Why do photons naturally travel at c?

[Gamish]

Why do photons naturally travel at c? Do photons have an actual size to them? If so, what is it? I am trying to figure out the nature of the photon.

:zzz:

There are 3 types of knowledge in the world, knowledge, understanding, imagination

 

[misogynisticfeminist]

size is an extremely elusive thing i guess. The reason why photons travel at c is because that they are massless and they have infinite range in space as well (right?).

 

[russ_watters]

Well, you can use the anthropic principle: light travels at the speed of light because it is light. That's pretty axiomatic.

 

[1100f]

Well, you can use the anthropic principle: light travels at the speed of light because it is light. That's pretty axiomatic.

The question was not why light goes at speed of light, but why light goes at c?
Although c is called "the speed of light" it is not only the speed of light but of other "things" also. Since, historically, light was the first thing to be discovered that goes at c, from this c got its name. A more appropriate name would be for example "velocity invariant under Lorentz transformation". As such the question of why light travels at c is a legitimate question.

 

[cepheid]

Although c is called "the speed of light" it is not only the speed of light but of other "things" also.

Such as...?

 

[dextercioby]

Such as...?

Try gravitons,if they're ever to be found.And any other massless representation of the restricted Poincaré group.Neutrinos,maybe...Though they're pretty much thought to have a mass...Anyway...

 

[Garth]

This might help, my post #3 in the thread "space-time and the speed of light ".

"Time exists to prevent everything happening all at once; space exists to prevent everything happening at all the same place." (G.B)

Indeed events require a space and a time, for we cannot measure the position of anything except at a particular time and we cannot measure the time anything happens except at a particular place. This insight led Einstein to conceive a model of a four dimensional space-time continuum.

But how are these different dimensions, length, width, height, and time, to be connected? How can you measure the space-time separation or interval between two events?

Think first of two dimensions, the surface of a sheet of paper, for example, of length x and width y.
The shortest distance s between two opposite corners is given by Pythagoras' theorem:
s^2 = x^2 + y^2
If we now go to three dimensions, the shortest distance between two opposite corners of your room, for example, one on the floor and the other in the opposite corner on the ceiling, where the room is x long, y wide and z high, is given by:
s^2 = x^2 + y^2 + z^2.
So what happens if we go to four dimensions, and measure the space-time interval between two camera flashes, for example, one happening at one corner of your room and other happening at the opposite corner but a few seconds t later?

We might think the answer would be:
s^2 = x^2 + y^2 + z^2 + t^2, but we would be wrong. There are two things wrong with it.

First of all Einstein had been working on a problem, how to make Maxwell's equations independent of the observer's frame of reference, which would mean the velocity of light is equal for all observers as discovered by Michelson and Morley. He realized that these problems would be resolved if he adopted an idea of his colleague and lecturer, Minkowski, that the t^2 term should be subtracted, not added.

Secondly we are adding "dollars and euros", the dimensions of the terms in the equation are not right, we need a conversion factor, "a rate of exchange", to convert one into the other before we can add or subtract them. The conversion rate that turns time into distance is a velocity we call it c, so if c is measured in kilometres/second and t is so many seconds, then ct will be so many kilometres.

So the correct equation giving the space-time separation s between two events becomes
s^2 = x^2 + y^2 + z^2 - c^2t^2, and now there is one more refinement to make.

Rather than measuring the distance across an extended interval of space-time, it is important to deal only with the separation of adjacent events separated by a infinitesimal change in the coordinates, dx, dy, dz, dt. This allows the possibility that space-time might be 'curved', just as the surface of the Earth is curved into a sphere, although it looks flat on a small scale, or alternatively like the surface of a saddle. Or perhaps like that of a Popperdom, all 'hills and hollows'. So the correct expression of the infinitesimal separation of two adjacent events is:

ds^2 = dx^2 + dy^2 + dz^2 - c^2dt^2, this is called the metric of flat space-time. If we want to include 'curved' space-time then we put a coefficient, not equal to one, in front of each term dx^2 etc. In order to find out the separation between two events such as a distant super nova exploding and the event being observed on Earth you have to add up, or integrate all the infinitesimal ds intervals along the light path.

So you can see that the answer to

Why do photons naturally travel at c?

is that a velocity has to be built into the geometry of space-time to adjust the dimensions of the terms in the equation for the metric. Now it so happens that when we work out the kinematics of moving particles using this metric we find that for a moving object the mass measured by another observer increases as the velocity relative to that observer increases. When the velocity approaches the velocity c the mass approaches infinity, in other words no particle with rest mass can travel at the velocity c. However, photons of light have no rest mass and in a vacuum they have to travel at this velocity. Therefore the velocity c is the velocity of light in a vacuum, and we find it has to be built into the geometry of the space-time continuum.

