Spontaneous Energy Loss in Light

[Les Sleeth]

I have a question I hope someone can help me with. Does light ever spontaneously lose energy? What I mean is, would, say, an infrared wave be traveling along and suddenly shift to microwave radiation, and maybe later to a radio wave, without any outside influences having affected it? If external influences do cause EM to lose energy, what natural circumstances would do that?

Thanks in advance for anyone's help.

 

[Alexander]

I never heard about LOSS of energy (=frequency) by a photon.

 

[anil]

Loss of energy?

Light can loose energy but it depends on the medium. Wave spontaneously doesn't loose or gain energy. If that's the case we would have radiowaves from X ray machines and Gamma rays from radio stations. Only time photons loose energy is when they collide with a particle

 

[Les Sleeth]

So I might assume then, that if the universe were to expand eternally, any light that's ever been emitted and not colliding with a particle will continue forever at the same frequency?

 

[anil]

Yeah and No! most likely

Remember tired light theory. The wavelengths of the all photons lost energy through interactions with matter en route to earth. This means that almost all photons that interact with Earth must have atleast once collided with a matter or even a particle. So it's extremely rear you find a photon not collide with a matter continuing at their original frequency. I believe you will never find a photon not come in contact with matter ever since they have been emmited. If at all a photon never comes in contact with matter it should continue to move in the same frequency that they have been emitted. But let's wait till astrophysicists tell their ideas. Matter of fact Universe is expanding and galaxies are moving farther away from each other.

 

[drag]

Greetings !

Originally posted by LW Sleeth
So I might assume then, that if the universe were to expand eternally, any light that's ever been emitted and not colliding with a particle will continue forever at the same frequency?

Compared to the Universe as a whole - yes.
BUT, to us, being inside the Universe, the
frequency of EM waves decreases due to the
Universal expansion. An example is the Cosmic
Microwave Background Radiation. Shortly
after the BB the CMBR was EXTREMELY energetic
but then as the Universe expanded the frequency
and hence energy of these waves decreased until
it reached its current microwave frequency
spectral range (and it will keep decreasing).

Live long and prosper.

 

[anil]

drag: really?

So you are saying as the universe expands the frequency decreases?

 

[Alexander]

Originally posted by anil
Remember tired light theory. The wavelengths of the all photons lost energy through interactions with matter en route to earth. This means that almost all photons that interact with Earth must have atleast once collided with a matter or even a particle.

Not true. It simply means that you are in moving differently than the source reference system (Doppler effect as a mathematical coordinate transformation result).

 

[Alexander]

Originally posted by LW Sleeth

I was thinking about that because I wonder if there are lower states of energy possible for light than that of a radio wave, and if so, what might light be like in its absolute lowest state of energy.

Obviously only speculation is possible about most of that, but I thought if light had ever been observed spontaneously dropping to a lower energy state, that might be clue. [/B]

Lower than radiowave state is still radiowave (just with lower frequency: 1000000 Hz, 60 Hz, 0.00001 Hz). No additional speculations needed.

 

[anil]

Oh...ok!

So in reality a photons cannot loose all of it's energy?

 

[Integral]

Photons exist due the change in energy created when an electron changes orbitals. Likewise all photons cease to exist when they induse the reverse transition within an atom. Thus photons are pure energy, they are our conceputalization of how atoms exchange energy, this is the case for all photons. So do not attempt to consider photons without some understanding of the nature of atoms.

 

[Alexander]

Originally posted by anil
So in reality a photons cannot loose all of it's energy?

Nope. Energy concerves.

 

[Les Sleeth]

Originally posted by Integral
So do not attempt to consider photons without some understanding of the nature of atoms.

 

[quantumdude]

Originally posted by Integral
So do not attempt to consider photons without some understanding of the nature of atoms.

I don't think all that is necessary. Sure, atomic transitions emit light, but that is not the only source. There is also Brehmssrahlung, which is readily understandable without the gory details of atomic physics.

 

[marcus]

Originally posted by anil
So you are saying as the universe expands the frequency decreases?

Anil, this is true.

this post of yours did not get a response in the thread.
You were replying to what Drag said about some very old light that has declined in frequency and quantum energy over the years:

Drag said:
[[...the
frequency of EM waves decreases due to the
Universal expansion. An example is the Cosmic
Microwave Background Radiation. Shortly
after the BB the CMBR was EXTREMELY energetic
but then as the Universe expanded the frequency
and hence energy of these waves decreased until
it reached its current microwave frequency
spectral range (and it will keep decreasing).]]

So you said "does the frequency really decrease as space
expands" and yes indeed it does. The frequencies in the
CMBR have declined by a factor of 1000.

Space stretching out makes their wavelengths longer and
that lowers their frequency by the same factor. Space
has expanded by a factor of 1000 since those photons were emitted
and so their wavelengths are 1000 times longer and
their frequencies correspondingly lower.

That light which drag was talking about is believed to be
almost as old as the universe----having been emitted only
some 300 thousand years into its history.

interesting stuff, i think

 

[marcus]

Originally posted by anil
So in reality a photons cannot loose all of it's energy?

Maybe not all, without being absorbed, but a photon can lose 999/1000 of its energy and there are
trillions of photons in the space around us which have already done that----Cosmic Microwave Background photons.

Someone already replied to this post, but in a misleading way, suggesting that a photon can NOT lose any energy because
of something called "energy conservation" but this is untrue.
They CAN lose nearly all their energy.

