The "NOW". How is it described in Physics?

I have been having difficulty in discussing this concept "NOW" for some time and wondered that maybe I should have first asked the question about how physics defines or describes this concept of pheno.
Is it simply that zero duration event that exists between the past and future?
An event horizon maybe?

Albert Einstien seemed to get it right with his light cones yet even in those cone diagrams there is a reluctance to use the word "NOW" or present, etc. Why is this so?

Care to discuss?

 

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Well, for one thing SRT says that now for one inertial frame doesn't even qualify as a now in another, if the two are in relative motion.

 

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Which is a position I find facinating, given that the NOW is all there is as far as existing is concerned. It is the ongoing moment that everything happens in and yet it is considered as "nothing special compared to other times"

[ this is not a criticism but mere puzzlement ]

 

Quantum Quack, which theory of time do you identify with? I, myself, am a presentist, a division of the A-theory. There are also the B-theories of time. I would imagine most of the Relativists belong to the B camp camp, since presentism does not recognize relativity of simultaneity as being a fundamental 'truth', but only a particular method of clock synchronization (Einstein clock synchronization). How you define NOW is related to how you how you think of time. Here is an interesting draft of a paper by Dean Zimmerman that may be of interest to you:
http://fas-philosophy.rutgers.edu/zimmerman/PrivilegedPresent.pdf

 

I think after skimming through the linked dcument that I must be aligned with A- theory [ presentist ]

I tend to believe that as the author mentions that both camps are "talking past each other" in many respects. For example to calculate non-simultaneity one must assume a base line of simultaneity to perform the calculations.

So using absolute time between observers we can measure relative time.

Without the absolute time reference we would be unable to derive meaningful results.

In another thread I ask the question about two ships at different velocities within a light field of a nearby star. Do they experience reflecting in the same ongoing moment? [ignoring observations of the other ship] and regardless of velocity or distance relative each other and the star.

In other words I am asking if they experience light events in the same ongoing Now?

Using the humble photon has some advantages.

At all times in a photons travels it has a potential to realise an event. From what I understand that event potential is always betweeen past and future.

Whether the event occurs or not is not so important. What this says to me is that no matter where you are in the universe photons have an event potential that must be between the past and future [ NOW]
Therefore regardless of relative velocity or distances any object would experience reflecting a light event always in between not only the photons past and future but also the reflectors past and future.

[edit: I have since been informed by JamesR that to use the wording event potential would be incorrect possibly reflection potential may be better?]

Thus in this manner as an example a rock or other solid object also is existing only between the past and the future. [As it is with our photon]

To say that one photons time potential is different to another would invalidate the lights invariance postulate I feel.
But I am ahead of myself......

If one takes a snap shot of a star and it's light field and showed every wave of photons in that snap shot, which photon wave does not have a event potential of being between the same past and future? [ when considering all waves within that snap shot]
If one goes on to take a snapshot of the entire universe would the results be any different?

Any way, I find it puzzling that physics has not fully defined these events other than to claim them as events and not some other more technical term, that includes a time component. It sort of feels foolish to use the words "present" or "now" when discussing physics and I wonder why this is so.

 

I guess what I am suggesting is that as far as our photon is concerned time is absolute. Therefore if time is absolute for our photon then so to must it be for our reflectors. Thus if I am correct in my assessment, the NOW is uniformly simultaneous throughout the universe.
edit:
Note: I am not referring to the passage of time due to dillations, though I see relative time passage [ due to dilation] and absolute time [ simultaneous NOWS ] co-existing quite easilly.

 

I've probably fudged what I wanted to say but any way, thems the breaks...

 

This thread reminds me of a long past philosophy course. The text book had a 15-20 page discussion of time. The professor elaborated on the text and other notions. I did not understand the points being made by the text or the professor, nor do I understand the article linked by a previous poster. What is the great concept being described? The sentences are syntactically correct, but seem to be lacking semantic content.

That long ago philosophy course resulted in my looking for something more meaningful, which I found in some book or article by Albert Einstein. Albert’s comments make far more sense to me than any other view of time. The following is a paraphrase (I do not remember his exact words).

• When an individual thinks about the events in his life, he seems to be above to order them using the criteria of before and after. He can assign numbers to the events in such a way that events associated with smaller numbers occurred before events associated with larger numbers. A device called a clock can be used to assign numbers to events in a consistent fashion.
  
  In expressing the laws of physics, it is convenient to use a continuous variable called time consistent with the above notion.
  
  There is little more that can be said about the concept of time.

It is interesting that Albert italicized or put the words before & after in quotes to indicate that he viewed them as undefined primitive terms (Id est: They could not be defined using more primitive words). From the view of axiomatic logic, if you and I need a further definition of some primitive term, we are not likely to be able to discuss the subject at issue.

Words like past, present, future, et cetera (definable using before & after) can be handy for discussing concepts associated with the notion of time. However, it seems to me that such words & additional notions do not add anything to the basic understanding of time as presented by Einstein.

 

Unfortunately, it is not that simple, Fraggle Rocker. Take a look at a comet, for instance. When you point your finger at the comet, is that where the comet is 'NOW'? Or is that where the comet was when the photons you are seeing were emitted? Have you and the Earth changed locations since the light was emitted? When did the Supernova 1987a erupt? It took the light from the supernova about 170,000 years to reach us. It is very difficult to determine an 'absolute' space and time mathematically. Some of the things we 'see' may no longer exist, or may have moved in an unexpected manner. Special Theory generally avoids this difficulty by placing the observer in a 'rest' frame and the observation of the event as the 'NOW', something like a snapshot taken by the observer.

