[Alan McDougall (OP)]

The expression "A moment in time" does it exist in physics?

Some say this is really a philosophical question, but I disagree, why can't physicists also address this enigma?

Or do we ever reach a moment in time, it seems to me that a moment never really happened, because when we try to measure or capture it, it has already gone.

Alan

[Bill S]

I'm not a physicist, but I suspect that one reason why physicists are reluctant to talk of a moment in time is that it is not an exact term.  Another is that, in physics, "moment" has its own definition that is not quite the same.

When you talk of a moment in time, do you mean "that infinitesimal point of time that is the present", or were you thinking of a definable period of time, as in: that was the moment at which it happened?   

[Alan McDougall (OP)]

I'm not a physicist, but I suspect that one reason why physicists are reluctant to talk of a moment in time is that it is not an exact term.  Another is that, in physics, "moment" has its own definition that is not quite the same.

When you talk of a moment in time, do you mean "that infinitesimal point of time that is the present", or were you thinking of a definable period of time, as in: that was the moment at which it happened?   

Yes I mean the present or "Now" if you like my dilemma, is that in reality there does not seem to be a fixed moment or now, because just when you think you have reached it, it is already in the past.

There is a concept called "presentalism" which postulates that only the present is real and both past and future do not exist

[evan_au]

The Greek philospher Zeno effectively argued that there must be some smallest unit of time, and since nothing can change in this time, motion is impossible(!)

Given the speed of human neurones, the shortest perceptible "moment in time" must be in the region of 10ms to 100ms.

Physicists can measure and control events that are in the order of femtoseconds, or 10^-15 seconds.

But this is still far larger than what some theories suggest is the smallest unit of time, at around 10^-44 seconds.

[Alan McDougall (OP)]

The Greek philospher Zeno effectively argued that there must be some smallest unit of time, and since nothing can change in this time, motion is impossible(!)

Given the speed of human neurones, the shortest perceptible "moment in time" must be in the region of 10ms to 100ms.

Physicists can measure and control events that are in the order of femtoseconds, or 10^-15 seconds.

But this is still far larger than what some theories suggest is the smallest unit of time, at around 10^-44 seconds.

With the help of supercomputers the ZENO paradox has been solved.

[AndroidNeox]

"A moment in time" can be defined for any set of observers sharing a single reference frame... a static system with no movement or acceleration. Einstein proved that there is no such meaningful concept for the physical universe, this is called "non-simultaneity".

[Alan McDougall (OP)]

"A moment in time" can be defined for any set of observers sharing a single reference frame... a static system with no movement or acceleration. Einstein proved that there is no such meaningful concept for the physical universe, this is called "non-simultaneity".

My idea of  "a moment in time"  is how an individual observes a discrete moment in time, if such a reality exist in physics? From "different reference points" such as a set of observers sharing a single reference frame, in my opinion such a frame simple does not exist because of general relativity. Even the slightest movement of one observer relative to another alters the "moment/now" for each observer , because of movement between them.

I agree, therefore, that in reality there no fixed moment in time for any observer, because even different, referent points on our own tiny bodies move differently relative to each other,and occupy different nows  albeit almost so infinitesimal tiny difference that that the effect is impossible to measure or quantify.

[Alan McDougall (OP)]

"A moment in time" can be defined for any set of observers sharing a single reference frame... a static system with no movement or acceleration. Einstein proved that there is no such meaningful concept for the physical universe, this is called "non-simultaneity".

I repeat this comment I made on another similar topic because it addresses the same problem.

Before our universe emerged out of the primordial singularity, time did not exist, so I think that in reality only in this state, the only true, "actual single discrete moment" that ever existed, was then. After all if time did not flow, then, the primordial singularity, was stuck in a "moment" (Sloppy wording sorry for that.). Until something caused it to emerge and create our universe, (Big Bang/Emerge/ you choose?). No third party was there as far as I know?

Just a note of mine! There was never such an events a "big bang", this is as best,  loose terminology, in my opinion.

Alan

[Bill S]

Before our universe emerged out of the primordial singularity, time did not exist,

This is not unanimously held, I'm not sure it is even the majority view any more among cosmologists.  There is certainly no proof of which I am aware.

After all if time did not flow, then, the primordial singularity, was stuck in a "moment"

There have long been two schools of thought about time and motion.  In one time flows and everything else is static, while in the other time is static and everything moves through time.  These two ideas are sometimes referred to as tensed and tensless time.

Tensed time is “now relative”; it assumes an ever moving now, which progresses towards the future, always leaving more past behind it.  Time is moving.

Tensless time relates to clock time, dates etc. and is regarded as being static.  For example, 11.30 (GMT) on the 25th October 2007 is a tensless time; in relativistic terms it is an unchanging spacetime event.  In this view there is no objective passage of time. 

[bizerl]

I've always imagined that a "moment in time" is as indistinct as a "location in space". Whatever scale you use, there is bound to be some sort of blobbiness that cannot be refined.

However thinking about it further, I wonder if the uncertainty principle could somehow apply to time in the same way it applies to space? I know that the exact location of any given object is difficult to pin down (or is it something to do with direction it is travelling in?)...

Anyway, I just felt there could be a limitation on time due to the uncertainty principle somehow, but i'd need a physicist to verify/shoot me down.

[Pmb]

The expression "A moment in time" does it exist in physics?

Of course it does. Who said otherwise?

[yor_on]

Any time you define a event, in some coordinate system, you must have defined a 'moment in time', I should think? Any description of some passed event also consist of you, lifting up a moment in time for common inspection. If you mean that we only can observe what already passed I think you have a point, but all those events, logic craves to exist, must have had a 'instant' existence inside a arrow to create that 'past'. A event can be seen as a 'outcome' as I think, and outcomes are what we measure on.