SWaB: Ask Ethan: Does light really live forever?

Discuss Astrophysics.
Post Reply
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

SWaB: Ask Ethan: Does light really live forever?

#1

Post by notFritzArgelander »


Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
User avatar
pakarinen United States of America
Inter-Galactic Ambassador
Articles: 0
Offline
Posts: 4014
Joined: Fri Jun 14, 2019 3:33 pm
4
Location: NE Illinois
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#2

Post by pakarinen »


A simple question with an answer that teaches a lot of physics. I like it!
=============================================================================
I drink tea, I read books, I look at stars when I'm not cursing clouds. It's what I do.
=============================================================================
AT50, AT72EDII, ST80, ST102; Scopetech Zero, AZ-GTi, AZ Pronto; Innorel RT90C, Oberwerk 5000; Orion Giantview 15x70s, Vortex 8x42s, Navy surplus 7x50s, Nikon 10x50s
User avatar
helicon United States of America
Co-Administrator
Co-Administrator
Articles: 585
Offline
Posts: 12280
Joined: Mon May 06, 2019 1:35 pm
4
Location: Washington
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#3

Post by helicon »


I do suppose that in a vacuum it indeed would go on forever. JWST is almost seeing back to the origins of the universe, and the light has traveled for billions of years already what's to stop it from traversing forever? The only "impediment" would be obscuring gas/dust like the coal sack.

But since light shines in every direction, such as a 365 degree sphere at its point of origin it would be difficult for the gas/dust to obscure the light completely.
-Michael
Refractors: ES AR152 f/6.5 Achromat on Twilight II, Celestron 102mm XLT f/9.8 on Celestron Heavy Duty Alt Az mount, KOWA 90mm spotting scope
Binoculars: Celestron SkyMaster 15x70, Bushnell 10x50
Eyepieces: Various, GSO Superview, 9mm Plossl, Celestron 25mm Plossl
Camera: ZWO ASI 120
Naked Eye: Two Eyeballs
Latitude: 48.7229° N
User avatar
AntennaGuy United States of America
Milky Way Ambassador
Articles: 0
Offline
Posts: 1409
Joined: Sun May 19, 2019 1:20 am
4
Location: Tyler, TX USA
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#4

Post by AntennaGuy »


Hmm. A photon doesn't have a rest mass. Suppose we ignore expansion of the universe and consider only the perspective of special relativity. Let's consider an inertial frame passing by an observer at a speed that is infinitesimally smaller than the speed of light. From the observer's perspective, time passes very, very slowly in that fast-moving frame passing by. In the limit, as the speed of that frame approaches c, time heads to a standstill. One (you) can never be in an inertial frame standing next to a photon at rest, since you will always see (well, detect if you prefer) that photon to be traveling at c. From that point of view, it seems to me that a photon has no definable lifetime at all. After all, in a reference frame moving at 99.9999999999...etc% the speed of light, you could cross the entire universe in the blink of an eye (in your time). Alternatively, the whole universe is observed to be compressed in depth toward the limit of zero thickness (via Lorentz contraction) along the photon's path. So, it is tempting to perceive a photon as not being subject to the passage of time at all. Whether its "lifetime" is infinite or finite, or (and in my view) not even rigorously definable, both the age and size of the universe are basically irrelevant (IMHO) to it. By traveling at c, it neither ages at all, nor observes any thickness at all to the universe. Now, somehow, all this gets understood (by smarter people than me) a bit differently when accounting for the perspective of a curved spacetime and an expanding universe. My understanding breaks down before all that, but nFA can fill in the rest (at least, as far as he understands it, which is beyond me, but still not nearly far enough to be "complete"). Future physicists will presumably finish solving this problem, very likely long after we are all dead. :shrug: (and yes, I have ignored what happens in collisions of photons with other things..)
p.s. In particle physics, the observation of a particle changing in any of its observable properties, as it travels in free space, is considered evidence that its rest mass is non-zero -- since any change requires the passage of at least some time in the particle's rest frame. Re: "neutrino oscillations."
* Meade 323 refractor on a manual equatorial mount.
* Celestron C6 SCT on a Twilight 1 Alt-Az mount
Prof. Barnhardt to Klaatu in The Day the Earth Stood Still: "There are several thousand questions I'd like to ask you.”
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#5

Post by notFritzArgelander »


And yet despite having no rest mass the photon has a (frame of observer dependent) frequency. If the photon had a rest mass Maxwell's equations would have to be replaced by Proca's and the dispersion relations for EM waves in a vacuum would differ. This permits interesting astrophysical limits on the photon rest mass.

