Richard Johns's Blog

Is it Crazy to Ride without a Helmet?

2011-08-17T21:36:00.000-07:00

IN a provincial court last Friday morning, Vancouver resident Ron van der Eerden was defending his right to ride a bike without a helmet. Many people have concluded that Mr. van der Eerden is a loon. Yet a surprising fact is that, in a recent poll of British doctors, conducted by the British Medical Association, 68% agreed with him that helmets should not be compulsory for cyclists.

What can account for such craziness -- among doctors of all people?

To begin to find an answer, let’s ask a simple question. Compare an average person who cycles for transport, riding about 300 hours per year, with a typical motorist, also driving about 300 hours per year. Who is at greater risk of injury and death?

Answer: the motorist. This may be surprising, but it is a fact. While certain risks are very slightly higher for the cyclist, namely the risks from road accidents and air pollution, these increases are massively outweighed by lower risks of cardiovascular disease, cancer and diabetes, among other risks. A study published just this week in the British Medical Journal found that the risk-reductions of cycling (in Barcelona) compared with driving outweigh the risk increases by a factor of 77. Other studies have estimated the cost-benefit ratio at 20, or as little as 7, but no study has found that driving is safer. Cycling is far safer than driving.

This is something to bear in mind when thinking that someone is crazy to ride a bike without a helmet. If you’re an average Canadian, then that cyclist’s risk of death and injury is far lower than yours. Who is the crazy one here?

These studies take no account of whether a bicycle helmet is worn or not, since it makes very little difference. To see why, we can describe the situation with some rough, but reasonably accurate, numbers. Let’s compare two people, Chris and Mike. Chris cycles for an hour, while Mike drives his car for an hour. As a result, Chris’s life expectancy is reduced by about 5 minutes, due to the risk of death in a road accident. Mike’s life expectancy is also reduced by about 5 minutes, for the same reason. However, exercise is known to increase life expectancy, so Chris gains about 100 minutes in life expectancy from that. Overall, Chris gains 95 minutes, and Mike loses 5 minutes. Now, what about the helmet? A head injury is the sole cause of death in about 30% of cyclist deaths, and in the recent scientific literature helmets are estimated to prevent about 10-30% of such injuries. Hence wearing a bike helmet might reduce Chris’s chance of death by 3-10%, corresponding to an increase in life expectancy (per hour of cycling) of 9-30 seconds. In comparison with the general gain of 95 minutes, from cycling itself, the further gain of a few seconds from wearing a helmet is trivial.

Looking at these figures, it is little wonder that helmet laws, wherever they have been introduced, have had no noticeable impact on the rates of cyclist injury and death. And that the safest countries to cycle in are those where almost no one wears a helmet. Helmets are a non-issue for cycling safety.

There is also something perverse about fining someone for taking a personal risk, when the majority who take much larger risks are not fined. Given the clear fact that cycling (helmeted or otherwise) is far safer than driving a car, why are cyclists fined? As shown above, leaving one’s helmet at home costs a cyclist a few seconds of life expectancy, per hour in the saddle. If that same cyclist chose instead to drive a car for that hour, he or she would miss out on some 95 minutes of added life expectancy. The law guides us toward the more dangerous action. We should also remember the “second-hand risk” in driving a motor vehicle, i.e. the risk imposed on third parties. Driving for an hour costs other people a total of about 5 minutes of life expectancy as well. The law apparently prefers us to put others at risk rather than exposing ourselves to a much smaller risk.

Now, one might argue of course that even a small gain is a gain, and so wearing a helmet still a smart thing to do. At the same time, however, it’s not something to get worked up over. And it certainly isn’t a matter for legislation.

Is Safety Now a Religion?

2011-06-20T22:06:00.000-07:00

For many years I've been bugged by the constant appeals to "safety" in our society. It seems that, if a policy has nothing at all to recommend it, on rational grounds, then it's invariably supported by an appeal to safety. The very word has a kind of magical power, and is almost impossible to resist. "I'm sorry, but it's a safety issue." Who can argue against that?

More recently I've started to wonder about the sources of this practice, and begun to suspect that it has its roots in our post-religious culture. It is, I am starting to believe, the result of a religious impulse, the need for salvation.

