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Is there such a thing as true darkness?

  1. Jul 19, 2012 #1
    Considering that what most people think of as light is just the visible spectrum of electromagnetism, while much more exists on either end, are the frequencies beyond either end of the visible spectrum still light? If so, since electromagnetic fluctuations permeate the known universe, is there any such thing as true darkness?
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  3. Jul 19, 2012 #2


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    If you consider the radiated thermal spectrum of an object at finite temperature the you could expect the occasional photon of visible em radiation even inside a coal mine with the lights all turned off. But, to an Engineer this is near enough darkness for Jazz!
  4. Jul 19, 2012 #3
    Yes, if humans define darkness as the absence of photons in the visible spectrum that can be received by retinal cells.

    Of course if you lock yourself in a room shut off from enough receivable energy in the visible spectrum your brain will eventually start making things up for you.
  5. Jul 19, 2012 #4


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    This is a very complex question to my thinking.
    Do you mean subjective or objective darkness? If the former, I submit that a totally blind person might be considered to be in true darkness regardless of environment. Even that might depend upon the pathology. If a sighted person is in an enclosed room or simply sticks his head into a box, he'll "see" spark-like dots due to random interactions of cosmic rays or other irritating things with the photoreceptive cells in his eyes. They're called "phosphenes". That, to me, counts as vision, but can it be considered "darkness" simply because no photons are involved?
  6. Jul 19, 2012 #5


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    I wouldn't class 'seeing things' that aren't there as vision. It's more like 'imagination'.
  7. Jul 19, 2012 #6


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    The whole point is that they are there. They're subatomic particles. Would you consider a track in a cloud chamber to be imaginary just because you can't see the particle that created it?
  8. Jul 19, 2012 #7
    The whole EM spectrum is light, and the light that we see is visible light. There is such a concept as true darkness, but based on what we observe it's very likely that this true darkness doesn't exist anywhere in the known universe. Even if you put someone in deep space, inside a black box, the box material would still emit thermal radiation inward because its temperature is above zero.
  9. Jul 19, 2012 #8
    This is more to the point which i was after. The others were digressing into human biological interpretation of the visible spectrum as opposed to the full natural spectrum. This seems a relatively definitive argument in my favor.

    With that being said, I'll pose a new question:

    If there is no true darkness in the known universe due to electromagnetic fluctuations, are the resultant radiations from said fluctuations which fall outside either end of the visible spectrum still photons? or is the visible spectrum alone defined by the manifestation of photons?

    I am just now getting into physics with more depth than before so I may be posing an ignorant question here, but it will help me know whether or not to ask the next question which I have in mind.
  10. Jul 19, 2012 #9
    The photons can be in the visible range or outside of it. I would hazard a guess that most are in the microwave region (the cosmological background radiation), but there are still some in the visible region from non-blackbody sources, eg. starlight. I'm not sure whether a human eye could detect it in the darkest regions, but it's still there.
  11. Jul 19, 2012 #10


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    Stars are black bodies. And everything is actually a black body. The name is misleading. As everything emits radiation. Cooler objects you can't see it, because the intensity of the light is very weak in the visible spectrum. But as they get hotter, the intensity of the light increases in the visible spectrum and you can see the light. First they glow red and then white hot (white hot as it's called - and if it is glowing white then it is hot).

    There is always visible light everywhere - there's just not always enough to see it. Even deep space is full of starlight.

    Here's a graph of black body radiation, and the visible spectrum. You see you need quite a bit of heat to see the heat.
  12. Jul 19, 2012 #11
    Even if you shut yourself in a perfectly opaque box in deep space, the box will come in thermal equilibrium with the cosmic microwave background, rise to 3 kelvin, and reradiate that heat continously according to a (mostly) blackbody curve. Even at just 3K, there is some amount of radiant intensity at visible wavelengths.
  13. Jul 19, 2012 #12
    If you dont digress into how humans perceive the world about them through EM waves, chemical means, or pressure receptors with the question that was posed in this case, one can go way off into fantasy land. In fact at the base of any scientific activity one must always be cognisant of the limitations of human perception. People completely forget so many filtering mechanisms, etc... that the brain has given us through natural selection that dont have a darn thing to do with finding photons.

    We merely got rid of that rubbish for you to get to the serious stuff :)

    Thank god I cant always feel my clothes touching all parts of my body... it would be damn difficult for me to type in this particular discussion.
  14. Jul 19, 2012 #13


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    All electromagnetic radiation propagates by way of photons.

    'Light' is a tricky term; it tends to be human-centric. Bees and birds and many other animals can see well into the IR and UV range, using optical sensors, so that is one way of defining light that is not human-centric.

    If you go much farther outside the visible spectrum, you can no longer use optical sensors. You get into microwaves on one end and X-rays on the other.
  15. Jul 19, 2012 #14


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    Thanks a lot, you saboteur! Now I'm starting to itch. :grumpy:
  16. Jul 19, 2012 #15
    So now you can think about your breathing ;)
  17. Jul 20, 2012 #16


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    (I don't know whether or not you actually realize how funny that is, given my circumstance.) :biggrin:
  18. Jul 20, 2012 #17
    You got me on this one. "non-blackbody" was an awful word for me to use.
  19. Jul 20, 2012 #18
    This is great. I had suspected this would be the case. Thank you all for helping me work through this. Now I must pose yet another question which will take me closer to my end goal:

    Since, in a matter of speaking, there is "no true darkness," because there are photons being propagated within every vestige of the known universe, would that imply that the universe as a whole is made entirely from and full of substance?

    To qualify, this touches on a still raging debate on whether photons actually have mass or not, or whether photons have both mass and no mass at the same time(in a similar way that photons act as both wave and particle). From the standpoint that photons do indeed have mass, and from the technical definition of substance as the mass or tangibility of a thing, then it would follow that the answer to my above question would be yes. But it could also be argued that as photons have no mass, and hence, no literal substance, and that literal substance is scarce in the universe. This would leave us with a no to my above question.

    Now, touching on the metaphysical side, and re-qualifying what substance is outside of it's literal and physical definition, even if photons have no mass, could it be argued that they still have substance? Which would allow for a yes to my [first] above question in either case.

    Now to pose my suggested re-qualifying of what substance is: can something which has function, lack substance? For photons, whether mass-less or not, have function. If something has function, would that not imply, philosophically, that it must have substance?

    I fear though that I may be posing a question which cannot be properly answered in a general physics forum, so pardon my digression, against my own previous resistance, into human perceptions.
  20. Jul 20, 2012 #19


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    No, it would not.
  21. Jul 20, 2012 #20
    And your reasoning is...?
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