# Thread: Does light have a mass?

1. Crisp,

From James R statement:

The energy doesn't become rest mass in this case - that would require a nuclear reaction. The energy becomes kinetic energy of the ball, which you can look at as relativistic mass if you want to.
James R is indicating that the increased energy in a collision of a particle in a particle accelerator is due to the increased kinetic energy of the particle, and is not the result of the increase in mass of the particle.

Tom

2. Hi c'est moi,

From a few posts back:

"I'm interested to know what Crisp etc. think of my last idea, ie there is no increase in energy nor increase of mass during the motion of a body, there is only increasement of the momentum
it is *when* the IMPACT happens and a small (or a big) amount of mass is converted into energy that virtual particles have the possibility to remain (and are no longer virtual because of the energy that has been added) and that ONLY this accounts for the observed increase in energy which cannot be explained with the rest mass simply because nor the rest mass nor the inertial mass have anything to do with this only the amount of energy (converted from mass) that was supplied to enable virtual particles to remain is important"

Increase of energy, or increase of (relativistic) mass or increase of momentum are all equivalent statements. Depending on the situation, it is sometimes easier to work with only one of the interpretations.

Also, the "remainder" of a collision does not consist of virtual particles: they are real, we can observe them. An amount of energy is supplied for a collision, during this collision unstable or stable particles can form, and (unstable particales can) decay into the products we see from the collision. The concept of virtual particles is not needed here (there is no energy to borrow somewhere, which is where virtual particles come in -- even though I personally find the concept very dubious ).

Bye!

Crisp

3. Hi Tom,

"James R is indicating that the increased energy in a collision of a particle in a particle accelerator is due to the increased kinetic energy of the particle, and is not the result of the increase in mass of the particle."

Yes, and I totally agree with him. Have I ever argued that an increase in mass is per definition an increase in restmass ? It is exactly this what I was trying to explain in my formula posts: what physicists call "increase of mass" is another way of saying "increase of kinetic energy".

Bye!

Crisp

4. Crisp,

Either there's an increase in mass OR there's an increase in kinetic energy.

Which one is it????

Tom

5. """Increase of energy, or increase of (relativistic) mass or increase of momentum are all equivalent statements."""

this is getting messy, I mean, what are they doing?? three terms with different equations who are basically the same ... okay the kinetic and relatvistic you showed it to be the same
but now it's also the momentum who is supposed to be the same (p=mv) so P is really an increase in energy according to you ... why call it momentum?

Frankly, I am really poundering about this whole kinetic energy-thing. Maybe it is obvious that I am deadwrong on this, but here are some thoughts that occure to me right now:

What is the purpose of kinetic energy? As far as I get it, it is there because a body which has been accelerated should, in this line of thinking, receive energy from its source ... or not? Let's stay with the baseball. Has mechanical ENERGY been added? I was thinking, doesn't your arm work like a kind of elastic collision ... you turn around with your arm and when you stop it and release the ball, all the momentum from your arm has been transferred to the baseball. Okay, you have used energy to swing your arm. Where did the energy go? --> all of it went away in the form of heat (sweat ...). It didn't go in the ball (or maybe the ball absorbed a small amount of heat). I see further no transfer of energy to the ball to let me believe that the ball absorbed this 'kinetic' energy (btw, how would this happen in this case). I also don't see why momentum is the same as kinetic energy. It seems to be a distinct kind of thing to me.

But that's not all ... I need to explain the excess of energy that is released (and measured) in the collisions in particle accelerators. (I don't see how you can measure the energy of a baseball that hits a wall so ... and I simply see its greater impact because of the momentum it is carrying, not because it has more energy.)

"""Also, the "remainder" of a collision does not consist of virtual particles: they are real, we can observe them."""

Your are missing the point, what I know is that virtual particles are quite real, i.e. add energy and they remain. It's that simple. How much energy is there released in such a collision you think? A huge amount I thought.