The world is a fairly dangerous and unpredictable place. Of course life in Canada is relatively predictable, compared with most other places, and the dangers are relatively few, but it remains true that any of us could be killed, or be permanently crippled, tomorrow. This is scary. And I'm not sure that the fact that these risks are low in Canada helps reduce the fear that much. (I get the impression that we learn to tolerate the level of risk that we're exposed to.)

So what do we do with that fear? Our Christian ancestors had one strategy, and now we have another. The Christian approach (shared by other religions) is to acknowledge that there is no safety in this world, which is arbitrary and chaotic. Yet we have nothing to fear, since our safety is with God, who guards our life and will bring us into his kingdom. "And as surely, sinner, as thou canst put thy trust in God, thou art safe" (Charles Spurgeon, "Salvation and Safety", p. 4) It's this understanding of safety that comforted early Christians as they waited to be eaten by lions, for example.

The present approach is quite different, of course. With no God, our safety cannot reside in another world. We must be safe in this world. And here we see the source of the irrationality in the present safety religion. For the world just isn't safe. The Christians are right on this point: the world is arbitrary, chaotic and unpredictable. Our psychological need for a safe world just doesn't make it so.

I realise that, these days, saying that the world isn't safe (and cannot be made safe) is some kind of blasphemy. Our modern scripture says that there are no accidents, but only "preventable injuries", with the obvious consequence that injury-free living is a real possibility. (We just need more regulations, more inspectors, more safety gadgets ...) By denying the possibility of safety I am saying, to our culture, that there is no heaven.

For some time now I've been following the debate about bike helmets, looking at the arguments for and against. This case provides a very good illustration of the safety religion in Canada (and no doubt elsewhere). The arguments against wearing bike helmets, and especially against legislating their use, are simple and rational. The absolute risk of injury and death from cycling is very low. And wearing a helmet doesn't make much difference to that risk, if any. These arguments would appear to be irrefutable, yet the majority reject them. How do they respond? Judging from various internet discussion forums, that I've been reading, the typical response is emotional, fear-driven, and grounded in the need to be safe.

The human skull is only a quarter of an inch thick! Imagine that smashing into the hard asphalt, or the hood of a truck. You might think you don't need a helmet to ride to the corner store, but that could be the very time disaster strikes. Think about all those dead cyclists, and drooling scores of others with permanent brain damage. Strap on a helmet and be safe! (It sounds eerily like Spurgeon. "There is a hell for the wicked, but none for the righteous.")

I reject all false gods, including the modern idol of safety. I refuse to worship it. I prefer to live in the real world, and worship the real gods of reason, truth and love.

[Caveat: I'm not necessarily against all measures that are justified on grounds of safety. Some can be justified on rational grounds. I don't, for example, eat raw hamburger bought from a supermarket! I am trying to foster the virtue of prudence, in myself and my children. I object only to the worship of safety, and the irrationality that goes with it.]

New Cycle Analyst

2010-06-11T21:34:00.000-07:00

Having received a free Cycle Analyst from ebikes.ca, as part of their twice-a-month giveaway, I was inspired to put in a proper dashboard as well. A piece of walnut veneer plywood looks a lot nicer than white coroplast!

I like the large screen of the new unit, which is much easier to read. It also has a lot of extra features, being version 2.2, such as enabling the throttle to control the motor current rather than the voltage.

New Bike Details

2010-05-23T18:07:00.001-07:00

The basic features of my new bike were described in a previous post. In this post I'll show some close-up shots, and explain some of the technical details.

This first shot shows the dashboard. Right now it's just a mockup made from twinwall plastic. (I have plans for a nice piece of plywood.) The computer is the Cycle Analyst 2.0 from ebikes.ca. Their new large-screen model would work better here. The switches control two front lights.

The second shot shows the front panniers, with the child seat base between them, and the power switch for the motor and lights behind that. The child seat itself fits into one pannier, as you see.

Now we see the child seat installed, held in place by a single bolt into the base. The seat is by Wee Ride, but most of it has been chopped off. The foot rests were very small, for example, whereas the panniers allow even a six year old to ride comfortably, in something like a kneeling position.

The seat base also houses most of the electrical connections, including the Cycle Analyst shunt, as can be seen when the lid is flipped up.

Here you see the motor gearhead, mounted inside the frame triangle. The motor is available from Cyclone, with the gearhead attached. This is the 24V, 360W version, and weighs about 3kg altogether. The controller is inside the motor case.

