Editing Philosophy of physics (section)

==Philosophy of space and time==
{{Main|Philosophy of space and time}}

The existence and nature of space and time (or space-time) are central topics in the philosophy of physics.<ref name="MaudlinSTa">{{cite book|last1=Maudlin|first1=Tim|title=Philosophy of Physics: Space and Time|date=2012|publisher=Princeton University Press|isbn=978-0691143095|page=xi|url=https://books.google.com/books?id=siyNQV8F0y8C&q=central+topic|access-date=3 October 2017|language=en|quote=...the existence and nature of space and time (or space-time) is a central topic.}}</ref> Issues include (1) whether space and time are fundamental or emergent, and (2) how space and time are operationally different from one another.

===Time===
{{Main| Time in physics}}
[[File:Wooden hourglass.jpg|thumb|right|Time, in many philosophies, is seen as change.]]
In classical mechanics, [[time]] is taken to be a [[fundamental quantity]] (that is, a quantity which cannot be defined in terms of other quantities). However, certain theories such as [[loop quantum gravity]] claim that spacetime is emergent. As [[Carlo Rovelli]], one of the founders of loop quantum gravity, has said: "No more fields on spacetime: just fields on fields".<ref>Rovelli, C. (2004). Quantum Gravity. Cambridge Monographs on Mathematical Physics. p. 71.</ref> Time is [[define]]d 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 structure|hyperfine]] transition in the 133 [[caesium]] [[atom]]. ([[ISO 31-1]]). What time is and how it works [[Logical consequence|follows from]] the above definition. Time then can be combined mathematically with the fundamental quantities of [[space]] and [[mass]] to define concepts such as [[velocity]], [[momentum]], [[energy]], and [[Field (physics)|fields]].

Both [[Isaac Newton]] and [[Galileo|Galileo Galilei]],<ref>[[Roger Penrose]], 2004. ''[[The Road to Reality]]: A Complete Guide to the Laws of the Universe''. London: Jonathan Cape. {{ISBN|0-224-04447-8}} (hardcover), 0-09-944068-7 (paperback).
</ref> as well as most people up until the 20th century, thought that time was the same for everyone everywhere.<ref>{{Citation |last=Rynasiewicz |first=Robert |title=Newton's Views on Space, Time, and Motion |date=2022 |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |url=https://plato.stanford.edu/archives/spr2022/entries/newton-stm/ |access-date=2024-07-28 |edition=Spring 2022 |publisher=Metaphysics Research Lab, Stanford University}}</ref> The modern conception of time is based on [[Albert Einstein]]'s theory of relativity and [[Hermann Minkowski]]'s [[spacetime]], in which rates of time run differently in different inertial frames of reference, and [[space]] and time are merged into [[spacetime]]. 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 [[Age of the universe|13.8 billion years ago]] in the [[Big Bang]]. Einstein's theory of special relativity mostly (though not universally) made theories of time where there is something metaphysically special about the present seem much less plausible, as the reference-frame-dependence of time seems to not allow the idea of a privileged present moment.

===Space===
{{Main|Space}}

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 metre or simply metre, is defined as the [[speed of light|distance traveled by light in a vacuum]] during a time interval of 1/299792458 of a second (exact).

In [[classical physics]], space is a three-dimensional [[Euclidean space]] where any position can be described using three [[coordinate system|coordinate]]s and parameterised by time.  Special and general relativity use four-dimensional [[spacetime]] rather than three-dimensional space; and currently there are many speculative theories which use more than three spatial dimensions.