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a reply to: joelr
Even if you intended to omit dark energy from your explanation, I don't understand why you would say that "Spacetime expands slowly on a local scale". The Hubble constant infers something twice as far away is receding twice as fast (in a statistical "best fit" of data) which implies the rate of expansion is not any slower on a local scale, where local in this case would be say 30 million light years or so, near the lower end of where Hubble's law applies.

As for the Hubble constant dealing with distance or time, the units are inverse time, thus the inverse of the Hubble constant is the Hubble time so I'm not sure what point you're trying to make by asking if it's a distance scale. Distance and time are related in ways that can get a little confusing in cosmological terms, since we don't observe things where they are, we observe things where they were.

More of a personal question regarding your backgrounds. I am assuming most of the contributors in this topic do not work in physics or science even though they have a physics background (at the undergraduate or graduate level). Is this the most use you guys will get out of your physics background or does it come into your daily work at all? Just curious.

a reply to: Diablos

A number of us ARE in STEM fields, and use it constantly.

originally posted by: Bedlam
a reply to: Diablos

A number of us ARE in STEM fields, and use it constantly.

Im in the alfalfa fields if that counts!

a reply to: ImaFungi

Motion (and complex, multi layered, multi qualitative, 'particle decay-able' motions at that) is continuous, so every time one attempts to pint point a locality it has already moved/changed/interacted with other moving parts in other ways;

I totally concur. Quantizing things does not tell you true nature only how we can apply a 'ruler' to it. In short, distance is not made of millimeters just because science came up with the way to measure it.

a reply to: Arbitrageur

So if Earth is at the center of the universe, that might possibly explain observations without dark energy according to these mathematicians, but most scientists don't think Earth has such a special place in the universe.

From being in the center of observer idea, why not7 We are at the center but not in a sense you were trying to depict it. That's what perspective means, the point of view.

Space-time has its scape to it. It is up and down, left and right back and forward and everything in between. Just go distance outdoors and look at the beautiful surroundings and try to estimate the distance between your coordinate and the distant village. Did you forget to conceder valley and a plane between two points? Light takes exactly that complex path.




cheers] Thank you.

removed

originally posted by: greenreflections
From being in the center of observer idea, why not7 We are at the center but not in a sense you were trying to depict it. That's what perspective means, the point of view.

In the mainstream Lambda-CDM model, two different perspectives from observers in widely separated galaxies yield very similar expansion patterns with dark energy, while in the mathematician's model two such different perspectives yield very different expansion patterns because their model has no dark energy and one observer is at the center of the universe while the other isn't. Your comment is so unclear I can't understand what it's supposed to mean, but it does seem to infer you don't understand the difference.

a reply to: delbertlarson

By the way, the paper is not something I personally wrote; no I'm not at that level. I think about QM for fun as a hobby, but my day job (like many physicists) is software and data analytics. I worked on chaotic signal analysis in academia.

a reply to: Diablos

I don't use physics at my work sadly but I use all sorts of things I learned on the way.

a reply to: delbertlarson

I agree that not all interactions mean a creation of particles. But I believe that in QFT the fields are more fundamental than the particles which are more ephemeral sometime things. If we think about fields first, and time evolution on fields (more like classical EM but with quantum fields) there seems to me to have less difficulty conceptually, after you swallow the Big one, that the quantum fields live in functional space and not just over x,y,z,t. So if E & B classically live in x,y,z,t; the quantum verision is a wavefunction of a function.

Go from point particle, to a wavefunction: QM of point particles goes from classical physics of a point in a vector space (x,y,z,t) to a function.

Next level: true quantum mechanics of electromagnetism is a function of a function, i.e. the wavefunction of the electromagnetic field. And similarly QFT of SM fields are functions of functions.

originally posted by: Arbitrageur
a reply to: joelr
Even if you intended to omit dark energy from your explanation, I don't understand why you would say that "Spacetime expands slowly on a local scale". The Hubble constant infers something twice as far away is receding twice as fast (in a statistical "best fit" of data) which implies the rate of expansion is not any slower on a local scale, where local in this case would be say 30 million light years or so, near the lower end of where Hubble's law applies.

