Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 29/12/2009 23:55:18

It seems as Planck scales (http://en.wikipedia.org/wiki/Planck_scale) decides what exist and doesn't exist inside SpaceTime.
So my question is, is a photon also inside that Planck scale?

A photon can't be characterized as having a size. It has a wavelength that can be infinitely short or infinitely long, an infinitely large amount of energy or infinitely small amount of energy. Therefore it can be inside the Planck scale.

Ah ah :)
Infinities but, inside SpaceTime?
You're sure about that?
If you're thinking of virtual photons I agree but they are not inside SpaceTime, only their 'work' is.

Perhaps you are considering photons to be particles? While electromagnetic energy transmission through free space can exhibit some particle like behaviour, I don't think it has been established that this is the result of movement of actual particles.

Current quantum theory consists of laying a quantum framework over a classical spacetime. All the forces except gravity are part of the quantum framework. Gravity is accounted for by the classical behavior of spacetime. As I understand it (from reading the wiki and other sources), when you work with things on the Planck scale, the assumption that spacetime is classical breaks down, and you have to deal with a quantum theory of gravity. Until we have a tested theory of quantum gravity, we won't be able to model what happens at the Planck scales. It might well be that something that small can exist, but it would be described by something beyond our current models.
As to particle size limits, our current models predict that fundamental particles interact as infinitely small points. However, in between interactions, they are described by quantum wavefunctions which are spread out over some small region of space (called the Compton wavelength). So long as the wavefunctions are spread out over regions of space larger than the Planck length, current quantum field theory can describe them. If the particles were to have Compton wavelengths smaller than the Planck length, a new theory would be needed.
I believe this also applies to photons, where the Compton wavelength is simply the wavelength of the photon. This means in practice that at small enough wavelengths (high enough frequencies), current quantum field theory probably doesn't do a great job of describing photons.
There is one issue with this that I'm not 100% sure on. You can always change the frequency of light relative to yourself by simply accelerating which introduces a Doppler shift. You could imagine a photon had high enough energy that it was just on the verge of the Planck scale. If you accelerated towards it, you would see its wavelength decrease relative to you and drop below the Planck scale, but someone who hadn't accelerated would still see the photon above the Planck scale. I'm not sure how to interpret that...

Well, there you have it. I think JP summed up my views on the subject quite accurately. [;D]

So sweet :)
I love it, makes my head spin, wondering over all the possibilities involved here.
Isn't the Compton wavelength where the electron (http://en.wikipedia.org/wiki/Black_hole_electron) breaks down to a black hole
So if I sum up JP then you say that either a photon needs to be inside Plank size or we will need a new theory, and that goes any which way? 'Wave' as well as 'point like particles'??
Or is it that when seen as 'point like particles' they have no size at all?
Thanks to both of you.
And a good new year.

And this goodie (http://www.thenakedscientists.com/HTML/content/questions/question/2395/) from here.
"Any photon energetic enough to precisely measure a Plancksized object could actually create a particle of that dimension, but it would be massive enough to immediately become a black hole (a.k.a Planck particle), thus completely distorting that region of space, and swallowing the photon. ""
This also seems to point to that a photon is expected to be over Plank size?
The Quantum Mass Spectrum of the Kerr Black Hole.pdf (http://arxiv.org/pdf/0812.5012%20%3Cem%3E(*crosslisting*)%3C/em%3E)

I think it's important to remember that the Planck units don't limit what can exist, only what can be worked out within current theories. For example, if the shortest possible distance is the Planck distance and the smallest time period is the Planck time unit, it would appear that everything must move at the speed of light 'c'; moving one Planck distance in one Planck time unit = 'c'. To move slower than 'c' then, requires things to happen in smaller periods than the Planck time or over shorter distances than the Planck distance.
An interesting aspect to this discussion though, is the inverse square law, applicable to EM waves/photons, which implies that area i.e. two dimensions are relevant.

Interesting idea LeeE. "For example, if the shortest possible distance is the Planck distance and the smallest time period is the Planck time unit, it would appear that everything must move at the speed of light 'c'; moving one Planck distance in one Planck time unit = 'c'. "
If I get it right (?) you're implying that the speed of light in a vacuum then should have a direct relevance to those two 'constants'? like if something move one Planck distance in one Planck time, then it could represent 'c' as that is the constant for the fastest velocity ever (Inside SpaceTime)?
Never thought of it that way, but it makes a weird sort of sense :)
It should be testable if it was correct, so, is it?
If it was so, then all other things moving slower should need more Plank time per Planck distance, right?
And it would be interesting to know what 'C' would come out as, translated into that concept, even if there is several Planck time to every Planck distance for it. Although to be perfect :) the first suggestion would be cool. One PD per PT == 'C'

But íf it turns out that 'c' would be too slow though?

I think it's probably more a case of those two constants being derived from 'c', in conjunction with a few other 'natural' constants. The idea behind the Planck units is a bit like using the Metre, Kilogram, Second units, as opposed to feet, pounds and hours etc; when you use these unified sets of constants things are easier to work with. However, some aspects of physics theory are based upon some of the Planck constants, so they don't seem to work with smaller values.
Don't forget that the Planck units are not exclusively small; some of them are very large indeed i.e. the Planck Temperature is 1.416 785 × 10^{32} K and the Planck Density is 5.15500 × 10^{96} kg/m3

Yeah, but I liked it, it was a new one to me :)
I mean, if we do have a symmetry to SpaceTime I would expect it to be reflected in our 'constants' too?
On the other hand we have those CP violations (http://en.wikipedia.org/wiki/CP_violation) so?

As you say Planck constants is where Quantum physics ends, right?