Barney Calhoun is a character in the Half-Life series, in Half-Life 1 he was a security guard for the Black Mesa facility. In Half-Life 2, he is working as an inside man for Doctors Isaac Kleiner and Eli Vance, spying on the Combine.

No it is not. It requires one of the games in the half life 2 series such as TF2 portal or half life 2.

Gordon Freeman

Half life one is more arcadish and was the first in the series, although Half Life 2 is much better. Truly it is if you want to go by the story line or just play because of the fun

If you wanna play for fun go with 2, if you wanna play with a good storyline go with 1 first and then 2

A half-life is the time taken for the radioactivity of a material to fall to half its original value. A material can undergo infinite half-lives because each time it falls to half the next half-life falls to half of that half:

No half-lives have elapsed when radioactivity is at the original amount; 1/1.

1 half-life is when radioactivity is at 1/2

2 half-lives is when radioactivity is at 1/4.

3 half-lives is when radioactivity is at 1/8.

4 half-lives is when radioactivity is at 1/16.

And so on.