Physics 1910W Freshman Seminar Fall 2017 What is Time? M,W 10:10-11:00 pm, Tate B Instructor: J W Halley Tate 130-15 612 624 0395 woods@woods1.spa.umn.edu Course web site: http://www.physics.umn.edu/classes/2016/fall/Phys%201910W.001/index.html Please use this email address: halle001@umn.edu

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.

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?

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?

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 10000 years old.

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.

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.

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.

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.

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.