"The public domain is the realm of material — ideas, images, sounds, discoveries, facts, texts — that is unprotected by intellectual property rights and free for all to use or build upon." [1]
In truth, the public domain is essential to a healthy copyright system. Invention is recreation and everything is a remix, as Kirby Ferguson maintains in his fascinating video essay series at https://www.everythingisaremix.info/watch-the-series. A robust public domain provides the raw material by which artists assemble, transform, and otherwise produce new variations on old themes, and it works in tandem with copyright to ensure that those creators enjoy a time-limited monopoly on their creative expression and can make a living from their efforts.
[1] Excerpted from Public Domain Day 2022 (https://web.law.duke.edu/cspd/publicdomainday/2022/faqs/) by Duke Law School’s Center for the Study of the Public Domain, licensed under a Creative Commons Attribution 3.0 Unported License (http://creativecommons.org/licenses/by/3.0/).
Many things are always in the public domain under US copyright law, which only protects the original expression in creative works: Ideas, facts and unoriginal compilations of facts (like phone books), genres, titles, names, short phrases, slogans, lists of ingredients, plotlines, themes, basic chord progressions, scientific principles, theories, formulae and laws of nature, and utilitarian objects are all automatically in the public domain, as is anything published by the US Government. [1]
But much that's culturally relevant is locked behind copyright, largely unavailable to the cultural commons. In the United States, the basic copyright in most materials remains in force until 70 years after the death of the creator, or 95 years after publication (or 120 years from creation, whichever expires first) for works owned by corporations, which means that material published from 1928 (95 years ago) on is not in the public domain unless:
[1] Excerpted from Public Domain Day 2022 (https://web.law.duke.edu/cspd/publicdomainday/2022/faqs/) by Duke Law School’s Center for the Study of the Public Domain, licensed under a Creative Commons Attribution 3.0 Unported License (http://creativecommons.org/licenses/by/3.0/).
[2] Derived in part from Copyright Services: Copyright Term and the Public Domain (https://guides.library.cornell.edu/copyright/publicdomain) © 2004-2022 Peter B. Hirtle. Last updated 4 January, 2022. Use of this chart is governed by the Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/legalcode)
If you were born after the Beatles broke up, almost nothing published in your lifetime will enter the public domain in the US before you die unless the copyright holder specifically releases it, as Tom Lehrer did with all of his music and lyrics earlier this year (although sound recordings and architectural works have a different copyright structure and term from other published works). Lehrer's tactics (he created a trust to inherit his copyright and directed that 'All the lyrics herein should be treated as though they were in the public domain') illustrate the difficulty of prematurely placing works in the public domain. "There are a lot of conflicting theories about how and if one might dedicate a work to the public domain, but one of the problems is that any such dedication could be seen as an 'unenforceable promise' and one that could be revoked at any time."[3]
[3] Bailey, Jonathan. October 21, 2020. Tom Lehrer: The Public Domain Tango: A Copyright Carol.... Plagiarism Today. Accessed December 21, 2022. https://www.plagiarismtoday.com/2020/10/21/tom-lehrer-the-public-domain-tango/