# Light can't travel at the speed of light?

Gold Member
Hello;

For something to travel at the speed of light, mass has to be zero. Light is a form of electromagnetic radiation, which travels at 299,792,458 m/s. Because it is a form of electromagnetic radiation, it is carried by the force-carrier particle, the photon. However, photons have a very small mass that is almost zero (but not zero) so it is not massless, but very close to being massless. If light is simply photons, then that means that light cannot reach 299,792,458 m/s; in other words, light cannot reach the speed of light.

Where is the error here?

Thanks.

The error here, is that you are wrong in your thinking that photons "have a very small mass". Unfortunately, photons are massless. One can sometimes argue that we can assign a "moving mass" to photons, but at the end of the day, they have a rest mass of zero.

Gold Member
Why is their rest mass zero?

I thought photons were particles... how can a particle have no mass?

Why is their rest mass zero?

I thought photons were particles... how can a particle have no mass?

Then, would you please explain what is mass?

Then I will pose the reverse question: why must a particle be massive? :p
According to commonly accepted consensus, well the answer is simply that it does not interact with the Higg's field, and thus does not acquire a mass.

photons do not have mass. photons is a "particle" representative of energy but it is massless just like a wave is massless.

I am going to need some footnotes on this one!

"A photon is a particle that does not have a mass."
a) Does not the designation if it as a 'particle' imply it has a mass?
b) It interacts with the Higgs field. But, but, but the Higgs field/particle may or may not exist. Both are real possibilities - so where does that leave our little photon - still massless (or very small) and still zipping about at the speed of light. Yet if it is achieving the speed of light .... it cannot have a mass.

The rest mass is zero.

a) It only has mass while in motion?
b) Is there some kind of transformation associated with accel.. or decel - eration?

Are not photons particles, assuming a mass, when, as per Heisenberg, they are measured?

Help! Help! Help! I am trying hard not to make a pun about how I cannot see it. :yuck:

diazona
Homework Helper
I am going to need some footnotes on this one!

"A photon is a particle that does not have a mass."
a) Does not the designation if it as a 'particle' imply it has a mass?
Nope. Not at all. (I'm talking about invariant mass a.k.a. rest mass here, which is what physicists mean by "mass")
b) It interacts with the Higgs field.
Assuming the Higgs field is real (and the Standard Model is correct), it doesn't. The photon has no coupling to the Higgs field. If the Higgs field is not real (and the Standard Model which predicts it is incorrect), that still doesn't change the fact that photons appear to be massless.
The rest mass is zero.
a) It only has mass while in motion?
No, you're thinking of energy. A photon only has energy while in motion - then again, photons are physically required to always be in motion, so it's kind of meaningless to talk about what would happen to a photon if it stopped.

"Mass" a.k.a. "rest mass" a.k.a. "invariant mass" is a property of the type of particle, and it does not depend on what speed it's moving at (which is why it's called "invariant mass"). For particles that do not travel at the speed of light, it's the mass you would measure if the particle were at rest with respect to you. It turns out that you can calculate it from the formula
$$m^2 = p^\mu p_\mu = \biggl(\frac{E}{c^2}\biggr)^2 - \biggl(\frac{p}{c}\biggr)^2$$
whether the particle is moving relative to you or not (this can be experimentally verified for massive particles). For particles that do move at the speed of light (like photons), being at rest is not a well-defined concept, but if you use that formula to calculate $m$ of a photon, you get zero.

b) Is there some kind of transformation associated with accel.. or decel - eration?
What sort of transformation would you mean?

Are not photons particles, assuming a mass, when, as per Heisenberg, they are measured?
As above, saying something is a particle does not imply a mass. In that sense we take it to mean some notion of locality, i.e. it's basically "concentrated" in one spot rather than being spread out like a wave.

ZapperZ
Staff Emeritus
I am going to need some footnotes on this one!

