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How is one supposed to work with physical constants in Mathematica V9? When I first open Mathematica, physical constants such as SpeedOfLight are not recognized. If I load the PhysicalConstants package using:

<< PhysicalConstants`

I get the following error message:

General::obspkg: "!(\"PhysicalConstants`\") is now obsolete. The legacy version being loaded may conflict with current Mathematica functionality. "

However, at this point SpeedOfLight is properly defined!

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Welcome to Mathematica.SE! I suggest the following: 1) As you receive help, try to give it too, by answering questions in your area of expertise. 2) Read the faq! 3) When you see good questions and answers, vote them up by clicking the gray triangles, because the credibility of the system is based on the reputation gained by users sharing their knowledge. Also, please remember to accept the answer, if any, that solves your problem, by clicking the checkmark sign!. – DavidC Jan 18 '13 at 15:25
@dude seen this meta:… ? – Sjoerd C. de Vries Jan 18 '13 at 16:36
@Sjoerd Yes. By coincidence, I tried to revert to my former and actual name, David Carraher, this morning, but I'll have to wait till Feb. 1. (Users can only change their names once every 30 days.) For now, it looks like I'm trapped in my pseudonym. :) – DavidC Jan 18 '13 at 16:44
@dude the site moderators can change this for you. I changed to 2013 for the new year without realizing the 30 day limit but rm kindly changed me back. – Mike Honeychurch Jan 18 '13 at 21:32
@MikeHoneychurch Sounds good. How do I contact a monitor? (I really don't know). – DavidC Jan 18 '13 at 22:22

In v. 9, speed of light is a quantity. Try


The output will look like the following:

1 c

However, its FullForm is actually

Quantity[1, "SpeedOfLight"]

You will find addition information about how to use physical constants as quantities in the Compatibility Tutorial and in the Units Overview.

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