Archive for May, 2012

I remember a technology TV show in the mid 90’s showing a roller coaster simulator ride. The audience is shown a simulation or video of the view out the front of a roller coaster, and the seats jostle and tilt in concert with the footage. I was only 11, but I concluded that it was the coolest thing ever.

Why they are almost convincing

There is a good physics reason why these rides are almost convincing. Galilean relativity says that inertial reference frames are indistinguishable using local experiments. In layman’s terms, if you are in an enclosed plane traveling in a straight line at a constant speed, then there is nothing you can do inside the cabin to work out how fast you are travelling1. The plane could be stationary or it could be doing a thousand miles per hour, and you won’t notice any difference between walking up the aisle and down the aisle.

In a car, we gauge speed by looking out the window and watching the scenery fly past. Ride simulators can simulate a fast moving roller coaster by showing a simulation of scenery going past. They also simulate the bumps and shunts by jostling your seat – the faster your car is going, the more you will feel the small deviations from uniform motion due to potholes.

I’ve been on a few of these rides, and I’m not fully sucked in. Speed is fine, bumps are fine, but the most exciting part of a real roller coaster ride is the “stomach in your throat” feeling as you go over a crest, or being thrown to one side as you take a corner at speed. Unlike speed, acceleration can be measured locally, so it can’t be simulated with a video and a shaky chair.

How to make them fully convincing

There is a way to simulate acceleration. Einstein’s equivalence principle roughly states that freely falling is locally indistinguishable from zero gravity. We can illustrate this point with a thought experiment. Suppose you wake up in an elevator which is freely falling (i.e. ignore wind resistance etc). There is nothing you can do inside the elevator to determine whether you are freely falling, or whether someone has turned off gravity2. If you want to know what it would be like if there were no gravity, then go jump off a cliff (in your mind, of course). (more…)


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I recently posted on Arxiv a paper titled “The Fine-Tuning of the Universe for Intelligent Life”. A slightly shortened version has been accepted for publication in Publications of the Astronomical Society of Australia. The paper is primarily a review of the scientific literature, but uses as a foil Victor Stenger’s recent book “The Fallacy of Fine-Tuning: Why the Universe Is Not Designed for Us” (FoFT). Stenger has since replied to my criticisms. The following is my reply to his reply to my article criticising his book which criticises fine-tuning. Everybody got that?

A few points before I get into details:

  • There isn’t much in this post that wasn’t in my original article. I write this to summarise the important bits.
  • “Barnes does not challenge my basic conclusions.” Not even close. Re-read.
  • “Barnes seems to want me to reduce this to maybe 1-5 percent.” Nope. I didn’t say or imply such a figure anywhere in my article. On the contrary, the cosmological constant alone gives 10^{-120}. The Higgs vev is fine-tuned to 10^{-17}. The triple alpha process plausibly puts constraints of order 10^{-5} on the fine-structure constant. The “famous fine-tuning problem” of inflation is 10^{-11} (Turok, 2002). The fine-tuning implied by entropy is 1 in 10^{10^{123}} according to Penrose. For more examples, see my article. Or just pull a number out of nowhere and attribute it to me.
  • “He fails to explain why my simplifications are inadequate for my purposes.” Red herring. My issue is not oversimplification. I do not criticise the level of sophistication of Stenger’s arguments (with one exception – see my discussion of entropy in cosmology below). Stenger’s arguments do not fail for a lack of technical precision. Neither does the technical level of my arguments render them “irrelevant”.

Point of View Invariance (PoVI)

A major claim of my response (Section 4.1) to FoFT is that Stenger equivocates on the terms symmetry and PoVI. They are not synonymous. For example, in Lagrangian dynamics, PoVI is a feature of the entire Lagrangian formalism and holds for any Lagrangian and any (sufficiently smooth) coordinate transformation. A symmetry is a property of a particular Lagrangian, and is associated with a particular (family of) coordinate transformation. All Lagrangians are POVI, but only certain, special Lagrangians – and thus only certain, special physical systems – are symmetric. Stenger replies:

“PoVI is a necessary principle, but it does not by itself determine all the laws of physics. There are choices of what transformations are considered and any models developed must be tested against the data. However, it is well established, and certainly not my creation, that conservation principles and much more follow from symmetry principles.”

Note how a discussion of PoVI segues into a discussion of symmetry with no attempt to justify treating the two as synonymous, or giving an argument for why one follows from the other.

Of course conservation principles follow from symmetry principles – that’s Noether’s theorem. It’s perfectly true that “if [physicists] are to maintain the notion that there is no special point in space, then they can’t suggest a model that violates momentum conservation”. The issue is not the truth of the conditional, but the necessary truth of the antecedent. Physicists are not free to propose a model which is time-translation invariant and fails to conserve energy1. But we are free to propose a model that isn’t time-translation invariant without fear of subjectivity.

And we have! Stenger says: “But no physicist is going to propose a model that depends on his location and his point of view.” This is precisely what cosmologists have been doing since 1922. The Lagrangian that best describes the observable universe as a whole is not time-translation invariant. It’s right there in the Robertson-Walker metric: a(t). The predictions of the model depend on the time at which the universe is observed, and thus the universe does not conserve energy. Neither does it wallow in subjectivity.

Watch closely as Stenger gives the whole game away: (more…)

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