What is time? (Again)

[Markus Hanke]

  On 8/10/2020 at 1:47 PM, joigus said:

I can develop the point in case you're interested, but won't press it otherwise.

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I think I get what you're saying
BTW, why do LaTeX formulas get screwed up when you quote them?

  On 8/10/2020 at 1:47 PM, joigus said:

I don't know what you think about that

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In general I think you are right...I'm just not sure about this particular instance. We first postulate that time might be a result of symmetry breaking; but symmetry breaking can't happen unless there is already time prior to that process. This kind of seems problematic, no? Maybe I am missing your point here somewhere...

  On 8/10/2020 at 5:41 PM, The victorious truther said:

As far as that interpretation goes but this depends on whether you identify spacetime as distinct from matter and its configurations (classical substantivalism), that it cannot exist in the absence of matter with its accompanying configurations (relationism), or that matter is fully identified with a specific spacetime region so an empty spacetime is basically a spacetime devoid of quantum field looking spacetime configurations (super-substantivalism).

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Yes, I get what you are saying. But the thing here is that GR has nothing whatsoever to say about spacetime as such - it is only about the geometry of spacetime. I think this is a really important distinction. The question as to the nature and small-scale structure of spacetime itself is outside the remit of GR. It simply connects energy-momentum distribution to certain aspects of local geometry (and even then it's only a constraint, you still need external boundary conditions), so it is about the dynamics of spacetime, not its nature.

  On 8/10/2020 at 5:59 PM, joigus said:

One of the most important lessons of modern physics is that there is no simple way to define vacuum, or empty space-time. In GR "vacuum" is filled with structure, or has room for it. So is QFT's "vacuum".

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Yes, and there is a fundamental problem here, in that the QFT vacuum is observer-dependent, whereas the GR vacuum is not.

  On 8/10/2020 at 10:48 PM, StringJunky said:

Space is a function of what's in it otherwise it can't exist

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I don't think I would agree with this. At the very least it would be the other way around - what's in it is a function of space. I'd also argue that existence in no way requires either space or time - but that would only lead us down yet another rabbit hole

  On 8/11/2020 at 11:30 AM, michel123456 said:

You know what a function is

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According to its formal definition - a relationship between two or more sets, domain and codomain.

[StringJunky]

  On 8/11/2020 at 3:19 PM, Markus Hanke said:

 

I don't think I would agree with this. At the very least it would be the other way around - what's in it is a function of space. I'd also argue that existence in no way requires either space or time - but that would only lead us down yet another rabbit hole

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This makes my head hurt but that would be a non-sequitur with your second statement, which I agree with... without evidence.  By 'is a function of' I mean it is 'dependent on' or 'arises from' something else. I could be using 'function' in a different/non-standard way to you.

Edited August 11, 2020 by StringJunky

[Markus Hanke]

  On 8/11/2020 at 3:30 PM, StringJunky said:

By 'is a function of' I mean it is 'dependent on' or 'arises from' something else. I could be using 'function' in a different/non-standard way to you.

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I am not sure which definition you use...when I speak of 'functions', I always use the standard textbook definition as a relationship between sets, or more accurately between elements in sets.

[michel123456]

  On 8/11/2020 at 3:30 PM, StringJunky said:

  On 8/11/2020 at 3:19 PM, Markus Hanke said:

I don't think I would agree with this. At the very least it would be the other way around - what's in it is a function of space. I'd also argue that existence in no way requires either space or time - but that would only lead us down yet another rabbit hole

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This makes my head hurt

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Mine too. To me, the "what's in it is a function of space" is totally impossible.

My opinion is that physicists are used to encounter weird things and when they end up with nothingness full like an egg, they say nonchalantly "here another weirdness", with this little smile that you should read as "I don't know what is going on". Physicists have stopped reacting like Erwin Schroedinger with his cat. Schroedinger's cat is an attempt to directly link a macroscopic event (living/dead cat) with a weird event of the microcosm. If scientists could always link  to everyday phenomena the weird results they get, well ...the weird results should vanish. IMHO physicists should think more like in 1935 and be astonished more & more. Not asking everyone to accept the incomprehensible because of lack of education.

Edited August 11, 2020 by michel123456

[joigus]

  On 8/11/2020 at 4:43 PM, michel123456 said:

Mine too. To me, the "what's in it is a function of space" is totally impossible.

