More Research on (Small) Multiple Dimensions

travisbecker writes "As a follow-up to this ./ article, take a look at this U. of Washington study (article courtesy of SpaceDaily.com) that shows that *if* other dimensions exist, as postulated in string theory, these dimensions would have to occupy a space smaller than 0.2 millimeter. Research is continuing in the 0.1 millimeter regime. The findings will be published in the Feb. 19 issue of Physical Review Letters."

Comments

Huh?

[waldoj]

I'm not totally dumb about the basic concepts of multi-dimensional theory, but I've got to admit: I have absolutely no idea of what this means. Could somebody explain this in layman's terms?

-Waldo

Re:Huh?

[slcdb]

I'm no physicist, but my take on it is like this:

a) They measured the gravity due to these small (0.02mm) objects to be just as weak as Newtonian physics says it will be.

b) The multi-dimensional theories think that gravity is so weak because after it "travels" a small distance some of it "leaks" off into these other dimesions.

c) If they can later measure gravity to be much STRONGER than normal using objects that are, say, 0.002mm in size, then they know that these other dimensions are either between 0.002mm and 0.02mm in size, or between 0.002mm and 0.02mm away from our dimension (depending on which theory you're specifically using). This is because the objects (in their entirety) would be close enough to each other that none of the gravity has yet "leaked" away.

(Theoretical physics from this programmer's perspective.)

Re:Huh?

[krlynch]

I'll try...

Strictly speaking, what this U. Wash. group has done is to confirm that the Newtonian formula for gravity (or the Einsteinian weak field limit, if you prefer), is correct for objects separated by more than a certain distance. In other words, in the weak field limit, gravitational force falls off exactly as 1/r^2. Although this may not be the way the experiment is done and analyzed, you can think of it this way: suppose gravity "really' acted like (1-exp(-r/a))/r^2. Then, if r >> a, you would find that gravity at larger distances fits the 1/r^2 model. In essence then, this experiment has put a limit on how big "a" can be, since if it was any larger, they would have seen the deviation.

Now, why is this interesting? One reason is that it is yet another confirmation of the weak field limit of General Relativity (this is not that interesting a confirmation, really). Another is that it tells us that there aren't additional dimensions that are "curled up" which are bigger around than a few millimeters. If there were, they would lead to specific types of deviations in gravity at small distances (curled up, or compactified dimensions are kind of difficult to get your head around, but here is a model to think about: get out a garden hose, and stretch it out straight. Then, go a few hundred feet away. That there garden hose would appear to be a one-dimensional object (a line segment). But if you go closer, you would see that it had extent in a second "curled up" direction. Just like this experiment does with the more topologically complicated world we live in).

Frankly, if this experiment had been done a few years ago (say five or six), it would not have been very interesting. "Hold on!", you might say, "Doesn't string theory expect extra dimensions?" And the answer would be "Yes, But", since the dimensions in string theory are expected to be so small that they would never be seen in this type of experiment. What is interesting about this result is that there are new (last five years or so) types of models which remove (in a sense) a certain "aesthetically unpleasant" aspect of the Standard Model of Particle Physics by adding relatively large extra dimensions (meaning fractions of millimeters, not fractions of fractions of Angstroms). So, what is interesting in this new result is that it directly accesses the most relevant size scales for these new models, placing (potentially) strong constraints on some of the most interesting (and easy to understand) versions.

[This unpleasant aspect is that the current version of the Standard Model receives corrections that are proportional to the next highest energy scale above the Standard Model's highest scale, and those corrections are quadratic in the higher energy scale...and the next energy scale is the scale of gravity, some 20ish orders of magnitude above the Standard Model scale. Corrections of this size would be in conflict with the predictions of the Standard Model. In order to eliminate them, you have to assume that certain parameters have ratios which are finely tuned to within 1 part in 10^30 (for example). This is the classic "hierarchy" problem. But, if you put in extra dimensions in a certian way, you lower the fundamental scale of gravity to the point where the corrections to the Standard Model are not large, and hence there is no "hierarchy" to become problematic. Personally, I don't think these models will survive the next few years, but I've been wrong before!]

Re:Time?

[caffeinated_bunsen]

Time is not required to be all that different from a spatial dimension. Some theories describe perception of time merely as a consequence of increasing entropy, which is itself a consequence of an expanding universe. If I understand the explanation I read correctly, time is simply a name for a direction in which a certain characteristic of the universe changes predictably. Specifically, a 4-dimensional (3 space, 1 time) universe is described as the surface of a 5-dimensional sphere. Most theories today use plenty more than 4 dimensions, but this case is easy to explain.