Because I think some people have the impression that because distant parts of the Universe can seemingly expand faster than light or even near light speed then they get an idea that you could go to that location and actually see nearby galaxies expanding near light speed. But from that point of view expansion would not be any different from our local point of view. So on a local scale expansion seems slower.

originally posted by: mbkennel
a reply to: delbertlarson

I agree that not all interactions mean a creation of particles. But I believe that in QFT the fields are more fundamental than the particles which are more ephemeral sometime things. If we think about fields first, and time evolution on fields (more like classical EM but with quantum fields) there seems to me to have less difficulty conceptually, after you swallow the Big one, that the quantum fields live in functional space and not just over x,y,z,t. So if E & B classically live in x,y,z,t; the quantum verision is a wavefunction of a function.

Yes QFT starts with a classical field equation then it's quantized and you get excitations in the field which is the particle part.The purpose isn't fields themselves, it's just a better way to deal with multi-particle systems that also incorporates other important concepts like Feynman diagrams and QM.

originally posted by: mbkennel
a reply to: delbertlarson

I agree that not all interactions mean a creation of particles. But I believe that in QFT the fields are more fundamental than the particles which are more ephemeral sometime things. If we think about fields first, and time evolution on fields (more like classical EM but with quantum fields) there seems to me to have less difficulty conceptually, after you swallow the Big one, that the quantum fields live in functional space and not just over x,y,z,t. So if E & B classically live in x,y,z,t; the quantum verision is a wavefunction of a function.

Go from point particle, to a wavefunction: QM of point particles goes from classical physics of a point in a vector space (x,y,z,t) to a function.

Next level: true quantum mechanics of electromagnetism is a function of a function, i.e. the wavefunction of the electromagnetic field. And similarly QFT of SM fields are functions of functions.

I have never believed in the idea of point particles. While relativity requires point-like events in four space, and while the modern "in" thing is both relativity and point-like particles, to me it is absurd. The problem comes in with all the infinities associated with a point-like particle. Gravity, Coulomb, and the magnetic dipole forces all go to infinity in such a view. It is far simpler if there is some finite extent to things. This is one of the attractions of string theory, but for me I always believed there should be finite extent in all three dimensions. Call it a ball theory if you wish.

I don't swallow the Big One either. My understanding of Hilbert Spaces is that each function within that space is an eigen function of the relevant quantum operators and that any arbitrary state can collapse to any one of those functions. Prior to a measurement a state may be in the superposition of several such functions. But each function is itself a function of x, y, z and t, and so also is the superposition.

My view is that each entity is a diffuse, finite thing. It has a density given by the square of the wave function. Wave functions collapse upon exchange of momentum. Everything exists in an absolute three dimensional space, time is the parameter that orders events, and no point-like entities exist. It then becomes our job as physicists to explain experimental results based on those precepts.

originally posted by: delbertlarson

I don't swallow the Big One either. My understanding of Hilbert Spaces is that each function within that space is an eigen function of the relevant quantum operators and that any arbitrary state can collapse to any one of those functions. Prior to a measurement a state may be in the superposition of several such functions. But each function is itself a function of x, y, z and t, and so also is the superposition.

Yes, that's the "point"

What is the state of the system, "now"? A state, ever since Newton, is a value in some space of all possible states. The state of a point mechanical particle in Newtonian physics is s(t) = (x,y,z,x',y',z',t). It's a point, a dot, in a *finite* dimensional vector space. Given s(t), you can get s(t+delta t).

Maxwell: the state now is a tensor of the electromagnetic field (ignore charges for the moment). This is a function over x,y,z of all possible electromagnetic field values. The state space is not a finite dimensional vector space, but a function.

QM of a single point particle: QM of a Newtonian particle is a function. Raise up from finite dim vector space to a functional space.

QFT: Raise up from classical fields (functions) to functions of functions.


QM: Since a superposition of functions is also a valid state, Base function = Phi1(x,y,z,t), Phi2(x,y,z,t), Phi3(x,y,z,t).
Each one is a valid state. But countable and even uncountable mixtures of those are also valid states are also valid states of the quantum mechanical system.

What is that QM system's state space? The current state is 's', state space is 'S'. s in S.

What is Capital S? Von Neumann figured it out: it's a Hilbert space. Current state of the QM system is a point in this Hilbert space, and laws of physics are evolution operators operating on that space.

Here's the difference between QM and classical electromagnetism (somebody correct me if I"m wrong, please) Suppose the Phi1 and Phi2 etcetera happened to be the electromagnetic eigenstates of a classical waveguide/cavity. Then by superposition of Maxwell's equations any combination of them is also a valid state. Except that you could write the time evolution operator (i.e. the dynamical laws of electrodynamics) on the linear sum of those, i.e. the net electromagnetic field. You can exchange time evolution operators on the individual decompositions, and then mix them, or you can mix them, and apply a time evolution operator on the net result and get the same answer. Because there is just one evolution operator on THE electromagnetic field.

