photon size

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Re: photon size

Postby doogly » Mon Jul 18, 2011 8:49 pm UTC

I tend to think of them much less as points in configuration space than as rays in hilbert space.
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Re: photon size

Postby ultramadscientist » Thu Nov 10, 2011 12:59 am UTC

Wouldn't the photon's "size" be approximately the plank length? A string would vibrate to create a particle, and a string is at the minimum a plank length in diameter. Being massless I would hope that a photon holds the minimum size.
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Re: photon size

Postby POMPEYEAGLE » Fri Nov 18, 2011 7:19 pm UTC

Hi,

having originated the original question a long time ago, the conclusion I get from this debate is that nobody knows the answer. Is that a fair conclusion?

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Re: photon size

Postby Yakk » Fri Nov 18, 2011 7:59 pm UTC

No, that is not correct.

A better answer is you are asking a question akin to "how tall is the economy". You are using an ambiguous term (tall, or in your case size) on something (the economy, or in your case a photon) that makes your question move between meaninglessness and ambiguity.

The "nobody knows the answer" implies that your question was well formed.
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Re: photon size

Postby oxoiron » Fri Nov 18, 2011 8:40 pm UTC

UK_system7 wrote:It [a photon] has a curious relation with electron orbitals in that it likes interacting with electron clouds of similar cross-section. Thus visible light has a wavelength roughly of 10 to the minus 10 metres, which happens to be the size of related electron clouds in an atom.
When you say "roughly", how many orders of magnitude error are you giving yourself? I ask, because you seem to be implying that visible light has a wavelength of about 0.1 nm, which is clearly not true (unless you live in a world with a vastly different refractive index than the one in my world, in which case I retract my statement).

EDIT: Changed 1 nm to 0.1 nm
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Re: photon size

Postby Charlie! » Fri Nov 18, 2011 9:48 pm UTC

ultramadscientist wrote:Wouldn't the photon's "size" be approximately the plank length? A string would vibrate to create a particle, and a string is at the minimum a plank length in diameter. Being massless I would hope that a photon holds the minimum size.

If a photon behaved like a wavepacket with a diameter of a planck length, what size orifice would we need in order to see photons diffract? At what size orifices do we start to see photons diffract in the laboratory? This is the sort of empirical gut check that should have told you that this guess wouldn't work.

And remember that size is not mass. In our everyday life lighter things are usually smaller, but the fact that size and mass are different units should make you stop and think "is that a universal law, or is there some mediating factor that might not be present for fundamental particles?" The mediating factor is density, and density indeed does not work intuitively for fundamental particles. If you wanted to make a sculpture of a fundamental particle you would use clay or wood or metal, and all have pretty much the same density, so a light particle would have to be smaller. But a photon isn't made out of clay, it's made out of electromagnetic fields, which can have any density at all - and so the relation between size and mass breaks down.

POMPEYEAGLE wrote:Hi,
having originated the original question a long time ago, the conclusion I get from this debate is that nobody knows the answer. Is that a fair conclusion?

Given the state of the thread it's not an unreasonable conclusion, but it's incorrect. "What is the size of a photon" is like the question "what is the depth of a poet?" It doesn't make quite enough sense to have one exactly right answer, but it has several almost right answers ("about a foot and a half", "the wisdom she imparts to her writing"), and unlimited wrong answers ("a lightyear"). You may have to do a bit of legwork to separate the almost right answers from the wrong answers in this thread, but you should think of it like an adventure :P
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Re: photon size

Postby Xami » Fri Nov 18, 2011 11:36 pm UTC

Charlie! wrote:Given the state of the thread it's not an unreasonable conclusion, but it's incorrect. "What is the size of a photon" is like the question "what is the depth of a poet?" It doesn't make quite enough sense to have one exactly right answer, but it has several almost right answers ("about a foot and a half", "the wisdom she imparts to her writing"), and unlimited wrong answers ("a lightyear"). You may have to do a bit of legwork to separate the almost right answers from the wrong answers in this thread, but you should think of it like an adventure :P


There are exactly the same number of right answers as wrong answers to any question. I say nothing of how useful the answers are. I also don't know in the case that the question does not have a correct answer.
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Re: photon size

Postby Charlie! » Sat Nov 19, 2011 1:34 am UTC

Xami wrote:There are exactly the same number of right answers as wrong answers to any question.

