APOD: A December Summer Night (2023 Dec 23)

[APOD Robot]

A December Summer Night

Explanation: Colours of a serene evening sky are captured in this 8 minute exposure, made near this December's solstice from New Zealand, southern hemisphere, planet Earth. Looking south, star trails form the short concentric arcs around the rotating planet's south celestial pole positioned just off the top of the frame. At top and left of center are trails of the Southern Cross stars and a dark smudge from the Milky Way's Coalsack Nebula. Alpha and Beta Centauri make the brighter yellow and blue tinted trails, reflected below in the waters of Hoopers Inlet in the Pacific coast of the South Island's Otago Peninsula. On that short December summer night, aurora australis also gave luminous, green and reddish hues to the sky above the hills. An upper atmospheric glow distinct from the aurora excited by collisions with energetic particles, pale greenish bands of airglow caused by a cascade of chemical reactions excited by sunlight can be traced in diagonal bands near the top left.

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[VictorBorun]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

[VictorBorun]

my question: the Solar system is peculiar-moving at the speed of 20 km/s toward an apex between Lyra and Hercules.
Now does the interstellar media headwind play any part in this night sky shine? (I know this APOD says it is just Sun's stellar wind and light)

[VictorBorun]

A December Summer Night (2023 Dec 23)..jpg (36.85 KiB) Viewed 23332 times

my second question: why some 8 minute exposure arcs are thin lines but others are parallelograms?

[johnnydeep]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

Um, yes? Unless I misunderstand the question, one path is straight and the other is the sum of the other two sides of a triangle:

[Chris Peterson]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

Well, if you want to be really technical, the photon that comes from Alpha Centauri never makes it to the sensor at all, regardless of the path it takes. That photon is scattered (probably many times) by air, by water, by camera optics. And the scattering of a photon actually involves the absorption of that photon and the emission of a new one.

[Christian G.]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

Well, if you want to be really technical, the photon that comes from Alpha Centauri never makes it to the sensor at all, regardless of the path it takes. That photon is scattered (probably many times) by air, by water, by camera optics. And the scattering of a photon actually involves the absorption of that photon and the emission of a new one.

Damn! This means I can never marvel again at a galaxy and think "Wow, these photons entering my eyes this very instant come straight from that galaxy and have been travelling for 40 million years!"?

[johnnydeep]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

Well, if you want to be really technical, the photon that comes from Alpha Centauri never makes it to the sensor at all, regardless of the path it takes. That photon is scattered (probably many times) by air, by water, by camera optics. And the scattering of a photon actually involves the absorption of that photon and the emission of a new one.

And even before the photon enters the atmosphere it will likely already have been absorbed and a different one reemitted many times, yes?

[Chris Peterson]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

Well, if you want to be really technical, the photon that comes from Alpha Centauri never makes it to the sensor at all, regardless of the path it takes. That photon is scattered (probably many times) by air, by water, by camera optics. And the scattering of a photon actually involves the absorption of that photon and the emission of a new one.

Damn! This means I can never marvel again at a galaxy and think "Wow, these photons entering my eyes this very instant come straight from that galaxy and have been travelling for 40 million years!"?

'Fraid not. The photon you see probably came off an atom making up the vitreous a fraction of a millimeter in front of your retina! You'll have to be satisfied with cause-and-effect: had the photon not been emitted by that distant galaxy, you'd never have seen anything at all.

[Chris Peterson]

a kid's question: is the path covered by a photon from Alpha Centauri any longer if that photon is registered by the sensor in the waters' part of the scene?

Well, if you want to be really technical, the photon that comes from Alpha Centauri never makes it to the sensor at all, regardless of the path it takes. That photon is scattered (probably many times) by air, by water, by camera optics. And the scattering of a photon actually involves the absorption of that photon and the emission of a new one.

And even before the photon enters the atmosphere it will likely already have been absorbed and a different one reemitted many times, yes?

That seems reasonable, but I don't know this for sure. We'd have to do a (messy) calculation that considers the very small interaction cross-section of particles in the low density intergalactic (and interstellar at each end) space, but also the very long path length.