Along the 'time' dimension, the entropy of the universe varies roughly with size. That is, as the universe increases size, entropy increases. As the universe decreases size, entropy decreases. Our perception of time is proposed to be simply a function of increasing entropy, so we must always observe increasing entropy and an expanding universe.

So time is not fundamentally different from other dimensions, but simply happens to be the property of the universe on which our consciousness depends. It is consciousness, not time, that introduces all the wierdness in relating the different dimensions. Or so goes that particular theory. Such theories are quite popular for speculation, but have no real evidence in their favor thus far. As I previously mentioned, this is where philosophy begins to take over from science. If you want to do anything useful, you still have to treat time and space as being fundamentally different.

Quantum Vaccuum & virtual particles

[Carlk]

I've suspected that strings, and the Higgs Bozo are gimmicks to get funding. Check the Feb 3 New Scientist. Alfonso Rueda is reported to have derived Newton's 2nd from Quantum Vaccuum & virtual particles! It seems virtually sensible! But no one's mentioning Rodger Penrose's Twistors.

Time?

[QuantumG]

So someone in the know: is time a dimension or not? When string theory talks of 10 dimensions it is talking about 10 physical dimensions. So the question is, if time is a dimension, is it a physical dimension and we just experience it differently to the other physical dimensions, or is it inheriently different?

Re:Time?

[Christopher Thomas]

Time is not required to be all that different from a spatial dimension. Some theories describe perception of time merely as a consequence of increasing entropy, which is itself a consequence of an expanding universe.

While entropy does indeed determine the direction in which time appears (to us) to "flow", timelike dimensions _are_ physically distinct from spacelike dimensions. There's a - sign instead of a + sign in front of them in one of the SR equations, which has important consequences.

A more detailed explanation is left to someone with a better background than I have.

Re:Time?

[a humble lich]

I am not a string theorist, but when string theory talks about 10 dimensions it is usually understood to mean 9 space-like dimensions and one time-like dimension. Now time-like dimensions are inheriently differenent than space-like dimensions, and that is what special relativity deals with--the wierdness of the time-like dimension.

Re:Time?

[ruck]

Actually, time is fundamentally different from the spatial dimensions. This can be easily seen by looking at a Lorentz transformation equation from relativity. These describe how things "rotate" (in a way analogous to rotation in 3 dimensions) through the 4 dimensions of spacetime, and basically, in such a equation there's a minus sign in front of the time term which one doesn't see in front of the others (x, y, and z, in cartesian coordinates). This minus sign indicates a certain asymmetry which is very important. In a way, the geometry of spacetime is hyperbolic rather than spherical. This ensures handy things like the preservation of causality, which is really a separate issue from the entropy arguments you bring up.

The hypersphere (4-D sphere) you mention, by the way, is more of an image that cosmologists like to use for picturing a closed universe. Time would generally behave the same way whether the universe is closed or not, however. Also note that it's only a 4-D rather than 5-dimensional sphere (the definition of a sphere is just the surface of a "ball," not everything inside). You're probably picturing the hypersphere residing in a 5-d space the same way a 2-D beachball sits in our 3-D world. It's important to remember that spacetime is all there is, though, so there really is no place for it to sit and in this case it's important that we're only refering to the surface.

Also, to respond to two other comments in this thread:

1. Brane theory (or M-theory as it's more often called) really is the same thing as string theory these days. No one really focuses only on strings (or 1-D "branes") anymore, and the name you call the theory is really a matter of taste (and publicity). Also, it's important to keep in mind that there are severaly formulations of string theory (though most have been shown to be equivelant).

2. Unless I'm mistaken, the 10 or 12 or more dimensions you hear advertised always either include time or are quoted as being in addition to time. In short, time is still treated as another (albeit special) dimension.

I don't claim to be an expert, by the way, and welcome corrections.

Re:Time?

[hackus]

It is proposed in some of the more daring research pieces out there that time really, doesn't exist at all. In fact, it would be nice to just get rid of the whole concept of time.

In case you are wondering, the whole idea about string Theory is that we have no way to link observational results of experiments that deal with the very small and the very large.

Here is an example:

A solar system, or a future solar system starts to form out of a gas cloud. Billions of miles across, it steadily collapses, planets start to form, and finally at its center gravity induces Nuclear Fusion.

The problem is that we have equations that can describe N body problems...(Although we cannot solve them.) By body I mean, planets, stars moons, in orbit about themselves and the newly formed star.

The problem is these physical observations start to break down when we want to explain the fusion, subatomic and even quantum mechanical aspects of the very small and what part it played during the collapse of the dust cloud.

So, we have to use a different sort of mathematics and set of equations to describe the orbits of the planets, then another completely different set of mathematics assumptions to explain how fusion works.

