Scientific Method / Science & Exploration

Physicists find insecurity in string theory

One of the great hopes for string theory has been dashed and physicists are …

Physics, and in particular particle physics, has had a dirty little secret for the past 50-odd years. Quantum mechanics allows the vacuum to have energy — either positive or negative — but is silent on what energy the vacuum should have. The response of most physicists has been to say that the vacuum energy is zero and on the assumption that this means there is no physics to be found, all head for the pub. However, results have proven otherwise and this weeks Science magazine has a couple of articles on the consequences.

Ten years ago, cosmologists proved that the rate of expansion of the universe was increasing, which means that the vacuum energy is not zero. Any non-zero value is problematic because it returns us to the dark days of the fine-tuned universe, where certain constants had to be improbably precise to result in the universe we observe. Fine-tuning resulted in the anthropic principle, which can be summed up as "we exist and therefore natural laws must include us." Most physicists responded to the anthropic principle with a hearty "well duh" and went back to work. That work went a long way towards showing how the universe might not be so finely tuned and the anthropic principle fell out of favor.

The nonzero vacuum energy didn't particularly bother physicists at the time because although quantum mechanics couldn't provide the answer, it was thought that string theory would. But a few years ago, physicists discovered that string theory was also unable to tell us what value the vacuum energy should be. Now we really have a problem: observations tell us that the vacuum energy must have a precise, positive value. None of our theories tell us why that should be so or what value it should be. This has lead some physicists to return to the anthropic principle. Others are traveling further afield, deciding that the real meaning is that there is a universe for every possible value of vacuum energy. These two view points, when viewed from the "testable hypothesis" section of science, fail spectacularly.

My personal point of view is that these physicists, having come so far and finding that the journey has mostly been down a blind alley, are feeling a moment of despair. Eventually we will discover why the vacuum energy is what it is and that will probably not require an infinite number of universes or some impossibly precise value.

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