Forget Stardust: You are made of spacetime

Saturday, June 16, 2007

The most accurate theories ever invented by humankind are quantum mechanics and the general theory of relativity. Both were—and are— somewhat contradictory to common sense and hence widely disputed. Both have been tested extensively and their predictions, common sense aside, have proven accurate in both cases out to the twelfth decimal place or better. Quantum mechanics deals with the physics of the very small and general relativity with the very large, and so neither is particularly observable in everyday life. Unfortunately, each of these theories retains older assumptions about the nature of reality which the other contradicts. Quantum mechanics retains a Newtonian view of space and time; general relativity a Newtonian view of the relation between observer and observed. They can't both be right. Reconciling these views has been the principal unfinished task of physics for at least half a century.

For a while, all theories attempting reconciliation rested on extensions of the huge unification achieved with the introduction of quarks around 1964, that theory now being called the "Standard Model". Unfortunately, several reasons have been found to make it necessary to try to replace the Standard Model with a better theory.

At least three possibilities have been mooted in the last quarter century, each of them having partially slipped into the public consciousness at least peripherally: string theory, supersymmetry, and quantum gravity. None of the three are yet subject to experimental verification or denial and hence all three could on legitimate grounds be spurned as non-scientific in nature. But obviously science, particularly hard science, involves a theoretical stage as well as a verification stage and it is probably quite reasonable to view these as scientific theories which happen to remain in the theoretical stage at the moment. Other theories including relativity and some predictions of quantum mechanics have historically required a long hiatus between theory and empirical verification.

Another related puzzle which has long tormented contemporary physics is the nature of the "zoo" of fundamental particles discovered rather haphazardly during the Twentieth Century. While the Standard Model has allowed us to discover much about these particles and their relations, there is no a priori reason known why the mass of the electron, say, should be exactly what it is, or how many other fundamental electron-like particles may exist. Such very basic questions remain completely unanswered. In addition to trying to reconcile relativity with quantum mechanics, all three of the newer theories mentioned attempt to explain the nature of the "fundamental" particles currently known empirically, using even simpler notions as the building-blocks. The well-known string theory, for example, simply rests on the assumption that particles are not 0-dimensional points but rather are the oscillations of little tiny "fundamental" 1-dimensional curves, called "strings".

Lately, despite the popularity of Brian Greene's The Elegant Universe, physicists have started to abandon string theory for other more promising pastures. The alternative theory of quantum gravity has begun to attract more adherents as some of its ideas begin to be fleshed out and some surprising results obtained. This theory posits the existence of "fundamental" particles called "preons", which can be conceived of as ribbons that can somehow interact by wrapping around each other. It is supposed that these ribbons can cross over and under each other to form a braid when three preons come together to make a particle. Individual ribbons can also twist clockwise or counter-clockwise along their length. The theories predict that such structures, essentially individual building blocks in the fabric of spacetime, are so ephemeral as to disappear after 10^{-44} seconds, yet electrons for example are much longer-lived.

In a radical reworking of the theory, quantum loop gravity, these braids of particles can be viewed as qubits (quantum bits), the entire universe of spacetime being thought of as one huge connected network of isolated units, functioning in essence as a single enormous quantum computer. It turns out that such a conception allows the braided preons to have a much longer-lasting lifespan than individual preons, essentially because of mutual harmonic resonance when they are together in their braided state. The leading theorists in this field have lately been able to show that this model can produce the lightest particles in the standard model—the electron, the "up" and "down" quarks, the electron neutrino and their antimatter partners. That is not "proof" but it is a huge hint from the universe that they might be on the right track. The theory has some startling implications.

Meanwhile, Markopoulou's vision of the universe as a giant quantum computer might be more than a useful analogy: it might be true, according to some theorists. If so, there is one startling consequence: space itself might not exist. By replacing loop quantum gravity's chunks of space with qubits, what used to be a frame of reference - space itself - becomes just a web of information. If the notion of space ceases to have meaning at the smallest scale, Markopoulou says, some of the consequences of that could have been magnified by the expansion that followed the big bang. "My guess is that the non-existence of space has effects that are measurable, if you can only see it right." Because it's pretty hard to wrap your mind around what it means for there to be no space, she adds.


Another clue that this exciting new theory may be on the right track is its ability to predict Newton's laws, essentially from nothing at all.

For loop quantum gravity to succeed as a fundamental theory of gravity, it should at the very least predict that apples fall to Earth. In other words, Newton's law of gravity should naturally arise from it. It is a tall order for a theory that generates space and time from scratch to describe what happens in the everyday world, but Carlo Rovelli at the University of the Mediterranean in Marseille, France, and his team have succeeded in doing just that. "Essentially we have calculated Newton's law starting from a world with no space and no time," he says.


To summarize, the new theory of loop quantum gravity imagines a universe which is little more than a huge quantum computer, whose individual computing elements form the basis of spacetime, and when they become wrapped up together into mathematically precise braids whose symmetry groups are the well-known "braid groups", they form what we have come to know as matter. If you are interested in further reading, you might find Roger Penrose's new book The Road to Reality useful, as it provides a good explanation of some parts of the new theory, based on spinors.

14 comments:

Luther McLeod said...

Excellent MHA. Now what else can I say without making an idiot out of myself?

That was as good an explanation as I've seen in a while. Though honestly I get most of my info from SciAm. But I'm behind on those due to their leftist tilt recently.