Robust Limits on Photon Mass from Statistical Samples of Extragalactic Radio Pulsars

https://arxiv.org/pdf/1803.07298.pdf
The photon zero-mass hypothesis has been investigated for a long time using the frequency-dependent time delays of radio emissions from astrophysical sources. How- ever, the search for a rest mass of the photon has been hindered by the similarity between the frequency-dependent dispersions due to the plasma and nonzero photon mass effects. Considering the contributions to the observed dispersion measure from both the plasma and nonzero photon mass effects, and assuming the dispersion induced by the plasma ef- fect is an unknown constant, we obtain a robust limit on the photon mass by directly fitting a combination of the dispersion measures of radio sources. Using the observed dispersion measures from two statistical samples of extragalactic pulsars, here we show that at the 68% confidence level, the constraints on the photon mass can be as low as mγ ≤ 1.51 × 10^−48 kg ≃ 8.47 × 10^−13 eV/c2 for the sample of 22 radio pulsars in the Large Magellanic Cloud and mγ ≤ 1.58 × 10^−48 kg ≃ 8.86 × 10^−13eV/c2 for the other sample of 5 radio pulsars in the Small Magellanic Cloud, which are comparable with that obtained by a single extragalactic pulsar. Furthermore, the statistical approach presented here can also be used when more fast radio bursts with known redshifts are detected in the future.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
User avatar
helicon United States of America
Co-Administrator
Co-Administrator
Articles: 585
Offline
Posts: 12280
Joined: Mon May 06, 2019 1:35 pm
4
Location: Washington
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#6

Post by helicon »


I hadn't really thought about it but I suppose light exhibits both wave - like properties and particle properties (photons). Kind of takes me back to high school physics.
-Michael
Refractors: ES AR152 f/6.5 Achromat on Twilight II, Celestron 102mm XLT f/9.8 on Celestron Heavy Duty Alt Az mount, KOWA 90mm spotting scope
Binoculars: Celestron SkyMaster 15x70, Bushnell 10x50
Eyepieces: Various, GSO Superview, 9mm Plossl, Celestron 25mm Plossl
Camera: ZWO ASI 120
Naked Eye: Two Eyeballs
Latitude: 48.7229° N
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#7

Post by notFritzArgelander »


helicon wrote: Tue Aug 09, 2022 3:08 pm I hadn't really thought about it but I suppose light exhibits both wave - like properties and particle properties (photons). Kind of takes me back to high school physics.
Here's another little factoid. ;)

Another way of testing the photon rest mass is to look for deviations from Coulomb's law. :)

In cgs units Coulomb's force law between two electrical charges 'z1' and 'z2' separated by r is

F = z1z2/r^2

If the photon had a mass, mγ, then there is a parameter with units of inverse length, λ = mγc/ħ where c is speed of light and ħ is Planck's constant divided by 2pi, then Coulomb's law becomes:

F = z1z2exp(-λr)/r^2.

Diligent schoolboys have looked for the exponential length dependence in Coulomb's law and found nothing. But the dispersion measure in a vacuum is much more sensitive.

The theoretical work on massive photons was done by Proca.

https://arxiv.org/abs/physics/0508195
We commemorate 50 years from A. Proca's death. Proca equation is a relativistic wave equation for a massive spin-1 particle. The weak interaction is transmitted by such kind of vector bosons. Also vector fields are used to describe spin-1 mesons (e.g. ρ and ω mesons). After a brief biography, the paper presents an introduction into relativistic field theory, including Klein-Gordon, Dirac, and Maxwell fields, allowing to understand this scientific achievement and some consequences for the theory of strong interactions as well as for Maxwell-Proca and Einstein-Proca theories. The modern approach of the nonzero photon mass and the superluminal radiation field are also mentioned.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
User avatar
AntennaGuy United States of America
Milky Way Ambassador
Articles: 0
Offline
Posts: 1409
Joined: Sun May 19, 2019 1:20 am
4
Location: Tyler, TX USA
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#8

Post by AntennaGuy »


notFritzArgelander wrote: Mon Aug 08, 2022 11:45 pm And yet despite having no rest mass the photon has a (frame of observer dependent) frequency. If the photon had a rest mass Maxwell's equations would have to be replaced by Proca's and the dispersion relations for EM waves in a vacuum would differ. This permits interesting astrophysical limits on the photon rest mass.

