Time by itself does not exist. It must not be claimed that anyone can sense time apart from the movement of things. ~ Roman philosopher Lucretius
What is time? If nobody asks me, I know; but if I were desirous to explain it to one that should ask me, plainly I know not. ~ Augustine of Hippo
Classical physics regarded time in the way we are familiar: a vector of past, present, and future; subject to subjective interpretation, but ultimately an objective metric.
Absolute, true and mathematical time, of itself, and from its own nature, flows equably without regard to anything external, and by another name is called duration. ~ Isaac Newton
Near the end of the 19th century, Austrian physicist Ludwig Boltzmann assaulted the temporal vector, arguing that time had no built-in arrow; but that its application in space gave time meaning, as entropy was irreversible.
The only reason for time is so that everything doesn’t happen at once. ~ Albert Einstein
Temporality lost its absoluteness with special relativity, which rendered time as relative to a frame of reference. 2 events may occur simultaneously or sequentially, depending upon point of view.
The past, present and future are only illusions, even if stubborn ones. ~ Albert Einstein
Under special relativity, causality is a point of view. The implication is that time, as a matter of perspective, is only an apperception.
Equivalently, time has no meaning in the quantum realm, nor does causality. The world simply incessantly is. Existence itself as an emergent phenomenon: spacetime itself coming into being. Continuity is a perspective, not a phenomenon.
General relativity posits that gravity warps spacetime: not just space, and not just time. Under general relativity, time stops upon entry into a black hole, and space ceases to exist. A singularity of infinite mass collapses space upon itself and squeezes time to a standstill.
Whereas space comprises equivalent dimensions that must be unified for manifestation, time stands alone. Relativity’s twining of spacetime goes only to gravity’s effect: inferring elasticity in time as a geometric fluid medium.
Heisenberg’s uncertainty principle posits an inherently nondeterministic reality through time. Under uncertainty, space and time are in a flux: where and when are probabilistic.
Everything in the future is a wave. Everything in the past is a particle. ~ English physicist William Lawrence Bragg
The only particle of time is the present moment, when wavelike interconnections in space have crystallized. Nonlocal simultaneity simply is; its recognition merely a perception by a mind insisting upon continuity.
Nothing in known physics corresponds to the passage of time. ~ English physicist Paul Davies
Between past and future, the present is an instantaneous confluence; the only moment of existence. Despite having no accounting for its passage, physics depends entirely upon time.
You could make different choices of what you mean by time and get any laws of physics you like. ~ American theoretical physicist Andreas Albrecht
As the potentate of entropy, gravity plays with time. There is more to it than that. Spacetime and gravity define each other.
Many physicists believe that on the very smallest scale of size and duration, space and time might lose their separate identities. ~ Paul Davies
At the quantum level, gravity is assumed to exist in granular form. Similarly, time is simply taken for granted as having tiny steps. If the measures become too tiny, time simply stops. This is the Zeno effect.