As example, 'preons' are new too me. What puzzles is do not preons occupy 'space-time'? I'm having a difficult time with the concept here. If they don't occupy anything then how do they exist? Are they constantly evolving(?) changing, morphing in and out of existence? Creating their own space/time? Just talking out my butt here.

I do think I understood most of what you said. But the concepts may take awhile to process.

I mainly just wanted too say that I really enjoyed the post.

MeaninglessHotAir said...

Thanks Luther. In this view, as I understand it, preons are spacetime, or individual units of such, and it is in their braiding and other interactions that we perceive what we call spacetime. In this view, what we call spacetime is therefore an illusion, or rather a meta-structure on top of the totality of preons. Keep in mind this is all theoretical and still very much in flux.

chuck said...

Go here for a different take on this variety of sushi theory.

I am not a theoretical physicist, but I think Smolin is something of a politician who promises great things but delivers few concrete results. LQG, for instance, has still not correctly computed the black hole entropy without introducing various fudges and ungrounded assumptions, a technique that seems endemic in that subfield of physics.

The New Scientist was fun to read 20 years ago but seems to have regressed. I blame Bush. I certainly wouldn't trust it for 'real' science.

chuck said...

Speaking of Motl, did you know Terence Tao had a blog? It looks like a fun read for those mathematically inclined. For those not familiar with the name, Terence Tao is one of the best mathematicians around today. He was also a child prodigy who scored 760 on the math SAT at age 8.

Luther McLeod said...

Thanks for the further explanation MHA. I understand the caveats re preons. But I'm still having conceptual difficulty with their 'being' space time. As I said, more time needed for digestion.

Skookumchuk said...

Amazing. You have been hitting home runs lately, MHA.

alvinwriter said...

The discovery of a new baryon particle (made of the three types of quarks) called "cascade b" only shows that quarks, regarded by many to be the fundamental constituents of matter, can and actually do make other strange forms that last only for an instant before decaying into other forms. Einstein made it clear that energy and matter are actually the same, and that space and time are really two sides of the same coin. Preons, if they are space time, should just as easily fall under matter energy. It may just depend whether you look at them from a quantum perspective or a relativistic one. But for all practical purposes, there's always the Newtonian to take comfort in. More about "cascade b" in the following. http://www.thenewsroom.com/details/411775/Science+and+Technology?c_id=wom-bc-ar

- Alvin from The Sci-Tech Desk at TheNewsRoom.com

Seneca the Younger said...

Originally there is no tree of enlightnment,/Nor is there a stand with a clear mirror./From the beginning not one thing exists;/ Where, then, is a grain of dust to cling?

Luther McLeod said...

SyT, but yet it happened.

"From the beginning not one thing exists"

And you present the conundrum.

OT, but an interesting discussion at slashdot:

Is it all for naught?

I don't think so myself. We will hang on to life as long as it exists. Its just our nature.

Seneca the Younger said...

Luther, the kicker here is "fundamentally" ... there's no such thing as "things" independently, they're all observations of apparently stable points in a process. But there's damn little of the 275 lbs of "me" today that was "me" ten years ago.

Luther McLeod said...

Point taken and previously understood SyT. Permanence is a trap. We're just riding the wave to splash on a non-existent shore.

danwinslow said...

Well, probably OT, but here goes :

The real question, for me, is why observation seems to enter in to these issues in a special way. I think a lot of what we consider the 'real' world is distorted by the fact that our consciousness seems to be restricted to a single moment in time. Time really is space-like, that is, the future already exists in the same way that Florida exists (even though as kids on a family vacation it seemed to be inifitely far away). A lot of the issues of the copenhagen and multiverse lines of thought seem to be based on missing information...information that does exist, but can only be apprehended by a consciousness that can span more than a single unit of time. The causality information and the 'real' path that the quantum collapse follows is only mysterious due to the fact that we can only see an instantaneous 'slice' of the real event. That is the origin of the collapse: the sudden restriction to a single slice of time when we 'observe'.
Note that classical physics and even the meaning of the word 'thing' in a macroscopic sense seem to be based on statistical mechanisms over time, ie., a time span must be taken into consideration.

Ok, I am officially out of brains now.

Luther McLeod said...

At least you had some to start with Dan :-) Not that lucky here.

"observation seems to enter in to these issues in a special way"

Isn't that all we have though? The more minutely we fine tune, in the quantum world, the more narrow our focus to just that sliver of 'eventness'. I don't think it is our consciousness so much as our machinery.

As too the future and Florida I'm not sure I understand your thought. Even though I like Florida, I grew up there :-) Yes, the future does exist, but I see it as potential, not filled space. Unless the end game is somewhere known and the major pieces placed so as to ensure our arrival there.

Actually I just want too know what's on the other side of the CMB.

danwinslow said...

I guess what I mean is that causality as we know it is just a kind of illusion produced by the fact that we can only deal with a single moment of time. The future is an actual place that already exists; it is meaningless to talk about causality in the same way it would be meaningless to ask whether one spatial location 'caused' another spatial location. There's no 'causing' required, it all just *is*.

The reason we have this illusion of causality is that we are constrained to a single slice of time; fundamentally unable to even think about things any other way. The only information we can perceive is missing a dimension..we are staring at shadows on the wall.

I stray far afield into philosophy here, but it's interesting to note that our consciouness, our idea of 'free will', depends on causality and the flow of time. The essence of free will is that we *don't know * what is coming next. If we knew everything that will happen, we would not be able to have anything we would consider free will.