Detection of the cosmic gamma ray horizon: Measures all the light in the universe since the Big Bang

May 24, 2013
This figure illustrates how energetic gamma rays (dashed lines) from a distant blazar strike photons of extragalactic background light (wavy lines) and produce pairs of electrons and positrons. The energetic gamma rays that are not attenuated by this process strike the upper atmosphere, producing a cascade of charged particles which make a cone of Čerenkov light that is detected by the array of imaging atmospheric Čerenkov telescopes on the ground. Credit: Nina McCurdy and Joel R. Primack/UC-HiPACC

(Phys.org) —How much light has been emitted by all galaxies since the cosmos began? After all, every photon (particle of light) from ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever existed throughout cosmic history is still speeding through the Universe today. If we could carefully measure the number and energy (wavelength) of all those photons—not only at the present time, but also back in time—we might learn important secrets about the nature and evolution of the Universe, including how similar or different ancient galaxies were compared to the galaxies we see today.

That bath of ancient and young photons suffusing the Universe today is called the (EBL). An of the EBL is as fundamental to cosmology as measuring the left over from the Big Bang (the ) at . A new paper, called "Detection of the Cosmic γ-Ray Horizon from Multiwavelength Observations of Blazars," by Alberto Dominguez and six coauthors, just published today by the Astrophysical Journal—based on observations spanning wavelengths from to very energetic , obtained from several and several ground-based telescopes—describes the best measurement yet of the evolution of the EBL over the past 5 billion years.

Directly measuring the EBL by collecting its photons with a telescope, however, poses towering technical challenges—harder than trying to see the dim band of the Milky Way spanning the heavens at night from midtown Manhattan. Earth is inside a very bright galaxy with billions of stars and glowing gas. Indeed, Earth is inside a very bright solar system: sunlight scattered by all the dust in the plane of Earth's orbit creates the zodiacal light radiating across the optical spectrum down to long-wavelength infrared. Therefore ground-based and space-based telescopes have not succeeded in reliably measuring the EBL directly.

So, astrophysicists developed an ingenious work-around method: measuring the EBL indirectly through measuring the attenuation of—that is, the absorption of—very high energy gamma rays from distant blazars. Blazars are supermassive black holes in the centers of galaxies with brilliant jets directly pointed at us like a flashlight beam. Not all the high-energy gamma rays emitted by a blazar, however, make it all the way across billions of light-years to Earth; some strike a hapless EBL photon along the way. When a high-energy gamma ray photon from a blazar hits a much lower energy EBL photon, both are annihilated and produce two different particles: an electron and its antiparticle, a positron, which fly off into space and are never heard from again. Different energies of the highest-energy gamma rays are waylaid by different energies of EBL photons. Thus, measuring how much gamma rays of different energies are attenuated or weakened from blazars at different distances from Earth indirectly gives a measurement of how many EBL photons of different wavelengths exist along the line of sight from blazar to Earth over those different distances.

Observations of blazars by NASA's Fermi Gamma Ray Telescope spacecraft for the first time detected that gamma rays from distant blazars are indeed attenuated more than gamma rays from nearby blazars, a result announced on November 30, 2012, in a paper published in Science, as theoretically predicted.

Now, the big news—announced in today's Astrophysical Journal paper—is that the evolution of the EBL over the past 5 billion years has been measured for the first time. That's because looking farther out into the Universe corresponds to looking back in time. Thus, the gamma ray attenuation spectrum from farther distant blazars reveals how the EBL looked at earlier eras.

This was a multistep process. First, the coauthors compared the Fermi findings to intensity of X-rays from the same blazars measured by X-ray satellites Chandra, Swift, Rossi X-ray Timing Explorer, and XMM/Newton and lower-energy radiation measured by other spacecraft and ground-based observatories. From these measurements, Dominguez et al. were able to calculate the blazars' original emitted, unattenuated gamma-ray brightnesses at different energies.

The coauthors then compared those calculations of unattenuated gamma-ray flux at different energies with direct measurements from special ground-based telescopes of the actual gamma-ray flux received at Earth from those same blazars. When a high-energy gamma ray from a blazar strikes air molecules in the upper regions of Earth's atmosphere, it produces a cascade of charged subatomic particles. This cascade of particles travels faster than the speed of light in air (which is slower than the speed of light in a vacuum). This causes a visual analogue to a "sonic boom": bursts of a special light called Čerenkov radiation. This Čerenkov radiation was detected by imaging atmospheric Čerenkov telescopes (IACTs), such as HESS (High Energy Stereoscopic System) in Namibia, MAGIC (Major Atmospheric Gamma Imaging Čerenkov) in the Canary Islands, and VERITAS (Very Energetic Radiation Imaging Telescope Array Systems) in Arizona.

Comparing the calculations of the unattenuated gamma rays to actual measurements of the attenuation of gamma rays and X-rays from blazars at different distances allowed Dominquez et al. to quantify the evolution of the EBL—that is, to measure how the EBL changed over time as the Universe aged—out to about 5 billion years ago (corresponding to a redshift of about z = 0.5). "Five billion years ago is the maximum distance we are able to probe with our current technology," Domínguez said. "Sure, there are blazars farther away, but we are not able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us—so weakened that our instruments are not sensitive enough to detect them." This measurement is the first statistically significant detection of the so-called "Cosmic Gamma Ray Horizon" as a function of gamma-ray energy. The Cosmic Gamma Ray Horizon is defined as the distance at which roughly one-third (or, more precisely, 1/e – that is, 1/2.718 – where e is the base of the natural logarithms) of the gamma rays of a particular energy have been attenuated.