In principle one can lose everything but a tiny remnant like a millionth of its energy. Or even all but a billionth. This is simply with the expansion of space---not by interacting or being absorbed by anything.

 

[marcus]

Originally posted by Integral
Photons exist due the change in energy created when an electron changes orbitals. Likewise all photons cease to exist when they induse the reverse transition within an atom. Thus photons are pure energy, they are our conceputalization of how atoms exchange energy, this is the case for all photons. So do not attempt to consider photons without some understanding of the nature of atoms.

Radiowaves are a form of light which is made by antennas.

Shall I not attempt to consider photons unless I have made a study of antennas?

X-rays and radiowaves are forms of light. X-rays are not made by orbital-change in atoms but by a beam of electrons hitting the wall and having to stop abruptly.

I think I must understand abrupt stopping before I try to understand light.

It seems that light has something to do with the acceleration and deceleration of charge---electrons in particular.

Changes in momentum---maybe there are changes in momentum inside atoms when they emit and absorb light.

Might be understandable in a general way without studying atoms specifically.

 

[Les Sleeth]

Originally posted by marcus
The frequencies in the CMBR have declined by a factor of 1000. Space stretching out makes their wavelengths longer and
that lowers their frequency by the same factor. Space
has expanded by a factor of 1000 since those photons were emitted and so their wavelengths are 1000 times longer and
their frequencies correspondingly lower. . . . interesting stuff, i think

Is this true? I'm unfamiliar with it . . . . could you recommend something I might read to get up to speed about it? I agree, intersting stuff, to me anyway.

 

[Les Sleeth]

Originally posted by Tom
There is also Brehmssrahlung, which is readily understandable without the gory details of atomic physics.

Brehmssrahlung? . . . I'll do a Google search and read up on it.

EDIT: No hits at Google . . . is that the correct spelling?

 

[wimms]

Originally posted by marcus
So you said "does the frequency really decrease as space
expands" and yes indeed it does. The frequencies in the
CMBR have declined by a factor of 1000.

Space stretching out makes their wavelengths longer and
that lowers their frequency by the same factor. Space
has expanded by a factor of 1000 since those photons were emitted
and so their wavelengths are 1000 times longer and
their frequencies correspondingly lower.

Wait, just only 1000 times?? 13billion lightyears radius that was once singularity, expanded just 1000 times?
And, if even photon looses energy due to expansion, then anything should loose energy to expansion. Or, even, which is first, expansion causing loss of energy, or loss (dissipation) of energy causing expansion?

LW, google for bremsstrahlung

 

[Alexander]

Originally posted by marcus
Maybe not all, without being absorbed, but a photon can lose 999/1000 of its energy and there are
trillions of photons in the space around us which have already done that----Cosmic Microwave Background photons.

Someone already replied to this post, but in a misleading way, suggesting that a photon can NOT lose any energy because
of something called "energy conservation" but this is untrue.
They CAN lose nearly all their energy.

In principle one can lose everything but a tiny remnant like a millionth of its energy. Or even all but a billionth. This is simply with the expansion of space---not by interacting or being absorbed by anything.

Incorrect. Light can not lose its energy without interaction with something. Energy and momentum must conserve.

I think, Marcus misunderstands how energy transforms from one reference system to another (coordinate transformation results in what we label as Doppler shift).

 

[marcus]

Originally posted by Alexander
Incorrect. Light can not lose its energy without interaction with something. Energy and momentum must conserve.

I think, Marcus misunderstands how energy transforms from one reference system to another (coordinate transformation results in what we label as Doppler shift).

Heh heh
The cosmological redshift is not a Doppler shift
I am pretty sure you misunderstand from the things you keep
saying.

The "coordinate transformation" you refer to is probably
a Lorentz transformation.

The "Doppler shift" formula resulting from that is

1+z = sqrt [(1+beta)/(1-beta)]

This is correct in the context of Special Relativity---as the doppler shift resulting from relative radial velocity beta.

But you would be laughed at on Usenet for suggesting that formula be applied to the cosmological redshift.

You might like to read some threads on sci.physics.research about this very misconception---a lot of people have it including, apparently, yourself. A google search should do it for you.

In fact, why don't you go on use net and argue that this is the appropriate formula to use for the redshift? Treat yourself to a real argument.

 

[Alexander]

Originally posted by marcus

X-rays and radiowaves are forms of light. X-rays are not made by orbital-change in atoms but by a beam of electrons hitting the wall and having to stop abruptly.

Not correct. X-rays which are used to study crystal structure ARE made by orbital change (see Mozely law). They call them K, L, M... lines. Very bright lines, by the way. They are orbital transitions of electron between inner shells. Electron states in inner shells of heavy atoms are way deep in Coulomb potential of almost unscreened (by outer electrons) strong charge of nucleus, thus their energy states are no longer in few eV but often in few KeV range. Photons originated in such orbital transitions thus also have energies in KeV range, thus we label them "x-rays".

Also, when electron beam is hitting a "brick wall" it does not stop abruptly. Instead, electron wanter in Coulomb fields of other electrons and nucleii around and begin to chenge its original direction of propagation and magnitude of its velocity (kind of randomly wandering and wiggling around). The varying electric field (between this electron and surrounding electrons and nuclei) is what we call "e/m radiation" - in the case of fast moving in the brick wall electron the electric field changes so rapidly that we call this it "x-rays".

