At the beginning of the 20th century, Albert Einstein (1879-1955) proposed a series of connected theories on relativity, including special relativity and general relativity. These theories revolutionized the field of physics and are some of the most influential and celebrated developments in modern science. Alongside quantum mechanics, the theories of relativity are central to modern physics.

Einstein published his theory of special relativity in several papers in 1905. This theory concerns the relationship between space and time, describing characteristics of light that contradict everyday experience but have been supported by countless experiments on subatomic particles and fast-moving objects. The famous E=MC2 equation summarizes the major principles of this theory, which states that energy is equal to mass multiplied by the speed of light squared. Einstein showed that the speed of light is a constant, and that mass and energy are different forms of the same thing. As a moving object’s speed increases, the passage of time affecting the object slows, and the object’s mass increases. These revelations pertain to many of the futuristic technologies that Kaku discusses in Physics of the Impossible, particularly faster-than-light travel (Chapter 11). Einstein published his theory of general relativity in a series of papers in 1915. General relativity relates to gravity, one of the four fundamental forces of the universe that affects phenomena on a macro scale. This theory describes gravity as a geometric phenomenon arising from curvature in the fabric of space-time. Countless observations of the cosmos have experimentally verified this theory.

A single theory to unify the four fundamental forces of the universe (gravity, electromagnetism, strong nuclear force, and weak nuclear force), providing a framework that includes all the known laws of physics, remains undiscovered, though many great scientific minds throughout history have attempted to develop a convincing model. One of the most significant individuals to bring the concept of a theory of everything into the public consciousness was Einstein, who died before completing his attempt to unify theories of relativity with quantum mechanics. Reports on Einstein’s last unfinished theory inspired Kaku to pursue a career in science, and one of his primary professional goals as a physicist is to contribute to the completion of a theory of everything through his work on string field theory. A theory of everything would provide unparalleled insight into the fundamental truths and characteristics of the universe, and allow physicists to make accurate predictions about the boundaries of possibility.

The field of physics that studies energy, entropy, and temperature is thermodynamics. It was developed alongside and partly due to the quest for a perpetual motion machine (Chapter 14). Four laws define the parameters and characteristics of thermodynamic systems. The first law is that energy cannot be created or destroyed. The second law is that total entropy (disorder) always increases. The third law is that entropy in a system reaches a constant value as the system approaches absolute zero. The zeroth law was added later, but it is the foundation of the other three laws and defines thermal equilibrium. Thermodynamics is key to understanding how energy and matter interact and thus is relevant to many different branches of science.