[VictorBorun]

Well, if you want to be really technical, the photon that comes from Alpha Centauri never makes it to the sensor at all, regardless of the path it takes. That photon is scattered (probably many times) by air, by water, by camera optics. And the scattering of a photon actually involves the absorption of that photon and the emission of a new one.

And even before the photon enters the atmosphere it will likely already have been absorbed and a different one reemitted many times, yes?

That seems reasonable, but I don't know this for sure. We'd have to do a (messy) calculation that considers the very small interaction cross-section of particles in the low density intergalactic (and interstellar at each end) space, but also the very long path length.

well that's a turn I never expected.
1) I thought a photon is a quantum of electro-magnetic radiation that can be traced and counted as one quantum even after some collisions or passing a thick media (temporarily slowing down say 2 times); if it's counted as 1 quantum still, it is traceable and its life goes on.
2) The diagram with a triangle looks nice however I think it's misleading. You can not in fact say that off-waters photons are longer travellers with any statistical significance considering the size of Alpha Centauri or the distance covered by Earth's orbiting and Solar system's peculiar movement in 8 minutes. What fun it could be if there were light echo phenomenon in such pictures: we could watch a flash in Alpha Centauri time and again…

[VictorBorun]

A December Summer Night (2023 Dec 23)..jpgmy second question: why some 8 minute exposure arcs are thin lines but others are parallelograms?

offtopic
To my eye there is some resemblance between the fuzzy reflections and the the Sparkler Galaxy

[johnnydeep]

And even before the photon enters the atmosphere it will likely already have been absorbed and a different one reemitted many times, yes?

That seems reasonable, but I don't know this for sure. We'd have to do a (messy) calculation that considers the very small interaction cross-section of particles in the low density intergalactic (and interstellar at each end) space, but also the very long path length.

well that's a turn I never expected.
1) I thought a photon is a quantum of electro-magnetic radiation that can be traced and counted as one quantum even after some collisions or passing a thick media (temporarily slowing down say 2 times); if it's counted as 1 quantum still, it is traceable and its life goes on.
2) The diagram with a triangle looks nice however I think it's misleading. You can not in fact say that off-waters photons are longer travellers with any statistical significance considering the size of Alpha Centauri or the distance covered by Earth's orbiting and Solar system's peculiar movement in 8 minutes. What fun it could be if there were light echo phenomenon in such pictures: we could watch a flash in Alpha Centauri time and again…

2) Well, sure the extra length isn't "statistically significant", but the path the reflected photon takes (ignoring particle/wave duality, etc) is certainly not shorter than the path a "direct" photon takes!

And in reply to Chris, for photons being absorbed and reemitted (the "re-" being misleading since it's not the same photon), to complete the picture, we'd have to ask at what point does a photon leave a star on it's way to us? From the instant its released as a byproduct of fusion in the core, it starts being absorbed and reemitted uncountably (1e9, 1e12, 1e15,...?) many times before it reaches us! And even once it leaves the photosphere, it still has to make it through the extensive corona.

[Chris Peterson]

That seems reasonable, but I don't know this for sure. We'd have to do a (messy) calculation that considers the very small interaction cross-section of particles in the low density intergalactic (and interstellar at each end) space, but also the very long path length.

well that's a turn I never expected.
1) I thought a photon is a quantum of electro-magnetic radiation that can be traced and counted as one quantum even after some collisions or passing a thick media (temporarily slowing down say 2 times); if it's counted as 1 quantum still, it is traceable and its life goes on.
2) The diagram with a triangle looks nice however I think it's misleading. You can not in fact say that off-waters photons are longer travellers with any statistical significance considering the size of Alpha Centauri or the distance covered by Earth's orbiting and Solar system's peculiar movement in 8 minutes. What fun it could be if there were light echo phenomenon in such pictures: we could watch a flash in Alpha Centauri time and again…

2) Well, sure the extra length isn't "statistically significant", but the path the reflected photon takes (ignoring particle/wave duality, etc) is certainly not shorter than the path a "direct" photon takes!