This bald assertion cannot withstand the power of... Multiple Choice Exaaaaaaaaams!
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Re: photon size

Postby thoughtfully » Sat Nov 19, 2011 6:27 pm UTC

Charlie! wrote:The mediating factor is density, and density indeed does not work intuitively for fundamental particles.

It's not only for fundamental particles that the usual intuitions about density fail. Degenerate matter behaves pretty oddly, too. For instance, brown dwarfs and large gas giants are all roughly the same size, although their masses can vary by a couple orders of magnitude. That's why Jupiter and Saturn aren't so dissimilar in size, but Jupiter is over three times as massive, although they're made of basically the same stuff!

Saturn is actually less dense than water.. it would float in a giant ocean, if such a thing made sense!

Degenerate stellar remnants such as white dwarfs and neutron stars actually shrink as they get heavier.
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Re: photon size

Postby Xami » Mon Nov 21, 2011 4:11 pm UTC

Charlie! wrote:
Xami wrote:There are exactly the same number of right answers as wrong answers to any question.

This bald assertion cannot withstand the power of... Multiple Choice Exaaaaaaaaams!


Xami wrote:I say nothing of how useful the answers are. I also don't know in the case that the question does not have a correct answer.


Given C is what gets you the good stuff
"Correct" answers
Not A
Not B
C
Not D

"Wrong" Answers
A
B
Not C
D
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Re: photon size

Postby eSOANEM » Mon Nov 21, 2011 10:26 pm UTC

Xami wrote:Given C is what gets you the good stuff
"Correct" answers
Not A
Not B
C
Not D


If by "correct" we mean a necessary and sufficient answer for success, then three of these are "incorrect" and would have to be merged into "not A, B or C" in which case, again there are more incorrect answers than correct ones.
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Re: photon size

Postby Xami » Tue Nov 22, 2011 7:24 pm UTC

Xami wrote:I say nothing of how useful the answers are.


Not A is a distinctly different answer than Not B. Both are true statements, not that you would get points though.
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Re: photon size

Postby Carlington » Wed Nov 23, 2011 10:12 am UTC

Xami wrote:
Charlie! wrote:
Xami wrote:There are exactly the same number of right answers as wrong answers to any question.

This bald assertion cannot withstand the power of... Multiple Choice Exaaaaaaaaams!


Xami wrote:I say nothing of how useful the answers are. I also don't know in the case that the question does not have a correct answer.


Given C is what gets you the good stuff
"Correct" answers
Not A
Not B
C
Not D

"Wrong" Answers
A
B
Not C
D


If not A is correct, then B, C and D are correct. [B,C,D] > C.
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Re: photon size

Postby Xami » Tue Nov 29, 2011 3:45 pm UTC

Carlington (The Aussie) wrote:
Xami wrote:
Charlie! wrote:
Xami wrote:There are exactly the same number of right answers as wrong answers to any question.

This bald assertion cannot withstand the power of... Multiple Choice Exaaaaaaaaams!


Xami wrote:I say nothing of how useful the answers are. I also don't know in the case that the question does not have a correct answer.


Given C is what gets you the good stuff
"Correct" answers
Not A
Not B
C
Not D

"Wrong" Answers
A
B
Not C
D


If not A is correct, then B, C and D are correct. [B,C,D] > C.


No, B, C, OR D would be correct.
And equally, Not B, C, or D would be incorrect.

Am I getting trolled here?
Every question has equal right and wrong answers. For every wrong answer, you can match it to a right one by placing 'not' in front of it and vice versa.
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Re: photon size

Postby Copper Bezel » Tue Nov 29, 2011 4:00 pm UTC

You're not getting trolled. You're assuming that A, B, C, and D are binary, true-false statements that can be negated with "not", while no one else is.

