Reading suggestions about the "nature of time"

[Lapidus]

There are quite some pop-sci books (by Greene, Smolin, Carroll and others) that deal with the "nature of time". Why does time appear to flow? Why is there a special moment, the "now"? Does simultaneity in SR imply a block universe? Why time-symmetric laws but a time-unsymmetric universe? Does the past exist, but the future not and why? These kind of questions.

But I could not find any good textbooks or papers that give a more precise summary of what physicists think of time. Or just some pages that sum-up the conundrums associated with time.

Or is that just a too philosophical topic for physics to bother with?

(I stumbled upon Ellis "evolving block universe" ideas, which look interesting. But I could not find out what others think about it.)

 

[PeroK]

When you study physics as an academic or practical subject you are generally concerned with learning about a specific topic. Classical mechanics, quantum mechanics, electromagnetism particle physics, relativity.

Each of these may require an understanding of the nature of time and space. But, generally that's part of the background to learning the actual subject.

There may be philosophy of science textbooks that deal more with these wider, cross discipline questions. You could look for those.

 

[Dale]

But I could not find any good textbooks or papers that give a more precise summary of what physicists think of time.

Time is what a clock measures.

 

[Lapidus]

Time is what a clock measures.

What is a clock? Can you define it without the concept of time?

 

[Nugatory]

What is a clock? Can you define it without the concept of time?

"Time is what clock measures" is a quote from Einstein. It is useful to compare it with the analogous statement about distance: "Distance is what a meter stick measures".

 

[Lapidus]

"Time is what clock measures" is a quote from Einstein. It is useful to compare it with the analogous statement about distance: "Distance is what a meter stick measures".

This I knew, too.

But I wanted to know what a clock is. Can you explain it without time?

 

[Ibix]

A clock is a repeating process (a pendulum), typically with a counter attached.

 

[Dale]

What is a clock? Can you define it without the concept of time?

I can describe plans to build a clock without defining time. But in the end it doesn’t matter too much. All systems of definitions are eventually circular or rely on undefined primitives.

For a physicist time is what a clock measures. A physicist knows what a clock is and can just have the purchasing department order them as needed.

You did want to know what a physicist’s view is, right? Not a philosopher’s.

 

[Lapidus]

A clock is a repeating process (a pendulum), typically with a counter attached.

Repeating? You mean, repeating in time?

 

[Ibix]

Repeating? You mean, repeating in time?

Repeating in whatever it is that it is measuring, yes. If you want to give that "whatever it is" the label "time", then we have an operational definition of the word.

 

[Nugatory]

Repeating? You mean, repeating in time?

Yes, and as Dale points out above, there's no way of avoiding the circularity - every dictionary must either contain undefined terms or circular definitions. The point of the analogy with distance and meter sticks is that although the same circularity is present in "Distance is what a ruler measures", we can use our intuition for what distance is to break the circularity. Clocks and time work the same way - we just have to recognize that the analogy works.

 

[Lapidus]

Thanks so far for your answers.

I am still interested in thoughts about the nature of time from physicists that go beyond the pure operational approach. Again: Does anybody know of papers, talks or chapter of books where trained physicists speculate about what time is (or might be) besides being what clocks measure?

As I said there are many pop-sci books that do that. But is there something more precise out there?

 

[Nugatory]

I am still interested in thoughts about the nature of time from physicists that go beyond the pure operational approach... As I said there are many pop-sci books that do that. But is there something more precise out there?

Precision comes from the operational approach, which is why physicists use it. So the answer to your question is probably "No". However, you may find the geometrical approach to spacetime ("Spacetime Physics" by Taylor and Wheeler is a good start) more satisfying than the coordinate-based approach found in many intro texts.

 

[Dale]

I am still interested in thoughts about the nature of time from physicists that go beyond the pure operational approach.

I don’t think there is any such thing, particularly not with “precision”. You seem to want philosophy and you want it to be precise, and it just isn’t.

We have given you a physicists perspective on time: time is what a clock measures. I don’t understand why you feel that is unacceptable, why you want it to be more complicated than it is, but that is in fact the answer to your question.

 

[George Jones]

I could not find any good textbooks or papers that give a more precise summary of what physicists think of time. Or just some pages that sum-up the conundrums associated with time.

Have you seen the book "The Physical Basis of the Direction of Time" by Zeh?

https://www.amazon.com/dp/3642087604/?tag=pfamazon01-20

Use "Look Inside" to see the table of contents. The short first chapter is "The Physical Concept of Time". Also, you might find that some the references given at the end of this book interest you.

 

[Drakkith]

Does anybody know of papers, talks or chapter of books where trained physicists speculate about what time is (or might be) besides being what clocks measure?

As I said there are many pop-sci books that do that. But is there something more precise out there?

Well, speculative things aren't generally precise, and precise things aren't generally speculative, so you're probably going to be disappointed.

 

[PAllen]

If you want to read speculations about time including from some serious physicists (e.g. Sean Carroll, George Ellis (of Ellis and Hawking fame) ) a good place to start is the following 2008 essay contest:

winners: https://fqxi.org/community/essay/winners/2008.1
all accepted submissions: https://fqxi.org/community/forum/category/10

 

[Lapidus]

Have you seen the book "The Physical Basis of the Direction of Time" by Zeh?

https://www.amazon.com/dp/3642087604/?tag=pfamazon01-20

Use "Look Inside" to see the table of contents. The short first chapter is "The Physical Concept of Time". Also, you might find that some the references given at the end of this book interest you.

