Category Archives: Physics

Overexcited about beam circulation

A lot of science web sites seem to be excited about plans for the initial beam circulation in the Large Hadron Collider on September 10. While it's a great step into getting the particle accelerator online, I'm a bit less excited about it than others. Personally, I'm waiting for the start of actual collisions between opposing beams a month or two later, which is when we'll start to get some real scientific data.

I wonder if this makes me a dour wet blanket?

Is fine-tuning really fine-tuning?

A paper speculating about the probability of stars forming in alternative universes mentioned at Cosmic Variance got me thinking again about whether our universe is fine-tuned for the emergence of intelligent life. I have always found fine-tuning arguments to be a sort of argument from ignorance, in that they tend to ignore the possibility of other forms of intelligence. While they might be correct in claiming that if the physical constants were significantly different then our form of carbon-based organic life could not arise, they don't really succeed in excluding other possibilities.

Sometimes I imagine that there could be universes with intelligent life that imagine that it's their own universe that's fine-tuned, and them unable to imagine life emerging in a universe that is almost entirely empty like our own. (Greg Egan's Schild's Ladder deals with similar concepts.) Or perhaps galaxy-wide life forms that somehow emerge from the interactions of entire stars acting similarly to cells might be unable to conceive of intelligence at an incredibly microscopic and fast scale like us. Thoughts like these sometimes makes me wonder whether we could even recognize some forms sentience even when it's in plain sight.

Stephen Baxter has a good grasp on how fragile the fine-tuning argument is, and he draws a vivid picture of what alternative forms of life could have arisen throughout the development of the universe in his Xeelee Sequence of stories, from those that form from defects in space-time itself to carbon-based life forms like ourselves. And really, given only the fundamental laws of physics we know today, could anyone have predicted the rise of carbon-based intelligent life like humans? Just because we don't know how intelligence could arise from different laws of physics doesn't mean it's impossible.

Light: from particle to wave

A number of people seem to believe that if a certain body of knowledge keeps on changing, then it must be worthless. But at least for science, which tries to understand how our reality works, it would be worthless if it didn't change to match reality. If science didn't throw away models that did not conform to reality, which is checked by performing experiments and seeing if the results actually agree with the models instead of by pure philosophizing, then it's not doing its job as a body of knowledge for understanding reality.

We see this happening in the history of science multiple times, where scientists had to adjust their theories according to what reality showed them, not the other way around. The history of science is made up of a series of approximations that get better with time. While most of the changes in science are small refinements to theories, every once in a while a drastic overturning of existing models is necessary to get a better approximation to reality.

The change of our understanding of light as particles to waves is a good example. In the 18th century, most scientists believed that light was a stream of particles. There was also a competing theory that light was a wave, but this was discounted in favor of the particle theory because while both conformed to experiment, the particle theory had the weight of Newton's approval behind it. If science was like a religion, where some sort of authority is the arbiter of truth, we'd probably still subscribe to this theory.

François Arago

However, this all changed in the 19th century as new experiments showed results that were more consistent with the wave theory of light. The results of an experiment which attempted to measure the speed of light through refraction and stellar aberration, which Skulls in the Stars has a fascinating account of, were not consistent with the particle theory. Instead, the results were consistent with the wave theory as long as one assumed that the "aether", the postulated medium through which light was supposed to form waves, was partially dragged with matter.

There was also Poisson's spot, which is a dramatic example of how wave theory was more consistent with reality than the particle theory. According to the particle theory of light, a sphere would block the light particles so that it would cast a completely dark, circular shadow. But when Fresnel advocated the wave theory of light, Poisson ridiculed him by showing that such a shadow would have a bright little spot right in the middle according to the wave theory of light. The reason for this is that because the edge of the sphere would all be the same distance from the middle of the shadow, the waves propagating from the edge would all bunch up into a bright spot.

Poisson spot
Poisson spot

At first glance, there being a bright spot right in the middle of the shadow of a sphere sounds like a ridiculous notion, and Poisson put it forward as such. So when Arago did the actual experiment and showed that there actually was such a spot in a sphere's shadow, that was a really strong indication that the wave theory of light was better than the particle theory. It's a bit ironic that the bright spot is more often known as "Poisson's spot" after the person who thought it didn't exist, although occasionally it's also called "Arago's spot" after the person who actually observed it.

