Do Photons Accelerate From Rest?
Most recent answer: 10/22/2007
- Draken-Korin
Mike W
(published on 10/22/2007)
Follow-Up #1: photon acceleration?
- Mark
Wichita, Kansas USA
The energy and momentum just come from other particles and fields.
I think it would be fair to say that there is not any known reason that the universe has a finite speed limit rather than an infinite one. However, at least the mathematical description with a finite limit is logical and self-consistent.
Mike W.
(published on 10/22/2007)
Follow-Up #2: energy and light speed
- B K Chandrasekhar (age 53)
Bangalore, India
The velocity-dependent energy formula (mv2/2) familiar from classical physics only applies to particles with rest-mass traveling at much less than the speed of light.
In the vacuum, you can classically describe the light energy as entirely consisting of electric and magnetic field energies. In the glass, the field energy is reduced but the energy is still there in the kinetic energies of particles (mainly electrons) oscillating in response to the fields.
Mike W.
(published on 10/22/2007)
Follow-Up #3: Speed of deflected light
- Amol Jagtap , age 25
Pune , Maharashtra , India
LeeH
You might also be wondering what happens as light directly approaches or leaves a massive object. It would seem like its speed would change, but as far as any local observer sees, it always travels at the speed of light. There's no way to patch this up without using a different geometry of spacetime than we intuitively believe. The new geometry is called General Relativity. Mike W.
(published on 07/24/2009)
Follow-Up #4: photon momentum in glass
- Indrajit Kuri
New Delhi, India
Let's say that the glass has anti-reflection coating, so we don't have to worry about any photons being reflected. The wavelength λ of the light in glass is reduced. Using the quantum relation p=h/ λ, the momentum p per photon is increased.
How can that be? As the light enters the glass, there's an attractive electromagnetic interaction between the light and the glass. The glass is pulled back toward the direction the light came from. This tiny motion of the large glass mass has the necessary p to keep total p conserved. An opposite recoil occurs as the light leaves.
Mike W.
(published on 07/20/2010)
Follow-Up #5: photon momentum in glass
- Indrajit Kuri
New Delhi
Mike W.
(published on 08/19/2010)
Follow-Up #6: How far will light travel?
- Prabhakar (age 47)
Hyderabad,Andhra Pradesh,India
LeeH
(published on 07/28/2011)
Follow-Up #7: Speed of Light vs Speed of Sound
- Chris (age 57)
Glen Cove , NY 11542
I'm not sure I understand your other questions exactly but you're definitely concerned with the constancy of the speed of light, and a similar phenomena with sound.
The first thing I need to correct is your assumption that the speed of sound is exact. This isn't true. The speed of sound varies greatly from one material to another. For instance, the speed of sound in water, approximately 1480 m/s, is around 4 times greater than that in air at about 340 m/s. Things like solid metals often carry sound even quicker, iron for example at about 5100 m/s. But even in air the speed of sound is not constant, factors that affect it include humidity, temperature, and pressure among others.
In most materials the speed of sound is defined as
s=(P/ρ)^(1/2)
Where P is the bulk modulus (or bulk elasticity for gasses) which is basically a measure of stiffness for a given material, and ρ is the mass density.
Next, the speed of light isn't actually constant either. When we say the phrase, the speed of light, we're usually talking about the speed of light in a vacuum. This value is accepted to be constant, but light does slow down when it propagates through matter. It slows by a factor called the Refractive Index, which typically increases with a materials density. Light however can act as a particle on its own and doesn't need any sort of matter to be present in order to propagate so we say it has no medium.
It used to be thought that light traveled through a medium, the so called "luminiferous aether", in the same way sound travels through air or some other material. Without getting too much into it, experiments like the have shown strong evidence against the medium and in support of Einstein's special relativity.
