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Episode 81: Is the Speed of Light Constant?

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Recap: Astronomical objects are an astronomically large distance away. So far, in fact, that for light, at its current speed, to get from there to us so we can see them, that the universe must be more than 6000 years old. Young-Earth creationists have a problem with that. This epiosde discusses one of the ways they try to solve that problem.

Puzzler for Episode 81: There was no puzzler for this episode.

Answer to Puzzler from Episode 80: The short answer is "no." And it's an emphatic "no," and yet this is an e-mail that tends to go around every few years, starting back in 2003 when Mars was at its closest to Earth in something like 60,000 years. Warwick traces it to a poorly worded article that stated at the time that if you look at Mars through a telescope at 75x magnification, Mars would look as big as the full moon does with the unaided eye.

To figure out if Mars could look as big as the Moon, you can use very basic trigonometry, or you can use ratios since the angles are very small. The basic math is that the angular size of an object is twice the inverse tangent of the radius of the object divided by its distance. For the average distance to the Moon, I get about 0.52°, which is the correct, known value of about half a degree. Yes, it varies a bit, but we'll use that number.

To figure out the closest that Mars can get, take the aphelion of Earth's orbit and the perihelion of Mars' orbit (farthest Earth gets from the sun, closest Mars gets to the sun). That's your distance. Put in Mars' diameter, and you get an angular size of 0.0071°, or about 26 arcseconds, where there are 3600 arcsec in a degree, so the Moon is around 1800 arcsec across. Putting it succinctly, the largest Mars can appear to Earth is about 1.4% the size of the Moon.

That's on the current orbit. To get it to appear as large as the Moon, it has to be about twice as far away as the Moon. Since I used the geometry way for the first part, I'll use the ratio way for the second part. Roughly, Mars is about 2x the diameter of the Moon. And for small angles, these things are linear. So, since it's twice the size, it will appear the same size if it's about twice as far away. In actuality it's about 140x farther away.

Q&A: There was no Q&A for this episode.

New News, Related to Episode 69: Neutrino 'flavour' flip confirmed

  • New news this episode relates to episode 69, "The Solar Neutrino 'Problem.'" The jist of that episode was that the neutrinos that should form during fusion in the sun weren't detected at Earth, but when it was found that neutrinos could change type, all the neutrinos were accounted for.
  • The new result from a large collaboration of over 500 scientists shows that, while we've known for a decade or so that neutrinos could change - or oscillate - in some ways, we didn't know if they could in another way. This experiment shows they can. I know that may sound somewhat vague, but going into more detail to understand it would take several minutes, so I've linked to the article in the shownotes and if you're interested, head over and take a look.

Additional Materials:


Claim: First, to get it out of the way, yes, the speed that light travels will change based on the substance that it is moving through. Light in a vacuum moves faster than light in atmosphere moves faster than light in water. What I'm talking about in this episode is the claim made by some people - especially by young-Earth creationists - that the speed of light in a vacuum changes, and that it has changed A LOT in the 6000-year history of the universe. That's really one of the only ways that they can get modern astronomy to fit into a 6000-year timeframe.

Speed of Light Evidence for an Old Universe

Because this episode is geared more towards debunking the young-Earth creationist claim, I think it's important to understand some of the context: Why, if the speed of light is 300 million meters per second, does the universe have to be old?

The most basic reason is that stuff is far away. And, light, traveling at the speed of light, takes time to get from there to here so we can see the stuff. We know the sun is at least 8.3 minutes old because we see its light and it is 8.3 light-minutes away. We know the next-closest star is at least around 4.3 years old because we see it, and it takes 4.3 years for light to get there from here. We can directly measure the distance to stars through the geometry of Earth's orbit out to around 1000 light-years, so we know it's at least 1000 years old. By using some standard types of stars, we can use those direct geometric distances to get distances out to many millions of light-years. Or, even just the size of our own galaxy, we can see stars 10s of thousands of light-years away with very little assumption other than the speed of light being constant, or physics working the same way here as it does there.

And hence the problem for young-Earth creationists: If you have objects that are over 10,000 light-years away, how can the light get here and the universe still be under 10,000 years old?

Some might say that the "smart" creationists simply don't try. If pinned down, then they say that God created the light en route to Earth to give the appearance of age -- this is known as the Omphalos Hypothesis and was formally proposed in 1857. Or generally just falling into that broad category of not caring about the HOW, just the blind faith that it did work.

