Magical Realist
Valued Senior Member
Can someone explain to a layman the concept of rest mass of a photon and how. if a photon has mass, it is able to move at the speed of light?
Rest mass of a photon
- Thread starter Magical Realist
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Photons are force carriers of the electromagnetic force, which has infinite range. As such, they must be massless. The same thing goes for gravitons, if they exist. The carrier particles for the strong and weak forces have mass, which means those are short-range forces.
Another thing to say, in light of the discovery of the Higgs field, is that photons don't couple to the Higgs field, so therefore they are massless.
As for moving at the speed of light, every massless particle must move at the speed of light.
This might be a bad way to think about it, but consider Newton's law a=F/m. If you apply force F to mass m, its acceleration is a. If m happens to be zero, then you get infinite acceleration with any applied force. But relativity prevents anything from moving faster than light, so massless particles all move at the speed of light - the fastest speed there is.
Looking at some ideas about photons having a tiny microscopic itisy bitsy wee bit of mass with a decay 1/2 life longer than the estimated life of the Universe
Along with a WHY
Why is the speed of light what it is? I can understand it cannot be Zero or Infinity but why has it settled for the speed it has? Why not say 100 klm faster, or slower?
Is there some reason/restriction which makes the speed of light what it is?
Magical Realist
Valued Senior Member
If a photon has no mass, then how can it be affected by gravity?
Janus58
Valued Senior Member
One way to answer this is that gravity doesn't just couple via rest mass, but the source of gravity is the stress energy tensor, of which rest mass is just one contributing factor. Photons have energy/momentum and these also contribute to the stress energy tensor, which means that photons will interact via gravitational fields.
Another way to answer this is that gravity is an alteration of the geometry(curvature) of space-time. And light always follows the geodesic( shortest path). While in Euclidean geometry, a straight line is the shortest distance, space-time geometry near a source of gravity deviates away from the Euclidean ideal.
Write4U
Valued Senior Member
Please do not post pseudoscience to our Science subforums.
IMO, the speed of light is the fastest possible way reality is able to refresh itself via quantum change.Looking at some ideas about photons having a tiny microscopic itisy bitsy wee bit of mass with a decay 1/2 life longer than the estimated life of the Universe
Along with a WHY
Why is the speed of light what it is? I can understand it cannot be Zero or Infinity but why has it settled for the speed it has? Why not say 100 klm faster, or slower?
Is there some reason/restriction which makes the speed of light what it is?
Quantum change needs time, it cannot happen all at once. This upper refresh limit turns out to be at SOL, the quantum refresh rate of massless particles. IOW, reality unfolds at SOL ?
I believe this is the foundation for CDT (causal dynamical triangulation), the smallest physical constituent of spacetime.
https://en.wikipedia.org/wiki/Causal_dynamical_triangulation
This is heady stuff!
But what intrigues me most, is quantum change itself. This is a remarkable phenomenon that reality itself changes form from quantum moment to quantum moment. If CDT evolution of spacetime itself cannot exceed SOL what are the implications?
When a change happens in a finite period of time, where is the line when the change happens? Is reality a chronology of ON/OFF moments. A Dynamic duality?
Would this result in the strange condition that at any given time half of the physical mass in the universe is expressed physically and the other half is in a non-physical state of quantum suspension (universal quantum evolution) ?
Einstein painted a picture of riding a light beam and time appears to stand still in the physical world. IOW, in the middle of a quantum moment. Which reality do we see?
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Possibility. Not heard of that as explanation
I read this some time ago and filed it away under Nuggets to Keep
Speed of Light is Planck Velocity
The interesting part about Planck units and the speed of light is that if you then divide Planck length by Planck time, you will get exactly the speed of light in a vacuum
Does that fit with your nugget?
Might have reference to it somewhere
Bit more
So what does that even mean about the speed of light? This means that the fastest anything can travel is exactly 1 length unit per 1 time unit.
Only change this Minion, with no qualifications in the field, would be to change 1 length unit per 1 time unit to combine the two into one and write as one age unit
How about that for 7 am Saturday morning and no breakfast or coffee yet?
Will go fix that now and come back later to this fascinating subject
DaveC426913
Valued Senior Member
This is a hard science forum, and the OP is asking for a basic explanation of a basic principle. Perhaps we could forego the more ... speculative opinions?
