The Question

I am writing an article that will attempt to refute a creationist claim that all supernova remnants in our galaxy appear to be less than 10,000 years old, based on the "well-established" decay pattern of a supernova's light intensity in the radio-wave frequency range. I would appreciate any information that you could give me, even just a reference or two, or a referral.

The Answer

I. My Personal Caveat

Before I give you your scientific answer about supernova remnants, I want to say from my personal experience you are probably bound to fail in your attempt to change the mind of any true creationist. The reason is simply that the evidence against the age of the universe being only 10,000 years old is so overwhelming, that yet another piece of evidence will not sway any "dyed in the wool" creationist.

In addition, no scientific argument can counter the non-scientific "theory" that God made the world 10,000 years ago (or any other length of time) exactly as it was then. That is to say, with the particular abundance ratios of radioactive elements, stars that appear to be billions of years old, and an expanding universe. By definition this "theory" is untestable, therefore it is unscientific in the true sense of not being testable.

In my experience, the only arguments that have worked with my creationist friends are ones that are completely non-scientific, but theological, in nature. Arguments that imply that the creation stories in the Bible were not written to actually explain how, or when, the Earth was created (why would there be 2 of them if that were the case?), but were written to say something to us about our own human condition, our faith, or even why the Earth was made. Science explicitly makes no effort to answer these "why" questions, so whatever answer a person comes up with is compatible with the scientific data.

II. The Scientific Answer to your question

The gist of the creationist's argument is right, observations of ongoing radioactive decay in supernova remnants can only date the very young ones. The expansion method of dating a supernova remnant (as describe in the WWW page from my last message) similarly only works for young supernova remnants (up to about 10,000 years old).

There are ways to date older supernova remnants (ages > 10,000 years), however they are not very accurate. These methods involve X-ray observations which measure the temperature of these supernova remnants. From the temperature, one can estimate the speed of the shock wave, from the speed of the shock wave one can estimate the age. Using these methods, we observe supernova remnants up to abound 100,000 years old, when they fade into the interstellar medium.

Now if the goal of this is to find the age of the universe, supernova remnants are not the objects to look at. This is simply because they become mixed up with the interstellar medium after only about 100,000 years. The universe is much older than that, which we know from the oldest stars (on the order of 10,000,000,000 years old). In addition, we observe distant objects that are billions of light years away.

The most solid evidence for the Earth being old are the products from long half-life radioactive decay found in meteorites and rocks on the Earth. For example: Potassium 40 (40K) decays into the gas Argon 40 (40Ar) with a 1/2 life of 1.3 billion years. As long as a rock remains a rock, this 40 Ar remains trapped. If the rock melts, the Argon escapes. So, by measuring the amount of 40K and 40Ar in a rock, geologists can measure its age.

Common elements used for this are Potassium 40 (1/2 life = 1,300,000,000 years), Uranium 238 (1/2 life = 4,500,000,000 years), Rubidium 87 (1/2 life = 47,000,000,000 years).

These studies clearly show that the Earth is at least 3.9 Billion years old, because that is the age of the oldest rocks. The oldest meteorites are about 5 Billion years old.

Good luck,

Jonathan Keohane
-- for "Ask a High-Energy Astronomer"

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