My opinion is that physicists are used to encounter weird things and when they end up with nothingness full like an egg, they say nonchalantly "here another weirdness", with this little smile that you should read as "I don't know what is going on". Physicists have stopped reacting like Erwin Schroedinger with his cat. Schroedinger's cat is an attempt to directly link a macroscopic event (living/dead cat) with a weird event of the microcosm. If scientists could always link  to everyday phenomena the weird results they get, well ...the weird results should vanish. IMHO physicists should think more like in 1935 and be astonished more & more. Not asking everyone to accept the incomprehensible because of lack of education.

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WADR,

I don't tell architects (or people who've had proper training in the matter) how to make buildings because I know better. But if I did, I would have no complaint if they call me to task and ask me about structures, beam tensions, corrosion, aesthetics, etc. "I think your corrosion argument is a clumsy attempt to link chemistry with aesthetics" would not be good enough an argument by any standards.

That would be a completely silly, indefensible intellectual position.

Edited August 11, 2020 by joigus

[michel123456]

  On 8/11/2020 at 5:01 PM, joigus said:

WADR,

I don't tell architects (or people who've had proper training in the matter) how to make buildings because I know better. But if I did, I would have no complaint if they call me to task and ask me about structures, beam tensions, corrosion, aesthetics, etc. "I think your corrosion argument is a clumsy attempt to link chemistry with aesthetics" would not be good enough an argument by any standards.

That would be a completely silly, indefensible intellectual position.

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Sure. But if you had a look in your basement and see that the foundations hover 1 meter over the ground, wouldn't you say "what kind of a weird architect did this"? Or if your house suddenly crumbles for no apparent reason, wouldn't you blame the architect?

Or to paraphrase I don't remember who: I cannot pilot a helicopter but when I see one crashing I know that something went wrong.

Edited August 11, 2020 by michel123456

[joigus]

  On 8/11/2020 at 5:19 PM, michel123456 said:

Sure. But if you had a look in your basement and see that the foundations hover 1 meter over the ground, wouldn't you say "what kind of a weird architect did this"? Or if your house suddenly crumbles for no apparent reason, wouldn't you blame the architect?

Or to paraphrase I don't remember who: I cannot pilot a helicopter but when I see one crashing I know that something went wrong.

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OK. But, "something went wrong!!" or "that guy did it!!" doesn't help much, does it?

What does it add to the ongoing argument?

That's my question for you.

[MigL]

  On 8/11/2020 at 3:30 PM, StringJunky said:

This makes my head hurt

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  On 8/11/2020 at 4:43 PM, michel123456 said:

Mine too.

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I don't know about Sringy, but in Michel's case it could be all the bells he keeps hearing

[joigus]

May I also add, @michel123456, that the concepts of vacuum that you so much dislike is also what allows you to formulate plausible scenarios for known experimental physics?

Vacuum Einstein field equations --> The Sitter universe

Vacuum in QFT --> anomalous g-factor of the electron

In the first case, it gives a prediction that escaped Einstein and that's been confirmed by measurements on supernovas.

In the second case, it's the most astonishingly precise prediction that's ever been. (1 part in a billion)

You're clutching at straws here.

[michel123456]

  On 8/11/2020 at 5:21 PM, joigus said:

OK. But, "something went wrong!!" or "that guy did it!!" doesn't help much, does it?

What does it add to the ongoing argument?

That's my question for you.

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Nothing. Bad moment I suppose.

[joigus]

  On 8/11/2020 at 3:19 PM, Markus Hanke said:

BTW, why do LaTeX formulas get screwed up when you quote them?

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I don't know. They must be in PS format when they come out, so LaTeX compilation no longer works on them. That's my guess.

[michel123456]

  On 8/11/2020 at 5:37 PM, joigus said:

May I also add, @michel123456, that the concepts of vacuum that you so much dislike is also what allows you to formulate plausible scenarios for known experimental physics?

Vacuum Einstein field equations --> The Sitter universe

Vacuum in QFT --> anomalous g-factor of the electron

In the first case, it gives a prediction that escaped Einstein and that's been confirmed by measurements on supernovas.

In the second case, it's the most astonishingly precise prediction that's ever been. (1 part in a billion)

You're clutching at straws here.

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Did i say that?

What I dislike is the "structure" of spacetime, the "fabric" of spacetime. The fact that some scientists prefer the empty plate of the balance.