Now if light didn't work exactly the way it did, well the universe as we know it today wouldn't exist. But if in its place were something just a tiny bit different and in that quasi-universe there evolved something a bit like the animals we have here on earth then they would likely evolve a similar sense of sight to some animal currently on earth. Evolution is a process that rewards the ability to pass on genetic code. Pretty much all matter interacts with light in one way or another, and things that want to eat you, or you want to eat, are no exception. So being able to see your food/predators is very beneficial in living long enough to pass on one's genes.
So to put it in a sentence. Evolution follows physics, not the other way around.
Thanks for the question,
Mike Boehme
(published on 03/31/2012)
Follow-Up #8: photons in water
- chris (age 58)
Glen Cove , NY,Nassau
As photons go into materials, they become something a little different, dressed in excitations of electrons. Assuming no reflection, the net energy E of the whole packet stays essentially the same, since the frequency f doesn't change and the relation E=hf is universal. However, the wavelength λ becomes shorter, so the momentum p of the packet goes up, since p=h/λ is also universal. (The momentum conservation is maintained by a slight recoil of the whole material.)
So since p goes up and v goes down, if you define inertial mass as p/v, it goes up by a factor (c/v)2=n2, where n is the index of refraction. In a typical material with electric but not magnetic susceptibility at optical frequencies, this is just a factor of ε, the electric susceptibility.
Mike W.
(published on 08/01/2012)
Follow-Up #9: Does light need a medium?
- George Mavros (age 68)
NYC, NY. USA
As for dark matter, we know where it is because of its gravitational effects on visible matter. Light propagates just fine through regions where it's dense and where it's very sparse. Other than the weak gravitational effects, light and dark matter don't influence each other.
What about dark energy? Since at least in the current epoch, it seems to be everywhere, we can't make the same sort of argument we made about dark matter. Conceivably a full understanding of dark energy and a deeper understanding of light could both end up pointing to some Calabi-Yau geometry of some underlying string field, which I guess you could call a "medium". That is not only way over my head but also beyond the current state of the art.
Mike W.
(published on 09/02/2012)
Follow-Up #10: Is there an ether
- George Mavros (age 68)
NYC, NY. USA
In a standard coordinate frame, gravitational fields are very weak in most places. In some places they are zero. You can pick other coordinate frames which shift the places where the field is zero. None of that does anything to the transmission of light.
Because of the large variability in the density of dark matter, we can conclude with a lot of confidence that it's essentially irrelevant to the transmission of light.
We've discussed elsewhere that the question of whether or not there can be a perfect vacuum relies too much on semantic choices to be worth pursuing, at least at our current level of understanding. Maybe once a quantum theory of gravity is developed that will change.
Perhaps more importantly, what we mean by exploring these issues is not to just look for some satisfying words but rather to look for some mathematical structures that tell us something correct about what to expect to see in the world.
Mike W.
(published on 09/04/2012)
Follow-Up #11: motion and gravity
- Rodger (age 27)
Mesquite, TX. U.S.
Relativity doesn't say "time moves differently for those of us moving slower than those of us moving faster" because it denies that the terms "moving slower" and "moving faster" have any objective meaning, except within a particular choice of reference frame. Nobody is moving in their own frame. It is true that the time intervals assigned to events differ between frames moving with respect to each other, but which way the differences go depend on how the events are moving with respect to the frames.
We can assign a velocity to everything we see without changing the laws of physics describing things. Nevertheless, the most obvious convenient choice is to set the average velocity of everything within range of us to zero. The easiest way to do that is to say that the CMB coming in from all directions is equal in the best frame. Using that natural frame, our galaxy is in motion at about 600 km/s. (see )
Mike W.
(published on 03/06/2013)
Follow-Up #12: photon basics
- Nathan (age 25)
Casa Grande, Az, USA
You can treat a photon as having a specific mass, its energy divided by c2, so long as you're careful about the meaning of 'mass'. Here you'd mean the ratio of the momentum to the velocity. Some people use 'mass' just to mean rest-mass, and the rest-mass of photon is zero.
