Talk:Time in physics

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(William M. Connolley 22:01, 10 Jun 2005 (UTC)) This article is weird. Most of it is about the history of time in physics (which is how it started off) which is splendid stuff but really belongs in an article called... history of time in physics. Some of it (eg the lagrangian stuff) appears to be "some equations which have time in them" and I can't see the point. Charles, this is all your fault :-)

Perhaps it should be called "the history of time in physics", but then again the subject of what time has meant to physicists has changed over time. The reason for this is because just thinking about time one can come to so many conclusions. And the issue of the physics tied in with the philosophy of time has not even been suggested by the title. For anyone too impatient to wait for the cleanup I suggest reading "Time" by Craig Callendar. Dessydes 16:34, 19 May 2006 (UTC)

Contents

[edit] To the reader

Time is a fundamental subject in physics which has taken thousands of years to understand and master. In distinction to Space, which is obvious to our eyes, it took some technological developments to make the subject of Time easier, and we are not yet done with the subject.

If you do not understand part of the article, just let it pass; your mind will return to that which you did not understand, if you are still interested, and you have prepared yourself for the next encounter.

[edit] Time in computational physics

I left this section alone, during the rewrite. Does it make sense to delete it? It is covered in a previous sentence of the article: time is a parameter in physical models Ancheta Wis 10:25, 1 February 2007 (UTC)

Unless others object, I'm gonna delete it. This section contains unreferenced statements originating from an erroneous identification of the update step in an algorithm with time. The two are unrelated. JocK (talk) 14:00, 2 February 2008 (UTC)
Done. -- JocK (talk) 09:02, 10 February 2008 (UTC)

[edit] To the editors

I have retained the cleanup notice; it sits at the foot of the article. If the community deems it fit, we can then remove the notice, in time. Removed the tag as a bot got hold of it. --Ancheta Wis 18:36, 6 January 2007 (UTC)

[edit] The physics of a conversation

Since working on the cleanup, I realize that I have never seen research on the physics of a conversation. What I mean is that the world line of a system can be known, but what about two intercommunicating systems? Are world lines doomed never to interact, except thru dynamical influences? The closest thing to this I know about is A Mathematical Theory of Communication which embodies encoding and therefore protocol, but I would appreciate a citation for a basic, from the ground-up, analysis. --Ancheta Wis 14:40, 10 September 2006 (UTC)

See Rudolph Carnap (May 1957) Chapter G (sections 48-51: pages 197-212), "Axiom Systems of Physics", Introduction to Symbolic Logic and its Applications, a translation by William H. Meyer and John Wilkinson of Carnap's January 1954 Einführung in die symbolische Logik.
Carnap utilizes the concept of a signal line which finesses the need for contiguity of world lines, but which instead allows for a signal chain between world points.
Ancheta Wis 19:04, 25 December 2006 (UTC)

[edit] why time causes paradoxes in physics

Math is a highly compressed language that seeks to symbolize experiences in a very precise way. In many calculations time is treated as an abstract term, just a number, and detatched from the experience that is beyond the symbol, namely a cycling countable event. because of this detatchment, silly notions like backwards in time and time travel can arise. It is not so easy to make time into a negative if you see it for what it is, namely some perceived stable event that one can use as a gauge for other less orderly events. Speed is Distance/time, but this gets all hairy in relativity where its not so nice and neat as the mass of the object and its speed begins to effect the time factor, which mathematically seems to go negative beyond light speed...but of course that is only because the numbers have lost their connection to the events they are symbolizing.Jiohdi 21:44, 5 February 2007 (UTC)

I will get more concrete and refer to John Cramer [http://seattlepi.nwsource.com/local/292378_timeguy15.html is preparing an experiment] to determine whether quantum entanglement is also nonlocal in time as it is in space. This can also be stated as 'sending a signal back in time'. The experiment is still in preparation as of 10:57, 16 November 2006 (UTC).