We know that the way we are observing things through spacetime is observer dependent. One observer will observe the "structure" bending like this, the other observer will see the "structure" crash upon itself, the 3rd observer will notice nothing. Doesn't that mean that there is no underlying "structure"? Isn't that some sort of evidence?

Edited August 11, 2020 by michel123456

[joigus]

  On 8/11/2020 at 5:50 PM, michel123456 said:

We know that the way we are observing things through spacetime is observer dependent. One observer will observe the "structure" bending like this, the other observer will see the "structure" crash upon itself, the 3rd observer will notice nothing. Doesn't that mean that there is no underlying "structure"? Isn't that some sort of evidence?

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I don't see the connection.

Observations are observer-dependent ==> There is no underlying structure in the vacuum

???

But this was about time, wasn't it?

[michel123456]

  On 8/11/2020 at 5:59 PM, joigus said:

I don't see the connection.

Observations are observer-dependent ==> There is no underlying structure in the vacuum

???

 

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Don't you? If there are 3 observers and they observe 3 different ways the "structure" behaves, is it possible for a "structure" to actually behave differently at the same time . IOW that there exist 3 different "realities" at the same time?

Edited August 11, 2020 by michel123456

[joigus]

  On 8/11/2020 at 6:09 PM, michel123456 said:

Don't you? If there are 3 observers and they observe 3 different ways the "structure" behaves, is it possible for a "structure" to actually behave differently at the same time . IOW that there exist 3 different "realities" at the same time?

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No, sorry. I don't. It's not the structure that's behaving differently; the observers are.

What that has to do with the vacuum goes through my mind like neutrinos through a paper sheet.

You had a very interesting observation (IMO) about macroscopic observers being deprived of completely reconstructing their own past world-lines (the way I understood your argument). I thought so and I still do. Debatable perhaps, but made sense to me.

But then you went farther and farther afield into a dark territory I know nothing of, nor do I have any intuition of what you mean. I've never been there. Sounds to me like you're trying to build a nuclear power station with tinker-toy assembly pieces and you forgot the plutonium.

Edited August 11, 2020 by joigus

[Markus Hanke]

  On 8/11/2020 at 5:50 PM, michel123456 said:

We know that the way we are observing things through spacetime is observer dependent. One observer will observe the "structure" bending like this, the other observer will see the "structure" crash upon itself, the 3rd observer will notice nothing. Doesn't that mean that there is no underlying "structure"? Isn't that some sort of evidence?

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I should point out here that not all relevant quantities are observer-dependent. Locally, all proper quantities - such as proper acceleration, proper length etc - are invariant, and all observers agree on them. Furthermore, all those quantities that characterise the geometry of spacetime and the distribution of energy-momentum therein are tensors, so all observers agree on them, too. So there is only one reality, which is characterised by suitable invariant and covariant quantities (which may not always be obvious to us).

What I do find fascinating though is that the vacuum ground state of quantum field theories is not one of those quantities - it is explicitly observer-dependent. Hence, where one observer sees a vacuum, another observer may see a thermal bath of particles. This raises some interesting questions about the ontology of what we usually consider to be the fundamental building blocks of our universe (particles). It also makes it obvious that there is a deep link there somewhere between GR and QFT.

[joigus]

  On 8/11/2020 at 3:19 PM, Markus Hanke said:

In general I think you are right...I'm just not sure about this particular instance. We first postulate that time might be a result of symmetry breaking; but symmetry breaking can't happen unless there is already time prior to that process. This kind of seems problematic, no? Maybe I am missing your point here somewhere...

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Yes, you're right. Don't take me seriously. What I was saying there was in the spirit of parametrizing a problem, see if I can understand it better in terms of parameters. That's all. I have no idea if there could be a meaningful way to talk about something "happening" in that context. It would be very, very iffy. Just a "what if".

Edited August 12, 2020 by joigus

[michel123456]

  On 8/11/2020 at 6:25 PM, joigus said:

No, sorry. I don't. It's not the structure that's behaving differently; the observers are.

What that has to do with the vacuum goes through my mind like neutrinos through a paper sheet.

You had a very interesting observation (IMO) about macroscopic observers being deprived of completely reconstructing their own past world-lines (the way I understood your argument). I thought so and I still do. Debatable perhaps, but made sense to me.

But then you went farther and farther afield into a dark territory I know nothing of, nor do I have any intuition of what you mean. I've never been there. Sounds to me like you're trying to build a nuclear power station with tinker-toy assembly pieces and you forgot the plutonium.

