Is the Universe Expanding or Has it Already Expanded?

[C. Bernard]

Should not we say "the universe WAS expanding" rather than "IS expanding" since
the red shift augments as we go back in time to the farthest and therefore the oldest
galaxies?

 

[Drakkith]

What do you mean? To our knowledge the universe is still expanding.

 

[C. Bernard]

What i mean is:is the speed of the expansion slowing or accelerating?
It seems to me that it is slowing since it is greater as we go back in time.

 

[AlchemistK]

The expansion is increasing, and that too at an exponential rate. In fact, even faster than the speed of light.

 

[dpa]

i think i got C.bernard's argument.
The red shifts we observe are of light from stars far distant and hence are older than present. So should we not say cosmos was expanding?

 

[Drakkith]

What i mean is:is the speed of the expansion slowing or accelerating?
It seems to me that it is slowing since it is greater as we go back in time.

The recession velocity of galaxies increases as we look further back in time. The acceleration of the expansion turns out to be increasing.

The expansion is increasing, and that too at an exponential rate. In fact, even faster than the speed of light.

The rate of expansion causes objects to accelerate in velocity away from us. Since the speed of light is a velocity, not a measure of acceleration, nothing can accelerate at the speed of light.

i think i got C.bernard's argument.
The red shifts we observe are of light from stars far distant and hence are older than present. So should we not say cosmos was expanding?

In the absence of something stopping the expansion I don't see how you can say that.

 

[C. Bernard]

The speed of the expansion could be greater than the speed of light since it's the universe that is expanding, nevertheless we are measuring it with galaxies that no longer exist!
So i am still of the opinion that we should speak of it as being greater in the past.

 

[alexg]

So i am still of the opinion that we should speak of it as being greater in the past

But observation show us that it wasn't.

 

[marcus]

The speed of the expansion could be greater than the speed of light since it's the universe that is expanding, nevertheless we are measuring it with galaxies that no longer exist!
So i am still of the opinion that we should speak of it as being greater in the past.

My take on it is that we "measure the speed of expansion" by fitting a mathematical model to the data. We compare what the model says we ought to see with what we actually do see, and adjust the parameters (tweak a few knobs) to get the simplest best fit.

Are you comfortable with calculus? The model is a couple of simple equations and it generates curves---like there is one called a(t) the "scale factor". It is a number that increases with time.

the time derivative da/dt of a(t) could be written a'(t). Are you used to that prime-for-slope notation?
That is the closest thing I can think of that corresponds to the idea of "speed of expansion".

It is not a speed that you could write down in meters per second. Or write in terms of lightyears per century or whatever.

a(t) is a curve, at each time t it is a definite number that is currently around 1, and it's currently increasing by about 1/140 of one percent in a million years.
Right now today it is 1.00000
and in exactly one million years from now it will be 1.00007.

Right now today the time derivative of the scale factor is a'(t) = 0.00007 per million years. (a kind of "James Bond" number, if you like.)
When people say "expansion is accelerating" they mean that a'(t) is increasing. They do not mean that some uniquely defined SPEED is increasing. A speed is something you can express in meters per second.
You could call a'(t) a *rate* I guess, and say the *rate* is increasing.

At some times in the past we are confident that a'(t) has been extremely much bigger than the "James Bond" size it has today. And we are certainly confident that in the relatively recent past it has been LESS than today's value.
So the expansion rate---correctly expressed as a'(t) the timederivative of the scalefactor--has in the past been both bigger and smaller than it is today.

We can be pretty confident in the model (nothing in science is completely sure but this is unusually well supported) because it agrees well with masses and masses of data, millions of datapoints with more coming in all the time. And because the model is a straight shot derivation from the Einstein 1915 law of gravity, an equation which has been checked to exquisite precision by numerous experiments in the solar system.
So we don't look out and measure some particular speed which is "the speed of expansion of the universe". there is no such speed. We fit a model to a huge amount of data, we get a snug fit, and we calculate a curve a(t) and the slope of that curve is a'(t). It is not a speed but it is what popularizers and journalists call "the speed of expansion".

And that bad translation of a math quantity into words is responsible (along with other bad verbalizations) for much of the confusion.

