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If the speed of light were half its actual value, would the universe have developed very differently?
Ron Dippold
San Diego, California, US
Absolutely – because the speed of light isn’t just the speed of light itself, it’s also the fundamental speed limit of everything else, including the forces that hold atoms together. But let’s ignore atoms ripping apart for now – stiff upper lip!
Because the apparent edge of the universe we can see is limited by how fast it is expanding and the speed of light, our visible universe would be half the radius it is now, which means one-eighth the volume. The “actual” universe would be the same size, we just couldn’t see the rest of it because the light hasn’t reached us yet. Because the speed of light is also the speed of gravity, galaxies would form a little differently. But at this point you could squint and it wouldn’t look all that different.
However, then there’s Albert Einstein’s famous equation, e = mc2, which says that an object’s energy is equal to its mass times the square of the speed of light. If c is suddenly cut in half, the amount of energy you get from converting any given mass into energy is only a quarter as much! Since stars like our sun rely on nuclear fusion to keep from collapsing under their own gravity, they would have to burn four times as much gas, making them die much faster. So the universe would be colder and darker on average.
Physical constants relating to electricity, magnetism, the force holding atoms together and many other things are defined by the speed of light. If c is halved, the fine-structure constant holding atoms together doubles, which means any atoms heavier than barium become so unstable you won’t see them outside of labs. Platinum, gold, silver, iridium, plutonium, uranium, lead – all gone! We would still have iron and tin, but some elements that weren’t radioactive would now be radioactive.
Finally, we’ll kill off all life as we know it – if the speed of light is cut in half, the magnetic constant and the dielectric constant get cut significantly. If electrostatic forces weaken, atoms would balloon in size and the chemical bonds holding molecules together would weaken significantly. We would dissolve.
You can try to fix this with various patches like doubling the Planck constant to compensate for the half-speed light, but this makes atoms 64 times larger than they were before and the molecular binding force is still a quarter what it was before. Again, we dissolve. If you had a sufficiently large clump of sufficiently dense matter in this new universe, there might be enough gravity to hold things together enough to start new life with some radically different biochemistry.
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