The planetary gearhead provides about 9.5 : 1 speed reduction, and the chain provides another 2.3 or so (32/14), for a total of about 22 : 1. This is using the middle (32 tooth) chainring. I can also use the large (42 tooth) chainring, for a total of 29 : 1, so that the motor runs at higher rpm. This means less current, and more range, but less power and a higher-pitched sound from the gearbox.

The motor chain tension is adjusted simply by sliding the motor up and down the mounting.

Here the motor itself is visible on the right of the gearhead. Rather conveniently, all the places where water can get into the motor are between the mounting plates, basically inside the frame. This is a nice feature, since the motor itself isn't supposed to be used in the rain. In this installation, however, it's fully protected. (I recently found out that the duct tape above the motor is needed for this.)

You see how the wire entry point is protected.

The battery is the black thing at the top of the rear panniers. This one is a LiFePO4 made by Li Ping of Shanghai. The specs were said to be 24V 20Ah, but actually they measure a little better than this on the Cycle Analyst. Ping had to put the cells together in a different way in order for the battery to fit into the space. The price was also very reasonable, so (to this point, only 1300 km so far) I'm very pleased with it. (The weight is about 5kg.)

Finally we see, from above, the attempt to make the whole bike aerodynamic. The idea was to approximate the familiar "tear drop" shape of aircraft wings and so on. I can't say I've noticed much drag reduction though! I wonder if joining the front and rear panniers together, with side panels of some sort, would make much difference?

Latest Bike

2010-05-16T10:03:00.000-07:00

This is a continuation of my "Bike Project" post.

About two years ago my cheap Raleigh frame reached the end of its life, as the pivot bushing for the rear swing arm wore out, and couldn't be replaced.

So I searched for a new bike, looking for another full suspension frame as this is needed when carrying hard-shell panniers at high speeds on rough city streets. I also needed a frame that fitted my existing panniers, and with replaceable suspension bearings. The Norco Fluid met all these requirements. (See below.)

A new bike meant an opportunity to try new ideas. Some problems with the Raleigh were:

1. The low placement of the motor exposed it to water and mud.

2. The 8 speeds were fine for city commuting, but hauling a heavy trailer around the Gulf Islands required additional low gears.

3. The WeeRide front child seat mounted to an ugly, heavy steel bar clamped between the headset and seat post. It had to go.

These three problems were solved by a single new structure, formed by a pair of aluminium plates clamped around the frame by about ten small bolts. (See below.)

1. Holes were cut to mount the motor to one plate, in the middle of the frame triangle, a location that protects it from the elements. All the potential entry points for water are actually between the plates.

2. The motor was turned around, putting its drive sprocket on the left instead of the right (the rotation reversed). The left crank was replaced with a crankset, so that the motor chain could still drive the cranks as before. But the existing triple crankset on the right was left intact, so that the standard 27 gears remained.

3. At the top, these plates were joined by a sturdy piece of aluminium U-channel to form a base that the WeeRide attaches to by its usual bolt.

Detail of how the carrier struts attach to the frame.

The front panniers then re-attached easily to the aluminium plates, and the fairing went back onto the panniers. I made fibre glass rear panniers to match the front as well.

And that's basically it (for now). More technical details to follow ...

Bike Project

2010-05-15T21:32:00.000-07:00

I've been meaning to document some of my ideas and designs concerning bicycle transportation, and now seems like a good time.

I've commuted by bicycle since I was eleven years old, when I passed my Cycling Proficiency Test and received a permit to bike to secondary school. Until around 2000 I rode classic 12-speed sports bikes with drop handlebars, but then I started to experiment.

After my frame broke for the second time, I thought I'd try a mountain bike, but with road tires and drops. It worked ok. (See picture.)

For bringing things to and from work I attached some steel boxes, intended for hanging file folders. They locked, and were totally waterproof. They were a bit noisy though, going over bumps in the road, especially when carrying metal objects. I wondered if full suspension might be a good idea.

At about the same time I started to experiment with rain ponchos. The idea I had was for one that was as small and tight as possible (no big flapping thing). More like a tent than than poncho, hence the provisional name "bike tent". It attached to the frame of the bike, by means of a bar that projected forward from the headset. The hem of the poncho was stiffened by a rather elaborate contraption consisting of a chain threaded through short pieces of plastic tubing. (When the chain was put under tension, the hem became stiff.) It worked well, although I still needed to wear waterproof boots and gaiters.

