged particles, and a few electron neutrinos. The fundamental forces (electromagnetic, strong interactions, weak interactions) remain shattered, their symmetry broken by a lack of pressure and temperature in the same way that girls and boys at junior high school dances are clutched up against opposite walls, waiting for hormones to overcome fear and the inevitable humiliation of rejection and ridicule. To cause the forces of electromagnetism and weak nuclear interaction to combine together requires energies on the order of 100 GeV (about 10000x 1 MeV); unification of strong interactions to electroweak interactions is estimated at about 10[sup]14[/sup] Gev, vastly more than anything we can hope to produce in the foreseeable future. So nuclear fusion bombs, as much destruction as they can wage, are many, many orders of magnitude lower in peak energy density than anything resembling the nascent stages of cosmological evolution.
How will see see these hypothetical extra energies? When you force normal particles together in the compact, highly focused beam of a particle collider, they should form some of these more exotic particles. They don’t last very long, of course; under normal conditions, most really exotic particles like muons or pions have lifetimes on the order of microseconds or picoseconds. Occasionally we get a glimpse of one, identified by how far it penetrates or how it curves in response to a magnetic field; more often, it is identified by the products it generates (and where they go) when it breaks up. Still, the really fun stuff, like Higgs bosons, are only available at very very high energy levels. The energy levels capable by the Large Hadron Collider only cover the lower range of the Higgs, so failure to find it is really inconclusive.
As for these other (and at this point, completely hypothetical) dimensions, there are various theories for where they are and why we don’t see them. One of the most popular notions, stemming back from Oskar Klein, is that the extra dimension (later dimensions) are contained in a compact topographical set, bounded and separate from the 3+1 dimensions of normal spacetime. This is mathematically convenient because the extra energy is just stored away like gasoline or a compressed spring, only released when you reach some threshold where the dimension unwraps and is combined in symmetry with normal spatial and temporal dimensions. Others believe that the dimensions are actually bigger and that our universe simply floats inside of them, or that they’re all around us but we just can’t see them because we’re orthogonal to them. However, we have absolutely no experimental evidence that these extra dimensions exist, and many people believe that the entire notion of extra dimensions (10, 11, or 26, depending on how you work the math and how small your handwriting is) like listening to a hophead go on about leprechauns stealing his gold. On the forefront (at least, in popular consciousness) is the lovely Lisa Randall, who has with colleagues proposed models that do not require extra dimensions. I haven’t even made an attempt to read up on this (even her pop-sci book, Warped Passages, sits on my bookshelf as yet unread) so I can’t say anything unintelligible about this other than that simplifying the theory into something that doesn’t require experimentally unobservable dimensions is probably a step in a direction that, if not exactly right, isn’t going to squish and smell like cow manure.
If we do manage to crack open one of these extra dimensions, no worries about the world coming to an end; lacking sufficient pressure to keep the symmetry working, they tend to come apart like a cheap gold watch. Fears of apocalyptic doom should be allayed by the fact that cosmic particles at these energies and higher regularly impact the Earth’s upper atmosphere and do nothing more malevolent than briefly disrupt your t.v. signal.
As for string theories, they have the appeal of being able to unify all of the fundamental force interactions; not just strong, weak, and electromagnetic, but potentially also gravity, which is otherwise a very elusive force on the level of fundamental particles, and thus, bring cohesion between general relativity and quantum mechanics in the same way that QED brought electromagnetic force into special relativity. This doesn’t mean that they actually work, only that it would be a great relief if they did, allowing relativists and particle physicists to sit shoulder to shoulder at the bar without a knee-jerking, eye-poking, ear-pulling fight breaking out.
Greene, by the way, is an articulate speaker and unquestionable very intelligent man; he is also an unvarnished advocate for superstring theories, which is understandable because this is his life’s work, and also he makes a fair amount of money from books and popular lectures on the subject. He should not be considered an unbiased source, though. As as Pasta notes, confirmation of a few aspects of string theories is but one small part of the objects of the LHC and SLHC programs; indeed, it is unlikely that any results therefrom will confirm any string theories (which themselves are seriously incomplete at this point) to the exclusion of other speculations (although it could potentially undermine them). String theories are the in thing in pop science at this point, and thus receive a lot of attention, but it is hardly the case that even a significant fraction of physical science funding is going toward researching this area.
Stranger
