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Jeff Root
2014-Aug-03, 12:51 AM
Another thread just made me wonder ...

Until acceleration of the cosmic expansion was discovered in
1998, it was thought that the farther away a galaxy is, the
faster we and it are moving apart. Roughly graphed, speed
versus distance would be a line rising to the right.

Since it takes light time to reach us from those galaxies,
acceleration of the expansion beginning about 5 or 6 billion
years ago would mean that the closer a galaxy is to us, the
more acceleration it would show. Graphed as a line falling
to the right.

Could the two cancel each other out? So that speed versus
distance would be nearly constant? How close do they come
to actually doing that, for at least some range of distance?

-- Jeff, in Minneapolis

Jens
2014-Aug-04, 05:07 AM
Sorry, I can't give an answer, but wanted to say that this is also something that I think about and am interested in seeing an answer to. . . In the first place, I find it hard to wrap my brain around the idea that the further things are from us, the faster they are moving away from us, but at the same time, the further we look the further back in time it is, so the closer the galaxies should be to each other, even though they're moving away faster

WayneFrancis
2014-Aug-04, 09:02 AM
My understanding is they factor that in. IE the rate of expansion for any given global T would be constant but looking back across space and time the expansion isn't constant because of increased gravitational effect. So galaxies do indeed have a higher observed red shift for their given distance from us as you get closer to us. We can compute for any given T what their recession rate should be but observation needs to factor in the different conditions earlier on in the history of our visible universe.

My understanding also says that galaxies further away have a larger angular size then they really do because of the expansion. The effect makes the density look different then if you could just take a snap shot and look at the visible universe as a whole.

Cougar
2014-Aug-04, 11:34 AM
...acceleration of the expansion beginning about 5 or 6 billion
years ago would mean that the closer a galaxy is to us, the
more acceleration it would show. Graphed as a line falling
to the right.

This doesn't seem right. The acceleration is an increase in expansion. If expansion is "a line rising," wouldn't accelerated expansion also be "a line rising," but a bit more?

WaxRubiks
2014-Aug-04, 11:35 AM
I find it hard to wrap my brain around the idea that the further things are from us, the faster they are moving away from us,

if you think of sand stuck on a rubber band, and then stretch the rubber band back and forth, you will see the sand grains near both hands are moving the fastest away from each other......and the grains just to the side of any othe grains isn't moving away from its neighbour, so fast.

Or did you mean something else?

Cougar
2014-Aug-04, 11:52 AM
...the further things are from us, the faster they are moving away from us, but at the same time, the further we look the further back in time it is, so the closer the galaxies should be to each other, even though they're moving away faster

You've shifted your frame of reference mid-sentence. First, you're considering earth's point of view, then you take the point of view of galaxies far back in time. The simple relationship only relates the distance between two objects to their relative recessional velocities, with the Hubble constant actually being a (slow-rolling) function of time.

mkline55
2014-Aug-04, 07:16 PM
Another thread just made me wonder ...

Until acceleration of the cosmic expansion was discovered in
1998, it was thought that the farther away a galaxy is, the
faster we and it are moving apart. Roughly graphed, speed
versus distance would be a line rising to the right.

Since it takes light time to reach us from those galaxies,
acceleration of the expansion beginning about 5 or 6 billion
years ago would mean that the closer a galaxy is to us, the
more acceleration it would show. Graphed as a line falling
to the right.

Could the two cancel each other out? So that speed versus
distance would be nearly constant? How close do they come
to actually doing that, for at least some range of distance?

-- Jeff, in Minneapolis

If you graph velocity vs. distance and you graph acceleration vs distance, you are graphing two different things. They won't cancel out.

Jeff Root
2014-Aug-04, 10:26 PM
The graph is speed versus distance. Increasing speed in the
upward direction (naturally), increasing distance to the right.
The curve is up to the right.

But the acceleration of the expansion means the speed is
greater at small distances and less for larger distances.
The curve is down to the right.

The combination must be more nearly flat than the base
expansion alone, for the recent era. Closer to constant
speed with distance than would otherwise be the case.

-- Jeff, in Minneapolis

Jens
2014-Aug-04, 11:22 PM
if you think of sand stuck on a rubber band, and then stretch the rubber band back and forth, you will see the sand grains near both hands are moving the fastest away from each other......and the grains just to the side of any othe grains isn't moving away from its neighbour, so fast.

Or did you mean something else?

Yes, I meant something slightly different. If light traveled instantaneously, I'd have no trouble. But taking the example of the rubber band, the light reaching you from a part of the rubber band that is far away is actually light that left a long time ago, when the rubber band wasn't stretched as much. So in other words, when you see the part of the rubber band that is far away, it should be less stretched than what you see nearby. I find that difficult.

Jens
2014-Aug-04, 11:28 PM
You've shifted your frame of reference mid-sentence. First, you're considering earth's point of view, then you take the point of view of galaxies far back in time.

I'm assuming I suppose that if we see an object that is 12 billion light years away, that that object will look like what the universe was like 13 billion years ago for us. Is that not so? Even if the light travels 13 billion years, do the objects appear in the same state of evolution of the universe that we see today? That would seem weird to me.

Jeff Root
2014-Aug-04, 11:32 PM
The description Jens gives seems accurate to me. I'm not
so sure his "frame of reference" shifted. I think it is all as seen
from Earth at the current time.

-- Jeff, in Minneapolis

Jeff Root
2014-Aug-04, 11:36 PM
Whoop -- Jens, did you mean to use a consistent distance
in your last post? It shifted from 12 billion light-years to 13!

-- Jeff, in Minneapolis

WayneFrancis
2014-Aug-05, 02:35 AM
Yes, I meant something slightly different. If light traveled instantaneously, I'd have no trouble. But taking the example of the rubber band, the light reaching you from a part of the rubber band that is far away is actually light that left a long time ago, when the rubber band wasn't stretched as much. So in other words, when you see the part of the rubber band that is far away, it should be less stretched than what you see nearby. I find that difficult.

For part of the light's journey, the beginning part, the light is less stretched. But the key is it has all the stretching including the stretching that occurred more recently and closer to us. So while what we "see" in one way is less stretched the light is more stretched. The stretching radial to us, for very distant objects, would make galaxies closer together. Also their angular size should seem larger and the angular distance between them might seem larger but I'd have to work out the maths. The ratio of their angular size to angular distance between them would be smaller then it really is.

My point is we can't just rely on the pictures we see for anything of an accurate understanding of the distances. We have to adjust sizes and positions based on the data to undistort (new word?) the view.

Ken G
2014-Aug-06, 02:19 PM
The frame of reference is indeed important to be clear about. If what you mean by the "speed" of the galaxies is the rate with age that that galaxy's distance from earth, at age 13.8 billion years of the universe, is increasing, then this will always obey the law that the "speed" is proportional to the distance. This is an immediate consequence of the cosmological principle, which states that all that is happening is the "scale factor" of the universe is increasing with age.

The reason other kinds of "speed" are being talked about is that a different version is being mixed in, which has more the flavor of "the speed that galaxy was moving away from us when it emitted its light". That's a kind of confused version of "speed" though, and it sounds motivated by the idea that the redshift we see is somehow fixed when the light is emitted, such that the redshift is an artifact of the state of affairs when the light was emitted. That's a misconception, redshift is not an artifact of "how things were", it is a measure of all the expansion that has occurred since the light was emitted. Pay heed: cosmological redshift does not care at all what rate that galaxy's distance was increasing when the light was emitted, it cares about exactly one thing: how much space has expanded over cosmological time since the light was emitted.

That's why it is so very wrong to think of cosmological redshifts as Doppler shifts, even though astronomers themselves are guilty of propagating that myth. Presumably it's because they think people can understand Doppler shifts better than they can understand cosmological redshifts, but there's really no reason to think that-- most people have a very tenuous grasp of Doppler shifts too! It's really just that they learn about Doppler shifts before they learn about cosmological redshifts, so it seems like a good idea to make contact with that prior concept-- but as it happens, it's not a good idea at all.