That looks very good and might be just what I wanted. I will check it out at my library on Monday!

Thanks George!

 

[Lapidus]

If you want to read speculations about time including from some serious physicists (e.g. Sean Carroll, George Ellis (of Ellis and Hawking fame) ) a good place to start is the following 2008 essay contest:

winners: https://fqxi.org/community/essay/winners/2008.1
all accepted submissions: https://fqxi.org/community/forum/category/10

For some strange reason I have not found this site before. Looks perfect!

Thanks PAllen!

 

[Dale]

Note that all of the fqxi essays are solidly philosophical, definitely not physics.

 

[PAllen]

Note that all of the fqxi essays are solidly philosophical, definitely not physics.

Agreed, but many are philosophy by leading physicists, which is what I guessed the OP is looking for.

 

[haushofer]

Did someone already mention Julian Barbour? :)

 

[PAllen]

Did someone already mention Julian Barbour? :)

First essay in the links I gave earlier.

 

[Lapidus]

I don’t think there is any such thing, particularly not with “precision”. You seem to want philosophy and you want it to be precise, and it just isn’t.

We have given you a physicists perspective on time: time is what a clock measures. I don’t understand why you feel that is unacceptable, why you want it to be more complicated than it is, but that is in fact the answer to your question.

Why do I perceive "now" as a special moment compared to all the other moments in my life, past and present? Which physics equation, concept, theory accounts for one of the most human conscious experience, the perception that time flows, that the past becomes the present, that the present becomes the future?

And mind you, this is not some minor point. The way we perceive time is central of how we understand the world.

Well, at least I am not completely alone with having an uneasy feeling about this.

„Rudolf Carnap recounts a wonderful conversation he had with Einstein on this subject: “Einstein said that the problem of the now worried him seriously. He explained that the experience of the now means something special for man, something essentially different from the past and the future, but that this important difference does not and cannot occur within physics. That this experience cannot be grasped by science seemed to him a matter of painful but inevitable resignation.“

From: Brian Greene. „The Fabric of the Cosmos.“

 

[Dale]

Why do I perceive "now" as a special moment compared to all the other moments in my life, past and present?

That isn’t a physics question, it is a psychology question. To my knowledge we don’t yet have a general theory of psychology that can answer this question.

Personally, I think that the idea that “now” is perceived as special is a fallacy. Every moment you are awake is perceived as “now” just as much as any other moment. When your watch reads 9:00 you feel it is “now” and when your watch reads 9:01 you also feel it is “now” and so on. There is nothing particularly special about any moment in that psychological sensation as it is a sensation that merely indicates that you were awake at that time and does not in fact single out any particular moment.

 

[PAllen]

Let me make an analogy.

Many practicing lawyers could care less about "philosophy of law" (e.g. what might constitute an ideal legal system, what does ideal mean, etc. in this context) because they need to focus on the functioning of the legal system they operate in, and this takes all of their time to do well.

However, many other practicing lawyers are (sometimes highly) interested in such philosophical questions irrespective of practical utility.

The same goes for physicists and "philosophy of physics".

 

[Nugatory]

@Lapidus, we can provide answers but we can't guarantee that you will like them.

As the "time is what a clock measures" discussion above suggests, physics approaches the idea of time starting from the proposition that the "time" is something we measure; physics is about using these measurements to make useful and interesting predictions about how the universe behaves. You don't have to like it that nothing in physics will give you an answer that satisfies you... but it won't.

This thread is closed because it's reached the point of diminishing returns. As with all thread closings, if anyone has more that needs to be said PM any mentor to ask that it be reopened for your contribution.

 

[robphy]

Here's something that may be of interest
https://cla.umn.edu/mcps/publications/minnesota-studies-philosophy-science8. Foundations of Space-Time Theories. 1977. Editors: J.S. Earman, C.N. Glymour, and J.J. Stachel.
Table of Contents. Open Access (all chapters).

Foundations of Space-Time Theories
Volume 8: Minnesota Studies in Philosophy of Science

Some Philosophical Prehistory of General Relativity Howard Stein ...3
Indistinguishable Space-Times and the Fundamental Group Clark Glymour ...50
Observationally Indistinguishable Spacetimes: Comments on Glymour's Paper David Malament ...61
Prediction in General Relativity Robert Geroch ...81
Time in General Relativity C. J. S. Clarke ...94
Till the End of Time John Earman ...109
The Causal Theory of Space-Time John A. Winnie ...134
Facts, Conventions, and Assumptions in the Theory of Space-Time Lawrence Sklar ...206
Geometry and Observables Peter G. Bergmann ...275
The Curvature of Physical Space Wesley C. Salmon ...281
Absolute and Relational Theories of Space and Space-Time Adolf Grünbaum ...303
On Space-Time Ontology: Extracts from a Letter to Adolf Grünbaum Howard Stein ...37
Simultaneity in Newtonian Mechanics and Special Relativity Michael Friedman ...403
On Conventionality and Simultaneity—Another Reply Peter A. Bowman ...433
Indexes ...451