Physicists Clarify Exotic Force

Azazel writes "A research group, including Purdue University physicist Ephraim Fischbach, has completed an experiment which shows that gravity behaves exactly as Isaac Newton predicted, even at small scales. Unfortunately for those in search of the so-called "Theory of Everything," the finding would seem to rule out the exceptions to his time-honored theories that physicists believe might occur when objects are tiny enough."

Comments

Re:Did I miss something?

[Nos.]

Well, this is the first time I've heard of the "Casimir force" force, but reading the article linked to in the article (how's that for RTFM) explains that pretty well. Now, the prevailing "ToE" is to the best of my knowledge, string theory. This theory was developed to explain the inconsistancies with Newton's and Einstein's laws when you got down to a sub-atomic level. This study is basically saying that string theory is wrong, and that Newton and Einstein (for the most part) were right.

Now, the interesting there here is the "Casimir force" which basically, is the force of photons striking an object. We touched on this actually in high school physics. We were experimenting to find out if light was a wave of a particle. (its a wave of particles). I started to ask questions like, if that's true, wouldn't most stationary objects eventually gain mass due to a build up of photons. We never quite got into that... probably a little advanced for most people in high school physics. Sorry, back on topic. This force, becomes very powerful (comparatively) at sub-atomic levels. The force of a particle travelling at the speed of light can become very significant. In fact, it becomes more significant than gravity. So, everything as usualy, I'm not sure I'd agree, but it hopefully does get us one step closer to the ToE.

Re:Did I miss something?

[TripMaster Monkey]

Yes and no...what's noteworthy about this experiment is what they didn't find. Much like the Michelson-Morley experiment in 1887, which set out measure the 'aether', and instead failed utterly to detect any such thing, this experiment was devised to detect exceptions to the behavior of gravity on a quantum scale, and found no such exceptions.

Ephraim's not giving up yet, though...he plans on developing another experimental apparatus that is a million times more sensitive than the one that was used in this experiment. Also, even though this experiment was nominally a 'failure', the fringe benefit of clarifying the Casmir force is a big success.

Gravity at small length scales

[dr. loser]

IAAP (I am a physicist), and here's the deal:

There are suggestions out there that one way to test for the existence of extra "compactified" spatial dimensions (the kind of stuff needed in string theories) is to look for deviations from Newton's 1/r^2 gravity at small distance scales. See, for example, here.

The problem is, it's very hard to measure just the gravitational interaction between two objects separated at micron scales. Gravity is incredibly weak compared to common forces like electrostatics and magnetic interactions, and even more exotic things like Casimir forces (related to the van der Waals interaction).

The Purdue team has shown that the measured Casimir force in their experiment acts just as expected, setting a new limit on how screwy gravity can be at these distance scales.

For what it's worth, there are two other big efforts in this area. The one at Stanford is led by Aharon Kapitulnik, and is so sensitive that their apparatus can detect the different forces on Au and Si in the earth's magnetic field due to diamagnetism (!). The one at Washington is reportedly even more sensitive, and there are rumors circulating that they may have seen something exciting.

The really cool thing here is how table-top solid state experiments may have something profound to say about high energy physics, without any big accelerators.

Re:Gravity at small length scales

[exp(pi*sqrt(163))]

Well **** me! I was going to call your bluff on the van der Waals force being related to the Casimir force but wisely I did a bit of web searching first and found that they are related and that this has been known since 1955. On the one hand I've studied quantum field theory and read papers on the Casimir force, and on the other hand I've worked with computational chemists who put the vdW force into their models all the time. But I had no clue these things were related. I had merely assumed that the vdW force was simply what you got when you summed together a few electrostatic potentials for the kinds of dipoles you might expect to find in atoms. Is there a good source to read up on this? (Go easy, it's years since I actually did any physical calculations.)

Sometimes science can get a little too compartmentalized. the vdW force is more in the domain of chemists than of physicists so physicists don't get taught it.

Re:Gravity at small length scales

[dr. loser]

Try this, particularly the external link to the 1999 hep-th paper.

In short, when you assume "action at a distance" and calculate the instantaneous forces between fluctuating dipoles, you get the van der Waals interaction. When you do full local treatment of the quantum EM fields, including retardation effects, you get the Casimir force.

Re:Gravity at small length scales

[thermopile]

IAAAP (I Am Also A Physicist), and let me (humbly -- your explanation was really good) add some more meat to your description.

Physicists have a really, really hard time explaining *why* gravity is 10^42 times weaker than all other forces. (If you really want to split hairs, it's about 10^38 times weaker than the Weak Force, but what's an order of magnitude among friends?) Gravity appears to be a completely different manifestation than the electromagnetic, weak, and strong forces of nature. This irks many, and they try to rectify that by a Grand Unifying Theory (GUT).

One recent shot at explaining all this was well laid out in this article in Physics Today (subscription required, sorry) from 2002. In short, it theorized that gravity exists in 11 dimensions, not just 3, over short distances. Over some distance, the force known as gravity would "collapse" back down to our traditional 3. The fact that it acted over 11 dimensions, not 3, made gravity drop off as something like 1/r^10. This could help explain the apparent weakness of gravity.

IIRC, the authors predicted that gravity would get measurably stronger at small distances, as it was acting in many dimensions at once. Towards the upper end of their estimates, they predicted that gravity could be measurably stronger at distances around 3-5 millimeters.

As I read this latest discovery, it appears to throw water on that attempt to unify gravity with everything else. Back to the drawing board.

Yes. . .everythings normal BUT. . .

[robotkid]

The reason why this is interesting is not because "oh look, gravity still works". Most physicists have no doubt that whenever you test it, gravity will still work. In that sense, it is kindof what another poster said, "everything's normal, big deal!"

But what's eating all the theorists is that they have absolutely no idea why. The venerable laws of gravitation are empirical, in the sense that noone knows where it comes from other than the fact that it is associated with mass. All the other forces of nature have a quantum explanation, and have a particle that transmits them (most notably electromagnetism and photons). Noone has been able to satisfactorily reconcile gravity with any fundamental (quantum mechanical) nature of a particle.

It's almost scary that we know more about what binds subatomic particles together than what keeps the moon orbiting the earth. It's also ironic that most people's only introduction to physics is newtonian physics which is presented in textbooks as complete and understood. It's true we have the math to predict the effect of gravity to arbitrary precision, but I'm sure engineers can back me up that just because something has a robust empirical law doesn't mean anyone really understands how it works.

Accumulation of photons

[jd]

Actually, that is a really important point and is partially answered by the photoelectric effect predicted by Einstein. (Photons have mass, they strike electrons, this generates current, the photon is not absorbed but now has much lower energy)

Particle/wave duality is not fully explained by thinking of light as a wave of particles, as this conflicts with observations of diffraction gratings at extremely low light intensities. It is my understanding that a "refinement" is to describe light as a single photon that exists with varying probabilities across the wave. (The wave is then a probability wave.)

QM allows objects to exist at multiple points or in multiple states simultaneously, until directly observed. If you do try to directly observe a photon, you do indeed see a single packet of energy. But if you look only at the results, you see a wave.

By looking at light as a probability wave, a lot of apparent paradoxes don't "go away" but do fit a lot better with other known apparent paradoxes, which (to me) indicates the phenomena are related and not distinct.

Getting back to gravity, we could be in for an interesting dilema here. With no variations so far detected, the theory of gravity being an exchange of particles seems less likely. Einstein's model of a distorted space/time would seem to be the more probable, at this point.

This is important, as the predicted QM model for gravity could not be compatible with Enstein's model of gravity. They could not coexist, one had to be wrong. At this point, it seems likely that the particle-exchange model is the one that is wrong, which means QM in its eventual form will likely not be 100% particle based. It may need to be a heterogenius model.

