# Why does light travel at light speed?

## Main Question or Discussion Point

Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?

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mathman
Photons travel at light speed because that's the way it is. Photons hitting the sun (doesn't matter where they came from) will interact with the material of the sun (electrons, protons, etc.). Being the sun itself is irrelevant.

It's not as simple as "that's the way it is." Why are they the only things that can travel at c (not including neutrinos which are theorized to travel at c). What property gives them this ability to travel so fast? Is it the no rest mass?

jcsd
Gold Member
photons are not the only things gthat travel at c (neutrinoes are no longer beleived to travel at c tho'), it's a general property of massless particles.

Staff Emeritus
Gold Member
Dearly Missed
You can derive the relativitic equation for energy $$e^2 = p^2c^2 + m^2c^4$$. Here p is the magnitude of the three dimensional momentum, m is the invariant mass of the particle, and c, of course is the speed of light. I repeat that this equation follows from the postulates, the basic definition of relativity.

Now suppose p = 0; then the particle has no momentum => you are looking at it in its rest frame and the equation reduces to $$e = mc^2$$ which you might have seen before.

On the other hand suppose m=0; in this case the particle is massless and the equation reduces to $$e = pv$$, the energy is the size of the three-momentum times the speed of light. Plug this into the Lorentz transformation for energy and you get the velocity of the particle is c.

So these two predictions come from the relativistic energy equation, the energy of a particle at rest is $$mc^2$$ and the speed of a massless particle is c.

Of course in more general states you don't have either of these conditions, but the energy equation is still true.

hi,
we know that nothing can travel faster than the speed of light.....but what is it about light that gives it this special status?? i mean it could have been sound...could have been some other thing......n if we if we think logically then anything travelling opposite to the direction of light at velocity "v" will see light going away from it at an speed " c+v"......i know this point has been time n again been said to be false when light comes into picture......but some how 1+1 will always be 2!!
regards, niranjan

jackpot337 said:
Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?
Nothing needs to be pushed for it to move. The speed of light can be calculated from Maxwell's equations. See

http://www.geocities.com/physics_world/em/mawell_eq.htm

It starts with the permittivity and permiability of free space. They appear in Maxwell's equations as the reciprocal of the the product of the two. That is written as the square of a quantity which is labeled "c". The equations for the E and B fields are then shown to be the solution of a wave equation and c is the speed of the wave.

Pete

jcsd said:
photons are not the only things gthat travel at c (neutrinoes are no longer beleived to travel at c tho'), it's a general property of massless particles.
Not true. Photons have a mass, just theoretically no rest mass. So, particles with mass can travel at c as long as they have no rest mass.

niranjan, I like the way you think. Light is relative to the object, but physicists always find some mathematical way to prove it is constant. Simple logic says that if you are moving towards a light source the velocity of light would be dependent on that source.

$$\vec{v}_{light}=c+\vec{v}_{observer}$$

Or if the object is moving away from the light source

$$\vec{v}_{light}=c-\vec{v}_{observer}[/itex] Maybe they are the other way around, but logic says light speed should be dependent on velocity of the source of the observer or light source. So, neutrinos are said to not travel at the speed of light? So, this means they have a rest mass, right? Sorry, mentors, if my expression of opinion shouldn't go here but rather in a thread in Theory Development. Last edited: marcus Science Advisor Gold Member Dearly Missed selfAdjoint said: You can derive the relativistic equation for energy [tex]e^2 = p^2c^2 + m^2c^4$$. Here p is the magnitude of the three dimensional momentum, m is the invariant mass of the particle, and c, of course is the speed of light. I repeat that this equation follows from the postulates, the basic definition of relativity.
...

On the other hand suppose m=0; in this case the particle is massless and the equation reduces to $$\inline e = pc$$, the energy is the size of the three-momentum times the speed of light...
so at some level there is this basic assumption or observation about the universe:

"there is a certain speed that is the same to all observers"

nothing said about light or massive or massless or any kind of boson, that would be too clunky----just that the universe has a preferred scale of speed that reads the same in all frames

later, after some algebra and thinking about particulars, we learn that massless things actually GO this speed, and that, as it so happens, light goes this speed

Correct me if I am wrong, but it seems as if the basic postulate does not even assume that anything travels at this speed, not to mention saying anything about light. I just says that in the universe there IS this speed which looks the same regardless of your perspective.

and the equations you mention derive from that, by some algebra

I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.

wolram
Gold Member
By MARCUS.
I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.
----------------------------------------------------------------------------------------------------
So what could this other type of physical quality be, the best i could
think of is a" physical meter", yard stick, universal mile ,what ever,
im sure time is irrelevant to physical quality, so what is missing?