WaxRubiks
2014-Aug-06, 02:52 PM
is there any real difference between saying that the universe is expanding, and on the other hand, that matter is shrinking...?

We always talk about the former, but I would think the are effectively the same..

Jeff Root
2014-Aug-06, 03:10 PM
Frog,

When Ken and I fought about that before, he said they are the
same, I said that they are not, because an expanding Universe
is a single thing happening -- expansion of all space -- while
shrinking matter would involve gazillions and gazillions of
separate, individual shrinkings of separate, individual particles
and objects all throughout the Universe, all co-ordinated to
shrink simultaneously and uniformly without any apparent cause
for them to do that.

All in one sentence.

-- Jeff, in Minneapolis

WaxRubiks
2014-Aug-06, 03:23 PM
Frog,

When Ken and I fought about that before, he said they are the
same, I said that they are not, because an expanding Universe
is a single thing happening -- expansion of all space -- while
shrinking matter would involve gazillions and gazillions of
separate, individual shrinkings of separate, individual particles
and objects all throughout the Universe, all co-ordinated to
shrink simultaneously and uniformly without any apparent cause
for them to do that.

All in one sentence.

-- Jeff, in Minneapolis


but take Uranium; wherever that is in the universe, it will be decaying at the same rate(allowing for time dilation).....is that co-ordinated?

Strange
2014-Aug-06, 03:34 PM
is there any real difference between saying that the universe is expanding, and on the other hand, that matter is shrinking...?

We always talk about the former, but I would think the are effectively the same..

I believe you can change the coordinate system you use and describe things that way. It isn't as simple as you might think, though. You end up with things like the speed of light not being constant. And I'm not sure how it describes the fact that expansion only happens on cosmological scales; I suppose it would mean that space locally contracts at the same rate as matter... or something.

Basically, it is more complex and counter-intuitive so no one does it.

Jeff Root
2014-Aug-06, 03:40 PM
Frog,

I'll admit that you seem to have a good point, there.
I'll have to cogitate on it further to know what it means...

-- Jeff, in Minneapolis

Cougar
2014-Aug-06, 04:53 PM
All in one sentence.

Aha. So I'm not the only one. :p

But back to the OP, I think the more pertinent graph to consider is the following, with rate of expansion the vertical axis, and time the horizontal....

http://user.xmission.com/~dcc/expansion%20rate%20history.jpg


Courtesy PennState Eberly College of Science. (http://science.psu.edu/news-and-events/2012-news/Schneider11-2012)

Ken G
2014-Aug-06, 11:44 PM
is there any real difference between saying that the universe is expanding, and on the other hand, that matter is shrinking...?

We always talk about the former, but I would think the are effectively the same..To my mind, they are the same. Indeed, I've even used the "shrinking matter" picture to answer that pesky Big-Bang question, "what is space expanding into?" If we regard matter as shrinking, the question goes "poof." It is perfectly normal to talk about matter as shrinking, by the way, we do it all the time in special relativity when we speak of "length contraction." No one is ever bothered by the "coordination" required there!

WaxRubiks
2014-Aug-07, 02:52 AM
suppose you can look at it lots of ways; matter AND space could be expanding, but at different rates, or shrinking at different rates....without a reference size, you can argue it anyway you want.


But does the expansion of space(or the shrinking of matter) mean that the Moon is getting further away, and all the planets in the solar system are getting further from each other?

Jeff Root
2014-Aug-07, 03:50 AM
But does the expansion of space(or the shrinking of matter)
mean that the Moon is getting further away, and all the
planets in the solar system are getting further from each other?
That's a big part of what I meant by the shrinking having to
be co-ordinated. All the atoms in the Earth could shrink, and
all the atoms in the Moon could shrink, but the space between
the Earth and Moon would have to also shrink at the same rate,
or they would get farther apart ... for reasons other than the
tidal momentum transfer we know is happening....

-- Jeff, in Minneapolis

WaxRubiks
2014-Aug-07, 04:01 AM
Well, I guess the moon must be getting further away, due to the Universe expanding(or matter shrinking), or is that incorrect?

It would only be a few millimetres a year wouldn't it, so difficult to measure?

Shaula
2014-Aug-07, 04:15 AM
Well, I guess the moon must be getting further away, due to the Universe expanding(or matter shrinking), or is that incorrect?

It would only be a few millimetres a year wouldn't it, so difficult to measure?
The simple conceptual response normally given to this question is that the Moon and the Earth are gravitationally bound - therefore you don't see any expansion happening between them. The more complex but more accurate response is that the scale factor change is described using the FLRW metric which describes a homogeneous isotropic system. The Earth Moon system is certainly not homogeneous and isotropic, meaning we cannot use that metric here.

Jeff Root
2014-Aug-07, 04:21 AM
It would actually be less than a millimetre per year, if
everything in the Universe moved apart due to the cosmic
expansion. But the Moon doesn't move away from the Earth
due to cosmic expansion because gravity keeps them together.

If the cosmic expansion is everywhere throughout space, not
just outside of galaxies or between clusters of galaxies, then
there is a kind of force pushing the Earth and Moon apart,
but the effect would not be to make them keep getting farther
and farther apart. Instead, it would just keep them a small,
constant amount farther apart than they otherwise would be.
Gravity holds them together like a spring. Cosmic expansion
stretches that spring ever so slightly, but the stretching does
not increase over time.

At least, it wasn't thought to increase over time until the
acceleration of the expansion was discovered in 1998. The
acceleration should cause the Earth and Moon to move apart
very, very, very slowly at the current era. Much less than a
millimetre per year.

-- Jeff, in Minneapolis

WaxRubiks
2014-Aug-07, 04:47 AM
well, if the moon wasn't getting further away from the Earth due to expansion, then that would rule out the matter shrinking perspective..

But do we know, experimentally that the moon isn't getting further away, or any other object.....what is the difference between the moon in orbit, due to gravity, and the gravitational effect of galaxies billions of light years away? The moon doesn't know it is in orbit around the Earth, so what is the difference?

Jeff Root
2014-Aug-07, 05:24 AM
The effect of cosmic expansion is too small to detect on the
scale of the Earth and Moon, or Sun and planets, even if it
exists, and even if the acceleration of the expansion causes
the orbits to increase. Recession of the Moon due to tidal
interaction with matter in the Earth is only measureable
statistically, by laser ranging over the course of many orbits.
Recession due to cosmic expansion would be far smaller.

What we're talking about here is a simple difference of
expansion rate due to the difference in distance. If the
expansion is proportional to distance, the short distance
to the Moon, to Pluto, or even to the other side of the
Milku Way galaxy doesn't result in much expansion. That
is why gravity is able to prevent the expansion at those
scales.

-- Jeff, in Minneapolis

WaxRubiks
2014-Aug-07, 06:01 AM
If the moon did become further away due to expansion, then its orbital speed would be too large for the orbit, so does that mean it would find a lower orbit, its original orbit?

Strange
2014-Aug-07, 08:35 AM
well, if the moon wasn't getting further away from the Earth due to expansion, then that would rule out the matter shrinking perspective..

It wouldn't rule it out. It just wouldn't be simple; there would have to be a local shrinking of space to match the shrinking of matter so that the "illusion of expansion" only occurred at cosmological distances.

Jeff Root
2014-Aug-07, 10:56 AM
If the moon did become further away due to expansion, then
its orbital speed would be too large for the orbit, so does that
mean it would find a lower orbit, its original orbit?
The expansion applies continuously, so there would always
be a balance between the expansion and the orbital speed.

But in general, I think you are probably right.