What String Theory promises, is to combine these two worlds into one set of compact equation/equations that you can derive or describe the orbit and interaction of the dust clounds characteristics from the formation of the planets, to the ignition of the star at its center, to its death as a strange object as a white dwarf, neutron star, or even a black hole in a Super Nova explosion.

String Theory promises this but it is not clear it can deliver yet. With new computing technology in the next 100-300 years, many of the kinds of mathematical computations required to solve some of the equation sets to eliminate certain dead ends in the paths of String Theory will emerge.

In reality, computing power limits what we can do now at the moment, with this sort of question.

I spend nights looking at the twisted code of my own organizations enourmous effort to track neio's Near Earth Impact Objects (NEIO.ORG) and it would be beautiful to posess a mathematics that is more elegant than Newtons theory, or even Einstein's.

One Equation or set of Equations that describe Gravity, Time, Electromagnetism, Quantum Mechanics, and basic behaviour of particle matter would be the supreme achievement of our species.

If we can prove, we have the intelligence to posess the keys to the Universe, no lock will be out of our reach.

Re:Time?

[caffeinated_bunsen]

>Time does not exist. It is an illusion.

And lunchtime doubly so, right?

Seriously, though, what theory are you using that completely does away with time? Every one I've heard of incorporates time in the same way as spatial dimensions, or at least a very similar way. Perception of time is another matter entirely, and statements like "Time does not exist" are still strictly the domain of philosophy and metaphysics. Take some of your own advice.

Also, this is not an article about string theory. The subject of the article is a test of a particular brane theory. This is somewhat similar to string theory, but is definitely not the same thing. At the very least, read the article to which this is a follow-up.

If you're just trolling (which is a definite possibility, seeing as how you appear to know nothing of what you speak), I think "All your string are belong to us!" or something similar would have been more effective.

Re:That big?!

[Eryq]

I'm not a physicist, but if I remember correctly, the reason the unextended dimensions are thought to have "remained small" after the Big Bang is that they were prevented from expanding due to having strings would *around* them, like lassos.

The analogy is that a loop of string can exist on a cylinder in two basic ways: wound around it, or not. The strings we encounter in our 3 extended dimensions are the latter kind, but we'd expect to see the former kind in the "curled up" dimensions.

This gets back to why we have extended dimensions in the first place. One theory [as I understand it] is that at one point *all* the dimensions had string wound around them, and then enough wound pairs cancelled each other out in some dimension that it was able to expand and become our first extended dimension.

So why only 3? Probability. As the topology of space changed with each new extended dimension, it became less and less likely for enough wound-string pairs to cancel each other out (just like it's far more likely for 2 randomly-moving pool balls to collide on a 1-meter-square table than it is for them to collide in a 1-cubic-meter space.

So basically, I agree with you: I'd expect Planck-sized dimensions. Although the 0.1 mm is an interesting number: I seem to recall that the Planck mass is a lot larger than one would think, and that string theory had to explain why subatomic particles are not more massive than they appear to be. Maybe there's some relationship?

Time enough.

[Kibo]

There is at least one physicist who claims that time truly does not exist and is taken seriously. His view is that when you try to make the simplified wave equation for the universe you can do so if you factor out time. This has lead at least him to believe that time is something of an illusion caused by our consiousness slipping between what he calls "nows". Really all he's doing is claiming that the multiple worlds/universes theory for QM is fact and the undisputed underlying reality and the fact that you can factor out time and get a simpler model is evidence of this. It's pretty flakey, but he's taken fairly seriously, and puts his work up for peer review. Scientific American had an article on him a while back. Personally, I find his ideas a bit fruity, but it really all boils down to personal taste. I prefer decoherence, but technically it doesn't even matter. If a certain point of view is more illuminating in a certain aspect then that view is useful for investigating those aspects. The underlying reality is a veil that isn't readily pulled back, so anything that maps the another process onto time just becomes a question of semantics. In the end Neils Bohr would disagree with everyone (and he rarely lost an argument). He would concede one point; however, this whole jag is the domain of philosophy.

But why should a time dimension exist at all?

[TheLink]

Is there any scientific proof that time is a dimension?

I can understand the concept of time as a measurement.

If an object moves from point A to point B, it's now at B and took T seconds to do it. But why should there be a "past"?

Time being a dimension will make things more interesting, but is there proof of it?

That big?!

[Gyl]

I'm surprised the size is still that large, that's practially within the visible range. I would have expected the extra dimensions to be "obviosly" around Plank lengths. (10^-43m ?) anyone know why they could be so large? or what does string (or brane) theory state? 0.1 mm would seem to be a size that could become useful for Star Trek like inventions in the near future!