Existing without mass?

[Ganymede12]

If photons don't have mass then how do they exist?

[George Jones]

They have energy. I very much related question is "How do electromagnetic waves (light, TV, radio, etc.) exist?" Electromagnetic waves don't mass, but they do have/carry energy and momentum.

[Ganymede12]

So photons are energy?

[George Jones]

Each photon is a quantum of the electromagnetic field that has a specific energy and a specific momentum.

[Ganymede12]

eh?

I haven't started my GCSE's yet in school (that will be next year) so I'm lacking quite a bit of knowledge!

[Capricorn]

Physics is/can be a strange field.

Why not existing without mass ?

Nothing seems to prevent it.

In physics you will have to get comfortable with far stranger ideas then this one.

As I recall a photon is defined as having zero rest mass, but they are not at rest, they wander around at c, which by relativity means they could have infinite mass. It depends on what 0 divided by 0 is. :grin:

So you may have to just accept they can have a mass anywhere from zero to infinity.

Starting to get the idea of strange ?

[sologuitarist61]

Physics is/can be a strange field.

Why not existing without mass ?

Nothing seems to prevent it.

In physics you will have to get comfortable with far stranger ideas then this one.

As I recall a photon is defined as having zero rest mass, but they are not at rest, they wander around at c, which by relativity means they could have infinite mass. It depends on what 0 divided by 0 is. :grin:

So you may have to just accept they can have a mass anywhere from zero to infinity.

Starting to get the idea of strange ?

This is a strange conundrum isn't it? If a photon has energy then surely it must have some mass as well, hence E=mc2 (sorry don't know how to do superscript!!), so I understand Capricorn stating that a photon can have infinite mass as it moves at the speed of light, but don't understand why it doesn't at rest - surely a photon is never at rest, so must always have mass?

Perplexed of Cornwall

[George Jones]

eh?

Sorry. Yes a photon has energy (and monetum).

I haven't started my GCSE's yet in school (that will be next year) so I'm lacking quite a bit of knowledge!

Now you have confused me! As my avatar on the left indicates, I am not in Britain, and I am not entirely sure what "haven't started my GCSE's yet" means. You givve your age in your profile, so I think that where I am it would mean something like "haven't started high school yet".

Electromagnetic radiation is made of electromagnetic waves of different wavelengths. You are familiar with many examples of electromagnetic waves: each colour of visible light has a different wavelength; gamma rays have wavelengths shorter than light; infrared has wavelengths longer than light; microwaves have wavelengths longer than infrared; TV and radio signals have wavelengths longer than microwaves.

Qunatum theory applied to electromagnetic radiation shows that electromagnetic wave are "not continuous", they are "grainy". Each elctromagnetic wave is made of zillions of individual bits called quanta (singular, quantum), that is, made of photons. All thes littles bits of energy add together to give the total energy of the wave.

Keep asking questions, and keep reading!

[sologuitarist61]

Thanks George that helps explain it very well.

Just for your information Ganymede is probably 13 or 14 if he is takijng his GCSE's next year.

[themos]

If a photon has energy then surely it must have some mass as well, hence E=mc2

No, that does not follow. What you can say is that you can bang together two photons, their energies will add and their momenta will cancel (as they add like two opposite vectors). That energy can be turned into two particles with mass going in opposite ways. E=m*c*c then tells you how much energy you would need to create particles of mass m.

[themos]

surely a photon is never at rest, so must always have mass?

It's precisely the other way: if something can never be at rest then it has no mass and vice versa.

[sologuitarist61]

It's precisely the other way: if something can never be at rest then it has no mass and vice versa.

I think I am getting out of my depth here, so will retire .... exit stage right!

[George Jones]

I am going to try to explain some of this using some maths.The level of the maths is CGSE (sorry, Ganymede12), so give it a go, folks. If, howver, this not your cup of tea, ignore.

One of the most famous equations, E = mc², is actually a special case of a somewhat less famous equation. This famous special case applies only to massive particles that are at rest (not) moving. The more general equation,

E² = (mc²)² + (cp)²,

applies to this case, and also to moving particles and to massless particles. Here, p is the momentum of the particle.