If something gave the Moon a sudden, very brief push away
from the Earth (say, lasting a few minutes), then, depending
on where it was in its orbit, both its apogee and perigee
would likely change slightly, but I think the orbital period
should remain almost unchanged. Were you involved in the
discussion a year or two ago of what happens to an orbit
when a satellite is pushed in the various possible directions?
Giving a satellite a brief outward or inward push from an
initially circular orbit should make it start going up and down
in an elliptical orbit without greatly changing the average
speed over the whole orbit ... I think....

-- Jeff, in Minneapolis

WaxRubiks
2014-Aug-07, 11:06 AM
no I wasn't on that discussion, I don't think.

Reality Check
2014-Aug-08, 12:21 AM
But does the expansion of space(or the shrinking of matter) mean that the Moon is getting further away, and all the planets in the solar system are getting further from each other?
No - they are gravitationally bound: Why doesn't the Solar System expand if the whole Universe is expanding? (http://www.astro.ucla.edu/~wright/cosmology_faq.html#SS)

Ken G
2014-Aug-08, 01:16 AM
It wouldn't rule it out. It just wouldn't be simple; there would have to be a local shrinking of space to match the shrinking of matter so that the "illusion of expansion" only occurred at cosmological distances.
In fact, there is a local shrinking of space, it's why the Moon orbits the Earth-- and it isn't simple, but that's general relativity for you.

WaxRubiks
2014-Aug-08, 05:18 AM
In fact, there is a local shrinking of space, it's why the Moon orbits the Earth-- and it isn't simple, but that's general relativity for you.

maybe a local shrinking of space could be driving inflation? To compensate, and equal out?

Jeff Root
2014-Aug-08, 05:21 AM
A neat idea.

-- Jeff, in Minneapolis

Ken G
2014-Aug-08, 05:20 PM
maybe a local shrinking of space could be driving inflation? To compensate, and equal out?That would be a different theory from GR, though. Usually we try to understand inflation and acceleration within the context of GR, though it also involves QM, so it's a pretty uneasy alliance and I can't comment on whether it really holds together. The fact is, there is something going on here that we have not figured out yet, so all bets are on the table.

WayneFrancis
2014-Aug-09, 04:38 PM
To my mind, they are the same. Indeed, I've even used the "shrinking matter" picture to answer that pesky Big-Bang question, "what is space expanding into?" If we regard matter as shrinking, the question goes "poof." It is perfectly normal to talk about matter as shrinking, by the way, we do it all the time in special relativity when we speak of "length contraction." No one is ever bothered by the "coordination" required there!

I only have 1 problem with the "shrinking matter" as a real concept is that it requires all the forces to change at the exact same rate. Well 2..it also has me doubting that if I have 10x10x10 grid of blocks and shrink them to stay as a connected at least 999 of them would also have to have their position change. I kind of understand it is a coordinate choice thing but not fully happy with it.

With space/ time expanding it is just 1 property changing. Space/ Time. Not ALL the other forces, particles etc all changing in some weird lock step. occam's razor and all

WayneFrancis
2014-Aug-09, 04:40 PM
suppose you can look at it lots of ways; matter AND space could be expanding, but at different rates, or shrinking at different rates....without a reference size, you can argue it anyway you want.


But does the expansion of space(or the shrinking of matter) mean that the Moon is getting further away, and all the planets in the solar system are getting further from each other?
I would say, by a very small bit yea.

WayneFrancis
2014-Aug-09, 04:46 PM
It would actually be less than a millimetre per year, if
everything in the Universe moved apart due to the cosmic
expansion. But the Moon doesn't move away from the Earth
due to cosmic expansion because gravity keeps them together.

If the cosmic expansion is everywhere throughout space, not
just outside of galaxies or between clusters of galaxies, then
there is a kind of force pushing the Earth and Moon apart,
but the effect would not be to make them keep getting farther
and farther apart. Instead, it would just keep them a small,
constant amount farther apart than they otherwise would be.
Gravity holds them together like a spring. Cosmic expansion
stretches that spring ever so slightly, but the stretching does
not increase over time.

At least, it wasn't thought to increase over time until the
acceleration of the expansion was discovered in 1998. The
acceleration should cause the Earth and Moon to move apart
very, very, very slowly at the current era. Much less than a
millimetre per year.

-- Jeff, in Minneapolis
I tried to do the back of the napkin calculation in another post about 12 hours ago and was about to get like 2cm which I thought was about 2 orders of magnitude to high from gut feel.

I thought tho that was only for things like the strong and electroweak force. where the distance would just change by about 1x10-20 give or take a few orders of magnitude.
For gravity it would add up but so what. Our moon is receding at what? 2.2cm/year because of tidal interactions.

WayneFrancis
2014-Aug-09, 04:49 PM
The expansion applies continuously, so there would always
be a balance between the expansion and the orbital speed.

But in general, I think you are probably right.

If something gave the Moon a sudden, very brief push away
from the Earth (say, lasting a few minutes), then, depending
on where it was in its orbit, both its apogee and perigee
would likely change slightly, but I think the orbital period
should remain almost unchanged. Were you involved in the
discussion a year or two ago of what happens to an orbit
when a satellite is pushed in the various possible directions?
Giving a satellite a brief outward or inward push from an
initially circular orbit should make it start going up and down
in an elliptical orbit without greatly changing the average
speed over the whole orbit ... I think....

-- Jeff, in Minneapolis
Ah thanks for that...It makes more sense now.

Ken G
2014-Aug-10, 05:30 AM
I only have 1 problem with the "shrinking matter" as a real concept is that it requires all the forces to change at the exact same rate. Imagine you are in a spaceship that accelerates toward Earth until it is moving at 99% of the speed of light. Do you realize that every meterstick on the planet Earth that is pointed toward you will appear to you to have shrunk in length, all at the same rate? What made them do that? Length contraction is a common notion in special relativity, and no one is bothered in that context about "what makes it happen." We don't think anything is needed to make it happen, it is just how length works!

Well 2..it also has me doubting that if I have 10x10x10 grid of blocks and shrink them to stay as a connected at least 999 of them would also have to have their position change. I kind of understand it is a coordinate choice thing but not fully happy with it.Again, that's just what happens, from your perspective, to an array of blocks when you move toward them at high speed. Your problem is, you are asking "how can that really happen," but relativity says it doesn't "really happen" in an absolute way, it's just how you look at the situation. So when I say "matter is shrinking", I don't mean that's what is really happening, any more than I mean that space is really expanding. It's all in how we look at it, in the coordinate choice. The reason you are not happy with it is you have come to put too much faith in coordinates-- relativity tells us to let go of that faith, and see coordinates for what they are: just a language for talking about the same thing in a way that sounds different, like English vs. French.


With space/ time expanding it is just 1 property changing. Then how does all the space know how to expand at the same rate? It's just that it starts out with the same initial condition-- but the same can be said for all those masses!

Shaula
2014-Aug-10, 05:55 AM
Imagine you are in a spaceship that accelerates toward Earth until it is moving at 99% of the speed of light. Do you realize that every meterstick on the planet Earth that is pointed toward you will appear to you to have shrunk in length, all at the same rate? What made them do that? Length contraction is a common notion in special relativity, and no one is bothered in that context about "what makes it happen." We don't think anything is needed to make it happen, it is just how length works!
Alternatively your own meterstick is expanding by a different amount depending on what you are using it to measure. Again, identical results but one has a fairly simple description whereas the other one requires a quite complex scheme to get it to work conceptually. Which is why we choose to use length contraction rather than subjective expansion. Ditto space expansion versus matter contraction in cosmological terms.

WayneFrancis
2014-Aug-11, 02:47 AM
...
Then how does all the space know how to expand at the same rate? It's just that it starts out with the same initial condition-- but the same can be said for all those masses!

I don't disagree with anything you've said in the first part. Just my personal hatred of that "coordinate choice"


As for the latter....Well how do a protons all over the universe know to have the mass 1.672621777(74)10−27kg? It is just a property of the proton..

