Stephen Speicher

Spectacular Hubble image of a spiral galaxy

83 posts in this topic

What I hear as a layman is that they infer a black hole by what they don't see; i.e., they don't see anything and that is a black hole.  Lately I've heard that they've have empirical proof that this nothing exists (which to this layman's ears means they've found a contradiction and ought to check again).  What is this supposed proof?

Since a black hole cannot be observed directly, the evidence is inferred from observations and measurements of objects in the vincinity. So, for instance, observations of the high orbital speed of a star or gas near the center of a galaxy is taken to imply a massive central object capable of creating a strong gravitational field. Or the observation of accretion disks (rotating matter around a gravitational source) is taken to indicate the existence of a compact object with a strong gravitational field. Or, observation of jets of particles travelling at ultra-relativistic speeds (speeds greater than 99% the speed of light), is taken to imply a massive object capable of accelerating to such speeds.

And what is it that they are identifying as a black hole?  Is there an actual anomoly that they are trying to identify or is it like string theory, just something someone made up?

No, not made up. These are inferences from observations or measurements, and in some cases the supposition of a black hole is the only known explanation, while in other cases it was later discovered that alternative explanations existed. For instance, the ultra-relativistic jets I mentioned up above were for some time taken solely as evidence for black holes, but two years ago an ultra-relativistic jet was directly connected to a neutron star instead. Before this further observation it was thought that only black holes could be responsible for these ultra-relativistic jets.

Share this post


Link to post
Share on other sites
What are gravitons?

Interestingly, the notion of gravitons, if not the exact name, preceded particle physics and quantum theory. In 1873 Lord Kelvin analogized the gravitational theory of Le Sage and spoke of the "gravific corpuscles infinitely small." ("On the Ultramundane Corpuscles of Le Sage," William Thomson [Lord Kelvin], Philosophical Magazine, V. 45, pp. 321-332, 1873.)

Anyway, the graviton is a hypothetical particle which in the standard model of particle physics carries the gravitational force, much as the photon carries the electromagnetic force. More specifically, each particle mediates its respective interaction. There are several differences in these particles. though, not the least being that we can detect and count photons but we have never directly detected a graviton. However, since the gravitational force is about a millionth billionth billlionth billionth as strong as electromagnetism, the difficulty in direct detection is understandable.

If they are particles emanating from an object, then how can they cause one object to be attracted to another?

How that occurs (and how realistic it is) depends on each of the many, many different theories. From Newton's own musings of the excess of pressure of ether surrounding bodies with rarefied ether between, to a mechanical Le Sage-type gravity in a sea of "ultramundane corpuscles" where two bodies shade the corpuscles and thereby attract each other, to some new quantum field theory of gravity creating an attractive force by a graviton interaction with negative momentum transfer. Historically there exist a seemingly endless array of theories, but there is no generally accepted particle theory of gravity today.

If gravity acts between objects, can one speak of gravity in the space between objects?  What is the gravitational attraction of the moon on the earth 10,000 miles above the earth?

The effect of gravity falls off as the inverse square of the distance.

If gravity is a force between two objects, what happens when there are more than two objects?

Without simplfying assumptions this becomes the complex perturbations of a classical n-body problem.

Share this post


Link to post
Share on other sites
Jay, for physicists a black hole commonly represents a certain mathematical solution to the Einstein field equations...

Thanks for the explanation. Clearly, the term "black hole" means more than I reallized it did.

My understanding now is that, for increasingly massive stars:

1. Those that are least massive end up as white dwarfs. Their gravity is not strong enough to overcome the forces that keep atoms from collapsing, so what you'd end up with is a ball of cold atoms.

2. More massive than that, and the gravity is strong enough to squash the atoms down into neutrons. But it isn't strong enough to overcome whatever forces are responsible for the integrity of neutrons. So what you end up with is a ball of neutrons, densely packed together.

3. Even more massive, and the gravity is strong enough even to overcome the forces responsible for keeping the neutrons together and..... as I see it, here's where we really don't know what happens. The theory that predicts black holes in effect says that, once you get past this point, all of this matter just keeps on getting squashed down until it takes up absolutely no volume.

What I see as the problem to be solved is that: one can rule out a physical singularity on philosophical grounds, but we still don't have an explanation for what happens physically to stop the collapse somewhere short of becoming a singularity.

Is this approximately correct?

Share this post


Link to post
Share on other sites
Is this approximately correct?

Yes. Though there are proposals for intermediate stages such as the "quark star" that I previously mentioned.

