# B How are Photons massless? How can a particle have no Mass?

1. Aug 25, 2018

### AlvisPrabhu

I did look it up , but couldn't find anything that explains this in a simple manner.

2. Aug 25, 2018

### MathLover69

Photons are not particles, rather they are packets of energy. Energy does not have mass, therefore, photons do not have mass. It can rather be counter-intuitive considering how photons behave like particles (bounce off surfaces, collide with each other). If you want to read more, go search for Wave-Particle duality online.

3. Aug 25, 2018

### sophiecentaur

Why would you expect “simple”? It’ a very sophisticated non- classical idea.

4. Aug 25, 2018

### ZapperZ

Staff Emeritus
Here's the problem. You are somehow stuck in your mind, that something that has been described as a "particle" must have a "mass".

You need to realize that this is not based on science, but rather on your personal prejudice, i.e. that's what you are familiar with. I remember chatting with a young man who can't imagine anyone not having bread or potatoes for a meal, because that is what he grew up with from where he came from. In science, it is not unusual that we discover that some things that we have accepted, turns out to not be the complete picture. So we readjust our understanding, and discover how we have only seen a rather narrow view of certain things. You need to do the same thing and re-examine why you are insisting that a "particle" must always have mass.

Secondly, the word "particle" used in this case has connotations that are NOT accurate to describe entities such as "photons". A "photon" is not a particle that you are familiar with. It is not something like a ping-pong ball. We used the word particle sometime to describe it because, under certain conditions, it has particle-LIKE behavior. We lack better words to describe it, so we use existing terminology to call it something. If we call it JumboMumboDa, instead of a particle, you won't be asking this question.

So the problem here isn't "why does that particle has no mass?", but rather your conceptual understanding of what it is and your understanding of our world. The "particle" doesn't have to explain itself.

Zz.

5. Aug 25, 2018

### waves and change

Just to add to the answers already given. The real issue is that math can not be translated in many aspects into the English language. The math is a language of its own that describes a photon. But trying to describe this using another “basis” - English..... it doesn’t translate.

6. Aug 25, 2018

### Staff: Mentor

Well, it is actually pretty simple. By definition of mass (in units where c=1): $m^2=E^2-p^2$. And for a photon $E=p$. So by substitution $m^2=p^2-p^2=0$

7. Aug 25, 2018

### weirdoguy

Wave-particle duality is not part of modern quantum physics! Your whole post is very misleading.

Last edited: Aug 25, 2018
8. Aug 25, 2018

### MathLover69

I never said that wave-particle duality was "part of modern quantum physics". I said " If you want to read more, go search for Wave-Particle duality online.". ?

9. Aug 25, 2018

### davenn

And that is the problem part of your earlier post that @weirdoguy was pointing out when he said ……

Agreed,iIt's not part of quantum or classical physics

as it is likely to lead them down the path of believing that it is still a practical approach.

Dave

10. Aug 25, 2018

### AlvisPrabhu

Thanks , but what is p here?

11. Aug 26, 2018

### PeroK

$p$ is the momentum of a particle.

Light, despite having no mass, does have energy and momentum. You may have seen the equation $E = mc^2$, which equates the rest energy of a particle with its mass. That equation, however, is valid for particles with mass - and not valid for photons. A more general version of that equation is:

$E^2 = p^2c^2 + m^2c^4$

That equation equates the energy of a particle to its momentum and mass. And this is valid for massless particles, where the equation reduces to:

$E = pc$

Last edited: Aug 26, 2018
12. Aug 26, 2018

### Delta2

Ok probably you ll throw me stones here for mentioning this , but why does the higg's field gives mass to every other particle of the universe but does not give mass to photons?

But I agree to some of the posts posted in this thread, that the main problem is that our intuition guided by our everyday experience ,is thinking that whenever we hear the word particle we always think that the particle must have mass. It is rather better to see mass as a property of a particle (like for example the electric charge Is another property and some particles have charge but some others do not) and not as part of the definition of a particle.

13. Aug 26, 2018

### MoreAndMore

Why is it no longer a practical approach if you don’t mind me asking?

14. Aug 26, 2018

### Staff: Mentor

$p$ is momentum. So in units where c=1 mass is defined as $m^2=E^2-p^2$ meaning that the square of the mass is equal to the square of the energy minus the square of the momentum. This is the definition of mass.

For photons $E=pc$ so in units where c=1 we have $E=p$ and therefore $m=0$.

Last edited: Aug 26, 2018
15. Aug 26, 2018

### weirdoguy

@MoreAndMore wave-particle dualism is part of the old quantum theory that died around 1924-26. It was replaced by modern quantum physics that 'starts' with nonrelativiatic quantum mechanics, Schreodinger equation, Dirac formalism, etc. Simply there is no place for it in that formalism.

Last edited: Aug 26, 2018
16. Aug 26, 2018

### PeroK

If you study QM, then the behaviour of a particle (an electron, say), is described by the quantum theory. This is a single theory, with one concept of an electron and one governing equation (Schroedinger equation).

There is no formal division into: a) when an electron behaves like a particle, it follows these equations ... and b) when it behaves like a wave, it follows these equations. Everything is simply "quantum" behaviour. There is no point in the theory where wave-particle duality rears its head.

However, it lives on in particular in popular-science because quantum behaviour includes phenomena that were previously associated with particles and phenomena that were previously associated with waves. In this sense, wave-particle duality is alive and kicking!

17. Aug 26, 2018

### davenn

basically, that's why

18. Aug 30, 2018

### David Lewis

So technically is an electron a particle?

19. Aug 30, 2018

### sophiecentaur

Why not?
Why would a photon not be a particle either? But we need to re-think our definition of a 'particle' - that's all. Forget little bullets for a proper description of many situations.

20. Aug 30, 2018

### Mister T

The simplest possible answer is that the expectations that led you to your question are flawed. You may expect nonzero mass to be a property necessary for the existence of a particle, likewise for a nonzero size. When the word particle is used by a physicist it has a meaning that differs from other meanings. There are particles that appear to have zero mass and there are particles that appear to have zero size.