# I want to know that light has mass or not.

1. Feb 18, 2010

### YOOKUNG

I want to know that light has mass or not.

็ำHelp me ! Can you explain little?

2. Feb 18, 2010

### nicksauce

No light does not have mass.

3. Feb 18, 2010

### physx_420

Light consists of packets of energy called photons which are massless particles, so the short answer would be no. However, it really depends on what kind of mass we're talking about. According to E=mc^2 mass and energy are the same thing, and since light has energy, it should have a mass. But this mass isn't what we would call "normal" mass, rather, it's a relativistic mass (i.e. a measure of the energy of particle that changes with velocity). I think it's convention not to call this relativistic mass the same thing as a kilogram, gram, etc. because the kg type of mass is invariant.

4. Feb 18, 2010

### I_am_learning

Nope. But it does have momentum!

5. Feb 18, 2010

### YOOKUNG

Concluded that no light mass. Yes/No ?

6. Feb 18, 2010

### nicksauce

Yes.

7. Feb 19, 2010

### YOOKUNG

I think light is a photon from the electron and the electron have mass is not.

8. Feb 19, 2010

### ZapperZ

Staff Emeritus

Zz.

9. Feb 19, 2010

### YOOKUNG

ok I am very sorry.

10. Feb 19, 2010

### Pythagorean

This will help the lazy ones:

https://www.physicsforums.com/showpost.php?p=1285138&postcount=6 [Broken]

Last edited by a moderator: May 4, 2017
11. Feb 19, 2010

### Rasalhague

By the first definition, "invariant mass of a particle is defined as the total energy of the particle measured in the particle's rest frame divided by the speed of light squared", a reader of the FAQ would have to conclude that the question of whether a photon has invariant mass is undefined, since no rest frame is defined for a photon. Only by the second definition, expressed

$$m_0 = \sqrt{\frac{E^2}{c^4} - \frac{p^2}{c^2}}$$

can the reader conclude that a photon has no invariant mass. The second definition is called more general though, so I suppose it trumps the first. But still, the impression I get is that it's saying that whether a photon has invariant mass depends on which of two currently used definitions of invariant mass you use (even if you reject the terminology of "relativistic mass"), which contrasts with the more definite "no" that people are giving here.

The FAQ doesn't mention that a system of more than one photon, not all travelling in the same direction, can have an invariant mass, since we can define a frame in which the vector sum of their momenta is zero, and in this frame, its (nonzero) energy will equal its invariant mass.

Last edited: Feb 19, 2010
12. Feb 19, 2010

### ZapperZ

Staff Emeritus
That's because, for the overwhelming majority of this type of question being asked in this forum, it is the simplistic and most naive version that keeps popping up. Including such more complex scenario will simply confuse the reason that presumably does not know (or even want to know) about such a thing. The FAQ aims to answer those questions within the context of what typically appears in this forum.

Zz.

13. Feb 19, 2010

### Rasalhague

It's inevitable that, as a student learns more physics, they have to trade in some of the simplified definitions they're given at first for more refined and general definitions. It just gets a bit dizzying when that whole process is compacted into one paragraph! "Mass is defined thus," with no explicit statement that this is not a general definition, or what special case it's restricted to (a particle with mass), followed immediately by "or more generally..."

14. Feb 19, 2010

### Staff: Mentor

There is only one definition of invariant mass.

15. Feb 19, 2010

### Rasalhague

Then perhaps the FAQ could give that definition (i.e. the general one) first, adding that for a particle with mass this is equivalent to "the total energy of the particle measured in the particle’s rest frame divided by the speed of light squared".

16. Feb 19, 2010

### ZapperZ

Staff Emeritus
Don't you think this gets very confusing for someone JUST trying to learn something simple, especially if we have to do this for every single thing we talk about? Do you see constant qualifier for the Photoelectric effect that this is strictly applicable ONLY for single-photon photoemission without any appreciable Schottky effect? Which is more "dizzying"?

Zz.

17. Feb 19, 2010

### Rasalhague

I think it'd be less confusing after "the invariant mass is defined as" to give the one, general definition that's applicable to a photon, and therefore doesn't need any qualification.

If it's felt necessary to give another, less general definition as well, I think I'd find it less confusing if this wasn't introduced first as "the definition", and if the limits of where that subdefinition applies were made explicit. That said, I appreciate that there's a tricky balance between saying something generally true and saying something that's comprehensible to a lay reader and doesn't have too many distracting detours.

In the section on $E = mc^2$, it seems to me ambiguous whether you're saying that, in the case of a photon, the $m$ should be interpreted as what you call "inertial mass" (= $\frac{E}{c^2} = \frac{p}{c}$?), so that "there's nothing inconsistent" with applying the famous equation to a photon because $m$ means something other than rest mass here, or whether you mean that $m$ in the famous equation should be interpreted as "relativistic mass", which by the definition that follows would be undefined for a photon, so that "there's nothing inconsistent" because it's undefined for a photon, and so $E = mc^2$ can't be applied to a photon, or whether you're reading $E = mc^2$ as $E_0 = m_0 \, c^2$, in which case the famous equation does apply to a photon and "there's nothing inconsistent" because a photon has neither rest mass nor rest energy, although it does have kinetic energy. (The latter two interpretations both being widespread, the third--I gather--the current favorite.)

The statement that the famous equation is "derived from" $E^2 = (pc)^2 + (m_0 c^2)^2$ might suggest that you're treating $m$ in $E = mc^2$ as equivalent to $m_0$. But "All of the photon's energy is in the term $pc$. Some people would say that this is the photon's 'inertial mass'" make it sound like $m$ in the famous equation isn't $m_0$, but related somehow to the $pc$ term, at least for a photon. On the other hand, the only explicit definition of $m$ without a subscript, in this section, is $m = \gamma m_0$.

Sorry, I must be in a quibblesome mood today ;-) I should say that I'm fairly new to relativity myself, with fresh memories of the things that have confused me over recent months. I appreciate that you have far more understanding of physics than I do, and more experience of how to present these ideas.

18. Feb 20, 2010

### DrZoidberg

It depends on how you define mass.
Nowadays mass in physics usually means "invariant mass", also called "rest mass".
And since light can never be at rest it can not have rest mass.
However in reality light does of course have mass because it has energy. Light even possesses a gravitational field. An incredibly strong laser puls can attract matter and light due to gravity.

19. Feb 21, 2010

### JerryClower

No, light has does not have real mass. The medium of which is emitting the light has mass, but not the light itslef. I can shine a flashlight at you and the falshlight has mass, but not the light that is shining from it.

20. Feb 22, 2010

### Brown399

lmao. Are all of you simply compelled to confuse this lad? Just curious. =]