Has the speed of light significantly slowed down over the past 6,000 years?

Question

One of the problems for young-Earth creationism is that we are seeing light from stars more than 6,000 light-years away, which implies that those stars pre-date the alleged date of the creation.

To get around this creationists have proposed that the speed of light used to be much higher:

The author claims that computer analysis of the measurements of light velocity recorded since 1675 shows otherwise and points to a beginning—a creation of light and a subsequent slowing down of this created light. The date of this beginning appears to support the short Biblical chronology.

(Note: I have seen this claim made in many places, and I am not concerned here with refuting the specific evidence on that page. I am merely establishing that this is a notable claim).

Edit: The article quoted above is dated 1981, although the page is still live on Answers in Genesis. Creationists have since backed off from the claim that historical measurements demonstrate that light has slowed down. Some still argue that it might have slowed down, but that scientists are ignoring this possibility. For instance the Institute for Creation Research has an article dated 2003, but still available on their website now, which seems to argue (if I understand the word-salad correctly) that this is possible. Other creationists have taken refuge in more complex theories involving a geocentric or sun-centric universe in which the speed of light or the expansion of space varies in a way that puts Earth in a privileged position such that the essential asymmetries are only invisible from the point of view of Earth.

At first sight the proposition that light has slowed seems to be an irrefutable theory. However it occurs to me that there may be astronomical phenomena which provide evidence for the speed of light at or near the time the light was emitted. An example would be a nova in a gas cloud: we would be able to see the light-sphere from the explosion growing through the cloud. Another would be a pulsar in a gas cloud, in the same way.

If we can see that light from an object 10,000 LY away was travelling at the same speed when it left the object then it would be evidence to refute the creationist argument that the speed of light might have changed in the last 10,000 years.

Is there good evidence showing whether the speed of light has slowed down over the past 6,000 years?

Answer 1

It turns out that SN1987A did pretty much what I suggested. It had a ring of material that had been thrown off the star some 20,000 years ago. When the star exploded the light from the explosion illuminated this ring 8 months later. Simple measurements of the size of the ring and the known distance to SN1987A yield a speed of light that is the same as the one we see today.

Pulsars also present serious problems to c-decay theories, because we see light and radio waves from pulsars as they spin. If the speed of light has changed then this should show up as changes in the time of arrival of the pulsar emissions today.

Edit: As Gordon Davidsson (below) points out, SN1987A is not a good test here. Suppose that when SN1987A exploded the speed of light was faster than now. Whatever that speed was, the light from the ring would still be 0.6 LY behind the light from the supernova. Then both sets of light slow down to the current speed, and hey presto, we see the ring illuminated 0.6 years after the supernova explosion.

However by the same token pulsars are a good test. Suppose a pulsar is spinning once a second. Each pulse of light directed towards Earth is a second behind of the last one. If the speed of light is much faster than now then the distance between the pulses will be much greater. Then if light slows down the pulses will still be the same distance apart, but because the light is travelling slower an observer on Earth will see the interval between the pulses increase. Since the pulses we are seeing today must have been in flight during this supposed slow-down of light, we would see pulsars seeming to slow down by significant amounts. We don't see this.

Answer 2

TL;DR

Here's the gist of my answer:

• Some physicists have been working for decades on theories involving a variable speed of light.
• These theories are not widely accepted within the physics community.
• Empirical measurements have placed strict constraints on some of the predictions.
• Even if the theories are correct, there's only significant change in the speed of light over billions of years, and in the early universe - not in the last 6000 years.

Therefore, the answer to your question is a no.

A theoretical introduction: variable speed of light theories

The creationists have stumbled into an issue that has proved to be a slight problem in physics. For several decades, a few scientists have worked to set up consistent theoretical frameworks involving a speed of light that varies in time, originally motivated to avoid the issue of cosmological inflation and related issues, which are now a bit better understood. Many of these variable speed of light (VSL) theories are extensions of general relativity. See an article by Magueijo for more; I mention it here only because Magueijo is one of the major proponents today in VSL theories. I believe one of the latest major papers in this vein is Afshordi & Magueijo 2016.

