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.
Actually, he’s saying it’ll be the other way around. We’ll collide particles with each other, with a certain known amount of energy going in. Then we collect the particles that come out of the collision, and measure their energies. If the total energy after is less than the energy before, presumably, that lost energy went into an extra dimension.
By definition, the other dimensions postulated by the various string theories are *not * contained within our four dimensions. The experiments proposed are to try to finesse out the extremely tiny and subtle variations that the extra dimensions would cause. Atomic tests are therefore the worst place to look for them. How could you find the existence of a sub-atomic particle lasting for an extremely brief brief of a second in the midst of all that chaotic superabundance of energy?
Honestly, most “theories of everything” (even if I could grok the multi-dimensional math, which I can’t) currently being bandied about contain so much pure, unadulterated conjecture they have about as much empirical basis as debates about the origin of Superman’s powers. Unless some real breakthrough comes down the pike the notion of “proving” string theory in the foreseeable future seems pretty remote.
It’s the energy per particle that matters. The energies involved in atomic bomb explosions are on the few MeV per particle scale. (1 MeV = 1 million electron-volts = 1.6×10[sup]-13[/sup] joules.) The upcoming Large Hadron Collider will produce particle collisions millions of times higher in energy. The atomic bomb wins out in total energy release simple because there are lots of particles undergoing fission all at once (~10[sup]24[/sup] of them).
I continually come back to these string theory and darkmatter/energy concepts, but each time I find myself discusted at the amount of energy (seemingly the majority of physics research funding) going into concepts that are based on such leaps of logic.
You think its a ton of money? The US spending on high-energy physics hasn’t kept pace with inflation for years. It’s $688.3 M for the full year. Most of that goes towards salaries, not fancy gizmos. Even then, it’s pocket change - that’s roughly five F-22 fighter jets.
High energy physics also yields knowledge that helps in many, many other areas than research on dark matter and string theory. Understanding the fundamental properties of matter on tiny scales is necessary for a more complete understanding of quantum mechanics, which has the potential to revolutionize computing with quantum computers.
It also will probably yield benefits we haven’t even thought of - that’s the pattern of scientific discovery. We can only apply new knowledge after we’ve discovered it.
And I’m not entirely sure what these leaps of logic are. If the leap of logic you are referring to is that its unproven, how do you propose we go about any scientific inquiry? Sure, its all hypothetical at this point and unproven with experiment. But would you rather have us blindly do experiments at random without a hypothetical explanation to test?
If that isn’t the leap of logic you are referring to, then what is? The theory is rigid and unique in its explanatory power, even though it isn’t finished. It’s quite mathematically rigorous - what are your specific problems? I’m not as optimistic as Greene, who is one of string theory’s most active advocates, but I think it is silly to say that it is filled with “leaps of logic” that would “disgust” me.
Essentially, yes. Theorists are careful to construct models that respect the current laws of physics, but when you give yourself complete freedom, it’s not too hard to do. In many models, the particles that we are familiar with are confined to the usual 3-space, while other particles may be free to roam the higher N-dimensional space.
The “motivations” that find their way out into the open are a small, and usually lesser, part of the whole suite of reasons for pushing the energy frontier. (Brain Greene, of your link (the video in which I didn’t watch), bugs me to no end partly for this reason.) These speculations make for exciting-sounding news stories and lecture topics, but it’s not that a couple of people had some hairbrained ideas and then particle physicists thought they should break the bank going after them. On the contrary, the LHC will answer a lengthy list of questions that we already know we have about how things work. At the same time it will very likely introduce new questions. Theorists try to guess ahead of time about these unexpected things, but until there is actual data, every crazy idea is as good as the next. Unless you ask the theorist.
The only thing that is a virtual certaintly: we are going to learn something.
The reason the LHC is worth the Benjamins is that it will bring us across a natual energy boundary of our current understanding. The “standard model” of particle physics creaks and groans above ~300 GeV, and there is a lot that suggests new physics beyond this scale. The LHC is a 14000 GeV proton collider. (Aside: since protons are composite particles, you only get about a third of the energy on average, but that still means interactions on the scale of 5000 GeV.) This will bring us well into uncharted – but known relevant – waters.
