Q & A: Do photons accelerate from rest?

I read your answer that light does not accelerate and it seemed too make sense but if you turn on a light bulb the photons emmited must accelerate to the speed of light in air since they were not moving before, don’t they?
- Draken-Korin

Those photons didn't really exist before. There aren't a fixed number of photons. They can be destroyed and created. The energy, momentum, and angular momentum they have were there beforehand, but not in the form of photons.

Mike W

(published on 10/22/2007)

It is incomprehensible to me how a photon of light can go from 0 to 67 million miles per hour instantaneously. What is the motive force that propels the photon? Where does the energy come from? Why is c "limited" to 186,000mps? Why is the speed of light finite? What prevents the light from traveling even faster than it does? I realize that the answers to these questions are unknown, but I would be interested to hear the speculation of the physicists in the scientific community on this subject.
- Mark
Wichita, Kansas USA

We've addressed part of that question , about 'acceleration', above.

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 08/21/07)

Assume visible light traveling in vacuum and entering a transparent glass with refractive index 1.5. Because the glass is transparent, the light gets transmitted and comes out from the other side. Prior to entering the glass, the velocity of light was c. Just after the vacuum-glass interface, its velocity is c/1.5 (therefore there is deceleration!). After it comes out of the glass-vacuum interface, its velocity is again c (now there is acceleration from velocity c/1.5 to c). Where is the energy source for driving this deceleration and acceleration of the photons?
- B K Chandrasekhar (age 53)
Bangalore, India

That’s a really interesting question. It turns out that a blip of light has the same energy in the glass as it had outside. There is a universal quantum relation between energy and frequency, E=hf, where h is Planck’s constant. The frequency doesn’t change in the glass, so the energy per quantum doesn’t change, and neither does the number of quanta.
 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 09/01/07)

I was taught in school that Photons are both a particle and a wave, and that photons have a specific mass. You are saying this is not true? If not, then why are our schools teaching it and if photons do have mass then how do they get around the acceleration part? If they are just energy and are "already there in another form" Then how does electricity effect metal or gas to transform that energy that is "already there" into photons/light coming from a light bulb.
- Nathan (age 25)
Casa Grande, Az, USA

I don't think that what you were taught in school is really wrong. Quantum objects (photons are an example) show wave-like behavior in some circumstances and more particle-like behavior in other circumstances.

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 01/26/09)

According to theory of relativity ,When light rays travels around the massive object,their path gets deflected.My Question is that whether this deflection causes some deceleration of photons or not ? and how ?
- Amol Jagtap , age 25
Pune , Maharashtra , India

No, the speed of light remains the same.   Only the direction is changed.  It's like riding in a car on a circular race track at constant speed.

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 07/27/09)

In follow up question #2, how momentum of photons is conserved when light enters glass and again when it leaves it? Does photons become more massive as they enter refractive medium which slows its speed?
- Indrajit Kuri
New Delhi, India

This is a great question. It's a matter on which there's not quite unanimous consensus, but I'll give my own view, which seems in accord with recent experimental evidence and also is probably the closest to a consensus view. (See http://focus.aps.org/story/v22/st20)

 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 08/12/10)

It is a well known fact that speed of light decreases as it passes through any refractive medium like glass. But in your reply you say that 'the momentum p per photon is increased' as the light enters the glass. If photons mass is unchanged, shouldn't then the speed of light increase in glass? if momentum of photon increases. Isn't it contradictory?
- Indrajit Kuri
New Delhi

It's a little tricky to talk about the "photon's mass", but if you were to assign an inertial mass (p/v) to the photon, it would be larger in the glass. You can think of the bare photon as being dressed in a disturbance of the electrons in the glass, with some inertial mass attributed to that disturbance.

Mike W.

(published on 08/24/10)

light emitted from a star for example, how long and how far will it travel if it encounters nothing on its path? What happens to light in a black hole when it cannot escape?
- Prabhakar (age 47)
Hyderabad,Andhra Pradesh,India

If a photon doesn't bump into anything it will travel forever.  The age of the universe is thought to be 13.7 billion years old.  This number comes from astrophysical observations and is a consequence of the so-called 'Big Bang Theory'.   We can observe light coming from this time by looking at the Cosmic Microwave Background (CMB).  So that light has been happily traveling for 13.7 billion years.

LeeH

(published on 07/29/11)

This "light speed thing" has been buzzing around in my mind.Something about the "speed limit" idea does not set right with logic as i know it.Everything i have read about photons seem unique to other particles. In spite of what I know about evolution and adaptation to the environment, it seems too coincidental That the two most important,sound waves and photons have exact speeds. How would our perception of sight ever work well or evolve properly if photons did not behave exactly the way they do ? Yet it is accepted at least right now that electromagnetic waves do not propagate in any kind of "substance" as sound does. Could it be possible that there is some kind of "medium" here and in space that we are not aware of yet that might limit it's speed? And if we do find it someday all of this would make total sense. Another interesting thing about light (and owning plenty of LED flash lights)is that the question comes up, Does a higher watt plash light produce more photons or photons that carry more energy. The same question seems more amplified in the case of different powered lasers.
- Chris (age 57)
Glen Cove , NY 11542

Your last couple questions are much simpler to answer than the rest so I'll address them first.  Two similarly designed flashlights, emitting light with different power, will basically just emit different amounts of approximately the same energy photons.  Similarly, lasers of the same color but greater power simply emit more photons than those of lesser power.  However, lasers of different color emit photons of different frequencies.  The higher the frequency, the higher the energy of the photon.  Frequency follows color, from lowest to highest, in standard ROY G BIV rainbow order.  This means a blue photon carries more energy than a red photon, so a red laser emitting the same power as a blue laser will emit more photons of less energy.