 

[C. Bernard]

Thanks Markus for your effort and trouble in trying to make me understand that it is the rate of expansion and not the speed. Unfortunately i am very unconfortable with calculus.
Furthermore i was under the impression that the theory was a question of cosmological reshift and the stretching of wavelength discovered by Hubble.
Since it seems to be based on thousands of measures of something else i confess my ignorance and can only hope that not everybody aggrees with your explanation.

 

[marcus]

...
Since it seems to be based on thousands of measures of something else

Getting the simplest best fit model is based on many (hundreds of thousands) measurements of redshift. Once you have a model that predicts in broad outline what has been observed then that model tells you things like "speed of expansion" --- it gives you the curve a(t) which is the expansion history of the universe.

What one directly measures is redshift (and some other things like angular size, luminosity, correlation with microwave sky temperature, periodic behavior but for simplicity let's just focus on redshift). And one makes COUNTS of how many galaxies one sees in a particular ranges of redshift. How the count varies with redshift. So you make direct measurements and you get a kind of "census data". The model has to predict that. In broad outline it has to reproduce nature.

And the model also has to derive from the law of gravity that is the most accurate we known so far (1915 Gen Rel).

So then, at the end of all that---the best fit model gives you a curve. Actually because of uncertainty it gives a range of very similar curves. that curve a(t) is the expansion history. It is a picture of how the scalefactor has increased over time. The slope of that curve, at any given time, is the "speed of expansion" at that time. It started off very steep and then leveled off slightly and is now increasing gradually. It has always been positive slope---a(t) has always been climbing---but the slope has varied.

The point I'm trying to make is that we measure redshifts directly. We do not measure the slope of the scalefactor curve a(t) directly. We fit a model that matches and summarizes all that redshift census data (and other direct observation data) and then that model gives us the expansion history curve.

"Speed of expansion" has no other meaning.

 

[Khashishi]

What we are measuring is the amount of expansion that occurred between when the light was emitted and when the light finally reaches us. We assume that the redshift occurs because the light is expanded mid-flight. So yeah, this is measuring the expansion that occurred in the past, but not just at the point of emission.

 

[marcus]

What we are measuring is the amount of expansion that occurred between when the light was emitted and when the light finally reaches us. We assume that the redshift occurs because the light is expanded mid-flight. So yeah, this is measuring the expansion that occurred in the past, but not just at the point of emission.

Exactly right! When you measure a redshift number z, for some object, then
1+z is the ratio of scalefactor now to scalefactor back then when the light was emitted
1+z = a(now)/a(then)

That is one small bit of information about the curve a(t) the expansion history of the universe, and from many many such measurements one reconstructs the whole a(t) curve.

The "speed of expansion" has no other meaning besides the slope of that curve (which we do not measure directly but are able to construct by a kind of curve-fitting, I suppose you could call it, more exactly I'd call it model-fitting, to the data.)

The present slope of the the a(t) expansion history curve is
0.00007 per million years.
that is the fractional increase of any distance between two wide-separated stationary observers that occurs over the course of a million years.

 

[C. Bernard]

It seems i am beginning to see the light (no pun intended). So can we say we measure the expansion that occurred during the time it took the light to get to us?
And that it is an integral of all the various expansions that took place (which could have varied one way or the other)? And the overriding trend is an augmentation of the rate of expansion?

 

[marcus]

It seems i am beginning to see the light (no pun intended). So can we say we measure the expansion that occurred during the time it took the light to get to us?
And that it is an integral of all the various expansions that took place (which could have varied one way or the other)? And the overriding trend is an augmentation of the rate of expansion?

That sounds good to me. You could try it on other folk and see if works for them too.
Translating from a partly mathematical scheme into a purely verbal english language description is often awkward. It's likely to be either inconveniently wordy or else imperfect in some other way. But IMHO you got it right.

 

[eltodesukane]

i think i got C.bernard's argument.
The red shifts we observe are of light from stars far distant and hence are older than present. So should we not say cosmos was expanding?

It was, and it is. And it's accelerating.

 

[bill alsept]

My take on it is that we "measure the speed of expansion" by fitting a mathematical model to the data. We compare what the model says we ought to see with what we actually do see, and adjust the parameters (tweak a few knobs) to get the simplest best fit.