This latest result confirms that the kinds of galaxies observed today are responsible for most of the EBL over all time. Moreover, it sets limits on possible contributions from many galaxies too faint to have been included in the galaxy surveys, or on possible contributions from hypothetical additional sources (such as the decay of hypothetical unknown elementary particles).

Explore further: Astronomers find evidence of water clouds in brown dwarf atmosphere

More information: Link to the paper in The Astrophysical Journal as accepted is arxiv.org/abs/1305.2162

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VENDItardE
1.5 / 5 (12) May 24, 2013
every photon (particle of light) from ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever existed throughout cosmic history is still speeding through the Universe today.

really? not a single one of them has ever been absorbed? really?
sstritt
1.7 / 5 (12) May 24, 2013
Photons colliding with other photons? Really?
DavidW
1 / 5 (9) May 24, 2013
It sounds brilliant. I can't wait to see this graphed and illustrated to see what it reveals. The understanding of the limits, that they indicate may be evolving from their work, seems to indicate we should get some very interesting pictures. Many theories many face new questions or end up disproved.
Q-Star
2.6 / 5 (15) May 24, 2013
every photon (particle of light) from ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever existed throughout cosmic history is still speeding through the Universe today.

really? not a single one of them has ever been absorbed? really?


Yeppers, it was poorly worded. I think they should have said that energy is conserved, what is absorbed one place, has to be emitted somewhere to keep the conservation balance.
wavettore
1 / 5 (14) May 24, 2013
Gamma rays, over time, diminish their energy level (and their frequency) to become X rays, from X rays they will become ultraviolet and so on. The original quantum is not lost but distributed into other forms of energy through "spontaneous symmetry breaking".
Once reached an almost flat longitude (and lower critical energy level) these waves solidify into hydrogen atoms breaking up their energy in opposite elements, like the split ends of a broken hair. When the hydrogen atoms are reached by the heat of other
incoming waves they fuse together to create more complex forms of energy.

http://www.wavevo...ing.html

Pressure2
1 / 5 (8) May 24, 2013
I wonder how a photon can be red-shifted and change its frequency and still conserve its energy and momentum?
Does it become 2 photons at 1/2 the energy and momentum if it is red-shifted by a factor of 1Z?
GSwift7
1.9 / 5 (13) May 24, 2013
I wonder how a photon can be red-shifted and change its frequency and still conserve its energy and momentum?


I would guess that it doesn't. Once a photon gets stretched to the point that its energy is below the Plank limit, it basically doesn't even exist any more. The stretching of the Universe actually can be interpreted as a decrease in the energy/mass density of the Universe. The total mass/energy remains constant while the volume increases. The Universe as a whole will conserve energy/momentum, but local areas will become less dense. It's as though the photon is trading its momentum in exchange for space itself.

That's an interesting thought, as it implies a basic equivalence between space and energy just like the equivalence between mass and energy.
Fleetfoot
5 / 5 (2) May 24, 2013
Photons colliding with other photons? Really?


There is a summary and some references here:

http://en.wikiped..._physics
Fleetfoot
4 / 5 (4) May 24, 2013
I wonder how a photon can be red-shifted and change its frequency and still conserve its energy and momentum?


Energy is frame dependent, if you are hit by a stone, it hurts less if you are running away from it. You get the same effect with photons in the Doppler effect.
Pressure2
1 / 5 (9) May 24, 2013
If you are hit by a stone while running away it would hurt less but the stone would still contain some energy and momentum.

I don't see how this applies to a photon, unless we see a part of a photon and the other part still exist at a lower frequency.

It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe.
Shootist
1.5 / 5 (11) May 24, 2013
every photon (particle of light) from ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever existed throughout cosmic history is still speeding through the Universe today.

really? not a single one of them has ever been absorbed? really?


Quite a Feynman diagram.

Q-Star
2.9 / 5 (17) May 24, 2013
It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe.


The system's energy is conserved. The photon which is part of the system can take on or pass off part it's energy.

In the case of cosmological redshift, the photon imparts some of it's energy to the expansion that the system is undergoing. Anytime a photon changes it's frequency it exchanges energy with something not of it's self, ie, another particle, a field, the environment it passes through.
ValeriaT
1 / 5 (13) May 24, 2013
The gamma ray photons materialize with photons of CMBR under formation of leptons. This brings the GZK limit for observation of gamma rays from distant sources (the vacuum essentially protects the terrestrial life against gamma rays from distant sources in similar way, like atmosphere blocks the harmful UV radiation). At the case of distant intensive gamma ray burst the GZK limit is violated, because the energetic photons tend to interact gravitationally each other in AWT.
Pressure2
1 / 5 (13) May 24, 2013
It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe.


The system's energy is conserved. The photon which is part of the system can take on or pass off part it's energy.

In the case of cosmological redshift, the photon imparts some of it's energy to the expansion that the system is undergoing. Anytime a photon changes it's frequency it exchanges energy with something not of it's self, ie, another particle, a field, the environment it passes through.

Oh brother, another layer of magic introduced to save the BB theory from the web it has weaved!
Q-Star
2.8 / 5 (16) May 24, 2013
It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe.