As an aside, let us assume gravity does bend space/time. Since information cannot travel infinitely fast, and as no two events can occur simultaneously, when a massive object moves, space cannot restore itself the moment the object has left. Thus, there must be something analogous to a restoring force within space/time, and therefore some parallel to Hooke's Law.

By implication, an object moving fast enough should leave a trail, where the effect of gravity on space/time is apparent, even though there is no longer any source of that gravity present. A massive-enough object may even leave some sort of "wake", similar to that of a boat, only in gravity rather than in water.

Hooke predicts an upper limit to expansion, though. Something stretched beyond a certain point cannot be restored to its original dimensions, but will rather be restored to some other state, with a much lower restoring force existing.

By implication, a sufficiently massive black hole should result in a region of permanently deformed space/time, as the expansion would exceed the Universe's ability to restore.

As far as I know, no such "massless holes" have been found, but the more the Einstinian model is verified, the more certain I am that such a thing must exist.

Re:Accumulation of photons

[Nos.]

Wow, loved that comment. I had never thought of the "wake" effect of a massive object moving (or disappearing) and space/time not being able to completely restore itself afterwards. Even though my understanding of physics is quite limited (high school level plus some reading and disucssions since then), I do tend to grasp most concepts, and the idea that gravity was a force travelling as a particle never quite felt right. However, the idea that a massive object could bend space/time did.

I'm part way through Hawking's "The Universe In A Nutshell". I started reading it on a trip my wife and I took to Hawaii (she grabs popular fiction, I buy Hawking's to read on the beach - I think she's starting to question my sanity). For the most part I really enjoy the book, and its definitely given me something to think about, though I'm starting to disagree with parts of it. It seems that he, and most other physicists are picturing the universe as a closed system. Given some of the things he says in that book, I'm starting to wonder if it is. Of course that just begs the question, If its not, what's outside it?

Anyways, thanks for pointing out some ideas I hadn't come across yet, and it looks like I'm finally going to have to start reading more on Quantum mechanics. I just don't seem to be getting much farther in my understanding of the universe without going down that road.

Correction to the above....

[dr. loser]

I succeeded in tracking down the actual paper from the Purdue folks. What they've really done is come up with a clever experimental scheme that measures the gravitational interaction independent of the Casimir force - basically it's a background-free measurement. Very slick.

Explaining Gravity

[Squiffy]

Slightly off-topic:

So they still haven't observed the graviton and are still having trouble explaining why.

What I'd like to know is, why aren't physicists trying harder to explain gravity as a "pseudo-force" like the centrifugal "force" and the Coriolis effect? That's not just a rhetorical question. What makes physicists so sure that the graviton even exists? I trust that there must be some deep reasoning involved -- what is it?

Re:Explaining Gravity

[jpflip]

You're right that we've never observed a graviton. However, most physicists would say that this is hardly a surprise. There's no trouble explaining why - any effects of quantum gravity (any behavior where you'd have to know about gravitons and not just about general relativity) probably shouldn't kick in until the Planck energy scale (the energy scale associated with the observed strength of the gravitational force), which is something like 10^16 times greater than any energy ever achieved in an accelerator. Some theorists have come up with ways in which quantum gravity effects become manifest at lower energies (such as the extra-dimension theories the experiments in this post are designed to test), but your naive guess would be that we shouldn't have seen quantum gravity yet.

What you describe (gravity as pseudoforce) is actually something like the way gravity works in general relativity. In that theory, mass warps the fabric of spacetime. Objects travel in the straightest lines they can in this curved space, and we perceive the bends in those paths as being because of a "force" between masses. This theory has been extremely successful in explaining all sorts of large-scale phenomena (not to mention the fact that it is very theoretically beautiful).