I think what a lot of people are saying and it seems more every day is that what we are actually seeing is not what’s happening but is being interpreted that way. The greatest scientist in the world use to look up at the heavens and knew for sure that Earth was the center of it all because it was so obvious.

 

[Chronos]

Science is about model building. It is imperfect, but, the most productive approach to date. Just because certain observations are inconsistent with a model does not mean the model is flawed, perhaps incomplete, but, not necessarily flawed. It's not like there is no such thing as inaccurate deductions drawn from observation, or simple observational errors. If you put your pants on backwards it is tempting to blame the tailor for the poor fit.

 

[John15]

I have the same problem with redshift. I have seen the graphs and yes speed over distance shows acceleration but change distance for time and you get a deceleration i.e. redshift in light say 10 million years old is much less than that for 10 billion year old light. so the question is should we be using distace or time?

 

[space_buster]

Charts of supernova as a function of red shifts are used to determine the speed of expansion of universe. Adam Riess first looked at the results, he was quite surprised–the expansion of the universe was not decelerating, but accelerating–it was expanding faster and faster! The most likely explanation was that old cosmological constant term of Einstein!

 

[Drakkith]

I have the same problem with redshift. I have seen the graphs and yes speed over distance shows acceleration but change distance for time and you get a deceleration i.e. redshift in light say 10 million years old is much less than that for 10 billion year old light. so the question is should we be using distace or time?

I don't see how your are getting a deceleration. In either time or distance the redshift has been increasing.

 

[John15]

The highest redshift is the CMB at 13.5 billion years, andromeda at 2.5 million years is blueshifted, the further back in time you go the higher the redshift therefore the closer in time the lower the redshift.

 

[Drakkith]

The highest redshift is the CMB at 13.5 billion years, andromeda at 2.5 million years is blueshifted, the further back in time you go the higher the redshift therefore the closer in time the lower the redshift.

Of course, the closer the galaxy is the less time the light has been in transit to us and the less the force of expansion is on it, resulting in a slower recession velocity. Andromeda is so close that expansion has pretty much zero effect on it and the gravitational force between our galaxies are drawing us closer to gether, resulting in the blueshift we see. Using either distance or time is perfectly acceptable as long as you are consistant and clear about it. Something that is 11 billion years back in time, meaning the light has been traveling for 11 billion years to reach us is NOT 11 billion light years away, it is much further thanks to expansion.

Lets be clear by what we mean by acceleration. The expansion rate has been observed to be increasing over time, meaning that the recession velocity at a particular distance is greater now than it used to be. This is what we mean by "the expansion is accelerating". The redshift itself has always been increasing the further away an object is due to expansion.

 

[John15]

Further clarification would be appreciated.
Granted the object from which light has been traveling from for the 11 billion years is now further away but how do we know it still exists. Also how do you know at what point in that 11 billion years the light was redshifted?
The expansion rate has been observed to be increasing over time, by this am I correct in assuming that a certain galaxy has been watched over say the last 100 years and the redshift is now greater than it was then as this is obviously the only way to be certain that redshift is increasing, comparing 5 and 10 billion year old light cannot in any way be accurate.

 

[Drakkith]

Further clarification would be appreciated.
Granted the object from which light has been traveling from for the 11 billion years is now further away but how do we know it still exists.

It doesn't exist NOW as we see it, as it has had 11 billion years to change. It may have merged with another galaxy or something, but the matter that makes it up definitely still exists. Matter, mass, and energy do not simply disappear for no reason.

Also how do you know at what point in that 11 billion years the light was redshifted?

The light has been redshifted over the entirety of its journey by the expansion of space. There is no single event that caused the redshift.

The expansion rate has been observed to be increasing over time, by this am I correct in assuming that a certain galaxy has been watched over say the last 100 years and the redshift is now greater than it was then as this is obviously the only way to be certain that redshift is increasing, comparing 5 and 10 billion year old light cannot in any way be accurate.

No, we look at the recession velocity of galaxies at many different distances from us, since doing so allows us to look at many different points in the past and observe how the universe was back then. I think a single galaxy has such a small increase in recession velocity over 100 years that it is not capable of being measured. Even if we can, we have not been able to measure redshift of galaxies accurately enough for more than a few decades if that. I believe it has only been within the last decade or so that we have been able to measure the velocity of nearby stars to within a few meters per second relative to us.