I thought about putting panniers at the front, the idea being that they would block some of the rain hitting my shins, as well as improving the bike handling. Here's a picture of the styrofoam plug I made, intending to make the part from fibre glass. The plug was shaved down to the point where I could ride with hitting my knees on it!

Unfortunately the bike itself was stolen at this point. Time for a new bike, and chance to experiment with full suspension.

Here are the front panniers, and I stuck a bit of a fairing on the front of them, a bit like a motorbike. I thought it might look nice, as well as perhaps making it more aerodynamic. You'll also see the aluminium tubes added to strengthen the rear carrier.

It looked better with a lid, and a paint job.

Now, at about this time my wife and I had our first child, and I wanted to bring him with me. Throwing him in the cargo box made a good picture, but wasn't practical for the roads.

But products are available for this sort of thing, fortunately. One doesn't have to invent everything!

So the bike was developing nicely into a pretty useful machine for getting to work, carrying groceries, children, etc. But for a while I'd thought about adding an electric motor to the bike, to see how that would be.

One of the ideas driving these experiments is that cycling has a lot to offer developed countries, as well as developing ones, in terms of reduced congestion, improved health and fitness, reduced cost, less pollution, etc. Of course cycling is rare in Canada, due to numerous barriers, some of which can be addressed by changes to the bike itself. The main barriers are:

1. It's perceived as dangerous.
2. It's too strenuous (especially going uphill).
3. The range is too limited.
4. You get wet, or otherwise have to put on a full-body rain suit.
5. You can't bring enough stuff with you.

#1 is of course a misperception. The best measure of the total risk of an activity is its effect on life expectancy, and cycling increases life expectancy (as well as general health). We just have to get the word out.

#2 is addressed by adding an electric motor. The motor's output is roughly that of a fit human, about 1/2 horsepower, which makes a big difference.

#3 is also addressed by the motor. It allows higher speeds, at the same or lower level of effort by the rider, so that longer commutes become feasible.

#4 is addressed by the bike tent (see above and below). It's quicker and easier to put on than a rain jacket, pants and overmitts, and it's much better ventillated, so you don't get sweaty. (Especially with a motor as well.) You can ride to work in your work clothes, even in heavy rain, and arrive perfectly dry.

#5 is addressed by stay-on-the-bike, waterproof storage panniers, as well as by trailers (see below).

As you see, I chose a mid-drive system, where the motor is mounted to the frame and drives the crankset. The engineer in me liked the idea of the motor driving the wheel through multiple gear ratios, so that the motor is always close to its peak power and efficiency. (This is the Cyclone motor, by the way.) This first setup had 8 speeds, from the cassette on the back wheel. I figured I didn't need any low gears, with the extra 360 Watts on board.

However, once you have some extra power, it's tempting to increase the loading. A second child appeared, and sometimes they want to bring friends along! My flat bed trailer, intended for cargo, sprouted a back rest for children. (I also continued to experiment with the "bike tent" concept, now adding what look like umbrella tines.)

The next post should take us up to the present ...

Building Codes and "Safety"

2010-04-09T22:00:00.000-07:00

I just spent a couple of hours trying to figure out why I can't live in my own basement. Of course I know the reason, that the ceiling height of 6'6" (less in some spots) doesn't meet the minimum for the building code, or planning bylaw, or some such thing.

But I wanted to know the reason for that reason. Why is this the minimum height, rather than some other value? And why is there a minimum ceiling height at all?

I had heard that it was something to do with fire safety. So first I connected to the UBC library VPN, giving me access to the world's knowledge stored in electronic academic journals. Then I searched for factors influencing fire safety, especially relating to ceiling height. What did I find? Nothing. Zip. Zilch. Nada. Plenty about fuel loads, fire exits, fire barriers, sprinklers, etc. etc. Nothing about ceiling height.

So, turning my back on the academic world, I did general internet searches, visiting discussion boards, planning documents, etc. Finally I found THE ANSWER.

I found a 2006 report by the Acting Commissioner, Department of Development Services, City of Oshawa, Ontario, Canada. (It can be read here.)

Apparently the Property Standards Committee had requested that proposed that the minimum ceiling height be reduced in existing residences, presumably to increase the supply of cheap housing. How dare they!!!