The key point is that photons do not exist in fixed numbers. They can be created and destroyed. They do not sit at v=0 and then somehow accelerate up to v=c. When they exist, they are always traveling at speed c.
The conversion of other forms of energy to electromagnetic (photon) energy can be described in some cases simply as driven by the acceleration of charged particles, just like in the classical theory of electromagnetism. In quantum electrodynamics there are other specific rules governing the likelihood of energy present in charged particles getting converted to EM field energy, and you need these more subtle rules to predict, for example, how long a hydrogen atom typically spends in an 'excited state' before it falls to the lowest-energy state, emitting a photon.
Mike W.
(published on 05/16/2013)
Follow-Up #13: Many cosmic question
- Kevin Henriksen (age 42)
Australia
Q: "If neutrinos are heavier and slower than photons, why do neutrinos go straight through objects while photons can be refracted and slowed down in different mediums?"
A: Neutrinos don't participate in electromagnetism. The interact only via gravity and the weak nuclear force. Those are both far too weak to cause the sort of slowing that affects photons.
Q: " Why did neutrinos arrive from Supernova 1987A before the photons, after traveling for 168,000 years? Shouldn't the photons have had plenty of time to catch up and pass the neutrinos if they travel slightly faster?"
A: No. They were catching up, but not nearly enough to make up for the initial photon delay in getting out.
Q: "Is the speed of light actually constant or does it change with time as universal gravity lessens as the universe expands?"
A: So far no measurement has indicated that the fundamental constants are changing.
Q: "Did the universe start with a slow bang and gradually but very slowly speed up as universal gravitation lessens?"
A: Is there a theory along those lines? Does it give anything like the extraordinary detailed fit to the data (e.g from the Planck satellite) given by the standard inflationary Big Bang theory?
Q: "Do photons have mass?" A: see
Q: "Can entrophy decrease inside black holes?"
A: No.
Mike W.
(published on 08/22/2013)
Follow-Up #14: photon physics
- Kevin Henriksen (age 42)
Australia
Let's start with one I know:
"Is there a way to record the temperature of a photon?: A photon has no temperature. Temperature describes a probability distribution for being in states with different energies. "A photon" sounds like it's in a single definite state.The only single state that has a definite temperature is the state of lowest energy, with T=0, which has zero photons.
" Why were photons delayed in getting out of Supernova 1987A? ": I guess the photons had to work their way out through a plasma of charged particles. They would scatter and be absorbed/emitted many times, unlike the weakly-interacting neutrinos. Something like that happens with photons, but not neutrinos, coming from the Sun.
"With mass constant and the universe expanding, wouldn't universal gravitation lessen over time? " : The gravitational tension pulling the whole universe back together would (and did) lessen over time. However, unless there's some completely new physics not yet understood and not evident in the behavior of galaxies, that has no effect on the more familiar symptoms of universal gravitation- the attractions of stars, galaxies, etc.
"How do you know what is happening entropy-wise inside black holes? " I think it's fair to say that, at least at the moment, we don't quite know what's going on entropy-wise inside a black hole. (see the many recent arguments about black hole firewalls) Viewing the black hole from the far outside, several lines of argument (beyond my ability to reproduce) say that its entropy is A/4, where A is the horizon area in Planck units. (see , ) I'll update this answer if I can find a good explanation.
"Aren't black holes re-organizers of matter?" Yes, but so is everything else interesting, e.g. you. The question is whether, as viewed from the outside, black holes do something special, that is change quantum states in a "non-unitary" way, in which a single state turns into a probabilistic mixture of states, not just a more complicated-looking single state. The current consensus leans toward answering "no, it's unitary" but the confidence on that answer has recently dropped. If the answer changes to "yes, it's non-unitary", then maybe we'll also have to re-evaluate whether less exotic systems (e.g. you) are also somewhat non-unitary. That would mean an enormous revolution in basic physics.