 

[GeorgeDishman]

Further clarification would be appreciated.

The expansion rate has been observed to be increasing over time, by this am I correct in assuming that a certain galaxy has been watched over say the last 100 years and the redshift is now greater than it was then as this is obviously the only way to be certain that redshift is increasing, comparing 5 and 10 billion year old light cannot in any way be accurate.

Actually, measuring the wavelength of spectral lines is one of the most accurate measurements possible in astronomy. The ratio of the wavelength received to that transmitted is the "redshift".

The following are 'toy' figures intended to illustrate the method and aren't particularly accurate. Suppose we look at three galaxies which are at distances of 100, 200 and 300 million light years and the light from them has been redshifted by 1.1%, 2.1% and 3.0% respectively. We can deduce that the light was "stretched" by 1.1% in the last 100 million years, by 1.0% from 200 to 100 million years ago and by 0.9% in the preceding 100 million years (from 300 to 200). From those we can say that the rate of expansion is increasing by 0.1% per 100 million years.

Does that help explain how a set of specific redshifts can be used to determine not only the current rate but also the history of the rate? In reality of course we have thousands of results each adding a point to the graph as well as a theoretical curve which fits very well.

 

[Naty1]

You guys agonizing about acceleration might take a look here at the 'deceleration
parameter' , q, ...and note the accompanying diagram:

http://en.wikipedia.org/wiki/Hubble_expansion#.E2.80.98Ultimate_fate.27_and_age_of_the_universe

In a link:

... observations of distant type Ia supernovae indicate that q is negative; the expansion of the universe is accelerating.

Type Ia are a 'standard candle' ...a fixed brightness from which we can calibrate things...becuase the fixed brilliance is related to distance...roughly like car headlights from the same model car at varying distances...

 

[SHISHKABOB]

The highest redshift is the CMB at 13.5 billion years, andromeda at 2.5 million years is blueshifted, the further back in time you go the higher the redshift therefore the closer in time the lower the redshift.

just so you know, Andromeda is blueshifted because it is moving towards us due to the gravitational forces between the Milky Way and Andromeda. At small distances, things like gravity are much more influential than the expansion of the Universe.

It's like what if the Moon had a charge of +1C and the Earth had a charge of +1C too. There'd be a repulsive force between the Earth and the Moon due to electrostatic forces, but the force due to Gravity is much stronger and so has much more influence.

 

[John15]

George the first explanation I have seen along those lines, I can see how the accumulation can work, it still assumes though that space is expanding i.e. if light was redshifted as it left the 300 million light year galaxy with the full 3% redshift because it was moving faster then then the redshift from the closer galaxies would be smaller if there had been a deceleration in that time with no expansion of space. Your explanation seems to rely on the expansion of space rather than the movement of the galaxies. Is redshift caused by the expansion of space or the movement of the galaxies?
Also redshift must be relative to the moving bodies i.e. take 3 bodies moving in the same direction along the same axis 1 moving at 100 2 (middle) moving at 70 3 moving at 40. These are all moving apart in the same direction and I think I am correct in saying the redshift from 2 - 3 and 2-1 would be the same.
I am not disputing redshift just questioning if the interpretation is correct and all other possible causes have been checked and ruled out, possibly because I cannot see how space can just materialize out of nowhere unless it worked out like our tectonic plates, maybe the great attractor is an area of dissapearing space.
Regarding andromeda, the BB theory relies at least in part on the fact that galaxies seem to be moving apart so they must have been closer together in the past, of course if andromeda is moving towards us now then by the same reasoning it must have been further away in the past which is a bit contradictory.

 

[bapowell]

andromeda is moving towards us now then by the same reasoning it must have been further away in the past which is a bit contradictory.

The big bang theory is consistent with a uniform and isotropic expansion of the universe on sufficiently large scales where the energy density itself is close to uniform and isotropic. However, not all galaxies are at rest with respect to this expansion (simply being carried along with it), nor should they necessarily be. Interactions with other galaxies, as well as small perturbations during galaxy formation, can lead to non-negligible galactic velocities relative to the expansion (called peculiar velocities.) So, in fact, Andromeda's motion towards our galaxy is perfectly consistent with a full consideration of gravitational dynamics that includes the effects of local structure. As shishkabob says above: the local interaction dominates the tendency for Andromeda to move along with the expansion of the universe. It is not even a bit contradictory.

