The Communist's Constant

I don't really know why, but I feel like explaining something today. Maybe it's because the semester is over, which means that I'll have to wait another four months before I can start teaching again? Well, I could also be in a nerd mood...

Here's a question: apart from the number of fruits that allegedly fell on Isaac Newton's head, what do you believe to be the best-known number in physics? I'd say it's the speed of light, with its astronomical value of (approximately) 300.000 kilometers per second. Most people are probably familiar with the fact that Albert Einstein postulated that the speed of light is constant - for any observer, regardless his or her frame of reference, whether you like it or not - but the subtlety and counterintuitive character of this claim, dating back to 1905, is often misunderstood.

First of all, this means that the speed of light neither depends on your position in the universe, nor on today's date; you, hopefully still reading this, Genghis Khan and the Emperor of Antares, a red supergiant star in the Milky Way galaxy, would all measure the same speed of light. And apart from physics textbook printing companies, who would have loved it if the speed of light were to change over the years, nobody should be bothered by this. Nor suprised. What is harder to believe however, is the fact that your measured value for the speed of light does not depend on the speed of the source, nor on your own movement (away from or towards the source). As this is probably the more difficult side of the equation, let me give you an easy analogy to illustrate the weirdness of Einstein's assumption, on which his celebrated theory of special relativity is based.

Imagine you are about to be chased by a killer hamster, ready to plunge his widely underestimated pointy fangs into your juicy calf muscles and reduce you to a pile of undigestible leftovers within mere seconds. The angry creature is 100 meters away from you and will approach you at 5 meters per second.

Your options are:

a) Stand still, which is the lazy way of spending the last 20 seconds of your life. Note that a lot can be done in 20 seconds, as I will elaborate in a further post.

b) Run towards the creature, which is in fact the best way to make your life even shorter. Before you all start posting the same comment: no, wearing an armoured body suit will not change this; although killer hamsters are known to prefer fresh food to canned food, this will absolutely not prevent them from violently murdering you.

c) Try to run away from the creature. Bear in mind though that recent research conducted by a team of world leading authorities seems to suggest that killer hamsters have no sense of “giving up”. This is a preliminary result however, as the rodents under observation and - therefore - the experiment are still running. Some say that this exceptional stamina is due to the fact that killer hamsters are sent to special Shaolin treadmill camps once they reach puberty. We refer the interested reader to the recent paper “If what they say is true: the Shaolin and Killer Hamsters could be dangerous” (W. Tang et al.), to appear in Journal of Bloody Scary Animals which are not Rabbits.

Anyway, my point is the following: the time it takes this fluffy monster to reach you heavily depends on your own speed and direction of movement. Or, put differently, insofar as you'd be interested in trying to determine the killer hamster's speed at the moment it launches its famous final blow - which seems like a rather unconventional way to spend the last moment of your life, you'd not always find the same value. If you'd be running at more than 5 meters per second away from your certain death in furry disguise, for instance, you'd have the impression that its speed would be a negative number as it would appear to be moving away from you.

However, and this is the upshot of today's lesson, for photons (light particles) this is fundamentally different: regardless of the direction in which and the speed at which you are moving, light will always move towards you with the very same speed. I am not convinced God is a DJ, especially not when tuning into the wrong radio station, but I'm pretty sure that if He is behind all this, God is definitely a commie...