The speed of light or c is 299,792 kilometres per second. This speed is one of the most basic constants of our universe. It's an unyielding rule that stands no matter where you are or how fast you move. That’s correct, from any viewpoint, the speed of light is always c. This must sound very strange- after all, isn’t all motion relative? And aren't most speeds dependent on where you're standing or how quickly you are moving (i.e. reference frames)? But c isn't like that.
Why is the speed of light constant, and what would happen in a universe where it wasn't?
Albert Einstein's theory of special relativity changed how we think about space and time. It's built on a couple of key ideas, one being that the speed of light is always constant, no matter what the speed is of the source of light or the observer. This in itself is surprising because it is not how ordinary speeds work. For example, if you are driving your car towards your friend, from your perspective, it is your friend who is moving closer to you. So, in a way, your friend is the one who is in motion.
So which one is it? Are you moving towards your friend, or are they moving towards you? Well, relativity states that it can be both. In fact, from either one of your reference frames, it can be determined that the other is moving.
If you’re in a car moving toward another vehicle, the relative speed between you and the approaching car is the totality of your speed. Yet with light, whether you’re moving toward or away from the source, its speed remains unchanging at c. To change the speed of light would have required rewriting the nature of space and time themselves. Not coincidentally, besides its other bizarre behaviours, the speed of light represents the universe's speed limit: nothing in our known universe can travel faster than c.
This speed limit makes the universe understandable. It puts cause and effect in their right order so that effects don't precede their causes. Without it, the universe would literally and logically fall apart. The multiverse, or alternate universes, postulate that there could exist other realities. Those universes could have completely different laws of physics. One of the possibilities is that the speed of light might not be constant or have a different value in these hypothetical universes.
In a universe with a different speed of light, everything would change. Perhaps time would run at a different pace, and the whole balance between energy and matter would shift. Maybe stars would never form, or life could never be possible in such a universe. It might be an entirely alien universe, incomprehensible or chaotic. The big question is whether our universe's speed of light is special or just one possibility out of many. If the multiverse exists, it's probably that c varies across different universes.
But that leads to deeper questions. Are these laws of physics chiselled in stone, or are they just one version among many? If they're arbitrary, what caused them to be the way they are in our universe? For now, light's speed in our universe is always the same. One day, perhaps science could prove the notion of alternate universes and even give an idea as to the deviations found in universal constants. Space telescopes and colliders are already trying to find new and unseen answers at the boundary of human understanding, and every day, we know more about the universe than we knew before.
The speed of light is more than just a number; it's a basic rule that sculpts our universe. Its constancy makes space, time, and causality possible. But the idea of alternate universes invites us to imagine places where this rule might not hold. Exploring these possibilities helps us better understand the universe and ourselves. Whether we will finally find these alternate universes or not, the journey of asking the questions deepens our appreciation for the cosmos. It pulls us back to the realization that the mysteries of light and space are not just scientific puzzles but rather windows into the profound nature of reality.
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