Mass of photons

[abskebabs]

Hi everybody. I was just wondering do all photons and neutrinos really have zero mass or is this just to make equations work? It just confused me because my physics teacher told me that photons have zero mass, and this allows them to travel at the speed of light without becoming infinitely heavy; yet i know that photons do have a momentum, so it would seem that them having zero mass would be impossible. Can anyone help me resolve this? Thanks in advance.

 

P.S. if anyone has a good knowledge of quantum physics, please could you have a look at the Quantum evolution thread I've posted in the physics forum. I'd just like to know what you think.

[insane_alien]

neutrinos have mass photons do not.

 

photons however do have momentum according to the equation p=h/wavelegnth

just because momentum can be derived by m*v doesn't mean that it is the only way it can have momentum.

[RyanJ]

Hi everybody. I was just wondering do all photons and neutrinos really have zero mass or is this just to make equations work? It just confused me because my physics teacher told me that photons have zero mass' date=' and this allows them to travel at the speed of light without becoming infinitely heavy; yet i know that photons do have a momentum, so it would seem that them having zero mass would be impossible. Can anyone help me resolve this? Thanks in advance.

 

P.S. if anyone has a good knowledge of quantum physics, please could you have a look at the Quantum evolution thread I've posted in the physics forum. I'd just like to know what you think.[/quote']

 

As far as I know all photons have 0 mass, I'm not shure if this is just too make the equations work but due to [math]E=mc^{2}[/math] nothing can travel at light speed if it has mass.

 

As for Neutrinos I'm not shure (Last I read it was under debate) according to Wikipedia (http://en.wikipedia.org/wiki/Neutrino) they do have mass though.

 

I'm bad at explaining and I really don't understand quantum physics myself so I can probably not be a lot of help here I am afraid

 

Cheers,

 

Ryan Jones

[Klaynos]

As far as I know all photons have 0 mass' date=' I'm not shure if this is just too make the equations work but due to [math']E=mc^{2}[/math] nothing can travel at light speed if it has mass.

 

As for Neutrinos I'm not shure (Last I read it was under debate) according to Wikipedia (http://en.wikipedia.org/wiki/Neutrino) they do have mass though.

 

I'm bad at explaining and I really don't understand quantum physics myself so I can probably not be a lot of help here I am afraid

 

Cheers,

 

Ryan Jones

 

[math]E=mc^{2}[/math] <--- the correct form of this is the equation in my signiture, it shows that 0 mass particles have wavelength.

 

Photons have no mass. Neutrinos have a very small mass.

[swansont]

As insane_alien touches upon, mv is a classical equation for momentum, and as the photon is not a classical entity, its momentum is not described by that equation.

[timo]

I assume your confusion about "how can anything have momentum if it doesn´t have mass" comes from the momentum in classical newtonian mechanics which is given as [math] \vec p = m \vec v[/math], there. The terminoligy in classical newtonian mechanics normally uses the parameter [math] \vec v [/math] to describe kinematics. The big advantage is that the formulas become somewhat suggestive.

 

 

From a more modern standpoint it is better to consider momentum being the parameter which describes the kinematics:

- In contrast to [math] \vec v [/math], for which [math] | \vec v |<c [/math], the momentum [math] \vec p [/math] can, in general, have any value.

- Momentum is conserved under any processes (like the collision of two cars), velocity isn´t.

- You don´t run into the problem with massless particles

 

Using the relativistic equations, energy and velocity as a function of momentum are calculated as:

[math] E = \sqrt{ m^2c^4 + | \vec p |^2 c^2 } [/math]

[math] \vec v = \frac{\vec p c^2}{E} [/math]

Using these, you can easily see that massless particles have a velocity of c (regardless of their momentum) and that particles with m>0 have a velocity <c (also regardless of their momentum).

 

IN SHORT: Think of momentum as the fundamental property, not velocity.

 

 

Hi everybody. I was just wondering do all photons and neutrinos really have zero mass or is this just to make equations work?

You cannot change nature (the masses of the particles are given by nature, not chosen by man) to make it fit your equations. It´s done the other way round. Neutrinos almost certainly have a mass. It is not known if photons have a mass; they are treatened as massless in all cases I know of. The equations in which photons and neutrinos are massless are good aproximations as long as the momenta of the particles are large compared to their masses. For photons, this has always been the case, so far.

 

It just confused me because my physics teacher told me that photons have zero mass, and this allows them to travel at the speed of light without becoming infinitely heavy;

See above for why only particles with zero mass can have a velocity of c.

This "mass increases with velocity"-thing is a very bad habit. I don´t want to go into detail here, I´ve seen too many discussion about it. If you´re interested in this, the terms "rest mass" and "relativistic mass" are the one to look for. I use "rest mass" in this post.

[abskebabs]

thanks a lot everybody you've helped clarify things.

[5614]

Also the fact that with Klaynos's sig it says [math]m_0[/math] the subscript 0 means it is referring to the rest mass of a photon.

 

This means its mass when it is not moving.

 

As a photon is always moving at c (speed of light) relative to any observer it cannot not move. It cannot be at rest and therefore cannot have a rest mass.

 

=====

 

It was once thought that neutrinos had a mass, but it was then proved that they did indeed have mass. Because of this they do not travel at the speed of light, they travel a bit slower than it.

[anglepose]

Photons Must have mass or how come a black hole is black

gravity doesnt attract nothing

[ydoaPs]

what? try rephrasing in english.

[anglepose]

black holes suck in light and light is bent by planets. There for light is affected by gravity. Light is if im correct made of photons.

photons like electrons have a mass.

that is why there affected by gravity.

[the tree]

I think what he's saying is that photons must have mass to be affected by gravity. I guess he forgot that the equation for graviaty doesn't involve the mass of the attractted body.

[anglepose]

then how come gamma beta and alpha radiation escape from a black hole this is shown in radio pictures of black holes

 

 

 

 

 

sorry if im wrong.

[anglepose]

go to http://www.llnl.gov/pao/news/news_releases/2005/images/barish_figure359x194.jpg

[JustStuit]

I wasn't aware they had shown alpha and beta emitted, is this correct?

[JustStuit]

Candidates for stellar-mass black holes were identified mainly by the presence of accretion disks of the right size and speed' date=' without the irregular flare-ups that are expected from disks around other compact objects. Stellar-mass black holes may be involved in gamma ray bursts (GRBs), although observations of GRBs in association with supernovae or other objects that are not black holes [2'] [3] have reduced the possibility of a link.

I suppose gamma is possible but I'm pretty sure there isn't alpha radiation.

[anglepose]

Also isnt it defieng the laws of relitivity to say a photon has no mass

becuase how on earth does a photon have energy if it has no mass.

even an electron Pure energy has a mass.Theres no dennying a phton has energy

[Severian]

Black holes suck.

[swansont]

Also isnt it defieng the laws of relitivity to say a photon has no mass

becuase how on earth does a photon have energy if it has no mass.

even an electron Pure energy has a mass.Theres no dennying a phton has energy

 

Quite the contrary; it is conforming to the laws of relativity to say that the photon has no mass. Photons are different from other particles in several ways, e.g. they always travel at c. One of the many strange things about relativity and quantum physics, and just serves to remind us that all of nature is not a simple linear extrapolation of our everyday experiences.

 

Electrons aren't pure energy.