The system's energy is conserved. The photon which is part of the system can take on or pass off part it's energy.

In the case of cosmological redshift, the photon imparts some of it's energy to the expansion that the system is undergoing. Anytime a photon changes it's frequency it exchanges energy with something not of it's self, ie, another particle, a field, the environment it passes through.

Oh brother, another layer of magic introduced to save the BB theory from the web it has weaved!


No, that little bit of magic predates the BB theory by many years. Ya can thank Mr. Maxwell for the magic of how photons display their energy, and what they do with it. (Planck and Boltzmann filled in gaps.) The BB theory was built, partially on their work.
Q-Star
2.9 / 5 (17) May 24, 2013
Oh brother, another layer of magic introduced to save the BB theory from the web it has weaved!


See ya need to get this thing ya hate in proper perspective. Redshift, electromagnetics, spectroscopy, quanta of energy, wave functions, SR and GR, expanding space, all this stuff came before the BB theory.

Not, as ya incorrectly think, after the BB theory to prop it up. It would make it much easier for ya to argue against if ya didn't get everything in the wrong order.
Pressure2
1 / 5 (12) May 24, 2013
Oh brother, another layer of magic introduced to save the BB theory from the web it has weaved!


See ya need to get this thing ya hate in proper perspective. Redshift, electromagnetics, spectroscopy, quanta of energy, wave functions, SR and GR, expanding space, all this stuff came before the BB theory.

Not, as ya incorrectly think, after the BB theory to prop it up. It would make it much easier for ya to argue against if ya didn't get everything in the wrong order.

Sure magic predates the BB theory. Ya ever hear of flat earth society? What about alchemistry, ah ya on the right track!
Tektrix
1 / 5 (2) May 24, 2013
"It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe."

I was flummoxed by this too, and then someone explained it to me like this (it sounds good to me but I'm not an astrophysicist, so caveat emptor):
"Expansion is stretching spacetime and with it, the electromagnetic field along the path of observation. Any photons traversing this stretched region are concomitantly stretched, that is, their wavelengths are lengthened. As a photon is constantly exchanging energy with the electromagnetic field, energy is automatically conserved. The amount of energy exchanged with the field is exactly as much as is required for the wavelength of the photon. The result is that it all balances out- it has to or the photon wouldn't be there in the first place."



Pressure2
1 / 5 (10) May 24, 2013
"It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe."

I was flummoxed by this too, and then someone explained it to me like this (it sounds good to me but I'm not an astrophysicist, so caveat emptor): Expansion is stretching spacetime and with it, the electromagnetic field along the path of observation. Any photons traversing this stretched region are concomitantly stretched, that is, their wavelengths are lengthened. As a photon is constantly exchanging energy with the electromagnetic field, energy is automatically conserved. The amount of energy exchanged with the field is exactly as much as is required for the wavelength of the photon. The result is that it all balances out- it has to or the photon wouldn't be there in the first place. What do you guys think?

To me it is just more magic, but I must add I could always be wrong.
Fleetfoot
4 / 5 (4) May 25, 2013
If you are hit by a stone while running away it would hurt less but the stone would still contain some energy and momentum.

I don't see how this applies to a photon, ...


For a photon, both energy and momentum are proportional to frequency. Moving away from a source reduces the frequency via Doppler so the energy and momentum are reduced but we still detect the whole photon.

It also doesn't explain how a photon can change frequency and conserve energy and momentum caused by the accelerating expansion of the universe.


Depending on your choice of frame, energy isn't conserved by cosmological redshift but it is a complex topic:

http://math.ucr.e..._gr.html
Fleetfoot
3.7 / 5 (3) May 25, 2013
The gamma ray photons materialize with photons of CMBR under formation of leptons. This brings the GZK limit for observation of gamma rays ...


GZK applies to protons ("cosmic rays") specifically, not gamma, but you have the right idea, the effect is similar.
sstritt
1.4 / 5 (10) May 25, 2013
Photons colliding with other photons? Really?


There is a summary and some references here:

http://en.wikiped..._physics

Thanks. I was unaware of this
vidyunmaya
1 / 5 (10) May 26, 2013
Sub: Search Origins-Cosmology Vedas-Knowledge base-creative spirit
Change in concepts to understand the cosmic fnction of the Universe-helps proper East-west dialogue.
Frequency shift has several layers in log-scale dimensional universe. Comprehension helps cosmology studies. Latest addition: knowledge base cration-Jnanam udbhavam has updated prjections.vidyardhicosmology[dot]blogspot[dot]com
ValeriaT
1 / 5 (9) May 26, 2013
Ya can thank Mr. Maxwell for the magic of how photons display their energy, and what they do with it
Nope bro - Mr. Maxwell's has nothing to do with photonzz.. (Maxwell's theory doesn't support quantization).
Code_Warrior
not rated yet May 26, 2013
From this lay person's perspective they must have some assumptions about the amount of dust and gas out there that lead them to conclude that they can accurately measure the reduction in energy due to the EBL photons. Perhaps they assume that the number of EBL photons in any given path >> dust & gas in the path, which seems like a reasonable assumption to make. Also, how accurately are they able to measure the energy of a gamma ray through the measurement of the products of it's collision with the Earth's atmosphere? I can understand such measurements inside a collider because the collider's detector surrounds the products of the collision, but this is not the case with atmospheric collisions. Just how much of the energy in the incident gamma ray are we missing based on what we add up with our ground observations and how does this affect the result?
Q-Star
2.8 / 5 (16) May 26, 2013
From this lay person's perspective they must have some assumptions about the amount of dust and gas out there that lead them to conclude that they can accurately measure the reduction in energy due to the EBL photons.