"How do polarized sunglasses block 50% of the light and polarize the other 50% no matter what angle they are tilted to the incoming light? " : A better way to think of this is that the incoming light was already a mixture (usually about 50/50) of the two orthogonal linear polarizations. The polarizer absorbs one but not the other. (See ) In practice, they don't work perfectly, and will transmit a little of the blocked polarization and block a significant amount of the transmitted polarization. Those percents will actually depend some on the whether the polarizer directly faces the light or is tilted. Perhaps what you mean is how can they work the same even as they're turned at different angles, still directly facing the light. Here the key idea. Because the equations describing the wave propagation are linear, you can get the behavior of the whole wave by summing the behaviors of any parts you break it into. You can take the same wave and break it into orthogonal polarization components along any two right-angle axes. That's because the electric fields are vectors, and you can express vectors in any basis set that you choose. So you just pick a basis where one axis is along the direction the polaroid absorbs and the other is at right angles to it.
Mike W.
(published on 08/22/2013)
Follow-Up #15: photon energy and frequency
- Mohan (age 61)
chennai, tamil nad, india
This is one of those simple-sounding questions whose answer will have to go deep into the unfamiliar world of quantum mechanics. As a result, you will probably find it unsatisfying. If some reader can think of a better way to convey the ideas, we'd love to hear it.
Q: "What oscillates in a photon? "
A: A photon, like any other quantum state, has a "phase", a complex number that rotates in the complex plane. The frequency is the frequency of that rotation.
Q: "Also, how such an oscillation decide the energy of the photon?"
A: This one is easy. That quantum frequency is exactly the same thing as energy, always, for everything, not just photons. They're just two different names for the same thing. Often different units are used when the name "energy" is used and when the name "frequency" is used. The unit conversion factor is called Planck's constant.
So the interesting question becomes: what does that abstract quantum frequency have to do with the more familiar frequencies you measure, such as the rate at which a classical electrical field might oscillate? All those classical frequencies come from beat frequencies between quantum states with different frequencies (or, you could say, energies). As they go in an out of phase, interference terms between them change, so "things" move around and classical fields change. It's easier to visualize this idea with states that represent the fuzzy positions of particles rather than photon states. It turns out that electrical field oscillations come from beats between states with different numbers of photons. That sounds like it means that a state with one photon (or any other definite number) would not have an oscillating electrical field. And that conclusion is indeed right- a state with a definite photon number has an unchanging distribution of possible fields centered on zero.
Mike W.
(published on 11/10/2013)
Follow-Up #16: photon mass in matter
- Hisham Ragheb (age 32)
Bolkely, Alexandria, Egypt
Hi Hisham- Good question. I've placed it as a follow-up to some related questions.
The weight of a photon is just given by ghf/c2, where g is the gravitational field, h is Planck's constant, f is the photon frequency, and c is the speed of light. That weight just depends on one property of the photon: energy, hf. That doesn't change as the photon goes into some material and changes into a different sort of particle, moving more slowly.
Mike W.
(published on 11/21/2013)
Follow-Up #17: understanding why light travels at c
- Patrik Amethier (age 18)
Stockholm, Sweden
Yes, the arguments go exactly the way you surmised.
Mike W.
(published on 03/05/2014)
Follow-Up #18: speed of light and ether
- Charlie (age Cur)
Toronto, Ontario, Canada
I'm not sure what you mean by "the speed of temperature", but probably this is what you're getting at. In many familiar waves (plucked string, water ripples,...) the wave travels at some speed but the stuff that is doing the waving (string, water,...) isn't going anywhere fast. You're thinking that although electromagnetic waves travel at c, the "stuff" that's waving isn't traveling, and you want to call that stuff "light".
The key problem with that is that what we mean by light is the wave itself. We know when and where it starts, and when and where it arrives. So you can measure the speed. It's "c".