Okay but in accelerators the energy given to, let us say an electron, is through an electric field. The electric field is seen as virtual photons carrying momentum and energy (or energy AND energy in your view Crisp???). When they hit an electron, does the electron really absorb it? Let me put it in a different way: we are talking about VIRTUAL photons who carry energy, i.e. they remain for a split second. If the electron absorbs one it will not remain there till the collision. The MASS of this VIRTUAL photon is gone. Only the last photons absorbed by the electron will add mass, but I don't expect this to be much. The excess in energy observed afterwards can be explained with (1) the energy of virtual photons that have *JUST* been absorbed before the collision takes place (2) the much larger part is because of virtual photons that remain due to added energy from the collision

damn, I'm already wrong --> a smaller part *is* kinetic energy
nonetheless, the kinetic energy concept seems to be needless in everyday scenarios such as throwing a baseball (the source of energy is different ... that seems to be the key here)

"""even though I personally find the concept very dubious"""

I know many people who find it dubious
for some reason i like it
It's not illogical for me

6. Hi all,

Tom,

"Either there's an increase in mass OR there's an increase in kinetic energy. Which one is it????"

A few posts back I gave the following formula for the relativistic kinetic energy:

E<sub>k</sub> = (m - m<sub>0</sub>)c<sup>2</sup>

Since m<sub>0</sub>c<sup>2</sup> is constant, an increase in kinetic energy is equivalent to an increase in m, the (relativistic) mass. I know it's hard to accept, but relativity is really consistent .

c'est moi,

"...now it's also the momentum who is supposed to be the same (p=mv) so P is really an increase in energy according to you ... why call it momentum?"

Several reasons: first of all, it has the dimensions of a momentum (kg m / s), and one can derive a conservation relation for that quantity which is (if you interpret P right) exactly the same as the classical conservation of momentum.

Let me give you one adive: don't. Energy (kinetic, potential, or whatever you want to call it) is a concept. It just happens to be that certain combinations of observable physical quantities remain the same before and after something happens. For example, in a classical collision experiment:

mv<sup>2</sup><sub>before</sub>/2 = mv<sup>2</sup><sub>after</sub>/2

One simply defines the "kinetic energy" as the quantity that remains the same in this scenario. Ofcourse there's a lot more to "energy" than this simple example, but you shouldn't look for something that - IMHO - isn't intentionally there.

"nonetheless, the kinetic energy concept seems to be needless in everyday scenarios such as throwing a baseball (the source of energy is different ... that seems to be the key here)"

I disagree, the source of energy might be mechanical or biological, the concept of kinetic and potential energy make it very easy to calculate where the baseball will touch down.

Bye!

Crisp

7. Light does have mass, it's just very little. 4 1/2 pounds of sunlight hit the earth every day. Think about how much light hits us per day. Not very heavy stuff, is it?

8. """Several reasons: first of all, it has the dimensions of a momentum (kg m / s)""""

hum, silly I didn't see that (mv and mc²) BUT still, why is there any notion of momentum then? Why saying that a photon has energy and momentum? That's plain stupid.

+ an increase in momentum means an increase in veloctity (v) and not mass so the concept is not really the same like that
of course, in relatvity they start playing with maths and there you get relativistic mass (which is not really mass but energy but they call it mass but you can't measure the mass ... go figure)

"""Let me give you one adive: don't."""

to late I pounder about everything

"""Energy (kinetic, potential, or whatever you want to call it) is a concept. It just happens to be that certain combinations of observable physical quantities remain the same before and after something happens.""""

yeah but here it is different ---> how is the mechanical energy transferred to my baseball? how is this possible? it can't be done through photons ...

9. Lesion42,

Light does have mass, it's just very little. 4 1/2 pounds of sunlight hit the earth every day. Think about how much light hits us per day. Not very heavy stuff, is it?

Don't let the Einsteinians hear you say that!!

Tom

10. Hi all,

c'est moi

"hum, silly I didn't see that (mv and mc²) BUT still, why is there any notion of momentum then? Why saying that a photon has energy and momentum? That's plain stupid."

That's not stupid, it's practical. By giving the photon energy and momentum the relations for conservation of energy and momentum are preserved in situations where photons are involved. Even better: it turns out that in this is consistent for other situations aswel.

Concerning the unit of momentum: in relativity one usually works in special units (where c = 1 and becomes dimensionless). In these special units, the momentum is expressed in MeV/c. I hope this confuses you enough ... But nevertheless, even by inserting all proper units in eg. the expression for E<sup>2</sup>, you should find that the unit for momentum is kg m/s.

"yeah but here it is different ---> how is the mechanical energy transferred to my baseball? how is this possible? it can't be done through photons ..."

I guess this was why the concept of energy was introduced in the first place: to cover up the difficult situations where one does not know exactly what happens. But even in the baseball throwing scenario there is a nice explanation: since you're doing work on the ball (you're pushing/throwing it away) you give it more energy.

Tom,

"Don't let the Einsteinians hear you say that!!"

LOL! Wait a minute ? Light HAS mass ?

Bye!

Crisp

11. The concepts of momentum and energy and relativistic mass are abstract concepts. I think part of the problem some people (c'est moi, for example) are having here is that they want to be able to visualise momentum or energy as some kind of substance which is transferred between objects. It isn't. Energy not some type of glowing "stuff" passed from one thing to another. It is more of an accounting system used by physicists. It is useful because (whatever it is) it is a conserved quantity. The total energy that an isolated system has before some interaction is equal to the total energy it has after the interaction. Momentum is another, slightly different conserved quantity, making it useful. Why do we need <i>two</i> quantities - energy AND momentum, rather than just one? Because there are situations in which momentum is conserved and energy is not, for example.

The relativistic quantities of momentum, relativistic mass and relativistic energy are all related. Here are the equations for a single object of rest mass m<sub>0</sub>:

Relativistic mass is:
m = <font face="symbol">g</font>m<sub>0</sub>

Relativistic momentum is:
<b>p</b> = m<b>v</b> = <font face="symbol">g</font>m<sub>0</sub><b>v</b>

Relativistic energy is:
E<sup>2</sup> = (m<sub>0</sub>c<sup>2</sup>)<sup>2</sup> + (<b>p</b>c)<sup>2</sup>

Relativistic kinetic energy is:
K = (m - m<sub>0</sub>)c<sup>2</sup>

The bold symbols are 3-vectors, and <font face="symbol">g</font> is the relativistic factor:

<font face="symbol">g</font> = 1/sqrt(1-(v/c)<sup>2</sup>)

12. I think I'm getting what's going on here.

In electronics/electrical stuff, we say something has a certain amount of charge. Sometimes we can say it has positive or negative, or we can say the baseline is zero and all is above that. Either way achieves the same result, but in truth charge is nothing but a matter of equilibrium. Positive and negative are just opposite sides of a set of scales/balances, but we might just as well use positive numbers as measured against zero. Same thing.

So it seems to me that when discussing mass vs relativistic mass and such, we may be using different terms and different methods, we may even be using entirely different concepts such as acceleration and gravity, yet the end result is the same. In electronics we can use a few different forumlae and methods in some situations to achieve a correct answer. We can even convert a current source to a voltage source, or vice versa, when analysing a circuit, and either way get the same answer.

I think what I'm trying to get at is that the universe is about equilibrium, yin and yang, and even if two things are not exactly the same, they can be seen as the same in terms of their opposition or relationship. Conceptually the same in terms of what they achieve.

I know I'm not explaining my thoughts very well here, but I think I'm starting to get the reasons behind this difference.

13. It seems that my post yesterday never arrived ... and i've got a headache now so I'll keep it short

"""I guess this was why the concept of energy was introduced in the first place: to cover up the difficult situations where one does not know exactly what happens. But even in the baseball throwing scenario there is a nice explanation: since you're doing work on the ball (you're pushing/throwing it away) you give it more energy."""

yes you are right
sneaky
you perform work
but I am still poundering,
my thought is simply that only momentum should be used in this case (and as you said yourself, sometimes one interpretation fits better than another one) and not kinetic energy

simply analyse what you do:
you are moving around your arm
you arm is having momentum, it is doing work and it is applying a force on the ball so the ball is accelerating
You think, aha, there is input of energy and this must be conservated ..... but why should the ball carry it further?
The energy is lost, I think, through Heat and as friction between the muscle tissues (and some other things probably ...). The kind of energy that the ball is taking with it, is the momentum of your arm once it stops swinging and releases the ball.

"""The concepts of momentum and energy and relativistic mass are abstract concepts. I think part of the problem some people (c'est moi, for example) are having here is that they want to be able to visualise momentum or energy as some kind of substance which is transferred between objects. It isn't."""