Eckard Blumschein [1] claims that there is no signal back in time but perhaps just a mistake. E. Schroedinger wrote in 'Quantisierung als Eigenwertproblem', 4th Mitt. in Ann. Phys. (4)81, 109ff (1926), p. 112, 'one may consider, if nessessary, the real part of psi the real wave function". He omitted the aspect of required apparent symmetry. Only positive elapsed time can be measured. Our usual time scale is bound to an arbitrarily chosen event. It has been abstracted and extrapolated from elapsed time which is bound to a real object. This abstraction replaces one-sided integral relationships by differential equations. Pertaining time-symmetry requires exclusion of the unphysical advanced part of solution in order to obey causality: Future quantities cannot influence current processes being causally embedded into the integral effect of past influences. Invariance against shift and even reversal of time reversal is only possible at the level of abstracted usual time.

Since time and frequency are related to each other via complex Fourier transform, function of either time or frequency must exhibit Hermitean symmetry, i.e. positive as well as 'unphysical' negative arguments. Fictitious negative elapsed time is required as to encode merely positive frequency chosen by Schroedinger and also by Dirac. Weyl did not understand this in 1931. He wondered about PCT-symmetry.

One has to humbly accept that both imaginary and apparently negative quantities are the tribute one has to pay for abstraction into IR anstead IR+ and use of complex calculus. Apparent symmetries must be interpreted as unreal. They would disappear with correct inverse transform as do imaginary quantities, too. Blumschein 17:22, 5 March 2007 (UTC)

Your paper suggests an experiment based on Cramer. Cramer might try dumping energy into both future and past. Your paper suggests all the energy will wind up in either the past or the future, and not in both directions. But this would require careful accounting of the energy in each pulse which he is transmitting. --Ancheta Wis 19:17, 5 March 2007 (UTC)

Do not get me wrong. I just picked up Cramer's idea as an example of ignored essentials. Nobody can analyze future data. They simply do not yet exist. While past events are evident from left traces, future events are only predictions, no matter whether they will come true or not. Albert Einstein wrote: 'For us believing physicists, the division into past, present and future has merely the meaning of an albeit obstinate illusion.' I do not see any reason to share this belief with him, Newton, Minkowski, Hamel, Hilbert, Noether and many others. At least there is no doubt: While past is unchangeable, future is uncertain. All measurable reality exclusively belongs to the past. It makes a serious difference whether one deals with abstract time as did A. E. and John Cramer or with reality-bound elapsed time. I am pointing my finger squarely to most serious consequences of the neglect of this essential difference between abstract time in IR and concrete elapsed time in IR+. While obviously nobody can go back in elapsed time, mainstream physics and technology do not yet understand the implications for signal processing and quantum mechanics, including fourfold redundancy, non-causality, arbitrariness, and misinterpretation of apparent symmetry. Blumschein 07:07, 6 March 2007 (UTC)

[edit] relativity

An interesting concept arises when relativity is analyzed. The fact that an object's speed and mass have an effect on time itself is a rather unique, interesting concept. Perhaps the best explanation for this, in my opinion, is that time is not a stable, concrete quantity. In other words, time can be thought of as simply a manifestation of matter and its properties. For instance, in a universe which has no beggining and no end, and in which all matter remains in a fixed, constant position and does not change in any way, time can be thought of as inexistent, since matter and the universe it inhibits doesn't exhibit any modification. Therefore, all observable changes in time can be thought of as simply changes in matter. When we consider time for an object sitting still, one might say that the object's time progresses even though the object exhibits no change. While this statement is valid, one could also say that this is simply because the surrounding matter (surrounding the observed object), and, consequently, the universe itself, is in continuous motion and constantly exhibits observable changes which influence the observed time for the observed object. In a universe in which all matter remains constant (and the universe itself did not, will not and does not exhibit any change whatsoever), time for any object can be considered as being inexistent. Nevertheless, time is significant and its presence is influential. The answer to the question "what is time?," however, is, I believe, more complex than a simple 'AM-PM' differentiation. —Preceding Signatures comment added by 24.219.29.168 (talk) 08:09, 18 January 2008 (UTC)