So if the Hubble constant is just a property of the vacuum there isn't any choice but for space to expand at the same rate. But If it isn't just the vacuum expanding then we have to take all the other properties of everything else and have them change but in a very coordinated manner. Mass has to change, strength of the strong and electroweak force has to change. Gravity has to change. But photons don't change...if they did there would be no red shift. See what I'm getting at. I know it can be viewed as a coordinate choice but it raises to many "but what about" questions where if it is just the vacuum expanding these questions of coordination and exclusion aren't raised. It is one thing changing that results in a given set of observations.

Ken G
2014-Aug-11, 03:13 AM
Alternatively your own meterstick is expanding by a different amount depending on what you are using it to measure. Again, identical results but one has a fairly simple description whereas the other one requires a quite complex scheme to get it to work conceptually. Which is why we choose to use length contraction rather than subjective expansion. Ditto space expansion versus matter contraction in cosmological terms.Yet do you not see that matter contraction in the Big Bang is actually the direct analog of length contraction in special relativity? So why favor one in one application, and reject its direct analog in another? It is just like taking the perspective of an alien 10 billion years ago, who is in a frame that is moving rapidly relative to the local mass frame, and asking that alien, what will happen to future rulers compared to your ruler? The alien, if they use a length contraction picture to describe all the other rulers in their own time as being contracted, is equally justified in saying that all future rulers will be even more length contracted than those are.

Ken G
2014-Aug-11, 03:31 AM
I don't disagree with anything you've said in the first part. Just my personal hatred of that "coordinate choice"
It often depends on the application. For example, if you want to understand Kepler's laws of planetary motion, you probably don't choose coordinates where the Earth is stationary. But if you want to understand the phases of the Moon, that's exactly what you do!


As for the latter....Well how do a protons all over the universe know to have the mass 1.672621777(74)10−27kg? It is just a property of the proton..
Yes, they were all told they were protons at some point in the past, and somehow they essentially retain that information. But they could also have been told what size they are, and how that size is evolving in time based on the gravitational influences, so that's how they would "know" what to do.

So if the Hubble constant is just a property of the vacuum there isn't any choice but for space to expand at the same rate.So it's natural that "the vacuum" can act all as one thing, across vast spaces, but the protons cannot?
I know it can be viewed as a coordinate choice but it raises to many "but what about" questions where if it is just the vacuum expanding these questions of coordination and exclusion aren't raised. It is one thing changing that results in a given set of observations.It isn't one thing, it's the vacuum everywhere there is vacuum-- that's a lot of things!

WayneFrancis
2014-Aug-11, 04:04 AM
It often depends on the application. For example, if you want to understand Kepler's laws of planetary motion, you probably don't choose coordinates where the Earth is stationary. But if you want to understand the phases of the Moon, that's exactly what you do!
Yes, they were all told they were protons at some point in the past, and somehow they essentially retain that information. But they could also have been told what size they are, and how that size is evolving in time based on the gravitational influences, so that's how they would "know" what to do.So it's natural that "the vacuum" can act all as one thing, across vast spaces, but the protons cannot?It isn't one thing, it's the vacuum everywhere there is vacuum-- that's a lot of things!

No that is not what I'm saying at all.

Here is my point. We can look at cosmic inflation from 2 basic view, I'm just limiting it to just these 2 for simplicity.
A) Space is expanding.
or
B) Matter is shrinking.

Now given those 2 choices (not saying that either "IS" what is happening just describing WHY I like A over B for a point of reference)

If A is true, because it is a just a property of the vacuum that it expands, increase, reproduces itself, what ever, we observe distant objects recede at increasing rates proportional to the distance. We observe light from distant object red shifted because of this recession.


If B is true and matter is shrinking then all the fundamental forces need to change. I'm not sure how this would apply to QM if at all.


So with A we have 1 thing changing. With B we have multiple things changing to support the same view.

it just boils down to that. I'm more comfortable saying "the vacuum expands" rather then saying "Gravity, the electromagnetic force, the weak force and the strong force all change at the same rate allowing matter to "shrink"

Perhaps you could tell me a more simple model where those forces change so that a proton could change its "size" on the order of 1x10-20ish/s
I'm looking at the 4 forces as 4 properties of "matter". I guess "gravity" could be replaced by "mass". It is like saying a rain drop doesn't fall to the ground but everything else rushes up to it. I know you both are coordinate choices. I just prefer to deal with the 1 thing changing rather then leaving it alone and having everything else's position changing.

so I'm saying I treat the vacuum everywhere the same as I treat protons everywhere.

Sorry if I'm not clear with my meanings.

Shaula
2014-Aug-11, 04:43 AM
Yet do you not see that matter contraction in the Big Bang is actually the direct analog of length contraction in special relativity? So why favor one in one application, and reject its direct analog in another? It is just like taking the perspective of an alien 10 billion years ago, who is in a frame that is moving rapidly relative to the local mass frame, and asking that alien, what will happen to future rulers compared to your ruler? The alien, if they use a length contraction picture to describe all the other rulers in their own time as being contracted, is equally justified in saying that all future rulers will be even more length contracted than those are.
If you look at what I said I do not reject either. What I said was that we tend to pick the co-ordinate system or conceptual model that makes the physics simplest and use that. I made no judgement as to whether this is right, it is merely convenient.

Ken G
2014-Aug-11, 05:18 AM
If A is true, because it is a just a property of the vacuum that it expands, increase, reproduces itself, what ever, we observe distant objects recede at increasing rates proportional to the distance. We observe light from distant object red shifted because of this recession.
But what I'm asking you is, why do you think saying "it is a property of vacuum to expand" is any different from saying "it is a property of matter to shrink"?

If B is true and matter is shrinking then all the fundamental forces need to change. I'm not sure how this would apply to QM if at all.
Why would the fundamental forces need to change? Consider an atom. The electron orbitals occupy a certain number of proton widths. If the protons shrink, and the electron orbital shrink, you still have the same number of proton radii inside the orbital. If rulers shrink, and c stays the same, then a tick of a light clock is still the time it takes light to cross a ruler, but compared to a previous eon, that will be a shorter time. So we say light emitted in olden times appears redshifted to a clock that ticks faster today. But no laws of physics are any different, no change to the fundamental forces. To the person holding the shrinking ruler, it isn't shrinking, to the person holding the shrinking clock, the clock is not ticking faster. That's because what that person means by distance is defined by that ruler, and what they mean by time is defined by that clock. So they can never see any change, so they will never see anything wrong with any fundamental forces or laws. This is already how special relativity works, would you say that special relativity requires the fundamental forces to change when we consider a frame to be length-contracting?

It is like saying a rain drop doesn't fall to the ground but everything else rushes up to it. I know you both are coordinate choices. I just prefer to deal with the 1 thing changing rather then leaving it alone and having everything else's position changing.Yet we do that kind of thing all the time, whenever it is convenient. When you are on an amusement park ride that spins you around and presses you against the wall of the ride, do you not say that some kind of invisible force is pressing you against that wall? As soon as you do that, you are saying that you are the one thing that is stationary, which requires that everything else (including the people on the ground outside the ride) are moving! If you see nothing wrong with language like that when you are on a ride, why do you see something wrong with it now? We don't always choose our coordinates to make the least number of things move, sometimes we prefer to regard ourselves as the standard to which all other things are compared, even if we are just one thing in a universe of a lot of other things.

