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

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

[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.

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.

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.

Free Link

[Anonymous Coward]

Physical Review Letters is pricey. Here's the free copy off arXiv. Anonymous for no karma whoring.

Photons have mass?

[Nasarius]

Photons have mass

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

Re:Photons have mass?

[Moekandu]

Photons have mass

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

Actually, it more than simply implies that they have mass. You cannot have momentum without mass. You can have velocity without mass (in the case of neutrinos, I think), but not momentum. In fact, this is how you calculate momentum:

p = mv

p = momentum
m = mass
v = velocity

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:Explaining Gravity

[Anonymous Coward]

Maybe because they move too fast.

The minimum observed limit on the speed of gravity is >= 2*10^10c.

http://www.ldolphin.org/vanFlandern/gravityspeed.h tml

Yes, 20 billion times the speed of light.

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: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.