Garth

 

[juju]

Hi,

Photons travel at the velocity they do because of the properties of the space in which they are traveling. The two properties known today which determine the speed of light are electric permittivity and magnetic permeability.

juju

 

[Gamish]

Maybe photons trabel at c because it is a "particle" which allows "space" to "freely" enter it, so we don't interpit photons to have mass.

 

[yogi]

Gamish: "Im trying to figure out the nature of the photon."

Near the end of his life Einstien wrote to his friend Besso "...all these years of conscious brooding about the photon have brought me no closer to the truth. Nowdays every Tom, Dick and Harry thinks he knows the answer, but he is mistaken."

 

[Gamish]

I guess your right on that one. In all of Einstines understanding, he did not know it all. There have been times in my life where I said "I understand", and then later on I find out that I didn't. But, "imagination is greater than knowledge", as Einstine said.

Im the master at time!

 

[Chronos]

The whole 'c' thing revolves around the very basic premise of causality. Without an invariant, background independent point of reference, such as invariant speed of light, causality breaks down. Which is to say the very concept of time itself vanishes, as Garth pointed out. To truly understand this [in my opinion], you also need to be familiar with the mathematics of Galilean and Lorentzian transformations. They are not as terribly complicated as you might think. In many ways, they are more elegant and logical than any verbal explanation.

The only assumption required to make these models work is that light speed [time] is measurable and has a finite speed: which agrees well with observation. Try this link to see why:
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/ltrans.html#c2

 

[marlon]

And if there are not enough explanations yet...here is an other version from the QFT people...photons travel at the speed of light because they have no restmass. They have no restmass because they do NOT interact with the Higgs-field. The reason for this : well, the U(1)-symmetry of the EM-interactions is never broken after the contineous breakdown of symmetry...(ie when nature has "chosen" one of the possible vacuum-states). This system is used to generate mass of elementary particles which are massless in QFT because a mass term mixes up the different chiralities which correspond to two different fundamental properties...To exemplify : left and right handed chiralities do NOT behave in the same way when they undergo weak interactions...

regards
marlon

 

[jcsd]

As has been mentioned, given that it has no rest mass, it travels along a null geodesic (it's a fairly simple excerise to show this) and hence travels at c. Of course many will say this is just circular reasoning as we often use the assumption that it travels on a null geodesic to show that it has no rest mass!

 

[Andrew Mason]

Asking why the speed of light is c is a little like asking 'why is the lowest temperature -273 C? Our concept of temperature relates to another concept: kinetic energy of atoms. -273C or 0 Kelvin is the natural limit of kinetic energy of matter.

The 'photon escape' phenomenon necessarily defines time and distance for matter. No object having rest mass can attain the state of a photon. As matter tries to approach the photon state, the photon continues to escape. Thus c represents a natural limit on the rate at which matter can interact. As matter approaches the photon state, time and distance approach 0.

c, which defines the photon state, is the absolute zero of space and time.

AM

 

[donjennix]

I think that the question is a bit more profound than may be immediately apparent. I am intrigued that Special Rel is basically determined by the behavior of a quantum entity (the photon, of course)-- the disconnect between GR and quantum may ultimately hinge on the fact that there is a subtle connection between quantum properties and relativity that we are not quite getting.

To say that light is on the "null geodesic" is, I think, just a tautology. The speed of light is not like absolute zero -- a minimum temp is predicted by the definition of heat. What in the universe predicts that EM travels at a specific vel? It is just a discovery.

 

[jcsd]

I think that the question is a bit more profound than may be immediately apparent. I am intrigued that Special Rel is basically determined by the behavior of a quantum entity (the photon, of course)-- the disconnect between GR and quantum may ultimately hinge on the fact that there is a subtle connection between quantum properties and relativity that we are not quite getting.

To say that light is on the "null geodesic" is, I think, just a tautology. The speed of light is not like absolute zero -- a minimum temp is predicted by the definition of heat. What in the universe predicts that EM travels at a specific vel? It is just a discovery.

The fact that the original formulation of SR depends on the behaviour of light is merely an accident of history, you should recognize that it is NOT necessary to formulate it in such a way.

A geodesic can be thought of as a (locally) distance (using the word distance in the sense of the metric function) minimizing path, a null geodesic is simply a geodesic where the distance between all points on the path is zero, any massless particle can be shown to travel along a null geodesic.

 

[donjennix]

OK - I know that you can get to SR from several directions; but, really, what is a null geodisic other than a path where (ct)^2 - x^2 - y^2 - z^2= 0 --- i.e., where something is traveling the (constant) speed of light. Again, all the formulations of SR end up equavalent to "c is constant" (e.g., the British textbook approach that the frequency shift follows a constant K or 1/K depending on direction of v, regardless of which emitter is taken to be "at rest").