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.
Thanks a lot; your writing is always very complete.
I am unclear on how they establish the presence of particles such as muons or pions from mechanical artifacts and other anomalies. When instruments with higher electromagnetic resolution and such are developed, then they sense the presence of energies producing lower values than the electrons, neutrons, or protons - which we non-particle physicists are more suited to imagine. Can you direct me to some experiments that show me what we really do know about these particles?
It is not my intention to imply that the LHC has more funding that it should get - I think all ‘scientific’ (i.e. not money to big pharma and the like which is often masqueraded as scientific spending) research deserves much more funding that it is getting. I mean to say that I am disappointed in the extent of string theory and dark matter attention when I know there is much more out there - and surely even great postulates that I have never heard.
Why are you combining string theory and dark matter? How do they relate?
Is there really a “notion” of proving string theory or of being able to find strong evidence of it? Science can build on the latter without doing the first.
The things that the LHC disproves will be at least as important as anything it proves. Conjecture needs boundaries to build around, and over. Experiments provide those boundaries. If you follow your math far enough, it eventually describes one thing well enough that if you find it, you can recognize it. If your math says the thing has to be there, and the experiment says it ain’t, then you have progress! You have progress if it is, too, although not quite so much.
Let me clear up a few misconceptions (upon refresh it looks like some of this may be covered above, but I’ll give my version anyway):
The really expensive stuff is high-energy experiments like the LHC. But the LHC isn’t really supposed to test string theory at all (although I can see how you could get that impression from the way it’s been covered in the media). The main purposes of the LHC are:
(1) To search for the Higgs boson, which is the one particle needed to complete the Standard Model of particle physics. Basically, the Higgs is what causes all the other particles to have mass. If we don’t see it at the LHC, it basically means physicists seriously need to rethink where mass comes from.
(2) To search for evidence of physics beyond the standard model. This could include “superpartners” of the known particles, or evidence of extra dimensions, or God only knows what else. Finding anything that goes beyond the Standard Model would be tremendously useful in guiding the course of future particle physics research.
There’s a chance the LHC will find some evidence that string theory is correct, but really there’s no way to know if it will or not. Like I said, that’s not really why they’re doing the experiment. The most important thing is completing the understanding of mass in the Standard Model, which unlike string theory is an extremely well tested theory.
I should also clarify that unlike String Theory, which is motivated by theoretical calculations, both dark matter and dark energy were discovered in experiments, namely astronomical observations. We don’t know what dark matter is (maybe it’s the aforementioned superpartners, maybe something else), but we know for sure it exists, since we can see its gravitational effects by looking through telescopes and what not. (See here for a blog post discussing one of the more definitive observations.) Likewise, we don’t know for sure what dark energy is, but we know it exists. “Dark energy” essentially means “whatever it is that’s causing the expansion of the universe to accelerate.” We can say for sure the universal expansion is accelerating, even if we don’t know exactly why.
Research in “pure theory” (meaning string theory, M-theory, loop quantum gravity, etc.) is comparatively cheap, due to the very fact that you can’t build a particle accelerator or a space telescope to test the theories. No one knows how to do a definitive test of those theories yet. (Whether they ever will know how depends on who you ask.) So basically the researchers’ grants are paying for the cost of pens and paper.
(OK, they’re also paying for grad student and post-doc salaries, travel funds, PC’s and software, etc., but my point is they aren’t using billions of dollars to build huge experiments, because no one would know what to build.)
Ultra-short version (for anyone whose eyes may have glazed over from the above):
(1) The LHC is important to look for the Higgs boson, the particle that is believed to give everything mass. This is really about completing the well-tested Standard Model, and doesn’t actually have anything to do with string theory.
(2) There’s already experimental evidence of dark matter and dark energy, in fact suggesting that they make up the bulk of our universe. So understanding them is a pretty important goal in the physics world.
(3) There’s not a lot of money being spent on string theory experiments, because at this point there’s really no such thing as a string theory experiment. (There are some experiments that might have the fringe benefit of shedding some light on string theory, maybe.) And theory without experiments is comparatively quite cheap.
