I want to cite a paper that has a lot of authors. The list of authors take up half a page in my reference list, and considering the prospect of paying for extra pages, I would like to shorten it, if possible. Is is okay to shorten the author list? If so, how many authors should be included?
Every citation style has some threshold at which it is OK to change a long list into an "et al." citation ("et al." is an abbreviation of the Latin "et alia" meaning "and others"). The particular threshold depends on the citation style, but if you've got half a page of authors it's certainly over any reasonable threshold.
You then simply convert your citation of, e.g.:
A, B, C, E, ... , Z, "Massive Joint Study," Journal of Big Science, 1(4), 2012.
A, et al., "Massive Joint Study," Journal of Big Science, 1(4), 2012.
As mentioned by @jakebeal, the correct approach is dependent on your citation style. Here is an example of 6th Edition APA Style.
Quoting the following post from the official APA style blog, the following rule applies to articles with more than 7 authors:
The first six authors are listed; all subsequent authors except the last are omitted and replaced with an ellipsis; and then the name of the last author is listed. Here’s an example of the new reference list style, for a study with 87 authors (yes, 87!):
Terracciano, A., Abdel-Khalek, A. M., Adam, N., Adamovova, L., Ahn, C., Ahn, H., . . . McCrae, R. R. (2005, October 7). National character does not reflect mean personality trait levels in 49 cultures. Science, 310, 96–100. doi:10.1126/science.1117199
You should follow a style guide. If your editor, instructor, etc. didn't tell you what style to use, pick one. In this case pick one that doesn't insist on full author lists.
When in doubt I use the Physical Review style. On page 7 of the linked guide you'll find the example for a listing a large collaboration of authors:
J. M. Smith et al. (XYZ Collaboration), Phys. Rev. D 46, 1 (1992).
For the paper you link it would be:
B. P. Abbott et al. (The LIGO Scientific Collaboration), Rep. Prog. Phys. 72 (2009).