You're writing a dissertation. As the dissertation's copyright holder, you now have exclusive rights to reproduce copies of it, prepare journal article or book adaptations from it, distribute copies of it on the Web, read the dissertation aloud to a public paying audience (!!!), and even make posters of each page of text and display it if you wanted to.
The exclusive rights are not indefinite, however, because indefinite protection would quell the very progress that copyright law was meant to protect. So, copyright protection expires. The length of protection is determined by various factors. In general, however, you can expect that the length of copyright protection is at least the author's life + 70 years from the author's death.
Not everything that authors create is subject to protection under the Copyright Act, though. Copyright protects only expression, not ideas or facts. If you are writing a dissertation and incorporating population statistics, you need to cite the statistics' source by professional standards, but you don't need to ask permission to use them because facts are not subject to copyright protection to begin with. Note the distinction between using the statistics vs. using the language with which an author describes them: The author can have copyright in his/her particular expression, but not the underlying facts, themselves.
A Copyright Office circular offers additional examples and explanations of exclusions from copyright protection. Note that while someone can't hold copyright in short phrases and titles, or in ideas and methods, someone might be able to get trademark or patent protection, respectively.
Copyright protects "original works of authorship" that are fixed in a tangible form of expression. Note that being "fixed" implies that it has been captured in something that is either human or machine or device-readable--for example, if you've written it on paper or stored it on a computer hard drive.
Lastly, before we turn to permission questions, it's important to know that once you create an original work of authorship that is fixed--like your dissertation--copyright immediately attaches. You are the copyright owner, unless you entered into a funding or employment arrangement that vests copyright ownership in someone else. The UC page on student copyright ownership explains more.
Moreover, you do not need to register your copyright with the U.S. Copyright Office to hold copyright; copyright attaches automatically. There are certain benefits of registering your copyright, however, identified in Step 4's discussion of Copyright Registration.