And in reply to Chris, for photons being absorbed and reemitted (the "re-" being misleading since it's not the same photon), to complete the picture, we'd have to ask at what point does a photon leave a star on it's way to us? From the instant its released as a byproduct of fusion in the core, it starts being absorbed and reemitted uncountably (1e9, 1e12, 1e15,...?) many times before it reaches us! And even once it leaves the photosphere, it still has to make it through the extensive corona.

Indeed, the initial photon must spawn countless trillions more through scattering in its parent star, until it finally reaches our retina or camera.

Something to consider in the path length discussion, is that a photon travels at exactly one speed in a vacuum, but in air its speed is defined by a statistical distribution. So it's possible for a photon (let's just treat it as one for this discussion) to travel the longer path in less time than one on the shorter path. It will only be when we average the flight time of many photons that we'll see the average time is less on the shorter path.

[VictorBorun]

2) Well, sure the extra length isn't "statistically significant", but the path the reflected photon takes (ignoring particle/wave duality, etc) is certainly not shorter than the path a "direct" photon takes!

And in reply to Chris, for photons being absorbed and reemitted (the "re-" being misleading since it's not the same photon), to complete the picture, we'd have to ask at what point does a photon leave a star on it's way to us? From the instant its released as a byproduct of fusion in the core, it starts being absorbed and reemitted uncountably (1e9, 1e12, 1e15,...?) many times before it reaches us! And even once it leaves the photosphere, it still has to make it through the extensive corona.

A gamma photon from a nuclear fusion reaction in the core of a star would not stay ONE photon passing the radiation zone (if there is such zone) or heating up the convection zone (if there is such zone). One photon of one kind is absorbed and then multiple photons of much lower energy re-emitted; no way to point out one of them it say that it is the original photon reborn.

Elastic scattering of a photon is another tale.The photon may somewhat change the direction, energy, polarisation and phase, but all in predictable way. And, typically, it is still one photon.
For example, a reflection from a mirror will not change the plane in which the incoming path and the perpendicular to the surface of the mirror lie; it also preserves phase. If the mirror is moving to or from the observer, the reflection would cause a violet or red shift, but it could be taken in account and would not stop us from detecting an emission line and such.
All in all it stays to reason to trace that photon and talk about it as one photon, however dramatic its living path was.

[johnnydeep]

2) Well, sure the extra length isn't "statistically significant", but the path the reflected photon takes (ignoring particle/wave duality, etc) is certainly not shorter than the path a "direct" photon takes!

And in reply to Chris, for photons being absorbed and reemitted (the "re-" being misleading since it's not the same photon), to complete the picture, we'd have to ask at what point does a photon leave a star on it's way to us? From the instant its released as a byproduct of fusion in the core, it starts being absorbed and reemitted uncountably (1e9, 1e12, 1e15,...?) many times before it reaches us! And even once it leaves the photosphere, it still has to make it through the extensive corona.

A gamma photon from a nuclear fusion reaction in the core of a star would not stay ONE photon passing the radiation zone (if there is such zone) or heating up the convection zone (if there is such zone). One photon of one kind is absorbed and then multiple photons of much lower energy re-emitted; no way to point out one of them it say that it is the original photon reborn.

Elastic scattering of a photon is another tale.The photon may somewhat change the direction, energy, polarisation and phase, but all in predictable way. And, typically, it is still one photon.
For example, a reflection from a mirror will not change the plane in which the incoming path and the perpendicular to the surface of the mirror lie; it also preserves phase. If the mirror is moving to or from the observer, the reflection would cause a violet or red shift, but it could be taken in account and would not stop us from detecting an emission line and such.
All in all it stays to reason to trace that photon and talk about it as one photon, however dramatic its living path was.

Point taken regarding the original fusion-produced photon having extremely high energy (gamma), and spawning lower energy photons. After all, we don't only receive gamma ray photons from the Sun! Does fusion only produce gamma ray photons? Also, do any of the source gamma rays make it out of the Sun? I know gamma rays can come from solar flares per Wikipedia - https://en.wikipedia.org/wiki/Sunlight# ... _and_power

[Chris Peterson]

2) Well, sure the extra length isn't "statistically significant", but the path the reflected photon takes (ignoring particle/wave duality, etc) is certainly not shorter than the path a "direct" photon takes!

