It’s easy to think that a healthy young heart should beat more regularly than an old heart. Not so. Numerous studies have shown that the variability in heart rate is a good indicator of cardiac health and older, sicker hearts tend to show less variability, not more.
The variability is in the time between beats. Plotting this change over time shows a clear pattern. In sick people, the beat is more regular. But in healthy people, small changes are very common and big changes also occur, albeit less often.
This pattern is important. Mathematicians call it a “heavy-tailed” or “power law” distribution, and it has significant features. Perhaps the most surprising is scale invariance. However closely you study such a plot, it always looks the same. So the variations that occur at one scale look exactly the same as the variations at another scale.
This pattern is different for older people with more fragile hearts. And it is not just the heartbeat that varies in this way. Neural spiking patterns and human gait patterns also vary in the same way with age. And that suggests a new way to study aging by better understanding the role that scale invariance plays in biological processes.
There is a problem, however. Humans are hard to study in this respect. The aging process takes decades, and humans are difficult to monitor over these time scales.
So researchers would dearly love to study the nature of scale invariance in less complex organisms that are easier to look after. But the existence of these patterns in lower life forms has yet to be established.
Enter Luiz Alves at Northwestern University in Illinois and a few pals, who have found evidence of scale invariant patterns in the behavior of nematode worms. What’s more, these guys say the patterns change as the worms age and that this provides a powerful new way of studying the biological processes and mechanisms behind aging.
First, some background. Scale invariance is the notion that some things look the same regardless of the scale at which they are viewed.
Lots of things have this property. A coastline is scale invariant or fractal—it looks equally jagged whether examined at the scale of centimeters, meters or kilometers. An infinite straight line is scale invariant—there is no way of knowing how close you are to such a line simply by looking at it.
By contrast, rectangles, cars, and galaxies are not scale invariant. These objects look entirely different depending on their distance.
Scale invariance is not just confined to objects. In recent years, physicists have discovered lots of phenomena that are scale invariant, too. For example, forest fires are scale invariant. There are lots of small forest fires and a few big forest fires, but the relationship between size and frequency is scale invariant.