The nature of time. Which books to read?

[Robert100]

There is a discussion on the nature of time in the General Physics forum, originally titled "What is time?" I know that physicists have no firm conclusions on this issue, but I want to read about the various views that are taken seriously by physicists. Can anyone comment on the books/authors mentioned here in this discussion? (See post #42.)

https://www.physicsforums.com/showthread.php?t=177858&page=3

(I believe that replies should go in that forum, and not here, to avoid cross-posting.)

Thanks,

Robert

 

[pmb_phy]

There is a discussion on the nature of time in the General Physics forum, originally titled "What is time?" I know that physicists have no firm conclusions on this issue, but I want to read about the various views that are taken seriously by physicists. Can anyone comment on the books/authors mentioned here in this discussion? (See post #42.)

https://www.physicsforums.com/showthread.php?t=177858&page=3

(I believe that replies should go in that forum, and not here, to avoid cross-posting.)

Thanks,

Robert

A very well known and well respected book on this topic is called

The Philosophy of Space & Time, Hans Reichenbach, Dover Pub

I have another book I'm anxious to get around to reading but I can't find it right now and I forgot the exact name, something like The Nature of Time which is a collections of essays.

Pete

 

[Robert100]

Someone reccomended "A Brief History of Time", by Stephen Hawking, but this book is a mile wide, yet not deep. When it comes to discussing the nature of time itself, Hawking shows little knowledge of the great debates about this subject. He doesn't mention any of the mainstream ideas about what time is, or how we should understand it.

http://en.wikipedia.org/wiki/The_Unreality_of_Time

http://en.wikipedia.org/wiki/B-Theory_of_time

http://en.wikipedia.org/wiki/Presentism_(philosophy_of_time)

http://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)

http://plato.stanford.edu/entries/time-experience/

http://plato.stanford.edu/entries/change/


I am distressed by the way that many physicists seem to have done little or no research on the very subjects that they write about. Even if a physicist somehow has come to disagree with the views of (apparently) every single philosopher, and all of their varied views, shouldn't the physicist at least briefly explain why?

I'd like someone to point me to a book or article written by a physicist that is at least conversant with the subject, or written by a philosopher who is conversant with modern physics.

Robert

 

[George Jones]

The Physical Basis of the Direction of Time is a well-known book that has had some good reviews.

 

[pervect]

I am distressed by the way that many physicists seem to have done little or no research on the very subjects that they write about.

Robert

Philosophy is mostly irrelevant to physics. If you really want to talk about the philosophical aspects of time, try the philosophy forum.

The fundamental difference between science and philosophy is that science makes testable predictions. This allows scientific arguments to eventually end, because one can actually go out and make a measurement that settles the question one way or the other.

Purely philosophical questions, though, don't have any way to be tested. Because they can't be tested, they can't be proven wrong. They can't be proven right, either. This implies that philosophical arguments tend to go on forever. It also means that because they can't be tested experimentally, they are not in the domain of science.

[add]
So, in short, as far as science is concerned (i.e not philosophy), the important aspects of time are the aspects that can be measured. This is why "Time is what you measure with a clock" is a good philosophy for physics, it keeps the focus on what can be measured, and it avoids the pitfalls of endless philosophical debate.

 

[lalbatros]

Well I am pretty sure that time does not exist.

Describing a system at a certain time t1 or at another time t2 makes no difference since there is a perfect reversible correspondance between these two descriptions.

Why, then are we losing all our ... to solve the ...-dependent equations of physics?
What do we gain by this difficult exercice?
Nothing, the information is the same.

Where is the flaw?

 

[Cusp]

Tine is defined so that all event don't happen at once. That's all you need.

I agree that most philosophy of science has no baring on science itself. From what I have seen, most of it stems from the philosophers inability to properly understand quantum mechanics.

 

[SpitfireAce]

Yeah, well I think the "why observation leads to wave-function collapse" is an endless philosophical debate, go tell all of them that they are wasting their time

 

[Cusp]

Yeah, well I think the "why observation leads to wave-function collapse" is an endless philosophical debate, go tell all of them that they are wasting their time

Why does there have to be a why? Things interact - all interactions occur at points so all interactions (including observations) collapse wavefunctions.

 

[SpitfireAce]

I was referring to a specific thread, I guess your talking about the environmental decoherence theory... has that been worked out to everyone's satisfaction yet?

 

[russ_watters]

Well I am pretty sure that time does not exist.

Describing a system at a certain time t1 or at another time t2 makes no difference since there is a perfect reversible correspondance between these two description.