It's the creationists who are at all sorta kinda science-minded who care about this issue, and the main way they try to solve it is to argue that light used to be really really fast, and now it's really really slow, relatively speaking. The issue is known as The "Starlight Problem."

Special Pleading / Wishful Thinking

Since I think most of you will forgive me if I reach my conclusion first and then go into their evidence, what this really is is a form of special pleading or wishful thinking. There's really no reason to think that the speed of light has changed. And we've looked, but that's a bit I'll talk about later. The issue is that creationists NEED it to have changed based on a literal reading of a book compiled over the course of several thousand years by pre-scientific people. Therefore, to get science to somehow sorta fit into that interpretation of that compilation, the speed of light must have been faster in the past.

Hence, this is a very clear case of special pleading and wishful thinking.

The Argument: c-Decay

But, they do have an argument to make. Several different versions exist with more or less complicated math allegedly backing them up. I'm only going to go into the most basic one in this episode to illustrate the point. More complicated versions exist like claiming we were sprung from a black hole so time was slower or something in the past closer to the center of the black hole so that light could travel faster than time moved for us near the center but the light from millions of light years away could have time to reach us. That was an idea known as the White Hole Cosmology advocated starting in 1994 by Russell Humphreys.

The simplest argument that creationists make is similar to their argument about Earth's magnetic field: They look at a few measurements for the speed of light that have been made in the past, show that people got larger numbers back then than we do now, and say therefore it's gotten slower. This is known as the "c-decay" model. It was first systematically formulated by the creationist Barry Setterfield in his 1981 book "The Velocity of Light and the Age of the Universe." It was championed for quite awhile, and I've linked in the shownotes to an Answers in Genesis article that appears to be Setterfield's original, or at least one that was written at the same time the book came out.

What is really needed for this to work is for light to have been literally many millions of times faster in the distant past - 6000 years ago - than it is today. And happily, Setterfield is happy to comply with his fit reaching a value of 5·10^11 times faster back in 4004 B.C., or 500 billion times faster.

If you look at and then graph the data in his table, there is one measurement from 1675 that is much faster than we get today -- by about 0.5%. It has large error bars, but not large enough to overlap today's value. I would say they just underestimated them. There's another measurement from 1728 with no error bars that also gets a value 0.4% faster than today's. There's nothing else in his table until 1871, nearly 150 years later, where it's 0.2% faster than today. After that, the values are slightly faster for a decade or two and then overlap with today's measurements.

Seems kinda to make sense: Based on his table of historic measurements, it's gotten about 0.5% slower since the very first attempts to measure it only about 340 years ago. And, you can fit a decay function to it, or an exponential sinusoidal decay function which is what he did. The nature of any decay function is that the decay is slower as you go forward in time, and it's much faster as you go farther back. This is all based on his table.

As you might have suspected, this is called cherry-picking. As in, choosing the data that fits his pre-conceived notion or the story that he wants to tell. For example, there are at least two estimates of the speed of light that were done between his first two - the ones in 1675 and 1728. Christiaan Huygens, a very famous Dutch astronomer, estimated it at 26% slower than the current value. Sir Isaac Newton in 1704 estimated it as about 5-15% slower than the current value. Or there's an 1849 measurement that was faster than the 1675 measurement, and an 1862 one that's slower than today's value. Yet, Setterfield just picked the 1675, 1728, and then goes right to 1871. Why didn't Setterfield include those other ones.?

One could also ask why he chose to fit an exponential-sinusoidal function. I mean, the reason is that's what's needed to get it a lot faster in the past while still kinda fitting the data now. But when I look at his data table, it fits a sigmoid function very well, where it was stable and fast a few hundred years ago, then dropped rapidly just at the time where we started to be able to measure it well in the late 1800s, and now it's very stable today. My point is that not only did he cherry-pick the data he used, but he cherry-picked the way to fit the data to get the answer he wanted.

And yet, this was VERY popular when it came out, being really the main way creationists explained the starlight problem for around a decade. It's also still sometimes used today, though most creationists don't adhere to it.


One reason that creationists today don't follow it is contained within a debunking article from -- you'd never guess! -- the Institute for Creation Research itself. The ICR article doesn't have a publication date, but it clearly goes through the issues I just related of cherry-picking the data and fit function. You know that you're in trouble when your own group of crazies is against you.

The second reason is more direct: Since the invention of the laser, we've been able to measure the speed of light to very, very high precision. Lasers were invented over 50 years ago. We have had more than enough accuracy since the invention of the laser to measure the decay that Setterfield modeled. It hasn't happened. That's the nice thing from this: You actually can make a prediction - a simple one - from his model, and test it. And that test supports the null hypothesis that he's wrong, that the speed of light is not changing.