Redshift.

Perhaps they assume that the number of EBL photons in any given path >> dust & gas in the path, which seems like a reasonable assumption to make.


Spectroscopy can give ya good idea with absorption lines, though not perfect.

Also, how accurately are they able to measure the energy of a gamma ray through the measurement of the products of it's collision with the Earth's atmosphere?


Once ya capture the gamma photon in a detector (telescope. ccd. photometer, spectroscope etc), just just crunch the numbers. The physics of photometry and spectroscopy don't change with the source or place of observation, only the conditional nuances.
Q-Star
2.6 / 5 (15) May 26, 2013
From this lay person's perspective they must have some assumptions about the amount of dust and gas out there that lead them to conclude that they can accurately measure the reduction in energy due to the EBL photons. Perhaps they assume that the number of EBL photons in any given path >> dust & gas in the path, which seems like a reasonable assumption to make.


I think ya may misunderstand what EBL is. It ISN'T the stuff between the galaxies, that's the IGM (Intergalactic medium). The EBL (extragalactic background light) are the photons emitted during the very early (pre-galaxy forming era) universe, between the recombination era and reionization era. It's the residual radiation responsible for the reionization.
Code_Warrior
not rated yet May 27, 2013
I think ya may misunderstand what EBL is. It ISN'T the stuff between the galaxies, that's the IGM (Intergalactic medium). The EBL (extragalactic background light) are the photons emitted during the very early (pre-galaxy forming era) universe, between the recombination era and reionization era. It's the residual radiation responsible for the reionization.

Yes, I did misunderstand. However, the gamma photons still have to travel through the IGM to get to us and I assume that there is an effect of some sort due to that. The article stated that the interaction between a gamma photon and an EBL photon would produce an electron-positron pair that would fly off into space and never be heard from again. I am left wondering if the IGM can produce a similar result, or does interaction with the IGM produce a sufficiently different result that it can be quantified separately?
Code_Warrior
not rated yet May 27, 2013
Once ya capture the gamma photon in a detector (telescope. ccd. photometer, spectroscope etc), just just crunch the numbers. The physics of photometry and spectroscopy don't change with the source or place of observation, only the conditional nuances.

The article implies that they don't obtain direct gamma photon measurements and states that when a high energy gamma ray hits the upper atmosphere it produces a shower of charged particles travelling faster than light in air and this produces Cerenkov radiation that they detect with the special ground based telescopes. I assume that these charged particles are scattered by the atmosphere and many of them go undetected, so I am wondering what kinds of assumptions they make about the shape and density of the particle trajectories that allow them to estimate the total gamma photon energy based on the fraction of the products that they can detect?
Fleetfoot
5 / 5 (2) May 27, 2013
I assume that these charged particles are scattered by the atmosphere and many of them go undetected, so I am wondering what kinds of assumptions they make about the shape and density of the particle trajectories that allow them to estimate the total gamma photon energy based on the fraction of the products that they can detect?


I think Q-Star missed the point here, what you are really asking is
a) what aspects of the detection correlate to the original incident energy, and
b) how do they calibrate the measurements to get specific values

I don't know the details either but higher energy particles will produce a different mix and quantity of secondary particles and they may have higher kinetic energy too. Precisely what the process is for using that information is detailed engineering.
Fleetfoot
5 / 5 (2) May 27, 2013
.. the gamma photons still have to travel through the IGM to get to us and I assume that there is an effect of some sort due to that. The article stated that the interaction between a gamma photon and an EBL photon would produce an electron-positron pair that would fly off into space and never be heard from again. I am left wondering if the IGM can produce a similar result, or does interaction with the IGM produce a sufficiently different result that it can be quantified separately?


That effect is known as reddening rather than redshift and both will occur. However, reddening due to matter should produce a smoother effect over frequency while photon-photon interactions will have a sharper cutoff. An electron has a mass of 511keV so to create a pair needs just over 1MeV, below that they can't interact. I haven't read the paper yet but I would expect that separating different effects is a significant part of the analysis.
Fleetfoot
5 / 5 (1) May 27, 2013
This latest result confirms that the kinds of galaxies observed today are responsible for most of the EBL over all time. Moreover, it sets limits on possible contributions from many galaxies too faint to have been included in the galaxy surveys, ...


Since we know the mass/light ratio for galaxies, this also provides an upper limit for the total visible mass of a few percent of the critical energy density. Since we also know from other tests that the universe is close to critical density overall, this is further evidence for the existence of a non-visible component, i.e. "dark matter".

The cranks and skeptics often claim that galaxy velocity curves are the "only" evidence for DM but in reality there are numerous such observations that require it.
Q-Star
2.8 / 5 (16) May 27, 2013
I assume that these charged particles are scattered by the atmosphere and many of them go undetected, so I am wondering what kinds of assumptions they make about the shape and density of the particle trajectories that allow them to estimate the total gamma photon energy based on the fraction of the products that they can detect?