The problem with applying VSL theories to the present case is that these ideas arose to explain phenomena in the earliest stages of the universe - not the last few centuries. As such, predictions being derived are rather cosmological in scale, and Afshordi and Magueijo are much more interested in checking variations in the cosmic microwave background than phenomena in our astronomical backyard. The point of this is that some - some - physicists - have been working on these ideas for decades, but not in the direction these creationists are trying to go.

Just before posting this answer, I noticed that one of your articles mentions Magueijo, as supposed evidence that the creationists' theories are viable. They're not (see the next section), because even Magueijo's theory, which is pretty far out on the fringe, doesn't predict the effects they're looking for.

Back to experiments

In recent years, we've had excellent cosmological measurements, primarily thanks to the WMAP and Planck satellites. Qi et al. 2014 (arXiv version) use these to attempt to constrain VSL theories as well as possible. They make the assumption that the speed of light varies as a power law with a, the scale factor of the universe, which describes the size of the universe as a function of time; this in turn means that c varies with redshift, and thus time. Using this form of c(t), they fit the free parameters (chiefly, the index n) to calculate quantities relating to Type Ia supernovae (a common cosmological tool), the cosmic microwave background, the Hubble constant, and something called baryon acoustic oscillations. Qi et al. they came to several conclusions:

• The index n = -0.0033.
• At redshifts less than z ~ 0.1 (which corresponds to a lookback time of over 1 billion years), the variation in c is essentially zero.
• Even at redshifts up to z ~ 10-100, the variation in c is less than about one part in 100.

The fine-structure constant

Physicists have also been interested in determining whether a variety of other physical constants are in fact constant in time. One key target is a quantity called the fine-structure constant, α, so called because secondary "fine" splitting of energy levels in the hydrogen atom are proportional to the constant squared. Understanding whether α is truly constant is important because even small variations in it could have catastrophic effects. What's important here is that the fine-structure constant depends on the speed of light, and variation in α could indicate variation in c.

Some of the earliest - and possibly the most famous - measurements in the change in α - involved the natural fission reactor in Oklo, Gabon. A variety of studies found the fine-structure constant changing by as little as one part in 10¹⁷ per year (see Uzan 2002 (arXiv version) for a review) at present. Other geological results, typically involving alpha and beta decay, have produced upper limits a few orders of magnitude higher, but still quite low. As these studies give us limits applicable on geologic timescales, they show that α certainly could not have varied much over the past 6000 years; ergo, unless the other constants that make up α (the elementary change and Planck's constant) have also been changing, the speed of light cannot have changed significantly over the same timescales.

(Though it's not strictly related to the claim because it involves going much further back in time, we also have astronomical and cosmological constraints on the variation in α; see again Uzan 2002. By studying spectral line splitting in quasar spectra, this variation has been explicitly calculated up to redshifts of z ~ 2-3, rather than the mere placement of upper bounds. Assuming certain systematic errors are not at play, this could be significant; however, there seems to be conflict with various geological results. At any rate, these are not widely accepted as definitive evidence of any variation, and certainly not of a recent one.)

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Putting all of the above together, the creationists' claims - which require enormous variations in the speed of light over recent timescales - seem to be ruled out completely, even if smaller variations in c or α cannot be ruled out farther back in time. Even if the fringe science they cite is correct, it still doesn't support their claims.

The Setterfield-Norman paper

I read a paper by Setterfield and Norman from 1987 that was responsible for some of this mess. I noticed several problems with their analysis:

• Some of their data is grouped together, and presented as if they were all taken in one year - the mean of the actual years of data collection. It's unclear if this is also used in the fitting.
• The functions they attempt to fit the data to are poorly presented. r² values are missing from three fits, and at least two of the nine fits show a monotonically increasing speed of light - not a decreasing one.
• Three fits have r² values similar to within one part in 10⁴, which is suspicious. The choices of functions to fit are not explained (and look quite bizarre), leading me to believe that ones leading to fits that did not agree with the authors' conclusions were omitted.
• Table 22 shows a) both a decrease in the measured decay rate as one reaches the present, The authors interpret the decrease in decay rate as implying that the decay is slowing; most of us would interpret that as showing that there simply is no decay.
• Finally, it's not clear that a valid statistical analysis was undertaken, or that r² is the right value to be relying on here. They claim to reject hypotheses of constant speed of light, but in many cases by percentages that are less than 3 sigma, and very much below the standard of proof for a discovery.