 

Ahh!! but the observation is of course NOW as the photons being observed by their effect are always NOW. It is only the source of those photons that is no longer necessarilly in the location it was when that information was emmitted in the form of photons.
If we could see past the illusion created by those historic photons we would find our object is also NOW but not necessarilly in the same location.[ assuming that the object has not been destroyed since those emmissions etc] The light delay creating an illusion of location is still recorded NOW. But of course this is the illusion that delayed information generates, even if that information is always NOW.

If we ignore information delays I think we can safely say that everything is existing in a uniform simultaneous ongoing moment often refered to as NOW, it's just some objects would be invisible, but certainly material due to the delay in their information reaching us.

 

It could be argued, at least philosophically, that Albert E. has defined time with the use of photons. The photon itself defines time as it defines the past and the future by it's attributes.
"Where it was is the past and where it will be is the future, but where it is, is the present".

And because it is central [present, NOW] it could be argued that the photon defines time for everything else as well.
Humourously:
Instead of using words like NOW or Present may be we should just simply use the word Photon instead, ie everythng happens in the photon.... replacing the word NOW with photon.

 

This is quite fascinating.

The only problem with a simultaneous "NOW", is deciding how long it lasts, since all frames of reference regarding the imperceptible nanosecond we experience as now would be different. Another frame of reference might experience that nanosecond as a year with 3x10 to the sixteenth power of those moments to choose between as "NOW".
Another completely different frame of reference would experience our now as lasting a year and wonder why we didn't split it up into smaller segments of "NOW".

Supposedly.

I think.

I really couldn't say whether that is true or not, sorry.

 

I agree it is an interesting subject...

The only problem I have with your post Cole is that the "now" is actually zero duration as the photon has no rest frame. Thus the magical thing about all this is that one could argue that no-thing actually exists [ zero duration ] but can only be said to be existing.
As the photon is constantly moving and is centrally positioned in time it also means that the "now" is constantly changing at the same rate as the photon changes it's location. Which means that change is happening at 'c' universally. No faster and no slower than that of our photon. IMHO

question:
How can we dilate a zero duration moment?
Answer: Seems to me that we can't.

 

Very good, old chap.
But if the NOW is of zero duration, what is the difference between choosing the zero duration "moment" by arbitrarily referencing a frame between what you call past and present, and choosing a different zero duration moment? Aren't both no time at all?
How can you call something of no duration be a "moment" - isn't that a measure of time in some way?
And wouldn't an infinite amount of those zero duration "moments" fit in between the past and the future, since they take up no time at all and could be added to each other with no increase in duration?
Which of those infinite "moments" will you choose as the NOW???
This reminds me of zeno's paradox.

And how can something of zero duration be simultaneous with anything but itself?

 

You are quite right. Using the word moment is mis-leading. .Setting up a contradiction "zero duration moment" and this is why this issue is so difficult to describe. Zero duration~ what? maybe position would be better. Zero duration position or location....hmmmmm...but even this would eventually lead to a problem I feel.

Sure why not but in the final wash it is still the same zero duration "moment"...opps position.....hmmm location...sheesh!!

eh??

 

So if you turn the word "time" into the word "space", aren't you going to have a relativistic dilation of perception of position, similar to the relativistic perception of time (as in days, etc.)?
I.e. - isn't space going to have a hard job becoming concrete enough to describe all positions throughout the universe at a particular "moment"?
Maybe not.

 

It is tough to get the 'ole head around this subject I must admit.
Possibly it may be better understood if we take a look at just one emmitted wave of photons.
From the moment of creation the wave of photons moves outwards from it's source. As it journeys onwards it's potential to reflect is always between past and future. At no time in it's journey is it anything else but between past and future.

At approximately 299,792.458 kms it is now 1 second old. the emmission point is also 1 second older. As the photon and all the photons behind it continue to move outwards, some photons are older than others but at all times their reflective "moment" is the same "moment". If one photon is 299, 792 kms away from the source it is 1 second old when compared to a photon wave that is twice this distance [ 1 second old and 2 seconds old ] you can see that even if they are of different ages they still are simultaneous in their reflective potential in time. That is to say that a photon at 299,792 kms has a reflective potential at the same zero duration moment as every other photon regardless of their age.

So if we look at the field of all light emmitted from a star and suspend animation [with out imaginations ~ take a snap shot] it would be correct to say that all the photons define the same moment. [ even though they are of differring age]

Assuming that if the universe is flooded with photons it could then be said that the photonic field is simultaneous at a universal scale even though some photons may be 1000000 years old and some only 0.5 of a second old they all share the same moment of reflective potential.

That is to say that if a reflector was placed at any location within that field the act of reflection would occur between the past and future of every photon and not just those that were reflected.

So an object at 1 lys distance from the source of light reflects light simultaneously as an object only 0.5 lys distance from the source. [ the information may be historically different but the moment of reflection is simultaneous.]

Therefore the photons of the universe define the NOW. And by this reasoning the NOW must be absolute or simultaneous for every photon emmitted and still existing.

well.....thats the reasoning I'm using I guess....

 

Doesn't measuring the point of NOW, or putting a reflector at any location in the field, require that the location you are proposing be at rest?
And isn't this the big problem with defining simultaneity in the first place, i.e. how to decide which point is at rest?

Maybe not.

I guess all points are at rest during the "NOW" period of zero-duration, since nothing can travel any distance during that "time".
But is it really ok to say that the point at the nose of the rocketship is at rest during the moment between the past and the future, if it will be at another location once any time at all has passed?