I think Q-Star missed the point here, what you are really asking is
a) what aspects of the detection correlate to the original incident energy, and
b) how do they calibrate the measurements to get specific values

I don't know the details either but higher energy particles will produce a different mix and quantity of secondary particles and they may have higher kinetic energy too. Precisely what the process is for using that information is detailed engineering.


I did miss his point. I thought he understood that direct observation of EBL gamma photons can only reliably be done from space observatories.
Q-Star
2.8 / 5 (16) May 27, 2013
.. the gamma photons still have to travel through the IGM to get to us and I assume that there is an effect of some sort due to that. The article stated that the interaction between a gamma photon and an EBL photon would produce an electron-positron pair that would fly off into space and never be heard from again. I am left wondering if the IGM can produce a similar result, or does interaction with the IGM produce a sufficiently different result that it can be quantified separately?


That effect is known as reddening rather than redshift and both will occur. However, reddening due to matter should produce a smoother effect over frequency while photon-photon interactions will have a sharper cutoff.


Correct, additionally the intervening IGM MAY be excited enough to show it's presence by emission/absorption spectra in UV, Visual or Infared bands. When it does, the gamma radiation can be inferred.

Q-Star
2.6 / 5 (15) May 27, 2013
The cranks and skeptics often claim that galaxy velocity curves are the "only" evidence for DM but in reality there are numerous such observations that require it.


So true. For me the most compelling is the lensing phenomena. This is beginning to produce "maps" of the DM that can be reliably tested against direct observation of gravitational redshift / blueshift.
Fleetfoot
not rated yet May 27, 2013
The cranks and skeptics often claim that galaxy velocity curves are the "only" evidence for DM but in reality there are numerous such observations that require it.


So true. For me the most compelling is the lensing phenomena. This is beginning to produce "maps" of the DM that can be reliably tested against direct observation of gravitational redshift / blueshift.


For me, it isn't any one but the fact that so many different observations all give the same result.
Q-Star
2.5 / 5 (16) May 27, 2013
For me, it isn't any one but the fact that so many different observations all give the same result.


Yeppers, the cranks, crackpots and internet Einsteins don't realize that their pet "theories" have to explain a million observations, not the single exception. And that it is unphysical to turn nature on and off for different phenomena/observations/locations.

I mention lensing because it was the nail in the coffin for most competing theories. What I find particularly interesting is the fact that the 1st lensing observed was in 1919. Another lens wasn't observed for 60 years, in 1979. Sixty years of discussion, but no observations until we had the proper telescopes.
Code_Warrior
5 / 5 (1) May 27, 2013
I think Q-Star missed the point here, what you are really asking is
a) what aspects of the detection correlate to the original incident energy, and
b) how do they calibrate the measurements to get specific values

I don't know the details either but higher energy particles will produce a different mix and quantity of secondary particles and they may have higher kinetic energy too. Precisely what the process is for using that information is detailed engineering.

Yes, this is exactly what I was wondering.
Code_Warrior
5 / 5 (1) May 27, 2013
That effect is known as reddening rather than redshift and both will occur. However, reddening due to matter should produce a smoother effect over frequency while photon-photon interactions will have a sharper cutoff.


Correct, additionally the intervening IGM MAY be excited enough to show it's presence by emission/absorption spectra in UV, Visual or Infared bands. When it does, the gamma radiation can be inferred.

Thank you. Excellent explanations.
Benni
1 / 5 (9) May 27, 2013
really? not a single one of them has ever been absorbed? really?


.....what is absorbed one place, has to be emitted somewhere to keep the conservation balance.


Not if "transformation" occurs. Do you know what I mean by this? (hint: E/c*2= m). In such a case there is not a re-emission of a photon for "conservation of energy" to occur.

Q-Star
2.5 / 5 (16) May 28, 2013
Not if "transformation" occurs. Do you know what I mean by this? (hint: E/c*2= m). In such a case there is not a re-emission of a photon for "conservation of energy" to occur.


@Benni,

For a nuclear engineer (ha, ha, ha) ya sure do hate to miss an opportunity to post some dumb stuff.

A photon isn't energy, it transfers energy. It is the mediator of the electromagnetic force. It does not turn into a matter particle, it only transfers the energy needed to alter it's momentum/mass. Ya really should go read some elementary physics stuff (pre-college level) Ya seem to know nothing beyond parroting E = mc^2. To be sure ya don't know what that little formula means or what it's used for. It's been pointed out to ya on numerous occasions.

E = mc^2 is not a recipe for creating matter, it is a momentum equivalence equation. The only place E = mc^2 has ever been realized by a particle with mass is in the pre-inflation era, if ya can work those physics out ya'll get a Nobel.
Q-Star
2.6 / 5 (15) May 28, 2013
..the cranks, crackpots and internet Einsteins don't realize that their pet "theories" have to explain a million observations, not the single exception..
Well, exactly. The massless photon doesn't explain why photons materialize, why massive objects are losing mass with radiation of photons, why photons fall into black hole in similar way, like other massive bodies, etc. If you apply such a criterion consequently, you'll realize, that the relativistic model is just one extreme approximation of the physically realistic situation.


Zeph, ya're making the same mistake that the "nuclear engineer" is making. Mass, momentum, energy and matter are closely related, but they are not four words which means the same thing.

Go to the google,,,, or go to a good physics dictionary, study the nuances of what each of those four things ARE, then maybe ya and that other guy won't keep getting them crossed up in your "lectures" to us mere mortals.
GSwift7
1.4 / 5 (11) May 28, 2013
Very interesting, and surely good work, but it is surprising how many assumptions must be made to get the results.