 

[Alexander]

Originally posted by marcus


But you would be laughed at on Usenet for suggesting that formula be applied to the cosmological redshift.

You might like to read some threads on sci.physics.research about this very misconception---a lot of people have it including, apparently, yourself. A google search should do it for you.

In fact, why don't you go on use net and argue that this is the appropriate formula to use for the redshift? Treat yourself to a real argument.

Markus, I think your "losing energy light" hypothesis is layman crackpottery laughted at: http://www.astro.ucla.edu/~wright/tiredlit.htm

My advice - start reading good textbooks. And not just reading, (and cutting and pasting). Do homework exercises - they help way way better than just reading.

Heh heh heh...

 

[marcus]

Originally posted by Alexander
Markus, I think your "losing energy light" hypothesis is layman crackpottery laughted at: http://www.astro.ucla.edu/~wright/tiredlit.htm

My advice - start reading good textbooks. And not just reading, (and cutting and pasting). Do homework exercises - they help way way better than just reading.

Heh heh heh...

You sound as if you have completely missed the point.

If you believe in the "tired light" hypothesis and that
"tired light" explains the cosmological redshift, then it
is hopeless. Cant have a reasonable discussion.

Likewise if you believe it is a Doppler redshift.

Sorry but must cut off discussion. Bye.

 

[Les Sleeth]

Originally posted by wimms
LW, google for bremsstrahlung

Thanks . . . found it.

 

[Les Sleeth]

Originally posted by Alexander
http://www.astro.ucla.edu/~wright/tiredlit.htm

Interesting.

So do you say photons do not lose energy due to the universe's expansion? If so, is the low energy state of CBMR due only to it having collided with so many particles during its long journey?

 

[marcus]

Originally posted by LW Sleeth
Interesting.

So do you say photons do not lose energy due to the universe's expansion? If so, is the low energy state of CBMR due only to it having collided with so many particles during its long journey?

Sleeth, Alexander's "tired light" idea is totally wacko!
Do not listen to him. He only seems to want to argue and
believe himself to be right.

Indeed the loss in energy in the CMBR is due to the expansion of the universe---best seen as having stretched out the wavelengths of CMB photons. There is a very broad consensus among cosmologists about this.

The "tired light" people must be a very small minority indeed, one never hears from them directly but only indirectly through references to the idea

 

[marcus]

Originally posted by LW Sleeth
Is this true? I'm unfamiliar with it . . . . could you recommend something I might read to get up to speed about it? I agree, intersting stuff, to me anyway.

Hello Sleeth,
sorry I just noticed your reply to an earlier post of mine
I had said:

[[The frequencies in the CMBR have declined by a factor of 1000. Space stretching out makes their wavelengths longer and
that lowers their frequency by the same factor. Space
has expanded by a factor of 1000 since those photons were emitted and so their wavelengths are 1000 times longer and
their frequencies correspondingly lower. . . . interesting stuff, i think]]

And you replied to that, asking for a reference.
Ned Wright's tutorial and cosmology FAQ are the easiest places to begin, probably.

The 1000 figure (sometimes given more precisely as 1100) is the factor by which the universe has expanded since the event called "recombination" or "last scattering" which is ordinarily dated as being 300 thousand years after the beginning.

Let's say 1100 instead of 1000.

This is the same as 1+z where z is the redshift.
1+z is the ratio by which wavelengths have been stretched out and so it is the same as the factor by which space has expanded.

I have to go for the moment but will get back to this. Glad you asked. Perhaps I can find a link for you to something online.

 

[Les Sleeth]

Originally posted by marcus
Sleeth, Alexander's "tired light" idea is totally wacko!
Do not listen to him. He only seems to want to argue and
believe himself to be right.

Indeed the loss in energy in the CMBR is due to the expansion of the universe---best seen as having stretched out the wavelengths of CMB photons. There is a very broad consensus among cosmologists about this.

The "tired light" people must be a very small minority indeed, one never hears from them directly but only indirectly through references to the idea

I am a little confused I think. The article Alexander referred to reasons against the tired light theory. It says:

"Tired light models invoke a gradual energy loss by photons as they travel through the cosmos to produce the redshift-distance law. This has three main problems:

* There is no known interaction that can degrade a photon's energy without also changing its momentum, which leads to a blurring of distant objects which is not observed. The Compton shift in particular does not work.
* The tired light model does not predict the observed time dilation of high redshift supernova light curves. This time dilation is a consequence of the standard interpretation of the redshift: a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift z=1
* The tired light model can not produce a blackbody spectrum for the Cosmic Microwave Background without some incredible coincidences. The local Universe is transparent and has a wide range of temperatures, so it does not produce a blackbody, which requires an isothermal absorbing situation. So the CMB must have come from a far away part of the Universe, and its photons will thus lose energy by the tired light effect. The plot below shows what happens if the CMB comes from z = 0.1. "

So from what Alexander is saying, it seems he disagrees with it too. Are you advocating it or disagreeing with it? Or are you saying the tired light issue has nothing to do with CMBR and the loss of energy by photons due to expansion?

 

[marcus]

Hello Wimms, just saw your post. Only time for a quick answer. You replied to this frome me:

[[... "does the frequency really decrease as space
expands" and yes indeed it does. The frequencies in the
CMBR have declined by a factor of 1000.

