Philosophy of Time

a-theory (dynamic)

time objectively passes, but only the present is real

b-theory (static)

time does not flow; all moments are equally real

presentism

a-theroy, only the present exists

growing block (possibilism)

variant of a-theroy, the past and present exist

shrinking block

variant of a-theory, only the present and future exis

moving spotlight

variant of both a and b theories, whole block is real; spotlight of “now” moves

argument against presentism

1. Presentism says only present events exist

2. The present events are simultaneous with the here and now

3. But simultaneity is relative (Einstein)

4. So, “what is present” depends on your reference frame

5. Existence can’t depend on your perspective

6. Therefore, presentism is false

special relativity

1. the laws of physics are the same for all inertial observers

2. The speed of light is constant

Einstein’s train though experiment

1. Bob (middle of the train): Light flashes from both ends reach him at once —> simultaneous

2. Al (outside observer): Train is moving —> light from the front travels farther —> not simultaneous

3. Conclusion: Simultaneity is relative to the observer’s motion

inertial reference frame

a system in which objects are moving at a constant velocity

Minkowski spacetime

4D manifold of events

events

points in time

worldlines

object’s history through time

Godel spacetime

the entire universe rotates, light cone tip allowing return to the past, any point can be reached again

wormholes

tunnels connecting two spacetime points

personal time

traveler’s own clock

external clock

background of the universe

time travel

a mismatch between personal and external time

grandfather paradox

the idea that you can’t change history by David Lewis

closed timelike curves

loops in space time