First, the actual distance to the blazars is infered from indirect means, so there's quite a wide margin for error there.

Then the calculation of the original brightness introduces another possible error margin of unknown size. (the unknown distance and original brightness in combination create a difficult problem, but oh well)

Lastly, they are using cerenkov radiation as a proxy for gamma rays, so there's another source of uncertainty there.

Then of course, there's instrumental uncertainty as always, especially since they are attempting to get statistically significant results at the limit of detection. I haven't looked at the actual study. I wonder what the CI is?
Fleetfoot
5 / 5 (1) May 28, 2013
A photon isn't energy, it transfers energy. It is the mediator of the electromagnetic force. It does not turn into a matter particle, ...


Well if you read the article, that's exactly what they are measuring. If two photons have sufficient energy in the frame in which they have zero momentum, then that energy can 'transform' as Benni put it into an electron-positron pair removing the photons from the gamma flux we observe. The electron's mass is 511keV so it takes a rest frame energy in excess of 1022keV for pair production.

E = mc^2 is not a recipe for creating matter, ..


It tells you how much energy is needed to create that amount of mass though.
Fleetfoot
5 / 5 (1) May 28, 2013
The photon is localized particle and it removes the matter from radiating body - not just energy ..


Mass is a relationship between energy and momentum. Photons have equal energy and momentum hence their mass is zero.

at least in its own reference frame.


It is not possible to construct a reference frame for any massless particle, the axes become degenerate.

Of course the mass density of visible light photon is very subtle due the large E/m = c^2 factor.


Visible light have energies around 2 to 3 eV, the experimental upper limit is conservatively about 10^-14eV so even allowing for the factor of c^2, you are wrong by at least 14 orders of magnitude.
Q-Star
2.6 / 5 (15) May 28, 2013
@ Fleetfoot, from the article:

When a high-energy gamma ray photon from a blazar hits a much lower energy EBL photon, both are annihilated and produce two different particles: an electron and its antiparticle, a positron, which fly off into space and are never heard from again.


That is a dramatic oversimplification. Don't ya think? And not entirely complete/accurate.

Be that as it may, this is what I understood from reading the article:

When a high-energy gamma ray from a blazar strikes air molecules in the upper regions of Earth's atmosphere, it produces a cascade of charged subatomic particles. This cascade of particles travels faster than the speed of light in air (which is slower than the speed of light in a vacuum).


So they would have a number in theory to use to balance out the observations made from space based observatories. Did I read it wrong? Maybe so sometimes they lose something when writing the for a general audience. I'm willing to be wrong.
GSwift7
1.3 / 5 (12) May 28, 2013
It is not possible to construct a reference frame for any massless particle


That isn't true. Even massless particles are subject to the rules of physics in whatever reference frame they inhabit. For example, the massless photon cannot leave a black hole. This would not be true if the reference frame did not exist.

Q-Star:

I am interpreting it a bit differently, though I could also be reading it wrong. I think they used space based observatories to measure in certain wavelengths, while the ground based proxy using cerenkov radiation is supposed to measure gamma rays that would have interfered with different frequencies of photons.

Keep in mind that they are basically trying to measure the shadow cast by gamma rays passing through photons, and using that to try to figure out how many and what kind of photons got in the way of the gamma rays. It all hinges on whether they are correct about how bright the source was. I think they're walking in a gray area here. No pun intended
GSwift7
1.7 / 5 (12) May 28, 2013
continued:

That sounds so strange that I have to repeat it. They are looking for the shadow of light being blocked by other light. What an odd thing to contemplate. It's not very often that you get to play with GR and QT in the same experiment. That's kinda cool.
ValeriaT
1 / 5 (8) May 28, 2013
the massless photon cannot leave a black hole. This would not be true if the reference frame did not exist
This is just an evidence, that the photon is massive, so it has a reference frame...;-) Newer mind, the guys like you will parrote everything, what the mainstream will tell him.
Mass, momentum, energy and matter are closely related, but they are not four words which means the same thing.
Just because of it the E = mc^2 equation IS NOT a momentum equivalence equation. E stands for energy, not a momentum.
Q-Star
2.6 / 5 (15) May 28, 2013
Q-Star:

I am interpreting it a bit differently, though I could also be reading it wrong. I think they used space based observatories to measure in certain wavelengths, while the ground based proxy using cerenkov radiation is supposed to measure gamma rays that would have interfered with different frequencies of photons.


I'm somewhat confused, this is new science and not well known to me. I'm thinking they want to find a way to determine how much the gamma radiation of a certain frequency is of the lower energy type of the EBL, and how much is actually coming from high energy gamma (from GBR's, quasars, blazers etc) being changed in to this same low energy gamma radiation. So they can subtact the "false" gamma radiation from the gamma radiation they (hope to) receive from the EBL.

But this is so new, it is more than possible that I am completely wrong.
Q-Star
2.6 / 5 (15) May 28, 2013
Just because of it the E = mc^2 equation IS NOT a momentum equivalence equation. E stands for energy, not a momentum.


Only to a person to uninformed/lazy to do enough reading to understand where that formula came from, what it was devised to be used for. Non-physicists will never become physicists until they know know how energy, momentum and mass relate.