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Interesting, and maybe right. I can live with that.*

  On 8/12/2020 at 4:41 AM, Markus Hanke said:

I should point out here that not all relevant quantities are observer-dependent. Locally, all proper quantities - such as proper acceleration, proper length etc - are invariant, and all observers agree on them. Furthermore, all those quantities that characterise the geometry of spacetime and the distribution of energy-momentum therein are tensors, so all observers agree on them, too. So there is only one reality, which is characterised by suitable invariant and covariant quantities (which may not always be obvious to us).

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Yes. One reality. You should have come here much earlier. There are a bunch of people here that argue about multiple reality and that drives me out of mind.

  On 8/12/2020 at 4:41 AM, Markus Hanke said:

What I do find fascinating though is that the vacuum ground state of quantum field theories is not one of those quantities - it is explicitly observer-dependent. Hence, where one observer sees a vacuum, another observer may see a thermal bath of particles. This raises some interesting questions about the ontology of what we usually consider to be the fundamental building blocks of our universe (particles). It also makes it obvious that there is a deep link there somewhere between GR and QFT.

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And when an observer sees space, another may see time.

* Take into consideration that I am erasing continuously my darkest thoughts🙂

 

Edited August 12, 2020 by michel123456

[dimreepr]

  On 8/11/2020 at 5:19 PM, michel123456 said:

Sure. But if you had a look in your basement and see that the foundations hover 1 meter over the ground, wouldn't you say "what kind of a weird architect did this"?

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Not unless I'm an architect. 😉

[joigus]

  On 8/12/2020 at 4:41 AM, Markus Hanke said:

What I do find fascinating though is that the vacuum ground state of quantum field theories is not one of those quantities - it is explicitly observer-dependent. Hence, where one observer sees a vacuum, another observer may see a thermal bath of particles. This raises some interesting questions about the ontology of what we usually consider to be the fundamental building blocks of our universe (particles). It also makes it obvious that there is a deep link there somewhere between GR and QFT.

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OK. I've been racking my brain about this for a while. You mean the Unruh effect, or the Rindler coordinates for flat space-time. Those are accelerated observers. The bare vacuum (non-interacting theory) is invariant under Lorentz transformations, and the dressed vacuum (dressed with interactions) I think also is, if I remember correctly.

$$U\left(\Lambda\right)\left|0\right\rangle =\left|0\right\rangle$$

$$U\left(\Lambda\right)\left|\Omega\right\rangle =\left|\Omega\right\rangle$$

(you can also assume a phase á la Wigner)

Mind you, temperature in GR is something that still has to be understood, I hope you agree. I also think there must be a deep connection between GR and QFT that requires understanding entropy in GR and accelerated observers in QFT more thoroughly.

It seems that some people in the scientific community share your worries:

http://www.scholarpedia.org/article/Unruh_effect#Persisting_controversies

  On 8/12/2020 at 7:21 AM, michel123456 said:

Yes. One reality. You should have come here much earlier. There are a bunch of people here that argue about multiple reality and that drives me out of mind.

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Count me in. Multiple universes makes me very unhappy too. But that's in the area of interpretation of the theory. There's ground for reasonable disagreement, I surmise.

  On 8/12/2020 at 7:21 AM, michel123456 said:

* Take into consideration that I am erasing continuously my darkest thoughts🙂

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I do too.  

[michel123456]

  On 8/12/2020 at 1:03 PM, joigus said:

You mean the Unruh effect, or the Rindler coordinates for flat space-time. Those are accelerated observers

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The Unruh effect: under the prism of what I posted you in PM, it may be (although I thought it would be impossible) that the observer under ridiculously high acceleration is "kicked out" of his own time line in such a way that he can observe particles of his own future (otherwise unobservable) or belonging to his own past, depending on the direction of acceleration.

[joigus]

  On 8/13/2020 at 6:59 AM, michel123456 said:

The Unruh effect: under the prism of what I posted you in PM, it may be (although I thought it would be impossible) that the observer under ridiculously high acceleration is "kicked out" of his own time line in such a way that he can observe particles of his own future (otherwise unobservable) or belonging to his own past, depending on the direction of acceleration.

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Or for just any acceleration. It is not a crazy idea at all that non-locality/non-causality are present at a very small scale that cannot have consequences farther away than a certain tiny range. So you could have both pre-images and post-images of your local universe that your mind integrates in a "solid picture", so to speak. I think that's possible.

