Dark matter, Accidence and substance

cy · Apr 8, 2006

Dark matter, Accidence and substance
Two possible explanations for the bulk of reality

"THE unknown pervades the universe. That which people can see, with the aid of various sorts of telescope, accounts for just 4% of the total mass. The rest, however, must exist. Without it, galaxies would not survive and the universe would not be gently expanding, as witnessed by astronomers. What exactly constitutes this dark matter and dark energy remains mysterious, but physicists have recently uncovered some more clues, about the former, at least."

http://www.economist.com/science/displaystory.cfm?story_id=6768214

InfidelCastro · Apr 8, 2006

Economists talking general relativity and particle physics. That's a new one.

Economists tend to think they know everything, when in reality their own profession is a guessing game.

cy · Apr 8, 2006

The Economist is a well respect mag that dives into lots of topics. not just economics.

InfidelCastro · Apr 8, 2006

cy said:
The Economist is a well respect mag that dives into lots of topics. not just economics.

Maybe they could change the name to "The Knowitall"?

TedTheLed · Apr 10, 2006

TedTheLed said:
3-21-06

funny, we're looking for alot of oil that may or may not be here on earth, while at the same time we are also looking for most of the matter in the universe, which we have also somehow misplaced...

( hey, you don't suppose...???? .......naaaah........ )

:wave: Darrell

...

TinderBox (UK) · Apr 10, 2006

how fast is gravity, does anybody know.

what i mean is does the effect of gravity travel faster than light.

regards.

BB · Apr 10, 2006

The speed of gravity (measured in 2002 using radio telescopes):

As it turned out, the Jovian weather cooperated, and everything did go well, until the big day itself. On September 8, the telescope at Saint Croix malfunctioned because of serious tape recording problems. Fortunately, it turned out that the data from other telescopes could compensate for the loss. Although Kopeikin and Fomalont also had to discard about 15 percent of their data because of bad weather on Earth, this still left enough data to carry out the analysis. They compared the position of J0842+1835 on September 8, 2002, with its average position on the off-Jupiter days. Plugging this into Kopeikin's formula for the gravitational field of the moving Jupiter gave them the answer they were looking for. Kopeikin and Fomalont became the first two people to quantitatively measure the speed of gravity, one of the fundamental constants of nature. They found that gravity does move at the same speed as light. Their actual figure was 1.06 times the speed of light, but there was an error of plus or minus 0.21. The results were then announced at the 2002 American Astronomical Society annual meeting in Seattle, Washington.5

The result rules out the possibility that gravity travels instantaneously, as Newton imagined. If it did, a minutely different shift in the position of the quasar would have been visible on the night of September 8. This vindicates Einstein's instinct when formulating his general theory of relativity, which was to assume that the speed of gravity was equal to the speed of light.6

-Bill

MichiganMan · Apr 10, 2006

TinderBox (UK) said:
how fast is gravity, does anybody know.

what i mean is does the effect of gravity travel faster than light.

regards.

Its my understanding that gravity itself is more of a theory than anything else. I mean we can observe a phenomena where things tend to move towards one another, and mass, density and distances consistently factor into the degree to which it happens, but short of calling the phenomena "Gravity," we don't really know what it is. We have a much better handle on light, especially around here.

I love the discussions on dark matter. Dark matter is basically just a theoretical place holder, and an unsatisfying one at that, that we have to use until someone with a better idea comes along. It gives one a handy perspective on what it was like living in earlier ages when so much less was known about why or how things worked, ie. lightning, bacteria, etc, but enough was known to know that there had to be an answer, just barely out of reach.

AlexGT · Apr 10, 2006

I think gravity must be faster than light, because why is it that Black Holes in space have so much gravity that not even LIGHT can escape them?

Another thing I was wondering, what would happen if you travel at the speed of light and turn on a flashlight? would the beam be traveling at Zero times the speed of light or at 2 times the speed of light to the observer?

Hmmmm...

AlexGT

TinderBox (UK) · Apr 10, 2006

How about this one.

could the pull of a black hole cause light to travel faster than it`s normal speed by being pulled in, if gravity is slightly faster than light.

regards.

BB · Apr 10, 2006

Using Google will probably get you better answers than CPF... Try Wikipedia for the Faster than Light question as a start.

As I understand, many of the questions that can be asked (like "does a black hole evaporate over time") are unknown at this time and mostly discussed using higher order theoretical math (like trying to figure out 1/1/10<>1/0.1

) that us normal taxpayers are never going to understand.

Without the math, there is know good way of having a discussion as the theory (really theories) lies in the manipulation of the math--not by using a Watt meter plugged into a black hole on a desk. And any non-mathematical analogies that are attempted are almost so inaccurate in describing the math as to be virtually wrong from the get-go.