Seeing that reality was better approximated by the wave theory of light rather than the previously widely accepted particle theory of life, scientists didn't just stubbornly cling to the particle theory taught by their forebears. They yielded to reality and accepted the wave theory of light, which eventually merged with the science of electricity and magnetism to give Maxwell's theory of electromagnetism.

And the story doesn't end there. Further developments in the 20th century eventually got us to beg the particle aspect back into our theories of light, and we got quantum mechanics where everything is both a particle and a wave. We also got the theory of special relativity. These two theories were much better approximations of reality than the ones that came before. And it's not unlikely that we'll replace our theories yet again in the future with even better approximations. Perhaps even in the near future with results from instruments such as the Large Hadron Collider, GLAST, or LIGO.

All about quantum mechanics

For the average person who is interested in quantum mechanics, Sean Carroll and David Albert will be talking about the subject at Sean Carroll is a physicist at CalTech, while David Albert is a philosopher of science at Columbia University.

Sean Carroll is soliciting questions about quantum mechanics that you might be curious about. One of the things they will probably talk about is how superpositions in quantum mechanics makes things different from classical mechanics. And from the comments so far, they will probably also talk about the various interpretations of quantum mechanics (and there are a lot of them, almost all of which give the same predictions in practice), and there will probably be a lot of discussion of wave collapse, which is the same as measurement in quantum mechanics.

If you have anything else that you have always wondered about quantum mechanics, leave a comment on their blog post.

Building space-time

The July issue of Scientific American has an interesting article on how our four-dimensional space-time could arise from basic building blocks that self-organize in a quantum superposition. Their approach, called causal dynamical triangulations, is an extension of Euclidean quantum gravity. But instead of just seeing what a superposition of self-organized building blocks that assemble arbitrarily looks like, which turns out to be a bunched up and very messy space-time with plain Euclidean quantum gravity, they imposed causality on each building block so that they can only assemble in specific ways. Their computer simulations show that the result would be a space-time that looks much like our own on large scales.

They show that at large-scales space-time should be a smooth four-dimensional space-time, but at microscopic scales the dimension of space-time shrinks down to two, which is a concept I have difficulty wrapping my head around. The concept of dimension they used is the spectral dimension, which is determined by how fast something spreads over time. For integer values of the spectral dimension I think I can understand how it relates to the more conventional notion of dimension (the number of values necessary to locate something uniquely), but fractional values of the spectral dimension is rather hard to understand.

And if the theory is correct, then the built-in causality would imply that wormholes and time travel would not be possible. While it's cool that ordinary space-time could be built from first principles, it would be a bummer in that faster-than-light travel or direct observation of historical events would not be possible.

Causal dynamical triangulations is an interesting approach, independent from string theory, which shows promise given their simulation results. It's also nice to see that string theory has not completely monopolized theoretical physics, despite the lack of any experimental predictions or confirmation. While I don't think that string theory is a dead end, I do like to see a more balanced allocation of resources to alternatives, considering that all of the current theories of quantum gravity are somewhat lacking in experimental support. I also have to admit that I find background-independent theories, where space-time itself arises from the laws of physics instead of being a pre-defined background, more aesthetically pleasing in a philosophical sense.

Some of the questions I have about causal dynamical triangulations are:

  • With space-time being built out of basic building blocks, do these constitute a sort of "aether" which defines an absolute space-time? What happens to Lorentz invariance?
  • Do the simulations only show an empty vacuum universe, or does it reduce to the theory of general relativity in the classical limit?
  • Would the incorporation of causality in the building blocks imply a built-in arrow of time in the laws of physics?

And some questions that they still need to figure out:

  • Is there a fundamental building block to space-time, or is space-time infinitely divisible?
  • Would it have implications for the origin of the universe?
  • Are there any feasible experimental predictions that would differentiate causal dynamical triangulations from other theories?