I couldn't wait to read the response from the Acting Commissioner, a Mr Hodgins, MA. Why can't I live in my own basement? Here are the arguments:

1. The existing minimum height matches the Ontario Building Code.

Yes, yes, I know that! One code justifies another. No doubt the Ontario code cotes the BC code, which cites the Quebec code, which cites ...

2. "It promotes a reasonable level of comfort, convenience and dignity for occupants..."

Whaaa..? Do we now need laws to ensure that people look out for their personal comfort, convenience and dignity? Will there be laws forbidding tight shoes, or itchy sweaters? This would be pretty hilarious, were it not for the fact that my family of 4 is crammed into 600 square feet while a 900 square foot basement lies empty beneath our feet. Where's the "comfort, convenience and dignity" in that? Those values would be enabled, rather than hindered, by a spare bedroom, an office, a play room, a home movie theatre with a 60 inch plasma screen and surround sound (oops, daydreaming!)

Better get to the 3rd and final reason:

3. "It advances safety and safe access for emergency services personnel who need sufficient height to undertake their work"

Aha! Now, finally we have it. No one could question the appeal to safety, now could they! The argumentum ad salvum (taking a guess at what the latin might be). Here's the argument structure:

1. Option X would be cheaper, more convenient, etc. etc.
2. But option Y would be safer
-------------
Hence, we must do Y

Apparently I can't live in my basement because of the remote possibility that a copper might visit me while I'm down there, and have to stoop a little! (You know how tall those fellas are!)

Does anybody do any cost-benefit analysis of this sort of thing? Apparently not. Since I and my family are here 7 days a week, year round, and we haven't had a single visit from the emergency services since ... well ... our second son was born 4 years ago, such an analysis is unlikely to weight the convenience of those workers higher than ours.

Why should self-organisation be limited?

2009-07-21T15:32:00.000-07:00

My main research project of the past decade has been an investigation of physical constraints on self organisation. This sounds like a dry topic, I know, but it is rich with practical applications as well as being of great philosophical interest. My latest paper on the subject may be downloaded here.

Certain kinds of object, even rather large, intricate ones, will appear spontaneously under fairly general conditions. Crystals are the most obvious example of this. A sodium chloride crystal, for example, is a very specific arrangement of sodium and chlorine atoms. The chance of a large crystal forming by chance is too remote to consider. If, for example, you were to arrange black counters and white counters randomly on a large square grid, you would not produce the perfect pattern of alternation, like a chess board, that we see in sodium chloride. The appearance of sodium chloride is therefore a case of self-organisation.

Probably the most impressive examples of self organisation are fractals. The Mandelbrot set, for example, is enormously intricate, one might even say infinitely so, yet it is generated by a rather simple rule. Also, while the Mandelbrot set is a mathematical abstraction rather than a natural object, there are apparently natural fractals.

These examples, while impressive, point to a possible limitation of self organisation, however. For crystals and fractals share the feature of being highly self similar. If you move a crystal over by one atom, it maps exactly onto itself. It is "invariant", as they say, "under spatial translation". Fractals don't have this kind of self similarity. But they are self-similar, or invariant, under changes of scale. As you progressively "zoom in" on the Mandelbrot set, for example, you just see more and more of the same. So, despite being very intricate, crystals and fractals are kinda boring. This raises the following questions:

Do self-organised objects have to be self similar?
If they do, then why is this?

These are hard questions that I have long studied, and can now finally answer with some confidence. The answers are:

Yes
It results from symmetries in the underlying dynamics, specifically causal locality and invariance under spatial translation.

If these answers are indeed correct, then there are some important consequences for biology, concerning theories of the origin of life. For living organisms are quite unlike crystals and fractals in this respect. While there is a lot of self similarity in animals and plants, there is also a lot of differentiation. It appears then that living organisms cannot be products of self organisation, as is usually supposed.

I'm not going to repeat the whole argument here, as it can be found in my paper. But I will reflect on the argument, from three different perspectives. The argument can be seen supporting a limitative principle, analogous to Gödel's incompleteness theorem, the Löwenheim-Skolem theorem, or Bell's theorem. The argument is a fairly typical symmetry argument. The argument also stands in a tradition of arguing that a cause can produce an effect only if the effect is somehow "present" in the cause.

