Light (and heat) (split from What exactly is energy?)

[Dekan]

 

In relativistic kinematics, there is mass energy and there is kinetic energy. The sum of which give you the total energy.

 

 

Heat is not always photons, and sometimes light is heat. Sometimes it isn't. The situation is much more nuanced.

 

Thanks Swansont. Could you just clarify this please. Is "light" an actual "thing", or just a human-invented collective noun for lots of photons?

 

 

 

 

1

[swansont]

Light is electromagnetic radiation that we can often view as a bunch of photons. Sometimes the classical picture offers a better/clearer explanation.

[Dekan]

Light is electromagnetic radiation that we can often view as a bunch of photons. Sometimes the classical picture offers a better/clearer explanation.

Do you mean that there aren't really any photons? Just a continuous spectrum of radiation.

 

And the "photons" are only a kind of Physicists' invention. Devised to break the spectrum into imaginary discrete bits. Which can then be used to do Physics calculations.

Like breaking the continuous surface of the ocean, into imaginary discrete lines of Latitude and Longitude. Which can then be used to do Navigational calculations.

 

I can groove with that. But are so-called "photons" really in the same class as electrons - ie real, genuine, point-like particles?

[swansont]

Do you mean that there aren't really any photons? Just a continuous spectrum of radiation.

 

And the "photons" are only a kind of Physicists' invention. Devised to break the spectrum into imaginary discrete bits. Which can then be used to do Physics calculations.

Like breaking the continuous surface of the ocean, into imaginary discrete lines of Latitude and Longitude. Which can then be used to do Navigational calculations.

 

I can groove with that. But are so-called "photons" really in the same class as electrons - ie real, genuine, point-like particles?

 

Whether photons are real is more philosophy than science. They are our way of modeling the observation that the energy and momentum of light comes in discrete bundles. The energy depends on the frequency and you can't substitute (in most cases) having a higher intensity but lower frequency of light in an interaction. So the effect of having the electromagnetic field act as if photons exist is very real. i.e. nature behaves as if they are.

[Strange]

Do you mean that there aren't really any photons? Just a continuous spectrum of radiation.

 

Depending on what you mean by "really", I would say photons are just as real, and just as much an invention of physics, as electromagnetic waves are. But, as swansont says, that is philosophy not science.

[Sensei]

And the "photons" are only a kind of Physicists' invention.

 

Devised to break the spectrum into imaginary discrete bits. Which can then be used to do Physics calculations.

Do you feel heat on face when going outside in sunny day or while sunbathing.. ?

 

That are billions of billions photons per second hitting your body, and increasing temperature of your atoms.. Increase of temperature is detected by nerve cells and informing brain about it.

Put hand to strong laser > 1 W, and it will burn your body - large quantity of photons per small area unit per second will heat your body, and brain will be informed about damage, causing alarm, and reaction "run away".

 

Continuous spectrum = infinite energy = black body/ultraviolet catastrophe..

http://en.wikipedia.org/wiki/Ultraviolet_catastrophe

 

Average visible photon has wavelength ~532 nm

so energy of such photon is h*c/wavelength = 6.62607e-34 * 299792458 / 532e-9 = 3.73e-019 J

Sun is emitting at Zenith 1050 J/s of energy per m^2 (at least such amount is reaching ground at max). (I am simplifying calcs to not take care of UV/IR/MW - let's hypothetically assume the all is from visible spectrum)

 

1050 J / 3.73e-019 J = ~ 2.8e+21 photons per m^2 per second.

That's average 2812 photons per 1 nm^2 per second of your face.

 

More photons per area unit per second = more total energy = solid materials are melting, liquids are turning to gas, and in extreme situation gas is ionized (f.e. explosion of nuclear weapon, creates plasma inside).

(and it can be used to cutting metals or other materials in laser cutting machines )

 

Some of these photons are absorbed, some are reflected (therefore other people can see you in day light), others are refracted.

If your cloth is black, you feel hot, black surface is good visible photons absorber.

while wearing white cloth, you feel better at summer, visible photons are reflected.

Edited July 22, 2014 by Sensei

[Enthalpy]

[...] But are so-called "photons" really in the same class as electrons - ie real, genuine, point-like particles?

 

You shouldn't take "point-like" as a definition of "particle". Our real need for particles is that some of their properties can't split: the electron's charge, spin, the photon's energy... so we count these things with integer numbers.

