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- Thread starter junglebeast
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This is not correct. You can have change wrt time (d/dt) but you can also have change wrt space (d/dx).But without time, there can be no change, because change is something that occurs over time.

That said, The Grand Design is a pop-sci book, not mainstream scientific literature. Physicists writing pop-sci books often make ambiguous statements that try to convey highly technical material in a overly simplified manner.

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I agree with that: an inflation over distance without the existence of a time dimension appears to imply no change over a time dimension - a "frozen" universe. Thus no physical change (= a change over time) of the universe.

But perhaps they mean something else (if so, what?).

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Simply because you arbitrarily choose to label d/dt as a "physical change" and d/dx as a non-"physical change" does not alter the results of any experiments in the least. According to you Maxwell's equations and the Einstein field equations and QM are highly successful physical theories that are all full of non-"physical changes".Thus no physical change (= a change over time) of the universe.

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No. Since Newton* and even before, theories of physics are "mathematical physics": mathematical descriptions that allow to make predictions about observations of physical phenomena.[..]According to you Maxwell's equations and the Einstein field equations and QM are highly successful physical theories that are all full of non-"physical changes".

Note that I followed the OP's formulation in my answer; debating words instead of trying to answer his question is not very helpful.

*http://en.wikipedia.org/wiki/Philosophiæ_Naturalis_Principia_Mathematica

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Most of which involve what you arbitrarily deem non-"physical changes" in the course of making said predictions.theories of physics are "mathematical physics": mathematical descriptions that allow to make predictions about observations of physical phenomena.

The OP's formulation is wrong, as is yours. d/dt is not the only change in physics. Correcting errors is helpful, even if it is not appreciated.Note that I followed the OP's formulation in my answer; debating words instead of trying to answer his question is not very helpful

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I'm pretty sure that most people will disagree with your formulation which confounds mathematical relationships with physical changes. But instead of entertaining debates about words, do you have any useful answer to the OP's question?[..] The OP's formulation is wrong, as is yours. [..]

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I call BS on this. I challenge you to find a mainstream scientific reference that supports the idea that only d/dt can be considered a "physical change" let alone one that supports the claim that most people agree with that idea. I have not come across either type of claim in my studies.I'm pretty sure that most people will disagree with your formulation

Yes, see post 2 where I provided both the correction and a useful answer.But instead of entertaining debates about words, do you have any useful answer to the OP's question?

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ghwellsjr

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Didn't we just have this conversation less than a month ago?

Yes, we did, starting with post #37 (page 3) on this locked thread:

https://www.physicsforums.com/showthread.php?t=537940"

And I'll repeat my comment from post #82:

Yes, we did, starting with post #37 (page 3) on this locked thread:

https://www.physicsforums.com/showthread.php?t=537940"

And I'll repeat my comment from post #82:

Maxwell's equations contain two spatial derivatives, the curl and the divergence, which are vector operators, along with the gradient which calculate changes in vector fields that do not involve time. Wikipedia says the gradient "Measures the rate and direction of change in a scalar field". Maybe you should edit the wikipedia article so the world can be in line with your opinion.

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Yes, and harrylin was unable to justify his position then either.Didn't we just have this conversation less than a month ago?

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DrGreg

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Even outside the technical language of maths and physics, it's quite normal in everyday speech to talk about, say, the

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This is not correct. You can have change wrt time (d/dt) but you can also have change wrt space (d/dx).

Ok, fair enough. It is true that change can be measured (mathematically) with respect to any dimension, but your observation leaves much to the imagination with respect to my question.

It seems that you are saying the the inflationary changes of the three spatial dimensions can be measured relative to the fourth dimension. Moreover, the meaning of the fourth dimension changes from being spacelike to timelike with respect to itself.

From an intuitive perspective, the difference between a spacelike and a timelike dimension is that energy is constant relative to the timelike dimension (ie, energy is conserved), whereas energy is non-uniformly distributed throughout the spatial dimensions.

Thus, if the timelike dimension originally behaved like a spacelike dimension, then I assume it is implied that there was no conservation of energy during the initial phase of the big bang...and as the fourth dimension becomes more timelike, this is really just saying that conservation of energy gradually starts to apply.