Robust Limits on Photon Mass from Statistical Samples of Extragalactic Radio Pulsars

https://arxiv.org/pdf/1803.07298.pdf
The photon zero-mass hypothesis has been investigated for a long time using the frequency-dependent time delays of radio emissions from astrophysical sources. How- ever, the search for a rest mass of the photon has been hindered by the similarity between the frequency-dependent dispersions due to the plasma and nonzero photon mass effects. Considering the contributions to the observed dispersion measure from both the plasma and nonzero photon mass effects, and assuming the dispersion induced by the plasma ef- fect is an unknown constant, we obtain a robust limit on the photon mass by directly fitting a combination of the dispersion measures of radio sources. Using the observed dispersion measures from two statistical samples of extragalactic pulsars, here we show that at the 68% confidence level, the constraints on the photon mass can be as low as mγ ≤ 1.51 × 10^−48 kg ≃ 8.47 × 10^−13 eV/c2 for the sample of 22 radio pulsars in the Large Magellanic Cloud and mγ ≤ 1.58 × 10^−48 kg ≃ 8.86 × 10^−13eV/c2 for the other sample of 5 radio pulsars in the Small Magellanic Cloud, which are comparable with that obtained by a single extragalactic pulsar. Furthermore, the statistical approach presented here can also be used when more fast radio bursts with known redshifts are detected in the future.
If photons had mass, would one still maintain that E = hf, for photons? If so, it would seem to then follow that there must be a lower limit to the frequency of measurable electromagnetic waves, since it would seem difficult to argue that an experimentally measurable classical EM wave could itself consist of less than one photon! But so far, there does not seem to be any lower limit to measurable EM wave frequencies.
* Meade 323 refractor on a manual equatorial mount.
* Celestron C6 SCT on a Twilight 1 Alt-Az mount
Prof. Barnhardt to Klaatu in The Day the Earth Stood Still: "There are several thousand questions I'd like to ask you.”
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

Re: SWaB: Ask Ethan: Does light really live forever?

#9

Post by notFritzArgelander »


AntennaGuy wrote: Mon Aug 15, 2022 1:21 am
notFritzArgelander wrote: Mon Aug 08, 2022 11:45 pm And yet despite having no rest mass the photon has a (frame of observer dependent) frequency. If the photon had a rest mass Maxwell's equations would have to be replaced by Proca's and the dispersion relations for EM waves in a vacuum would differ. This permits interesting astrophysical limits on the photon rest mass.

Robust Limits on Photon Mass from Statistical Samples of Extragalactic Radio Pulsars

https://arxiv.org/pdf/1803.07298.pdf
The photon zero-mass hypothesis has been investigated for a long time using the frequency-dependent time delays of radio emissions from astrophysical sources. How- ever, the search for a rest mass of the photon has been hindered by the similarity between the frequency-dependent dispersions due to the plasma and nonzero photon mass effects. Considering the contributions to the observed dispersion measure from both the plasma and nonzero photon mass effects, and assuming the dispersion induced by the plasma ef- fect is an unknown constant, we obtain a robust limit on the photon mass by directly fitting a combination of the dispersion measures of radio sources. Using the observed dispersion measures from two statistical samples of extragalactic pulsars, here we show that at the 68% confidence level, the constraints on the photon mass can be as low as mγ ≤ 1.51 × 10^−48 kg ≃ 8.47 × 10^−13 eV/c2 for the sample of 22 radio pulsars in the Large Magellanic Cloud and mγ ≤ 1.58 × 10^−48 kg ≃ 8.86 × 10^−13eV/c2 for the other sample of 5 radio pulsars in the Small Magellanic Cloud, which are comparable with that obtained by a single extragalactic pulsar. Furthermore, the statistical approach presented here can also be used when more fast radio bursts with known redshifts are detected in the future.
If photons had mass, would one still maintain that E = hf, for photons? If so, it would seem to then follow that there must be a lower limit to the frequency of measurable electromagnetic waves, since it would seem difficult to argue that an experimentally measurable classical EM wave could itself consist of less than one photon! But so far, there does not seem to be any lower limit to measurable EM wave frequencies.
No. E=hf would no longer hold. You’d have the same dispersion relationship as for a massive particle. You would see departure from E=hf near the photon rest mass.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
Post Reply

Create an account or sign in to join the discussion

You need to be a member in order to post a reply

Create an account

Not a member? register to join our community
Members can start their own topics & subscribe to topics
It’s free and only takes a minute

Register

Sign in

Return to “Astrophysics”