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Author Topic: A photonic paradox or a dumb question?  (Read 4374 times)

Offline namaan

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A photonic paradox or a dumb question?
« on: 18/10/2010 22:07:38 »
Hey guys,

So yeah, I ask this knowing that there's a chance that it's a dumb question, but anyway, assuming we need photons of some sort to detect what's "out there" in the universe, then what if, hypothetically speaking, a chunk of the universe were just photons floating away from us such that they would never reach us. For example, referencing the pie graph at http://map.gsfc.nasa.gov/universe/uni_matter.html, what if 23% of the universe were photons that never reach us for detection? Couldn't these photons very well serve the place of "dark matter"?
« Last Edit: 19/10/2010 00:00:58 by namaan »


 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #1 on: 19/10/2010 02:44:21 »
Btw, the paradox in the title refers to the idea that we see/detect matter because it gives off light/photons, hence you could call it "light matter", in referenced opposition to "dark matter". So the paradox is that the only thing that doesn't give off light allowing us to detect it is light it self. As far as I understand it anyway...
« Last Edit: 19/10/2010 02:47:19 by namaan »
 

Offline Ethos

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A photonic paradox or a dumb question?
« Reply #2 on: 19/10/2010 04:57:36 »
According to present estimates, dark matter accounts for some 80 percent of the total matter in the universe. Quite unlikely that any source of stray photons could add up to that much mass................Ethos
 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #3 on: 19/10/2010 05:18:13 »
I thought dark energy accounted for that much? The link I provided in the original source on the NASA website, is it a dated source? I mean I think it was saying that scientists knew dark matter to account for 23% of total matter with high certainty.
 

Offline Ethos

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A photonic paradox or a dumb question?
« Reply #4 on: 19/10/2010 05:24:10 »
Wikipedia; "from these figures, dark matter accounts for 80 percent of the matter in the universe and ordinary matter accounts for 20 percent"..........The 23 percent you refer to is the mass energy density while ordinary matter accounts for only 4.6 percent. If you will read further, you will see that the ratio of dark matter to ordinary matter is 4 to 1 in favor of dark matter................Ethos
« Last Edit: 19/10/2010 05:29:38 by Ethos »
 

Offline Soul Surfer

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A photonic paradox or a dumb question?
« Reply #5 on: 19/10/2010 09:15:19 »
We never see or detect almost all of the photons in the universe.  We can only detect those that are passing through where we are looking at the time we are looking.  The fact that there may be photons elsewhere is immaterial.  Also Photons do not float around anywhere they are always travelling in a specific direction at the speed of light.

The density of photons throughout the universe IS included in the positive energy part of the equation in the same way that gravitating particles  (matter both visible and "dark") are included in the negative energy part of the equation.  The problem is that most popular science writers and TV programs do not explain things clearly and fully but cut to the spectacular bits for dramatic effect.
 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #6 on: 19/10/2010 13:37:07 »
Ethos, I was actually referring to the mass-energy density in the first place, though I could have been clearer about this. Given that context, 23% of the "stuff" out there would be dark matter, and yes dark matter exists along side ordinary matter in a 4-1 ratio (with ordinary matter consisting of 4.6% as per the NASA link).

Soul Surfer, while my terminology could have been more scientific, I understand the basic properties of photons so it really shouldn't affect what I'm saying. As you said, we detect only a tiny fraction of photons at any given time.

As for your second point, I guess I just find it strange that you could say that the *density* of photons is included in the positive energy part of the equation, when as far as I understand density at some point denotes mass of some sort. Taken together with (pseudo?) ideas about energy being made of the same basic "stuff" as matter, it seems that while photons have a negligible mass for more relevant measurements, when photons are taken together as a whole across the universe, it seems to me to be a case of drops adding to an ocean.

But I assume your response would be that photons don't show up in gravitation measurements of the universe unlike dark matter (being made of gravitating particles), and so photons really don't have mass to speak off in a traditional sense?
 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #7 on: 19/10/2010 18:41:08 »
Incidentally, I'm curious to know that if photons don't show up on gravitation measurements, then how are they detected? Is it indirect mathematical / logical derivations?

Oh, and if I appear to be someone high on popular science with no sound basis to make these arguments, then hit me with it, I can handle it. Physics is a hobby of mine so I don't have to worry about my career whilst thinking strange, potentially nonsensical, thoughts.
 

