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Boratssister
2010-Oct-28, 12:58 PM
If a light source flashed 1 photon in all directions, how far away from that light source would you have to be in order for that flash to be missed by detectors? In other words when will gaps appear between photons as they travel in a straight line,outwards,from a point to an ever increasing radius? just wondering

Strange
2010-Oct-28, 01:15 PM
I'm not sure what "in all directions" means. If it really was all directions then there would be a "solid wall" of photons however far away you were (otherwise there would be some direction that was not in "all"). Are you trying to understand how large a photon is or something else?

Boratssister
2010-Oct-28, 02:30 PM
I'm not sure what "in all directions" means. If it really was all directions then there would be a "solid wall" of photons however far away you were (otherwise there would be some direction that was not in "all"). Are you trying to understand how large a photon is or something else?

I suppose its comparable to the double split experiment, a bit.......
When I see a star and then move my eye slightly then I still see that star, even though I'm light years away. I assume this is because the stars rain photons. However if the star flashed a sphere of photons 1 photon thick then how could that sphere remain a ''solid wall'' light years distance from the source?Given that light travels in straight lines how far away from that source would a detector had more than a 50percent probability of missing a photon?an inch or a light year?

Hornblower
2010-Oct-28, 02:48 PM
I suppose its comparable to the double split experiment, a bit.......
When I see a star and then move my eye slightly then I still see that star, even though I'm light years away. I assume this is because the stars rain photons. However if the star flashed a sphere of photons 1 photon thick then how could that sphere remain a ''solid wall'' light years distance from the source?Given that light travels in straight lines how far away from that source would a detector had more than a 50percent probability of missing a photon?an inch or a light year?

It depends on how many photons were emitted in the flash. You have not given us enough information.

Boratssister
2010-Oct-28, 02:57 PM
It depends on how many photons were emitted in the flash. You have not given us enough information.

1 photon flash in all directions from a spherical source 1 kilometre in diameter.

Hornblower
2010-Oct-28, 03:08 PM
1 photon flash in all directions from a spherical source 1 kilometre in diameter."One photon flash in all directions" is numerically meaningless. It still does not tell us how many photons we have, so we don't know how far apart they are at any given distance.

Boratssister
2010-Oct-28, 03:19 PM
"One photon flash in all directions" is numerically meaningless. It still does not tell us how many photons we have, so we don't know how far apart they are at any given distance.

how many photons can you fit onto a sphere 1km in diameter?

Shaula
2010-Oct-28, 04:41 PM
Depends on the wavelength...

Photons are quantisations of the EM field. They can be described as waves or particles. They do not have an intrinsic size, they are not like billiard balls we can pack together. Where they are is also probabilistic, not a classically deterministic property of them. All you can say is that as the intensity of the light drops the probability of a quantum event in a detector (that can be described in terms of a photon hitting it) drops. You have to set a threshold for when you regard this probability as low enough that the flash will be missed.

George
2010-Oct-28, 05:11 PM
If a light source flashed 1 photon in all directions, how far away from that light source would you have to be in order for that flash to be missed by detectors? In other words when will gaps appear between photons as they travel in a straight line,outwards,from a point to an ever increasing radius? just wondering It is better to work in terms of magnitude. Once your object drops below a visual magnitude of about 31, then the Hubble Telescope's sensors will not "see" it.

The human eye and sensors can detect a single photon, however, so it is possible your object can be occasionaly seen (detected) at vast distances.

Jeff Root
2010-Oct-28, 05:53 PM
What I read many years ago is that, under the right conditions,
a human can detect six photons of green light hitting a small
spot on the retina at about the same time. However, a single
photon can cause a receptor in the retina to activate. That just
isn't enough to send a message to the brain that something
was seen.

Similarly, a single photon can register in a CCD like those in the
Hubble Space Telescope. But a single photon could be signal or
it could just be noise, so many photons are required to hit a spot
on a CCD before it looks like a real light signal, not just noise.
I don't know what that number is, but for the particular CCD that
has a visual magnitude limit of about 31, it is the number that
corresponds to that magnitude-- on average. An astronomer or
telescope technician working with the telescope probably would
not pay any attention to the number of photons, but would note
the level of noise in the least-affected cells and the degree of
saturation of the most-affected cells, and adjust the integration
time to provide the best compromise for obtaining the desired
information-- the same as adjusting the shutter speed on a film
camera. You want enough photons to hit a cell where there is
supposed to be a signal, without so many photons hitting cells
where there should not be a signal that the dark background is
fogged. Magnitude 31 is about the limit for HST to distinguish
between a cell that detected a signal and a cell that didn't.

