What is time? (Again)

[swansont]

19 minutes ago, michel123456 said:

A light beam was send from the Earth 100 years ago (1920) into space. The first photons of this beam are today 100LY away. They are still in 1920.

Because if they hit the eye of some E.T., the E.T. will see the image of 1920.

IOW the photons are traveling in space but not in time.

You could say the same thing about something written down and transported by other means.

It’s not time dilation.

[Ghideon]

35 minutes ago, swansont said:

lack the ability to test any model one might propose.

Thanks, that's the kind of answer I was looking for. 

I was somehow unable to think of that angle; lack scientific options in the context of my question.

[Janus]

36 minutes ago, michel123456 said:

A light beam was send from the Earth 100 years ago (1920) into space. The first photons of this beam are today 100LY away. They are still in 1920.

Because if they hit the eye of some E.T., the E.T. will see the image of 1920.

IOW the photons are traveling in space but not in time.

 A bell is rung at 12:00:00.    You are  686 meters from the bell. You hear the bell ring at 12:00:02.  But just because you are now just hearing a ring that occurred two seconds ago doesn't mean that the sound of that ring is "still at 12:00:00".
 

[Markus Hanke]

On 8/29/2020 at 8:44 PM, Ghideon said:

Our models of massless particles seems to prevent us from "assigning a clock" to ride along with anything massless, but how come that it is so?

Because any conceivable clock - even an ideal one - must be massive, and therefore it cannot be comoving with a photon.
Mathematically speaking, you can't parametrise the length of a photon's world line using proper time (because ds=0); however, that doesn't mean that their world lines don't have a well defined length in spacetime. They do, you just need to use a different affine parameter.

[geordief]

On 9/3/2020 at 5:40 AM, Markus Hanke said:

Because any conceivable clock - even an ideal one - must be massive, and therefore it cannot be comoving with a photon.
Mathematically speaking, you can't parametrise the length of a photon's world line using proper time (because ds=0); however, that doesn't mean that their world lines don't have a well defined length in spacetime. They do, you just need to use a different affine parameter.

OK ,that is interesting  and must deserve far more thought than I can provide.

 

But I have 2 questions (after racking my brain)

This is clearly based on the "time is what clocks measure" aphorism?

and : If  we are ,as we are here unable to provide a clock  can we say that time does not  apply to the scenario? Or is it just that it is a scenario
about which we cannot say anything vis a vis time?

Speculating, is it possible to build a clock  with massless objects? Can two massless objects (in a vacuum)interact with each other in such a way as to produce  an observable(and thus timeable) series of events?

 

Is it perhaps the "vacuum" itself which is as worthy of attention as the maximum speed limit in a vacuum (and the speed of massless particles in it)?

[swansont]

1 hour ago, geordief said:

OK ,that is interesting  and must deserve far more thought than I can provide.

 

But I have 2 questions (after racking my brain)

 

This is clearly based on the "time is what clocks measure" aphorism?

No, it's not trying to define what time is. It's simply a reality that to measure time you need a system with massive parts.

 

1 hour ago, geordief said:

and : If  we are ,as we are here unable to provide a clock  can we say that time does not  apply to the scenario? Or is it just that it is a scenario
about which we cannot say anything vis a vis time?

We don't have physics that predicts time from the perspective of a photon.

 

1 hour ago, geordief said:

Speculating, is it possible to build a clock  with massless objects? Can two massless objects (in a vacuum)interact with each other in such a way as to produce  an observable(and thus timeable) series of events?

You need to detect the photons somehow. They have to interact in some way. And you have to generate the photons.

Clocks use photons, but not exclusively photons.

 

 

[geordief]

19 minutes ago, swansont said:

We don't have physics that predicts time from the perspective of a photon

 Is it possible to say that a photon only exists if it interacts (initially or finally)?

 

In that sense does a photon "die"  if its trajectory does not include some massive object in its future?

 

For any photon ,can a "lifetime" be assigned to it and is that the only "beat of its clock"?

[swansont]

Just now, geordief said:

 Is it possible to say that a photon only exists if it interacts (initially or finally)?

I don't think one can justify that.