And in reply to Chris, for photons being absorbed and reemitted (the "re-" being misleading since it's not the same photon), to complete the picture, we'd have to ask at what point does a photon leave a star on it's way to us? From the instant its released as a byproduct of fusion in the core, it starts being absorbed and reemitted uncountably (1e9, 1e12, 1e15,...?) many times before it reaches us! And even once it leaves the photosphere, it still has to make it through the extensive corona.

A gamma photon from a nuclear fusion reaction in the core of a star would not stay ONE photon passing the radiation zone (if there is such zone) or heating up the convection zone (if there is such zone). One photon of one kind is absorbed and then multiple photons of much lower energy re-emitted; no way to point out one of them it say that it is the original photon reborn.

Elastic scattering of a photon is another tale.The photon may somewhat change the direction, energy, polarisation and phase, but all in predictable way. And, typically, it is still one photon.
For example, a reflection from a mirror will not change the plane in which the incoming path and the perpendicular to the surface of the mirror lie; it also preserves phase. If the mirror is moving to or from the observer, the reflection would cause a violet or red shift, but it could be taken in account and would not stop us from detecting an emission line and such.
All in all it stays to reason to trace that photon and talk about it as one photon, however dramatic its living path was.

Elastic scattering does not result in the same photon continuing. QM describes that scattering as the annihilation of one and the creation of another. That is different from an atom absorbing a photon and emitting another.

[VictorBorun]

Elastic scattering does not result in the same photon continuing. QM describes that scattering as the annihilation of one and the creation of another. That is different from an atom absorbing a photon and emitting another.

Quantum Mechanics + Quantum Field Theory may add virtual particles every way they like but if a photon is observable and say let the observer detect spectral lines in Alpha Centauri's surface, I prefer to call it one and the same photon: as emitted from Alpha Centauri's surface so registered by the sensor for this APOD

Talking about wording, consider a human being, like a person you are having a conversation with. They may change with time and even change their mind after hearing your argument but we all prefer to call them one and the same person, don't we?

[Chris Peterson]

Elastic scattering does not result in the same photon continuing. QM describes that scattering as the annihilation of one and the creation of another. That is different from an atom absorbing a photon and emitting another.

Quantum Mechanics + Quantum Field Theory may add virtual particles every way they like but if a photon is observable and say let the observer detect spectral lines in Alpha Centauri's surface, I prefer to call it one and the same photon: as emitted from Alpha Centauri's surface so registered by the sensor for this APOD

Talking about wording, consider a human being, like a person you are having a conversation with. They may change with time and even change their mind after hearing your argument but we all prefer to call them one and the same person, don't we?

Well, like I said, the QM treatment does not consider a photon to survive any interaction. The original one is always replaced. A photon can only travel in a straight line (really, a geodesic). It cannot have its direction changed.

Personally, I do not consider a person to exist for more than a fraction of a second. We are a new person over and over. It is convenient to act as if the person we are talking to is the same person throughout the conversation... but I would argue that at the level of the theory of mind and cognitive science, that is not actually the case.

[johnnydeep]

Elastic scattering does not result in the same photon continuing. QM describes that scattering as the annihilation of one and the creation of another. That is different from an atom absorbing a photon and emitting another.

Quantum Mechanics + Quantum Field Theory may add virtual particles every way they like but if a photon is observable and say let the observer detect spectral lines in Alpha Centauri's surface, I prefer to call it one and the same photon: as emitted from Alpha Centauri's surface so registered by the sensor for this APOD

Talking about wording, consider a human being, like a person you are having a conversation with. They may change with time and even change their mind after hearing your argument but we all prefer to call them one and the same person, don't we?

Well, like I said, the QM treatment does not consider a photon to survive any interaction. The original one is always replaced. A photon can only travel in a straight line (really, a geodesic). It cannot have its direction changed.

