Wednesday, September 19, 2007

Notes on Riddles of Existence Ch. 3: Time

First, for more discussion, see:

Time Travel: http://www.iep.utm.edu/t/timetrav.htm

For more advanced discussion, see:

Time Travel & Modern Physics: http://plato.stanford.edu/entries/time-travel-phys/

Some highly practical, concrete questions:

• Could we go back in time? Is this logically possible, or does the very concept of time make this an impossibility/
• Can our actions now cause things in the past to be different?

Obviously, these questions depend on the nature of time! But what is time anyway?

The flow of time (p. 44) [The “time moves” theory of time]

We tend to describe time as “moving” (it flows, marches on, flies, etc.)

But movement is defined in terms of time! (p. 45)

The present moment “moves.”

But then the present is defined in terms of other times. (Later, the present will be 6 PM).

Different times are part of hypertime? If so, then hypertime is part of hypertime2 and on and on!

Perhaps then the present moves with respect to itself. (p. 47).

“Noon is present at noon,” etc. Trivial and doesn’t explain how time “moves.”

The space-time theory (of time)

Time is like space. (p. 49)

· Space-time diagrams. See figures!

· Temporal parts= a temporal cross section of that object; that object at that time. (Compare spatial part to temporal part).

Person = sum of temporal parts.

Reality = a single unified space time (vs. the “time moves” theory). (p. 50)

Time does not flow; it is like space. (But only one direction: past to future)

1. Temporally distant objects are still real: past objects exist (??), i.e., they exist somewhere in spacetime. (p. 51), like spatially distant objects are also real.

2. Objects have temporal parts, like spatial objects have spatial parts.

3. Claims about “here” and “now” depend on who is saying them, where she is (in space and time) and what is said.

Arguments against the space-time theory: change, motion, causes (p. 52)

Does time flow? Or is like space?

Space-time doesn’t have the problems (paradoxes?) of flow theory. But there are objections to the space-time theory.

In opposition to (1)-(3) above, one might claim that (p. 53)

· past and future objects do not exist (in spacetime?);

· things do not have temporal parts; at any time the whole object is present,

· “now” is not like “here”: the present moment is special, unlike “here.”

Sider says he’ll skip these objections! (“Time is short!”)

Other objections that time is not like space (p. 53); disanalogies:

1. Regarding change. “Change” vs. “spatial heterogeneity”

2. Regarding motion: can spatially move in many directions; can only go one direction in time.

3. Regarding causes: an event at any place events can cause other events at any other place, but events can’t cause events at just any other time: later events can’t cause earlier events. The past isfixed.

Replies to these objections, these claims about disanalogy (p. 54).

· To objection 2, see 54-57 for the details. How’s this work?

· To objection 3 “most challenging and interesting”, see p. 57.

§ Could there be backwards causation? Could we cause ourselves to be present in the past (i.e., travel through time?). Is this logically possible?

· If time is like space, then perhaps!

Time Travel: (p. 58)

“Back to the Future”

First, McFly pushes the button in 1985 and then he is in 1955.”

· Story is contradictory. If contradictory, then impossible. Can a time travel story be told without contradiction?

o Eliminate that contradiction by appealing to McFly’s experience.

· Could McFly prevent himself from ever existing? (Another worry: Time travel to kill earlier self?!)