Ken G
2014-Aug-11, 05:23 AM
If you look at what I said I do not reject either. What I said was that we tend to pick the co-ordinate system or conceptual model that makes the physics simplest and use that. I made no judgement as to whether this is right, it is merely convenient.But I am saying that no such convenience has been demonstrated in regard to Big Bang cosmology. Where is the added convenience of saying that space is expanding versus matter is shrinking? All you have in cosmology is a(t), the time dependence of the scale factor. That's it-- that's the one number we are trying to justify with all our "space is expanding" language! Yet what is a scale factor? It is just one thing: the ratio of the distance between the galaxy clusters to the length of the standard ruler. So having the former increase, or the latter decrease, is precisely equally convenient. So it's not convenience that motivates expanding space, it is sheer ethnocentrism-- we prefer to imagine that the objects we bank on to measure lengths cannot be the thing that is changing, because we bank on them to not change. That's exactly what the ancients Greeks were doing when they banked on the Earth to not move, they were just asserting their own desire to live in a stationary place in such as way as to elevate it to a physical principle that did not exist, and was no more convenient than a moving Earth (indeed, in some situations it is more convenient to let the Earth move, in others, more convenient to let it not move, but in the case of the cosmological scale factor and the length of rulers, there's no difference either way).

WayneFrancis
2014-Aug-11, 06:32 AM
But what I'm asking you is, why do you think saying "it is a property of vacuum to expand" is any different from saying "it is a property of matter to shrink"?Why would the fundamental forces need to change? Consider an atom. The electron orbitals occupy a certain number of proton widths. If the protons shrink, and the electron orbital shrink, you still have the same number of proton radii inside the orbital. If rulers shrink, and c stays the same, then a tick of a light clock is still the time it takes light to cross a ruler, but compared to a previous eon, that will be a shorter time. So we say light emitted in olden times appears redshifted to a clock that ticks faster today. But no laws of physics are any different, no change to the fundamental forces. To the person holding the shrinking ruler, it isn't shrinking, to the person holding the shrinking clock, the clock is not ticking faster. That's because what that person means by distance is defined by that ruler, and what they mean by time is defined by that clock. So they can never see any change, so they will never see anything wrong with any fundamental forces or laws. This is already how special relativity works, would you say that special relativity requires the fundamental forces to change when we consider a frame to be length-contracting?
Yet we do that kind of thing all the time, whenever it is convenient. When you are on an amusement park ride that spins you around and presses you against the wall of the ride, do you not say that some kind of invisible force is pressing you against that wall? As soon as you do that, you are saying that you are the one thing that is stationary, which requires that everything else (including the people on the ground outside the ride) are moving! If you see nothing wrong with language like that when you are on a ride, why do you see something wrong with it now? We don't always choose our coordinates to make the least number of things move, sometimes we prefer to regard ourselves as the standard to which all other things are compared, even if we are just one thing in a universe of a lot of other things.

Because I view "Matter" as a much bigger umbrella of many components under it then the vacuum. Right or wrong. At the moment we can break down "matter", which must include dark matter for this to match observation, into many different components. Many components interact with 1 field but not others. So now for some reason this change can change a bunch of different forces but doesn't impact a photon in the same way. Leaving the photon to then red shift from our point of view.

Again Ken I think you are pushing way more then you need to. I've said I accept it is a coordinate choice but that I'm just not comfortable with it pure and simple. You seem to want to ram something I've said I already accept down my throat. I, me, myself, find it easier to think of expansion as space as expanding because to ME, ALONE, NOT CONCERNED ABOUT OTHERS INTERPRETATION, find it easier to deal with on a conceptual level. You don't have to agree that I think Occam's razor applies here by conveniently bundling up all that are not photons into one huge group. I don't make that distinction and since you and I will get the same answer at the end of the day it doesn't matter that I prefer to consider it one way rather then another. I'm not saying one is "reality". I'm not claiming either is "reality". I'm saying that one model works better for me. For me I tend to pick the model where I have to do the least amount of fiddling and that my brain seems comfortable with.

WayneFrancis
2014-Aug-11, 06:46 AM
But I am saying that no such convenience has been demonstrated in regard to Big Bang cosmology. Where is the added convenience of saying that space is expanding versus matter is shrinking? All you have in cosmology is a(t), the time dependence of the scale factor. That's it-- that's the one number we are trying to justify with all our "space is expanding" language! Yet what is a scale factor? It is just one thing: the ratio of the distance between the galaxy clusters to the length of the standard ruler. So having the former increase, or the latter decrease, is precisely equally convenient. So it's not convenience that motivates expanding space, it is sheer ethnocentrism-- we prefer to imagine that the objects we bank on to measure lengths cannot be the thing that is changing, because we bank on them to not change. That's exactly what the ancients Greeks were doing when they banked on the Earth to not move, they were just asserting their own desire to live in a stationary place in such as way as to elevate it to a physical principle that did not exist, and was no more convenient than a moving Earth (indeed, in some situations it is more convenient to let the Earth move, in others, more convenient to let it not move, but in the case of the cosmological scale factor and the length of rulers, there's no difference either way).

I can't speak for Shaula but if you are applying that to me then you are putting words into my mouth. Perfect world I could flip between various models no problem but I'm not even close to being at your level so I'm not comfortable with that. I've pointed out that there are things that I have to say "I don't know" with one model "shrinking matter" and you can say it doesn't make a difference all you want but unless I've got it all conceptualized in my head properly me saying "Yea, it is all the same" is lying to myself even if it is true.

WaxRubiks
2014-Aug-11, 07:04 AM
is one thing that is wrong with the matter shrinking perspective, time dilation? Would it mean that a neutron star should shrink at a slower rate than the moon?

WaxRubiks
2014-Aug-11, 07:12 AM
I wonder if time dilation would slow the expansion of a vacuum, locally to a neutron star..

WayneFrancis
2014-Aug-11, 07:25 AM
is one thing that is wrong with the matter shrinking perspective, time dilation? Would it mean that a neutron star should shrink at a slower rate than the moon?

These are examples of questions where I'd say "I don't know" even if they have a very definite answer.

caveman1917
2014-Aug-11, 03:22 PM
But I am saying that no such convenience has been demonstrated in regard to Big Bang cosmology. Where is the added convenience of saying that space is expanding versus matter is shrinking?

If my meterstick is shrinking, then why do i keep getting the same distance from here to the moon?

Shaula
2014-Aug-11, 04:26 PM
But I am saying that no such convenience has been demonstrated in regard to Big Bang cosmology.
And nowhere did I say that I was only interested in Big Bang cosmology. It is not ethnocentrism that motivates me it is the desire to avoid horrible questions like "How does a particle I model as a point shrink?", "What does this mean for a quantum field?" and "How do I incorporate matter shrinking into QM?". An expanding universe already gives us enough problems with its energy conservation issues and what that means for T invariance.

caveman1917
2014-Aug-11, 05:49 PM
And nowhere did I say that I was only interested in Big Bang cosmology.

The main reason it isn't used much in Big Bang cosmology is that expansion is a global property and providing a local explanation such as "matter shrinking" requires you to consider all bound (gravitationally or otherwise) systems as actually collapsing in on themselves but you can't detect that because your measuring rod is shrinking.

mkline55
2014-Aug-11, 06:10 PM
If my meterstick is shrinking, then why do i keep getting the same distance from here to the moon?
The counter-argument to that is, if space is expanding, then why is my meterstick staying the same size? The answer is the same. There is more than one force at play.


And nowhere did I say that I was only interested in Big Bang cosmology. It is not ethnocentrism that motivates me it is the desire to avoid horrible questions like "How does a particle I model as a point shrink?"
Really? Choosing to ignore all the spacial properties of a particle and treat it as a point just means you need to continue to ignore all the spacial properties. The center of gravity of any sphere mass is a point. Changing the diameter of the sphere does not change that.


An expanding universe already gives us enough problems with its energy conservation issues and what that means for T invariance.
I have a hard time accepting that as a valid argument for favoring one viewpoint over another. It almost sounds like, "My favorite theory only has one unsolvable problem with X, so why would I ever want to consider another option?"

caveman1917
2014-Aug-11, 06:37 PM
The counter-argument to that is, if space is expanding, then why is my meterstick staying the same size? The answer is the same. There is more than one force at play.

Because space only expands on global scales and it is not a force. Expansion is not "pulling" the ends of your meterstick apart that needs to be "countered" somehow.