But, perhaps even more important are radically different approaches that question the mechanisms that led to the problem in the first place. General relativity has proven to be spectacularly successful in predicting real-world facts, but not all solutions to mathematical equations necessarily reflect what actually exists. When we look at stars and galactic structures, gravitational effects have been taken as the dynamic. But perhaps electromagnetic effects play a role that has not been previously appreciated. Some aspects of plasma cosmology are quite interesting.

So perhaps what we need is a new perspective that will make better sense of the myriad observations and measurements.

Share this post


Link to post
Share on other sites

Stephan:

Thank you for taking the time to answer my questions (as well as those of the others, which I also appreciate). As usual, I find what you have to say infinitely fascinating. :)

Share this post


Link to post
Share on other sites

Stephen,

I'd like to second Janet's comment. I just wish I had had you as my physics teacher.

:)

Share this post


Link to post
Share on other sites
I like to go to the Hubble site every once and a while to look at the pictures. I try to conceive of the size of whatever I'm looking at, but it is too grand for my unschooled brain. Do those of you who have studied the physics and understand what a light-year is really have a grip on what it means to say that something is 170,000 light-years wide?

<{POST_SNAPBACK}>

Sure. "Conceptualization is a method of expanding man's consciousness by reducing the number of its content's units—a systematic means to an unlimited integration of cognitive data." (ITOE, p. 64). If you work with the units then the process of reduction from the greater to the smaller (light-years to inches) is an abstractly grasped process that becomes automated and therefore connected to the perceptual world. It is not direct perception, but it is grasped abstractly in a very meaningful way.

I have been meaning to say more about this for a long time.

A perceptual visualization of the enormous scope of measurements that we humans deal with is illustrated in the fascinating little book Powers of Ten: About the Relative Size of Things in the Universe, by Philip and Phylis Morrison and the Office of Charles & Ray Eames, pb 1994. A newer, 1998 version by Charles and Ray Eames alone is Powers of Ten: a Flipbook is also available. (I don't know what the difference is between the versions.)

There is also a very well done short film version of Powers of Ten available on DVD that is well worth watching. The DVD also includes the interesting earlier 1968 prototype version of the film that includes a scale showing relativistic effects.

The range of distances covered is from about a billion light years (~10^25 meters) across superclusters of galaxies all the way down to subatomic particles the size of .1 fermi (a unit of nuclear distance = 10^-16 meters, smaller than a proton). Each short "chapter" is devoted to illustrating a single power of ten, containing a photograph or other likeness together with a brief description of what there is at that size, measured in meters. You can start at a photograph spanning 10 meters of a man and a woman lying on the grass in a park, then proceed to zoom in or out one power of ten at a time, progressively looking at objects smaller or larger until you cover the whole range.

This allows you to directly visualize the relationship between sizes measured by adjacent and nearby powers of ten, but the mental unit economy made possible by the decimal system of numbers and counting powers of ten is what makes it possible to conceptually grasp the whole range and portions of it that are too much to directly grasp perceptually (except by supercrows :)). From Ayn Rand's Introduction to Objectivist Epistemology, "Chapter 7, The Cognitive Role of Concepts":

Observe the principle of unit-economy in the structure of the decimal system, which demands of man's mind that it hold only ten symbols (including the zero) and one simple rule of notation for larger numbers or fractions... Mathematics is a science of method (the science of measurement i.e., of establishing quantitative relationships), a cognitive method that enables man to perform an unlimited series of integrations. Mathematics indicates the pattern of the cognitive role of concepts and the psycho-epistemological need they fulfill.
This is how
Conceptualization is a method of expanding man's consciousness by reducing the number of its content's units—a systematic means to an unlimited integration of cognitive data.

A "light-year" is a unit of measurement that further reduces extremely large distances to smaller numbers of the larger light-year unit. A light-year is the distance travelled by light at its speed of 186,000 miles per second, or 3*10^8 meters/sec -- that means 3 followed by 8 zeros, or 300 million meters/sec, or 300 kilometers/sec -- which is a distance of about 10^16 meters in one year, i.e., ten thousand million million meters, or ten million million kilometers, that it takes light a whole year to traverse.

170,000 light years across a galaxy means it is so big that it takes light 170,000 years, or 1,700 centuries, to get across it, even though the speed of light is so enormous that it travels mundane distances between points here on earth almost instantaneously. The upper limit of 10^25 meters in the book Powers of Ten, which is almost the limit of the observable universe, is about 10,000 times greater: 10^9 light-years, or 10 million light-centuries.

These mathematical concepts are what allow you to conceptually grasp these enormous scopes, by numerically relating the units, that cannot be directly grasped perceptually.

Share this post


Link to post
Share on other sites