Just now, geordief said:

In that sense does a photon "die"  if its trajectory does not include some massive object in its future?

It continues to propagate until it interacts.

 

Just now, geordief said:

For any photon ,can a "lifetime" be assigned to it and is that the only "beat of its clock"?

Yes, a lifetime can be assigned, as long as it's from an observer's frame of reference.

[geordief]

1 minute ago, swansont said:

It continues to propagate until it interacts

Do we know some photons (the earliest ones in the life of the Universe ) which  can never interact ?

 

Can the Universe never catch up with them ?

 

Would such photons (if they exist) never weaken  or cease to exist by virtue of there being no possibility of any interaction?

[swansont]

1 minute ago, geordief said:

Do we know some photons (the earliest ones in the life of the Universe ) which  can never interact ?

Can never interact? All photons can interact, AFAIK.

1 minute ago, geordief said:

Can the Universe never catch up with them ?

This doesn't make sense. They are in the universe. 

 

1 minute ago, geordief said:

Would such photons (if they exist) never weaken  or cease to exist by virtue of there being no possibility of any interaction?

I don't know of a scenario where this would be true.

[geordief]

2 minutes ago, swansont said:

This doesn't make sense. They are in the universe

If they were the first objects formed at one epoch in the universe  would not some of them been "uncatchable"  by the other objects that formed ?

 

Were there no photons whose trajectory included no possibility  of meeting any massive  particles?(because all the massive particles were created in the photon's past)

[swansont]

18 minutes ago, geordief said:

If they were the first objects formed at one epoch in the universe  would not some of them been "uncatchable"  by the other objects that formed ?

No, not as far as I know.  You have high-energy photons creating particle-antiparticle pairs, so there would be matter around for the photons to interact with.

 

Quote

Were there no photons whose trajectory included no possibility  of meeting any massive  particles?(because all the massive particles were created in the photon's past)

"Photon's past" from whose perspective? Surely not the photon's — no valid frame of reference there. Massive particles were created by photons (the pair production mentioned above) and photons could scatter off of those massive particles. 

The CMB is consistent with the photon creation from the recombination era - when the universe became transparent, tells us that photons were scattering before that. 

 

Is there a photon out there that never scattered? Possibly. I don't know how to assess how unlikely that is, or if it could be detected if nothing happened to send it in our direction.

[Markus Hanke]

With all of the above being said, I think is also important to remember that the property of mass only exists at all because the universe is now in a state of comparatively low energy. If you go back to the early universe, prior to the point of electroweak symmetry breaking, the Higgs mechanism did not yet operate, so all elementary particles were massless. In that sense, mass is just the result of a broken symmetry.

[geordief]

1 hour ago, swansont said:

The rate slowing comes from c being invariant, not just being finite. That’s the basis of relativity.

Is there anything (apart from the consequences) that can be said about this invariance?

 

It seems counter intuitive but is clearly a thoroughly verified experimental finding.

 

Are there any "preconditions" that  make it unsurprising after all that this invariance should  exist or do we just have to accept it as a fact of life and a basic building block of physics?

[joigus]

1 hour ago, geordief said:

Is there anything (apart from the consequences) that can be said about this invariance?

 

It seems counter intuitive but is clearly a thoroughly verified experimental finding.

 

Are there any "preconditions" that  make it unsurprising after all that this invariance should  exist or do we just have to accept it as a fact of life and a basic building block of physics?

Maxwell's equations. They were historically the most powerful reason behind Einstein's faith in the special principle of relativity against all common intuitions. They express some kind of "duality" between the electric and magnetic fields that would be completely destroyed were the principle of special relativity not exact.

[dimreepr]

1 hour ago, geordief said:

Is there anything (apart from the consequences) that can be said about this invariance?

 

It seems counter intuitive but is clearly a thoroughly verified experimental finding.

 

Are there any "preconditions" that  make it unsurprising after all that this invariance should  exist or do we just have to accept it as a fact of life and a basic building block of physics?

Aren't you dancing around the issues?

 

[Markus Hanke]

1 hour ago, geordief said:

It seems counter intuitive but is clearly a thoroughly verified experimental finding.