Shaula
2014-Aug-11, 09:00 PM
I have a hard time accepting that as a valid argument for favoring one viewpoint over another. It almost sounds like, "My favorite theory only has one unsolvable problem with X, so why would I ever want to consider another option?"
I am not arguing for one viewpoint over another. I am merely saying one is simpler to use in the current scheme of things. As I had hoped was obvious I have considered the other viewpoint, it is not exactly a huge intellectual leap to come up with this duality. I considered it, realised that it didn't add anything to the model (GR) and made it far harder to work QM into GR. So I choose not to use it. It is totally up to you which viewpoint you use, something I have said on this forum before. You misrepresent what I am saying in your last sentence.

As for the point particle thing... We do not ignore its spatial extent, we purposefully remove it because it leads to problems. Instead we talk about interaction cross sections which vary with energy and by interaction. That is because we are not dealing with particles. We are dealing with excitations of a field. Which make the matter shrinking view a difficult way to represent these fields. Again, you are free to use this formalism if you want. Just like you are free to model the rotation of stars around the galactic centre using ECEF co-ordinates if you want.

Ken G
2014-Aug-12, 04:28 AM
Because I view "Matter" as a much bigger umbrella of many components under it then the vacuum. Right or wrong.Yet the current view that emerges from field theory is that all of matter can spring from the vacuum, if the energy is available. So vacuum is no less a single thing than matter is, they are intimately related to each other and it seems likely that any real distinction between them is largely artificial. Or consider the famous relationship in general relativity, described as "matter tells space how to curve, and space tells matter how to move." That last view raises the question, do we know there is matter there because of what the space is doing, or do we know the space is there because of what the matter is doing? Questions like this don't really have answers, it's like asking if a zebra is a black horse with white stripes or a white horse with black stripes. It just depends on your point of view, how you prioritize the ideas. There's nothing wrong with taking a perspective, like imagining the the Earth is at the center of the Big Bang expansion, but we also realize that other views are no more complicated and no less valid. If someone said to me, "I know I can't prove the Earth is at the center of the universe, but it just seems simpler to me to imagine that", I would say "you can certainly picture the universe that way, but you should not hold that it is a fundamentally simpler picture, because there are demonstrable contexts where a different view is simpler, such as if you are trying to explain retrograde motion of other planets."


At the moment we can break down "matter", which must include dark matter for this to match observation, into many different components. Many components interact with 1 field but not others. Yet look at general relativity-- all matter follows the same inertial paths, regardless of what type of matter it is. How do all those different types of matter know how to follow the same path? It's not a problem to say that a bunch of different types of matter are all doing the same thing, when you have no reason to expect them to do anything different. Indeed, we can ask, do they all do the same thing because "space makes them do that," or do we take the observed fact that they all do the same thing and use it to create the concept of space in the first place? All a matter of how you want to say it, none of these answers are simpler or more insightful than any others, they each contain their own lessons.


So now for some reason this change can change a bunch of different forces but doesn't impact a photon in the same way. Leaving the photon to then red shift from our point of view. If one takes the view that matter shrinks, nothing is happening to the photon at all, because it is not matter. Photons don't length contract either!


Again Ken I think you are pushing way more then you need to. I've said I accept it is a coordinate choice but that I'm just not comfortable with it pure and simple.I'm not finding any fault in anyone asserting what they are comfortable with, all my objections are around statements that one particular view is fundamentally simpler, or better follows Occam's razor, than another. Those kinds of claims are hard to support obectively, the case can only be made from the purely subjective standpoint. From that standpoint, you are welcome to hold any view you like, but it still may be helpful to be conversant in the other views-- you never know where the next insight might come.

I, me, myself, find it easier to think of expansion as space as expanding because to ME, ALONE, NOT CONCERNED ABOUT OTHERS INTERPRETATION, find it easier to deal with on a conceptual level. But that's not an accurate description of the above conversation. You claimed that the fundamental forces have to change if someone holds that future matter will shrink relative to present matter, and past matter is expanded, but that's not the case-- the fundamental forces do not change, because when we talk about fundamental forces, we always use our own rulers and clocks, and ancient or future aliens will always use their own rulers and clocks. So it's not just a matter of you describing your personal preferences, there are misconceptions here that I generally assume people do not want to carry.

For me I tend to pick the model where I have to do the least amount of fiddling and that my brain seems comfortable with.And that's a perfectly valid choice to make, but it can still be of value to recognize that perhaps the other models do not take more fiddling, they are just being misunderstood.

Ken G
2014-Aug-12, 04:47 AM
If my meterstick is shrinking, then why do i keep getting the same distance from here to the moon?The answer is the only possible one-- the distance to the Moon is shrinking as well. To clarify, what is meant by this, is someone else's version of the distance to the Moon is shrinking. Our own version of that distance will always use our own rulers, so a sequence of scientists over time, using their own rulers, would never regard their own rulers as being short. The "matter is shrinking" view is not how we talk about our own rulers, it is how we talk about past rulers. We'd say that in the past, aliens that used metersticks were using longer ones than ours. They got the same distance to the Moon because that distance was also longer-- the Moon-Earth system is a bound system, ruled by the laws of physics, and would then be viewed as shrinking for the same reason the metersticks would be viewed as shrinking, it would just be what gravity is doing to bound systems to keep the laws of physics the same over time.

Ken G
2014-Aug-12, 04:58 AM
And nowhere did I say that I was only interested in Big Bang cosmology. But that's the only context in which we would say that space is expanding! We never use language like that in any other context, so this whole conversation is only about Big Bang cosmology.

It is not ethnocentrism that motivates me it is the desire to avoid horrible questions like "How does a particle I model as a point shrink?", "What does this mean for a quantum field?" and "How do I incorporate matter shrinking into QM?". Those aren't the questions, we would not regard a point particle as shrinking, we would regard bound systems of point particles as shrinking. There's no issues with QM either, again because the only time the question even comes up is Big Bang cosmology.


An expanding universe already gives us enough problems with its energy conservation issues and what that means for T invariance.Sure, but shrinking matter adds no new problems, it's just a different language for talking about all the same issues. That's not a bad thing, it's a good thing-- more languages for talking about the same things just gives you more power to obtain insight.

speedfreek
2014-Aug-12, 12:19 PM
If matter were shrinking, wouldn't all the forces and constants have to also decrease, in order to maintain the view we see? Wouldn't we have a whole gamut of decreasing properties (nuclear force, gravity, speed of light etc) all maintaining the same relationship between them whilst shrinking, in order to keep everything looking as if our rulers weren't changing?

So, isn't shrinking matter necessarily a far more complicated model than an expanding universe? With the expanding universe, only the distance between highly separated galaxies increases, whilst all the forces and constants remain the same.

WaxRubiks
2014-Aug-12, 01:16 PM
Sure, but shrinking matter adds no new problems, it's just a different language for talking about all the same issues. That's not a bad thing, it's a good thing-- more languages for talking about the same things just gives you more power to obtain insight.

Like I said earlier, matter and space could be expansing, but at different rates, or shrinking at different rates, but I am still not sure about gravitational time dilation. If all matter is shrinking at the same rate, is shrinkage indepentant of gravitational time dilation?

Jeff Root
2014-Aug-12, 02:12 PM
I think Frog March is suggesting that a hydrogen atom
which spent the last several billion years in the Sun would
be significantly larger than a hydrogen atom which spent
that time on Earth, when brought together and compared
side-by-side.

Edit: Reverse Polarity!

-- Jeff, in Minneapolis

.

Shaula
2014-Aug-12, 02:19 PM
There's no issues with QM either, again because the only time the question even comes up is Big Bang cosmology.
Big Bang cosmology requires the use of QM. Nucleosynthesis, for example. Fluctuations in the CMB. Sooner or later the theories have to play nicely.

WaxRubiks
2014-Aug-12, 02:27 PM
I think Frog March is suggesting that a hydrogen atom
which spent the last several billion years in the Sun would
be significantly smaller than a hydrogen atom which spent
that time on Earth, when brought together and compared
side-by-side.