But it's not your fault,,,, it's the fault of the school systems. In that they teach physics in terms of forces, masses, and accelerations. It should be taught (as it is on the college level) in terms of energy, momentum and motions. It would sure make things easier for college instructors if they did, and the kids who don't go to college would be smarter too.
Fleetfoot
5 / 5 (1) May 29, 2013
When a high-energy gamma ray photon from a blazar hits a much lower energy EBL photon, both are annihilated and produce two different particles ..


That is a dramatic oversimplification. Don't ya think?


Yes.

And not entirely complete/accurate.


It accurately reflects the initial and final components, not the details.

When a high-energy gamma ray from a blazar strikes air molecules in the upper regions of Earth's atmosphere, it produces a cascade of charged subatomic particles. This cascade of particles travels faster than the speed of light in air (which is slower than the speed of light in a vacuum).


So they would have a number in theory to use to balance out the observations made from space based observatories.


I think that's right, various measurements were used to determine the spectrum in space and on the ground so that the deep space attenuation could be separated from local effects. I haven't read the paper yet though, just the article.
Fleetfoot
5 / 5 (1) May 29, 2013
I'm somewhat confused, this is new science and not well known to me. I'm thinking they want to find a way to determine how much the gamma radiation of a certain frequency is of the lower energy type of the EBL, and how much is actually coming from high energy gamma (from GBR's, quasars, blazers etc) being changed in to this same low energy gamma radiation. So they can subtact the "false" gamma radiation from the gamma radiation they (hope to) receive from the EBL.

But this is so new, it is more than possible that I am completely wrong.


The probability of a photon-photon interaction depends on the total energy so they can measure the the prevalence of EBL photons by measuring the fraction of gamma photons which are removed from the flux by that process. There is no conversion to other frequencies, either the gamma gets here or it changes to an electron-positron pair.

The trick is to measure attenuation versus frequency and calibrate out other causes of attenuation.
Fleetfoot
5 / 5 (2) May 29, 2013
It is not possible to construct a reference frame for any massless particle


That isn't true.


Try it, apply a Lorentz Transform in the +x direction with v=c and find out what happens to the x' and t' axes. You may find it more intuitive to consider how the axes rotate and calculate the angle between them as v -> c.

Or, if you don't want to take my word for it, how about the Physics FAQ:

http://math.ucr.e...hts.html

"There are no inertial reference frames in which the photon is at rest so it is hopeless to try to imagine what it would be like in one."
GSwift7
1.4 / 5 (10) May 29, 2013
But this is so new, it is more than possible that I am completely wrong.


They are observing light from blazars that reaches Earth. They are calculating what the original intensity of that light should have been and comparing that value to the light that actually reaches Earth. The reason gamma ray photons are important here is because they have enough energy to combine with lower energy photons (the EBL). When that happens, it essentially destroys both photons. Only certain energy levels of photons will combine in this way, so by looking at how many of those energy level photons are missing from the blazar light, they can calculate how many EBL photons were struck by the blazar gamma ray photons en route to Earth. By looking at blazars at different distances, they can see how the EBL changed over time.

I don't think anyone posting here understands what this experiment is doing.

As I said, it depends on a lot of assumptions. It's like a house of cards built on another.
GSwift7
1.4 / 5 (10) May 29, 2013
I think that's right, various measurements were used to determine the spectrum in space and on the ground so that the deep space attenuation could be separated from local effects. I haven't read the paper yet though, just the article


The space observations aren't a large enough sample, so they need ground based observations to increase the sample size (the whole objective here is to get measurements from a bunch of blazars at different distances). The problem with ground based observations is that the atmosphere is opaque to gamma ray photons. We can only see the "explosion" caused when they strike the atmosphere, so it's only a rough approximation of how many and what energy levels hit the atmosphere from a given blazar source (oversimplification warning). There should be no difference between the space observations of a given blazar and the calculated value derived from cherenkov radiation from that same blazar.
Q-Star
2.6 / 5 (15) May 29, 2013
@ Fleetfoot:

Very high energy gamma photons from blazars and other sources do in fact interact in many places as it travels to us. And the result of many of these interactions, it will become a phenomena that mimics what the EBL we want to study looks like. As I say, this is a new technique and science to me,,,, what I'm taking from it is they want to quantify how much these mimics contribute to the EBL observations.

In the creation of an electron-positron pair. The photons which created them, will be immediately replaced by the electron and positron attempting to interact with each other, or attempting to interact with their environment. (These are the mimes I am thinking) Which is why I stated that the article was over-simplistic and somewhat inaccurate.
GSwift7
1.7 / 5 (11) May 29, 2013
(These are the mimes I am thinking) Which is why I stated that the article was over-simplistic and somewhat inaccurate


I am 99% sure I'm getting this correctly.

They are simply trying to figure out how many of the gamma ray photons from the blazars AREN'T reaching Earth. It's kinda like reading absorption lines. If you know what the original signal should look like, and you can figure out which parts are missing, then you know what the light passed through. In this case, it's not a gas, rather it is other wavelengths of light. In the unique case of gamma rays, they can interact with other photons, causing an absorbtion pattern similar to the way a gas does. By comparing those absorption patterns from blazars at different distances, you can see how the EBL changed over time. In other words, by looking at which blazar photons are missing, you can infer what kinds of photons they must have run into along the way to have been 'destroyed'.
GSwift7
1.4 / 5 (10) May 29, 2013
Here's a link to a much better description of the work, which supports my interpretation:

http://astrobites...horizon/

Here's a quote from my link, basically saying that they are looking for an 'absorption spectrum', though that term isn't technically correct.