"A photon is a particle that does not have a mass."
a) Does not the designation if it as a 'particle' imply it has a mass?
b) It interacts with the Higgs field. But, but, but the Higgs field/particle may or may not exist. Both are real possibilities - so where does that leave our little photon - still massless (or very small) and still zipping about at the speed of light. Yet if it is achieving the speed of light .... it cannot have a mass.

The rest mass is zero.

a) It only has mass while in motion?
b) Is there some kind of transformation associated with accel.. or decel - eration?

Are not photons particles, assuming a mass, when, as per Heisenberg, they are measured?

Help! Help! Help! I am trying hard not to make a pun about how I cannot see it. :yuck:

There is a severe misunderstanding here on what is meant by the word "particle" as applied to photons especially. Would you still insist on the same criteria for photons if we instead call it "light corpuscular"? Would you also then want a light corpuscular to have mass?

So we instead call it a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical "particle", the latter of which has (i) a definite position (ii) a definite BOUNDARY in space of what it is (i.e. you can see where a ping pong ball ends), etc. This is not what a photon is. We didn't call it a "particle" because it has those properties. We call it a particle because its energy comes in quantized clumps. That's it!

And if you want to argue that "Hey, doesn't energy implies mass via E=mc^2?", then I'll point to you our FAQ that has dealt with this misunderstanding so many times already.

Moral of the story: never get hung up on the ordinary English words that we use to describe something in physics. The actual physics description of it is the overriding principle, and the words we use to describe it is a very weak attempt at understanding nature, and as can be seen here, is filled with unintended connotations.

Zz.

Zz. You are great! I totally agree with you! Good thing about a foreigner learning physics is as you said, hardly trapped into these ''take for granted' understanding.

if light had mass then we could slow it down , but we cannot slow light down at all.

So BASICALLY ... the word "particle" used with "photon" is only a description and should not be taken literally?

Born2bwire
Gold Member
So BASICALLY ... the word "particle" used with "photon" is only a description and should not be taken literally?

Yeah. It's not entirely inconsistent though. It's just that when we talk about particles in the quantum mechanical sense we are ascribing the word particle with a different set of properties than the classical particle. However, most people rarely deal with or even hear about other true quantum particles. For example, I would expect that people would have similar confusion and questions about the nature of gravitons if the theory was as widely known as photons.

I guess this makes my thoughts on the Casimir force off too. I had taken it that it was the momentum on the photons being communicated to the casimir plates - pushing them together ..

Oh well - back to the text book

Born2bwire
Gold Member
I guess this makes my thoughts on the Casimir force off too. I had taken it that it was the momentum on the photons being communicated to the casimir plates - pushing them together ..

Oh well - back to the text book

The Casimir force can be calculated by the momentum imparted by virtual photons, but these are not physical photons. They are mainly just mathematical tools but they generate accurate results.

Dale
Mentor
2021 Award
So BASICALLY ... the word "particle" used with "photon" is only a description and should not be taken literally?
I would say that the same is true with ANY term in physics. They are all simply labels for some concept which is actually defined in mathematical and experimental terms. The common-usage connotations are not relevant to the physical definition.

Because of this nonsense words like "quark" are actually very good physics terms because they don't come with any contextual baggage that gets in the way.

Sir Isaac Newton was the first to explain the mechanics of light, describing light as particles because his physics of particles being such a success was the first good explanation of light. Until he was corrected by the Dutch physicist Christiaan Huygens born in 1629 developed the idea that light moves like waves explaining the reflection and refraction just as well as the corpuscular theory.

But ultimately it was expressed by Neils Bohr in the 1920 as the “theory of complementary”, which holds that the wave and particle of (in this case) light are not mutually exclusive to one another but complementary. Both concepts are necessary to provide a complete description.

ZapperZ your crude approach to labelling light to particles is offensive to anybody with a passion for history. You are incorrect to provide this explanation the way you have done. You said “So we instead call it a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical "particle",”. You do not take a respected physicists explanation and make it your own, the explanation of light is both wave and particle described by Neils Bohr not ZapperZ.