If a particle is not moving, it has no momentum. Putting p = 0 into the above equation gives

E² = (mc²)².

Taking the square root of both sides of this equation gives the famouus equation E = mc².

What about massless particles? Putting m = 0 into the above more general equation gives

E² = (cp)².

Taking the square root of both sides of this equation gives E = cp. So, massless particles are possible as long as they have energy and momentum related by this equation. If massive particles have energy, they must have momentum. Massless particles must move, they cannot be at rest! A little more work shows that massless particles must move at the speed of light.

[Ronnie67]

Photons are basically the most visible portion of the electromagnetic spectrum. This was one of the major breakthroughs Einstein and the father of quantum physics, Planck made about the nature of light. This link is what is behind the photoelectric effect that makes solar power possible.Because light is another form of energy it can be transferred or converted into other types. In the case of the photoelectric effect the energy of light photons is transferred through the photons bumping into the atoms of a giving material. This causes the atom that is hit to lose electrons and thus make electricity.

Read more: http://www.universetoday.com/74027/what-are-photons/#ixzz203cNnaUu

[Ganymede12]

As I type this I am watching a Brian Cox documentary (the're the best!) and Prof Cox has just said that Mass is a very dens form of energy! Ergo, mass is energy!

For the record, yes I am 14 and I am just about to end year 9.

Thanks for all that math, I'm sure it will come in handy (where's my pen......)

[SionR25]

Taking the square root of both sides of this equation gives E = cp. So, massless particles are possible as long as they have energy and momentum related by this equation. If massive particles have energy, they must have momentum. Massless particles must move, they cannot be at rest! A little more work shows that massless particles must move at the speed of light.

Sorry to be awkward here , but isn't the definition of momentum; mass x speed. So if you are taking photons to not have any mass then surely p=0?

We just started this at school before summer, looking at (can't remember the name ) the thing with one black side and one silver that will spin in a vacuum when light shines on it. As it was the last week no one really felt like doing any theory on it .

Sion

[George Jones]

Sorry to be awkward here ,

You're not being awkward at all.

but isn't the definition of momentum; mass x speed.

This definition of momentum, which is only valid for massive particles, doesn't take into account special relativity. Remember your thread on the Lorentz factor? v involves distance over time, and time involves the Lorentz (gamma) factor. When special relativity is taken into account, spatial momentum is given by mv times the Lorentz factor. The Lorentz factor goes to infinty as v approaches c, so a massive particle's momentum goes to infinity as its speed approaches the speed of light. Momemntum can never actually get to infinty, so a masive particle's speed can never actually get to the speed of light. Relativity shows that a massive particles must move slower than the speed of light!

So if you are taking photons to not have any mass then surely p=0?

Not if if v goes to c. If v goes to c at the same time as m goes to zero, we get (taking into account the Lorentz factor) zero times infinity, which is undefined.

We just started this at school before summer, looking at (can't remember the name ) the thing with one black side and one silver that will spin in a vacuum when light shines on it. As it was the last week no one really felt like doing any theory on it .

Yes, this is a cool demonstration.

[JamesF]

We just started this at school before summer, looking at (can't remember the name ) the thing with one black side and one silver that will spin in a vacuum when light shines on it. As it was the last week no one really felt like doing any theory on it .

I think what you're talking about there is "Crookes Radiometer". Mind-blowing thing to see if no-one has explained how it's believed to work. Actually, I think it's a mind-blowing thing anyhow

For those interested in why E=mc^2 and why it doesn't quite apply, I recommend Brian Cox and Jeff Forshaw's "Why does E=mc^2?" book. It's not for the maths-phobic, but they do try to walk you through it as easily as possible and to be honest it's not really hard maths in the first place, though it might require the occasional pause and reality-check.

James

[SionR25]

Thanks for the explanation

I think what you're talking about there is "Crookes Radiometer". Mind-blowing thing to see if no-one has explained how it's believed to work. Actually, I think it's a mind-blowing thing anyhow

For those interested in why E=mc^2 and why it doesn't quite apply, I recommend Brian Cox and Jeff Forshaw's "Why does E=mc^2?" book. It's not for the maths-phobic, but they do try to walk you through it as easily as possible and to be honest it's not really hard maths in the first place, though it might require the occasional pause and reality-check.