 

[GeorgeDishman]

George the first explanation I have seen along those lines,..

I'm glad I could put in new terms, sometimes a different way of looking at a question makes the answer more understandable. You should understand I'm not offering anything original, just explaining the standard model in a different style.

Regarding andromeda, the BB theory relies at least in part on the fact that galaxies seem to be moving apart so they must have been closer together in the past, of course if andromeda is moving towards us now then by the same reasoning it must have been further away in the past which is a bit contradictory.

This relates to your other questions so I'll address it first. The effect of cosmological expansion is proportional to the distance between objects so for example within the Solar System the effect is virtually undetectable and if you dropped a brick, you would be very surprised if it fell upwards because the gap between it and the Earth "expanded".

This map shows our Milky Way, Andromeda and other smaller members of our local group. All these are so close together they behave like the planets in the Solar System (though not in a neat plane).

http://www.atlasoftheuniverse.com/localgr.html

Now click the "Zoom out x20" button. We are in the middle. To the right is the Virgo Cluster which has over 200 large galaxies. They too are gravitationally bound, they are too close for expansion to overcome their mutual gravity.

The same is true of the Fornax and Eridanus clusters (lower left) but they are so far from the Virgo Cluster that (to the best of my knowledge) the two sides will always be moving apart, the expansion is the greater effect at that separation.

Within bound groups, the motion within the cluster determines the spectral shift through the old fashioned Doppler Effect, with which I am sure you are familiar, which is why Andromeda has a blue shift.

I can see how the accumulation can work, it still assumes though that space is expanding i.e. if light was redshifted as it left the 300 million light year galaxy with the full 3% redshift ...

Take a sheet of paper and draw two dots close together at the bottom. That the distant galaxy and ours 300 million years ago. Now draw two dots at the top of the sheet farther apart for it and us now. Draw a line join the old and new location for the distant galaxy and another for ours something like this:

\ /

The angle between the lines is the relative velocity. If the angle is 1 degree, it doesn't make any sense to say that it occurs either "at" the left line or the right line. Redshift is a function of the angle between the lines.

... Your explanation seems to rely on the expansion of space rather than the movement of the galaxies. Is redshift caused by the expansion of space or the movement of the galaxies?

Both. If you think of a clusters far from us, the average for the group will be the cosmological shift but each individual galaxy will have a slightly different value due to it's motion within the group, that is called "proper motion". The latter is very useful since there is a well known relation between the average velocity within the group and the total mass of the cluster.

For cosmological redshift, the effect is equal to the change in distance between the galaxies. It is as if whatever stretched the space between us also stretched the wavelength of the light. We are forced to use the two different effects because the models say that galaxies for which the effect doubles the wavelength or more, the source was moving away at more than the speed of light. Trying to use Doppler shift simply doesn't work but the "expanding space" model is an exact match for what is observed.

Also redshift must be relative to the moving bodies i.e. take 3 bodies moving in the same direction along the same axis 1 moving at 100 2 (middle) moving at 70 3 moving at 40. These are all moving apart in the same direction and I think I am correct in saying the redshift from 2 - 3 and 2-1 would be the same.

Approximately, yes. Think in terms of angles between lines and it's fairly obvious (in relativity the angles add rather than speeds). You can think of galaxies "at rest" in expanding space like the points of drawing pins which are glued to the old "balloon model" (with the points outwards) and "proper motion" is like the point of one that is a little bent.

I am not disputing redshift just questioning if the interpretation is correct and all other possible causes have been checked and ruled out, possibly because I cannot see how space can just materialize out of nowhere ...

If you start with a little bit of 'nothing' and you want to have a lot of 'nothing' then you only need to add 'nothing' to it. What's the problem ;-)

There are many observational checks but two key ones are that cosmological redshift is the same for all frequencies (because the angle between the lines is the same) whicle all shifts caused by physical interactions varies with frequency (e.g. the colour of the light). The second key piece of evidence is that Type Ia supernovae near to use always last for a certain duration (they have a well defined light curve) while those at higher redshifts last longer. The reason is that by the time their glow is diminishing, they are farther away.