ZapperZ
Staff Emeritus
But ultimately it was expressed by Neils Bohr in the 1920 as the “theory of complementary”, which holds that the wave and particle of (in this case) light are not mutually exclusive to one another but complementary. Both concepts are necessary to provide a complete description.

ZapperZ your crude approach to labelling light to particles is offensive to anybody with a passion for history. You are incorrect to provide this explanation the way you have done. You said “So we instead call it a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical "particle",”. You do not take a respected physicists explanation and make it your own, the explanation of light is both wave and particle described by Neils Bohr not ZapperZ.

And the game you are playing that is avoiding the PHYSICS is deceitful. If you look at QM's formulation, there's no such thing as "duality". We do not have to switch gears to describe something as a wave, and then go to a different gear to describe something as a particle. These are not the ordinary particle, nor are they the ordinary waves. One only needs to look at the formulation, not what ZapperZ or Neils Bohr says!

Respect these physicists, but don't worship them. Their words are not the gospel.!

Zz.

You’re poorly mistaken, And your generalisation of QM increasingly offends me. I don’t worship physicists I do respect the input and understandings. Your attempt to Aristotle QM will live through the ages as a joke. Wasn’t it Niels Bohr and Rutherford that cemented Quantum theory with the model of the atom? and with this model nobody could doubt any longer the value of quantum theory as a description of the physical world of the very small. But one could go as far and say the world we see is a his interpretation and nothing is fact, But that would be ignorant wouldn’t it ZapperZ..for one to claim centuries of study and problem solving to mealy need respect is pitiful. I think it’s a way of putting your own spin on theories to claim intellectual points and to some degree feel like your contributing.

ZapperZ
Staff Emeritus
You’re poorly mistaken, And your generalisation of QM increasingly offends me. I don’t worship physicists I do respect the input and understandings. Your attempt to Aristotle QM will live through the ages as a joke. Wasn’t it Niels Bohr and Rutherford that cemented Quantum theory with the model of the atom? and with this model nobody could doubt any longer the value of quantum theory as a description of the physical world of the very small. But one could go as far and say the world we see is a his interpretation and nothing is fact, But that would be ignorant wouldn’t it ZapperZ..for one to claim centuries of study and problem solving to mealy need respect is pitiful. I think it’s a way of putting your own spin on theories to claim intellectual points and to some degree feel like your contributing.

This is very puzzling. You seem to think that QM has not progressed beyond what Rutherford and Bohr formulated. It's as if things like Feynman's path integral doesn't exist and neither does QFT.

QM, as it is now, has no "duality". I don't need to change my perspective to describe the wave-like observation, nor the particle-like observation. If you're "offended" by this, then you are offended by QM, not by me. You are welcome to double check the textbooks and show me where what I've said is wrong. That's all I care about, not that you were "offended".

Zz.

There you go again, how do you know what I’m thinking?. You think you know. But to better explain myself, you don’t dictate what QT or QM is, the principles of quantum theory stay the same”physical description of the very small“.

ZapperZ
Staff Emeritus
There you go again, how do you know what I’m thinking?. You think you know. But to better explain myself, you don’t dictate what QT or QM is, the principles of quantum theory stay the same”physical description of the very small“.

Again, nothing in here shows any understanding of QM. "Very small"? How about an entity consisting of the 10^11 particles? Still small? And why is this even an issue?

Your objection from the very beginning is puzzling. If what you want is nothing more than an argument, then go at it. But there's nothing in what I've said that has been contradictory to QM. On the other hand, you seem to be more fascinated with the shadow of the animal than the animal itself.

If I don't dictate what QM or QT is, then you certainly are not the representative of Bohr or Rutherford. I'll stop representing QM and QT if you stop representing the history of physics.

Zz.

What part in these series of posts did you interpret me trying to explain quantum theory. The original question was obviously asked by somebody who is trying to grasp an understanding of the speed of light not a big spill on quantum mechanics. Me giving history to this person will educate because it allows them to research the origans of the subject. Which when you go to school that’s how they teach, not that you would know.