Why, then are we losing all our ... to solve the ...-dependent equations of physics?
What do we gain by this difficult exercice?
Nothing, the information is the same.

Where is the flaw?

You are seriously asking what we gain by utilizing the concept of time in physics?

 

[lalbatros]

You are seriously asking what we gain by utilizing the concept of time in physics?

Yes, but this is nearly a paraphrase of the title of this threat.

I can safely say that all fundamental laws of physics are time-reversible.
Then we can -indeed- ask ourself why time does play this big role in our lifes as well as in theoretical physics.
We can ask ourself this question, even without considering the question of irreversibility, unless it would be essential for the answer.

In other words, what are we doing with an "Heisenberg picture" of our world?
The observables are functions of time, but the state of the system ignores time.
Does that mean that time is only about our observations but not about the reality of the world?

Sorry for expressing feelings instead of a rational point of view.

 

[chrisina]

I can safely say that all fundamental laws of physics are time-reversible.
Then we can -indeed- ask ourself why time does play this big role in our lifes as well as in theoretical physics.
We can ask ourself this question, even without considering the question of irreversibility, unless it would be essential for the answer.

In other words, what are we doing with an "Heisenberg picture" of our world?
The observables are functions of time, but the state of the system ignores time.
Does that mean that time is only about our observations but not about the reality of the world?

Sorry for expressing feelings instead of a rational point of view.

I am trying hard to understand what are your "feelings" on the matter of time, but can't. Can you please try to explain them once more ?
Thanks

 

[ZapperZ]

I can safely say that all fundamental laws of physics are time-reversible.
Then we can -indeed- ask ourself why time does play this big role in our lifes as well as in theoretical physics.
We can ask ourself this question, even without considering the question of irreversibility, unless it would be essential for the answer.

Then how do you account for all those broken time reversible symmetry results? Example: M. Covington et al., PRL 79, 280 (1997). And what about the CP violating decay of Kaons? That in itself implies a T symmetry-violating event.

Zz.

 

[lalbatros]

Then how do you account for all those broken time reversible symmetry results? Example: M. Covington et al., PRL 79, 280 (1997). And what about the CP violating decay of Kaons? That in itself implies a T symmetry-violating event.

Zz.

Yes, I agree completely, but I ommit this aspect because I don't see how this may affect the discussion about the nature of time. In addition, I have never read about CPT violations. If this is right, the CP violation is not fundamental to discuss the nature of time.

In our daily experience the T-violation obscures the T symmetry of most of the physics.
Once the T symmetry is accepted as a basis for discussion, it is hard to understand why time is so fundamental.

 

[ZapperZ]

Yes, I agree completely, but I ommit this aspect because I don't see how this may affect the discussion about the nature of time. In addition, I have never read about CPT violations. If this is right, the CP violation is not fundamental to discuss the nature of time.

How can it not be relevant since T reversal symmetry (or lack of it in certain situation) would imply an inherent characteristic of time? It is those small "exceptions" that is the source of a lot of new physics and understanding. Secondly, while there are no CPT violations (yet), the fact that CP (and T) symmetries are violated separately does not implie that these aren't "fundamental". In fact, broken time reversal symmetry is the KEY signatures of new merging phenomena. Remember, only when there's a broken symmetry somewhere do you get to see something "new". So such broken symmetry is extremely fundamental as signatures of a phenomenon. That is why such a thing is used in so many different studies. Broken gauge symmetry is a fundamental concept in our world and important enough that Phil Anderson got the Nobel prize for it.

In our daily experience the T-violation obscures the T symmetry of most of the physics.
Once the T symmetry is accepted as a basis for discussion, it is hard to understand why time is so fundamental.

See my discussion above. Just because the symmetry can be broken has no connection with something being "fundamental" or not. I don't see anyone here arguing about space and charge not being "fundamental". Yet, I see lots of people arguing about "time" being such-and -such. I've yet to see someone come up with the complete dynamical description of a system without using time. Until that occurs, I consider time to be an essential ingredient, without which, you cannot completely describe any system. To me, that is ample evidence that time is essential and fundamental. There are no evidence so far to the contrary.

Zz.

 

[lalbatros]

ZapperZ,

I can only agree.

But I like to take a classical physics point of view (first) on this question.
Even in classical physics time is the main subject, without time there would probably be no object to physics.
Yet, in classical mechanics all the information on a (autonomous) system is included in the initial conditions. Just as if the time evolution was simply a mystery for our conscience and a game for the physicists. I don't know if CP invariance could help in solving this paradox.

