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Time Passages IV

Posted by: Bogon, 6:37 PM GMT on February 13, 2012

The Past

A year ago I posted a series of three loosely affiliated blogs, which shared the same title and which discussed different aspects of the same subject. The first one talked about two trees newly planted in my yard. Both trees are potentially very long-lived, but what is their life expectancy here on this suburban lot? How much time will they be granted? You'll be pleased to hear that those trees survived their first year. They've had a chance to set roots. This (and each subsequent) year's summer heat and drought should pose less difficulty for them. There was a third tree, an Eastern redbud, which did not fare so well. It got struck by lightning. Wife and I had to replace not only the fatally zapped sapling, but also the main circuit board in our television. Ouch!

The second blog presented a personal overview of time. How do we speak about time? How do we experience its passage? The third installment addressed cosmology: time considered on the largest possible scale.

So what brings me back again after all this, uh, time? Well, life goes on. We live and learn. This year I learned of some new developments in physics. I am not a physicist, but I try to understand how the world works. It is the same curiosity about the world that brings me to Weather Underground. I'm not a meteorologist, but I like to learn what I can about the weather.

As long as we're talking about the past, it might help to set the scene by reviewing some history. It has been nearly a century since Albert Einstein announced his theory of relativity. General relativity remains our best tested and most trusted model of the operation of the universe at the large scales observed by astronomers and cosmologists. The theory describes the interactions of space, time and gravity. Einstein's equations treat space-time as a continuum which warps and flexes in response to the presence of mass and energy. The word 'relativity' refers to the way the theory relates moving objects. Each object has its own point of view. According to the theory there is no fixed background or point of reference. If two spaceships pass in the night, the passengers on each one will tell a different tale about what they saw.

At the same time Einstein was developing relativity, other scientists were working on quantum theory. This theory accurately models the universe at the very small scale of subatomic particles. Quantum electrodynamics describes the forces of electricity and magnetism perfectly (insofar as we are able to measure). There are also good working theories for the forces responsible for radioactivity and nuclear power. The word 'quantum' refers to the discrete discontinuous appearance of nature when viewed at the smallest possible scales. It's like when you zoom in on your computer screen you see jaggies and dots, the individual picture elements, that make up the displayed image. According to quantum theory matter is comprised of a small number of elementary particles, each of which has a fixed mass. Energy is exchanged in little packets. Events proceed stepwise. From any point of view those events are likely to look a little blurry, because there are definite limits on how finely you can measure. The act of measurement affects the thing being measured.

Throughout the remainder of the 20th century these two theories were tested and applied. Both work very well within the domain for which they were designed. For any real-world situation, physicists are obliged to choose one theory or the other or compare the predictions of both. Nobody has figured out a way to combine the two. The theory of relativity does not extend to situations involving high energy and small scales, and there is no quantum theory of gravity. The math is too hard. The techniques developed within one theory break down when applied to the other.

The Present

For the last few days my mother has been battling an eye infection. Her doctor prescribed some ointment and an eye patch. Mom complains that, when she goes to pour a beverage, without proper depth perception she's liable to miss the cup and spill her drink on the counter.

Physicists, too, are tired of peering at the universe through one eye or the the other. They think their view would be a lot clearer with full stereoscopic vision. So when I say there is no quantum theory of gravity, I mean there is no complete theory. The last ninety years have not been wasted. People are working on a variety of approaches. Some start with relativity and try to quantize it. Some start with quantum theory and try to develop a background-independent version which includes the graviton.

One approach that gets a lot of press these days is string theory. The basic idea behind string theory is that elementary particles should not be portrayed as dimensionless points. They are granted a finite size, which makes some of the mathematics more tractable. I say 'some' of the math; there is much more to string theory that is very hairy indeed. String theorists propose a group of elementary particles that have not been detected and postulate several extra spatial dimensions beyond the three that we observe. In support of their theories they offer weak circular arguments such as the anthropic principle. Whenever reality threatens to contradict the theory, they spawn a new version. In fact there are a zillion string theories with no obvious way of picking a winner.

From my point of view they are all losers. String theory is a monster that devours talented young mathematicians. It sucks up lifetimes and hefty research budgets and leaves nothing to show for them. It provides a convincing model for how to parley government grants into academic careers, but it has done nothing to shed light on how the physical universe operates. There are no verifiable predictions of string theory.

There are numerous competing theories that seem more promising. There's Roger Penrose with his spin foam and tensors. There's Lee Smolin, who champions a theory called loop quantum gravity. These guys are results-oriented. Unlike string theorists, they manage to refrain from wandering off into fanciful realms of elegant mathematical complication. They'll be content if they can explain known particles in three dimensions.

Loop quantum gravity theory asserts that space-time is quantized, i. e. there is some minimal unit of volume out of which space is constructed. Thus, on a very small scale it would be possible to view space as a kind of fluctuating grid. An elementary particle would have a finite size. It could be no smaller than a block in the grid. The particle would be represented as a set of properties (e. g. quantum numbers) assigned to some region of the grid. Gravity appears as curves and waves in the grid lines.

Here are links to two presentations from Perimeter Institute in Canada. You can watch Renate Loll describe causal dynamical triangulation, and/or view Fay Dowker as she expounds on the theory of causal sets. You may have noticed that the last two theories include the word 'causal' in their titles. That's where time enters the picture: the sequence of cause and effect. In addition to quantizing space, these theories add the notion that all the little grid lines must be aligned timewise. The arrow of time is built in from the start rather than being sought as an emergent property. That turns out to be a significant innovation. Calculations become easier, and believable results follow. If you listen to the videos, you'll hear how several lines of evidence from thermodynamics, information theory, black hole theory, astronomy and cosmology converge to indicate that these people may be on the right track.

The advantage of such theories is that they can reproduce some of the observed properties of three-dimensional space (plus time) with a minimal set of assumptions. A disadvantage is that each theory only provides an incomplete and unwieldy model. It is not an equation. It is an algorithm for simulating space-time. It runs on a computer much like one of the weather models here at WU.

The Future

None of these theories is ready for prime time. Progress is slow, because the problems are hard. Only in hindsight is it clear what questions to ask in order to get the right answers. Will it be easier to work from the top down by quantizing relativity? Or will the bottom up approach, adding the force of gravity to an existing quantum theory, yield the prize? Will it become possible to translate a computer-driven algorithmic model into a concise set of equations? Will string theory ever be good for something practical?

The goal of all this theorizing is perfect binocular vision. Physicists want a Theory of Everything. It doesn't mean that they'll immediately be able to solve all problems and write down all possible knowledge in a book. It means that we humans will finally have a single unified theory that accounts for all known particles and forces. Far from being an end to science, it will make a great beginning for whatever comes next.

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