James

I should probably finishing reading this before posting anymore questions .

Sion

[symesie04]

Isnt it just a lot easier to say that particles aquire mass by interaction with the Higgs field, photons never interact with the Higgs field and therefore never aquire mass?

[Ganymede12]

I asked my chemistry teacher about mass being a dense form of energy and he said that mass can only be turned into energy .

I also asked my Physics teacher about the Higgs boson and he said it's just one step down from quarks.

It is at this point where I'm a bit lost.

I don't normally say this but, Can you dumb it down for me?

[JulianO]

Mass and energy are interchangeable. Most of the time we convert mass into energy. Burning fossil fuels converts a tiny tiny bit of mass into energy. Nuclear reactions do a bit better, anti-matter matter annihilations go the whole way. You get the full mc²!

Going the other way is less common, but its what they're doing in the LHC. Bang together two particles with lots of kinetic energy and you can create mass. Given the c²'diness of the conversion - you need a lot of energy to create a little bit of mass, so thats why its not common in everyday life. It happened a lot in the big bang, and elsewhere now where there is lots of energy (usually in the form of gamma rays).

As far as Higgs goes, it's a force carrying boson.

So photons are the bosons that carry the electromagnetic force. Exchange of photons is what makes opposite charges attract and like repel.

Gluons carry the strong force - they bind together nucleons like protons and neutrons.

W & Z the weak force. Not really what I think of as a force, but it allows radioactive decay by exchange of these.

The Higgs carries the "mass" force. I think I'm right when I say the exchange of Higgs is what gives mass.

There is also the theoretical graviton that carries gravitational force. Gravitons exchanged cause the force of gravity.

[JamesF]

Some particle physics is stupendously counter-intuitive and very difficult to simplify because, well, at a simple level it just doesn't make sense. It helps if you work up to it by learning about things that are only a bit counter-intuitive first

I'd disagree that the Higgs Boson is "one step down from quarks". It's more of a step sideways. Basically, the "Standard Model" of particle physics has a number of different groups of elementary particles, where elementary means they can't be broken down any further. There are the quarks (up/down/top/bottom/strange/charm) which combine to form protons and neutrons, the leptons (electron, tau and one other that I can't recall the name of, plus their neutrino counterparts) and the bosons (W,Z,gluon,photon and Higgs, assuming it has really been found, plus possibly the graviton, if it even exists at all). The quarks and leptons tend to be associated with matter whilst the bosons tend to be associated with forces though that all seems to get a bit muddy in quantum theory. What the bosons are beyond that is, well, beyond me You might as well envisage them as different sorts of pixies. Photon Pixies manage electromagnetic fields and forces, whereas the Higgs Pixies deal with mass. W and Z Pixies make sure the weak nuclear forces work (I think and Gluon Pixies look after the strong nuclear force.

Pixies can be used to explain a lot of physics. They're really quite handy.

James

[Ganymede12]

_{Ok, so Higgs bosons are responsible for force and Mass can be converted into energy.}

_{I think I understanding it now.}

[themos]

The Higgs carries the "mass" force. I think I'm right when I say the exchange of Higgs is what gives mass.

I'm afraid that is wrong. The Higgs of the Standard Model is a "scalar" field (not a "vector" or a "tensor") which means it has spin 0, so has no polarisation states. It is very different in this regard from the photon, the "weak" bosons W and Z, the "strong" gluons and the graviton. The theory says that it is the only field that has a "frozen" component, which means the value of the field is non-zero even in the best vacuum and it is this "frozen" field that makes other particles have mass, provided they interact with the Higgs.

Additionally, because it is a "weak-spin" 1/2 particle, the field has 4 degrees of freedom. Three of those are "swallowed up" when the W+, W- and Z bosons become massive (massive bosons have 3 degrees of freedom, massless have only 2). The remaining one is what the physicists have been looking for, a Higgs "particle" of some mass. If it's not there, then we have trouble explaining why the photon stays massless.