The problem is that general relativity and quantum field theory (the theoretical framework of "particles" being exchanged that works so well for the other forces) seem to be fundamentally incompatible. General relativity is fundamentally a theory of the way the geometry of spacetime changes. Field theory is formulated on a pre-existing, static background spacetime. You get into mathematical trouble however you try to get these together.

You can continue in (at least) two ways. Particle physicists are usually more inclined to think that the field theory point of view is fundamental, and that whole geometry thing is just the way things look on large scales. This leads to string theory and the usual discussion of gravitons. If you treat the geometric point of view as more fundamental, you try quantizing spacetime and get loop quantum gravity. String theory is more popular, but no one knows what the right answer is (both may even be different points of view on the same thing!).

Re:Did I miss something?

[Nos.]

You are undoubtedly confused and I can't even begin to guess from where you gleaned this information

Well, as I said, I read the article about Casimir force linked to in the original article ( http://news.uns.purdue.edu/UNS/html4ever/030811.F ischbach.casimir.html) which contains this paragraph:
The Casimir force has to do with the minute pressure that real and virtual photons of light exert when they bump against an object. High quantities of photons are constantly striking you from all directions, emitted by everything from your stovetop to distant stars.

What?

[HerbieTMac]

Sorry, but you are misinformed. Gravity does not warp space-time, gravity is the warping of space-time.

So, no, you will not see a "wake" of gravity because you are an observer, you will be affected by the gravity of the object at a point. Since the object itself cannot move faster than the speed of light, the gravity well will always be able to restore faster than the object moves.

You may be thinking of frame-dragging, which is a different phenomenon.

BTW, what moderator decided that this comment was "Interesting"? What I wouldn't give for a "-1, Uninformed" mod.

Re:What?

[jd]

In order for that to be true, you must assume certain other things to be true:

• Space/time can bend without any means of exchanging information with the object (without some exchange of information, space/time would not be affected by any mass within it). Both the exchange of information and the consequence are referred to as gravity, but it would be more correct to describe the latter as a gravitational well or gravitational field, as distinct from gravity itself.
  
• Two events can occur simultaneously (the motion and the change in gravitational field) - problem is, this is expressly prohibited
  
• That the change of shape of space/time will occur the instant the information reaches it - problem is, this requires abnormally high densities of "quantum foam", creating a "spray" of Hawking Radiation, as it would be impossible for the particles to pair up correctly. Such a spray does not exist.
  
• That "quantum foam" has no repulsive forces, which would slow down any restoration of space/time - so far unknown, but not impossible.
  
• Velocity is relative, so what would you be measuring the velocity relative to? Space itself? But the whole point of relativity is that there is no absolute space to measure against. And even if there were, you're altering its shape, so there is no consistant point of reference to relate to, even if you could!
  
• All of this assumes, of course, that information is being continuously exchanged between the object and space. This is unlikely, on the face of it, as QM especially does not support the notion of anything being continuous. Thus, there will be some non-zero time interval between the motion of the object and the next exchange of data.
  
• Without a wake, the predicted gravitational waves for co-orbiting stars would be impossible.
  

In other words, there are many reasons for assuming some sort of gravitational wake, and some predictions that would seem to make such a hypothesis inevitable. I see nothing in your reasoning that suggests that this is impossible, so you may want to expand on it a little.

Re:What?

[HerbieTMac]

Sorry but you are confusing things.

• As an observer, you cannot perceive more than one point of gravity (or any other light-speed wave) at a time. In other words, you perceive the effects of gravity as a point source.
• If you want to argue that space and time are quantized, I am afraid that I cannot engage you in a discussion as you will have to assume a radical reworking of mathematical analysis to fit observational data. If you have some reason to assume this quantization, I will listen, otherwise you are engaging in a bit of intellectual masturbation.
• We are not talking about energy differentials as the gravitational energy is the result rather than the cause of space-time distortion. Without energy differentials, no quantum foam, no Hawking Radiation "spray." You need to be extremely careful when positing these sorts of causal structures.
• See above
• Velocity is measured as a fraction of the speed of light, relative to any observer. Since all observers see the speed of light as the same speed in their own reference frame, it is a useful measurement. It remains unproven that anything, including your moving object, can violate this.
• Again, you are arbitrarily quantizing space and time. No matter what the university drop-out on the cover of Wired magazine said, there is no evidence for this. Postulates without evidence mean nothing. Since all observational evidence points to time and space being continuous, your argument has no point.
• No. That is wrong. Binary star systems have no need of "wakes" to produce the observed eccentricities. Just non-instantaneous information exchange.