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 Michelson-Morley experiment 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 05/01/12)

leading up to my question are thoughts regarding what I have read previously. From what I have read it would appear that a photon has no RESTING mass because it is only observed in motion. This sounds like photons cease to exist if they were "stopped" . It has also been explained that in science if there is an observation then more of the same observations could be applied. One observation is that photons can be observed moving slower than their limit speed when they pass like through water and they still exist. The first question is if photons are slowed down do they loose some of their mass or intensity? If farther experiments could be done to farther slow down photons would photons as they are increasingly slowed start to increasingly loss mass (fade) until they almost do not move and virtually disappear until they stop and do disappear ? is there a a mathematical formula relating to the speed of the photon and it's mass?
- chris (age 58)
Glen Cove , NY,Nassau

I've moved your question to follow-up ones that lead into it better.

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 08/02/12)

Why is it that in this discussion of light and its speed you are stating that light travels through a vacuum and it does not need a medium to do so when we know that there are gravitational fields practically everywhere, not to mention the 97% Dark Energy & Dark Matter, which light must travel through ?
- George Mavros (age 68)
NYC, NY. USA

You can measure the effects of gravitational fields on light. They're usually very small, except near black holes. There's no indication that somehow light would stop propagating where the field goes to zero. (For insiders- yes we know that the gravitational field doesn't have a unique value, but for now let's use a conventional coordinate frame where it's about zero in most regions.)

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 09/03/12)

Thank you Mike. My understanding is that light is traveling through a medium at all times. The existence of a perfect vacuum seems inconceivable to me as you yourself state even weak gravitational fields exist everywhere, and as we have imaged through telescopes, within galaxies, clusters of galaxies and super galactic clusters. In fact as far as we can observe today everything in the presently detectable universe is connected at least gravitationally. I strongly suspect that Dark Energy & Dark Matter, indirectly detected and proposed theoretically today, might have a close relationship to gravity. The final point I would like to make is that, to my understanding,a perfect vacuum or 'nothingness' cannot exist.
- George Mavros (age 68)
NYC, NY. USA

George- You've misread a bit of what we wrote.

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 09/04/12)

I'm not quite sure how my questions really relate to the other questions I've read here about photons and such, but hopefully you might be able to shed light on them anyhow. My first question comes from the understanding (from Brian Greene's The Elegant Universe, pg. 52) that a muon traveling at 99.9 percent of light speed becomes 22 times more massive than a resting muon. Would this not create its own small gravitational field? If so, would larger objects also gain extra gravitational force by traveling faster? Provided the later question is yes, could we suggest that gravitational fields only exist because of motion? I have more questions in this line of questioning but I think I'll wait answers in case my future questions become moot. Also… As I understand it, according to Einstein's Special Relativity, time moves differently for those of us moving slower than those of us moving faster. Based on that idea, how can we understand or even approximate the age of the universe based on CMB since our movement in space is relative to the earth-sun-galaxy-other galaxies? How would we know if everything we observe visually and technologically isn't traveling through empty space at some immeasurable velocity?
- Rodger (age 27)
Mesquite, TX. U.S.

Yes, the motion of the muon (in our reference frame) adds to its energy and hence its contribution to the gravitational field, again in our reference frame. In its own frame its at rest, and hence its field is not enhanced in that frame. The situation is slightly more complicated because the momentum also contributes to the gravitational effects. At any rate, there is also a rest mass/energy for most familiar particles, and that rest energy is by far the most important source of gravity under familiar circumstances.

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 http://ned.ipac.caltech.edu/level5/March05/Scott/Scott2.html)

Mike W.

(published on 03/11/13)

the universe is estimated to be about 15 billion years old. one would assume a visible universe of roughly 30 light years across. yet it is more like 93 light years across. why is that?
- Tyler Newton (age 18)
Mississauga, Ontario, Canada

The first thing that's hard to grasp about this sort of question is that the answers are very highly dependent on your choice of coordinate systems, and the same laws of physics allow us many choices, convenient for different purposes. Here's a nice article on the coordinate system choices: http://en.wikipedia.org/wiki/Comoving_distance. Part of the problem is that if you say it's 93b l-yr across now, what you mean by "now" isn't well defined for distant objects.

Let me try to illustrate the issue using special relativistic coordinates, which are not actually part of the family of general relativistic coordinate systems but are simple enough to give you a feel for what's up. Let's say the universe is 14 b yr old. Let's approximate and say that there hasn't been acceleration since very shortly after the Big Bang. Then light getting here now from somebody who initially was moving away from us at almost c left about 7 bil yr ago, after the source got about 7 bil l-yr away from us. By "now" that same thing would be 14 bil l-yr away from us. So you'd be tempted to say we can see things that are "now"  as far apart as 28 bil l-yr. However, if by "now" you mean "14 bil yr old in its own frame" then there's a big difference. The clock in that source, moving away from us at nearly c, is running very slow, according to us. By the time it reads 14 bil yr, it's much father away from us.

In genuine coordinates, this special relativistic calculation doesn't apply. Nevertheless, when something is 14 bil yrs old in its own frame, that's much later than when its that old in the sort of frame you're tempted to use by intuition. So it's gone much farther away. Is that where it is "now"? As I said, it turns out that question isn't actually meaningful,  since "now" has no absolute definition. However, if we go with the  "same age in its own frame" definition, it does turn out to be ~46 bil ly away, just as you heard. You can see  https://en.wikipedia.org/wiki/Observable_universe for a fuller discussion.

Mike W.

 

(published on 04/29/13)