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

1. Mar 8, 2010

### FeDeX_LaTeX

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.

2. Mar 8, 2010

### Fightfish

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.

3. Mar 8, 2010

### FeDeX_LaTeX

Why is their rest mass zero?

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

4. Mar 8, 2010

### luxiaolei

Then, would you please explain what is mass?

5. Mar 8, 2010

### Fightfish

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.

6. Mar 8, 2010

### magnusrobot12

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

7. Mar 8, 2010

### croghan27

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:

8. Mar 8, 2010

### diazona

Nope. Not at all. (I'm talking about invariant mass a.k.a. rest mass here, which is what physicists mean by "mass")
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.
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.

What sort of transformation would you mean?

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.

9. Mar 8, 2010

### ZapperZ

Staff Emeritus
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.

10. Mar 8, 2010

### luxiaolei

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.

11. Mar 9, 2010

### cragar

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

12. Mar 10, 2010

### BL4CKCR4Y0NS

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

13. Mar 10, 2010

### Born2bwire

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.

14. Mar 10, 2010

### croghan27

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

15. Mar 10, 2010

### Born2bwire

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.

16. Mar 10, 2010

### Staff: Mentor

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.

17. Mar 10, 2010

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.

18. Mar 11, 2010

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.

19. Mar 11, 2010

### ZapperZ

Staff Emeritus
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.

20. Mar 11, 2010