Physics is the only science that explicitly studies time, but even physicists consider time to be one of the most incomprehensible properties of our universe. Even in the most modern and complex models of physics, time is often considered an axiomatic concept, not composed of or dependent on anything else. In science, time is often defined by measurement: it's just the reading of a clock. Physics in particular often requires extreme precision in the measurement of time, which requires time to be considered as an infinitely divisible linear continuum, rather than quantized (i.e. composed of discrete and indivisible units). According to modern atomic time standards, such as TAI and UTC and ultra-precise atomic clocks, time can now be measured precisely by about 10^-15 seconds, which is equivalent to an error of 1 second in about 30 million years. In non-relativistic or classical physics, the concept of time is usually that of absolute time, which is independent of any perceiver's time, develops at a consistent rate everywhere in the universe, and is inherently imperceptible . This is in line with most people's daily experience of the flow of time. However, since the advent of relativity in the early 20th century, it has taken into account phenomena such as time dilation for fast-moving objects, gravitational time dilation for capturing objects in extreme gravitational fields, and the important idea that time is actually just one element of four-dimensional spacetime . The theory of relativity also allows, at least in theory, time travel. There are several scenarios that allow for a theoretical basis for time travel. However, the concept of time travel also brings some paradoxes, and its possibility and physical utility have been questioned by many physicists. Quantum mechanics revolutionized physics in the first half of the 20th century, arguing that time may not be as central to quantum theory as it was in classical physics, and that there is actually no such thing as "quantum time". However, different interpretations of quantum theory (such as the Copenhagen interpretation, the many-worlds interpretation, etc.) do have some potentially important implications for our understanding of time. Most physicists agree that time has a beginning, measured from the Big Bang 138 billion years ago. Whether, how, and when time will end in the future is a more open question, depending on different concepts of the ultimate fate of the universe and other puzzling concepts such as the multiverse