I. Limitative Results: Gödel, Bell and Jeremiah

There is a fairly honourable tradition of limitative, or "no go" theorems in science. One of Gödel's theorems, for example, entails that there is no complete system of rules of inference in second-order logic, or indeed in any formal language in which a categorical set of axioms of arithmetic exists. This result is disappointing to many, since it shows that rational inference cannot be "captured", as it were, in rules that manipulate symbols. (Whether human inference can be so captured is another matter. I like to say that Gödel's theorem shows that God isn't a machine.)

Another delicious result is Bell's theorem, concerning quantum theory, which says that no local, hidden-variable theory can be empirically equivalent to standard quantum mechanics.

Each of these theorems, like all such theorems, is negative. It says that, while you might keep trying to do a certain thing, it will never work. You might say they are logical Jeremiahs, or prophets of doom. This would be a little unfair to Jeremiah, however, since even that much-maligned prophet did urge an alternative to defeat and exile. These results, by contrast, make no positive suggestion at all. Bell's theorem gives no clue about how to interpret quantum mechanics, and Gödel's theorem doesn't suggest any better understanding of rational inference.

My theorem is also pure doom and gloom. Let's be honest: It offers no positive suggestion at all.

Can such negative claims be part of science? It is often said that a scientist must propose hypotheses that are empirically testable. That's not really true, however. While that's a big part of science, a lot of good scientific work is indeed negative. Much useful work is done by experimentalists who show that, while hypothesis H might predict empirical result E, E doesn't actually occur. Also, while most theorists are busily showing that H predicts E, other theorists very helpfully point out that H doesn't really predict E at all, even though we thought it did. A really negative scientist might even show that no hypothesis of a certain type will ever predict E.

I'm afraid I'm one of those really negative scientists. I've shown that no hypothesis in a very broad class predicts the existence of complex living organisms. More precisely, life cannot self organise in any dynamical system whose laws are local and invariant under spatial translation.

At this point a worrying possibility emerges. This no-go theorem is so broad that it rules out just about any naturalistic theory of the origin of life! It certainly seems to rule out all the naturalistic theories presently proposed. Yet, the whole business of science is to provide natural explanations for phenomena, so this result is unscientific after all. (Even if it is technically correct, take note.)

Well, this is an awkward business! What are we to do?

1. It is surely irrational to disregard results that attack the viability of one's project, simply on the grounds that they attack the viability of one's project!

2. While I regard my limitative result as scientific, I myself am a philosopher (of science) by trade, so I don't mind too much if it's considered philosophy. But it is science really.

3. This kind of thing has happened before. Maxwell found that his equations for the electric and magnetic fields were incompatible with the mechanical philosophy of his day. In those days, to give a scientific explanation of something was to give a mechanical model for it. Yet Maxwell, despite much effort, was unable to understand his equations as describing displacement fields in any kind of solid. He had, in effect, a limitative theorem that no such model was possible. Yet science didn't end in the late 19th century! Instead, they realised that non-mechanical explanations are better than none. And if the world isn't a giant machine, then those are going to be the true explanations.

4. So I think we have to broaden our horizons, and be open to new kinds of explanation. Perhaps it won't be that bad? And we have no other choice, if we want our explanations to be true.

[A mischievous aside. I like to say that Maxwell and Einstein refuted the mechanical philosophy over a century ago, but no one told the biologists! Now, one might reasonably object to this, saying that a model that's been shown to be inadequate in one context might work perfectly well elsewhere. I see. So mechanical models are too clumsy to handle the subtleties of the electromagnetic field, but are adequate for such simple phenomena as life, consciousness and rational thought?]

II. Symmetry Arguments

Symmetry arguments are common in logic, mathematics and physics. In logic, it is generally held that logical consequence preserves symmetry, so that if the premises of an argument (considered together) are symmetric in some respect, then the conclusion must also be symmetric in that respect. I will take this for granted. For example, suppose you're investigating a murder, and you have some evidence that Fred did it. On the other hand, you have exactly the same sort of evidence that Mike did it. (Perhaps you know that the murderer is tall, left-handed and male, and both Fred and Mike have these attributes. They have equal motives, and so on.) In this epistemic situation, you cannot draw a conclusion about Fred doing the murder without drawing a similar conclusion about Mike. In other words, you should assign the same probability to each.

In some physical experiments, the initial situation is symmetric in some respect. For example, the state might have reflective symmetry about some plane, so that the state maps to itself under reflection in that plane. What should we believe about the outcome of this experiment, given this initial symmetry?

