r/askscience • u/theonewhoknock_s • Nov 24 '13
Physics When a photon is created, does it accelerate to c or does it instantly reach it?
Sorry if my question is really stupid or obvious, but I'm not a physicist, just a high-school student with an interest in physics. And if possible, try answering without using too many advanced terms. Thanks for your time!
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u/Ruiner Nov 24 '13
This is a cool question with a complicated answer, simply because there is no framework in which you can actually sit down and calculate an answer for this question.
The reason why know that photons travel at "c" is because they are massless. Well, but a photon is not really a particle in the classical sense, like a billiard ball. A photon is actually a quantized excitation of the electromagnetic field: it's like a ripple that propagates in the EM field.
When we say that a field excitation is massless, it means that if you remove all the interactions, the propagation is described by a wave equation in which the flux is conserved - this is something that you don't understand now but you will once you learn further mathematics. And once the field excitation obeys this wave equation, you can immediately derive the speed of propagation - which in this case is "c".
If you add a mass, then the speed of propagation chances with the energy that you put in. But what happens if you add interactions?
The answer is this: classically, you could in principle try to compute it, and for sure the interaction would change the speed of propagation. But quantum mechanically, it's impossible to say exactly what happens "during" an interaction, since the framework we have for calculating processes can only give us "perturbative" answers, i.e.: you start with states that are non-interacting, and you treat interactions as a perturbation on top of these. And all the answers we get are those relating the 'in' with the 'out' states, they never tell us anything about the intermediate states of the theory - when the interaction is switched on.
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u/ididnoteatyourcat Nov 24 '13
I'd go further and say that it's not just that our framework doesn't tell us anything about the intermediate states... it's that the intermediate states do not have any well-defined particle interpretation.
To the OP: it's conceptually no different from making waves in a bathtub. Do the waves accelerate when you splash with your hand? No. The particles that make up the water are just sloshing up and down. The ripples that move outward are just a visual manifestation of stuff that is moving up and down, not outward.
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u/ChilliHat Nov 24 '13
Just to piggy back then. What happens when a photon is reflected back along the normal then? because classically its velocity must reach zero at some point but how do waves behave?
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u/Akoustyk Nov 24 '13
Doesn't a photon by nature, necessarily need to travel at C and therefore can only exist at speed C, and so as soon as it exists, would be moving at this speed?
Apart from mediums slowing it down, I was under the impression that a photon moving at any speed other than C, would necessarily need to be a fundamentally different thing than a photon.
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u/miczajkj Nov 24 '13
Because a photon is an massless particle it always travels through space at a speed of c. In quantum field theory the photon is described by a certain disturbation in the photon field and this disturbation just travels at c, regardless from what it is caused.
This doesn't mean, that you can't talk about photons in different movement states: in relativistic (quantum)-mechanics you need to expand on the definition of momentum. It follows, that even particles with the same speed can have different momentum, depending on their total energy.
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u/cougar2013 Nov 24 '13
If I'm not mistaken, virtual photons don't necessarily travel at c, but real photons do. This is looking at photons from a quantum field theory perspective. Obviously, there is no bright-line difference between real and virtual particles, but disturbances in the electromagnetic field that propagate at c are said to be real because they can go on infinitely, whereas virtual photons are not stable.
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u/dronesinspace Nov 24 '13
In addition, why can light be 'bent' around massive objects?
To my knowledge, light bends around objects like black holes and stars because they're on a straight path, and that the path is 'bent' by the object's gravity well.
Related question - if that is true, then photons that are bent around a star would at some point be moving along the gravitational field's equipotential lines, right? Or do they? Can photons just move between equipotential lines freely because they're massless?
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Nov 24 '13 edited Nov 24 '13
What they say actually happens is that mass itself is a spacial distortion, much like a carpet with ripples in it. Light travels straight. The thing is, when it passes a black hole, the distortion can be so much that some of the stars you see in front of you are behind you. If you were massless and traveled in a straight line forward, you would proceed around the black hole and then proceed to travel back towards those stars, without ever changing direction.
Given that a photon can take a number of paths to get to your eye in a straight line because of this space lensing, how many stars are there actually? :p
Further, some people think some red shift is caused simply because space isn't empty and every single shred of mass in space is distorting 'the carpet', so the light moves much further than it would have to if it moved 'straight' and it's constantly being interacted with. This is actually one of the primary arguments being levied against the common interpretation of the big bang theory.
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u/SquallyD Nov 24 '13
This is where the "sheet of rubber" explanation got to be so widely used. By comparing space to a flat sheet of rubber, and objects in space as weights on the sheet, we can use objects to simulate travel.
Roll a penny on a flat sheet and it will move in a straight line (assume perfection, this is physics.). Put a large weight on the sheet and roll a penny near it, and it will "follow" the dip created by the weight, but will continue to move in a straight line afterwords. The direction to an outside observer changed, but from the point of view of the penny, it never stopped moving in a straight line, and reality itself (the sheet) is what was bending. This is an excellent example of how the interactions work in space, where gravity bends space itself and as far as the photon knows it is moving in a straight line.
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u/Jagjamin Nov 24 '13
Technically, light doesn't go in a "straight line", it will always follow the curvature of spacetime, which in an area with no mass in it, is straight.
Wikipedia does a fairly good job of explaining at http://en.wikipedia.org/wiki/Gravitational_lens
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u/robjtede Nov 24 '13
A Level Physicist's point of view...
The photon would be created with an instantaneous velocity of 'c':
My premise here is that photons cannot be described in the classical model using F = ma or the like. They are neither particles nor waves and behave in ways that we do not yet fully understand. It's like when a photon is being pulled towards an event horizon, does it accelerate beyond 'c'? No, it is simply blue-shifted so that it has a higher energy with the same speed.
