Tinkering with the speed of light - implications?

Recently in a discussion with a YEC friend who was making a case for a young earth.

He suggested that we cannot know that ancient star lift is truly ancient because we cannot know that it has always been constant. I’m sure I read on the forum that mucking about with the speed of light would have dire effects but didn’t have the knowledge at my finger tips.

So what would be consequences of a fluctuating speed of light on reality? Both fast enough to justify a 6K year old universe, But also, for curiosity, slowing it down?

Look forward to your replies.


Well let’s just take e = mc^2 for starters.

Double the speed of light, you quadruple the amount of energy hitting the Earth’s surface from the sun. Multiply it by a factor of a billion, and basically you end up with a supernova on your hands.


Interesting, thanks JC.

So if that is the case we would have to multiple this energy output by (for sake of argument) visible stars in the sky, yes? So, if I am following you correctly, there would be supernova levels of energy bathing the earth in all directions?

Would this also effect the energy output of decaying elements? For example, would plutonium decaying into lead (?) happen faster too and, presumably, give of more energy?

Most of this is over my head, but Casper (who was working on an astrophysics phD) wrote some articles about the speed of light and creationist arguments.


Yes and no. The binding energy of atoms is tied to E=mc^2, thus c increasing dramatically slows nuclear decay, but keeps the overall energy output constant. A 2,000,000 fold increase in c, which is the minimum needed for fitting starlight into 6,000 years, would make the half-life of 14C about 2x10^16 years, but would make each decay about 4x10^12 times more energetic. A 70 kg person normally gets about 40,000 Bq from the 14C in them, with a total energy of 10^-9 W. They would now experience one decay every three years, each of which would have an energy of 0.01 J. So, not likely to kill you, but not good for you either.

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Even if the speed of light changed a bit, the astronomical measurements would still be just off a bit also. And independent of the speed of light, astronomers see phenomena like this galaxy leaving a stream of gas 200 million light years long, like a contrail behind a jet plane, Measuring the length and looking at the speed the galaxy is traveling, you can see the event happened hundreds of millions of years ago NASA news: Breathtaking Hubble pictures reveal collapsing galaxy spewing trail of gas | Science | News | Express.co.uk. Much longer than any young earth explanation can give.

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I’m not a physics expert, but I would think that changing the speed of light would greatly alter how stars work. The size and energy output of the star is a fine balance between the fusion process pushing outwards and gravity pushing inwards. If you drastically change the speed of light I would expect very different looking luminosities, sizes, and so forth. On top of that, I would also expect different spectra from those stars. The fact that stars across the universe behave exactly like our sun sure seems like strong evidence that the speed of light has been the same for billions of years.

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You might mention the verse in Jeremiah:

This is what the LORD says: If I have not established my covenant with the day and the night and the fixed laws of heaven and earth
Jeremiah 33:25


I think he means that if something like the Andromeda Galaxy is 2.5 million light-years away, then the light could have gotten to us much faster if the speed of light was much faster? While it is certainly an interesting thing to theorize about the variable speed of light, and indeed, some physicists do this, outside of high energy theoretical physics or early universe cosmology, it’s not really relevant to most of our universe. The reason is that there are many independent lines of evidence that demonstrate the speed of light in a vacuum, well hasn’t changed lately:

Courtesy of @Antoine_Bret:


There is a polydactylic handful of measured, underived constants. C is one of them. They do not vary… Period. There is no reason to believe that they do in any circumstances. Any being the infinity of universes from eternity. Not just our insignificant, mediocre, infinitesimal one. They are what they have to be, not random, of natural necessity. Not by God playing dice, chance. God being God could vary c from its multiversal natural value, why on Earth, in Heaven’s name, the Hell, would He? To make a point, sure, but to cover His traces? I’ve never understood the need to make God a liar for the truth.

Asking if c weren’t is like asking if 6 were 9. What @jammycakes & @Paraleptopecten said about fusion and nuclear binding energy (are they not two sides of the same coin?) and decay are spot on.

I’m fairly confident that stars as we know them could not exist under such conditions. If core temperature continues to scale with star mass the way it does as low masses (which seems less than certain), then individual stars would need to weigh about as much as a large galaxy to induce fusion (and would emit about as much light as a large galaxy), due to the changes in atomic binding energy.


That is really cool.

But of course all the light from distant stars really comes from a heavenly sphere 6000 light years in radius. And the moment we step off the edge of our flat earth we are instantly teleported to the other side, so it just looks spherical. Ok that doesn’t really make a sphere but gives our earth the wrong topology (like a torus or a projective plane), but that is just to confuse all those know-it-all educated people.


Thanks for all the helpful answers everyone! Really useful and insightful.

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I understand the logic of saying when we look at a star 100 million light years from us, we’re seeing it as it was 100 million years ago (assuming the distance has been measured correctly). My question is, could the fact that the universe is continually expanding not affect how we understand that? What I mean is, if the ‘stuff’ in the universe is expanding and everything is continually moving away from other stuff (ie not just the earth moving away from a distant star but the stuff in between is moving and expanding as well, ie the stuff through which the light travels) could that not affect such calculations? The universe, after all, is not actually a vacuum but rather largely dark matter/energy (per current theories).

I suppose I mean if the space/time continuum is itself continually moving and expanding, is it as simple as proposed in my 1st sentence?

It does make some difference I am sure, but locally, we have galaxies moving toward us, away from us, and so forth. Also, the speed of those movements is really pretty insignificant compared to the speed of light for (relatively) nearby stars. Even at a million miles an hour, light travels a million miles in 5-6 seconds.

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It could possibly, but cosmologists take these factors into account.

Where there are uncertainties or potential sources of error, they take steps to quantify those sources of error. So you’ll see, for example, that the Earth is 4.54±0.05 billion years old – that tells us that the uncertainty in the age of the Earth, once absolutely everything is taken into account, is ±50 million years, or a little over 1%. The errors are typically round about that size – a few percent at most.

Cosmologists have two different ways to determine how fast the universe is expanding. One comes from measurements of distance and redshift in distant supernovae, the other comes from measurements of the cosmic microwave background radiation. These two measurements give results that differ by about 5% from each other, about twice the size of the error bars. This is a bit of a puzzle because it suggests that there’s something going on that they haven’t yet figured out, but the difference is far too small to justify any suggestion that both figures could be wildly out by a factor of a million.

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Yes, the distance typically given is the age of the light. The distance when the light was released and the current distance are significantly different once cosmic expansion starts to be significant (~1 billion ly). At the extreme of the CMB, it was released at a very low distance; the average distance, and hence the age of the light is 13.8 billion ly; and the current distance is 93 billion ly.

The given ages are calibrated to our frame of reference. There is no universal frame of reference, so all ages are going to be relative to a given frame of reference.

Of course, the same concepts apply to everything around us, not just to distant stars. The light coming from a car on the road takes time to reach you, so you are seeing the car as it was a few nanoseconds ago. If the car is moving away from you then the wavelength of the light reaching you will be slightly longer than it was when it originally left the car, and the wavelengths will be shorter if the car is moving towards you.

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Ah, that’s what makes the taillights so red! :sunglasses:

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