The third reason most don't quote it today has to do with the implications. I've said it before on this podcast and I'll say it again: Pseudoscientists consistently try to claim that scientists are in their ivory towers and don't look at the implications of what they do for other things. But it's them, the amateur scientists, who can be general enough in their studies to put everything together. I'm not saying that Setterfield claimed this, but this is an excellent example of how a pseudoscientist did just what other pseudoscientists claim scientists do.

As in, the implications of changing the speed of light aren't just to how long it takes light to get from point A to point B. There's a lot of other stuff. Like, say, how quickly fusion works. If the speed of light were 500 billion times faster 6000 years ago, that would mean the sun would have emitted energy at a rate nearly 1 billion times more than it does today. Similarly, the heat from the radioactive decay inside Earth that keeps it warm today would have heated the planet so much that it would still be liquid rock. There are numerous other implications, but I think I've made my point.

But Could it Be a Bit Different?

While lasers today have shown that, at least here, the speed of light hasn't changed, it is a valid question and area of research as to whether the speed of light was different in the past, even just a little, or if it's different in another part of the universe.

And many have tried to measure this. They can do it because the speed of light is part of what's known as the Fine Structure Constant, or "alpha," in physics. Alpha also has things like the charge of the electron, Planck's constant, and an electric constant or magnetic constant depending on how you write it. Any change in any of those parameters will change alpha. Any change in alpha will change many things in basic physics, such as the energy level at which different atoms and molecules absorb light. Hence, we can measure the spectra - the light emitted - by distant stars very carefully to determine if they show any sign of alpha varying.

Some scientists did that in the early 2000s and found no change. A team in 1999 looked at very distant quasars and found results suggesting alpha has very very slightly increased in the last 10-12 billion years. Some of the creationist sites I found while looking for information for this episode trumpeted that result. The change was five parts in around 100,000. Another study found it to maybe have changed in the last two billion years by five parts in 100 million based on experiments in nuclear reactors. Another group in 2010 found it may have been larger by 1 part in 100,000, 10 billion years ago.

While these suggest it might have changed, other research studies have found that it hasn't, by studying the same kind of distant phenomena. As a scientist, to me that means it needs more research in a systematic way to nail this down, since the results are too haphazard to say much of anything. As a skeptic, I would say that this is consistent with random scatter or noise and that it will likely converge to zero. As a podcaster talking about whether the speed of light is constant, I can say emphatically that the speed differences needed to get the results of Setterfield to make the universe under 10,000 years old is ruled out. All of the experimental evidence points to the conclusion that IF the fine structure constant has changed, and IF it's changed because of the speed of light changing, it's by a VERY small amount.

Provide Your Comments:

Comments to date: 5. Page 1 of 1. Average Rating:

Brian Davis   Silverado, CA

8:56pm on Monday, August 5th, 2013 

I wouldn't mind more C2C clips, but I feel guilty asking you to listen to more of it.

Jason Goemaat   Des Moines, IA

12:09am on Thursday, July 25th, 2013 

I think the best way to disprove c-decay is SN1987A. We can measure the angular size of its rings using a telescope, and the central ring brightens or dims 0.658 years after the star brightens or dims. If the speed of light has been constant, the angular size of of the ring's radius (0.808 arc-seconds) combined with the size using the speed of light travel time to the ring (0.658ly) means it is about 168,000 ly away.

The thing that kills the c-decay argument is that if the speed of light was faster in the past, the actual size of the rings would be larger because we know the distance based on the time it took the light to reach the rings from the star, meaning SN1987A would actually be much further away. The faster c was in the past, the further away SN1987A is, the longer light must have taken to get to Earth. If light was twice as fast when the supernova occurred for instance, we would know the rings were 1.316ly across. The angular size is 0.808 arc-seconds so we would calc... read more »

Jason Goemaat   Des Moines, IA

11:13pm on Wednesday, July 24th, 2013 

I vote for more coast to coast clips. I can't imagine having to listen to whole episodes...

Ben   Seattle

12:28pm on Wednesday, July 24th, 2013 

I don't mind the c2c clips at all. I use to listen to art bell and noory but now I take great pleasure listening to you dismantle them.

Gold (@Unifex)   Wellington, New Zealand

7:39pm on Tuesday, July 23rd, 2013 

More Coast to Coast snippets. Quite like them as it's difficult for people to say it was never said when you actually have recordings of them saying it.

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