Since no one has yet been able to rectify a quantized space-time with observation, I would have to say that I am skeptical that this breakthrough will come in a slashdot posting.

Re:What?

[Curtman]

"BTW, what moderator decided that this comment was "Interesting"?"

What deity bestowed the ultimate truth and power to judge the value of opinions upon you? I found the comment interesting. This is a great forum for discussion of news items, and that is what I come here for.

What I wouldn't give for a "-5, Callous Pedantism" mod.

Re:What?

[jd]

Let's start with the easy one: Is space/time quantized? To test this, you must point a telescope at a great enough distance that the angle produced by the increment is clearly visible and clearly uniform. Have telescopes seen clearly visible and "unnaturally" uniform regions of space? Uh, yes. That's exactly what is seen in the early Universe, despite the obvious problems this would create (such as no way for structures to form). The solution to this problem is to say that we are seeing a small enough region of quantized space that variation is impossible, that we are not seeing the variation that must exist because we aren't looking at a continuun. Ergo, yes, we have observations that can best be explained by quantized space/time.

Now onto the rest of my post: The physicists at Warwick University, for the Einstein Celebration, considered my theories on relativity to actually be pretty good. :) I had produced a summary of the derivation of relativity and from that derived what overall physical phenomena must underpin the entire theory.

The quantization of space/time is guaranteed. Why? Because matter is quantized, and matter and energy are simply different facets of the same thing, energy must be quantized. (Matter is merely condensed energy, it is therefore the same stuff, just in a different state.)

If energy is quantized, then fields must be quantized, as fields define energy. If fields are quantized, then space and time are quantized, as fields are defined over these.

The scale of quantization is extremely small. A Higg's Particle is the smallest unit of matter definable, but in order to have energy to condense, the scale on which a photon itself exists must be smaller still. There may well be smaller particles in the "quantum foam", which is fine as they don't have to be stable. The Higg's Particle is stable and is likely the smallest object that can be stable.

What does that give us for scale, though? Without knowing even the theoretical mass of the Higg's Particle, that is hard to even guess at, but a guesstimate based on existing data would imply quantization of space at around about 10^-50 m, and something comparable for time.

No, you do not perceive gravity as a point source, because you are not a point. That is why objects in a gravity well will stretch. EACH point of you will experience gravity differently and not from "one source" but rather from the composite value.

(If you are between Earth and the moon, you will experience gravity from each. At the right point, you will be held stationary because the interference will produce no net force. If that were not the case, the Universe would be in serious trouble. As would most of physics, as a lot depends on overlapping fields.)

We are talking about energy differentials. It is a grave mistake of the first order to distinguish between phenomena that are, in fact, the same thing. All objects travel at the speed of light, at different angles to space/time. It is the angle that produces relativistic mass, reliativistic length and relativistic time.

This can be proved by simple trigonometry, and is likely where Einstein got the equations in the first place. Relativity is just a restating of Pythagoras, as all equations are based on the same formula: R' = sqrt(1 - v^2/c^2), where R' is the relativistic version of the variable of interest.

When re-written, this becomes R'^2 + V^2 = C^2, which is basically Pythagoras.

However, the consequence of this is both simple and profound. If all objects indeed travel at the speed of light, at different angles to space/time, and indeed relativity is nothing more than the projection onto the plane of interest, then gravity is a direct consequence of this motion through space/time.