 

[John15]

Many thanks George.
As far as expanding space goes if you add nothing to something then you are taking something away or rather negating it. Take an apple add nothing to part of it you are turning part of the apple into nothing so you end up with less apple not more, it would of course work if space was like elastic.
The atlas of the universe is most interesting, still not fully convinced that redshift is due to space expanding though as you zoom out it looks like many galaxies are close enough to be gravitationally bound, you mention the virgo and fornax clusters while far apart there is a fair number of galaxies inbetween, of course it would be easier if the strength of the various gravitational fields was included.
Really need a proper discussion to work out the implications.

 

[elias2010]

Universe expansion



The sound waves are material (air) waves, so if we think that the Doppler Effect also acts on light waves, then light must be seen of the side of the particle (photon). Black holes in space prove that the light have a mass. Moreover the twin genesis effect shows us that a γ-photon has at least twice the mass of electron. In the phenomenon of fluorescence, for any absorbed photon a new one of lower energy is emitted. Lower energy (E) means lower frequency (ν): E=hν. But what is frequency meaning for one or a few photons? The question has been answered by Luis de Broglie since 1923: ν=mc2/h. So, it becomes obvious that frequency expresses the photon mass or size and thereby there are various photon sizes. That’s why its mass is not defined yet. Varying size is the reason for many observable effects, as the followings:
-Different refractive index of different colors
-Short radio waves reflection from the ionosphere
-Long radio waves large permeability (submarines radio)
Stars emit particles of all sizes such as UV-rays, X-rays, radio waves, γ-rays and ions. If an ion departs from a star and travels towards earth, except of a possible collision, it will arrive on Earth integer, regardless to star’s motion. That is also true for a photon: we do not expect change its size on the way because the star is moving. A γ-photon emitted from a far star will not arrive on Earth as an X-photon. An X-photon will not arrive as a UV-photon. Blue photons will arrive as they are and make man’s eye to feel the blue color. Therefore, any calculation based on redshift or blueshift is false. In 1980 Jan Claude Packard blames the cosmic dust for these. Now, his speculation becomes reasonable: Blue photons because of bigger size absorbed more by cosmic dust than the red ones. The bigger size, the much more collisions occur leading to higher absorbance. The longer distance among star and earth, there is more dust between them. The more dust between them, the less blue photons arrive on Earth than the reds.
The radiation is continuous. The width of the wave is the number of photons per period (T). The wavelength is the size of photon.
Consequently, the supposed acceleration of the universe and the attendant concept of “dark energy” is a mistake.

 

[Drakkith]

Sorry Elias, it is well known that light can be red or blue shifted by various methods. Most of your post contradicts known science and I advise you to learn more before attempting to tell us that a lot of what we know is incorrect.

 

[GeorgeDishman]

Many thanks George.
As far as expanding space goes if you add nothing to something then you are taking something away or rather negating it. Take an apple add nothing to part of it you are turning part of the apple into nothing so you end up with less apple not more, it would of course work if space was like elastic.

No, you are left with one whole apple, no more, no less. There was a wink at the end of that comment though because it's a somewhat unscientific discussion but basically there is no reason why more vacuum shouldn't just appear since vacuum is crudely the absence of anything.

The atlas of the universe is most interesting, still not fully convinced that redshift is due to space expanding though as you zoom out it looks like many galaxies are close enough to be gravitationally bound, you mention the virgo and fornax clusters while far apart there is a fair number of galaxies inbetween, of course it would be easier if the strength of the various gravitational fields was included.

The impression I tried to give was of the relation to scale. Obviously the stars with the Milky way are bound, as are the planets in the solar system. For very widely separated clusters, the gravity falls roughly as the square of the distance while expansion increase in proprtion so there is some gap beyond which expansion will dominate. The exact value of that isn't too important and it would take a decent simulation to predict accurately for specific galaxies depending on their current velocities, but from previous discussions I believe it is around the scale I indicated.

Really need a proper discussion to work out the implications.

This is a good forum to do that, there are people here who really know their stuff (and many more like me just trying to catch up).

 

[elias2010]

Dr Drakith,with all my respect,will you please answear my question instead of accusing me?
That was:
What frequency means for a photon or a few photons?
I only use existent theories.






"Sorry Elias, it is well known that light can be red or blue shifted by various methods. Most of your post contradicts known science and I advise you to learn more before attempting to tell us that a lot of what we know is incorrect. "

 

[marcus]

What frequency means for a photon...?