ZapperZ
Staff Emeritus
What part in these series of posts did you interpret me trying to explain quantum theory. The original question was obviously asked by somebody who is trying to grasp an understanding of the speed of light not a big spill on quantum mechanics. Me giving history to this person will educate because it allows them to research the origans of the subject. Which when you go to school that’s how they teach, not that you would know.

There's nothing in this discussion that is related to the original question. This is what you "objected" originally"

ZapperZ your crude approach to labelling light to particles is offensive to anybody with a passion for history. You are incorrect to provide this explanation the way you have done. You said “So we instead call it a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical "particle",”. You do not take a respected physicists explanation and make it your own, the explanation of light is both wave and particle described by Neils Bohr not ZapperZ.

So where in what I said THERE was it contradictory to the physics that we know of right now? This isn't a thread about history, the same way you tried to attack me in another thread because you didn't get the CONTEXT of the discussion? What was the error in that statement that you quoted?

EVERY concept in physics has an underlying mathematical formulation. That is the horse. What you are arguing appears to be based on the shadow of the horse. When I tell you that the horse have stripes, you felt "offended" because it wasn't in the shadow. Well tough! Learn QM! Or don't they teach you that in school?

Zz.

if light had mass then we could slow it down , but we cannot slow light down at all.

yes we can. light slows down in any non-vacuum environment.

yes we can. light slows down in any non-vacuum environment.
no it doesn't , that is just the time lag of the photons hitting atoms, getting absorbed and then re-emitted , but they always travel at c .

no it doesn't , that is just the time lag of the photons hitting atoms, getting absorbed and then re-emitted , but they always travel at c .

Good answer, cragar and like any good answer it bring up other questions ....

How long does it take the energy (it is the energy, is it not?) of the photon to be absorbed and then re-emitted? - is this the case or is the photon bounced off the mass of the atom??

Can I take it that the time is dependent upon the mass of the atom and the density of the 'atom cloud' (to invent a phrase). :shy:?

Does this influence astronomers looking at light from distant stars .... I know of the 'lensing effect' , that of nearby strong gravities, that of Einstein's (supported by the always delightful) Arthur Eddington - is there other interference?

Is there a similar sort of "Browning motion" from the interaction of the photons?

Good answer, cragar and like any good answer it bring up other questions ....

How long does it take the energy (it is the energy, is it not?) of the photon to be absorbed and then re-emitted? - is this the case or is the photon bounced off the mass of the atom??

Can I take it that the time is dependent upon the mass of the atom and the density of the 'atom cloud' (to invent a phrase). :shy:?

Does this influence astronomers looking at light from distant stars .... I know of the 'lensing effect' , that of nearby strong gravities, that of Einstein's (supported by the always delightful) Arthur Eddington - is there other interference?

Is there a similar sort of "Browning motion" from the interaction of the photons?

no it doesn't , that is just the time lag of the photons hitting atoms, getting absorbed and then re-emitted , but they always travel at c .

i stand corrected.

Hehehehehe ... it may cast light on my enquiries - even as it certainly makes murky the answers.

Is it the temperature that causes the 'delay' or the constituents of the BCE?

What is this BCE made of ... can I get some at the local Home Hardwear?

If temperature comes from the vibration of molecular/atomic sized particles and the closer to absolute zero you get is the closer to lack of vibrations, does this cause the 'light' or the energy of the photon, to delay its passage through the BCE?

I take it that: Her first trick was slowing a pulse of light to a crawl — 15 mph as it traveled through the BEC. was not referring to the photon itself (having been told they go at 'c' all the time) but of the admittance and rejection of the impulse provided by the photon through the BEC. But how does Pro. Hau 'freeze' a beam and then 'release' it?

Looks like the reporter was dazzled by reporting the story - as am I, in reading it.

jtbell
Mentor
no it doesn't , that is just the time lag of the photons hitting atoms, getting absorbed and then re-emitted , but they always travel at c .

That simple "time lag between absorption and re-emission" picture is incorrect. I suggest you take a look at post #4 in the "Physics Forums FAQ" at the top of this very forum.