Staff Emeritus
Gold Member
Dearly Missed
Marcus, I believe you are basically right. Modern texts will say on page 1 or 2, something like "in order to get all the components of our 4-vector into the same units, we have to multiply the time component by something to turn it into a length, like the space components. What do you multiply by a time to get a length? Obviously something with dimensions length over time: a speed! And this speed factor has to be a Lorenz scalar, so that there is only one conversion for everybody. Let's call it c for celeritas (Latin for speed). Hence we get Einstein's second postulate without the light dependence he was forced to by his "operational" framework. (By that last of course i mean he defined the physics in terms of operations people could actually do, an ideal of the radical physicsts, incuding Mach, of the late nineteenth century).

hemmul
marcus said:
"there is a certain speed that is the same to all observers"
great!

marcus said:
I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.
good question. actually it brought me to a slightly crazy idea right away: from this algebra, it can also be seen, that it is impossible to accellerate a massive particle above c, if initially it was moving slower than c. The same may be true for particles initially moving faster than c - that is they can't be deccellerated to sub-c speeds... Now what if we assume that there is another - greater - velocity? Kinda second "upper-bound" - for super-c particles we can't observe? say this speed equals 2c. and no super-c objects can't be accellerated to super-super-c speeds, and so on... :yuck:
Does it make any sense?
One of the "difficult" questions about that is "why do we live in [0,c] range?"

chroot
Staff Emeritus
Gold Member
Yes, hemmul, the particles you describe are called tachyons. Their mechanics are already well-understood, but there has been any evidence that they actually exist.

- Warren

chroot
Staff Emeritus
Gold Member
urtalkinstupid said:
Simple logic says that if you are moving towards a light source the velocity of light would be dependent on that source.

$$\vec{v}_{light}=c+\vec{v}_{observer}$$

Or if the object is moving away from the light source

$$\vec{v}_{light}=c-\vec{v}_{observer}[/itex] Maybe they are the other way around, but logic says light speed should be dependent on velocity of the source of the observer or light source. No, logic does not say this, or anything like this. Logic is a tool that allows you to derive conclusions from axioms. Please don't misuse terms. What you mean to say is that common sense says that the velocity of light is dependent on the velocity of the emitter. This concept is called "emitter theory" and has been as soundly defeated as any theory in the history of science. The most damning experiment is one that was done with neutral pions, a type of particle made from two quarks, travelling at nearly the speed of light in a particle accelerator. When these pions decay, they produce gammas -- high-frequency light. The speed of these gammas was measured directly, and was found to be -- drum roll, please -- c. Even though the pions were themselves travelling almost the speed of light, the light they emitted was still going c. The bottom line is simply that your common sense is based upon observations of things around you (chairs, desks, people and so on) that are not moving very fast at all. You have no common sense about things moving near the speed of light. You cannot therefore rely on common sense beyond its limitations. Experiments show us how the universe works, not common sense. And, no, your post does not belong here at all. Please don't make a habit of posting non-mainstream theories in the wrong forums here. - Warren Light speed = electric field / magnetic field This is considered to be always equal to 3 x 10^8 m/s in a vacuum for all wavelengths of light. marcus Science Advisor Gold Member Dearly Missed chroot said: Not quite kurious. I think you mean: [tex]c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$

- Warren
A physicist once explained in my hearing that
$$\mu_0$$
and
$$\epsilon_0$$
are not real physical quantities
they are not actual measurable properties of empty space
but are fictions, their formal existence established by convention within the SI system

therefore according to him it does not explain why the speed of light is c
if one merely demonstrates that putting $$\mu_0$$
and
$$\epsilon_0$$
into Maxwell equations causes one to get out a speed which is c

So I have come to regard this equation:
$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$
as a kind of tautology or a stacked deck

and then the meter got redefined as the distance light in vacuo travels in
exactly 1/299792458 of a Cesium133 atomic clock second which means if you use metric units the speed of light cannot logically be other than
299792458 of those meters per second. It is therefore fruitless to ask why it is what it is. It is the standard speed. (for the metric system and probably for mother universe as well)

I guess maybe the thing to ask is not why is it what it is (why is it 299792458 instead of 299792459?)
but to ask why is it constant? why is it AFAWK the same everywhere in universe? why is it the same for all observers even if the buggers are moving vis-a-vis us? why is there a constant speed standard in the U?