Don't you think I've been told this already?
I still think energy is "stuff". If it's not stuff then it is meaningless to talk about it. They can't simply be 'concepts', it must be 'something', it must be 'stuff'. Why do they visualise Em fields in QP as exchange of virtual photons?? as exchange of stuff ... not as some kind of invisible lines

"""Energy not some type of glowing "stuff" passed from one thing to another."""

yes it is
the glowing part --> light

"""It is more of an accounting system used by physicists."""

you can't count a concept
there must be "stuff" to count

"""It is useful because (whatever it is) it is a conserved quantity."""

you cannot have a quantity of a concept
it must be "stuff"

"""Because there are situations in which momentum is conserved and energy is not, for example."""

give me an example where energy is not conserved ...

14. we are talking about VIRTUAL photons who carry energy, i.e. they remain for a split second. If the electron absorbs one it will not remain there till the collision. The MASS of this VIRTUAL photon is gone. Only the last photons absorbed by the electron will add mass, but I don't expect this to be much (piece added now: I think scientists assume that the mass increases right from the beginning and goes right up till the end .... as you can see, this seems to be wrong, the electron will a lot less heavier than is assumed, if I am right in this then only the catching-up model can explain why it won't be able to reach the speed of the photons pushing it). The excess in energy observed afterwards can be explained with (1) the energy of virtual photons that have *JUST* been absorbed before the collision takes place (2) the much larger part is because of virtual photons that remain due to added energy from the collision
just a reminder + I added a piece

15. give me an example where energy is not conserved ...

Hmmm... the only example off the top of my head where energy is not conserved is when reference frames are changed. Other than that, energy can be transformed from one form to another, but is still conserved. Any other examples ?

16. An example where energy <b><i>wont</b></i> conserved is TIME TRAVEL.

bye!

17. excuse me if i may be out of context here. i read through page 3 and the last page of this thread. well a wise man once told me to state the obvious because thats what most people fail to observe, so here it goes.

pertaining to the photon having a mass issue, remember that there is only energy and mass is one of the forms energy takes. it goes then to say that mass is stored energy, and energy can take one form without another (ie a momentum but no mass)

also, someone stated that nothing exceeds the speed of light. i assume this meant matter? consider the shadow the earth makes crossing in front of the sun as seen from some distance. if the distanc were great enough, the speed of the shadow would excel that of light.

18. What I was trying to say in my last post, although not very clearly, was: There is a difference between "equivalent to" and "same as". Nature has equivalences all over the place, although not "same as".

19. ## Light must have a mass

I am a bit confused because i have read some very convinceing opinions as to why light must have mass. Let us say for arguements sake, that a photon particle is kind-of-like like a hollow sphere. That sphere must be made of some matter or else it cannot be there, so it must have mass. Also, people say that space is curved, but how can it be? Gravity is the attraction objects have on each other, why should this mean that it bends space. How can you even bend space? I know it is not a perfect vacumm because it is full of particles but essentially, there is nothing there to bend. I have seen diagrams that dipict space on 3-d plain where it showed a section of the flat plain bent by a planet. This cannot be because you can't think of space on those terms, there are no flat sections that can be bent because there is nothing to bend.

20. cman-87:

Welcome to sciforums.

Yes, you are a bit confused.

<i>Let us say for arguements sake, that a photon particle is kind-of-like like a hollow sphere. That sphere must be made of some matter or else it cannot be there, so it must have mass.</i>

The problem is that a photon is <b>not</b> like a hollow sphere, so your argument is based on a false premise, which leads to a false conclusion. A photon is more like a bundle of electromagnetic energy (though even that is not a very good description). It has no rest mass.

<i>Also, people say that space is curved, but how can it be? Gravity is the attraction objects have on each other, why should this mean that it bends space. How can you even bend space?</i>

The bending described by general relativity is a bending in the geometry of spacetime. Space is not made of some type of substance which bends. What "bends" is the coordinate distances and times between events in spacetime. And the bending doesn't cause attraction, it just makes things look like there is an attractive gravitational force.

<i>I have seen diagrams that dipict space on 3-d plain where it showed a section of the flat plain bent by a planet. This cannot be because you can't think of space on those terms, there are no flat sections that can be bent because there is nothing to bend.</i>

Those diagrams are analogies which attempt to depict 4 dimensional spacetime on a two dimensional diagram. You have to mentally add at least one space dimension to them to get a correct picture.

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