Rather than specifying a sphere a kilometer in diameter, I
suggest specifying a realistic star size, like the Sun. You can
still idealize it as a perfectly uniform sphere emitting photons
all over its surface.

You might want to choose between an emission of light having
a single wavelength and a distribution of wavelengths such as
a blackbody distribution.

You should also specify the characteristics of your light sensor.

What you see is what you get.

-- Jeff, in Minneapolis

korjik
2010-Oct-28, 08:42 PM
how many photons can you fit onto a sphere 1km in diameter?

All of them (mostly).

For you to see something, you really need to be receiving at least thousands of photons per second. All told, that means to see a star, it is going to be emitting ~1025 to ~1030 photons per second. At least....

For the Mk 1 eyeball, the distance you can see a star is dependent on class. Small M-class stars arent seeable at even a couple of light years, but big type-Ia supergiants can be seen at a couple thousand ly.

Concerning the OP tho, the question dosent make any sense. A photon goes in a certain direction. If one photon is emitted, it will go in one direction. If 1030 photons are emitted in random directions, then you will prolly be able to detect at least one many ly away.

George
2010-Oct-28, 08:58 PM
What I read many years ago is that, under the right conditions,
a human can detect six photons of green light hitting a small
spot on the retina at about the same time. However, a single
photon can cause a receptor in the retina to activate. That just
isn't enough to send a message to the brain that something
was seen. Good point. The practical range is about 5 to 10 photons during a time period of about 100 milliseconds. But the eye itself only allows about 10% of the photons to hit a receptor, so 50 to 100 photons per second are needed.

The Sun as seen from Earth produces a little less than 2x1018 photons per second per square cm (at 1 AU), so it's not too difficult to pick it out from the other stars. ;)

antoniseb
2010-Oct-28, 09:12 PM
If a light source flashed 1 photon in all directions, how far away from that light source would you have to be in order for that flash to be missed by detectors?

So by now, you probably have a better sense of things. Let me throw one more thing in your direction. A flash will give off some number of Joules (or Ergs) of energy. Visible light photons are somewhere around 2 eV (IIRC), and one Joule of these would be about 3x1018 photons. So now you just need to figure out how many Joules, and you can calculate how how big a detector you'd need to see it at a given distance.

astromark
2010-Oct-29, 05:32 AM
Intensification... Lol and magnitudel brightness... Lol and distance... All parts of your problem here.

A 'flash' one photon deep could actually be missed by your brains ability to process what it has been sent.

From the posts above mine... much information is gleamed.

fcunnane
2010-Oct-31, 10:09 PM
Hey guys, I'm pretty sure an omni-directional RF antenna spits light in all directions...

Photons certainly don't travel in a straight line though...

They look more like "sine-waves" when measured as a wave, and more like a particle with position when you can find it.

Practically, you would need a very powerful clocking circuit to determine the exact amount of time required to load a circuit to produce the light.

I don't exactly know how to go about counting waves other than frequency division, so if you want to count, you must use the particle math part of the wave-particle duality thing-a-ma-bobs...

Edit: Or at least I think...

2nd Edit: Pretty sure that is accurate now...

Infinitenight2093
2010-Oct-31, 10:53 PM
I suppose its comparable to the double split experiment, a bit.......
When I see a star and then move my eye slightly then I still see that star, even though I'm light years away. I assume this is because the stars rain photons. However if the star flashed a sphere of photons 1 photon thick then how could that sphere remain a ''solid wall'' light years distance from the source?Given that light travels in straight lines how far away from that source would a detector had more than a 50percent probability of missing a photon?an inch or a light year?

I don't think that the path of light can be limited to "1 photon" in all directions. Light particles behave as waves when unobserved (and "observed" doesn't necessarily mean to just look at light with your eyes, you would have to use a precise measurement to collapse the photons wave function) -This was proved by the "double slit experiment". So then if light behaves like a wave, we would be able to see the particle from all directions independent of distance.