Per the question about evaporating black holes:

Now why do black holes evaporate? Here's one way to look at it, which is only moderately inaccurate. (I don't think it's possible to do much better than this, unless you want to spend a few years learning about quantum field theory in curved space.) One of the consequences of the uncertainty principle of quantum mechanics is that it's possible for the law of energy conservation to be violated, but only for very short durations. The Universe is able to produce mass and energy out of nowhere, but only if that mass and energy disappear again very quickly. One particular way in which this strange phenomenon manifests itself goes by the name of vacuum fluctuations. Pairs consisting of a particle and antiparticle can appear out of nowhere, exist for a very short time, and then annihilate each other. Energy conservation is violated when the particles are created, but all of that energy is restored when they annihilate again. As weird as all of this sounds, we have actually confirmed experimentally that these vacuum fluctuations are real.

-Bill

MichiganMan · Apr 10, 2006

Now why do black holes evaporate? Here's one way to look at it, which is only moderately inaccurate. (I don't think it's possible to do much better than this, unless you want to spend a few years learning about quantum field theory in curved space.) One of the consequences of the uncertainty principle of quantum mechanics is that it's possible for the law of energy conservation to be violated, but only for very short durations. The Universe is able to produce mass and energy out of nowhere, but only if that mass and energy disappear again very quickly. One particular way in which this strange phenomenon manifests itself goes by the name of vacuum fluctuations. Pairs consisting of a particle and antiparticle can appear out of nowhere, exist for a very short time, and then annihilate each other. Energy conservation is violated when the particles are created, but all of that energy is restored when they annihilate again. As weird as all of this sounds, we have actually confirmed experimentally that these vacuum fluctuations are real.

Couldn't have put it better myself.

No, really, I couldn't. Honest. :drool:

TinderBox (UK) · Apr 10, 2006

what if the missing matter in the universe.

exists in multiple overlaping dimensions each ones mass adding to the cosmic whole.

another 24 co-existing universes will make up for you missing 96% matter.

it really very simple.:lolsign:

nisshin · Apr 10, 2006

AlexGT said:
I think gravity must be faster than light, because why is it that Black Holes in space have so much gravity that not even LIGHT can escape them?

IIRC, black holes trap light because the space around a black hole is very warped; light thinks it is traveling in a straight line but is actually going around in circles due to the warped nature of space around a black hole. As far as the speed of gravity goes, any change in the configuration of the gravity well around a black hole will propagate at the speed of light (for example, if the black hole tilted for some reason).

AlexGT said:
Another thing I was wondering, what would happen if you travel at the speed of light and turn on a flashlight? would the beam be traveling at Zero times the speed of light or at 2 times the speed of light to the observer?

The beam will still travel at the speed of light, relative to you. Also, the flashlight cannot reach the speed of light because it has mass, which tends to warp/distort space around it.

TedTheLed · Apr 10, 2006

I thought black holes turn into stars,or something, depending on the amount of mass they accumulate.. I'll call Stephen, and get back to you..

..and a flashlight traveling at the speed of light would itself have to become pure energy -- so it would either have to explode to manifest that energy, or else expand into infinity forever, because e=mc2

otoh maybe not.

BB · Apr 10, 2006

From that same link above about black holes:

Actually, nobody is really sure what happens at the last stages of black hole evaporation: some researchers think that a tiny, stable remnant is left behind. Our current theories simply aren't good enough to let us tell for sure one way or the other. As long as I'm disclaiming, let me add that the entire subject of black hole evaporation is extremely speculative. It involves figuring out how to perform quantum-mechanical (or rather quantum-field-theoretic) calculations in curved spacetime, which is a very difficult task, and which gives results that are essentially impossible to test with experiments. Physicists *think* that we have the correct theories to make predictions about black hole evaporation, but without experimental tests it's impossible to be sure.

Black holes form from collapsing stars--very massive stars (much larger than our sun--~10-1,000,000+ the mass of our sun) (see link above).

-BIll

thesurefire · Apr 10, 2006

TedTheLed said:
I thought black holes turn into stars,or something, depending on the amount of mass they accumulate.. I'll call Stephen, and get back to you..

..and a flashlight traveling at the speed of light would itself have to become pure energy -- so it would either have to explode to manifest that energy, or else expand into infinity forever, because e=mc2

otoh maybe not.

e=mc^2? Yes I agree that it does, but can you prove it?

BB · Apr 10, 2006

Why is it e=mc^2 and not e=(1/2)mc^2 which is the normal Newtonian formula for energy?

-Bill (went into engineering instead of physics) B

tvodrd · Apr 10, 2006

Read Steven's "A Brief History of Time" for some attempted lay explainations from the generally conceeded greatest physicist of *our* time. If I accelerate to 0.9C and shine my EDC ahead, what does somebody approaching me at 0.9C (from my frame of reference before I started my acceleration) see? Extreme blue-shift?

E=MC^2? is that MKS? E in Joules, M in KG and C in M/Sec? Where's js when you need him!

Larry

BB · Apr 10, 2006

tvodrd,

Not to be pedantic--but M=Grams, probably not KGrams... And, technically, the units would be any energy/mass/speed units, Not just metric. :dedhorse:

-Bill

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