DaveC426913
Valued Senior Member
Photons have no rest mass.
However, they do have energy. And Einstein's formula works both ways.
Since E=mc^2, it follows that m=E/c^2. That is not the same as saying photons "have" mass, but because of the energy they carry, they do interact with the world as if massive.
However, they do have energy. And Einstein's formula works both ways.
Since E=mc^2, it follows that m=E/c^2. That is not the same as saying photons "have" mass, but because of the energy they carry, they do interact with the world as if massive.
In Einstein's universe, gravity is not a force that affects mass; gravity is a curvature of space time. Photons travel along a geodesic through this curved space time - essentially, the geodesic is what is seen by the photon as "straight".
Magical Realist
Valued Senior Member
If a photon was standing still, would it still be affected by gravity?
https://io9.gizmodo.com/scientists-freeze-light-for-an-entire-minute-912634479
DaveC426913
Valued Senior Member
It can't - it wouldn't be a photon - so it's a moot question.
If someone asks how the laws of physics would be if the laws of physics were different, the standard answer is: "you'd get unicorns" (because changing laws means you can change it however you want.)
Seems a facetious answer, but it's not.
"Light is never really "standing still" despite the article headlines. It can be stored and sections of the wave nature, overtaking other parts of the spectrum...
The following puts it better then I.....
https://www.google.com/search?q=how can they make light stand still&oq=how can they make light stand still&aqs=chrome..69i57.10671j0j7&sourceid=chrome&ie=UTF-8
"By toggling the set-up in such a way that the waves can gain or lose energy, the lightwaves can be made to coalesce and freeze, or speed up and resume their journey out the other side. If we're to be particularly pedantic, we shouldn't imagine it as photons standing still waiting for the go signal."
In other words, its a trick to put it mildly. [Not that some of these "tricks"may not one day prove beneficial.
https://www.sciencealert.com/using-exceptional-points-coalescing-waves-stopping-light
While a photon is never standing still, photons/light do affect the geometry of spacetime [gravity] ever so slightly, due to their momentum. The same reason that light sails work.
Magical Realist
Valued Senior Member
Does a photon travel along a worldline in spacetime? I thought time ceased to exist at the speed of light.
Photons/light follow geodesics in spacetime. It is pointless asking about time from the frame of reference of a photon as there is no such thing in that frame. A photon [from its own frame] will cross the universe in an instant.
https://phys.org/news/2014-05-does-light-experience-time.html
Magical Realist
Valued Senior Member
If a photon follows a curved geodesic thru spacetime, then it is accelerating. How can it accelerate past the speed of light?
Is a geodesic 3 dimensional motion in space or 4 dimensional motion in spacetime?
Is a geodesic 3 dimensional motion in space or 4 dimensional motion in spacetime?
Because light is an exception, to the rule, in that it is already at and maintains constantly the universal maximum speed...the curvature of spacetime caused by photons/light, changes the geometry of the spacetime. I would also envisage time dilation and this may help.....
http://curious.astro.cornell.edu/ab...force-how-does-it-accelerate-objects-advanced
or this answer......
https://www.quora.com/Does-a-photon-pulled-by-a-gravitational-field-accelerate
We must first define acceleration to get a clear answer:
Acceleration(from Physics classroom):
- Acceleration is a vector quantity that is defined as the rate at which an object changes its velocity. An object is accelerating if it is changing its velocity.
Based on the information above, some people might quickly reach to the conclusion that light does accelerate near a strong gravitational field because it goes on a curved path.
But that is not right.
See the thing is that when light is travelling through the curved space-time, it is always going in a straight line, it just appears to be curved because of the curved space-time. This illustration shows you what I mean:
So the answer to your question is: No, light is does not accelerate near a gravitational field.
Hope you find it helpful!
https://en.wikipedia.org/wiki/Geodesics_in_general_relativity
A naive way to think about photons having zero rest mass and yet interacting with a gravitational field, is that the curvature in spacetime doesn't change the photon's energy, but it does make it appear to follow a longer path.
But that longer path depends on an independent observer who can 'see' the curvature in space around a massive object--at some remove, that is.
But that longer path depends on an independent observer who can 'see' the curvature in space around a massive object--at some remove, that is.
Magical Realist
Valued Senior Member
What is the source of the photon's momentum if it has no mass?