Jeff Root
2010-Nov-01, 12:05 AM
Hey guys, I'm pretty sure an omni-directional RF antenna spits light
in all directions...
I'm not sure how omnidirectional omnidirectional antennae are.
The antenna has a shape, and that shape influences the radiation
pattern. I think it would be rather difficult to make a radio antenna
that is omnidirectional throughout a complete sphere to a high
level of uniformity.

Photons certainly don't travel in a straight line though...

They look more like "sine-waves" when measured as a wave, ...
No, no, no, no, no.

Light travels in straght lines in a uniform medium, and ignoring
gravity. (Please, let's not drag gravity into this!)

The sine waves you see are a graphical representation of the
variation in the electric and magnetic fields of the light as it
goes past you. You never actually see or detect those waves,
only their effects, such as interference patterns. I believe that
individual photons have wave properties, but my understanding
is that the mainstream description of photons says they do not.
Wave properties only show up when you examine a statistically
large number of photons. Looking at an individual photon can
tell you the photon's energy, and how it interacts with matter,
but you won't see anything that looks like a wave. And it most
certainly does not travel a sinusoidal path!

-- Jeff, in Minneapolis

caveman1917
2010-Nov-01, 12:14 AM
I believe that
individual photons have wave properties, but my understanding
is that the mainstream description of photons says they do not.

Individual photons do have a frequency* for example, if that's what you mean by 'wave properties'.

*technically any observer will 'ascribe' a frequency to a photon he observes, it is not some invariant property of the photon itself. Multiple observers will ascribe different frequencies to the same photon, but all will ascribe some frequency.

caveman1917
2010-Nov-01, 12:18 AM
I don't think that the path of light can be limited to "1 photon" in all directions. Light particles behave as waves when unobserved (and "observed" doesn't necessarily mean to just look at light with your eyes, you would have to use a precise measurement to collapse the photons wave function) -This was proved by the "double slit experiment". So then if light behaves like a wave, we would be able to see the particle from all directions independent of distance.

The amount of that effect depends on how it was emitted. For example a laser will produce highly directional light - the path will be well-defined. One could reframe the scenario in terms of an array of lasers shooting out photons in different directions.

Jeff Root
2010-Nov-01, 01:28 AM
I believe that
individual photons have wave properties, but my understanding
is that the mainstream description of photons says they do not.
Individual photons do have a frequency* for example, if that's what
you mean by 'wave properties'.

*technically any observer will 'ascribe' a frequency to a photon he
observes, ...
How would you measure the frequency of an individual photon?
I'm pretty sure you won't be able to describe a method that works.

-- Jeff, in Minneapolis

caveman1917
2010-Nov-01, 01:09 PM
How would you measure the frequency of an individual photon?
I'm pretty sure you won't be able to describe a method that works.

-- Jeff, in Minneapolis

Do you mean in principle or as a practically working method?
In principle for example one can compare the energy content (gravitational effect) of a system before and after the photon was emitted, and then by E=hf one gets the frequency.
Though this is an indirect measurement - i don't think there is a method for directly measuring the frequency of a single photon, but that doesn't mean they don't have one.

Jeff Root
2010-Nov-01, 05:58 PM
How would you measure the frequency of an individual photon?
I'm pretty sure you won't be able to describe a method that works.
Do you mean in principle or as a practically working method?
Both.

In principle for example one can compare the energy content
(gravitational effect) of a system before and after the photon was
emitted, and then by E=hf one gets the frequency.
If you are going to use the formula E=hf, you need to measure
the frequency of an individual photon to show that E=hf. But the
formula comes from measurements of statistically large numbers
of photons, not individual photons.

Though this is an indirect measurement - i don't think there is a
method for directly measuring the frequency of a single photon,
but that doesn't mean they don't have one.
I also don't think there is a method for directly measuring the
frequency of a single photon, meaning that nobody has one.

-- Jeff, in Minneapolis

caveman1917
2010-Nov-01, 09:33 PM
If you are going to use the formula E=hf, you need to measure
the frequency of an individual photon to show that E=hf. But the
formula comes from measurements of statistically large numbers
of photons, not individual photons.

Whilst that may be true, mainstream physics does include the formula for individual photons thus one is allowed to use it.

I also don't think there is a method for directly measuring the
frequency of a single photon, meaning that nobody has one.