 

[lalbatros]

maybe I will read this sometimes:

Time and Free Will, Henri Bergson

 

[ZapperZ]

ZapperZ,

I can only agree.

But I like to take a classical physics point of view (first) on this question.
Even in classical physics time is the main subject, without time there would probably be no object to physics.
Yet, in classical mechanics all the information on a (autonomous) system is included in the initial conditions. Just as if the time evolution was simply a mystery for our conscience and a game for the physicists. I don't know if CP invariance could help in solving this paradox.

You still haven't addressed my question on why you seem to pick on 'time', yet you're leaving 'space' scot free. Can you define space in more "secure" terms than time? As far as I can see, the concept of space has the same importance and foundation as time, and GR/SR certainly treat them both as equals. So are you contradicting GR/SR? Why pick on time alone?

Zz.

 

[lalbatros]

You are right again:

You still haven't addressed my question on why you seem to pick on 'time', yet you're leaving 'space' scot free.

I think that for physics, space-time is a given stage.
Since 1905 the shape of this stage is an experimental topic.
Since more recently it seems that physics could go further in the investigations.

Still, there is no detailled answer to the original question, in the sense that space-time cannot be reduced to other concepts, but it can be observed scientifically and that's the most important.

Still, I think a huge number of people will never be satisfied with this answer: "space-time an ultimate concept that can be observed experimentally". Personally, I don't care much altough I can be interrested to learn.

Further, I think that in all the space-time dimensions, time is the most mysterious for a majority of people including physicist. That's probably because space can be perceived at will by using vision, the sense of touch, or even by hearing. In contrast, time cannot be perceived at will, for example you cannot "walk back in time" to measure some lapse of time.

Also, note that in SR, space has three components with signature 1, while time is unique with signature -1. Even in physics, time is apart.
In thermodynamics too, time as a very special role, even though thermodynamics can also be formulated in a covariant way.

In a local frame, time has its mysterious fingerprint that brings all these question.

 

[ZapperZ]

Er.. I have no idea what you just wrote here.

Time and space are NOT identical. That has never been my argument. Yet, they are treated on equal footing in SR/GR. That is the whole point that I'm trying to make.

So if anyone wants to argue that the time dimension is doesn't exist, or isn't necessary, then that person has to deal with SR/GR, not just classical physics. Furthermore, I will put it to you that you will have no way to "experience" space without time. Think about it.

Your claim that the majority of people, including physicist, find time to be mysterious is puzzling. No one that I know of in my line of work, and certainly in my Division at work, find time to be "mysterious", and certainly not enough to waste their time doing what we're doing here. In fact, open up any copy of Phys. Rev. Lett., for example, and see if you can find any serious research focusing on "time".

Zz.

 

[lalbatros]

Well, ZapperZ,

don't take my posts too seriously.

Actually, I don't care very much for this old question of time.
As an engineers, working in combustion, minerals, logistics, operations research and "things like that", I have a practical and pragmatic attitude in most things I do. Before studying physics and engineering (long ago), I was also a kind of "anti-phylosophy" guy, I had read Kant and had concluded that Kant would have been more successful if he had been a physicist.

But answering the question about the "nature of time" is first of all trying to understand why this question is asked and what is its meaning. Observe that the question was not about the "nature of space time". Also observe that this is not just a mistake by Robert100. I could even imagine that Robert100 is looking for a book on this topic simply because he has received that as topic for some personal work at university. Bergson that I gave as a possible bibliographic source also tackle specifically the time question?

The entangled nature of spacetime is certainly relevant or even essential in the discussion and the answer. But nevertheless, it remains that time is perceived as fundamentally different from space. It is also clear this even so in physics, just by the signature of the time coordinate.

Do you like phylisophy? I don't ... I think. But I will read Bergson ...

 

[ZapperZ]

There's a difference between "perception" and how it really is as treated in physics. Considering that most physicists have no care about the "perception" of time, then it is left for others to figure out why those who are outside that field are so enamored by time, while those who should know more, aren't! Could it be that, like most aspects of science, those who are not familiar with it really have some weird impression of it. After all, more than 1/2 of the general population doesn't even know that the Earth revolves around the sun, and that a substantial number believe in astrology.

So no. I do not go by "perception" especially when it is based on lack of understanding of something, and it certainly isn't a valid impetus to question or study something. Is the nature of space and time a continuing study? Sure it is, but not in the way that it has been discussed on here, and certainly not in the same vein as asking if it is an "illusion" or not.

Zz.

 

[lalbatros]

What you say is quite acceptable, specialy for an anti-philosophy as I am.

But you should admit that the relativistic invariance does not put space and time on the same foot.