 

When for instance an electron in a semiconductor detector absorbs a photon, the electron is very big (many thousand atoms volume) before, very big after, and the photon was generally very big, say the size of a pixel. Telling more accurately where (for instance, at which atom) the absorption occured would be provably wrong.

 

Whether particles are permanently points whose position is probabilistic (hi Swansont and the majority of scientists) or are the wave itself (my opinion presently, with good companions like Schrieffer) make little concrete difference. I feel a wave and nothing more is easier to figure, but maybe I get in trouble in the future.

 

What's certain and is agreed on: particles can concentrate and keep their fixed attributes like the charge, unsplit and available once. "Fundamental" particles like the electron can concentrate as much as our experiments permit to check, while composite particles like the proton show subcomponents if the experiment detail them enough.

 

This ability to expand or concentrate (or at least their waveform) is common to all particles, so you shouldn't distinguish between electrons and photons here. A 0.5µm wavelength photon coming from a star can be light-years wide; the telescope concentrates it to a pixel 5µm wide; it's absorbed by one electron 10nm wide, or if strayed, can again be meters wide. Exactly the same way, electrons can be large in a metal or a big molecule, but an electron beam can tell much more precisely where an interaction with them happened; the target electron "decides" then (and no earlier) if it interacts hence where it is.

 

Size and shape are just some variable attributes of particles among many. Other attributes like the momentum, the angular momentum... are uncertain (more or less uncertain depending on the particle's state) which evolve according to the subsequent interactions; sometimes these interactions are measurements, which try to determine accurately one of the particle's attributes (be it the position or an other) and then the particle's state becomes simpler for that attribute - other attributes become more uncertain as a consequence. For instance, if a measurement forces a particle to decide an accurate position, the momentum gets uncertain as a consequence.

[Dekan]

@ Sensei #6 and Enthalpy #7:

 

Thanks, and I appreciate your detailed and well-reasoned posts, which I've studied as best I can.

 

The arguments and information they contain are impressive. But sadly, I still retain a kind of "gut-feeling" that an "electron" is more a real thing than a "photon" is!

 

Cheers, and thanks again for your replies.

[Enthalpy]

One thought about particles being or not permanent points...

 

In a hydrogen atom (because we know algebraic solutions for that one), a nucleus supposed immobile gives 1/2000 error. One has to compute the orbitals using the center-of-mass of the electron and nucleus, and a reduced electron mass. This implies a difference between the spectra of protium and the heavier deuterium, which is observed, fits the mass ratios, and not the magnetic interactions.

 

Particle representations where the electron is permanently a point tell here that "when" the electron is at x, the proton is at -x/2000, done (only "when" is abstract here, since the electron has all positions at the same time). On the other hand, as I say "the electron is the wave", is seems harder to let the proton work as a counterweight of the immobile and centered wave.

 

So is that an argument for the permanent point and against the naked wave? I now believe not.

 

The trick is that QM writes a single wave for both particles: Wave(x_E, x_P) * phase_W( t). Then, if Psi(x_E) * phase_E( t) is a solution for the electron around an immobile proton, we can write the solution with a proton of finite mass like:

Wave(x_E, x_P) = Psi[x_E*(1+m_E/m_P)] * Psi[x_P*(1+m_P/m_E)] * Delta[x_E*m_E+x_P*m_P].

Delta is here Dirac's function and positions refer to the center of mass.

 

What remains is QM's abstract idea that two particles can have undefined positions, all over the time (orbitals are stationary), but both positions would correlate if they were measured: the proton being at -x/2000. This uneasy and fundamental idea is necessary in every representation of particles, and at least to my personal taste, permanent points being everywhere do not ease it as compared with naked waves.

 

Please keep in mind that the permanent point is the most usual representation (but I'm far from alone).

[...] I still retain a kind of "gut-feeling" that an "electron" is more a real thing than a "photon" is! [...]

 

What do you want to call a particle "real"?

 

Both particles are routinely observed individually. It's not a matter of colliders or huge physics lab: engineers see both with a perfectly banal setup.

 

Both particles are waves. This tells why matter has a volume.

 

Both particles can be created and can disappear. Individually and under common conditions for the photon, only with MeV energies and together with a positive particle for the electron.

 

Where do you feel a big difference?

Edited July 30, 2014 by Enthalpy

[swansont]

Whether particles are permanently points whose position is probabilistic (hi Swansont and the majority of scientists)

 

Hi yourself. I would prefer it if you did not try and represent my position on, well, anything. That's for me to do. It's annoying because people invariably get it wrong,as you have done here.