And even if the particular formulation "the speed of light is constant" is not in itself important, some sort of interaction between quantum and SR is happening at a very basic level.

Now maybe that is merely a banal trusim and I am not "getting it" somehow --- but I'm still not convinced that this speed limit donen't contain a bit more truth than we have yet.

 

[jcsd]

Yes, but your equating the constant c with the speed of light so of course the speed of light will be constant!

Infact some prefer not to call 'c' the speed of light for precisely this reason, i.e. it can lead to the confusion that light has some special almost magical role to play in relativity.

 

[Andrew Mason]

To say that light is on the "null geodesic" is, I think, just a tautology. The speed of light is not like absolute zero -- a minimum temp is predicted by the definition of heat. What in the universe predicts that EM travels at a specific vel? It is just a discovery.

If one were to attain the speed of light, time and space would collapse. As an observer approaches the speed of light (relative to the geometric centre of the universe, say), the size of the breadth of the universe approaches 0 and time slows so that the observer takes foreever to travel even that small distance.

If there was no matter in the universe there would be nothing to define a frame of reference. Location (ie. position in relation to another point in space) and time would have no meaning as these can only have meaning in reference frame. So a piece of matter, in effect, sucks back a frame of reference from an otherwise dimensionless universe. In this sense, light isn't moving at all (ie. to a matterless but illuminated universe). Matter simply defines location and time. Light will appear to move from one point to another. But that is just a consequence of defining a reference frame. To the photon, there is no such thing as motion, distance or time.

AM

 

[Wave's_Hand_Particle]

And if there are not enough explanations yet...here is an other version from the QFT people...photons travel at the speed of light because they have no restmass. They have no restmass because they do NOT interact with the Higgs-field. The reason for this : well, the U(1)-symmetry of the EM-interactions is never broken after the contineous breakdown of symmetry...(ie when nature has "chosen" one of the possible vacuum-states). This system is used to generate mass of elementary particles which are massless in QFT because a mass term mixes up the different chiralities which correspond to two different fundamental properties...To exemplify : left and right handed chiralities do NOT behave in the same way when they undergo weak interactions...

regards
marlon

The interaction rates(higgs-field) are dimensionally required?..thus the Parity of difference can be derived out of energy and mass: Energy is created at the expense of mass,E=Mc2.

Mass is created out at the expense of energy,E=MC2.

Photons are conversion products delivered between Mass and Energy, and Energy and Mass.

For energy creation, Mass decays at specific rates, delivered at a constant speed..C.

For mass creation, energy must be at a HIGH rate, and photons therefore must be delivered at a rate above C.

Detection of the Higgs interaction is reliant upon the Parity of dimensions of the where observer exists?

 

[jcsd]

If one were to attain the speed of light, time and space would collapse. As an observer approaches the speed of light (relative to the geometric centre of the universe, say), the size of the breadth of the universe approaches 0 and time slows so that the observer takes foreever to travel even that small distance.

In relativity any object traveling at the speed of light does not have instanously comoving inertial reference frames, so we cannot say about what would 'happen' at the speed of light. Also the universe has no geometric centre.

 

[Andrew Mason]

In relativity any object traveling at the speed of light does not have instanously comoving inertial reference frames, so we cannot say about what would 'happen' at the speed of light. Also the universe has no geometric centre.

Quite right. I should have said "if one were to attain a speed within an arbitrarily small neighbourhood of c, time and space would approach arbitrarily close to the point of collapse".

The universe has no single center because one cannot place absolute values on the positions of all matter in the universe at a single point in time (ie. other inertial observers would disagree on the simultaneity of those observations). But for each frame, there exists a unique geometric centre of the universe.

AM

 

[jcsd]

Quite right. I should have said "if one were to attain a speed within an arbitrarily small neighbourhood of c, time and space would approach arbitrarily close to the point of collapse".

AM

arbitarily close to c compared to what? Ceratinly when your traveling at speeds close to c relative to the background stars the unievsre looks very different, but collapse is the wrong word.

 

[donjennix]

Quite right. I should have said "if one were to attain a speed within an arbitrarily small neighbourhood of c, time and space would approach arbitrarily close to the point of collapse".AM

If you are in frame O and you measure your v compared to O' as (say) 0.99999c then one meter as measured in O' in the direction of v will seem (and the emphasis is on "seem") to be .0044 meter in the O frame. Since the universe has many frames -- some galaxies are moving away at high fractions of the speed of light --- there is no way the universe would appear to collapse if you are not a photon yourself.

Incidently, if there is any derivation of SR that doesn't use
1) constancy of the speed of light or
2) the dependence of the doppler shift totally on the relative velocities of the emitter and receiver

I would like to get a reference to it.