-- Jeff, in Minneapolis

except the other way around.

mkline55
2014-Aug-12, 02:29 PM
Like I said earlier, matter and space could be expansing, but at different rates, or shrinking at different rates, but I am still not sure about gravitational time dilation. If all matter is shrinking at the same rate, is shrinkage indepentant of gravitational time dilation?

I think if someone were to try to make a formal case, then time dilation would be a necessary consideration.

Jeff Root
2014-Aug-12, 02:55 PM
Akkkkk! I forgot they were shrinking! I got the time dilation
part right in my head, but then I stopped thinking...

-- Jeff, in Minneapolis

WayneFrancis
2014-Aug-13, 02:06 AM
Yet the current view that emerges from field theory is that all of matter can spring from the vacuum, if the energy is available. So vacuum is no less a single thing than matter is, they are intimately related to each other and it seems likely that any real distinction between them is largely artificial. Or consider the famous relationship in general relativity, described as "matter tells space how to curve, and space tells matter how to move." That last view raises the question, do we know there is matter there because of what the space is doing, or do we know the space is there because of what the matter is doing? Questions like this don't really have answers, it's like asking if a zebra is a black horse with white stripes or a white horse with black stripes. It just depends on your point of view, how you prioritize the ideas. There's nothing wrong with taking a perspective, like imagining the the Earth is at the center of the Big Bang expansion, but we also realize that other views are no more complicated and no less valid. If someone said to me, "I know I can't prove the Earth is at the center of the universe, but it just seems simpler to me to imagine that", I would say "you can certainly picture the universe that way, but you should not hold that it is a fundamentally simpler picture, because there are demonstrable contexts where a different view is simpler, such as if you are trying to explain retrograde motion of other planets."


I'll disagree with your semantics. When I say it is more simple for me I mean it is more simple for ME. You seem to keep trying to say I'm saying something I'm not here.
Focus on the "simpler to me" part.




Yet look at general relativity-- all matter follows the same inertial paths, regardless of what type of matter it is. How do all those different types of matter know how to follow the same path? It's not a problem to say that a bunch of different types of matter are all doing the same thing, when you have no reason to expect them to do anything different. Indeed, we can ask, do they all do the same thing because "space makes them do that," or do we take the observed fact that they all do the same thing and use it to create the concept of space in the first place? All a matter of how you want to say it, none of these answers are simpler or more insightful than any others, they each contain their own lessons.



They follow the same path because that field is created by the Higgs fields and there is only 1 of those topologies. Why do all cars in a roller coaster follow the same path? Because the roller coaster has tracks! It doesn't matter that all the cars are together or separate. I know you know this area 100x better then me and I find it disingenuous that you are doing this hand waving to make things seem very zen like and everything is equal. I don't agree that they are all equal. I bet there are better ways, more simple ways, at looking at various problems. That doesn't invalidate other ways that may be more complicated. It doesn't invalidate that for some people some ways are easier to arrive at the answer.

If I ask two people to solve the following math problem there are different ways to do it. Most of us would do it the first way if we had some paper. If they had to do it in your head most would still try to do it the first way. My son breaks down the problem into more manageable chunks. It is more complicated but it is easier for him, and honestly me, to do maths in our head this way.



61 61x57=(6x57) x 10 + 57
x57 (6x50+42) x 10 + 57
427 (3420) + 57
305 61x57=3477
3477


Saying they are both the same is your choice but to me they are different even tho they arrive at the same answer.



If one takes the view that matter shrinks, nothing is happening to the photon at all, because it is not matter. Photons don't length contract either!
I'm not finding any fault in anyone asserting what they are comfortable with, all my objections are around statements that one particular view is fundamentally simpler, or better follows Occam's razor, than another. Those kinds of claims are hard to support obectively, the case can only be made from the purely subjective standpoint. From that standpoint, you are welcome to hold any view you like, but it still may be helpful to be conversant in the other views-- you never know where the next insight might come.

Ken you keep putting words in my mouth. I say I'm applying Occam's razor for ME, MYSELF, NO ONE ELSE. It is more simple because of the way I've grouped stuff. Since I don't know many "Properties" of the vacuum I group that as 1 thing and "matter" is more complex and varied and includes dark matter. I can ramble off many more properties of various "matter" that are different between different types of "matter" Thus I don't group them all the same. Even if I didn't there are still more "properties" that for ME seem to need to be coordinated then the 1 property of the "vacuum" that needs to change within the mental model in my head.

That said I don't agree that given 2 models one can not be more simple then the other. They might produce the same answer but that doesn't mean they are of the same complexity. I'm not saying my mental model is the most simple one for a given question either. I'm saying for ME it is the most simple one based off of my knowledge. If you have the ability to look at 20 models that all come up with the same answer and see that they are all of the same complexity then great, you've got mental powers far beyond me. I look at GR and Newtons law and for most problems I'd say Newtons Laws are more simple. I can give the equation to a 5th grader and they can solve them. I give the 5th grader the same problem using GR and I bet you most of them would look at me like I'm crazy. I admit the GR gives a more accurate answer but ... hell NASA still using Newtonian formulas in some instances because they are more simple. Happy for you that GR is as simple as Newton Law but for most of us that is not the case. (See how putting words in peoples mouths isn't really productive)



But that's not an accurate description of the above conversation. You claimed that the fundamental forces have to change if someone holds that future matter will shrink relative to present matter, and past matter is expanded, but that's not the case-- the fundamental forces do not change, because when we talk about fundamental forces, we always use our own rulers and clocks, and ancient or future aliens will always use their own rulers and clocks. So it's not just a matter of you describing your personal preferences, there are misconceptions here that I generally assume people do not want to carry.

They do have to change. I agree we wouldn't be able to measure them because our "rulers" and "clocks" change too. But if you argue that they don't change then I'll flip that and say well then we can't be shrinking because we can't "measure" that shrinkage either besides the effect of red shift.



And that's a perfectly valid choice to make, but it can still be of value to recognize that perhaps the other models do not take more fiddling, they are just being misunderstood.

I disagree here. Different models are different models because they approach the problem differently. I'm sure you can quantify the number of variables. If you have 2 models with the same number of variables, meaning the same thing, then they are not really 2 different models. If you have 2 models and one requires you to use calculus and another just algebra to arrive at the same answer then I'd say the algebra model is the more simple of the 2. In my mental model I have 1 property changing when looking at it from some "god" point of view. When I look at the "shrinking matter" model, using a "god" view, I see many properties that need to change. That to ME is more complicated. Disagree if you want but I'm done playing "all models are equally valid and simple" because in my reality they are not and arguing they are is just semantics.

Again I don't disagree with your statement that knowing and looking at other models isn't a positive in most cases.

WayneFrancis
2014-Aug-13, 02:10 AM
The answer is the only possible one-- the distance to the Moon is shrinking as well. To clarify, what is meant by this, is someone else's version of the distance to the Moon is shrinking. Our own version of that distance will always use our own rulers, so a sequence of scientists over time, using their own rulers, would never regard their own rulers as being short. The "matter is shrinking" view is not how we talk about our own rulers, it is how we talk about past rulers. We'd say that in the past, aliens that used metersticks were using longer ones than ours. They got the same distance to the Moon because that distance was also longer-- the Moon-Earth system is a bound system, ruled by the laws of physics, and would then be viewed as shrinking for the same reason the metersticks would be viewed as shrinking, it would just be what gravity is doing to bound systems to keep the laws of physics the same over time.

I disagree. If the Moon and the Earth are shrinking the distance between the 2 is increasing. If it wasn't then you wouldn't get the red shift of photons from the "shrinking" matter. IF you really mean that the distance between the Earth and Moon is shrinking and the distance between them is shrinking then you have to explain why our galaxy and distant galaxies are shrinking but the distance between them is not.