In full QM with multiple particles (i.e. not Schroedinger equation) you can't do that. You can't average wavefunctions arithmetically, and time-evolve, and get the same answer if you did it the other way around.

The time evolution operator is in the Hilbert space, not the individual space. And entanglement lies in there.

There really is a difference: you can do QFT of electromagnetic fields, called "quantum optics" and get results which are not the same as Maxwell's equations in some limits, and these are experimentally confirmed.

originally posted by: joelr

originally posted by: mbkennel
a reply to: delbertlarson

I agree that not all interactions mean a creation of particles. But I believe that in QFT the fields are more fundamental than the particles which are more ephemeral sometime things. If we think about fields first, and time evolution on fields (more like classical EM but with quantum fields) there seems to me to have less difficulty conceptually, after you swallow the Big one, that the quantum fields live in functional space and not just over x,y,z,t. So if E & B classically live in x,y,z,t; the quantum verision is a wavefunction of a function.

Yes QFT starts with a classical field equation then it's quantized and you get excitations in the field which is the particle part.The purpose isn't fields themselves, it's just a better way to deal with multi-particle systems that also incorporates other important concepts like Feynman diagrams and QM.

I would have to disagree with the "purpose". The most fundamental equations, the Ansatz which reflects one's opinion about what the facts of Nature are, starts with writing down the Lagrangian density as a function of fields.

Particles are derived from it secondarily. They're important because that's how we measure stuff in experiments, but they seem less fundamental.

Feynman diagrams come from a procedure to enumerate terms in a perturbation theory to approximate the "true" physics which comes out of the underlying field interactions. You need them to adequately compute an experimentally accessible number.

That's like solving antennae by going to a finite mode approximation (each mode would be a 'particle') and deciding to cut off counting at some level of approximation.

But the real theory is full Maxwellian EM on a continuum, and everybody agrees that the EM field is more fundamental than the modes.

originally posted by: mbkennel
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I would have to disagree with the "purpose". The most fundamental equations, the Ansatz which reflects one's opinion about what the facts of Nature are, starts with writing down the Lagrangian density as a function of fields.

Particles are derived from it secondarily. They're important because that's how we measure stuff in experiments, but they seem less fundamental.

Feynman diagrams come from a procedure to enumerate terms in a perturbation theory to approximate the "true" physics which comes out of the underlying field interactions. You need them to adequately compute an experimentally accessible number.

That's like solving antennae by going to a finite mode approximation (each mode would be a 'particle') and deciding to cut off counting at some level of approximation.

But the real theory is full Maxwellian EM on a continuum, and everybody agrees that the EM field is more fundamental than the modes.

No I agree that the fields are more fundamental. When I say fields are not the purpose I mean it's used not because Physicists like fields it's used because of it's mathematical power. It can deal with multiple body systems that the Schrodinger wave function cannot.
The field theory also branches out and is used in many different areas of physics.

originally posted by: joelr
Because I think some people have the impression that because distant parts of the Universe can seemingly expand faster than light or even near light speed then they get an idea that you could go to that location and actually see nearby galaxies expanding near light speed. But from that point of view expansion would not be any different from our local point of view. So on a local scale expansion seems slower.

No, the expansion doesn't seem slower and isn't slower locally, not in that context.

Local recessional velocities near the low end of Hubble's law applicability are lower, but this isn't because of slower local expansion.

Look at the balloon analogy. Attach some little pieces of paper to spots on a balloon (see video), and inflate the balloon, where the surface of the balloon is a 2D model of the 3D expansion of the universe. From the perspective of any paper dot, the closest dots aren't receding as fast as the more distant dots, but this isn't because the expansion is any slower around the observing dot; rather, the expansion is relatively uniform across the surface of the balloon.

I didn't say anything about your statement "on a local scale expansion seems slower" when you first made it, because in an informal forum like this we can be a bit sloppy with our language and it's not a big deal. But when someone questioned if this was true I had to agree that it really isn't a correct statement, and I'm kind of surprised that you haven't just admitted this was sloppy language and not correct, because I suspect you know that, though I could be wrong so if I am I'm hoping this balloon analogy will help you understand why it's wrong. This video explains observations correctly (sans dark energy as in your simplified explanation) using the balloon analogy by saying the following:

Introductory Astronomy: Balloon Analogy for Expansion

-Galaxies are moving away from us.
-Galaxies that are further away are moving faster.