 

[jcsd]

The universe has no single center because one cannot place absolute values on the positions of all matter in the universe at a single point in time (ie. other inertial observers would disagree on the simultaneity of those observations). But for each frame, there exists a unique geometric centre of the universe.

AM

There are no cosmological models thta could be regarded as 'current' which feature a geometric centre of any sort.

 

[jcsd]

Incidently, if there is any derivation of SR that doesn't use
1) constancy of the speed of light or
2) the dependence of the doppler shift totally on the relative velocities of the emitter and receiver

I would like to get a reference to it.

There was threa don altrenative derivations of the Lorentz transformation, there may be one on there.

 

[Andrew Mason]

There are no cosmological models thta could be regarded as 'current' which feature a geometric centre of any sort.

Why can there not be a centre or centre of mass of the universe? Cosmologists talk about how wide across it is. If it has a finite size, which seems to be a necessary consequence of the Big Bang, there has to be a centre. Since all parts are moving relative to each other, many at relativistic speeds, there can be no absolute centre. But in each frame of refence 'centre of the universe' has a meaning.

AM

 

[Andrew Mason]

arbitarily close to c compared to what? Ceratinly when your traveling at speeds close to c relative to the background stars the unievsre looks very different, but collapse is the wrong word.

Compared to whatever neighbourhood of c you can suggest. What is the right word, if not collapse?

AM

 

[jcsd]

Why can there not be a centre or centre of mass of the universe? Cosmologists talk about how wide across it is. If it has a finite size, which seems to be a necessary consequence of the Big Bang, there has to be a centre. Since all parts are moving relative to each other, many at relativistic speeds, there can be no absolute centre. But in each frame of refence 'centre of the universe' has a meaning.

AM

Almost universally cosomolgical models that are isotropic and homogenous are prefered, this rules out that a geometric centre must appear in all frames a priori.

The big bang does allow universe of finite size though it is far from necessary, but a finite universe edoes not mean a geometric centre, to use a well-worn analogy the surface of a sphere has a finite area but is isotropic.

 

[jcsd]

Compared to whatever neighbourhood of c you can suggest. What is the right word, if not collapse?

AM

Collapse implie ssome sort of catastrohic/irrevrsible process, thoguh I suppose you use it in the sense of fold-up, to convey the fact taht most of the stars you would see would appear in a very narrow area.

 

[donjennix]

I apologize in advance for making an assumption --- but in another thread the writer seemed to be understandably confused by the famous balloon model where the galaxies are spots on the expanding balloon. You can get the impression that the universe IS the balloon and the center is the center of the balloon. But as jcsd points out, you have to consider the universe as the SURFACE of the balloon. If the center of the balloon exists, it is in hyperspace or some such; and, just like there is no "center" of the surface of the world, obviously there is no center to such a universe ... unless you take the trivial case that the "center" is where ever I am now. If you add to that the fact that the universe does not seem to have the curvature of a sphere (I can't rremember if that is positive or negative) and you get even more complications.

 

[Andrew Mason]

You can get the impression that the universe IS the balloon and the center is the center of the balloon. But as jcsd points out, you have to consider the universe as the SURFACE of the balloon. If the center of the balloon exists, it is in hyperspace or some such; and, just like there is no "center" of the surface of the world, obviously there is no center to such a universe ... unless you take the trivial case that the "center" is where ever I am now. If you add to that the fact that the universe does not seem to have the curvature of a sphere (I can't rremember if that is positive or negative) and you get even more complications.

I am talking about the 3 dimensional 'slice' that exists for each frame of reference and which we see (as would all other observers sharing our reference frame) as our spatial universe. That universe has a centre. I am not saying it is an absolute centre because other observers in other reference frames would disagree. But all observers in our reference frame would agree where the centre is located (assuming that we could do all the necessary measurements).

AM

 

[Garth]

I am talking about the 3 dimensional 'slice' that exists for each frame of reference and which we see (as would all other observers sharing our reference frame) as our spatial universe. That universe has a centre.

The standard cosmological models are unbounded, so the universe we observe has no centre, except that of the observer herself (Copernicus eat you heart out!).

Garth

 

[jcsd]

I am talking about the 3 dimensional 'slice' that exists for each frame of reference and which we see (as would all other observers sharing our reference frame) as our spatial universe. That universe has a centre. I am not saying it is an absolute centre because other observers in other reference frames would disagree. But all observers in our reference frame would agree where the centre is located (assuming that we could do all the necessary measurements).

AM

Yes we are talking about spatial slices, infact the discussion of the sphere fits in well with the spatial slice of the case of the RW metric which describes a finite universe which is essentially a 3-sphere (i.e. the surface of a four-dimensional sphere).

As Garth says the Copernican cosmological principle is a pretty fundamantal assumpion in neraly all areas of cosmology and a geometric cnetre is a total violation of this principal.