Expansion of the universe

bapowell

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.

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

The wavefunction of a photon is a plane wave.

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

GeorgeDishman

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/ca...ho/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/...oscopy.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.

el66

Doppler effect appears in waves produced by oscillators witch have “peaks” and “hollows”. These peaks can be condensed or diluted by the Doppler effect. In the light case, a peak represents an amount of photons. But photons is not only produced by oscillators (i.e. nuclear reactions). Radio oscillators pulsating emit photons “in waves”. Incandescent lamp and most stars (except pulsars and quasars) emit photons continuously and irregularly, not “in waves”.
But what is frequency meaning for one or a few photons?
Provided that the photon have not mass, frequency only means energy. So, by redshift effect photon looses energy. By blueshift effect it gains energy. Where this energy comes from? The stable motion of a star produces energy? If this energy is irrelevant to star’s motion, then we can’t make estimations for this motion from the change in energy.

bapowell

Please provide evidence for this statement, as it contradicts known science.
The photon does not gain/lose energy. In special relativity, we learn that energy depends on the reference frame. This really has nothing to do with light per se, but anything with energy. Take a baseball for example. A baseball has rest energy, [itex]E=mc^2[/itex], so an observer at rest wrt to the baseball measures its energy thusly. But there are also observers that can be in motion relative to the baseball -- they measure a total energy equal to the rest energy plus the kinetic energy of the baseball's motion. The point is that: observers at rest and in motion wrt to an object will disagree about its energy. This is not a violation of the conservation energy -- the energy of the baseball's in its rest frame is always the same; instead, it's a direct result of the postulates of relativity. The same is true for photons.

GeorgeDishman

Close, but to avoid confusion: a "wave" is by definition a series of alternate positive or negative peaks crossing zero in between. The "zero" might be a mean value such as sound waves whose peaks are relative to the mean air pressure but for EM waves, usually it is positive or negative field values relative to zero.

In the Doppler effect, one peak passes a detector (e.g. your ear) but before the next arrives, there is a change of distance between the detector and source so the next peak travels for a longer or shorter time and hence arrives either later or earlier. That difference adds to the original period of the waves to change its received frequency. They are not "condensed or diluted", rather their spacing is altered.

No. The simplest way to think of a photons is as a short burst of waves encapsulated in a form that can only interact as if it was a particle, i.e. all or nothing. It's equivalent to thinking of it as a particle which has an intrinsic phase which changes at a rate given by its angular frequency.

Light from an incandescent lamp is lots of photons all at random frequencies and phases - the peaks of one photon have no fixed relationship to those of another. Stars and pulsars are the same, the latter like lamps that are switched on for short periods regulary.

Light from a laser is lots of photons all at virtually the same frequency and with a fixed phase relationship, for example you could think of the peaks at the front of one photon being aligned with those at the end of the previous to produce a continuous wave. That's an analogy that has lots of problems, but it's better than your current misunderstanding.

Photons from radio transmitters are correlated like those from lasers but at lower frequencies. Radioactive gamma rays are random like photons from incandescent lamps but at higher frequencies.

Same as for the continuous signal, it is also a measure of energy when considered as a particle.

If someone throws a stone at you, it hurts more if you run towards it, less if you run away. The impact energy changes. Same if the thrower is running and you stand still. The same happens with photons, they have no mass (because their momentum is the same as the energy and mass is a function of the difference) but they still have energy and energy is frame-dependent. The energy measured by the thrower is not the same as that measured by the target, but the "change" in energy comes from the kinetic energy of the moving body, the target or the thrower depending on your viewpoint. It is a difference rather than a change though.

mufa

[most stars emit photons continuously and irregularly, not “in waves”.
"Please provide evidence for this statement, as it contradicts known science.]
Does n't star's light produced by random nuclear explosions? Have we evidence for something else?

bapowell

Stars do indeed emit photons continuously from nuclear reactions. My issue was with el66's claim that photons were not emitted "in waves", which I interpreted as a rejection of the wave nature of photons (a point he was making elsewhere in the thread.) It appears I've misread his statement.

GeorgeDishman

When we look at the spectrum of the Sun, it is close to a black body with a temperature around 5800K. Is it not more accurate to say the Sun emits thermal radiation due to the heat produced by the nuclear reactions? The photons produced directly by those reactions would have a spectrum characteristic of a much higher temperature.

Lino

All, I appreciate the this thread has moved on from the OP and earlier posts, but I hope that you don’t mind me reflecting back to those with a question. (If it is more appropriate to move this posting to another / new thread please feel free.)

Consider an ideal scenario whereby there are measurements from standard candles / type IIa supernova at regular cosmological distances (in a particular direction). Original wisdom tells me that the rate of universal expansion (between me and the various supernova) is proportional to the distance, and as I look at more distant ones I am looking back in time - representing the amount of time that the light has been travelling to reach me. More recent wisdom tells me that the rate of expansion has / is increasing (ie in the last 7bl years). Combining these wisdom’s, I think I’m right in saying that, the rate of expansion at any location is greater as the timeline approaches "now", but that the rate of expansion between any two locations increases as the distance between them increases.

Therefore, my questions is: how do I rationalise the measurements from one object (a more distant object) to another (more closer object) – given that it is not possible to get measurements from both that represent the same point in time? It seems a pretty basic question, so I assume that I am missing something, but it is one that I had not thought about previously!