Two answers have been considered here:

The outcome will have reflective symmetry about that plane.
The probability function over the outcomes will have reflective symmetry about that plane.

Which of these answers you should accept depends on whether or not you are a determinist. I shall first prove (2), and then derive (1) from (2) by adding the assumption of determinism.

Nothing interesting can be proved without premises, of course. What premise(s) will allow me to prove (2)? The first premise is the one above, that logical consequence preserves symmetry. The second premise is what philosophers call the Principal Principle, as follows:

If the chance of a future event E is known, then the epistemic probability of E is equal to it.

So, for example, if you know that a coin is biased in such a way that its chance of heads is 0.331 on each toss, then the subjective utility of a gamble that pays $1 on heads (and nothing on tails) is 33.1 cents. The Principal Principle, called PP for short, is almost universally accepted.

The third premise is no less certain. It says that the chance of an outcome in an experiment depends only on the possible causes of that outcome, i.e. on the causally relevant physical conditions. In other words, if you repeat an experiment exactly, perfectly duplicating all the relevant physical circumstances, then the same chances obtain for the outcomes. This doesn't mean that the outcomes will be the same, except in the deterministic case. For example, suppose you do some experiment with the Stern-Gerlach apparatus, and the chance of the particle going UP is q. Then, in every exact repetition of this experiment, the chance of UP will be q as well.

Let's refer to this third premise as the claim that chances are physical.

With these three premises in place, we now consider some ideal physicist who has maximal knowledge of the initial state of some experiment, together with maximal knowledge of all the relevant dynamical laws. Since chances are physical, it follows that this physicist will know the chances of all possible outcomes of the experiment. Now we assume that the initial conditions are symmetric in some way, and consider what this physicist can infer about the outcome of the experiment. Any such conclusion can only be inferred from what the physicist knows, i.e. from his knowledge of the initial conditions. Now, since logical consequence preserves symmetry, any symmetric outcomes A and B must have equal epistemic probability for this physicist. Then, using the Principal Principle, the outcomes A and B must have equal chances as well. (If they did not, then PP entails that their epistemic probabilities would be different.)

This concludes our proof of (2). Now suppose that the system is deterministic, so that the chance of each outcome is either 0 or 1. We will argue by reductio ad absurdum. Suppose that the initial state of the experiment has symmetry with respect to transformation R, so that the initial state maps to itself under R, but the outcome of the experiment does not. In other words, if the actual outcome is E, then R(E) ≠ E. Now, assuming determinism, whatever occurs has chance 1, and whatever doesn't occur has chance 0. R(E) doesn't occur, and so has chance 0. E occurs, and so has chance 1. But, using (2), symmetric outcomes have equal chance. This is a contradiction. Thus the result is proven, i.e. the outcome must be R-symmetric as well in the deterministic case.

My proofs of (1) and (2) show that symmetry arguments are valid in physics as well as in logic and mathematics.

My limitative theorem is established, in part, by just such a symmetry argument. I assume that the dynamical laws operate locally, and are invariant under arbitrary spatial translation. These are rather strong symmetries to impose, but they are well accepted by physicists. I assume a random initial state, so that every possible initial state has the same chance. Such an initial chance distribution has just about every kind of symmetry you might name.

To be continued ...

Column resistance -- some thoughts on Chandler, Szamboti and Bazant

2009-06-30T15:42:00.001-07:00

There has been some recent discussion on 911Blogger about one of David Chandler's videos on his YouTube channel.

http://www.youtube.com/watch?v=xG2y50Wyys4

I have some points to make here.

1. Chandler's argument, while correct as far as it goes, establishes only an upper bound on the column resistance. A better empirical estimate is roughly 0 - 0.1mg. This can be seen as follows.

The video measures the downward acceleration of the roofline of WTC1, during the first few seconds of collapse, and measures a downward acceleration of around 0.64g. Chandler argues, correctly, that this implies that the block is resisted by an upward force of (1-0.64)mg = 0.36mg, where m is the mass of the block. Yet this is puzzling, since the columns were (just a moment before) supporting the full weight (mg) of this block. (Moreover, assuming a safety factor of 3, they the columns were capable of supporting at least 3mg, almost 10 times their measured resistance.)

Chandler's video is, in my opinion, a useful device to attract attention to this important puzzle. However, while the reasoning is correct as far as it goes, the true situation is actually far more disturbing than even his argument suggests. (I assume that he is aware of this, but wants to keep the argument as simple as possible.) Let me explain why.