Personally, I do not consider a person to exist for more than a fraction of a second. We are a new person over and over. It is convenient to act as if the person we are talking to is the same person throughout the conversation... but I would argue that at the level of the theory of mind and cognitive science, that is not actually the case.

Though a human is quite unlike a photon in that a human changes from moment to moment even in complete isolation from all sensory input, whereas a photon in isolation is eternal. Hmm, does cosmic expansion count as affecting the photon since it's energy would be decreased over time? Would it still be the "same photon" in that case?

[Chris Peterson]

Quantum Mechanics + Quantum Field Theory may add virtual particles every way they like but if a photon is observable and say let the observer detect spectral lines in Alpha Centauri's surface, I prefer to call it one and the same photon: as emitted from Alpha Centauri's surface so registered by the sensor for this APOD

Talking about wording, consider a human being, like a person you are having a conversation with. They may change with time and even change their mind after hearing your argument but we all prefer to call them one and the same person, don't we?

Well, like I said, the QM treatment does not consider a photon to survive any interaction. The original one is always replaced. A photon can only travel in a straight line (really, a geodesic). It cannot have its direction changed.

Personally, I do not consider a person to exist for more than a fraction of a second. We are a new person over and over. It is convenient to act as if the person we are talking to is the same person throughout the conversation... but I would argue that at the level of the theory of mind and cognitive science, that is not actually the case.

Though a human is quite unlike a photon in that a human changes from moment to moment even in complete isolation from all sensory input, whereas a photon in isolation is eternal. Hmm, does cosmic expansion count as affecting the photon since it's energy would be decreased over time? Would it still be the "same photon" in that case?

I didn't say that photons can't be affected, only that they only can travel in a straight line (geodesic) which they cannot be deviated from. Which is why scattering involves the replacement of one photon with another.

[johnnydeep]

Well, like I said, the QM treatment does not consider a photon to survive any interaction. The original one is always replaced. A photon can only travel in a straight line (really, a geodesic). It cannot have its direction changed.

Personally, I do not consider a person to exist for more than a fraction of a second. We are a new person over and over. It is convenient to act as if the person we are talking to is the same person throughout the conversation... but I would argue that at the level of the theory of mind and cognitive science, that is not actually the case.

Though a human is quite unlike a photon in that a human changes from moment to moment even in complete isolation from all sensory input, whereas a photon in isolation is eternal. Hmm, does cosmic expansion count as affecting the photon since it's energy would be decreased over time? Would it still be the "same photon" in that case?

I didn't say that photons can't be affected, only that they only can travel in a straight line (geodesic) which they cannot be deviated from. Which is why scattering involves the replacement of one photon with another.

So is a photon that drops in energy while traveling along a space-time geodesic still the "same photon"?

[Chris Peterson]

Though a human is quite unlike a photon in that a human changes from moment to moment even in complete isolation from all sensory input, whereas a photon in isolation is eternal. Hmm, does cosmic expansion count as affecting the photon since it's energy would be decreased over time? Would it still be the "same photon" in that case?

I didn't say that photons can't be affected, only that they only can travel in a straight line (geodesic) which they cannot be deviated from. Which is why scattering involves the replacement of one photon with another.

So is a photon that drops in energy while traveling along a space-time geodesic still the "same photon"?

Sure. A photon only has a few properties. The invariant ones are its rest mass and speed (those are actually related). The only meaningful quantum property is its momentum. That changes due to its motion through expanding space. In fact, I don't think anything else can change the momentum of a photon without replacing it with a new one.

[johnnydeep]

I didn't say that photons can't be affected, only that they only can travel in a straight line (geodesic) which they cannot be deviated from. Which is why scattering involves the replacement of one photon with another.

So is a photon that drops in energy while traveling along a space-time geodesic still the "same photon"?

Sure. A photon only has a few properties. The invariant ones are its rest mass and speed (those are actually related). The only meaningful quantum property is its momentum. That changes due to its motion through expanding space. In fact, I don't think anything else can change the momentum of a photon without replacing it with a new one.