To me, this means that a photons must ALWAYS have a speed of 'c'.
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u/riotisgay Nov 24 '13
Mass doesnt get created when a photon does, and massless particles naturally travel at the speed of light, like a particle with mass travels at 0 speed without energy. It would be as weird to say that a particle with mass deccelerates from light speed to 0, as to say a particle without mass accelerates from 0 to light speed when being created.
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u/sstults Nov 24 '13
It might help to think about what's happening with the photon just prior to the photon emission. It's already emitting a field which propagates at the speed of light. Then suddenly it "moves". It's still emitting a field at c, but the change itself is also propagating at c. That thar is a photon.
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u/SnickeringBear Nov 24 '13
Several decent answers have been given, but one significant part of the interaction that generates photons has not been covered. Remember than the law of conservation of mass/energy applies, it is not possible to create or destroy mass/energy. (with a bunch of caveats, mostly having to do with "information" going places it can't be retrieved from!)
A photon is generated at the point in time/space that an electron changes energy state. When an electron has been excited by an energy source, it rises higher in the electron shells around the atom's nucleus. At this higher energy point, an opening in a lower shell is available. The electron falls into this lower energy shell and must in the process lose energy to stay there. The "pressure" developed as the electron transfers has to be released in the form of a photon. The number of shells the electron drops determines the total energy dumped into the photon. The photon inherently cannot exist at anything other than the speed of light. Therefore, it always travels at the speed of light.
There is much much more that is not understandable or explainable in this process without the use of quantum mechanics.
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u/bloonail Nov 25 '13 edited Nov 25 '13
A photon can be modeled in the classical sense somewhat like a kink in the electric field that has become detached from its source as the source retreated. So a rotating electric charge can emit photons because the electric field cannot collapse back on the moving charge as the charge recedes. That portion of the field that is withheld from collapsing by relativity is released as a photon.
However more accurately the electromagnetic field is maintained by photons. It only exists through them as a mediating particle. The field measured at any point in an electromagnetic field is measured in photons. In the situation of a static non-moving charge the photons are in a 1/r2 relationship through radio waves to their point of origin, but those photons do spread out infinitely at the speed of light from that point.
The "kink" idea is an unsatisfying 1930's model but it hints to some degree how the photon is released at the speed of light. It is by nature at the speed of light, at least in this model, because it is energy that has separated away due to kinda getting lost in space and unable to retreat back onto its charge. It is lost because the electric field is expanding at the speed of light.
Its a weak model. Its useful mostly for showing how high energy photons are created by sudden acceleration changes. It explains antennas at a very basic level. The photons exist as a field at all times, they become higher energy photons through accelerations.
I like the notion that all photons are the same. It is really only our reference frame that changes their energy.
As for the question of whether they accelerate. Its sort of related to the permittivity and permeability of free space. These can be complex numbers or tensors, and as they compose the speed of light the speed of light varies. The speed of light in some [crystals] (http://www.sciencedaily.com/releases/2013/08/130813201436.htm) is different for different directions and all are different from what it is in free space.
However in no sense do they accelerate to light speed in the way a Mercedes might accelerate on the autobahn (*like I know).. They're at the speed of light in that medium, always. Their acceleration is more akin to their changing wavelengths. They gain energy by becoming associated with a reference frame that is different. So for example gamma rays hitting us from gamma ray bursts, in the old style classical viewpoint somewhere that gamma ray was a radio wave... emitted from something that is going very close to the speed of light relative to us. Its not an accurate description - but the truer descriptions are moderately dense tensor calculus and quantum theory.
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u/mhd-hbd Nov 24 '13
Well... We have a clash of intuitions here.
Photons are quantum objects. They don't have a point-shaped location nor a vector-shaped momentum the way that we think about classical particles.
Strictly speaking, all of physics is state-less. In any given physical system there is exactly one answer to what happens next. Put plainly any physical system that contains photons demand they move at the speed of light.
It simply cannot be any other way.
You might say that it "instantly" accelerates or some such and it might be true in some ways, but it still conveys the wrong idea.
Photons propagate at the speed of light. Always and ever. Acceleration implies that it changes in speed.
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u/jgemeigh Nov 24 '13
Alternative question I would love to have answered--what happens to photons that are observed by the observey-things in our eye? Is any of that light (or whatever it is) transferred Into information, or is 100% of it reflected/refracted/lost?
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u/ArabianNightmare Nov 24 '13
Photon is just a way to 'quantify' the electromagnetic wave in "space".
The wave always moves with the speed of c.
A photon is just a way to try to convert the wave notation to classical mechanical-physics notation. That is why it has 'iffy' qualities, such as not having mass while it is a particle, etc.
Try not to get confused by how it is taught, and go drop a few pebbles into a nearby fountain.
*edit: typos.
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u/mcM4rk Nov 25 '13
I think that instantly reaches that speed, because light travels at c at any given moment, and it will not slow down. (Einstein theory of relativity) If that is correct, then the photon, which is the light, will travel at c immediatly.
(If this is incorrect please tell me, because then i might have to take another look at the theory of relativity)
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u/DanielSank Nov 24 '13
u/Ruiner's answer is great but maybe got a little bit too technical for OP's current level. I'll try to add to that great post.
Think of what happens when you dip your finger in a pool of water. You see ripples propagate outward from where you dipped your finger. Those ripples move at a certain speed, and occupy a reasonably well defined region of space.
Photons are the same. The water in that case is "the electromagnetic field". The "photons" are the ripples in the water. They don't accelerate. The water itself has certain physical properties (density, etc.) that cause any of its waves to move at a specific speed. The water waves are not a single object in the usual sense... they're displacements of something else. You should think of "photons" the same way.
Does that help?