But the priority, I think, is to understand where temperature comes from in GR (what degrees of freedom it's talking us about) and obtaining a generalisation of QFT workable for general coordinate systems (what's called in the lingo diffeomorphism-invariant). See how temperature arises in both contexts, then understand what both temperatures mean and relate them. Easier said than done... Keep in mind that whenever you have a temperature, it means that there are dynamical degrees of freedom that are not in your description, so your description is averaging over them.

Then you've got Maldacena's mind-blowing mathematical result that gravity inside a ball is describing classical gauge field theory on the surface of that ball, but at the price of having the metric be anti-DeSitter (something like an anti-universe or exponentially-contracting universe). This strongly suggests that any new physics should be capable of relating inside-outside quantities for any observers (that naturally perceive some kind of inside-outside distinction), which is what I was trying to connect with before.

More tame speculations later...

[Markus Hanke]

  On 8/12/2020 at 7:21 AM, michel123456 said:

Yes. One reality. You should have come here much earlier. There are a bunch of people here that argue about multiple reality and that drives me out of mind.

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Well, GR is a purely classical theory, so it doesn't support the idea of 'multiple realities'. There are of course lots of observable quantities which do depend on the observer (they are not invariant/covariant), but observer-dependence does not imply multiple realities.

  On 8/12/2020 at 1:03 PM, joigus said:

You mean the Unruh effect, or the Rindler coordinates for flat space-time. Those are accelerated observers. The bare vacuum (non-interacting theory) is invariant under Lorentz transformations, and the dressed vacuum (dressed with interactions) I think also is, if I remember correctly.

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Indeed - but it isn't so much the finer details of how exactly the observer moves, but rather the very fact itself that the observed vacuum depends on the motion of the observer at all. You travel in an accelerating rocket and see a sea of particles around you; then you stop, and pop! - they are all gone, though you are still in the same region of spacetime. If you really think about this, it poses very serious questions about what is really fundamental, and what is not.

  On 8/13/2020 at 11:51 AM, joigus said:

Then you've got Maldacena's mind-blowing mathematical result that gravity inside a ball is describing classical gauge field theory on the surface of that ball, but at the price of having the metric be anti-DeSitter

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The result has already been replicated for deSitter space (dS/CFT correspondence), and even Kerr spacetimes (Kerr/CFT correspondence). It suggests that the duality itself is an expression of some deeper connection between bulk and boundary - I bet there is some form of underlying duality that relates the two for any kind of spacetime, independent of its specific geometry (or perhaps for some physically significant subset of geometries). Finding this would be a major breakthrough.

  On 8/13/2020 at 11:51 AM, joigus said:

Keep in mind that whenever you have a temperature, it means that there are dynamical degrees of freedom that are not in your description, so your description is averaging over them.

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Good point +1  Never looked at it this way.

[studiot]

  On 8/13/2020 at 5:00 PM, Markus Hanke said:

The result has already been replicated for deSitter space (dS/CFT correspondence), and even Kerr spacetimes (Kerr/CFT correspondence). It suggests that the duality itself is an expression of some deeper connection between bulk and boundary - I bet there is some form of underlying duality that relates the two for any kind of spacetime, independent of its specific geometry (or perhaps for some physically significant subset of geometries). Finding this would be a major breakthrough.

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Indeed there is.

It's called the fundamental theorem of calculus.

[joigus]

  On 8/13/2020 at 5:00 PM, Markus Hanke said:

The result has already been replicated for deSitter space (dS/CFT correspondence), and even Kerr spacetimes (Kerr/CFT correspondence). It suggests that the duality itself is an expression of some deeper connection between bulk and boundary - I bet there is some form of underlying duality that relates the two for any kind of spacetime, independent of its specific geometry (or perhaps for some physically significant subset of geometries). Finding this would be a major breakthrough.

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I wasn't aware of this. I must have been sleeping all these years. Thank you. +1

  On 8/13/2020 at 5:56 PM, studiot said:

Indeed there is.

It's called the fundamental theorem of calculus.

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Mmmm...

Remindful of, suggestive of, rather than equal.

Reminds more of Cauchy's integral theorem of complex calculus.

And even more of Stokes' theorem for differential forms.

Because we always use analytic functions, things on the inside are determined by things on the surface. But I'm getting hopelessly vague and metaphorical.

Although the FTC is a particular case.