Thanks. Rest mass of a photon? I thought it was 0, but apparently, that hasn't been definitively confirmed!

Experimental checks on photon mass
Current commonly accepted physical theories imply or assume the photon to be strictly massless. If photons were not purely massless, their speeds would vary with frequency, with lower-energy (redder) photons moving slightly slower than higher-energy photons. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the upper bound on speed that any object could theoretically attain in spacetime.[31] Thus, it would still be the speed of spacetime ripples (gravitational waves and gravitons), but it would not be the speed of photons.

If a photon did have non-zero mass, there would be other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would allow the presence of an electric field to exist within a hollow conductor when it is subjected to an external electric field. This provides a means for precision tests of Coulomb's law.[32] A null result of such an experiment has set a limit of m ≲ 10⁻¹⁴ eV/c².[33]

Sharper upper limits on the mass of light have been obtained in experiments designed to detect effects caused by the galactic vector potential. Although the galactic vector potential is large because the galactic magnetic field exists on great length scales, only the magnetic field would be observable if the photon is massless. In the case that the photon has mass, the mass term ½ m²A_μA^μ would affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass of m < 3×10⁻²⁷ eV/c².[34] The galactic vector potential can also be probed directly by measuring the torque exerted on a magnetized ring.[35] Such methods were used to obtain the sharper upper limit of 1.07×10⁻²⁷ eV/c² (the equivalent of 10⁻³⁶ daltons) given by the Particle Data Group.[36]

These sharp limits from the non-observation of the effects caused by the galactic vector potential have been shown to be model-dependent.[37] If the photon mass is generated via the Higgs mechanism then the upper limit of m ≲ 10⁻¹⁴ eV/c² from the test of Coulomb's law is valid.

[Chris Peterson]

So is a photon that drops in energy while traveling along a space-time geodesic still the "same photon"?

Sure. A photon only has a few properties. The invariant ones are its rest mass and speed (those are actually related). The only meaningful quantum property is its momentum. That changes due to its motion through expanding space. In fact, I don't think anything else can change the momentum of a photon without replacing it with a new one.

Thanks. Rest mass of a photon? I thought it was 0, but apparently, that hasn't been definitively confirmed!

Experimental checks on photon mass
Current commonly accepted physical theories imply or assume the photon to be strictly massless. If photons were not purely massless, their speeds would vary with frequency, with lower-energy (redder) photons moving slightly slower than higher-energy photons. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the upper bound on speed that any object could theoretically attain in spacetime.[31] Thus, it would still be the speed of spacetime ripples (gravitational waves and gravitons), but it would not be the speed of photons.

If a photon did have non-zero mass, there would be other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would allow the presence of an electric field to exist within a hollow conductor when it is subjected to an external electric field. This provides a means for precision tests of Coulomb's law.[32] A null result of such an experiment has set a limit of m ≲ 10⁻¹⁴ eV/c².[33]

Sharper upper limits on the mass of light have been obtained in experiments designed to detect effects caused by the galactic vector potential. Although the galactic vector potential is large because the galactic magnetic field exists on great length scales, only the magnetic field would be observable if the photon is massless. In the case that the photon has mass, the mass term ½ m²A_μA^μ would affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass of m < 3×10⁻²⁷ eV/c².[34] The galactic vector potential can also be probed directly by measuring the torque exerted on a magnetized ring.[35] Such methods were used to obtain the sharper upper limit of 1.07×10⁻²⁷ eV/c² (the equivalent of 10⁻³⁶ daltons) given by the Particle Data Group.[36]

These sharp limits from the non-observation of the effects caused by the galactic vector potential have been shown to be model-dependent.[37] If the photon mass is generated via the Higgs mechanism then the upper limit of m ≲ 10⁻¹⁴ eV/c² from the test of Coulomb's law is valid.

Maybe not. But I'll be surprised if it turns out to be anything else. It is, of course, the rest mass that is zero. Which is why it cannot ever be at rest, or have any speed other than c. Because it is moving, it has what is an effective mass (which is consistent with having momentum). And that makes sense, given that mass and energy are equivalent.