Space stretching out makes their wavelengths longer and
that lowers their frequency by the same factor. Space
has expanded by a factor of 1000 since those photons were emitted and so their wavelengths are 1000 times longer and
their frequencies correspondingly lower.]]

In your reply you said:

Originally posted by wimms
Wait, just only 1000 times?? 13billion lightyears radius that was once singularity, expanded just 1000 times?
And, if even photon looses energy due to expansion, then anything should loose energy to expansion. Or, even, which is first, expansion causing loss of energy, or loss (dissipation) of energy causing expansion?

Since big bang there has been FAR more expansion than merely by a factor of 1100-----the factor for the CMBR.
The expansion has been by factor of 1100 since the epoch called "last scattering" or "recombination" which is when those CMB photons are thought to have been emitted.

The picture is of a clearing fog (actually hot plasma).
Above a certain temperature hydrogen gas is opaque, like the sun is opaque, because it is ionized into glowing plasma and is highly reactive with light. The electrons absorb and re-emit photons frantically.

Then as the plasma cools it suddenly becomes transparent because the neutral atoms form----no more free electrons.
The moment it becomes transparent, the photons are set free and fly essentially forever.

This cooling and transparency was reached at year 300 thousand, approx. This is the moment of origin of the CMB.

Matter behaves differently from light, in expansion. Matter too has an energy density (its mass-energy) but this falls off only as the cube------the energy simply spreads out into larger volume.
But light energy falls off as the fourth power. Because not only do there get to be fewer photons per volume but also each photon has its wavelength stretched out. This differential in the decline has an interesting effect. It changes the relative importance of matter versus light over time.

You ask which came first. The conventional view is that the expansion is a give and that the loss in CMB energy is simply one consequence of that.

Good question, thanks!

 

[Les Sleeth]

Originally posted by marcus
And you replied to that, asking for a reference.
Ned Wright's tutorial and cosmology FAQ are the easiest places to begin, probably.

Thanks, I found them and will read them today.

Originally posted by marcus
The frequencies in the CMBR have declined by a factor of 1000. Space stretching out makes their wavelengths longer and
that lowers their frequency by the same factor.

I've been known to question someone into a headache, so let me know when you've had enough. But . . .

If the expansion of space is lengthening radiation's wavelengths, doesn't this mean "space" and light are interconnected somehow? I have myself wondered if they might even be the same thing, with light simply an accentuated form of space. If space were in reality, say, some sort of polarized field that had constricted and then released to generate the big bang, and light is a ripple in that field, that might account for light's (and everything else's) polar nature, inflation, and the stretching of wavelengths as space expands.

 

[marcus]

Originally posted by LW Sleeth
Thanks, I found them and will read them today.

Great! If anybody knows some other online cosmology tutorial---entry-level like this one----and similar FAQ please post the URL!

Here's a helpful animation Wright has:

http://www.astro.ucla.edu/~wright/Balloon2.html

The photons start out blue and as space expands they get longerwavelength and they turn red.
And then as space recontracts they turn shorterwavelength and are colored blue again---the coloring is just an artificial way of
diagramming the change. Neat animation---movie of the universe.

Wright's isn't perfect or complete. His professor salary and research grants pay him to do other kinds of work--his tutorial is volunteer work, labor-of-love.

someone could make it a lot better if they could get a grant just for the purpose of creating a great online (un-popularizing) introduction to cosmology.

Wright will leave you with various confusions and missing pieces of the puzzle but it is still a wonderful start!

Originally posted by LW Sleeth

If the expansion of space is lengthening radiation's wavelengths, doesn't this mean "space" and light are interconnected somehow? I have myself wondered if they might even be the same thing, with light simply an accentuated form of space. If space were in reality, say, some sort of polarized field that had constricted and then released to generate the big bang, and light is a ripple in that field, that might account for light's (and everything else's) polar nature, inflation, and the stretching of wavelengths as space expands.

Deep question. Perhaps everything is connected to space because it HAPPENS in space. Space is, I guess, sort of defined by all the geometric relations between things.

In particular, the propagation of light is connected to space because it happens in that arena----no need for extra rubberbands or little wires to interconnect the two any further. IMHO.

Thought experiment: You and I discover an experimental place outside the universe where space is not expanding. We take a laser there and measure how far a flash of light can travel in a year. Maybe we put a mirror out a ways and have the pulse
fly to the mirror and back. We see that the pulse comes back with the same frequency and wavelegth it started out with.

Then we return to our real universe and set up the experiment putting the mirror intially the same distance away. what do we see? Does the pulse take longer to return? Is its wavelength lengthened when it gets back? If so how did that happen?

This is only a thought experiment because the expansion of space on a timescale of a year is way too slight to observe!

********

I do know that if space were eventually to begin contracting (as it does in some models) then the CMB which is now so cold would get so hot after a while that it would burn us up. It would go from present long-wavelength photons back to short wavelength sort of like sunlight, some even visible light, some infrared, some UV etc. Maybe this is a good picture of how expanding and contracting space changes the wavelengths of CMB photons.

Have fun. I cannot answer all the questions that you come up with, by a long shot! But they so far cause no sign of headache.

 

[Les Sleeth]

Originally posted by marcus
I cannot answer all the questions that you come up with, by a long shot! But they so far cause no sign of headache.

I think you better get your aspirin ready.