I admit that saying that "time does not exists" is pushing things much too far.
It remains however that for a large part of physics (may the whole physics), time is just a parameter describing the moment you observe a system, since the full initial conditions are enough to describe a system completely. Chosing a certain time is like chosing a specific representation of the system.

Ok, that was funny, but if I tell about that to my boss, I could lose some credibility ...

 

[ZapperZ]

I admit that saying that "time does not exists" is pushing things much too far.
It remains however that for a large part of physics (may the whole physics), time is just a parameter describing the moment you observe a system, since the full initial conditions are enough to describe a system completely. Chosing a certain time is like chosing a specific representation of the system.

You have said this a few times, and I don't understand why. "Initial conditions" are simply boundary conditions. You have boundary conditions for time, space, and even the first derivatives of any parameter. Just look at the Dirichlet and Neumann boundary conditions in solving the Poisson equation. So again, why pick on time when space also have the SAME identical property? When you solve a kinematic problem, you also have to specify "initial conditions". Nothing different here. So why are we singling out time?

Zz.

 

[lalbatros]

Why is time singled out is the space-time signature?

 

[Niv]

Why is time singled out is the space-time signature?

I think that in relativity it is not (in contrast to Classical mechanics).
But it has some unique characteristic:
1) The second law of thermodynamics.
2) Any object exist somewhere in any given time, but not the opposite.
I mean there is a function from time to x,y,z because in any t an object will have certain values of x,y,z, but for a certain x an object may have more then one combination of t,y,z.
I don't sure I explained myself well, so we can just say that all objects are moving in a certain direction in time, but not in space (because they can move backward).
* I don't sure if it valid in a black hole too.
3) Our subjective feel about it.
4) I think that uncertainty principle is taking place just in a certain time.
(I'll be glad if someone will confirm or contradict what I've said because I don't sure about it).

 

[lalbatros]

Niv,

The space time signature I reffered to was the form of the metric in relativity:

ds²=dx²+dy²+dz²-dt²

which is usually mentioned as the signature (1,1,1,-1) where time is indeed singled out.

 

[Niv]

My mistake, I didn't understand what you meant in "singled out".

Thank you for correcting me.

 

[ZapperZ]

Niv,

The space time signature I reffered to was the form of the metric in relativity:

ds²=dx²+dy²+dz²-dt²

which is usually mentioned as the signature (1,1,1,-1) where time is indeed singled out.

First of all, since this is the Philosophy forum, no one will realize that the c=1 simplification is in that equation. So you do need the c^2 term with dt or else you do not have a "length" and that equality is invalid.

Secondly, I do not consider this to be "singling out". We are writing a length equation here, so already we are biasing towards space and not time. But the FACT that you do need the term containing the time to actually have the complete metric means that you cannot have one without it. This is what I mean when I said that both space and time are on equal footing in SR. You can't have one without the other, or else your description is incomplete.

Thus, if they are on equal footing, why pick on space only? If time is an "illusion", so is space, and vice verse.

Zz.

 

[lalbatros]

ZapperZ,

Don't worry, I drop my claim about "non existence of time" without any feeling.
It is wrong even from the initial conditions point of view, since the initial conditions involve position and velocities.
Therefore, time is implicit already in the initial conditions.

... note that this could be further discussed if one considered position and momentum as coordinates and momentum as a time-independent concept ...
... so many points of view for the same thing ...
... Could we say that we obviously live in a 7-dimensional space?

Only remains:

- the fact that time has a unique signature as compared to the three other dimensions,
- and the macroscopic irreversibility that has no spatial equivalent

 

[ZapperZ]

ZapperZ,

Don't worry, I drop my claim about "non existence of time" without any feeling.
It is wrong even from the initial conditions point of view, since the initial conditions involve position and velocities.
Therefore, time is implicit already in the initial conditions.

But I've mentioned this explicitly way in the beginning, in terms of the equation of motion. In fact, if you think about it carefully, you have no way of defining "position" without an implicit assumption on the existence of time. Think about how you would measure position at the most fundamental level. If you buy SR, it can only be unambiguously done with light.

Only remains:

- the fact that time has a unique signature as compared to the three other dimensions,
- and the macroscopic irreversibility that has no spatial equivalent

I could easily say that it is space that has the "unique signature" when compared to time. Not only that, if you write down a coordinate position in terms of spherical polar coordinates, there are no "negative r" positions (r as in the scalar length, not the vectorial length), because that is unphysical. So I've just constructed a non "macroscopic irreversibility" for space as well!