It means its energy.

You know the electromagnetic field has both wavelike and particle-like aspects.
We can make a lightsource so dim that it only sends one photon at a time---say approx one photon per second.

We can send that thru a double slit and onto a screen where there are detectors. A detector is clicking only once or less per second.

By comparing which detectors where on the screen click we can discover an interference pattern of stripes as if each individual photon was acting like what we call a wave

And the wavelength we discover (from the stripes) to be proportional to the ENERGY of the individual clicks (a detector can be like a photocell that measures energy delivered to it)
as if the field were delivering energy in discrete bits, and nevertheless was like a wave.

From the wavelength, knowing the speed, we can tell the frequency. The frequency of that discrete bit of energy that was delievered by the field.
The frequency is proportional to the energy that was delivered---proportional by Planck constant.
Multiply the Herz by Planck's constant and you get the Joules. It's basically the same thing.

There are no particles really. There is only the field. The field acts like a wave (in the doubleslit apparatus) but it also delivers energy in discrete clicks. So it is not exactly like a wave we usually think of. Nor exactly like the little beans we naively picture when we speak of particles.
The field does not care how we imagine it. In reality it is neither naive picture, neither beans or ripples.

I think you know all this, so why would you ask? Of course you know what the frequency is. It is the energy delivered when there is a click!

I think this is off-topic in a Cosmology thread, Elias. You could get more discussion in Quantum forum, if you want it.

 

[elias2010]

Of course,and you know that E=mc2 and E=hν.De Broglie combined both equations at 1923:λ=h/mc or ν=mc2/h
Begining of this,I retype my question as you prefer:
If an ion departs from a star and travels towards earth, except of a possible collision, it will arrive on Earth integer, regardless to star’s motion. That is also true for a photon: we do not expect loose its energy on the way because the star is moving.
Thank you for the advices.

 

[marcus]

we do not expect loose its energy on the way...

energy conservation does not hold in expanding geometry
who is "we"?

CMB photons were emitted at 3000 kelvin and are now 2.75 kelvin blackbody radiation.
They have lost more than 999/1000 of their original energy.

We've discussed this before at the forum. We DO expect the photon to lose energy on its way.

Some folks might enjoy this blog post (it has a picture of Emmy Noether):
http://cosmic-horizons.blogspot.com/2011/11/conservation-laws.html

 

[Mark M]

To elaborate on what Marcus just said above, photons lose energy with the metric expansion of space. If you imagined a normal object traveling along expanding space, its speed would change - but as you know, the speed of light is invariant. Hence, as light travels through an expanding universe, its wavelength is increased. In terms of photons, wavelength is inversely proportional to energy by Planck's equation E = \frac {hc} { \lambda}

 

[GeorgeDishman]

Of course,and you know that E=mc2 and E=hν.De Broglie combined both equations at 1923:λ=h/mc or ν=mc2/h

E=mc² applies only to particles which have non-zero mass. De Broglie's forula tells us their wavelength but you can't use it for a photon, photons have zero mass.

If an ion departs from a star and travels towards earth, except of a possible collision, it will arrive on Earth integer, regardless to star’s motion.

The kinetic energy of the particle measured on Earth is less than that measured relative to the source.

That is also true for a photon: we do not expect loose its energy on the way because the star is moving.

Consider a photon emitted from a star in our own galaxy that happens to be moving away from us. It doesn't lose energy en route, but the frequency we measure here is less than that measured relative to the emitting star because of the Doppler effect.

In both cases, the energy is frame dependent. Cosmological redshift has a slightly different cause (see the posts by others for that) but the result is the same, the frequency received is less than that emitted. Inbetween, the frequency depends on the proper motion of the observer.

 

[Matt Benesi]

Bleehhhh... anyways. Durp.

 

[Drakkith]

Dr Drakith,with all my respect,will you please answear my question instead of accusing me?
That was:
What frequency means for a photon or a few photons?
I only use existent theories.

I'm sorry but the only questions I saw in your earlier post you answered yourself. However, as has been answered, frequency is the numbers of oscillations of the electric and magnetic field per second of a photon, which also corresponds to the amount of energy carried by said photon.