I don’t think it is counterintuitive - on the contrary, I would struggle to imagine what a universe would look like where this isn’t true. Saying that c is invariant between inertial frames is saying that all inertial observers experience the same laws of physics - whether you turn on your laptop in your living room, or while travelling in a very fast rocket, it will function the same in both cases. Intuitively, this is exactly what I would expect to happen. But maybe that is just me again  

[geordief]

 

17 minutes ago, Markus Hanke said:

I don’t think it is counterintuitive - on the contrary, I would struggle to imagine what a universe would look like where this isn’t true. Saying that c is invariant between inertial frames is saying that all inertial observers experience the same laws of physics - whether you turn on your laptop in your living room, or while travelling in a very fast rocket, it will function the same in both cases. Intuitively, this is exactly what I would expect to happen. But maybe that is just me again  

Yes I am familiar with that argument(I think Einstein uses it too but it has never really sunk home with me -clearly because I have not appreciated it correctly)

Are you saying that if we had an absolute preferred  frame of reference that the laws of physics would vary from place to place?

 

Btw is the lack of that preferred or absolute frame of reference equivalent to the invariance of c ?Does  the former imply the other or vice versa?

 

Perhaps I am OT now..

 

Edited September 8, 2020 by geordief

[dimreepr]

22 minutes ago, geordief said:

 

Imagine that...

 

[geordief]

2 minutes ago, dimreepr said:

Imagine that...

 

I was half way through answering you,when Markus replied.

I answered him and the half -composed answer to you went through inadvertently at the same time.

 

So I deleted it as I hadn't finished it 

 

Then it saw that my reply to Markus ,where I said I might be OT might cover it...

[joigus]

4 minutes ago, dimreepr said:

Imagine that...

 

The vacuum?

[The victorious truther]

10 hours ago, geordief said:

Are you saying that if we had an absolute preferred  frame of reference that the laws of physics would vary from place to place?

Depends on what relationship "c" has to do with physics and the ontological entities we familiarize ourselves with as well as the dynamics of spacetime itself. We could go the direction of some philosophers and postulate that the structure of spacetime is different to the dynamical symmetries of physical systems (in this case the Lorentz invariance of the laws of physics and the admittance of a fastest casual speed). All that is required here is Lorentz invariance as far as i'm aware and whether spacetime has a hand in it or not is a different interpretational discussion to the clearly experimental fact of Lorentzian invariance. 

[Markus Hanke]

18 hours ago, geordief said:

Are you saying that if we had an absolute preferred  frame of reference that the laws of physics would vary from place to place?

Yes, they would have to vary in some way between frames - in what sense could any one frame be considered 'preferred', otherwise?

18 hours ago, geordief said:

Btw is the lack of that preferred or absolute frame of reference equivalent to the invariance of c ?

The lack of a preferred frame means that no law of physics can vary if you go from one frame to another ('preferred' would need to mean that something about it is different compared to other frames), and that includes Maxwell's equations. 

[Markus Hanke]

19 hours ago, Eise said:

Hmm, you are already too long in relativity.

Maybe  It has long ago become such an integral part of my world-view that I find it difficult to understand why anyone would question or doubt the concept. After all, the world would look very different if spacetime was Euclidean.

My first contact with relativity was at a tender age of 11-12 years I think, in a (very well written) series of pop-sci books on modern physics. I distinctly remember it immediately making sense to me, though of course at that age - and lacking much of the physical and practically all of the mathematical background - I did not understand many of the finer details, and misunderstood some other aspects. Nonetheless, even then I was able to grasp the fundamental message - that space and time are local and depend on the observer. The whole thing made such an impression on me that it motivated me to teach myself calculus with books from my local library, so that I could understand more of it; so at the age of 13 or 14 I did differentials and integrals, and made my first clumsy attempts at messing about with tensors, much to the puzzlement of my classmates and astonishment of my teachers.

[swansont]

!

Moderator Note

Discussion of whether relativity is just light propagation delay has been split

https://www.scienceforums.net/topic/123006-time-dilation-is-propagation-delay-split-from-what-is-time-again/