Now you can claim that the light wave is in some underlying stuff, which people used to call "the luminiferous ether". Many experiments looking for this ether failed to find it. Still, if you're willing to have it obey some surprising properties (relativity), you're free to claim it exists. So then you'd have something connected with the light that's not traveling at c, just as the electrons in a wire don't travel as fast as the electrical signal. It doesn't matter, because the speed of light doesn't mean the speed of that hypothetical ether but rather the speed of the light itself.
Mike W.
(published on 03/22/2014)
Follow-Up #19: photons in matter
- Deborah (age 44)
U.K.
1) The space between the nucleus and the atom really isn't a vacuum. The quantum state of the electrons is spread out throughout that space. Within the standard current interpretations of quantum mechanics, the electron isn't one place in particular but really exists as a spread-out wave-like state. (see follow-up # 37 here but also see below.)
2) The same goes for the inside of the Sun. The quantum states of the particles are smeared out and overlapping.
3. The speed of light is reduced, because what travel is not bare light but ligt dressed in some shaking of those waves that were already in that space. To define a speed, you need to consider a volume big enough so that the various vibration modes of the background fields (mainly of electrons) can be specified.
This may be a minority viewpoint, but I did think one feature of Feynman's QED was peculiar. Although the explicit content all involves those spread-out waves and their interference, Feynman insists that the meaning of it all should be understood in terms of real point-like particles. I don't know why he wrote that, or even quite what he meant by it.
I should mention one very recent start toward a new interpretation of quantum mechanics in which particles do actually have positions. The wave-like properties arise from the average properties of a swarm of alternative universes which interact non-locally with ours. (This is called the Many Interacting Worlds interpretation: .) Weird as this one may sound, I guess it's no worse than any other interpretation of QM. The main drawback to it is that if somehow it were experimentally tested and found to have some supporting evidence, I'd feel obliged to find and change all of the old answers in which I claimed that no interpretation with specific particle positions could be viable.
Mike W.
(published on 01/28/2015)
Follow-Up #20: Can you accelerate an electron with a photon?
- jeff kozlin (age 35)
west warwick. ri usa
Hello Jeff,
I you bounce a high energy photon off of an electron the electron will recoil. The outgoing photon and electron will share the incoming momentum and energy. The problem is that a priori you don't know the direction and momentum of the recoil direction. Nevertheless if the incoming photon has an energy of a few MeV, from time to time the electron will be relativistic.
This whole process is called Compton Scattering. See for details.
LeeH
(published on 01/30/2015)
Follow-Up #21: How do we know that we cannot accelerate a photon?
- CS Nelson (age 68)
Springboro, Ohio, USA
Hi CS,
From an experimental point of view, of the many, many measurement made in the lab and in space, the result is always comes out to the same number, c. Even photons emitted from moving objects have velocity c.
From a theoretical point of view if the special theory of relativity is correct then all photons travel at the speed of light. They have zero rest mass but carry momentum, p, and energy E = hc where h is Plank 's constant. The value of c is our old friend the speed of light.
LeeH
(published on 12/16/2015)
Follow-Up #22: cosmology questions
- Marco Casteleijn (age 43)
Helsinki, Finland
In order of how clearly we can answer these:
1)The soup was everywhere, so the neutrinos, and later photons, started everywhere. They're still randomly buzzing around everywhere.
3) I don't think we said anything about entropy "slowing down" in a black hole. (I don't even know what that would mean.) Black holes are supposed to have the maximum entropy possible for that amount of energy confined to that amount of space (as measured from the far outside).
4) Since the universal expansion is accelerating, those black holes will get outside each other's horizons, unless something drastic changes in the background behavior of space. That means they can't all lump up. Isolated black holes will, instead, very slowly evaporate via Hawking radiation. That will leave a universe that is an extremely thin cold soup of photons.
2) As we discuss in many other questions, we don't really know how to extrapolate our understanding of the universe back to times les than the Planck time. What sort of mathematical manifold might extend beyond that remains to be discovered. There are some ideas, and they may have observational implications, e.g. for the presence of gravity waves in the early universe.
Mike W.
(published on 08/31/2016)