I meant my "that doesn't mean they don't have one" as "that doesn't mean individual photons don't have a frequency".

The double slit experiment works just as well with shooting out one photon at a time, so each individual photon must have the necessary wave properties such as frequency/wavelength.
Though again it's not direct measurements of the frequency - they must have a frequency to account for that behaviour.

ShinAce
2010-Nov-01, 09:51 PM
How would you measure the frequency of an individual photon?
I'm pretty sure you won't be able to describe a method that works.

-- Jeff, in Minneapolis

Can't the photoelectric effect serve this purpose? If it emits an electron, you know the energy was at least x.

As an aside, I'm fairly certain an onidirectional antenna is, ideally, omnidirectional in half space. It does have a ground plane.

caveman1917
2010-Nov-01, 10:00 PM
Can't the photoelectric effect serve this purpose? If it emits an electron, you know the energy was at least x.

Yes, there are many ways to directly measure the energy of a single photon. But it seems to me that he argues against the use of E=hf to convert that energy measurement into a frequency.

korjik
2010-Nov-02, 12:09 AM
Both.

If you are going to use the formula E=hf, you need to measure
the frequency of an individual photon to show that E=hf. But the
formula comes from measurements of statistically large numbers
of photons, not individual photons.

I also don't think there is a method for directly measuring the
frequency of a single photon, meaning that nobody has one.

-- Jeff, in Minneapolis

f=pc/h

Good enough?

Jeff Root
2010-Nov-02, 02:35 AM
korjik,

Can you show that f=pc/h works for individual photons?

-- Jeff, in Minneapolis

Jeff Root
2010-Nov-02, 02:53 AM
If you are going to use the formula E=hf, you need to measure
the frequency of an individual photon to show that E=hf. But the
formula comes from measurements of statistically large numbers
of photons, not individual photons.
Whilst that may be true, mainstream physics does include the
formula for individual photons thus one is allowed to use it.
I'd need to see an explanation of how/why it can apply to
individual photons.

I also don't think there is a method for directly measuring the
frequency of a single photon, meaning that nobody has one.
I meant my "that doesn't mean they don't have one" as "that
doesn't mean individual photons don't have a frequency".
Ah. That must be one of the most idiotic mistakes you've seen
all this month. Maybe *the* most idiotic.

So, you agree with me then? We are both ATM believers in
individual photons having frequency?

The double slit experiment works just as well with shooting out
one photon at a time, so each individual photon must have the
necessary wave properties such as frequency/wavelength.
Yes, but what actually *is* the frequency or wavelength of each
of those individual photons? I say that they are unmeasurable
and unknown.

Though again it's not direct measurements of the frequency - they
must have a frequency to account for that behaviour.
It isn't really even an indirect measurement-- It is only a
measurement of the average frequency or wavelength of a
population of photons. Each individual photon may vary from
that average by a random amount.

-- Jeff, in Minneapolis

korjik
2010-Nov-02, 03:04 AM
korjik,

Can you show that f=pc/h works for individual photons?

-- Jeff, in Minneapolis

Yes, I can.

To do it properly would require more equation work than I am willing to do here. Look it up in almost any QM book.

korjik
2010-Nov-02, 03:08 AM
I'd need to see an explanation of how/why it can apply to
individual photons.

Ah. That must be one of the most idiotic mistakes you've seen
all this month. Maybe *the* most idiotic.

So, you agree with me then? We are both ATM believers in
individual photons having frequency?

Yes, but what actually *is* the frequency or wavelength of each
of those individual photons? I say that they are unmeasurable
and unknown.

It isn't really even an indirect measurement-- It is only a
measurement of the average frequency or wavelength of a
population of photons. Each individual photon may vary from
that average by a random amount.

-- Jeff, in Minneapolis

Your opinion is irrelevant. You are wrong. Wavelength and frequency are definable quantities. You woud not be able to get coherent light if you could not define wavelength.

Jeff Root
2010-Nov-02, 03:24 AM
korjik,

I don't require that you work anything out, but I do need you
to show me exactly where anyone else has worked it out.

I have no doubt whatever that wavelength and frequency are
defineable quantities, or that they apply to populations of
photons. I just haven't been able to come up with a way to
measure the wavelength or frequency of an individual photon.
It doesn't seem to be possible.