I have a couple of more questions . . . I'll do the easy one first. I read Linder's FAQs first since it was shortest, and in it he brought up something I have been curious about. He says:

"What is meant by the edge of the universe where velocities reach the speed of light? There are a number of concepts put together in this question. By edge of the universe cosmologists mean the edge of the observable universe or the horizon. Just like on Earth the fact that you cannot see past the horizon doesn't mean there's nothing past there! (See the next question for more details.) This edge is equivalent to the distance light can travel in a time equal to the age of the universe."

Although he says what the edge means, he doesn't say what constitutes the edge. In other words, is the edge considered the the furthest reaches of matter (i.e., stars, nebula, etc.)? Or does it refer to how far light might have traveled beyond the matter it radiated from? If the edge is determined by the expansion of matter, could it be that any of it reaches the velocity of light? Finally, if it is light, then is light creating space? Or is space seen as expanding ahead of or with matter and/or light?

Originally posted by marcus
Deep question. Perhaps everything is connected to space because it HAPPENS in space. Space is, I guess, sort of defined by all the geometric relations between things.

In particular, the propagation of light is connected to space because it happens in that arena----no need for extra rubberbands or little wires to interconnect the two any further. IMHO.

I understand this is the prevalent view, but it doesn't make sense to me (I also know that a lot of things in physics are counter-intuitive but nonetheless true).

It doesn't make sense because of the mainly inviolable team of cause and effect. In virtually every other situation where an action consistantly results in a specific effect, we assume there is a relationship between the two. So, if space expands and EM's wavelength consequently stretches, that normally would mean there is some sort of causal relationship between the two. Right? Of course, it could be that aging EM would stretch whether or not the universe expands, but then we are back to the tired light theory. That is why I have thought space may be the whole ball of wax -- light, gravity, energy, expansion, matter -- all of it manifested potentials of a polarized field we call "space."

Originally posted by marcus
Thought experiment: You and I discover an experimental place outside the universe where space is not expanding. We take a laser there and measure how far a flash of light can travel in a year. Maybe we put a mirror out a ways and have the pulse
fly to the mirror and back. We see that the pulse comes back with the same frequency and wavelength it started out with.

Then we return to our real universe and set up the experiment putting the mirror intially the same distance away. what do we see? Does the pulse take longer to return? Is its wavelength lengthened when it gets back? If so how did that happen?

Well, according to what I understand about light speed, it should take the same amount of time for the pulse to return (assuming both pulses traveled in a vacuum), but according to what you are saying about light and expansion, the pulse inside the universe should return lengthened because during its travels the universe was expanding. As for how it happens, I don't know, but I do have my little theory I mentioned above.

 

[Alexander]

Originally posted by marcus
Sleeth, Alexander's "tired light" idea is totally wacko!
Do not listen to him. He only seems to want to argue and
believe himself to be right.

Markus, please apologise for associating this idea with me.

As you can clearly see from my post about "tired light" it was to show you that YOUR idea of "stretched photons" is wrong (as you said - totally wacko).

So, apologise for putting words I did NOT say in my mouth, ok? And don't do that again. PF is not the right place for personal attacks.

 

[marcus]

Originally posted by Alexander
Markus, please apologise for associating this idea with me.

As you can clearly see from my post about "tired light" it was to show you that YOUR idea of "stretched photons" is wrong (as you said - totally wacko).

So, apologise for putting words I did NOT say in my mouth, ok? And don't do that again. PF is not the right place for personal attacks.

I rarely read your posts but my impression is that whenever I do I am likely to find an error. If I remember correctly:

1. you invoked a global energy conservation law in Gen.Rel. and there is no such law.

2. you claimed the energy lost from CMB went into gravitational energy!

3. you claimed there could be no voltage difference between the wingtips of a plane flying thru vertical magnetic field

4. I believe you also made false assertions about the Hubble law and limitations of recession speed in GR, but I would have to look back to check.

In every thread I can remember where I've had occasion to read your posts, you have made errors and when you are wrong you tend keep adamantly reasserting the false claim.

My views are standard mainstream views. The cosmological redshift is not Doppler. Maybe in Russia or wherever you are words are used differently and it is called "Doppler" and that means something else. But the simplest explanation is that you are just eccentric and opinionated.

I am glad to hear you disavow the "tired light" notion!

Tell me your interpretation of the Hubble law v = H₀ D.
In what metric is the distance D measured? At what moment in time? The speed v is the change in what measure of distance?
At what time is the v considered to be measured?

This would be a good test of whether your understanding of what these things mean is an eccentric abberation on your part.

 

[Les Sleeth]

Originally posted by LW Sleeth
I read Linder's FAQs . . . in it he brought up something I have been curious about. He says:

"What is meant by the edge of the universe where velocities reach the speed of light? There are a number of concepts put together in this question. By edge of the universe cosmologists mean the edge of the observable universe or the horizon. Just like on Earth the fact that you cannot see past the horizon doesn't mean there's nothing past there! (See the next question for more details.) This edge is equivalent to the distance light can travel in a time equal to the age of the universe."

Although he says what the edge means, he doesn't say what constitutes the edge. In other words, is the edge [of the universe] considered the the furthest reaches of matter (i.e., stars, nebula, etc.)? Or does it refer to how far light might have traveled beyond the matter it radiated from? If the edge is determined by the expansion of matter, could it be that any of it reaches the velocity of light? Finally, if it is light, then is light creating space? Or is space seen as expanding ahead of or with matter and/or light?

