Speed Limit

by James Morris

Infinity is a hard concept to wrap our heads around. It’s difficult to imagine anything stretching on forever and ever. And yet, when we think about common aspects of our world – time, space, temperature – there is a general thought that these go on and on, that there are no real limits in nature. We might create all kinds of boundaries – fences, walls, borders – but nature, it seems, does not.

But that’s not the case. Nature is full of limits and boundaries, in places where we might not expect them.

Consider temperature. It can’t go down and down forever. There is a limit, and it’s absolute. It’s so absolute that scientists call it absolute zero. This idea was proposed at least as early as the 1600s.

Absolute zero is the lowest temperature possible. Temperature is measured using different scales. In Fahrenheit, absolute zero is about -460 degrees. In Celsius, it’s about ‑273 degrees. However, there is a scale where absolute zero actually takes on the value of 0. This is the Kelvin scale.

Why does this limit exist? Temperature relates to the motion of molecules. When molecules move, they have energy and disorder. Absolute zero is reached when there is no motion, and therefore no energy or disorder.

Scientists have actually never recorded temperatures as low as absolute zero, and it’s probably not even possible to get there. However, they have gotten very close. The current record is around 100 picoKelvin. This is a very small number indeed – 100 millionth of 1 millionth of 1 degree Kelvin above absolute zero.

At temperatures close to absolute zero, matter takes on unusual properties, ones that it doesn’t have at temperatures with which we are familiar. For example, superconductivity occurs, in which electrical resistance vanishes. As a result, there is no heat or energy loss associated with electrical currents. Superconductivity has a number of interesting applications, such as in particle accelerators (atom smashers) where protons are collided to answer longstanding questions in physics.

Is there an upper limit to temperature? This question is more difficult to answer. Physicists disagree whether there is an upper limit, and, if so, what this upper limit is. If it exists, at least it has a name – absolute hot.

Another well-known limit is the speed of light. Light travels quickly, but not infinitely so. As early as the 1600s (again), it was proposed that light travels at a finite speed. We now know that speed clocks in at about 186,000 miles per second, or 300 million meters per second. That’s fast, but not instantaneous. For example, the time it takes for light to get to us from the moon is about a second; from the sun about 8 minutes; from Jupiter about 45 minutes; and from Pluto about 5 hours. Light from the brightest star in the sky, Sirius, takes about 9 years to reach us, and from the North star several hundred years.

Because it takes time for light to reach us from distant sources like the moon, planets, stars, and galaxies, we are actually looking at the past when we look up at the sky. So, we are seeing what Sirius looked like 9 years ago, not what it looks like today.

There is an unusual property about the speed of light. The speed of light is always the same, regardless of the speed of the source from which it comes or the speed of the observer. For most moving objects, this is not the case. If I walk on a train, my speed is the speed I am walking plus the speed the train is traveling. But the speed of light is always the speed of light, even if it comes from a moving source. This was recognized by Albert Einstein and is part of his special theory of relativity.

The speed of light is the speed limit of the universe. What this means is that nothing travels faster than the speed of light.

RainbowWhen we speak of the speed of light, we are not only talking about light. Light is just one part – the visible part – of the electromagnetic spectrum, which also includes radio waves, microwaves, infrared and ultraviolet waves, and X-rays. Thus, these too move at the speed of light. The speed of sound is much slower, which is why we hear thunder after we see lightning, though the two occur at the same time.

For matter, the speed limit arises from the fact that, as the speed approaches that of light, it becomes harder and harder (requires more and more energy) to increase the speed. Eventually, nearly an infinite amount of energy is needed to approach 186,000 miles per second. Some of the fastest objects are particles in atom smashers. The acceleration process requires an immense input of energy.

As with temperature, the usual laws of physics break down as the speed of light is approached. Time slows down, an effect known as time dilation, and objects become smaller, called length contraction.

Temperature and light are both part of the physical world. Are there limits in the biological world too? Yes, there are all kinds of limits. We don’t live forever, and no organism really does. Organisms cannot live everywhere or eat anything, but are limited to (adapted for) different niches.

Some limits exist, even though we don’t know what they are. Consider the men’s 100 meter. The current record is 9.58 seconds, held by Usain Bolt. One day, this record will surely be broken, and that record will fall too. But a limit exists.

There is another limit worth exploring: a limit to perfection. What does this mean? It is sometimes said that organisms are so well adapted that they are perfect, or nearly so. We hear statements like “the wings of a bird make a perfect aerofoil, like the wings of a plane” or “the human eye is perfect, like a camera.”

But these statements don’t hold water. Organisms are far from perfect. It’s more accurate to think of them as good enough for the environment that exists right now.

Why is this the case? One reason is history. History constrains organisms. We can’t just sprout wings, or gills for that matter, because these would be a radical developmental change. We are still evolving – all organisms are – but we are stuck with developmental pathways that have been in place for millions of years.

Another is the environment. The environment in which organisms live is constantly changing. So, what’s well adapted today may not be well adapted tomorrow. Furthermore, when we speak of the environment, we are not just talking about the physical environment. We are also talking about other organisms, which too are changing. So, it’s not an exaggeration to say that the environment is a moving target.

And a third goes back to chemistry and physics – there are limits imposed by the physical world. We work with the molecules we have according to the laws of chemistry and physics, of which we are subject to and in which we live.

In other words, we are ultimately constrained by the same limits that exist in the universe. Which perhaps is a kind of freeing thought.

© James Morris and Science Whys, 2015.

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