Edit: That is, "Pickles are not blue" does not logically equal "Pickles are green."
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Re: photon size

Postby gmalivuk » Tue Nov 29, 2011 4:08 pm UTC

I think there's a bit of a misunderstanding of the difference between "true statement" and "correct answer". It may very well be the case that there are as many true statements as there are false statements to be made about any particular thing. This is not the same as saying there are as many correct answers as incorrect answers to any given question.

"What number is the sum of 1 and itself?" has exactly one correct answer and uncountably infinitely many incorrect ones, even though it's possible to say many many other true things about it. The point being that these other true things, like "it's not 23", aren't what anyone (other than Xami, apparently?) would really consider to be a correct answer to the question.
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Re: photon size

Postby sjorford » Tue Nov 29, 2011 4:52 pm UTC

gmalivuk wrote:aren't what anyone [...] would really consider to be a correct answer to the question.

Well, they're correct, but useless.

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Re: photon size

Postby eSOANEM » Tue Nov 29, 2011 5:04 pm UTC

They are correct statements (in that they are true), but they are not the correct answer. This is the point gmal is trying to make, it would appear that xami is equating "a true statement" with "a correct answer" even if that true statement is incomplete (as it includes incorrect answers).

In the case A, B, C, D where A is the answer you will get the marks for, not B, not C and not D all contain A so are true statements, but they are not the correct answer as they also contain incorrect statements (and, for a number of answers more than 3 with just 1 that will get you the marks, there will always be more incorrect answers included by a "not X" type answer than correct ones).
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Re: photon size

Postby Xami » Wed Nov 30, 2011 3:24 pm UTC

I guess I have been using correct and true interchangeably -
what's 1+2?
Not 7
To me this is correct because 7 is incorrect. Horribly unuseful of course.

So the difference here is most people would not say that NOT Incorrect is decidedly different from Correct?
But of course NOT False = True so there's some fundamental difference in how those work
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Re: photon size

Postby MHD » Fri Dec 02, 2011 11:25 pm UTC

At the very least nothing is smaller than the Planck length, approx. 10-35 m.
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Re: photon size

Postby doogly » Fri Dec 02, 2011 11:49 pm UTC

False. The Planck length is where we sorta kinda expect quantum gravity to be important. No one knows the full theory of quantum gravity, so nobody can say what happens at that scale, or if anything happens at all. It's just the scale at which sober caution tells you to pause the usage of your current theories, and either become a quantum gravity researcher or chill.
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Re: photon size

Postby RichM » Sun Feb 12, 2012 7:05 pm UTC

I think this is a way to look at the size of a photon.

The size of a photon can be defined by how it interacts with matter.

First, it has an infinite extent--or more accurately, the probability of where it is is infinite. However, a majority of it's energy (or the probability where it's at) is in a very small region of space. That region is related to the wavelength of the photon. The easiest way to see this is via diffraction. If I have a plate with a huge hole and fire photons through it at a screen, the photons will hit a the screen as if the plate wasn't there. As I decrease the size of the hole, the photons start to diffract as the edge of the hole clips the tails of the photon probablity function. This continues until the diffraction is dramatic. At this point, one can define the size of the photon using an arbitrary criterion using diffraction

d = lambda/sin(theta)

Pick the angle to define the size. As you see, this relationship allows for photons infinity large. As you make the angle of the diffraction approach zero, you have to have an infinite hole size which indicates the photon is infinite. However, for a reasonable situation, one could definite the angular spread to be pi/4 radians, making the size

d = lambda*sqrt(2)

The important thing here is the wavelength. For light, the hole needs to be really small, on the order of microns whereas, for radio waves, it can be on the order of meters.

If you look at it from antennas, you might see that that you need a bigger antenna to effectively catch bigger photons due to the wavelength scaling.

In fact, that could be another way to define the size. For a radio wave of wavelength lambda, the optimal length of an antenna is lambda/2. Thus, the size of a photon could be defined as this.
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