That's true but it doesn't imply local expansion is slower, because in the balloon analogy they show those observations hold even when the local expansion is the same as non-local expansion, that is, the balloon is expanding at the same rate everywhere on its surface.

Now of course if you go outside the context of your original statement, to parts of the universe closer than the lower end of Hubble's law (less than about 10 megaparsecs), then gravity dominates over dark energy and at such cosmologically close distances the expansion really is slower, or even zero when you get close enough (molecules aren't expanding), but your original statement and clarification don't suggest you were referring to this type of context.

originally posted by: Arbitrageur
No, the expansion doesn't seem slower and isn't slower locally, not in that context.

Local recessional velocities near the low end of Hubble's law applicability are lower, but this isn't because of slower local expansion.

Look at the balloon analogy. Attach some little pieces of paper to spots on a balloon (see video), and inflate the balloon, where the surface of the balloon is a 2D model of the 3D expansion of the universe. From the perspective of any paper dot, the closest dots aren't receding as fast as the more distant dots, but this isn't because the expansion is any slower around the observing dot; rather, the expansion is relatively uniform across the surface of the balloon.

I didn't say anything about your statement "on a local scale expansion seems slower" when you first made it, because in an informal forum like this we can be a bit sloppy with our language and it's not a big deal. But when someone questioned if this was true I had to agree that it really isn't a correct statement, and I'm kind of surprised that you haven't just admitted this was sloppy language and not correct, because I suspect you know that, though I could be wrong so if I am I'm hoping this balloon analogy will help you understand why it's wrong. This video explains observations correctly (sans dark energy as in your simplified explanation) using the balloon analogy by saying the following:

Introductory Astronomy: Balloon Analogy for Expansion

-Galaxies are moving away from us.
-Galaxies that are further away are moving faster.

That's true but it doesn't imply local expansion is slower, because in the balloon analogy they show those observations hold even when the local expansion is the same as non-local expansion, that is, the balloon is expanding at the same rate everywhere on its surface.

Now of course if you go outside the context of your original statement, to parts of the universe closer than the lower end of Hubble's law (less than about 10 megaparsecs), then gravity dominates over dark energy and at such cosmologically close distances the expansion really is slower, or even zero when you get close enough (molecules aren't expanding), but your original statement and clarification don't suggest you were referring to this type of context.

I get that, the balloon has the same rate of expansion all over it's surface. If the balloon were Universe size then it would be the same thing. People looking at pieces of paper taped on the balloon that were far away from their perspective would notice it looked like those pieces of paper were expanding much faster than the paper pieces around their paper. Then they might think that if they travelled to that area where the paper was expanding very fast, they would see fast expansion up close.

When they got there expansion would look no different that it did when they were at their home paper. From their new perspective they might see their old neighborhood to be expanding faster now.

So again, I was trying to explain this illusion in an informal way. This IS the Hubble Constant, for so many megaparsects apart the speed separating galaxies increases by "x".
So therefore on a smaller scale, or local scale, expansion seems slower.

Meaning you can't go to the place where galaxies are speeding apart near light speed. Because wherever you go, you are now LOCAL to that area, and you will not see lightspeed expansion. You can see it in a telescope but not in your local area.
Again, it was part of an attempt to show that the fast expansion speeds we see on a far away galaxy doesn't mean that the galaxy is expanding away like a rocket at 1/2 light speed. You have to take all the expansion from here to there and add it up little bits at a time.

a reply to: joelr

Time exists, it's one of the vectors of "position" in defining a location. There is a common mis-understanding about time that comes from some Physicists saying that the FLOW of time may be an illusion.
Flow of time may be psychological but time as an aspect of reality is 100% real. Not only can it be measured but we can predict exact changes in it's rate of change depending on it's speed and gravitational influence.

I too thinking time is a vector (no negative allowed).
And defining position only in 3D is never going to be enough if someone wants to be precise. Space-time....you know, the flow of time should be a value depended to the fact that space expanding, imo, and vise versa. If space expanding then time flows forward along too. It is 'space-time' after all))) When space stops expanding, time will cease to exist.
Hehe...that would mean space will never start contracting as it is a vector just like time.

Spece-time will preserve definition of 'forward movement' via new universe, white whole, what ever you call it.


cheers )