Expert: James Gort - 2/28/2007

Question
Hello Prof. Gort,
Over the decades as we read and learn about astronomy and physics one thing stands out and annoys me to some extent. The information varies according to who ever is doing the calculating. I ran across an article that addresses these discrepancies. http://www.swintons.net/deodands/archives/000075.html
I am wondering what you have to say about this issue.
Thanks so much for your time and have a great day.
Urania

Answer
Hello Urania,

I agree with what the article points out.  Especially in astrophysics.  I sometimes tell my class (only half jokingly) that if your answer is correct to within an "order of magnitude", that pretty much will agree with most of the research!  But to address the website you pointed out, there are a few things to keep in mind.

He started with the "maximum theoretical density".  That's fine, but is he comparing apples to oranges?  My reply to you stated an "average density" of a typical neutron star.  The two values differ by about 3 times.  So his 10^10 tonnes is my 3.3 x 10^9 tonnes.  And since the equation of state is not well defined (something I agree with) and you happen to agree with his cited University of Chicago 1991 estimate, you would get one billion tonnes (the same as the NASA estimate).

So one problem is the estimate of density itself.  Are we talking "maximum" or average?  Both can vary, depending on the equation of state chosen.

But we may even be talking about central density.  That may be the reason the Canadians' estimate is 50 times greater.  I don't know for sure.

Another problem is the word "teaspoon".  That can be a little ambiguous, since it's a measurement not often used.  I would have guessed a teaspoon was "about" one cubic centimeter, instead of the correct five cubic centimeters.  Easy to make a mistake there!

So I think a major problem is just ensuring you're comparing the same things, and then knowing there's some difference even in modern estimates (since the equation of state isn't well known).  I'll have to add, though, that the theory has been checked through a handful of eclipsing binary neutron stars, in which both the size and mass can be measured fairly precisely.  So we know we're not far off!

Finally, there's just the plan old typographical errors or web sites which do not sufficiently check their facts.  That's the reason to be wary of what you read on the web (or even in some textbooks!).  Many years ago, when encyclopedias were printed and not published on the web, I sat down and went through the entries which I knew something about.  I found more than one hundred errors.  I wrote the publisher, but he wasn't interested in my findings! Enough said.

So the web can be a great source of information, but, if possible, check the facts and use multiple sources.  That's a reminder for us all!

Prof. James Gort