Of course,and you know that E=mc2 and E=hν.De Broglie combined both equations at 1923:λ=h/mc or ν=mc2/h
Begining of this,I retype my question as you prefer:
If an ion departs from a star and travels towards earth, except of a possible collision, it will arrive on Earth integer, regardless to star’s motion. That is also true for a photon: we do not expect loose its energy on the way because the star is moving.
Thank you for the advices.

The energy of the ion will be different depending on if you are moving away from or towards the Sun (Or if the Sun is moving away from or towards you). Similarly the energy of a photon will change as well. In addition, the expansion of the universe causes a redshift of light over huge distances, resulting in light losing it's energy over millions to billions of years of travel.

 

[Naty1]

from a post above

we do not expect loose its energy on the way...

and a reply

energy conservation does not hold in expanding geometry
who is "we"?

CMB photons were emitted at 3000 kelvin and are now 2.75 kelvin blackbody radiation.
They have lost more than 999/1000 of their original energy.

These are two different ways of looking at the same phenomena:

https://www.physicsforums.com/showthread.php?t=162727&highlight=current+flow&page=4

[In the great 2007 thread Wallace, Chronos and Oldman take a different view than expressed here [and there] by Marcus...you can read the posts from the 40's thru 50's and see the pros and cons. ]

I do think it is better to think of (photons) as being redshifted by being observed in a different frame ...Now at t ticks along, the scale factor a(t) increases. Therefore two observers who are both at rest wrt to the CMB, but who have different times t will therefore be in different frames (have different metrics). This is what leads to photons being redshifted when observed and emitted at different times.

...two observers who are both at rest wrt to the CMB, but who have different times t will therefore be in different frames (have different metrics). This is what leads to photons being redshifted when observed and emitted at different times...

I tend to agree, photons are not redshifted by traveling through the universe, they are redshifted only because they are observed in a different frame from what they are emitted in.

Marcus: # 48] I am not comfortable with that because among other things I see cosmologists doing inventories of the energy density which are implicitly estimated IN A CMB FRAME...

These 'conflicting' viewpoints stem from this as explained by Chalnoth elsewhere:

" … You get some total redshift for faraway objects due to cosmological expansion. How much of that redshift is due to the Doppler shift# and how much is due to the expansion between us and the far away object is completely arbitrary."

# Doppler shift is based on relative velocity frame based differences.

You takes your pick!

 

[elias2010]

Did n't black holes prove that the photons have a mass?
Anyway,provided that they have not mass,why do you believe that the Doppler effect whitch acts in material waves has to act in non-material waves ( the light waves as you claim) too?

 

[bapowell]

Did n't black holes prove that the photons have a mass?

Where does this come from?

Anyway,provided that they have not mass,why do you believe that the Doppler effect whitch acts in material waves has to act in non-material waves ( the light waves as you claim) too?

Why do you think that the Doppler effect is only relevant to "material" waves?

 

[Whovian]

Just to make bapowell's comment perfectly clear:

Photons, in their rest state, are massless (though they do have a mass when moving at the only speed they're observed at, c, but this has nothing to do with the fact that they're affected by gravity.)

Define "material waves." Do you mean matter waves? In which case, why should the Doppler Effect only affect matter waves? A good understanding of why it arises should make it clear that this isn't the case.

 

[GeorgeDishman]

Just to make bapowell's comment perfectly clear:

Photons, in their rest state, are massless (though they do have a mass when moving at the only speed they're observed at, c, but this has nothing to do with the fact that they're affected by gravity.).

Just to make it clearer, mass is a scalar quantity hence is invariant. The formula for calculating mass for a moving particle of any type at any speed is:

m² = E² - |p|²

where E is the energy and |p| is the magnitude of the momentum.

For a photon, theory says E=|p| so m=0.

Actual measurements can only place an upper limit on this due to the finite accuracy of all instruments:

http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass

 

[elias2010]

In which case, why should the Doppler Effect only affect matter waves?
Because this only is experimentally assured.The other is a speculation whitch universe expansion is based on.

 

[Ich]

Because this only is experimentally assured. The other is a speculation whitch universe expansion is based on.

Huh?
Ever got a speeding ticket?

Is it really too much effort to read e.g. the Wikipedia article on the Doppler effect before you make such allegations?