(Relativistic mass, by this logic, is simply a force exerted at 90' to space. The distortion of space is the result of this. This means tha

Re:What?

[mattpalmer1086]

Hmmm... just finished reading an excellent book about cosmology (The Fabric of the Cosmos, by Brian Greene). In it, the author clearly states that all objects are constantly moving at the speed of light through space-time.

If you are stationary in space, then you are "moving" at the speed of light through time. Any motion through space reduces your velocity through time - but it always adds up to the speed of light.

I must admit, that idea made me stop and re-read that section of the book a couple of times, as I'd never heard relativity expressed that way before. I may have not entirely presented his arguments correctly (I am not a physicist), but that was the essence of it.

Re:What?

[AKAImBatman]

If you are stationary in space, then you are "moving" at the speed of light through time. Any motion through space reduces your velocity through time - but it always adds up to the speed of light.

That's more or less what relativity says. It's not so much that your velocity always adds up to the speed of light, however, as it is that we are travelling on a four dimensional vector. i.e. Just as a car traveling in a diagonal path has a slower southward velocity than a car travelling at the same speed but heading due south, so is our apparent velocity through time affected as we "turn" more into the three dimensions that we know.

I must admit, that idea made me stop and re-read that section of the book a couple of times, as I'd never heard relativity expressed that way before.

One important thing to remember is that there are two theories: The General Theory of Relativity (i.e. Gravity) and The Special Theory of Relativity (i.e. Space-Time Warps). Most people are referring to the former when they speak of relativity, thus causing no end of confusion. :-)

Photons have mass?

[Nasarius]

Photons have mass

No! Photons have momentum. This does not imply that they have mass.

Re:Photons have mass?

[WaterBreath]

In fact, this is how you calculate momentum...
Not for photons.

This is how you calculate momentum for photons:

p = h / lambda, where lambda is wavelength.

Alternatively:

p = hf / c, where E is energy, and f is frequency.

More info here:
http://scienceworld.wolfram.com/physics/Photon.htm l

And here:
http://scienceworld.wolfram.com/physics/Energy.htm l

You can "back-calculate" a supposed mass for a photon, once you know its momentum, by using the p = mv equation. But this often called a "fictional" mass, because it is purely relativistic. If you took away a photon's speed, it would have neither mass nor momentum, and would essentially cease to exist. Mass as an fundamental physical quantity exists even in the absence of velocity. This cannot happen with a photon...

Unless you subscribe to the view that photons do not always travel at c in vacuum. But I will not argue that here. Not enough space, and I don't want to be in a flamewar.

Re:Photons have mass?

[doppe1]

It is not generally accepted that photons have mass, they are generally believed to have no mass.

The bending of light around large objects is not due to the planet excerting a force due to gravity on the photon, but instead the presence of the planet bending the space-time around the planet, then the photon travels in a straight line through this curved space-time.

This means that the photon does not need to have mass to be bent by light.

Re:Photons have mass?

[WaterBreath]

Unless I'm mistaken, the general belief is that a photon does have mass.

No offense intended, but you are mistaken.

If photons do not have mass, why are they affected by gravity?

According to relativity, gravity bends space. It doesn't act directly on other mass. Rather space acts on mass, by telling "how to move", which is along paths called "geodesics". A geodesic is a path demarking the "shape" of spacetime in a region. Light moves along geodesics, which is basically a way of saying that it perceives itself to move straight through local space, though that space may not appear "flat" externally. It's similar to how driving on a straight road on the surface, we do not directly perceive the curvature of the planet. Mass bends space, which causes the local geodesics to be curved relative to distant space that is differently curved. So from here on earth, we perceive the light to bend, but in a local context, the light is travelling in a straight line.

Re:Photons have mass?

