Is the Universe Expanding or Has it Already Expanded?

[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?

 

[Whovian]

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.

a) The Universe's expansion is not based in any way on the Doppler Effect. The Doppler Effect does give us evidence for it, but General Relativity alone is enough to demonstrate the existence of the Big Bang.

b) The Doppler Effect applies to all wave phenomena, and as I have mentioned before, a good understanding of why it arises should make this clear. Simple mathematics are enough to prove its existence for all waves and wave pulses. It's not experimentally assured, it's mathematically assured. And while all our evidence of the physical world is based on what you can derive from theories, we have good evidence that waves exist in the Universe, in the case of both matter and light waves, which, again, the Doppler effect applies to.

 

[elias2010]

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.

Because matter waves don't propagate in vacuum like the light waves.They spread in a physical medium,the air for example.It is the medium whitch periodically pressed and depressed by the source.When the source is moving towards a receiver,the air between is temporarily pressed.
On the other hand,light waves is radiation.Neither they or empty space can be pressed.

 

[elias2010]

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

Where goes the lost energy?
In case of blueshift photons gain energy.Where does this energy comes from?
Can stable motion (moving star, not accelerating) produce energy?

 

[elias2010]

a) The Universe's expansion is not based in any way on the Doppler Effect. The Doppler Effect does give us evidence for it, but General Relativity alone is enough to demonstrate the existence of the Big Bang.

b) The Doppler Effect applies to all wave phenomena, and as I have mentioned before, a good understanding of why it arises should make this clear. Simple mathematics are enough to prove its existence for all waves and wave pulses. It's not experimentally assured, it's mathematically assured. And while all our evidence of the physical world is based on what you can derive from theories, we have good evidence that waves exist in the Universe, in the case of both matter and light waves, which, again, the Doppler effect applies to.

Zenon's paradox teach us that a "mathematical proof" can easily lead us to a mistake.

 

[Whovian]

Zenon's paradox teach us that a "mathematical proof" can easily lead us to a mistake.

Zeno's Paradox is completely unrelated to the topic at hand.

Because matter waves don't propagate in vacuum like the light waves.They spread in a physical medium,the air for example.It is the medium whitch periodically pressed and depressed by the source.When the source is moving towards a receiver,the air between is temporarily pressed.
On the other hand,light waves is radiation.Neither they or empty space can be pressed.

Okay, and when the source is moving towards you, the peaks of the wave get closer together, since the source has moved closer to you by the time it emits the second peak. This is the Doppler Effect.

 

[bapowell]

Where goes the lost energy?
In case of blueshift photons gain energy.Where does this energy comes from?
Can stable motion (moving star, not accelerating) produce energy?

First, energy is not globally conserved in general relativity. There is extensive information on this topic here on the forums and on the web.

With regards to blueshift, indeed, the photon must gain the energy from somewhere.

A moving star *has* energy, called kinetic energy. It doesn't produce it.

 

[bapowell]

Zenon's paradox teach us that a "mathematical proof" can easily lead us to a mistake.

On the contrary. Mathematically, series calculus provides a solution to the paradox. To which mistake are you referring?

 

[bapowell]

Because matter waves don't propagate in vacuum like the light waves.They spread in a physical medium,the air for example.It is the medium whitch periodically pressed and depressed by the source.When the source is moving towards a receiver,the air between is temporarily pressed.
On the other hand,light waves is radiation.Neither they or empty space can be pressed.

Okay, but what does the compression/rarefaction of the supporting medium have to do with the Doppler effect?

 

[elias2010]

First, energy is not globally conserved in general relativity. There is extensive information on this topic here on the forums and on the web.

With regards to blueshift, indeed, the photon must gain the energy from somewhere.

A moving star *has* energy, called kinetic energy. It doesn't produce it.

From where it gains the energy? From the (because of) star's motion?
If this energy is irrelevant to star's motion,then you can't make estimations for it's motion by the change in energy.

 

[elias2010]

Zeno's Paradox is completely unrelated to the topic at hand.



Okay, and when the source is moving towards you, the peaks of the wave get closer together, since the source has moved closer to you by the time it emits the second peak. This is the Doppler Effect.

In the case of one photon where do you see the waves?

 

[elias2010]

Okay, but what does the compression/rarefaction of the supporting medium have to do with the Doppler effect?

The compression/rarefaction needs(consumes)energy.

