Monthly Archives: December 2010

How does the damage caused by exposure to radiation vary as the dose of radiation increases?

Most people assume that if you double the amount of radiation you double the damage caused, and that there is no threshold below which no damage is done. This is called the Linear No Threshold (LNT) model and is represented by the graph below:

The LNT model has been the subject of some disagreement in recent years. The American Nuclear Society said in a 2001 Position Statement* that:

“There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent.”

This linear threshold model holds that there is a limit below which no damage is caused, but that damage then increases linearly beyond that limit.

There are other possible models. Damage may increase in an exponential way, with very low damage at low doses but increasing amounts of damage at higher doses.

In a logarithmic model the damage would be very large at first, but taper off as the dose increases.

So which model is correct?

The LNT model remains the most commonly used by regulatory bodies, but there is growing interest in threshold models and in the idea of radiation hormesis, the idea that a small dose of radiation is actually good for the body by somehow stimulating the body’s repair systems. I think the most likely candidate is a J-shaped curve with a significant threshold but then a fairly rapid linear increase in damage caused.

* American Nuclear Society, Health Effects of Low-Level Radiation, Position Statement 2001.

n-grams and the most popular science

I’ve heard it said that:

The 19th Century was the century of chemistry,
The 20th Century was the century of physics,
The 21st Century will be the century of biology.

And that’s probably accurate*.

Google’s Labs division has released a new product, the Books Ngram Viewer (an n-gram is a string of characters where n represents the number of characters). Google’s n-gram viewer is based on the enormous corpus created by the Google Books program that has digitised more than fifteen million books.

It’s quite interesting to look at the appearances of the sciences in the record; it seems to confirm the aphorism above.

“Natural philosophy” dominates until the 1850s when “physics” becomes the more common term. “Chemistry” clearly dominates over “physics” until the 1920s at which point there are a number of exchanges until the late 1950s after which point “physics” is the dominant term. You can also see “biology” overtaking “chemistry” in the 1990s.

* This is only because physicists have switched their attentions to areas of biology like bioinformatics and computational biology.

Your eyes are not good at seeing blue

Here is a image of the Bangkok skyline.

Here is the same image with the red, green and blue components shown separately.

Notice how white objects are bright in all three components, how the green park on the right of the image appears brightest in the green component, how the orange streetlights are bright in both the red and green components and so on.

Below is the same excerpt from the image but with the detail removed individually from the red, green and blue components by pixellating each by a factor of ten.


Red detail removed:

Green detail removed:

Blue detail removed:

Notice the difference? Getting rid of the blue detail makes no real difference to the image at all. You have to look very carefully to see the change that removing the blue detail makes:

The human eye contains about 120 million “rods” and 6 million “cones”. The rods, concentrated around the fovea centralis are responsible for fine detail but only detect light and dark, not colour (in physics terms, they are photodetectors).

The cones are split into three groups, making human beings trichromats who see in three colours, (as opposed to most non-primate mammals which are monochromats that see only in black and white).

It’s this mis-match in the sensitivity of the eye to blue that causes the eye’s inability to see detail in blue.

A public service announcement regarding paper snowflakes

As Christmas draws nearer decorations are starting to appear in windows, and some of those decorations are paper snowflakes. The problem is that most paper snowflakes are wrong.

Real snowflakes demonstrate hexagonal (sixfold) symmetry; each snowflake is composed of one part, repeated six times, with a separation angle of 60°.

The “standard” method of creating paper snowflakes, folding a piece of paper in half and then in half again, creates snowflakes with tetragonal (fourfold) symmetry, each piece separated by 90°.

How to Make A Genuine Paper Snowflake

1.) Get a square piece of paper and fold it in half diagonally twice, across both sets of corners.

2.) Using the centre of the piece of paper as a marker, fold the paper into equal thirds, and trim off the top to create an equilateral or isoceles triangle.

4.) You should now have a folded triangle of paper. Cut your pattern into the paper, making sure that it is symmetric (if it’s not symmetric you’ll end up with three-fold symmetry). If you cut off the edges of the triangle but not the centre you’ll end up with the desired “spokey” effect.

The UK Government wants to end evidence-based drug policy

This is what the Misuse of Drugs Act 1971 currently says about the make-up of the Advisory Council on the Misuse of Drugs (ACMD):

1.1 The members of the Advisory Council, of whom there shall be not less than twenty, shall be appointed by the Secretary of State after consultation with such organisations as he considers appropriate, and shall include:
(a) in relation to each of the activities specified in sub-paragraph 1.2 below, at least one person appearing to the Secretary of State to have wide and recent experience of that activity; and
(b) persons appearing to the Secretary of State to have wide and recent experience of social problems connected with the misuse of drugs.

1.2 The activities referred to in sub-paragraph 1.1a above are:
(a) the practice of medicine (other than veterinary medicine);
(b) the practice of dentistry;
(c) the practice of veterinary medicine;
(d) the practice of pharmacy;
(e) the pharmaceutical industry;
(f) chemistry other than pharmaceutical chemistry.

1.3 The Secretary of State shall appoint one of the members of the Advisory Council to be chairman of the Council.

This is what the text will read if the Police Reform and Social Responsibility Bill which is currently in Parliament is passed*:

1.1 The members of the Advisory Council, of whom there shall be not less than twenty, shall be appointed by the Secretary of State after consultation with such organisations as he considers appropriate.

1.2 The Secretary of State shall appoint one of the members of the Advisory Council to be chairman of the Council.

Notice the difference? Gone is the requirement for any sort of scientific expertise amongst committee members, and the Home Secretary is free to appoint anyone that he/she considers appropriate.

This change seems to be motivated solely by the fact that the Government doesn’t like the advice it’s been receiving. David Nutt was sacked in October 2009 after stating that more people die every year falling from horses than from taking ecstacy†. Six members of the council have resigned since, citing the Government’s failure to heed its advice and its employment of a decision-making process “unduly based on media and political pressure.”

This bill allows the Home Secretary to replace scientists with sycophants, and should be defeated. An Advisory Council should be made-up of well-qualified advisors.

* The changes to the ACMD are outlined at the bottom of page 108 of the bill.
† In the UK approximately 30 people die per year from conditions linked to taking ecstacy and about 100 per year in horse-riding accidents.