My point is that using such things to single out time is meaningless. It certainly hasn't bothered enough physicists since they use time reversibility symmetry (or broken time symmetry) with enough frequency to make such a thing common. The fact that the P and T symmetries are 2 of the 3 fundamental symmetries in elementary particle physics is my proof that time and space are inseparable, and you can't have one without the other.

Zz.

 

[lalbatros]

I agree, of course.
But time and space being inseparable does not mean that they are the same thing or that they are interchangeable.
Any event has 3 space coordinates and 1 time coordinates in any frame of reference.
Interchanging one spatial coordinate with another is perfectly meaningful.
But interchanging one spatial coordinate with the time coordinate make no sense.

I also doubt about your polar coordinates example.
The time coordinate could cover the range [-Inf,+Inf] even when describing irreversible physics.
Irreversibility in time has to do with how the system may change when it evolves from t1 to t2>t1.
I don't see any analogue for a spatial coordinate.
Well ... maybe I have an idea ...

 

[ZapperZ]

I agree, of course.
But time and space being inseparable does not mean that they are the same thing or that they are interchangeable.
Any event has 3 space coordinates and 1 time coordinates in any frame of reference.
Interchanging one spatial coordinate with another is perfectly meaningful.
But interchanging one spatial coordinate with the time coordinate make no sense.

You will note that I've never said that they are the same. In fact, I have mentioned the fact that they are not the same. But not being the same does not mean that they cannot be on equal footing. C, P, and T symmetries are all not "the same", yet, they are of equal importance in terms of symmetry principles. So again, no one here should think that they are "the same". This has never been in any of the arguments here, other than the omission of "c" that you made in your metric equation.

I also doubt about your polar coordinates example.
The time coordinate could cover the range [-Inf,+Inf] even when describing irreversible physics.
Irreversibility in time has to do with how the system may change when it evolves from t1 to t2>t1.
I don't see any analogue for a spatial coordinate.
Well ... maybe I have an idea ...

Er.. I have no clue what you are trying to do here.

Zz.

 

[Quatl]

I also doubt about your polar coordinates example.
The time coordinate could cover the range [-Inf,+Inf] even when describing irreversible physics.
Irreversibility in time has to do with how the system may change when it evolves from t1 to t2>t1.
I don't see any analogue for a spatial coordinate.
Well ... maybe I have an idea ...

I think this might help

Consider a 2d universe, as a piece of paper. On this page there is an object, say a square with a non symmetrical pattern of polka dots along it's edges. within the universe you can plot the square's location only if you use 3 values, it's X and Y position, and a rotation around an axis perpendicular to the page (that is Z.)

You might object that the rotation's magnitude isn't necessary for observers within the page (say the triangle guy some ways off) but it is critical if you really think about it.

A 2d person would be very confused about what this third axis was exactly since cannot experience translations along it but only rotations, where as for the other two dimensions they see translations but not rotations. (note that to the triangle if you move the square along Z it disappears from the universe. If you rotate the square along X or Y, the square either becomes a point or just changes texture, depending on the exact positions of the two shapes. A texture change in this case would be like your organs suddenly becoming visible to the person across the room.)

If you were to make a book by taking snapshots of each page from a higher (or lower) Z and stacking them upon one another, the book as a whole would describe the history of the universe. This does not mean that such a book "must" exist in order for the 2d universe to exist and go about it's business. Travel (that is translation) along this axis does not have to occur for this 2d world to have the symptoms of time. But were a very smart triangle to come up with an intellectual concept that allowed him to make such books in it's mind, the resulting tool would be very useful.

By analogy, our 3 dimensional history could be recorded in a 4 dimensional book without the universe "being" such a 4d object (in fact a list of positions and times for a moving object is a projection of such a 4d book.) This may be why space and time aren't interchangeable. If time is this type of abstraction we would see exactly what we do see. This would also explain why, while we can talk about time coordinates in a way that is meaningful, we can't necessarily traverse time the way we do space. (note certain statements made about relativity make this seem weird. I'm not qualified to say whether they make it in fact weird. Maybe I should make a topic about that discussion. Could be fun.)

We make books like this all the time, calenders as a simple example.
-----------------------------------------------
When you choose a point to make a measurement, or start a simulation you have to record which page you're talking about. That's the minimum that the time coord represents.

Is our universe book like? Or do things actually move along our "page?" For physics generally it doesn't matter, that's more of a cosmology question.

Many of these ideas are presented in a fun way in a book called "Flatland: A Romance of Many Dimensions" by Edwin A. Abbotte. (maybe all of them, I can't tell how much I've elaborated on it since I last read it years ago)