A question about Photons and Space-time

[SilverIcetail]

I would like to ask you all a question about Photons and the expansion of the universe, and how they relate to one another.

I thought about it after a long talk with a friend about how photons react to the universe, and how they basically don't experience time, since they are in fact traveling at the speed of light, where the time dilation is close to infinite. Then I wondered about what exactly happened after the big bang?

We have the microwave background radiation that we can observe, and red shift and blue shifting of bodies of light traveling towards and away from us, that I understand, but, here's the question that got me waking up at 5 in the morning: where exactly does the energy of the photons that are stretched out... go?

X-rays and Gamma rays are more powerful than radio waves, right? So if the big bang was a bright expansion of gamma rays, where does that energy go when it is stretched out? It doesn't exactly make sense to me. Then the thought came to me, is light traveling through space what causes the universe to expand at the expense of the energy they lose? Does the light lose energy as a result of space-time expanding, or does space time expand due to the force of photons? (Is photons losing energy during their travel through space actually the answer to dark energy?)

I am looking forewards to your responces, and if you may debunk my idea or explain why it doesn't work, for some reason I do not yet know, please do so. Thank you so much for your time!

[Mordred]

here's the question that got me waking up at 5 in the morning: where exactly does the energy of the photons that are stretched out... go....

The energy doesn't go anywhere. How we measure energy is also observer dependant.

 

What this means in the frame of reference of the emitter its frequency/energy remains the same. Yet in the observer frame we must account for redshift.

It is an observer quantity. In much the same way length and time is.

 

Onto the light aspects. First a quick intro into how we model photons influence on expansion. However more importantly is the temperature/pressure and density aspects.

Edited October 26, 2016 by Mordred

[SilverIcetail]

The energy doesn't go anywhere. How we measure energy is also observer dependant.

 

What this means in the frame of reference of the emitter its frequency/energy remains the same. Yet in the observer frame we must account for redshift.

 

It is an observer quantity. In much the same way length and time is

So the fact that we are moving away from the photon faster and faster is the reason that the photon looks to us as if it is more and more red shifted? Did I understand your answer correctly?

 

Now that you layed it out to me in such easy terms, I feel stupid for even having thought of it that way.

With sinserity: Thank you, you let my mind rest easier. Tired minds are not very rational at times.

[Mordred]

So the fact that we are moving away from the photon faster and faster is the reason that the photon looks to us as if it is more and more red shifted? Did I understand your answer correctly?

 

Now that you layed it out to me in such easy terms, I feel stupid for even having thought of it that way.

With sinserity: Thank you, you let my mind rest easier. Tired minds are not very rational at times.

Essentially correct but lets express it properly due to volume change or increased length change. This volume change affects the wavelength of the light path (redshift/blueshift).

[SilverIcetail]

Essentially correct but lets express it properly due to volume change or increased length change. This volume change affects the wavelength of the light path (redshift/blueshift).

I never understood exactly how volume changing can force a photon to change its wavelenght.

 

(Asking a seperate question from the original in order to understand something related)

 

How much space does a photon actually occupy? What is a single photon's diameter? (Does this question even make sense?) I always wondered this, because that's the only way I can see volume changes, such as space time expanding, change the attributes of a photon. In some of the older books I used to frequent about physics it showed the example of a bee, a nanometer and a sky scraper to show the size of the wavelenghts, which always confused me. Is it not more correct to say "The photon has (1 hertz) for every (unit of lenght) it travels"?

 

Sorry if these questions are absolutely annoying to those of you who knows better

 

Thanks for the quick responses though, I appreciate that!

[Mordred]

Well a photon is essentially an excitation of a field with both wave and pointlike properties. Defining the volume of the pointlike characteristics is meaningless.

 

However its wavelength is definable. It has numerous relations. These relations will help.

 

[latex]\frac{\Delta_f}{f} = \frac{\lambda}{\lambda_o} = \frac{v}{c}=\frac{E_o}{E}=\frac{hc}{\lambda_o} \frac{\lambda}{hc}[/latex]

 

You can see the energy to frequency relations above. For Doppler shift your formula is

 

[latex]f=\frac{c+v_r}{c+v_s}f_o[/latex]

 

For gravitational redshift its

 

[latex]\frac{\lambda}{\lambda_o}=\frac{1}{\sqrt{(1 - \frac{2GM}{r c^2})}}[/latex]

 

In cosmological redshift it is

 

[latex]1+Z=\frac{\lambda}{\lambda_o} or 1+Z=\frac{\lambda-\lambda_o}{\lambda_o}[/latex]

 

A good start point to learn this is start with Weins Displacement law for the blackbody temperature aspects.

 

https://en.m.wikipedia.org/wiki/Wien%27s_displacement_law

 

All three cases above redshift is a result of wavelength. The first set of relations above will show the energy influence. Weins Displacement law will help connect this to blackbody temperature.

 

Another useful study point is luminosity to temperature relations.

 

https://en.m.wikipedia.org/wiki/Luminosity

Edited October 26, 2016 by Mordred

[Sensei]

For Doppler shift your formula is

 

[latex]f=\frac{c+v_r}{c+v_s}f_o[/latex]

 

Relativistic Doppler effect has formula

[math]f=f_0\sqrt{\frac{1+v}{1-v}}[/math]

or

[math]f=f_0\sqrt{\frac{1-v}{1+v}}[/math]

 

which is the same as:

[math]f=f_0(1+v)\gamma[/math]

or

[math]f=f_0(1-v)\gamma[/math]

Edited October 26, 2016 by Sensei

[Mordred]

 

Relativistic Doppler effect has formula

[math]f=f_0\sqrt{\frac{1+v}{1-v}}[/math]

or

[math]f=f_0\sqrt{\frac{1-v}{1+v}}[/math]

 

which is the same as:

[math]f=f_0(1+v)\gamma[/math]

or

[math]f=f_0(1-v)\gamma[/math]

Thanks Sensei I forgot to include relativistic doppler

[Sriman Dutta]

A photon is a massless particle and I have never heard about a photon's volume.

[Mordred]

A photon is a massless particle and I have never heard about a photon's volume.

that's because volume is not a measurable property of a particle. Here is a list I might miss one or two lol.

 

Mass

Charge

Spin

color

flavor

isospin

Decays

Products

Lifetime

Scattering

Cross-section

Resonance

Resonance width and lifetime.

 

Energy and momentum cannot be used to define a particle but are measurable.

Edited October 26, 2016 by Mordred