Chandler's argument is usefully seen as being based on an idealised model of the tower, where the floors below the airplane impacts have zero mass, and hence no inertia. Also, in his model there is no energy consumed in the pulverisation of the concrete to a fine powder. The only force resisting the descent of the upper floors (above the plane impacts) therefore is due to the steel columns. In reality, of course, the lower floors have inertia, and concrete requires force to crush it. Hence Chandler's quick argument greatly over-estimates the column resistance, and so should be regarded as an upper bound rather than an estimate. The true situation is even more worrying than his argument suggests. But what really was the column resistance?

The best way to measure this from the video evidence, I think, is to use a different idealised model, based on what Kenneth Kuttler has called a "floating floor" model. In this model, the floors have inertia, which resists the acceleration of the falling block. One also includes an energy drain for the crushing of concrete, as well as a variable drain for the deformation of the columns. (For convenience, it's best to calculate using energies rather than forces, although of course the work done by a given force is simply the integral of that force with respect to displacement.) In such a model one easily calculates the downward acceleration of the top block, for a given value of the column resistance. One then adjusts that resistance value to bring the model's behaviour into line with reality, as seen in the videos. This is then a reasonably accurate empirical estimate of the mean column resistance.

What value is obtained by such a procedure? I haven't yet finished my analysis, but it is certainly very low -- much less than Chandler's upper bound of 0.36mg. I believe it is in the range 0 to 0.1mg -- somewhere around that.

2. The papers by Zdenek Bazant on the WTC collapses provide an alleged theoretical estimate that roughly agrees with this empirical estimate. Bazant compares the potential energy released as the "top block" descends one story with the energy absorbed by the plastic deformation of the columns over the same distance. He concludes (without, unfortunately, showing much of his calculation) that the energy released is at least 8.4 times the energy absorbed, so that the collapse will inevitably progress. In other words, the columns absorb about 12%, at most, of the energy released. Now, this energy fraction is easily converted to a force, using the fact that the work done to the columns must equal the integral of the force applied to them, with respect to the distance fallen. In other words, the average resistance of the columns is at most 12% of the weight of the block. This is at the upper end of what is allowed by the empirical data.

3. We see therefore that Chandler's argument will not disturb Bazant in any way, who has already accepted a much lower value for the column resistance. On the other hand, if Chandler's empirical upper bound for the column resistance is worryingly low, then Bazant's theoretical estimate is far more so! One may even wonder whether Bazant's estimate is genuine, given that he doesn't explain how he obtained it.

These darker suspicions are supported by the fact that Bazant's figure lies at the upper end of what is empirically possible. If one were to invent a figure, say to reassure the engineering community, then this would be the perfect value to choose. One would choose the highest, i.e. most plausible, value that was consistent with the data.

4. How is a non-specialist like me to evaluate the reasonableness of Bazant's upper bound? One obvious way is to compare it with the estimates of other experts. I am aware of two of these, one due to Gregory Szuladzinski in Journal of Engineering Mechanics and the other from Tony Szamboti in "The Missing Jolt", Journal of 9/11 Studies.

The estimates are similar in approach. They calculate the energy absorbed from the elastic compression, then plastic shortening, then plastic buckling of the columns. The resistive force of a column drops sharply after it begins to buckle, but it seems to remain above about 25% of the yield stress until the thing breaks ("fractures").

The outcomes of the two calculations are also similar, giving average resistances above mg, certainly far in excess of Bazant's alleged upper bound of 0.12mg. Szuladzinski and Szamboti both predict collapse arrest.

5. In JEM, Bazant replies to Szuladzinski, finding some ten or so errors in his calculation. (Although there seems to be some padding here, as correcting some of these "errors" would increase, rather than decrease, the resistance.) It is hard for me, as a non-specialist, to judge the validity of these objections, although I suspect that they are minor quibbles. It is telling, I think, that Bazant does not correct the calculation, showing what it ought to be. This indicates to me that the outcome would not be too different.

To avoid such trifling objections, if that's what they are, it would be useful to calculate a robust lower bound for the column resistance. Can we have that, please? By "robust" I mean that all reasonable allowances for lowering the column resistance have already been made, so that it is the lowest possible value. It is quibble-proof, so to speak.

I repeat (to all relevant experts): Can we have that, please?