Yes? No?

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Yes, and harrylin was unable to justify his position then either.

Wrong again: I feel no desire to start a dog fight in what should be a polite exchange of opinions. In both this and the other thread a debate about words is totally irrelevant and distracts from the subject at hand - it's a form of trolling.

Harald

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Yes that is rather standard in mathematics, but surely irrelevant for the discussion here.When I was first taught differentiation, many decades ago, I was taught that the symbol "df/dx" could be described in words as "the rate ofchangeoffwith respect tox". That was standard terminology then as I believe it still is now.

Not that it matters here but I have never heard anyone say such things in everyday speech; evidently I don't meet the people that you meet (nor does the dictionary know of it). :tongue2:Even outside the technical language of maths and physics, it's quite normal in everyday speech to talk about, say, thechangein temperature between the south and the north, or achangein height between a valley and a hill.

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My intuition seems to work differently from yours, for how can you think of "originally behaved" and "during", if there was no time to do the "during" in, nor for an "initial phase"?[..] It seems that you are saying the the inflationary changes of the three spatial dimensions can be measured relative to the fourth dimension. Moreover, the meaning of the fourth dimension changes from being spacelike to timelike with respect to itself.

From an intuitive perspective, the difference between a spacelike and a timelike dimension is that energy is constant relative to the timelike dimension (ie, energy is conserved), whereas energy is non-uniformly distributed throughout the spatial dimensions.

Thus, if the timelike dimension originally behaved like a spacelike dimension, then I assume it is implied that there was no conservation of energy during the initial phase of the big bang...and as the fourth dimension becomes more timelike, this is really just saying that conservation of energy gradually starts to apply.

Yes? No?

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From an intuitive perspective, the difference between a spacelike and a timelike dimension is that energy is constant relative to the timelike dimension (ie, energy is conserved), whereas energy is non-uniformly distributed throughout the spatial dimensions.

Although superficially it may look like what you say, this differentiation is not correct. Conservation of a physical quatity is expressed in terms of a continuity equation, where time and space enter on a nearly equal footing. The change of the quanity in question involves both space and time in like manner.

[tex]\frac{\partial \phi_x}{\partial x} + \frac{\partial \phi_y}{\partial y} + \frac{\partial \phi_z}{\partial z} - \frac{\partial \rho}{\partial t} = 0[/tex]

[itex]\rho[/itex] and [itex]\phi[/itex] form a spacetime vector [itex](\rho, \phi)[/itex].

"Energy conservation" implicitly assumes there is no flow of energy to or from the system evolving over time, automatically precluding changes with repsect to spatial displacement. The complete expression involves momentum terms and changes over space.

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My intuition seems to work differently from yours, for how can you think of "originally behaved" and "during", if there was no time to do the "during" in, nor for an "initial phase"?

These words are simply used in the English language to express relationships between events through the fourth dimension. Whether or not this dimension haves in a timelike or spacelike manner is irrelevant because we have no other word or tense to use, and it would only lead to more confusion if I tried to omit them.

Although superficially it may look like what you say, this differentiation is not correct. Conservation of a physical quatity is expressed in terms of a continuity equation, where time and space enter on a nearly equal footing. The change of the quanity in question involves both space and time in like manner.

[tex]\frac{\partial \phi_x}{\partial x} + \frac{\partial \phi_y}{\partial y} + \frac{\partial \phi_z}{\partial z} + \frac{\partial \rho}{\partial t} = 0[/tex]

[itex]\rho[/itex] and [itex]\phi[/itex] form a spacetime vector [itex](\rho, \phi)[/itex].

Ok...I get what you are saying...basically, the scale factor on the time dimension is so much bigger that it creates the appearance of energy being constant over time as particles bounce around in space, but in reality they can also bounce around in time causing the total amount of energy (as measured at any time) to fluctuate some small amount, but its just not as noticeable.

In that case it seems that the difference between these dimensions is that the scale factor on the time dimensions is so much more significant. When it is said that the fourth dimension behaved like a spatial dimension during the early phase of the singularity is this really saying that all four dimensions were treated with equal weighting in the "continuity equation"?

