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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?

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    Anecdotally: Decades ago, I personally met the cited author, Trevor Norman. Creationists often cite his university affiliation, but this study was unrelated to his role at the university. He was involved in IT administration in the Mathematics/Computer Science department, not research in the Physics department. – Oddthinking Jul 1 '18 at 12:52
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    Based on the title I though this question was was gonna be more serious en.wikipedia.org/wiki/Variable_speed_of_light – Fizz Jul 1 '18 at 15:38
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    @HagenvonEitzen. OK then, have meters been getting longer over time? – Paul Johnson Jul 2 '18 at 15:26
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    "Is there good evidence showing whether the speed of light has slowed down ". There is no evidence for that, neither good nor bad. – MichaelK Jul 5 '18 at 6:27
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    Unfalsifiable claims are those without predictable consequences. If light dramatically changes speed over time, that has consequences. e.g. by looking at galaxies billions of light years away we see emissions spectra consistent with earlier stages of galactic development. Such development takes billions of years so if the faraway galaxies are approximately the same age as nearby galaxies, they should not appear to be younger. Also, e=mc^2 so fusion in the sun should have produced dramatically more energy in the past. I'm sure if c changed there should be many observable consequences. – Qwertie Jul 8 '18 at 17:46
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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.

  • SN1987A isn't a good test here, because if the speed of light was faster when it happened the light would've taken less time to reach the ring, but then the time delay would've expanded (essentially redshifted) as the light slowed down on its way here. Ironically, this is exactly why the constancy of pulsars is good evidence for a (at-least-nearly) constant speed of light. BTW, for the same reason, the consistency of emission/absorption lines in spectra from distant stars & galaxies is also good evidence for a constant speed of light. – Gordon Davisson Jul 5 '18 at 3:02
  • Over what time period would you expect to "see pulsars seeming to slow down by significant amounts" if the speed of light has slowed? From what pulsar speed to what pulsar speed, from what year to what year? How many pulsars can we see? How far away are they all? – ErikE Jul 14 '18 at 23:38

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