Figure the odds

Humans are great at recognizing patterns and one of the patterns we do really well with is that of cause-and-effect. We like to know what causes what – and particularly what causes bad things to happen. So when something bad – like cancer – happens we want to find a cause; and we also want to know what causes cancer so that we can avoid getting it. We would like to believe that our bodies and our health are deterministic – that we can predict what will happen if only we can know enough. The problem is that this just isn’t the case and there are some fundamental limits of what we can know and of what we can predict. Or, to put it another way, people with the healthiest genes and lifestyles still come down with cancer, just as there are healthy octogenarians who smoke, drink, and eat fatty foods. Here’s why.

Physicists used to think that the entire universe was deterministic – that if we had enough information about every particle in the universe (position, velocity, mass, etc.) and the fundamental laws governing their motions and interactions then we could predict everything there was to know about the universe for all of time. On the surface this seems to make sense – after all, if we know we have perfect and precise knowledge of every aspect of, say, the balls on a pool table and if we have perfect knowledge of how we strike the cue ball then we should be able to predict exactly where every ball on the table will end up. If we consider all of the particles in the universe – every atom, every electron, every proton, and so forth – and if we know how each of these particles interact with each other then shouldn’t we be able to view the universe as the pool table writ large? Shouldn’t a big enough computer be able to calculate the future and fate of the universe?

Ideally yes – but…there are inevitable uncertainties that complicate matters. The universe is, at the most fundamental level, random, and this randomness creeps in everywhere.

Part of it is that we cannot know everything there is to know about a particle – this is the premise of the Heisenberg Uncertainty Principle. German Nobel laureate Werner Heisenberg realized that the very act of observing a particle changes it and he determined that the more precisely we know, say, a particle’s position the less precisely we can know it’s exact velocity (velocity is the particle’s speed and direction). So the first thing we need in order to have a deterministic universe – exact knowledge of every particle in the universe – is impossible to achieve. The deterministic universe doesn’t even get out of the starting blocks.

But the universe is even more random than that. Consider radioactive decay – my specialty. We can’t look at a single atom and know exactly when it will decay. We can look at a collection of atoms and predict how many will decay in a given amount of time, but the exact fate of any single atom is a mystery to us. And there are more examples – but this ought to suffice.

So what’s this got to do with radiation exposure and cancer? Funny you should ask….

It’s tempting to think of our bodies as the equivalent of the deterministic universe – that if we can have perfect knowledge of every molecule in our bodies and the rules behind how our cells operate then we can predict what will happen when we expose our bodies to potential harm. For example, if we know the precise path of a gamma ray through a body then we should be able to calculate exactly which cells it will pass through, exactly which chromosomes it will hit, exactly what genes will be damaged, and whether or not that damage will turn the cell cancerous. But we can’t do this either because of the fundamental physics mentioned earlier. We might know the precise path of a gamma ray, but we can only predict the probability that it will interact with any molecule (DNA or otherwise) with which it interacts in its passage through the body. Similarly, our bodies respond to DNA damage, but this is also probabilistic. So we can’t predict whether or not any particular gene will be damaged and, if so, we can’t predict with certainty whether or not the damage will be properly repaired. Radiation as a cause for cancer is fundamentally stochastic – random in nature – and we can only determine probabilities.

This was a point made by another Nobelist – physicist Erwin Schrödinger, in his landmark essay What is Life? As early as 1944 Schrödinger pointed out genetic information is transmitted by molecules, that molecules are made of atoms, and that the behavior of atoms is probabilistic. Given this, he pointed out that slight changes in all molecules – including the molecules that carry genetic information (they didn’t know that this was DNA at that time) – are inevitable and he proposed this as a mechanism for evolution. Schroedinger had made an inspired guess and, by so doing, he inspired the scientists who went on to become the first molecular biologists. And – again – he stressed the fundamental randomness of our genetics and our biology.

If we think about it this makes sense. Moving beyond the molecular level humans are tremendously complex and we react differently to just about everything. Expose two of us to the same pathogens and one will get sick while the other doesn’t. Expose one person to the same pathogen on two different days and we’ll see the same result. Some of us are lactose intolerant, some develop lactose intolerance later in life, and some happily chug milk for decades. So given all of this, what can we hope to figure out?

One thing we can do is to constrain the problem. With radiation, for example, I can tell you that the effects of a single x-ray will almost certainly be stochastic – it might (or might not) increase the risk of cancer, but it won’t cause skin burns. At the other extreme, the effects of a very high dose are entirely deterministic – expose a person to 1000 rem in one shot and they will certainly die of radiation sickness. So the response to radiation exposure transitions from purely stochastic to purely deterministic as dose increases from 0 – 1000 rem (but the deterministic part of this is death by radiation sickness, not by cancer).

So what does this all mean for us?

Well, first of all it means that a person with cancer can’t say exactly what caused it. We might calculate that there was a 95% probability that a particular cancer was caused by radiation exposure, but there’s still a 5% chance that the person would have got cancer anyhow from a random genetic flaw. Similarly, we can calculate that the risk of cancer from a low dose of radiation might be only a few percent, but it’s still not zero.

This does not mean that we have to abandon all hope, it just means that there are limits on what we can know. The random changes that Schrödinger speculated about are the same sorts of random changes that can give rise to cancer or to any other genetic disease – we might not be able to know them precisely, but we can at least understand what make them more or less likely to occur. We know that some factors increase the number of mutations – smoking is one – and that some factors (anti-oxidants, for example) help to reduce their likelihood. There’s a chance that, no matter how much we abuse our poor DNA, we’ll escape cancer and will live to a ripe old age – it might be a small chance but it’s still there. And there’s a chance that no matter how careful we are we’ll still get unlucky – this is the whole nature of stochastic effects. But by and large we’re in the same place we were in before – eating and living right might not be a guarantee of a long and healthy life, but it sure doesn’t hurt our odds.

Tags: , ,

No comments yet.

Leave a Reply