 

[Chronos]

A photon approaching a gravitational field is blue shifted, as it exits that same field it is redshifted by exactly the same amount, so where is the problem?

 

[Whovian]

In the case of one photon where do you see the waves?

And you appear to have a less than perfect understanding of Quantum Mechanics, too. (No offence.) In some situations, light must be treated as particles, and in some cases, it must be treated as a wave. In this case, a single photon, I believe, should be treated as a wave, so we still see the Doppler Effect.

 

[bapowell]

The compression/rarefaction needs(consumes)energy.

Not in a perfect fluid it doesn't. But again, I'm not sure what you're getting at. Are you arguing that the Doppler shift violates energy conservation? For the record, I badly misspoke before regarding the blue shift and energy gain. Blue shift does not correspond to a gain in energy. Rather, the energy of a photon is frame dependent.

It's good to be skeptical of physical results -- healthy skepticism drives science. But I would recommend that you do a bit more digging into the physics around the Doppler effect, like quantum mechanics (the wave function of a single photon gives it wavelike properties) and special relativity (to understand how energy is observer-dependent) before you take such a contrarian stance on the issue.

 

[elias2010]

A matter wave consists of an amount of e.c. air or water molecules.Do you think that a light wave consists of an amount of photons?
Do you think that a star emitts pulsed photons in waves like a radio oscillator?
In my initial question I consider one photon.How many peaks has it as a wave?
The following peak is the next photon emitted? I notify that energy is quantized.

 

[elias2010]

Zeno's Paradox is completely unrelated to the topic at hand.



Okay, and when the source is moving towards you, the peaks of the wave get closer together, since the source has moved closer to you by the time it emits the second peak. This is the Doppler Effect.

How many peaks has a photon? The second peak is a second photon emitted?

 

[elias2010]

A photon approaching a gravitational field is blue shifted, as it exits that same field it is redshifted by exactly the same amount, so where is the problem?

Do you mean that the energy amount is irrelevant to star's motion?

 

[Whovian]

How many peaks has a photon? The second peak is a second photon emitted?

Please take some time to understand Quantum Mechanics. On a single light wave, each photon is more likely to be at peaks than at troughs, but their location is pretty undetermined over the light wave.

 

[bapowell]

A matter wave consists of an amount of e.c. air or water molecules.Do you think that a light wave consists of an amount of photons?

This is really not the right way to think of a matter wave. The wave itself -- the vibrational disturbance -- isn't made of anything! In the case of compressional waves, these necessarily need a medium -- air or water or something else -- for transmission.

Electromagnetic waves are altogether different. Classically, we know form Maxwell's Equations that oscillating electric and magnetic fields work in concert to propagate the wave forward, even in vacuum.

Quantum mechanically, yes, we understand that light is actually a particle -- the photon. The photon's wavefunction gives it wavelike properties, recovering the classical wave mechanics of electromagnetism. But in certain situations, like the photoelectric effect and high energy collisions, the particle nature of the photon becomes evident.

Do you think that a star emitts pulsed photons in waves like a radio oscillator?

I don't know what a "pulsed photon" is. Do you mean, "does a star emit electromagnetic radiation?" Yes, it does.

In my initial question I consider one photon.How many peaks has it as a wave?

The wavefunction of a photon is a plane wave.

I notify that energy is quantized.

Yes, energy is quantized. Which is why light consists of photons.

 

[GeorgeDishman]

How many peaks has a photon? The second peak is a second photon emitted?

As has been pointed out before, we know have known the Doppler Effect applies to EM waves for decades, and the Doppler Radar speed detector is a good example.

Astronomers often use diffraction or reflection gratings to look at the spectrum of stars astronomers and can use it to measure how fast a star is moving towards or away from us. This page shows a diagram of the wavefronts which explains why the reflection angle depends on the wavelength:

http://wwwold.rmki.kfki.hu/plasma/castor/www_SUMTRAC2003/sumscho/grating.htm

The ruled lines on the mirror mean that the light will be reflected in a direction such that the wavefronts from different rulings arrive together.

This page includes an example of the spectrum of a bright source:
http://www.astro.sunysb.edu/fwalter/AST443/spectroscopy.html#gr

What may surprise you is that the gratings work equally well when the source is so dim, the photons arrive individually, perhaps seconds or more apart. Each photon arriving can be counted and the angle through which it was reflected defines its wavelength. You may struggle to understand this, most people do, but that's the way the world works. No matter how logical your arguments may seem to you, they won't change reality.