Photons lost in space

[CrossHair]

Thanks to all that have replied! I am clearer about certain things but I am still bothered by something. As with many thing about QM the solution is similar to the old "dead cat in a box" canundrum. The photons are not there unless you look at them, essentially. However, my question deals with the Universe and in this case study (humor me here) I could suppose that photons might be emitted directly out and never recieved by anything back in this Universe (for now we will ignore if the photon could, or would, be pulled back by gravity). If this synario happens then would the Universe, as we obserse it, be losing energy? (loss of photons)

 

[hammegk]

With regards to our universe, a photon in it has no access to "outside".

And in a sense a photon is everywhere in the universe until it is 'observed', although 'undergoes decoherence' is perhaps a better way to say it.

 

[hodgy]

It seems to me (not that I've really got a clue) that everything is fixed until you choose to measure it through a dimension.

Thus, every photon's path through time and space is in the wider sense, fixed. In no dimensions it appears as a solid model of its journey.

 

[John Bentley]

It seems to me (not that I've really got a clue) that everything is fixed until you choose to measure it through a dimension.

Thus, every photon's path through time and space is in the wider sense, fixed. In no dimensions it appears as a solid model of its journey.

Not sure what you're getting at here, but I think you've got it backwards - at least to most interpretations of QM.

1. Nothing is fixed until it is measured in the quantum world, as everything exists as a probability wave until then. Thus Schrodinger's cat is both alive and dead until you look. Classical or Copenhagen interp.

2. All possibilities for every single quantum event occur. Each possibility is a different universe unto itself. Many Worlds interp.

3. Then there is the Transactional interp which we have been discussing above, of which Molinaro gave a good summary.

I personally like #3 because it eliminates the need for an observation to be made before an event occurs. It also eliminates the proposition of infinite universes. And it eliminates the particle/wave duality, as every quantum particle is declared as both. It's only drawback is requiring travel backwards through time - but that's easier to get my head around than the others.

 

[Molinaro]

It's only drawback is requiring travel backwards through time - but that's easier to get my head around than the others.

What relieves me from worry there is that solutions to Maxwell's equations always give you the -ve side that everyone throws away.

 

[epepke]

When I talked about the spherical spread I was not refering to a collection of photons, but rather an individual photon -- or better yet -- each individual photon.

The photons, who appear as both particle and wave to us, are actualy the moving region of intersection of the 2 wavefronts. One is moving forward in time, the other backwards. From both the emmission and absorption points the 2 waves emminate, and their points of intersection along the path of least energy is that which we see as a photon moving forward in time at C. Everywhere else off that path the waves combine destructively and nothing is measured -- within the limits of the uncertainty principle.

You're quite right, except that it isn't exactly the path of least energy (I assume that you're referring to a Laplacian framework). It is most of the time. It's the path of highest absolute value of amplitudes integrated over all possible paths as if it were a classical particle (which, of course, it isn't). There are a number of interesting cases where this isn't the path of least energy.

 

[Molinaro]

I'm sure that there's plenty of room for correction. It's hard to decide how far to go when trying to keep it clear and simple for someone not familiar with the subject. I purposefully didn't look anything up while making the posts and instead relied on 'how I remember it', in general terms.

 

[Dilb]

I've been trying to think through this (and thanks for all the simplified summaries), and I've got a question. Since in this framework, all photons will interact with something, would this let us test for stuff "out there"? Suppose we have a positron source, so we get electron-positron production of 2 x-rays, which will have roughly opposite momentum. By detecting one of these x-rays we can say that the other x-ray must be going off in the opposite direction, or else momentum isn't being conserved.
So supposing the universe were non-infinite (say, we happen to be convinently close to the edge of all matter), would we detect no x-rays being sent inwards, because they can't be sent outwards, since there would be nothing to interact with?
I suppose it doesn't really matter with an infinite universe (or a finite universe is impossible, I don't know a lot of astrophysics), although I wonder if the finite-light cone of the universe could affect that. Information can't be transmitted faster than light (as far as I understand special reletivity), so for an photon to be sent towards something the electron has to have had the chance to "see" the other electron (they would have to be inside the light cone of each other, I'd think).
So, crazy? Wrong interpretation? Not a possible distinction from other models?

 

[epepke]

I'm sure that there's plenty of room for correction. It's hard to decide how far to go when trying to keep it clear and simple for someone not familiar with the subject. I purposefully didn't look anything up while making the posts and instead relied on 'how I remember it', in general terms.

Fair enough. I just think that it's important to point out that the highest probability is at a local minimum, and there are a number of interesting cases where there are several local minima e.g. frosted glass.

 

[Molinaro]

I've often thought of a similar test. Have the device on some satelite, point it right at the earth or out into distant 'voids' in our view of the distribution of galaxies and see if anything correlates.


And Epepke, yes that is a good clarification to refer to it as a local minimum rather than 'the minimum'.

 

[John Bentley]

Fair enough. I just think that it's important to point out that the highest probability is at a local minimum, and there are a number of interesting cases where there are several local minima e.g. frosted glass.

Yeah, IIRC, this also explains the rainbow effect on CD's and the same effect from oil on top of water. Feynman's "QED" explains it well.