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That there is a limit at all suggests that there are properties of spacetime that prohit faster speeds. What properties could this be? They probably have something to do with curvature.

What other properties can spacetime have? What you're suggesting is that anything other than dimensions and how they curve is an artificial device? Can this be right? Can you define fields (scalar or vector) on spacetime without it being a property of the background without being arbitrary? In other words, where could such fields come from if they are not derived from spacetime itself? And if they can not be derived from the spacetime background, can they be real?

Common sense is a branch of logic. Common sense says speed is dependent on velocity of source or object. My common sense is actually derived from reality based situations. For light to be constant, it would have to have the characteristic of being in two palces at once. Two trains traveling towards each other. Light is coming from two directions. Both light beams hit the two trains. Will the light hit the observers at the same time? It is said yes. Why can it not? Light can not go into the back and front of the train at the same time. By the emitter theory, do you mean how light is emitted and absorbed through different densities?

chroot, take a look at this. I'm sure you won't think much of it. It makes sense to me. After reading Einstein's work and reading this. I've found the link I'm fixing to provide you to make more sense. That's just me though. :rofl: Here it is

Nereid
Staff Emeritus
Gold Member
Sorry, mentors, if my expression of opinion shouldn't go here but rather in a thread in Theory Development.
chroot said:
And, no, your post does not belong here at all. Please don't make a habit of posting non-mainstream theories in the wrong forums here.
urtalkinstupid said:
Common sense is a branch of logic. Common sense says speed is dependent on velocity of source or object. My common sense is actually derived from reality based situations. For light to be constant, it would have to have the characteristic of being in two palces at once. Two trains traveling towards each other. Light is coming from two directions. Both light beams hit the two trains. Will the light hit the observers at the same time? It is said yes. Why can it not? Light can not go into the back and front of the train at the same time. By the emitter theory, do you mean how light is emitted and absorbed through different densities?

chroot, take a look at this. I'm sure you won't think much of it. It makes sense to me. After reading Einstein's work and reading this. I've found the link I'm fixing to provide you to make more sense. That's just me though. :rofl: Here it is
I think chroot includes material such as that in 'aliceinphysics' as non-mainstream theories. If you would like to discuss the misunderstanding and misrepresentation of relativity that are evident in 'aliceinphysics', please start a thread in Theory Development.

You might also look through previous posts in PF; IIRC 'aliceinphysics' has been clearly shown to be inconsistent with observational and experimental results.

chroot
Staff Emeritus
Gold Member
urtalkinstupid,

You absolutely must stop posting personal theories and crackpot websites to the general physics forums. Consider this your final warning.

If you'd like to discuss your train "paradox," please provide a little more detail -- about where the light is coming from, etc. -- and I will be happy to show you how there is no paradox at all.

- Warren

I'm sure you heard of this "paradox." It really isn't a paradox at all. It was the scenario that Einstein used to support his claims of light travelling at c. It is the old train and lightning scenario. I'll PM you it, if it's cool with you, chroot?

chroot
Staff Emeritus
Gold Member
No need to PM it. Post it here, and we'll help you understand it.

The simple fact is that tens of thousands of people in the scientific community have thoroughly examined the theory of relativity for over a century, and there are no paradoxes in it. Any paradoxes you claim to find are only evidence of your own misunderstandings.

- Warren

urtalkinstupid, I dont understand why u are trying to say that light speed isnt constant. It has been proven experimentally sereral times. For example, light coming from stars has been measured from different parts of the world, and every time the speed of light ended up being the same exact number. You might think it might be simply because the earth's orbiting speed is so little compared to the speed of light...well it doesnt matter because chroot pointed out that light has been measured from pions moving at nearly the speed of light, the speed of light measured was once again the same. If light speed wasnt constant, then the time dialation equation predicted by relitivity would be wrong. But the fact is that the model for time dialation is very right and has been proven several times with precise atomic clock. So please, just dont argue about something that has been proven true in so many ways and so many times.

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chroot
Staff Emeritus