Philosophy of space and time

Time
Main article: Time in physics

Time, in many philosophies, is seen as change.
Time is a fundamental quantity (that is, a quantity which cannot be defined in terms of other quantities, because at present we don't know anything more basic than time). Thus time is defined via measurement - by its standard time interval. Currently, the standard time interval (called "conventional second," or simply "second") is defined as 9,192,631,770 oscillations of a hyperfine transition in the 133 caesium atom. (ISO 31-1). What exactly time "is" and how it works follows from the above definition. Physicists use theory to predict how time is measured. Time then can be combined mathematically with the fundamental quantities of space and mass to derive concepts such as velocity, momentum, energy, and fields.
Both Newton and Galileo,[1] as well as most people up until the 20th century, thought that time was the same for everyone everywhere. Our modern conception of time is based on Einstein's theory of relativity and Hermann Minkowski's spacetime, in which rates of time at separate places run differently, and space and time are merged into spacetime. Time may be quantized, with the theoretical smallest time being the Planck time. Einstein's general relativity as well as the redshift of the light from receding distant galaxies indicate that the entire Universe and possibly space-time itself began about 13.7 billion years ago in the big bang. Whether and how the universe will ever end are open questions.
[edit] Time travel
Main article: Time travel
Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future. Concepts that aid such understanding include the closed timelike curve.
Albert Einstein's special theory of relativity (and, by extension, the general theory) predicts time dilation that could be interpreted as time travel. The theory states that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies: for example, a moving clock will appear to run slow; as a clock approaches the speed of light its hands will appear to nearly stop moving. The effects of this sort of time dilation are discussed further in the popular "twin paradox".
A second, similar type of time travel is permitted by general relativity. In this type a distant observer sees time passing more slowly for a clock at the bottom of a deep gravity well, and a clock lowered into a deep gravity well and pulled back up will indicate that less time has passed compared to a stationary clock that stayed with the distant observer.
These effects are to some degree similar to hibernation, or cooling of live objects (which slow down the rates of chemical processes in the subject) almost indefinitely suspending their life thus resulting in "time travel" toward the future, but never backward. They do not violate causality. This is not typical of the "time travel" featured in science fiction (where causality is violated at will), and there is little doubt surrounding its existence. "Time travel" will hereafter refer to travel with some degree of freedom into the past or future of proper time.
Many in the scientific community believe that time travel is highly unlikely, because it violates causality - logic of cause-effect sequence. What happens if you try to go back in time and kill yourself (or your grandfather, leading to the grandfather paradox)? Also, there are no experimental evidences of time travel. Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel— a variant of the Fermi paradox, with time travelers instead of alien visitors.
[edit] Space
Main article: Space

Spacetime, according to general relativity, is bent by objects with mass, causing time dilation.
Space is one of the few fundamental quantities in physics, meaning that it cannot be defined via other quantities because there is nothing more fundamental known at present. Thus, similar to the definition of other fundamental quantities (like time and mass), space is defined via measurement. Currently, the standard space interval, called a standard meter or simply meter, is defined as the distance traveled by light in a vacuum during a time interval of 1/299792458 of a second (exact). This definition coupled with the present definition of time (see above) makes our space-time to be Minkowski space and makes special relativity theory to be absolutely correct by definition.
In classical physics, space is a three-dimensional Euclidean space where any position can be described using three coordinates. Special and general relativity uses spacetime rather than space; spacetime is modeled as a four-dimensional space (with the time axis being imaginary in special relativity and real in general relativity, and currently there are many theories which use more than 4-dimensional spaces, both real and complex).
Before Einstein's work on relativistic physics, time and space were viewed as independent dimensions. Einstein's work has shown that due to relativity of motion our space and time can be mathematically combined into one symmetric object - spacetime, in which the time axis (multiplied by ic) is indistinguishable from space axes. (Distances in space or in time separately are not invariant versus Lorentz coordinate transformations, but distances in such so called Minkowski spacetime are - which justifies the name).