I see , interesting

There is a severe misunderstanding here on what is meant by the word "particle" as applied to photons especially. Would you still insist on the same criteria for photons if we instead call it "light corpuscular"? Would you also then want a light corpuscular to have mass?

So we instead call it a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical "particle", the latter of which has (i) a definite position (ii) a definite BOUNDARY in space of what it is (i.e. you can see where a ping pong ball ends), etc. This is not what a photon is. We didn't call it a "particle" because it has those properties. We call it a particle because its energy comes in quantized clumps. That's it!

And if you want to argue that "Hey, doesn't energy implies mass via E=mc^2?", then I'll point to you our FAQ that has dealt with this misunderstanding so many times already.

Moral of the story: never get hung up on the ordinary English words that we use to describe something in physics. The actual physics description of it is the overriding principle, and the words we use to describe it is a very weak attempt at understanding nature, and as can be seen here, is filled with unintended connotations.

Zz.

Dr. Z:

You have added a lot to this forum and I am in awe of your knowledge. But a couple of small points regarding your post.

First you say: "So we instead call it [the photon] a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical 'particle'..."

And then you say: "We call it a particle because its energy comes in quantized clumps. That's it!"

My guess is you wrote the latter statement in haste. We are really dealing here with the wave-particle duality of the photon which is a rather deep matter. If said photon of quantized energy terminated by spreading its energy and momentum over all available space paths then I don't think we would call it a particle; we would probably call it a "wave."
But of course the photon doesn't terminate over multiple locations; it terminates only at one space location. So we call it a "particle" because we have adopted the classical model that an entity of kinetic energy traversing space is a PROJECTILE that has a single space point of impact/termination. Whether this "projectile" model is valid for the photon is open to question [although not by most physicists], but that is another question.
But I would respectfully suggest that you reconsider the assertion that for the photon particle == quantized energy.

Physics Pfan

ZapperZ
Staff Emeritus
Dr. Z:

You have added a lot to this forum and I am in awe of your knowledge. But a couple of small points regarding your post.

First you say: "So we instead call it [the photon] a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical 'particle'..."

And then you say: "We call it a particle because its energy comes in quantized clumps. That's it!"

My guess is you wrote the latter statement in haste. We are really dealing here with the wave-particle duality of the photon which is a rather deep matter. If said photon of quantized energy terminated by spreading its energy and momentum over all available space paths then I don't think we would call it a particle; we would probably call it a "wave."
But of course the photon doesn't terminate over multiple locations; it terminates only at one space location. So we call it a "particle" because we have adopted the classical model that an entity of kinetic energy traversing space is a PROJECTILE that has a single space point of impact/termination. Whether this "projectile" model is valid for the photon is open to question [although not by most physicists], but that is another question.
But I would respectfully suggest that you reconsider the assertion that for the photon particle == quantized energy.

Physics Pfan

In a superconductor, the wavefunction for the Cooper pairs, which are paired electrons, are non-localized. This means that the cooper pairs, at any given instant before a measurement, are "spread out" all over the relevant space of the superconductor. Yet, these are "particles", meaning that these are composed of "electrons". It illustrates the case where even such "particles" CAN have the situation similar to what you deem as "classical wave", meaning they are spread out over all space the way a classical wave should.

It also means that photons can do the same thing even when we consider them as "clumps" of energy (and spin and momentum). So your assertion that only "wave" should be able to exhibit this spread in location doesn't hold true, because we have seen similar behavior in "particles" as well.

What people are missing here is that, once something can be described quantum mechanically, then hanging on to our classical biases no longer works! This is true be it for something we classically thought to be a wave (light), all the way to something we thought to be a classical particle (electrons, protons, neutrons, buckyballs, etc.). And the fact that these are consistently described via ONE single formulation (QM) and where all the wave-like and particle-like observations can be derived using just that one formulation, should dispel the myth that there is such a thing as a "wave-particle duality" in QM. There is a wave-particle duality in classical physics and in our minds, but there isn't any in QM! How can there be when you use the same set of description for everything?

Zz.