Offline Geezer

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A photonic paradox or a dumb question?
« Reply #8 on: 19/10/2010 19:10:41 »
Incidentally, I'm curious to know that if photons don't show up on gravitation measurements, then how are they detected? Is it indirect mathematical / logical derivations?

Well, your eyeball can be a pretty good detector for a start :D   Photons propagate through space as electromagnetic energy. When that energy interacts with matter there is a quantifiable reaction. That might be in your eyeball, your skin, a photocell, a radio antenna, etc., etc.

The detection mechanism will differ according to the frequency of the electromagnetic energy, and the frequency determines the quanta of energy. Only a little part of the possible frequencies produce light that is visible to humans.
 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #9 on: 19/10/2010 20:02:14 »
Ahaha, that's not exactly what I meant by "hit me with it", I'm aware of everything in your post. But appreciate the response none-the-less Geezer.

I guess I meant to ask how photons are detected aside from direct measurements (i.e. our eyeballs), and indirect measurements ala gravitation.
 

Offline Geezer

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A photonic paradox or a dumb question?
« Reply #10 on: 19/10/2010 20:34:20 »
Ahaha, that's not exactly what I meant by "hit me with it", I'm aware of everything in your post. But appreciate the response none-the-less Geezer.

I guess I meant to ask how photons are detected aside from direct measurements (i.e. our eyeballs), and indirect measurements ala gravitation.

You have to be careful what/how you ask around here. Too many "wise guys" I'm afraid  ;D

Actually, I'm not aware of any other methods of detection. I think there is a mechanical reaction too, but that's probably another form of direct measurement.

The tricky thing about photons is that, as far as I know, you can't detect them without destroying them. I don't think anyone has ever observed a photon "in transit" as it were. We have a sort of model for their behaviour, but it's really just a model. (Mind you, when it comes to that, so are a heck of a lot of other things in physics.)
 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #11 on: 23/10/2010 03:29:19 »
Let me rephrase the question a bit: what scientific evidence stands in opposition to the possibility of photons standing in place for dark matter?

Why, for example, would it be unreasonable for 23% of mass-energy density to be photonic density? I never received a response to the previous post where I questioned the simultaneous acceptance of photonic density and the apparent lack of photonic mass, and I'm not sure if it's because I'm so wildly uneducated on the subject that an explanation isn't possible (which I sincerely understand) or some other reason.

Also, why is it unreasonable to think that photons can't be detected in the same manner as dark matter can't be detected (doesn't react to / send light the way normal matter sends it).

Of course the all-encompassing answer to these questions might be that photons are not matter, so by categorical deduction, they certainly can't stand in place of dark matter. But what science shows that this is anything but a semantics issue, semantics which may change at some point unlike the foundations of good science?
 

Offline Geezer

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A photonic paradox or a dumb question?
« Reply #12 on: 23/10/2010 04:28:35 »
Ah! Well I'm probably not very well qualified to answer that. However, I can make a couple of guesses.

Dark matter is relatively stationary in relation to visible structures in space. I think it's responsible for many of those structures forming. Photons on the other hand, are always propagating through space at C.

If there were sufficient photons to exert the gravitational pull that dark matter exerts, I suspect we might be frazzled by all the energy that would be falling on Earth.  :D

BTW - I understand there is currently some speculation that there may actually be "dark photons". A form of electromagnetic radiation that is quite undetectable by us using normal methods.
 

Offline namaan

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A photonic paradox or a dumb question?
« Reply #13 on: 23/10/2010 04:35:54 »
That was my best guess as to why photons wouldn't make for good dark matter candidates too. I mean if someone coined the term something "matter", it's most likely for something that's relatively stationary...unlike our busy-body photons.

But dark photons...hmmmm :)
 

Offline Geezer

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A photonic paradox or a dumb question?
« Reply #14 on: 23/10/2010 04:52:45 »
But dark photons...hmmmm :)

I don't think they called them that exactly. But it seemed obvious to me that they should :D. It was a slightly casual reference anyway.

I just read it in SciAm today. I can dig up more info if you are interested.
 

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A photonic paradox or a dumb question?
« Reply #14 on: 23/10/2010 04:52:45 »

 

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