 

[CrossHair]

I've got a question. Since in this framework, all photons will interact with something, would this let us test for stuff "out there"? Suppose we have a positron source, so we get electron-positron production of 2 x-rays, which will have roughly opposite momentum. By detecting one of these x-rays we can say that the other x-ray must be going off in the opposite direction, or else momentum isn't being conserved.
So supposing the universe were non-infinite (say, we happen to be convinently close to the edge of all matter), would we detect no x-rays being sent inwards, because they can't be sent outwards, since there would be nothing to interact with?

OK, I am going to keep flogging this horse we all agree is dead! But this still seems like an interesting question.
Maybe I am just trying to attract more flies.
Same synario as quoted above but stated differently. Say that you could simple point a flash-light to someplace with nothing to receive the photons. The excited electrons in the filament will not emit photons. Does the energy simply go to heat only? or do they just get more and more excited? (could lead to funny jokes here...) Maybe this is why some flash-lights have to be beaten first before they work, they aren't excited enough yet to emit...

 

[hodgy]

Not sure what you're getting at here, but I think you've got it backwards - at least to most interpretations of QM.

1. Nothing is fixed until it is measured in the quantum world, as everything exists as a probability wave until then. Thus Schrodinger's cat is both alive and dead until you look. Classical or Copenhagen interp.

Unless you count measurement as a dimension. I can well imagine that everything exists as a wave. 2d through 3d, 3d though time, time through 4d etc.... That suggests to me that the probability (uncertainty) is the result of not knowing all the dimensions. In a world where all dimensions are perceived a photon's route is a solid artifact.

 

[kevin]

Does the energy simply go to heat only? or do they just get more and more excited?

Heat is photons too. just a different wavelength than visable light.

BTW, this seems similar to Schrödinger's cat. The cat exists in both states until observerd. The photon radiates everywhere until captured.

ETA: oh darn it, someone else made the Schrödinger's cat reference already.

 

[Roboramma]

Heat is photons too. just a different wavelength than visable light.

I thought heat could be described as something along the lines of "average kinetic energy" or something like that, of the particles involved. (of course I think my wording is even more off than my understanding!).

Anyway, from what I understand, some wavelengths of light are emitted by most forms of matter due to the fact that they are in a state of high kinetic energy (maybe when particles collide? I don't know). This represents some of the energy of that heat being dissipated in the form of photons, which, when they are later absorbed, will tend to increase the kinetic energy of whatever absorbs them.

Meaning that hot things emit photons, and whatever absorbs those photons becomes hotter.

But that's different from saying that heat is photons.

Do I understand this in any way resembling reality?

edited to fix quote

 

[epepke]

I thought heat could be described as something along the lines of "average kinetic energy" or something like that, of the particles involved. (of course I think my wording is even more off than my understanding!).

The words get very confused here. Temperature is related to the average kinetic energy of an ideal gas. Even this neglects some important factors, such as the angular motion of molecules and the band structure in metals.

Heat is more complex. Two objects can be at the same temperature, but much may have more heat than the other. I cooked a turkey yesterday. The turkey was actually at a lower temperature than the air in the oven but had a lot more heat.

Do I understand this in any way resembling reality?

I think so.

Obviously, the bit about photons is about radiated energy, which is sometimes also called heat, though it's really a way that heat gets transferred. For much of our experience, hot things radiate in infrared, but of course a hot light bulb filament also radiates in visible light.

 

[John Bentley]

Unless you count measurement as a dimension. I can well imagine that everything exists as a wave. 2d through 3d, 3d though time, time through 4d etc.... That suggests to me that the probability (uncertainty) is the result of not knowing all the dimensions. In a world where all dimensions are perceived a photon's route is a solid artifact.

Still not sure what you're getting at. How can measurement be a dimension? And your argument about a world where all variables are known is impossible in the real world, although even Einstein denied it. It's pretty much a fact that our reality comes with unmeasurable uncertainty. Look up Heisenberg uncertainty principle for the details. http://zebu.uoregon.edu/~imamura/208/jan27/hup.html

 

[Roboramma]

Thanks epepke, especially for the clarification of the distinction between heat and temperature! I feel dumb for not seeing that.
It sounds like heat is more of a measure of the total energy of a body whereas temperature is a measure of the average energy?
(in very broad terms? and of course realising that we're only talking about one specific type of energy.)

 

[Schneibster]

Stick your hand into the 350 degree oven. Does it burn you? OK, now do you dare touch the side of the oven? Of course not. That's the difference between temperature and heat; the air and the side are at the same temperature, but the air is far less dense and therefore can hold a lot less heat.

Essentially, temperature is a measure of the average kinetic energy per molecule; heat is a measure of the total kinetic energy of all the molecules together (note- not the vector sum- if the vector sum of the kinetic energy of the molecules of an object is not zero, the object is moving!).

 

[epepke]

Thanks epepke, especially for the clarification of the distinction between heat and temperature! I feel dumb for not seeing that.
It sounds like heat is more of a measure of the total energy of a body whereas temperature is a measure of the average energy?
(in very broad terms? and of course realising that we're only talking about one specific type of energy.)

In very broad terms, yes. Specifically, for ideal gases.

Heat, however, can be stored in other ways than temperature. For example, if you put heat into a piece of ice, it will warm up. Same with water. However, if the ice melts, it takes a lot of heat but doesn't change temperature (ideally, not at all; in practice, a little).