For a thousand years in thy sight are but as yesterday when it is past, and as a watch in the night.
Psalm 90:4
".....Rovelli turned next to the idea that time flows in only one direction,
from past to future. 
Unlike general relativity, quantum mechanics, and
particle physics, thermodynamics embeds a direction of time.
Its second
law states that the total entropy, or disorder, in an isolated system
never decreases over time.
Yet this doesn’t mean that our conventional
notion of time is on any firmer grounding, Rovelli said. Entropy, or
disorder, is subjective: “Order is in the eye of the person who looks.”
The equations for quantum gravity he’s written down suggest three
things, he said, about what “clocks measure.”
First, there’s a minimal
amount of time—its units are not
infinitely small.
Second, since a clock, like every object, is quantum, it can be in a
superposition of time readings. “You cannot say between this event and
this event is a certain amount of time, because, as always in quantum
mechanics, there could be a probability distribution of time passing.”
Which means that, third, in quantum gravity, you can have “a local
notion of a sequence of events, which is a minimal notion of time, and
that’s the only thing that remains,” Rovelli said.
Events aren’t ordered
in a line “but are confused and connected” to each other without “a
preferred time variable—anything can work as a variable.”
Light traces a cone, or consecutively larger circles, in
four-dimensional spacetime like ripples on a pond that grow larger as
they travel. No information can cross the bounds of the light cone
because that would require information to travel faster than the speed
of light.
“In spacetime, the past is whatever is inside our past light-cone,”
Rovelli said, ... “So it’s whatever can affect us. The future is this opposite thing,” ... “So in between the past and the
future, there isn’t just a single line—there’s a huge amount of time.” epocket
