Speed of Light question

Alzrius said:

Correct me if I'm wrong, but I thought the dark matter theory had recently been debunked?

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Well, that depends. There isn't just one "dark matter theory".

In the most basic sense "dark matter" is merely that matter which is not glowing brightly enough for us to see it with our devices at interstellar distances. By this definition, the Earth itself is "dark matter". The question is simply if there's enough of it or not.

Some attempts have been made to posit some exotic materials among the mundane dark matter, to effectively up the overall mass out there. None of those have stuck particularly well.

Harmon said:

I kinda scratched my head, and thought about black holes for a moment- "if what ever amount that isn't here has already been taken into black holes then would that mean that he's really got no clue about the amount of (matter/material/energy) in the universe?"

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It isn't really "no clue". You can get estimates on masses based upon how objects move. While dark, the holes exert gravitational effects on bodies that aren't dark. Which isnt' to say that we're 100% certain, but extremes can be eliminated.

What about time? I mean we are looking across a minunum of 5+ yrs to the nearest star to get information, and most of the cool universe stuff is decades, hundreds, and thousands of years older then that, so how can we make an educated understanding of the universe based on reactions and forces that are so much out of date.

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Well, what does it matter that the data is old?

If I assume the past works as the present, and I look at that old data, and that data is consident with my current rules, then I can take that as evidence that my current model also held at previous times.

If I assume the past works like the present, and I look at the old data, and that data is not consistent with my current rules, I can take that as evidence that the rules have changed, and I can start looking at how the rules have changed.

Either way, science wins.

Plus, understanding is not only about the here and now:

Consider, for a moment, that you knew nothing at all about cats. Never seen one, never heard of one. And I put you in a room with a 5 year old cat. You could watch it's behavior and learn a few things about it, but some things would remain a mystery. The whole thing about jumpiong in your lap, purring, kneading, and wanting to be stroked would be a bit baffling.

If, however, you look back in that cat's life, back to when it was a kitten, and at how it interacted with it's mother, you'd see where the purring and kneading and stroking come from, and in that context they make much more sense.

Which just goes to show that you need to know about the past in order to fully grok the present. So not only is the age of that data not a barrier to better understanding. It's rather a requirement. If all we saw was how the universe was today, at this instant, we would be able to say much less about how it functions.

Alzrius said:

Correct me if I'm wrong, but I thought the dark matter theory had recently been debunked?

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Well, I work in the field and I haven't heard about that.

It's true that we don't know what it is, although we've been chipping away at the problem: it's reasonably certain now that neutrinos have a little mass, which will contribute a few percent of the missing mass. Recent telescope surveys of our galaxy have turned up a larger population of brown dwarf stars (things more massive than Jupiter, but less massive than the sun, which burn very dimly and for a long long time) than was expected, and it's likely that such objects also contribute a few percent. That still leaves a big chunk of 'cold dark matter' unaccounted for, which is likely to be an exotic stable particle that we haven't discovered yet because it can only be created at energies higher than what we can produce in the lab. There is no shortage of candidates for such a particle, believe me!

There are also lots of people out there that believe no such particle exists, and that a modification of our theory of gravity is needed to explain the motion of galaxies. But no one is saying that galaxies are moving as they should given our current understanding of gravity and the tally of visible matter.

In galaxy clusters, for example, the galaxies are zipping about at hundreds of kilometers a second. When we add up all the mass in galaxies and hot gas (visible in the X-ray band), we find that these speeds are well above the escape velocity of the clusters. Conclusion: if there were no dark matter, the cluster would fly apart and disperse in short order. Since we see galaxy clusters everywhere and over a wide range of distances, this doesn't make sense. What does make sense is to conclude that there is about three times as more mass in the cluster than what we can see. If we do this then the galaxies and hot gas obey perfectly the laws of gravity as we understand them. The hot gas in particular is very useful, because it fills the entire volume of the cluster and its temperature and luminosity decrease as you get farther from the center. One can actually map out the gravity well as a function of radius this way.

What has been recently debunked is the idea that the total amount of matter in the universe (dark or otherwise) is just enough to stop the expansion of the universe, which was the prevailing theory (for reasons of elegance) for a long time. But this just means that we need less dark matter than was previously thought--the current observations have cut the amount of matter needed to 30% of this magic number.

Cheers,
Ben

Black Holes

Here's a figure I did a couple of years ago for my research. It shows photon paths emanating from a point not far from the center of a rotating black hole. The outer circle is the ergosphere of the black hole; within this radius everything is constrained to revolve in the same sense as the rotation, but may still escape. The inner circle is the event horizon, and the dotted circle is the inner radius of an accretion disk (a mass of hot gas swirling down into the black hole). The colors don't mean anything, they just help you tell one photon from another.

Ben

Speed of light (again)

I just thought I'd mention that I talked with a very intelligent person not too long ago who is working on the Speed of Light question. In his circles, at least, it is not generally assumed that the speed of light varies baed on the color of light in question.
The example he gave me (because of my limited science understanding) is that a blue ray of light will travel a given distance .00025% faster than a ray of white light.
So, the speed of light may NOT be constant, which would mean we would have to re-think both General and Special Relativity...

mps42 said:

I just thought I'd mention that I talked with a very intelligent person not too long ago who is working on the Speed of Light question. In his circles, at least, it is not generally assumed that the speed of light varies baed on the color of light in question.
The example he gave me (because of my limited science understanding) is that a blue ray of light will travel a given distance .00025% faster than a ray of white light.
So, the speed of light may NOT be constant, which would mean we would have to re-think both General and Special Relativity...

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I don't quite understand this example, but...

The speed of photons in vacuum is constant. However, a ray or pulse of white light consists of a spectrum of photons, and if it travels through a medium such as air or water then photons of different colors may travel at slightly different speeds (always less than c), and the peak of the pulse might therefore travel at a speed slightly different than c.

I'm not on the cutting edge of photonics. I am aware that some research groups have claimed to transmit pulses of light at speeds faster than c under highly specialized conditions, but it's not clear whether these speeds are real (in the sense of energy, momentum, or information are being transmitted faster than c) or illusory (in the sense that the separation between two points, such as two wave peaks, can grow faster than light even though no individual particle or photon is traveling faster). As far as I am aware, research in this field is still ongoing.

Ben

One thing I have noticed about posting here- when you write something that you think is a clear question the reply does not always fit the question, or as one of my old friends use to say- perception seems to be everything.