24.219.29.168, I like your contribution, but it does not follow that time is dependent on matter. All that can be said with certainty is that parties within a common light-cone can signal with each other, if one were to accept general relativity. We currently do not even have a way to tell if an object is 'sitting still', bcause our ideas about that are formed by our relationship to the massive earth on which we sit, and that earth was formed during the long passage of time. If our own earth was the result of accretion of iron-based accreta from a past star, and if we are currently too far away from objects like black holes to suffer huge gravitational effects, then we do not yet know very much about the relationship of time to matter, for lack of observations. The new space-borne observatories are remediating this lack, and we await the science which will be published from this data. But new physics can arise from new philosophy as well, as Newton so stunningly showed, so your contribution just might be indicative the new approaches that we need. --Ancheta Wis (talk) 12:34, 18 January 2008 (UTC)
Note that the article shows the dependence of the passage of time on the strength of a gravitational field. But matter is subject to other fields than gravitation, as well. An accounting of the factors in the accuracy of a time standard includes other terms besides the effect of the gravitational field on the satellites of the global positioning system. The last I read, there are at least a dozen other terms in the error budget of such a system. --Ancheta Wis (talk) 12:47, 18 January 2008 (UTC)

Ancheta Wis, you have an interesting point. Perhaps time is as you describe it, however, I am still curious as to whether or not a 'timeless' universe can exist. Theoretically, however, I doubt that we could ever discover such a universe (at least, not through the use of wormholes, as their existence requires the presence of time, and, therefore, 'hopping' from our universe to a timeless universe would be impossible with wormholes). What I find interesting from your statement above is that we do not have a concrete way to determine whether or not an object is 'sitting still.' This does raise a few questions, since the concept of standing still is present throughout various physics topics. I agree with what you said about time and the gravitational field, however, I have yet to see how my previous statement can be considedred a meaningful contribution. As far as I'm concerned, my, "philosophy" is, in the end, a compilation of uncomfirmed facts, which, as you have stated, is incorrect anyway. 24.219.29.168 (talk) 09:17, 20 January 2008 (UTC)24.219.29.168

[edit] Accuracy of UTC timestamp

The section "The state of the art in timekeeping" contains the statement: The UTC timestamp in use worldwide is an atomic time standard. The accuracy of such a time standard is currently on the order of 10(-15) seconds.. What accuracy is this? An absolute accuracy compared to what? I can hardly believe the statement to be correct. More likely the relative accuracy is of the order of 10(-15) (second per second). (This figure is typical for atomic clocks). JocK (talk) 14:16, 2 February 2008 (UTC)

My source was an NIST writeup. sigma of .5 * 10(-16) sec Yes, on the face of it, the accuracy is with respect to the system, because no absolute measurement exists, but measurements relative to the standards, i.e., to NIST-F1 will not vary by 1 second in 60 million years, which is 1 in 1.89 * 1015seconds --Ancheta Wis (talk) 20:57, 2 February 2008 (UTC)
Thanks, that's most useful. Have changed the text to reflect that this is a relative accuracy (measured in dimensionless units). JocK (talk) 07:11, 3 February 2008 (UTC)

[edit] Not canonical conjugate

This is certainly a difficult topic to write about and the article contains considerable confusion. For example, the statement "Energy and time are canonical conjugate variables of each other." is not true. I won't go into details as the matter is subtle but if this is disputed then provide a citation. Xxanthippe (talk) 09:15, 10 February 2008 (UTC).

I assume you are refering to the section on the Hamiltonian formulation of Newtonian mechanics? To me that seems a piece of text only complicating the issue. However, I can't find anywhere a remark like "Energy and time are canonical conjugate variables of each other." -- JocK (talk) 09:27, 10 February 2008 (UTC)
Ahh.. I see, you have already deleted the remark from the text. JocK (talk) 09:42, 10 February 2008 (UTC)

[edit] Modern perspective missing

The article lacks any discussion on the treatment (and emergence) of time in modern physics. No discussion on time in (loop) quantum gravity, no remarks on time emerging from spin networks, causal sets, no discussion on the twistor formulation of spacetime, etc. etc. As it is, the article should be entitled "The history of time in physics - from Galileo to early 20th century". I think the article needs a lot of work to become meaningful as a deepening into the physics not discussed in the general article Time. -- JocK (talk) 09:51, 10 February 2008 (UTC)

Be Bold with statements of additional content, backed up by citations with page numbers to the articles or books. If you were to get the ball rolling, I could help out. But the Penrose and Hawking equations are too hard for general consumption, so if you have an approach to make their exposition more accessible, how about giving it a go? --Ancheta Wis (talk) 10:42, 10 February 2008 (UTC)