Anyone care to answer?

 

[Alexander]

The visible edge of universe is just an expression. It has no physical "edge" meaning. Universe is same here, there (near "vision edge") and beyond the edge.

Main cosmological principle is Copernicus principle: our location in space is no special than any other. By other words, universe is spatially symmetric. This imposes severe limitations on mathematically possible "designs" of universe (cosmological models). Say, that positively curved universe shall not have boundaries, but nevertheless shall have finite volume and finite size. Negative or zero curved universe shall be infinite in size and in volume.

So, we live right at the edge of universe for observer(s) 13 billion light years away.

 

[marcus]

Originally posted by LW Sleeth
Anyone care to answer?

Sleeth I read Linder's FAQ and was shocked.
I like his Cosmology overview very much and admire him
both as a writer and researcher.
His Introduction to Cosmology may be a good textbook too, I haven't looked.

The FAQ was to an unmathematical lay audience and talked down a lot. Some of what he said, presumably in order to be understood, he simply should not have said.

There is a split between how cosmologists talk among themselves and what you get in popular accounts.
Within the community and with students they use the RW metric also called the FRW metric and a rest frame defined by the Cosmic Microwave Background.

Distance is often expressed as a redshift, and there is a standard way to convert this to a "comoving" distance----essentially applying the metric to give a current distance to the object.

They typically don't use "light travel time" because it is not a workable spatial distance---applies to a mishmash of different epochs.

In the FAQ he was using light travel time distance because, I guess, most general audience listeners understand distance that way.

Linder's overview was great but I would just disregard his popular-audience FAQ. I'll try to elaborate on this later.

Here's a url for Linder's good overview and for Wright's online calculator which you can use in conjunction with Linder's overview:

http://www.astro.ucla.edu/~wright/CosmoCalc.html

hmmm I've mislaid the Linder URL have to go hunt it

Here it is

http://panisse.lbl.gov/~evlinder/lcos.pdf

 

[marcus]

I see that Alexander has just replied. He illustrates the
common misconception that the "cosmological horizon" is
currently 13 billion LY away.

that is talking in terms of light travel time.

In fact the current distance to the farthest objects we could possibly see is now 40 billion LY, in principle.
This is measured by observers at rest with respect to background, in the present, using the standard metric.

The most distant quasar that has actually been observed
is 28 billion LY away, at the present time.

the light from this quasar (observed in 2002 at redshift 6.4)
took 13 billion years to get to us----traveling through regions which were in various stages of expansion when the light came thru. And during that 13 billion years----the distance to the quasar expanded out to 28 billion LY.

Its current speed of recession is 2c. The speed of recession is the rate of change in the comoving distance---which is what plugs into the Hubble law.

Wrights tutorial is the most accessible source on this
Linder glosses over too much

 

[marcus]

It is actually pretty simple and also at the same time interesting.

Toss out Linder's FAQ and look at something real, like that quasar---Wright discusses it and provides some links which I can get if you want them.

The z = 6.4 tells the story.
You plug that into a calculator (Wright has on online) using
the current value of Hubble parameter and other standard assumptions which are inputs to the metric. And it gives you 28 billion LY as the distance.

That is the distance to the thing NOW

If you want to know the distance to it when the light was emitted you just divide by 6.4.

Because space has expanded that much since the light was emitted. 28/6.4 is 4.4
So the quasar was 4.4 billion LY away when the light left it.

The light took 13 billion years to come.
But the distance THEN was 4.4 and the distance NOW is 28 billion LY.

The 13 billion year travel time is not too useful because it doesn't immediately give a good idea of the distance either then or now.

Anyway, we can't see quasars which are currently 40 billion LY away. So far we have only seen one 28 billion LY away. But IN PRINCIPLE the distance to the horizon is 40 corresponding to infinite redshift.

The CMB is at redshift 1100. You can try that in the online calculator and it will, I guess, give pretty near 40, but not quite.
Because the CMB comes from a while after the big bang, so it is not quite at the horizon.

Hope you come back with more questions and are not boggled.
It is terrible when cosmologists use light travel time distance in talking down to people. Leaves a residue of confusion in how the public thinks.

 

[Les Sleeth]

Someone may have already answered me and I do not realize it, but I still haven't quite understood what is being relied on as the "edge." Is it matter only, or light?

But maybe I don't understand how light travels in our universe. If, say, the universe is considered flat and so its matter is spread out spherically, is light traveling just around that sphere, and not outwardly perpendicular to the sphere? It seems like the measurement of the universe is of distant objects like the quasar, but I then wonder if the light it emits isn't traveling beyond it "outwardly perpendicular" to the direction of expansion, and if so then should that be considered part of the size of the universe.

The other related question I have is about how fast matter is traveling in expansion. I thought I read that some galaxies are receeding at nearly the speed of light. Can matter so massive really go that fast? And are we ourselves moving as quickly away from other galaxies?

 

[marcus]

the current model is that space is flat, infinite, and expanding
(which means spacetime is not flat, but that is another issue)

There is a spherical volume in space called the observable universe (radius about 40 billion LY) which contains the objects which we can in principle observe

their light has reached us, of course we can only see them
as they were in the past when the light was emitted but
those are the object we know about and can observe

they are contained in a ball currently 40 billion LY in radius.