WayneFrancis
2014-Aug-13, 02:12 AM
If matter were shrinking, wouldn't all the forces and constants have to also decrease, in order to maintain the view we see? Wouldn't we have a whole gamut of decreasing properties (nuclear force, gravity, speed of light etc) all maintaining the same relationship between them whilst shrinking, in order to keep everything looking as if our rulers weren't changing?

So, isn't shrinking matter necessarily a far more complicated model than an expanding universe? With the expanding universe, only the distance between highly separated galaxies increases, whilst all the forces and constants remain the same.

Not to Ken the 2 are just as simple as each other. But that is my point exactly, from a "god" point of view.

WayneFrancis
2014-Aug-13, 04:41 AM
Akkkkk! I forgot they were shrinking! I got the time dilation
part right in my head, but then I stopped thinking...

-- Jeff, in Minneapolis

I stop thinking all the time :p

caveman1917
2014-Aug-13, 04:30 PM
I disagree. If the Moon and the Earth are shrinking the distance between the 2 is increasing.

No, in those coordinates gravitationally (or otherwise) bound groups are constantly collapsing in on themselves, in such a way that we cannot detect that because our rulers are also shrinking the same way. It's technically fine, i just don't see the convenience over the normal way of doing it. Rather than wrapping your head around expanding space you now have to wrap your head around everything else collapsing in on itself without you being able to detect it.

Ken G
2014-Aug-14, 03:05 AM
Big Bang cosmology requires the use of QM. Nucleosynthesis, for example. Fluctuations in the CMB. Sooner or later the theories have to play nicely.
True enough, but shrinking matter has no issues with QM either, because QM is a local theory, and the local observer never sees the shrinking matter, all they see is the results of it on cosmological scales. In other words, all local physics is just as unaffected by the shrinking matter picture as it is by the expanding space picture, because the same number of rulers connect the Earth and the Moon, for example, whether cosmological space is expanding or all bound systems are shrinking. GR is a metric theory, which means all it cares about is how many rulers you need to get from point A to point B. QM is a local theory, which means it's all happening within the reach of your ruler, and if your ruler shrinks, and the QM system shrinks, nothing changes.

Ken G
2014-Aug-14, 03:14 AM
No, in those coordinates gravitationally (or otherwise) bound groups are constantly collapsing in on themselves, in such a way that we cannot detect that because our rulers are also shrinking the same way. It's technically fine, i just don't see the convenience over the normal way of doing it. Rather than wrapping your head around expanding space you now have to wrap your head around everything else collapsing in on itself without you being able to detect it.There is not necessarily any objectively demonstrable advantage, but there are some useful features:
1) it allows us to take "expanding space" less seriously, by seeing there is an alternative description
2) it makes certain "expanding space" related puzzles go away, like "what is the space expanding into"
3) It is one way to explain the Hubble law-- the rate of increase of distance to any object will obviously be proportional to the distance to the object because the rate of change of distance is the rate the ruler is shrinking, times the number of rulers it takes to get to that object, which is proportional to the latter quantity
4) It makes go away the question "why do photon wavelengths expand with space", because nothing is happening to the photon wavelength at all, it is the ruler that something is happening to
5) It just helps us see that all distances are actually ratios of distance to ruler length, they are not physically real things in and of themselves
There are useful advantages of the picture, though of course it would be just as wrong to say that "bound systems are really shrinking" as it would be wrong to say "space is really expanding."

Ken G
2014-Aug-14, 03:18 AM
I disagree. If the Moon and the Earth are shrinking the distance between the 2 is increasing. If it wasn't then you wouldn't get the red shift of photons from the "shrinking" matter. IF you really mean that the distance between the Earth and Moon is shrinking and the distance between them is shrinking then you have to explain why our galaxy and distant galaxies are shrinking but the distance between them is not.The explanation is simple-- that's what solves GR. It is no different from the problem having to explain why the space inside galaxies isn't expanding in the usual picture, as caveman1917 correctly points out.

Reality Check
2014-Aug-14, 03:51 AM
If all matter is shrinking at the same rate, is shrinkage indepentant of gravitational time dilation?
I would say that the problem with "shrinking matter" is more basic: What does the term actually mean?
What is actually shrinking - the length of bonds between atoms; size of atomic orbitals; size of atomic nuclei, all of them?

Gravitational time dilation is caused by a difference in gravitational potential between two points. It will never cause this shrinkage.

Ken G
2014-Aug-14, 04:05 AM
I would say that the problem with "shrinking matter" is more basic: What does the term actually mean?
What is actually shrinking - the length of bonds between atoms; size of atomic orbitals; size of atomic nuclei, all of them?All of them, it means that gravity is causing all bound systems to shrink. We have no idea why, but then we have no idea why space is expanding in the usual picture. Either way, it's just what the solution to GR says.


Gravitational time dilation is caused by a difference in gravitational potential between two points. It will never cause this shrinkage.Gravitational time dilation is just a language for talking about observable predictions made by the equations of GR. It never causes anything, nor is caused by anything, because language does not cause things, nor needs to be caused. We have no idea what gravitational time dilation is, nor if it even exists, we only know that it is one way to think about the equations of GR. But if we hold that being deep in a well slows down time, then of course we must also say it shrinks the rulers, or else the speed of light, a law of nature, would not stay the same.

WayneFrancis
2014-Aug-14, 04:18 AM
No, in those coordinates gravitationally (or otherwise) bound groups are constantly collapsing in on themselves, in such a way that we cannot detect that because our rulers are also shrinking the same way. It's technically fine, i just don't see the convenience over the normal way of doing it. Rather than wrapping your head around expanding space you now have to wrap your head around everything else collapsing in on itself without you being able to detect it.

Then explain to me why it doesn't work at the galactic scale. Gravity isn't just "local". If you want to say that the effect is to small to measure I'll agree but even in a empty universe bar 2 object 10 billions light years apart the gravity between them is still in affect. It is just VERY week. I'm OK with space expanding or contracting but what you are trying to say is that matter is shrinking AND the space is contracting and if that is true there would be no measurable affect. Remember the effect is on the order of 1x10-20ish give or take a couple of magnitudes. I understand that orbits might adjest themselves but 2 "points" in space would appear to increase. It is this "increase" compared to our rulers shrinking that would lead to the red shift.

Ken G
2014-Aug-14, 05:04 AM
Then explain to me why it doesn't work at the galactic scale. Gravity isn't just "local".Gravity obeys GR. So does shrinking rulers and shrinking galaxies. This is obvious-- GR is a metric theory, which means that all it tells you is the number of rulers that fit between two points as a function of time, using some way to coordinatize spacetime. The coordinates are arbitrary, that is the great accomplishment of GR. But once you choose your coordinates, you still only know how many rulers will fit, and if that number changes, you have no idea "why"-- you don't know if space expanded, or the rulers shrank, because GR is completely moot on the issue, and so is scientific testing. So we just arbitrarily choose a language to talk about what happened, and one of the most important things to understand about language is that it can be translated into other languages too, and if that translation is one-to-one, there is no increase in complexity and nothing that Occam's Razor could decide.

Swift
2014-Aug-14, 02:17 PM
As this thread has evolved from a simple question to a long discussion, I've moved it from Q&A to Astronomy

Strange
2014-Aug-15, 08:48 AM
3) It is one way to explain the Hubble law-- the rate of increase of distance to any object will obviously be proportional to the distance to the object because the rate of change of distance is the rate the ruler is shrinking, times the number of rulers it takes to get to that object, which is proportional to the latter quantity

I think that is equally obvious in the "expanding space" model as well. (Where "equally obvious" may mean that some people will never get it!) But I think some of the other points are good.

Ken G
2014-Aug-16, 08:52 AM
I agree-- so it's not a better or worse way to understand the Hubble law, merely an alternative way. I'm not actually trying to sell this perspective on its merits, I'm merely trying to point out it is a fully viable alternative that is not counter to Occam's Razor.