Here is a list of important unsolved problems organized into different areas of physics.
Some of these problems are based on ideas or theories, meaning current scientific explanations cannot fully account for certain observations or results from experiments. Others involve challenges in designing experiments to test proposed ideas or to study specific events in more detail.
Important questions remain unanswered in physics beyond the Standard Model, including the strong CP problem, finding the exact mass of neutrinos, explaining why there is more matter than antimatter in the universe, and discovering the true nature of dark matter and dark energy.
A major issue also exists within the mathematical structure of the Standard Model itself, which does not work well with general relativity. This mismatch causes both theories to fail in extreme situations, such as at the very beginning of the universe during the Big Bang or at the centers of black holes beyond their event horizons.
Historical problems and lists
Isaac Newton wrote down 31 questions about light in his book Opticks, which was first published in 1704.
In 1900, Lord Kelvin talked about two major problems in classical physics that were called "two clouds" because they confused scientists in the 20th century. These problems were: the lack of proof that a substance called the luminiferous aether existed, and the failure of the equipartition theorem to correctly predict how much heat solids can hold. These issues were solved within 10 years with the creation of special relativity and quantum mechanics.
In 1984, mathematician Barry Simon suggested a list of 15 problems about quantum operators. Around 5 of these were solved by 2000. A second list of 15 problems was shared in 2000, and at least 3 of these were solved.
Nobel Prize winner Vitaly Ginzburg gathered a list of open physics problems from 1971 to 2005. This list, called "the verge of the 21st century," included 30 problems. Ginzburg also listed three important questions: the problem of the arrow of time, the interpretations of quantum mechanics, and how life might arise from physics alone.
At the Strings 2000 conference, scientists proposed a list of 10 problems in theoretical physics. As of 2025, none of these have been solved.
Some problems from mathematical physics are part of famous lists of unsolved math problems. These include: