Light is a form of matter, so the question is strange. All zero mass particles share the property of going at that speed, and no other. No massive particle can go at that speed. Remember: mass means mass in the particle's test frame, which, for a photon, does not exist, so saying a photon has rest mass zero is, in fact, misleading. Rest mass is an attribute not appropriate to light.
Most simply, because momentum and energy wouldn't be conserved at the same time, assuming that "matter" is what has non zero rest mass.
Ok for you all, but wouldn´t be good to consider the last two elements of "that" equation: C2.?
“Why light, at "that" speed, does not become matter?
Remember E=mc2”
To answer on this question is necessary before to understand/to define – what is “matter”?
To make the above is necessary before to understand/to defile – what is the notion/phenomenon “Matter”?
The last is Meta-notion in the mainstream philosophy, where in the two main opposite doctrines “Materialism” and “Idealism” the existence of two main notions/phenomena, “Matter” and “Idea”/ “Spirit”/“Consciousness” [further “Consciousness”] are postulated. Since both notions/phenomena are Meta-notions, they cannot be properly defined/rationally understandable in any of the doctrines above, in both doctrines both, Matter and Consciousness, are transcendental, and, correspondingly, further in Nature sciences, including physics, they are transcendental undefined notions/phenomena also.
In the reality all what exist in the system “our Universe= Matter+Consciousness” is are some informational patterns/systems of the patterns that are elements of the absolutely fundamental and absolutely infinite “Information” Set/System; Matter and Consciousness are only some sub-Sets/sub-Systems. That is rigorously proven in the “The Information as Absolute” conception https://www.researchgate.net/publication/260930711_the_Information_as_Absolute
In the system “our Universe” both sub-systems above are fundamentally different, at that the sub-system “Matter” includes all/every informational objects that mutually interact by using a true information only, the light/photons are such informational objects [some uninterruptedly running close-loop algorithms], as any other material object/system, where constituting the object/system fundamental [as defined in physics] particles are some uninterruptedly running close-loop algorithms also.
Thus the light is material.
That is another thing that the notion “matter” in everyday life [and for a lot of physicists] relates to some objects that have some “measurable weight” masses since they consist of particles having “rest masses”. But in the reality the mass has two fundamental mentions - it is measure of inertia [the inertial mass] and it is a measure of the Gravity force [the gravitational mass]. And both masses are completely universal; including photons have both masses also.
The unique difference – when all/every particle [an so every system of particles, e.g., a galaxy] move uninterruptedly with the 4D standard speeds of light in the 4D sub-spacetime of Matter’s [5]4D absolute Euclidian spacetime, particles that have “measurable” masses move with 4D speeds [i.e. having obligatorily non-zero the speeds’ components in 4-th, the “coordinate time” dimension], when photons move in the 3D space only,
having, at that 4D [3D for photons, but that is in the same 4D sub-spacetime] momentums P=mc [“c” means a [here 4D] vector] and energy E=mc2. Historically the letter “m” in physics relate only to the having “rest mass” particles, including the rest mass is usually “m0”.
Besides, though, in recent physics only “invariant mass” is defined as the mass, since such convention is sometimes convenient at calculations; but, again, the mass is, first of all some measure of fundamental properties of material objects, i.e., of inertia and gravity force, when both – the inertia of and gravity force that acts on an material object depend on the speed of a 3D spatial motion of the object, when they have the masses “m” above, and thus this convention is rather artificial an has rather limited physical meaning.
Thus the formulae for the momentum and energy above are applicable to every particle, including to photons, in last case m=E/c2=ћω/c2, P= ћω/c2c.
By the way, returning to “measurable” masses – in every interacting system of particles, say, in an Iron brick, some, though very small, but non-zero, part of its mass is the mass of photons that constantly exist in the brick making it be solid.
Cheers
Article the Information as Absolute
The answer “why c2” is rather simple. That is because of Matter’s spacetime is [5]4D Euclidian manifold in physical theory and [5]4D Euclidian “empty container”, where all/every material objects and the system “Matter” as a whole exist and uninterruptedly change, in the objective reality.
This every material object constant changing exists/proceeds as changing of the objects’ spatial 3D positions and/or changing of the objects’ internal states
[the changing of the internal state is accompanied by changing of the 4-th “coordinate time” temporal interval]
as constant objects’ motion with the speed of light simultaneously in the 1D 5-th, i.e. “true time” dimension, and with 4D the speed of light in the 4D sub-spacetime.
At that Matter’s 4D sub-spacetime is not only Euclidian, it is fundamentally “Cartesian”, in this sub-spacetime the “coordinate time” axis is orthogonal to any spatial 3D line, including, every of 3 spatial axes.
The relation E=mc2 is applied first of all to the particles/objects that have “rest” masses, i.e. that were created by some momentums having non-zero 4-th component value
[usually, i.e. in the SR now, the temporal component is the zero component, but in the SR two “times”, i.e., “simply time”, which is zero dimension in the Minkowski 4D pseudo space, and the “proper time” act also, but the temporal dimension in the SR is a mix of really existent true and coordinate times. So 4-th axis/vectors’ component is the zero axis/component in this case also, that isn’t too inessential in this case though].
Thus if a having rest mass particle is at absolute 3D spatial rest, it moves in the coordinate time only with the speed of light, having the momentum P=m0c.
If after a spatial impact the particle moves in the space with a speed V also, its speed in coordinate time decreases, as that Pythagoras prescribed in (1-V2/c2)1/2, at that the momentum remains be P=mc, but the mass is as m=m0/(1-V2/c2)1/2.
The energy is a measure of the capability of an object to make some work; correspondingly to accelerate the object up to a speed V is necessary to make corresponding work also.
The work is W=∫Fdx=∫(dP/dt)dx=∫ c(dm/dt)dx and is equal to (more see, e.g. first 6 pages in https://www.researchgate.net/publication/259463954_Measurement_of_the_absolute_speed_is_possible)
Pfc-Psc, where Pf and Ps are the momentums of, say, a particle after and before acceleration; and, correspondingly, Pfc-Psc=Ef -Es.
I.e., again, E=Pc=mc2.
Cheers
Article Measurement of the absolute speed is possible?
Quarks, leptons, Higgs particle and massive gauge bosons, satisfy energy momentum relation E2= p2c2 + m02c4. This relation is satisfied by real particles on the mass shell. For photons on-shell condition therefore becomes, E= +/- pc, where p denotes it's 3-momentum. This is a fundamental difference.
Now why photon is mass-less? It turns out that by Higgs mechanism SU(2)L x U(1)Y symmetry breaks down spontaneously; however a residual U(1)em survives which forces photon to remain mass-less. Lagrangian density must therefore be invariant under U(1)em ; not just under the Lorentz symmetry which is a space-time symmetry in contrast with U(1)em which is an internal symmetry related with the invariance of Lagrangian density under field redefinition.
“…Quarks, leptons, Higgs particle and massive gauge bosons, satisfy energy momentum relation E2= p2c2 + m02c4. This relation is satisfied by real particles on the mass shell. For photons on-shell condition therefore becomes, E=pc, where p denotes it's 3-momentum. This is a fundamental difference…”, etc.
in the equations above there is no fundamental difference, including, for example, since p in the first equation is 3D spatial momentum also;
when in the Matter’s [5]4D Euclidian/ “Cartesian” spacetime, concretely in the 4D [t,X,Y,Z] sub-spacetime, where t is the coordinate time, all/every particle, independently on they have or not a “rest mass” and so are or not on “the mass shell”, move with the 4D speeds having absolute values be equal to the speed of light and so having 4D momentums P=mc. Photons simply have 4D momentums (0, P). [All/every material objects also always simultaneously move along 5-th, “true time” axis/dimension, with the speed of light]
Such situation in Matter happens since, though the Rules/Possibilities “Space” and “Time” are rather similar [including every fundamental change in all, i.e. of spatial position and of internal state of a particle is accompanied by “equal footing” spatial and [both] temporal [in “ct” metrics] intervals,
nonetheless they are different [more see the papers in the links in the SS posts above], and so the particles can be, and are, created by two different ways: by purely spatial impacts [with transmission of 3D spatial momentums], an example – photons, and by momentums having non-zero coordinate time component, in this case a particle has a rest mass m0 and always constant zero component of its 4D momentum pt=m0c [if it is impacted by some purely 3D spatial momentums, what happens practically always if the impacts aren’t at particles interactions, an example – acceleration of particles in the accelerators.]
As well as every particle always has the energy E =Pc=mc2; if it moves in the space also with a speed V, the spatial momentum’s component is equal p=mV, the 4D momentum is (m0c, mV).
Thus the first equation in the quote above is simply an application of Pythagoras theorem.
And, besides, the fact that photon hasn’t a rest mass isn’t a consequence of that “Higgs mechanism SU(2)L x U(1)Y symmetry breaks down spontaneously”
– such assertions are nothing more then some ad hoc claims that have no grounds, here – no physical reasons/grounds why this symmetry breaks and why just by such way. Besides from so called “Higgs mechanism” by no means follow the another “mass”, i.e., the gravitational mass; and, of course, there is no any even suggestions why the inertial and gravitational masses are equivalent at least at the statics.
When in the reality all here is rather simple, if one understands what are the Meta-notions/phenomena space/time/spacetime and that Matter is only some absolutely frank informational dynamic system, which, and every material object as well, uninterruptedly changes/moves in the [5]4D absolute Euclidian spacetime.
And, returning to the “mass” notion: since every particle is uninterruptedly running close loop algorithm, it is some gyroscope, which resist to attempt to change its 4D rotation rate value ω or/and direction, what is observed as the “inertia”, which has the measure “inertial mass”; and, since rather probably, every particle radiate the graviton at every the algorithm’s tact, both, the inertia and the gravity potential, are proportional to the same rotation rate and so are equivalent [equal, if corresponding mass units are equal by convention as that is in physics]
Cheers
Thierry is right.
Photons of energy 1.022 MeV or more will convert to pairs of massive electron-positron when grazing a massive particle such as an atomic nucleus.
In 1999, a team led by Kirk McDonald even succeeded in having electron-positron pairs created by colimating two photon beams to a single point in space.
See this paper titled "Positron Production in Multiphoton Light-by-Light Scattering":
http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-7444.pdf
I guess the argument that
gamma-> e+ + e-
is impossible, since energy and momentum cannot both be conserved, is unquestionably correct. In the immediate vicinity of a nucleus, neither momentum nor energy need be conserved, since the nucleus can, in fact, absorb both energy and momentum.
On the other hand, the paper quoted by André, undoubtedly very beautiful, is not a counterexample either, since it considers a system in which 4 or 5 photons join to produce an electron-positron pair.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Theoretical Physics