I think an equivalent question might be: What is the bandwidth
of an individual photon? I don't know enough about the relevant
physics to even begin to answer that question, but it might be a
useful way for you or someone else here to think about it.

-- Jeff, in Minneapolis

caveman1917
2010-Nov-02, 03:44 AM
So, you agree with me then? We are both ATM believers in
individual photons having frequency?

Since the mainstream position is that individual photons have a frequency (with the observer-dependence caveat), it is by definition not an ATM belief ;)

Yes, but what actually *is* the frequency or wavelength of each
of those individual photons? I say that they are unmeasurable
and unknown.

My (extremely limited) understanding is that it is the rate at which the photon 'switches' back and forth between having the energy in the magnetic component versus the electric component of the field - or something like that, i'm sure someone else can explain this better.
Just because there is no known way of measuring the frequency directly doesn't mean it isn't there, its existence is necessitated for the rest of the physics to fall in place.

Jeff Root
2010-Nov-02, 11:13 AM
Since the mainstream position is that individual photons have a
frequency (with the observer-dependence caveat), it is by definition
not an ATM belief ;)Maybe. I'm not convinced that it is
the mainstream position. I'll be glad if it turns out that it is.

Yes, but what actually *is* the frequency or wavelength of each
of those individual photons? I say that they are unmeasurable
and unknown.
My (extremely limited) understanding is that it is the rate at which
the photon 'switches' back and forth between having the energy in
the magnetic component versus the electric component of the field -
or something like that, i'm sure someone else can explain this better.
Heh. Now you're taking a turn at misunderstanding what I meant.

I wasn't asking what the physical meaning of the frequency is,
I was trying to ask ... well ... exactly what I asked: What is the
frequency of each individual photon? We can measure the mean
frequency of a large group of photons, but that measurement
doesn't tell us the frequency of the individual photons. If you
try to measure the frequency of an individual photon, you get
nothing-- no measurement.

I'm struggling to come up with a useful analogy...

Suppose I have several pictures of Albert Einstein, and I'm
rating them as to how good a likeness each picture is. I want
to put one on my website, and I want it to be a good likeness,
but I want to make it as small as possible. I make the pictures
32 pixels square, and rate each of them. Let's go smaller.
I make them 16 pixels square, and rate them. It's hard to tell
that they are supposed to be pictures of Einstein. Smaller?
Okay, 8 pixels square. Now it's hard to tell that any of them
are pictures of a person. Smaller yet?? Okay, one pixel each.
Each picture is now just a single dot of color-- mostly shades
of gray. How well does each pixel resemble Einstein? Not at
all, I'd say. Yet they are all still the same images, just reduced
in size.

A single pixel doesn't resemble Einstein at all, yet a whole
bunch of pixels, organized the right way, does resemble him.

A single photon has no recognizeable frequency or wavelength,
yet a whole bunch of photons, organized the right way, does.

Also, I think I can say a little bit about the wave picture of light:
It doesn't switch back and forth between the electric component
being strong and the magnetic component being strong-- the
two components are in phase, so are both strong together and
are both weak together, as they go past an observer. Imagine
the thing in the diagram below representing a single wavelength,
rather than a photon, moving from left to right. The observer
sees the intensity of both the electric and magnetic fields rise
from zero to a maximum positive value, fall back to zero, then
rise to a maximum negative value, and fall back to zero again
as it moves past. An individual photon, however, is like a point
in that it has no parts. You can say where and when it hit your
detector, and how much energy or momentum it had, but you
can't tell its frequency or wavelength, or even if it has those
properties.

-- Jeff, in Minneapolis

sirius0
2010-Nov-02, 01:02 PM

cjameshuff
2010-Nov-02, 10:11 PM
I just haven't been able to come up with a way to
measure the wavelength or frequency of an individual photon.
It doesn't seem to be possible.

Just off hand...diffraction gratings and perhaps metamaterials for rather direct geometric wavelength tests, dye absorption for energy level tests, etc.

What could possibly lead you to doubt that the energy-wavelength relationship holds for individual photons? There's a lot of single-photon physics that just wouldn't work if it wasn't true. Photons aren't untouchably small and undetectably weak...it's quite commonplace for equipment to detect and count individual photons. Some instruments even rely on absorption of exactly two photons of two distinct wavelengths by a target.

PetersCreek
2010-Nov-02, 10:28 PM