If we have a good number of high energy extragalactic sources and we know how many photons they emit as a function of energy, we can compare that intrinsic spectrum to the observed one. The difference will be caused by attenuation by the EBL


As I suspected, but didn't post above, if this can be done with enough accuracy, then it can become a new standard candle to measure distance to blazars.
Q-Star
2.6 / 5 (15) May 29, 2013
I am 99% sure I'm getting this correctly.


The more I read it, I'm also thinking ya are.

They are simply trying to figure out how many of the gamma ray photons from the blazars AREN'T reaching Earth. It's kinda like reading absorption lines. If you know what the original signal should look like, and you can figure out which parts are missing, then you know what the light passed through


Mmmmh, I think we agree, but we can already determine what they are passing through, because that something will then glow with it own characteristic emission spectra.

Determining attenuation of something like gamma photons is always going to be interesting,, because the medium doing the attenuating will remit the photons with specific emission lines. Since we can't do the spectroscopy at gamma wavelengths, the proxy emission lines might be a valid approach.

Gamma Gamma collisions is something I'll sure do some reading on now.

GSwift7
1.8 / 5 (10) May 29, 2013
BTW, the story here on phys. org is incorrect.

The only reason they compare the ground based to the space based is to callibrate the ground based observations. Since the ground based are a proxy, rather than a direct observation, they needed to callibrate them in order to know what a given measurement means.

To use a crude analogy, the space telescopes are catching bullets, while the ground instruments are catching the debris knocked loose from a bullet that passed through a wall. Without a database of what kind of bullets cause x type of debris, the debris information is useless. The more detailed and better written story I linked to accurately explains this.

Also explained in the other version of the story, as I pointed out above, is the fact that there is high uncertainty in knowing the exact output of the blazars at their source (which is required to get a good 'absorption spectrum')

GSwift7
1.8 / 5 (10) May 29, 2013
Mmmmh, I think we agree, but we can already determine what they are passing through, because that something will then glow with it own characteristic emission spectra.


Too faint to measure. That's the problem. If you could measure that emission, then you could also just measure the EBL directly. The light of our own solar system and galaxy prevents us measuring either of those directly. It's there, but hidden in the noise from local sources.

Gamma Gamma collisions is something I'll sure do some reading on now


These aren't gamma-gamma collisions, these are collisions of gamma from the blazars with low energy photons from the EBL. The whole point is looking for a way to count the low energy photons, which would otherwise be undetectable for us. This very old EBL light should be in extremely long wavelengths, and very low energy.
Q-Star
2.6 / 5 (15) May 29, 2013
Gamma Gamma collisions is something I'll sure do some reading on now


These aren't gamma-gamma collisions, these are collisions of gamma from the blazars with low energy photons from the EBL. The whole point is looking for a way to count the low energy photons, which would otherwise be undetectable for us. This very old EBL light should be in extremely long wavelengths, and very low energy.


Sorry, that's what I meant to say,,, gamma collisions of any sort is what I meant to type. I double typed the Gamma.
GSwift7
1.9 / 5 (9) May 29, 2013
Sorry, that's what I meant to say,,, gamma collisions of any sort is what I meant to type. I double typed the Gamma


lol, I have double typed several things today, but I keep catching them before I hit the button.

As for seeing any kind of radiation from the particle pairs created by one of these photon collisions, any particle pair created in this way would have extremely low energy. It would barely have enough energy to exist, as I understand it, which leaves very little energy to be radiated afterwards. I don't think it works like fusion, where you combine particles and then have extra energy that is radiated as part of the process. I think in this case, either you have the exact amount of energy needed to create some combination of particles, or it doesn't happen (though this stuff isn't a strong area for me). I'll bet antialias could offer some better perspective on that.
Q-Star
2.3 / 5 (15) May 29, 2013
As for seeing any kind of radiation from the particle pairs created by one of these photon collisions, any particle pair created in this way would have extremely low energy. It would barely have enough energy to exist, as I understand it, which leaves very little energy to be radiated afterwards. I don't think it works like fusion, where you combine particles and then have extra energy that is radiated as part of the process. I think in this case, either you have the exact amount of energy needed to create some combination of particles, or it doesn't happen (though this stuff isn't a strong area for me). I'll bet antialias could offer some better perspective on that.


Naa, I was thinking about the electron positron pair brought up earlier,,,, it got my interest up big time, particle physics and quantum theory is not my strong suit, (which I had fooled meself into thinking otherwise until pondering this article.) If ya don't actively keep ya, ya quickly fall behind.
Fleetfoot
5 / 5 (1) May 30, 2013
As for seeing any kind of radiation from the particle pairs created by one of these photon collisions, any particle pair created in this way would have extremely low energy.


An electron-positron pair has a mass of 2 * 511keV so that is the minimum for pair creation. The excess above that which the gamma and photon had before the interaction must either be carried off by a third particle or must be the kinetic energy of the electron and positron. In most cases, that means they will be relativistic and moving in opposite directions in the original null momentum frame. They will not interact with each other but the positron with probably annihilate with another IGM particle and any radiation from that is _very_ unlikely to be pointed precisely at us. The original gamma thus simply ceases to exist as far as we are concerned.