As a second question, and assuming that the above can be resolved, does this imply that there are two points (in the same direction) in the universe where any particular value of redshift applies – one “closer location” where the rate of expansion has “accelerated” sufficiently to produce the result, and one “more distant location” where the accumulation of time / distance produces the result?

Thanks in anticipation.

Regards,

Noel.

bapowell

You are probably right, but my intent was not discuss the specific mechanism of photon production in stars or the resulting spectral properties. As I said earlier, I was merely taking objection to the claim that photons were not wavelike.

mufa

I'm sorry, but I have many questions:
Can an electron beam be redshifted? A single electron can be?
A photon beam can be redshifted.A single photon can be? If so,it's energy not be reduced? Once a photon is emitted,then its energy can be reduced?

Drakkith

Yes and no. Redshift refers to the change in frequency of light. An electron doesn't have a spectrum like light does. HOWEVER, if I shoot an electron at you at 0.1c and you are moving away from me at 0.05c, then you will measure the velocity of the electron coming towards you at 0.05c as well. So the momentum and kinetic energy of the electron is less when measured by you, which is similar to the loss of momentum when light is redshifted. It's just not proper to use the term redshift when referring to matter.


Photons are absolutely redshifted. The key is to understand that the energy of the photon isn't being "lost", it is just similar to the electron example up above in that you are moving away from it and will measure it at a different energy level than someone moving towards it or staying stationary.

Naty1

Post#1:

That's a matter of semantics: Do you say to someone when playing hide and seek "I saw you hiding behind that tree." or do you say : "I see you hiding behind that tree"

All observations are of past events due to the finite speed of electromagnetic radiation. That goes for the cosmological relic radiation as well as an electron microscope.

A reason we say the universe IS expanding is that day after day,year after year, we keep getting more CMBR for a more distant emission sphere and it keeps showing a pattern of expansion....and we have no scientific reason to assume tomorrow will show "oops, expansion has suddenly stopped".

Sure, all particles have a particle and a wave nature: The wave nature of a particle is the Debroglie wavelength. for example, cosmological expansion causes things to lose momentum relative to the CMB....this means light redshifts but retains local speed 'c' at
all times while a matter particle loses momentum via a loss in velocity.

Carried along by the Hubble flow.
http://www.physicsforums.com/showthr...=614297&page=2


starting about Post #33...

GeorgeDishman

That is correct.

We use the measurements we can make from here to calculate the history of the scale factor. For any two randomly separated points, you can then use that history together with their comoving separation to calculate the Hubble distance between them at any given cosmological time.

No, the redshift is the ratio of the scale factor now to then. It has always been increasing though at a varying rate (about 7 billion years ago was a minimum but still positive) so the redshift always increases with distance.

Lino

Wow! Thanks George. I think that I understand what you are saying and it gives me a lot to target my reading at. But there is one concept that is very alien to me, so in prep, can I confirm, are you saying that when such measurements are taken here and now, it is the history of the input variables that cause the results - not just the "final" value of the variables. Is that correct? (I'm trying to understand / compare it to other measurement processes: for example, is this the equivilant to conducting a litmus test and based on the single result being able to understand the history of the acidity of the solution?)

Regards,

Noel.

mufa

[QUOTE=Drakkith;3963135]Yes and no. Redshift refers to the change in frequency of light. An electron doesn't have a spectrum like light does. HOWEVER, if I shoot an electron at you at 0.1c and you are moving away from me at 0.05c, then you will measure the velocity of the electron coming towards you at 0.05c as well. So the momentum and kinetic energy of the electron is less when measured by you, which is similar to the loss of momentum when light is redshifted. It's just not proper to use the term redshift when referring to matter.

Sometimes electron has wavelike behaviour,and photon behaves like a particle. I am still can't make out how photons loose momentum provided that the speed is stable, whereas in your example the relative speed of the electron decreases.Thanks.

Photons are absolutely redshifted. The key is to understand that the energy of the photon isn't being "lost", it is just similar to the electron example up above in that you are moving away from it and will measure it at a different energy level than someone moving towards it or staying stationary.
But photons speed is stable.Does n't frequency reduction meaning an energy reduction too?

MDEarl

This pertains, especially since alchemist mentioned an exponential expansion. Of course the universe continues to expand, but no model to date has been able to describe it without complicated addendums such as dark matter and energy and the cosmo. constant.
I have a mathematical question and hope to get some input from at least one knowledgeable person, and thought I might post it on PF.
In 1922 Alexander Friedmann came out with equations for an expanding universe which still form a basis for GR and cosmology today. To do this he employed Newtonian gravitation and conservation of energy principles, probably assuming they were universally applicable.
Hubble’s law has the form of a simple growth equation, HoR = dR/dt which mathematically requires an exponential value for R. When integrated, it produces an exponential radius of expansion of R=Ro e^Hot. This mandates an exponential volume expansion of the universe, leading to a positive radial acceleration of Hoc and a value for Ho differing from Friedmann’s by √2. The math appears straightforward to me, and I would like to know what I am doing wrong, or why the logic is faulty, especially since the results appear to fit the current picture of the universe perfectly. In my opinion Friedmann would have gone this way if he had the empirical evidence available today, and cosmology would be on the right track...a game changer.