By Marcel van der Veer
October 2022

Published in Science

More on Physics, Tutoring

Top picture: The Sun, captured by NASA's Solar Dynamics Observatory (source: NASA).

Hardly anyone will dispute that Einstein's general theory of relativity is well established now. Whether general relativity is the ultimate theory on gravity is a different discussion. Astronomers cannot find enough mass in the universe hence consider it full of elusive dark matter - or did Einstein not conceive the ultimate theory on gravity? Bright minds like Verlinde have proposed intriguing new theories on gravity without needing ghostly particles. Only history will tell, but let's return to today's subject.

Because of Einstein's success it is generally understood that light speed is the speed limit in space. Light speed is ridiculously high, roughly 300,000 kilometers per second. Hardly anyone can imagine such high speed. Me neither. Light speed is believed unattainable for matter - it would require an infinite amount of energy to propel the dot at the end of this sentence to light speed.

Can nothing move faster? Well, matter cannot, but space is not matter. Modern insights in the inner workings of a black hole suggest that space itself flows like a chaotic waterfall over the event horizon, exceeding light speed beyond it. That is why light cannot escape from a black hole. On a side note, Einstein believed that black holes were an artefact, an aberration of his theory.

To put light speed into perspective, let's compare astronomical scales to translate light speed into a value we can relate to. Suppose the Sun were a regular 32-panel football, some 22 cm in diameter, in the center of Amsterdam. On this scale, Earth is a grain of sand of about 2 mm size, orbiting the football Sun at a distance of 23 meters. In a year, light would travel the distance Amsterdam-Madrid.

The nearest star to the Sun is Proxima Centauri. Its light takes 4 years and three months to reach us. This means that our closest neighbor would be a same-sized football more or less in Detroit or Delhi. In our football universe, light speed would amount to circa 4 kilometer per day. The slowest turtle is twice as fast.

Fans of the Star Trek franchise know that warp 1 means light speed. After the original series, maximum speed was set to warp 10. At warp 10 you would be everywhere in the universe at the same time. Anyway, the creators of Star Trek thought up a way to travel faster than light. This brings us to the question why there would be a speed limit at all.

One of the best insights from physics lies beyond the why of light speed being the maximum speed for matter in the universe. In fact, light speed is not the maximum speed, it is the only speed. Many people do not realize that all matter travels at a same constant speed through spacetime. The reason for this is not known, for the moment we must accept that this is the way the universe works. Speed has a component in space and a component in time, but the resulting speed vector always has this same magnitude.

Photons, that make up light, are frozen in time so their speed must be the maximum speed attainable in space. We have come to call that maximum speed light speed, obviously. We ourselves move relatively slowly through space, so we must move at almost warp 1 through time. Think about this for a moment; this is about as cool as physics gets. The implied question is an old one - is space an arena in which events unfold in time?

Updated on 19-10-2022

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