Are you asking how we know that something we see is actually something? That is an insightful question, and there is no one answer. There are dozens of particle detector technologies in use, each worthy of a lengthy post itself, and these can be combined in limitless ways to build a composite detector for a specific set of goals. Each project thus requires its own approach to understanding and (believing) what the data are telling you.
When a new particle is suspected, you rarely learn everything about it all in one shot. Knowledge has to build over time. When particle physics was in its infancy, particle properties were explored in (relatively) crude ways. “Hey, this thin metal foil stops the radiation from this isotope but not the radiation from this other one.” Cloud chambers and, later, bubble chambers improved things quite a bit, allowing one to visualize particle trajectories. In these devices, ionization of the detector material by charged particles passing through it yields nucleation sites for cloud/bubble formation. A line of bubbles reveals the particle’s path. By putting the whole thing inside a uniform magnetic field, one can also measure the ratio of the particle’s charge to its momentum. (Here’s a bubble chamber picture and a drawing. How come that’s not just naturally occurring background activity? Because it goes away when you turn off or redirect the particle beam.)
(Bubble chambers aren’t used anymore, I should point out.)
Muons were originally discovered in cosmic rays as something:
(1) with the same charge as the electron (since it decayed to an electron and nothing else with charge)
(2) with significantly more mass than an electron (based on curvature in a magnetic field)
(3) with significantly less mass than the proton.
Since its discovery, the muon has become the most well-measured fundamental particle after the electron. Precision mass measurements, as an example, are obtained by creating muonium (an electron-muon bound state akin to the familiar electron-proton bound state called hydrogen) and measuring the transition frequency between the ground state and the first excited state, a quantity which depends directly on the muon mass.
Back to the more general topic of detection…
“Scintillators” are transparent materials that emit light in proportion to the amount of energy lost by a traversing charged particle. If you put a bunch of these scintillators together and sum the light output, you have a calorimeter with which to measure particle energy. Combine this with a momentum measurement via magnetically induced curvature and you instantly have a measure of the particle’s mass through the relativistic relation E[sup]2[/sup]-p[sup]2[/sup]=m[sup]2[/sup] (p=momentum, m=mass, E=energy). If you can measure the particle’s velocity (by how long it takes to go from A to B or by its Cherenkov properties), you can combine that with either the E or p measurement to calculate the mass, also.
For particles that you can’t observe directly (either because they are neutral or because they live too short a life), the above technique works just fine on the decay products. The neutral pion, for example, decays to two gamma rays very quickly. You can’t see the pion itself, but the gammas are easy to identify. Since energy and momentum are conserved in the decay, the total gamma energy and total (vector) momentum tell you the same for the pion. Plug these in to the above expression, and you have a mass. In practice, though, you may have all sorts of processes that produce gammas in your detector. So you select every interaction that seems to have two gammas, and you make a histogram of the corresponding masses. If some of the gamma pairs were indeed coming from a single particle, you’ll see a peak in the histogram at that particle’s mass. You can do this for other possible decay modes as well (whatever decays to whatever). Here’s an example for a more exotic particle, the D[sup]0[/sup]. In the left spectrum, the experimenters used two kaons to calculate an invariant mass. In the right spectrum, they used two charged pions. Notice that a peak shows up in the same place (~1.68 GeV/c[sup]2[/sup]) regardless of the decay mode.
I don’t want to give the impression, however, that mass is the all-important thing to deciding that you’ve found a particle. It’s just that mass is the easiest to discuss. There’s also spin, parity, lifetime, decay modes, etc., but all these are gotten with the same tools.
When the LHC comes on line, it will have its own set of (as you worded it) anomalies to contend with: detector and analysis artifacts that can mimic interesting stuff. So Step 1 (Step 0?) at the LHC is to measure all the things we already know about: pions, charmoniom, top quarks, W and Z bosons. All these “pedestrian” measurements will use the same components needed for the new measurements: the electron calorimeter, the muon tracker, the silicon vertex tracker, the magnetic field, the data readout system, the analysis software, etc. Once all the bugs are worked out of these subsystems, one can trust anything new that is found.