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Ok...I get what you are saying...basically, the scale factor on the time dimension is so much bigger that it creates the appearance of energy being constant over time as particles bounce around in space, but in reality they can also bounce around in time causing the total amount of energy (as measured at any time) to fluctuate some small amount, but its just not as noticeable.

Look at it this way. I mark off a particular volume of space. Over time, the amount of energy in this volume will be lumpy. Mark off a region in a busy intersection and it will very lumpy. So the same argument applies to lumpy changes over space as time. I don't know what scaling has to do with it. The phenomena would be the same, in either case.

When it is said that the fourth dimension behaved like a spatial dimension during the early phase of the singularity is this really saying that all four dimensions were treated with equal weighting in the "continuity equation"?

Sorry. I don't know what author claimed this behavior, or why.

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Then you completely lost me, as you stressed that nothing can happen in a universe without a time-like dimension. Never mind, I'll watch this thread to see if someone can clarify in what way time can behave like a spatial dimension (or, probablyThese words are simply used in the English language to express relationships between events through the fourth dimension. Whether or not this dimension haves in a timelike or spacelike manner is irrelevant because we have no other word or tense to use, and it would only lead to more confusion if I tried to omit them.

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The Grand Design is by Hawking, and I know that Hawking uses the Wick rotation. He calls it "imaginary time." So I think that is what he was talking about. I'm not the one to ask about the meaning of this, perhaps someone else can explain. But I get the impression that the point of the Wick rotation is to get rid of the peculiar qualities of the time dimension so it behaves like the three spatial dimensions.

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Asking someone to provide a mainstream scientific reference for a stated position is hardly trolling and is always acceptable. On this forum when you make non-scientific claims you should expect to have them challenged.Wrong again: I feel no desire to start a dog fight in what should be a polite exchange of opinions. In both this and the other thread a debate about words is totally irrelevant and distracts from the subject at hand - it's a form of trolling.

While it is true that this is a semantic debate, the

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Fair enough, as long as you understand that in physics "change" is not limited only to things that change wrt time.Let us not become bogged down in differences of arbitrary word definitions. I am seeking a better understanding of the actual process, not a better understanding of the definition of the word we call derivative...so please refrain from such trivial arguments and let us focus on the actual question.

I am not sure of this. I know that the FLRW metric does not change from timelike to spacelike nor vice versa, so I don't know what metric Hawking was refering to here. However, my understanding is that the signature is an invariant of the manifold, so I don't think it is possible for a (++++) manifold to evolve into the (-+++) manifold that we see today.Moreover, the meaning of the fourth dimension changes from being spacelike to timelike with respect to itself.

Unfortunately, as is common with pop-sci books, they are sloppy enough that a careful reader can come away with mistaken impressions. Unless someone knows what metric he was refering to then I suspect it is simply a misunderstanding of Hawking's intent.

Energy is not generally conserved in non-static spacetimes like the FLRW metric. There is a FAQ on the topic in the Cosmology forum:Thus, if the timelike dimension originally behaved like a spacelike dimension, then I assume it is implied that there was no conservation of energy during the initial phase of the big bang.

https://www.physicsforums.com/showthread.php?t=506985

and another one by Baez:

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

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Asking someone to provide a mainstream scientific reference for a stated position is hardly trolling and is always acceptable. On this forum when you make non-scientific claims you should expect to have them challenged.

While it is true that this is a semantic debate, thefactis that your definition is not the one used scientifically. You are, however, incorrect that it is distracting from the subject at hand. The OP shares the same mistaken idea as you do, so it is important to teach him the correct scientific definition so that he can better understand and communicate in the future.

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However, my understanding is that the signature is an invariant of the manifold, so I don't think it is possible for a (++++) manifold to evolve into the (-+++) manifold that we see today.

DaleSpam, you may be confusing metric with manifold.

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That could be. There is a signature for a metric and for a manifold, and I think they are both closely related and both are invariants. It is hard to know without an exact metric to discuss.DaleSpam, you may be confusing metric with manifold.

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