The surface or boundary of that ball is like a horizon

Maybe you could say it is an "edge" the way our ordinary
horizon on Earth is an edge. OK word, but maybe not the best word.

Does this help?

Originally posted by LW Sleeth
Someone may have already answered me and I do not realize it, but I still haven't quite understood what is being relied on as the "edge." Is it matter only, or light?

But maybe I don't understand how light travels in our universe. If, say, the universe is considered flat and so its matter is spread out spherically, is light traveling just around that sphere, and not outwardly perpendicular to the sphere? It seems like the measurement of the universe is of distant objects like the quasar, but I then wonder if the light it emits isn't traveling beyond it "outwardly perpendicular" to the direction of expansion, and if so then should that be considered part of the size of the universe.

The other related question I have is about how fast matter is traveling in expansion. I thought I read that some galaxies are receeding at nearly the speed of light. Can matter so massive really go that fast? And are we ourselves moving as quickly away from other galaxies?

 

[Les Sleeth]

Originally posted by marcus
The surface or boundary of that ball is like a horizon

Does this help?

Yes, it is mostly what I thought.

Here's what is still unanswered for me:

1. Light diverges. So does radiation travel outward and away from the horizon (essentially increasing the circumference of the horizon)? If so, is that light considered the horizen, or is it only the furtherest expanded matter that determines the horizen?

2. I'll just repeat my other question, "The other related question I have is about how fast matter is traveling in expansion. I thought I read that some galaxies are receeding at nearly the speed of light. Can matter so massive really go that fast? And are we ourselves moving as quickly away from other galaxies?"

Thanks for your patience.

 

[marcus]

Whoah! I see the problem! No, space is not like the surface of a balloon!

When people say space is flat they mean like ordinary 3D Euclidean space where the sum of the angles in a triange is 180 degrees. It is the infinite 3D space we are used to. this is on a large scale. Overlooking local bumps caused by stars and black holes and stuff. There are minor local abberations from flatness but in the large it is just our usual 3D infinite space.

The difference is that it is expanding. So when you put time into the picture it is more complicated. But anyway as far as space, at anyone instant of time, goes----think of standard 3D coordinates going to infinity.

Inside this Euclidean 3D space there is a finite ball-shaped volume containing all the stuff that has been seen with telescopes so far. This ball gets larger as time goes on and more news comes in and light from farther away and longer ago arrives.

Recession speeds in cosmology are routinely faster than light.
This doesn't mean spaceships can go faster than light it just
has to be because you have an infinite thing expanding uniformly. If you look far enough out you are going to see stuff receeding
faster than light.

The Special Relativity coordinates don't apply on large scale
and the speed limit you get in SR does not apply either.
The recession speed is not like an ordinary SR speed
(could not be used for sending information or for rocket travel, is not local but is only a change in distance between widely separated points in space).

Have to go, can't finish the discussion but will get back later

Originally posted by LW Sleeth
Someone may have already answered me and I do not realize it, but I still haven't quite understood what is being relied on as the "edge." Is it matter only, or light?

But maybe I don't understand how light travels in our universe. If, say, the universe is considered flat and so its matter is spread out spherically, is light traveling just around that sphere, and not outwardly perpendicular to the sphere? It seems like the measurement of the universe is of distant objects like the quasar, but I then wonder if the light it emits isn't traveling beyond it "outwardly perpendicular" to the direction of expansion, and if so then should that be considered part of the size of the universe.

The other related question I have is about how fast matter is traveling in expansion. I thought I read that some galaxies are receeding at nearly the speed of light. Can matter so massive really go that fast? And are we ourselves moving as quickly away from other galaxies?

 

[marcus]

Originally posted by LW Sleeth


2. I'll just repeat my other question, "The other related question I have is about how fast matter is traveling in expansion. I thought I read that some galaxies are receeding at nearly the speed of light. Can matter so massive really go that fast? And are we ourselves moving as quickly away from other galaxies?"

Thanks for your patience.

Thx enormously for YOUR patience, not always easy for me to say things concisely.

YES we are going away from the other galaxies at speeds near and even exceeding the speed of light. The uniform expansion of infinite space requires this. If there are conscious beings in other galaxies they would think of us as receding. But it is not like rocketship travel---it is just space spreading out. No one is going anywhere, is some sense. Just separation increasing in some cases at 2c in some cases 3c and so on----all different speeds depending on how far away.

Originally posted by LW Sleeth
Yes, it is mostly what I thought.

Here's what is still unanswered for me:

1. Light diverges. So does radiation travel outward and away from the horizon (essentially increasing the circumference of the horizon)? If so, is that light considered the horizen, or is it only the furtherest expanded matter that determines the horizen?

Have to postpone answer, busy. Be back

 

[marcus]

Originally posted by LW Sleeth
Yes, it is mostly what I thought.

Here's what is still unanswered for me:

1. Light diverges. So does radiation travel outward and away from the horizon (essentially increasing the circumference of the horizon)? If so, is that light considered the horizen, or is it only the furtherest expanded matter that determines the horizen?

Good thinking. It works both ways
this spherical volume containing things we have seen also
can be thought of as the ball of space which our light has reached
or which light from our location could have reached if something here had been making light from the beginning.

Ned Wright, in one example, looks at it that way.

He is explaining why the radius of the observable universe is about 40 billion LY and why the boundary is receding at speed around 3c. I will try to find the page reference and give a link.

