IT’S ANOTHER slow news day, so we’ll tackle an old issue. Creationists are forever claiming that the world — and the entire universe — started only around 6,000 years ago.
They wave away all of geology, claiming that everything is a consequence of the Flood. They wave away radiometric dating techniques, claiming that the decay rate of isotopes used to be different. They dismiss the hundreds of millions of years required for evolution because … well, because they don’t like evolution.
One of the creationists’ lesser-known difficulties is their Starlight problem, which asks this question: If the universe were only around 6,000 years old, then how is it possible that we see light coming from stars and galaxies that are millions of light-years away from us?
They have different responses to this, sometimes claiming that light used to move faster in the past. This argument invokes the concept of c-decay, which asserts that the speed of light was over a million times faster when the universe was created (within the last 10,000 years), and it has been slowing down ever since. This (they think) explains a universe that is seen to be billions of light years across, yet light from most distant objects has become visible in only a few thousand years. Their “solution” to the starlight problem ignores a great many things, including the uncomfortable fact that the Garden of Eden would have been a ridiculously high-energy environment (that “E” thing which equals mc2).
Confronted with that kind reality-denial, do we have any absolutely solid evidence that they can’t wave away? As it turns out, yes — we do. Well, we don’t have a sure-fire quickie that will prove to a creationist that the universe is 14 billion years old, but we can show that it’s many times older than their Genesis version, and that should be sufficient.
There’s a supernova with the catalog number SN1987A (so numbered because it was the first supernova discovered in 1987). Several months after the supernova was observed, the flash from its explosion was seen to illuminate a gas ring which existed around the exploding star. For those trained in such matters, that provided an opportunity.
With the known speed of light and the observed delay in seeing the illuminated ring, we can easily compute how far the ring is from the exploding star. Using geometry, we can then make a right-triangle — a very large, thin triangle. Earth is one point, the exploding star is another, and a perpendicular line from the star to its ring gives the third point. With the known distance from the star to its ring as the base of this right-triangle, and knowing the observational angle between the star and the ring as seen from earth, the distance to the star can be calculated — roughly 168,000 light-years.
There’s a nice, simplified diagram of that triangle at this website: The Age of the Universe and SN1987A, which we’ll copy here (if you don’t see it, click on the box):
But this depends on the speed of light long ago when the star exploded, because that’s how astronomers figure the distance light traveled in several months from the star’s explosion to the illumination of its ring. If the speed of light were much faster in the past and has been slowing down ever since, as the creationists insist, then the distance calculation would be wrong.
However, SN1987A gives us a nice little test of light’s constant speed. The explosion of the star and the later illumination of the ring were two separate events, several months apart. Yet the first light from the ring reached us exactly when expected, after a delay only due to the time it took the ring to light up. If light really were slowing down, then the time required for the first light from the ring to get here would be greater than time required for the original light from the exploding star; and that would indicate that the ring is much farther from us than the star. They wouldn’t appear to be part of the same system.
But there’s no discrepancy. The star-and-ring system, together, are both the same distance from earth. Thus, during almost a year from the time the star exploded to the time its ring was illuminated, the speed of light remained the same. Got that? Lightspeed stayed the same for a period of several months — a big chunk of time in a universe supposedly less than 10,000 years old.
“Okay,” respond the creationists, “maybe the speed of light didn’t change during that particular interval of time, but there’s still no evidence of what the speed of light actually was, and we say it was a lot faster then.”
Sorry, that won’t work. Hubble telescope observations are so good that we can actually measure the size of that illuminated ring around the supernova. This measurement is entirely independent of the speed of light. With that, and the the same triangle described above, we can once again compute the distance to the star. This is simple trigonometry, and the distance is still 168K light years — the same result as before. So light had to be traveling at today’s lightspeed back then (168K years ago), in order to reach the ring when it did.
Alas for the creationists, we have direct observational evidence that the speed of light hasn’t changed in 168,000 years. There’s no way the universe we observe could be only 6,000 years old.