The speed of light is as frustrating as it is fascinating. The speed of light in a vacuum is the fastest thing we can measure at 300,000km per second, although in a vast and ever expanding universe, it is painfully slow.
The universe currently measures 45 billion light years in all directions of observation. So if we shine a light from the international space station in all directions it would take 45 billion years to reach the edge of the observable universe.
Will this beam of light even reach the edge of the universe? Most likely not. Dark energy is accelerating the expansion of the universe at all times. Space is expanding at a rate of 72km/s per megaparsec. That’s roughly 32 million light years.
If we observe much farther out at 4200 megaparsecs, which we currently can do, that region of space is expanding away from us at the speed of light! Anything further than that is moving away from us faster than light.
The speed of light is only measured at how fast it travels through space. The universe can expand at whatever rate it wants to. That means light emitted from the galaxies at these distances will never reach us. The distance the light has to travel is much longer than the life expectancy of our planet and sun.
Given the expansion of the universe is faster than the speed of light at a particular distance, it leaves so much of the universe a mystery to us. Light from either reference frame will never reach us.
I suppose these points are undeniable proof that the speed of light actually has a limit. In times gone by it was assumed the speed of light was infinite. The equation E=Mc2 debunks this entirely because to keep the speed of light infinite, an infinite amount of mass would require an infinite amount of energy to keep the speed of light itself as infinite. Time, space, mass and energy could only travel at one speed – as fast as it possible to go.
If this were the case we would have no causality, no cause or effect and the universe would be one giant here and now. So sending signals to the far reaches of the cosmos may take centuries, millennia and longer! That is before we even have to await a response at the speed of light back to us.
This does pose an interesting final thought. Mathematically the chances of life in other galaxies is very likely. Should a signal reach a radio in another world, it could be billions of years from now. A response would take the same time in return. A civilisation could have become extinct prior to our signal reaching them. Or worse still, we could be gone by the time their reply gets here.