1. Physics 1910W Freshman Seminar Fall 2017 What is Time?
M,W 10:10-11:00 pm, Tate B
Instructor:
J W Halley
Tate
Course web site:
Please use this address:

2. What is time? Some possible answers:
Time is what clocks measure.
But what is a clock? .
You can't say it is something that measures
time. That would be circular.
We will see that clocks are made using
apparently regular repeating processes
occurring in nature. That fact gives an
idea of our implicit definition of time.

3. What is time? Some possible answers:
Time is the way we order and label events
in the past and predict events in
the future.
But does this mean that time is just
a human labelling mechanism?
Also, what do we mean by the
past? Is an event which occurred
5 minutes ago in the past? 5 seconds
ago? 5 millionths of a second ago?

4. Some ideas about the nature of the past:
It is just as real as the present.
The past is inferred from physical evidence in
the present (in our brains, in books and
computers, in the geological record, in
the stars).
Psychologically, it may not feel as real. It
'fades'. Some philosophers say it isn't real
at all.
This would mean that we build
up a picture of the past using a theoretical
model together with present evidence.
Does this picture represent something real?

5. The past is fixed and we can't change it.
This may be true. The models described in the last
paragraph usually assume so. But the human
account of the past can change dramatically:
300 years ago people thought the universe
was about years old.

6. Is the past fixed?
As noted, scientific accounts of the past change
with new evidence and new theoretical models.
Human history is notoriously biased by
political, religious and philosophical views of
its authors and by incomplete evidence.
Human mythology, now mainly represented by
religions, is constructed by a cumulative social
process determined more by the comforting or
morally instructive nature of the resulting stories
than by a desire for accuracy.
Personal past stories are, consciously or unconsciously
constructed partly from evidence and partly
by a desire to define a 'self' without a primary
emphasis on accuracy.

7. How might we try to establish that a definite series
of past events occurred?
One way is consistency of evidence:
One kind of consistency is just the consistency
of different observers:
If an accident occurs and
different witnesses tell the same story about what
happened, we think that it is likely that the story
describes a real event. (As a matter of fact,
witnesses almost never all tell the same story.)
In science, this requirement is more formal: Observations
, either of natural phenomena or in humanly arranged
experiments, are not regarded as reliably describing
a real event unless several observers report the same
thing.

8. If we build up knowledge of the past from
accumulated evidence in the present, keeping
consistent evidence and throwing out inconsistent
evidence then
The picture of the past will keep changing.
we will never definitely know the past (if indeed
there is a unique past).
We can say that the existence of a definite,
fixed past is an assumption, for which a
consistent picture provides a lot of evidence,
but not a definitely proven fact.

9. Is the future open?
It is a common view, at least in the US, that it is.
'Anything is possible.'
But science suggests that the future is constrained
by the present and past:
Newtonian physics: If the present and immediate
past were known with sufficient accuracy, the
future would be absolutely determined with
no openness at all. So if the real world were ruled
by Newtonian physics, the future would be
closed.
Would that mean that we could predict the future?
Not necessarily: The Newtonian models are
'chaotic' meaning that you would have to know
the present conditions with infinite accuracy
to predict the future.

10. We will address these and related questions in
the course as follows:
How we measure time. Clocks
Newtonian and human time. The difference
between what classical physics assumes
about time and the way humans and other
animals perceive it and how they are
related.
The science of heat (thermodynamics) and
time. It turns out that they are closely related.
The science of small objects (quantum
physics) and time. New complications come in,
The physics of relativity and time. Major revisions
to the Newtonian picture for rapidly moving objects
and in large gravitational fields.