[Khashishi]

Photons have no rest mass. Rest mass is the magnitude or length of the energy-momentum pseudotensor. Whether it has mass depends on how you define mass. If you use a semi-classical definition of mass as that which resists motion and creates gravity, then mass is energy, which photons have. But a lot of physicists use mass to mean rest mass, which is 0 for a photon. If you try to decelerate a photon, it keeps moving at c, but it decreases in frequency. If you try to bring it to a rest, you will red-shift it into oblivion.

Everytime gravity stops sucking...

[ignatz72]

Brian Greene loses a "brane" cell...

http://en.wikipedia.org/wiki/Brian_Greene

$\alpha \leq 10^{12}$? Why so large?

[menscher]

IAAP, and just skimmed the PRL on this. I'm a bit surprised to see they have only found (for $\lambda\approx 200nm$) that $\alpha\leq 10^{12}$. Here it's defined through $V(r)=V_N(r)[1+\alpha e^{-r/\lambda}]$ where V_N(r) is the expected Newtonian gravity.

So, as I see it, they've shown that this "other" interaction is less than a million million times stronger than Newtonian gravity, right? Until $\alpha \approx 1$, I wouldn't say they've "shown that gravity behaves exactly as Isaac Newton predicted". This is interesting, of course, but there's a long way to go. Fortunately they conclude their Letter by saying they expect to be able to get limits on $\alpha$ down to 10^6 with $\lambda\approx 100nm$. We'll be looking forward to it.

As a side note, it'd be really nice if /. learned to render TeX for any non-physicists who might be reading this.

String theory

[Pseudonym]

Nobody has ever adequately explained to me why string theory is popular. It isn't actually a "theory", since it hasn't made any testable predictions. There is no problem which it has (yet) solved. Its desirable features (e.g. supersymmetry) are not known to be useful in the first place. Even its motivating examples don't seem to fit the theory. (Hadrons look like strings, but no known string models look anything like hadrons.)

As far as I can tell, the argument seems to be that high-energy physicists need to be doing something other than sitting around twiddling their thumbs, and until someone comes up with an alternative, string theory is that something.

Re:String theory

[jpflip]

There is definitely a certain amount of what you describe. Particle theorists need something to do, and string theory is the best game in town. It has essentially no testable predictions currently, and it was motivated originally by hadron physics (which the current implementations have nothing to do with).

I think the thing that really got people excited about string theory was the fact that it's a quantum theory of gravity that works at all. That's pretty powerful, since people had been trying crazy tricks for decades to get particle physics and gravity to go together. This time they had come up with a particle physics theory via an entirely different road and later noticed that it happened to solve the gravity problem! This framework also has lots of attractive features. I believe it was the first framework for quantum gravity that was renormalizable (free of the "bad" infinities that screw up quantum gravity), and I think some current implementations are thought to be finite (no infinities at all!). It necessarily contains supersymmetry, a proposed symmetry of nature that helps with many issues of particle physics (i.e. the stability of the mass of the Higgs boson). It also has a very mathematically rich structure - even if it had no relation to the real world, mathematicians would still love to study its intricacies. It hasn't produced any testable real-world predictions, but it has been used for a few useful calculations - in particular, it has been used to account for the details of Hawking radiation and black hole entropy.

I think that string theory is a bit too popular, and that the general public and funding agencies have the idea that it just has to be right. It's very attractive, but still very speculative and far-removed from the real world. There are far too many posts on the slashdot boards, say, to the effect of "Q: What does this recent physics experiment say about string theory? A: Nothing."

Re:Business as usual; gotta keep looking closer

[mattpalmer1086]

Nope - this is nothing to do with when quantum effects "take over" from gravity. The reverse, in fact - those forces already swamp the measurement of such a tiny force as gravity - they had to find ways to rule them out to reveal the gravitational influence.

The experiment is looking for evidence that gravity does not follow Newton's law at very small scales. This is predicted by some theories (notably string theory). Confirmation that gravity behaves "normally" up to these atomic scales rules out some theories which require larger extra dimensions. As a side benefit, they managed to measure the casimir force really accurately too.