So it is the spherical volume containing things whose light has reached us and ALSO reciprocally the volume of where our light (if we had making it from the very start) would have reached.

Our galaxy didnt get started glowing exactly at the beginning, so its actual light hasnt quite reached that far, but that is the rough idea. It works both ways.

 

[marcus]

Yeah, here it is
http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

It is from Wright's Cosmology FAQ and is called

"If the Universe is only 10 billion years old, how can we see objects that are now 30 billion light years away?"

For easy numbers he is considering a Einstein de Sitter universe
(no extras) that is 10 billion years old

So he has the radius be 30 billion LY

Just as with a more realistic model with age something over 13 billion years
the radius is around 40 billion LY

He considers a galaxy out near the horizon and also
a photon of light passing that galaxy and heading outwards.
It could have been one emitted by us, if we had had a light on at the beginning.

He does a little arithmetic to let time pass a bit and then looks
at it again and the boundary has moved out and the photon is
right there keeping pace with it.

I think what you concluded intuitively is right and this calculation, tho simplified for easy numbers, confirms your intuition.

 

[Les Sleeth]

Originally posted by marcus
Good thinking. It works both ways
this spherical volume containing things we have seen also
can be thought of as the ball of space which our light has reached or which light from our location could have reached if something here had been making light from the beginning.

Thank you for clearing that up. The reason I wondered is related to an additional question I've been pondering which is, what is creating "space"?

Some have said that the expansion of the universe creates space as it goes along; that before the space of our universe gets there, there is nothing at all. Our space seems not a void after all with virtual particles popping in and out, dark matter, and the possibility of a cosmological constant (and the theoretical Higgs field?). In other words, while space may be the absence of matter, it also may be dynamic in certain respects.

So, I wondered if as the universe expands, what exactly defines the creation of new space on the horizon. Is it the light traveling ahead of matter expansion creating it? Or could possibly the qualities of space itself be expanding just ahead of light and everything else, creating the potential for the rest to follow?

(Getting that headache yet? )

 

[marcus]

Hello Sleeth thanks for the new question, more to think about. But there was still a bit of grist for my mill in what you said earlier

Originally posted by LW Sleeth The other related question I have is about how fast matter is traveling in expansion. I thought I read that some galaxies are receeding at nearly the speed of light. Can matter so massive really go that fast?
[/B]

this is the most important question because it involves RECONCILING the local SR picture with the global GR picture.

It is easy for distant galaxies to be receding at twice the speed of light or more and it does not make them more massive. It is inevitable that some galaxies would be doing this because of the uniform expansion of space. Yet this does not contradict Special Relativity, which is about relative speeds of things at the same point in space or in the same locale.

The coordinates of SR are called "Minkowski" and Minkowski coordinates don't apply in the large. The Minkowski spacetime
of Special Relativity has no ability to expand. In the real universe you can only apply Minkowski coordinates in a small local region,
like a bandaid. There is no bandaid that covers the whole person.

No rocket ship could PASS THE EARTH going over the speed of light. Indeed in our local coordinates as things approach the speed of light their energy (measured in our local laboratory coordinates) grows unboundedly. They become harder and harder to speed up any further---taking more and more energy to get the next little bit of speed.

But that is no reason why a distant galaxy can't be just sitting still in the space around it and receding from us at twice the speed of light. Thousands of galaxies are doing just that. It does not contradict SR because it is not in our laboratory, or in the sun's neighborhood, or in our local Minkowski coordinates----however you want to say it.

We did not invest energy to accelerate those distant galaxies. They don't have any extra energy or mass just because they are sailing away at twice the speed of light. They have to, because space is expanding, and they are not in the same local SR Minkowski coordinte frame with us.

The distance to them is measured with a METRIC, the robertson walker metric which is able to deal with spacetime curvature and cope with expansion and do stuff which the rigid Special Relativity metric ("minkowski metric") cannot do.

So...different idea of distance...leading to different idea of speed...NO PROBLEMO! Galaxies receding at twice the speed of light and yet no weirdness like tachyons and going backwards in time and killing your grandfather and circus freak infinite energy stuff. All those things are stories told in a Special Relativity surround.

They recede at twice the speed of light and more because they are outside the purvue and jurisdiction of Special Relativity and there is no weirdness.

Originally posted by LW Sleeth
Thank you for clearing that up. The reason I wondered is related to an additional question I've been pondering which is, what is creating "space"?

Now this is a REALLY interesting question. I will think a bit before answering. I think you would be satisfied if we just kept the DENSITY of mysteries like dark energy constant and space expanded.

If there is always the same amount of virtual X-particles PER CUBIC METER, then if your one cubic meter volume expands to be two cubic meters you now have twice as many virtual X-particles or "weirdons" or whatever. And twice as much of all the other familiar inhabitants of the vacuum as well.

That should be enough goodies for anybody. Let's not talk about space as if it were a commodity. If it expands and the per-volume quantity of all this mysterious stuff stays the same, then we get more goodies. the mystery remains, but phrased more acceptably IMHO.

It is definitely mysterious. 73 percent of the energy in the U is called "dark energy" and nobody has a convincing theory of what it is and it has never been detected. And the assumption is that the density of it is constant over space and time. So what makes more---to keep the density constant when space expands? [?]

It is one of the questions that make life worth living. I mean it.