What is the Maximum Possible Length of a Radio Wave?

[The Guru Kid]

I was researching about the highest possible EM wavelength and came across "the length of the universe" concept.

Is it just the length of the visible universe? or the universe possible?

thanx

 

[vincentm]

I think it maybe stating the known universe a.k.a the observable universe. :/

 

[Orion1]

Photon Phantom...


Longest photon wavelength?

Hubble photon wavelength:
$$r_u = \lambda_{\gamma}$$
$$\lambda_{\gamma} = \frac{c}{H_o}$$

$$H_o$$ - Hubble Constant

What is the 'wavelength' of this photon?

 

[SpaceTiger]

Check out this thread.

 

[turbo]

The highest possible wavelength is a standing wave (WRT to your frame of reference). It would be indistinguishable from the your local backgound, and therefor not measurable by you, regardless of how much energy the wave might contain.

Interestingly, quantum physics says that the energy of the vacuum is 120 OOM larger than we can measure. Sometimes, a UV cutoff is proposed to cut this discrepancy back by about 60 OOM (yeah!). I find it curious that these same folks do not propose an IR (actually much lower frequency!) cutoff where huge energies might be expressed at wavelengths too long for us to detect.

 

[Chronos]

Turbo, as you know, there is a theoretical IR cutoff, but not very firm. We run up against these kind of walls all the time - largely, IMO, because we have no viable quantum theory of gravity. Personally, I think both theories are incomplete. I hasten to add, however, I believe both theories are correct within their own domain [my inner crackpot is showing].

 

[dubmugga]

so the lowest EM wavelength would be an open 1d string vibrating as a photon at Planck level ?

 

[Chronos]

No. Empirical tests deny the 1d result at around 4 sigma levels.

 

[dubmugga]

I thought it would be lower than that !

 

[Orion1]

A photon is a 4 dimensional entity, (3 space + 1 time) therefore no photon can exist below D = 4.

the radius of the observable universe is about 3 times the Hubble length. (the current distance to galaxies and stuff with we are now getting light from goes out to 40 some billion LY or roughly 3 times)

'observable' Hubble Photon wavelength:
$$r_u = \lambda_{\gamma}$$
$$\lambda_{\gamma} = \frac{3 c}{H_o}$$

What is the 'wavelength' of this photon?

Reference:
http://universe-review.ca/F02-cosmicbg.htm
http://universe-review.ca/R02-16-universe.htm

 

[Chronos]

Huh? That makes no sense at all... unless you are trying to prove 2 = 3.

 

[Orion1]

Chronos, those are two different models, the first is based upon the Hubble Length, and the second is based upon the observable range.

It appears that you did not read the posted reference by SpaceTiger, post #4 reference #1, or checked post #10 reference #1.

Reference:
https://www.physicsforums.com/showpost.php?p=776595&postcount=4
https://www.physicsforums.com/showthread.php?t=68758

 

[SpaceTiger]

I'm a bit confused about the relevance here. Why are you talking about this in terms of the longest photon wavelength?

 

[Chronos]

Agreed, ST. I fail to see the relevance, which is the basis of my objection.

 

[Nereid]

One could approach the OP's question as some kind of 'what if?' thought exploration; most posts in the thread are of this kind.

Or one could approach this in terms of physical possibilities, within what we know of physics, the universe, etc.

Or one could simply go outside and count the horse's teeth

On the second approach. Leave aside how the longest wavelength EM (radio?) might be created, could they 'exist', in any meaningful sense? For starters, the propogation of long wavelength (low frequency) radio is limited by the plasma frequency, which is proportional to the square root of the electron density. So, the universe seems to set a low frequency bound on radio waves which can propogate, to the lowest density plasma. And where are these plasmas? What are their densities?

Is there an 'out'? Are the large regions of space which are NOT plasmas (or, more precisely, do not contain free electrons)?