Certain subsectors of the scientifically-oriented blogosphere are abuzz — abuzz, I say! — about this new presentation on Dark Energy at the Hubblesite. It’s slickly done, and worth checking out, although be warned that a deep voice redolent with mystery will commence speaking as soon as you open the page.
But Ryan Michney at Topography of Ignorance puts his finger on the important thing here, the opening teaser text:
Scientists have found an unexplained force that is changing our universe,
forcing galazies farther and farther apart,
stretching the very fabric of space faster and faster.
If unchecked, this mystery force could be the death of the universe,
tearing even its atoms apart.We call this force dark energy.
Scary! Also, wrong. Not the part about “tearing even its atoms apart,” an allusion to the Big Rip. That’s annoying, because a Big Rip is an extremely unlikely future for a universe even if it is dominated by dark energy, yet people can’t stop putting the idea front and center because it’s provocative. Annoying, but not wrong.
The wrong part is referring to dark energy as a “force,” which it’s not. At least since Isaac Newton, we’ve had a pretty clear idea about the distinction between “stuff” and the forces that act on that stuff. The usual story in physics is that our ideas become increasingly general and sophisticated, and distinctions that were once clear-cut might end up being altered or completely irrelevant. However, the stuff/force distinction has continued to be useful, even as relativity has broadened our definition of “stuff” to include all forms of matter and energy. Indeed, quantum field theory implies that the ingredients of a four-dimensional universe are divided neatly into two types: fermions, which cannot pile on top of each other due to the exclusion principle, and bosons, which can. That’s extremely close to the stuff/force distinction, and indeed we tend to associate the known bosonic fields — gravity, electromagnetism, gluons, and weak vector bosons — with the “forces of nature.” Personally I like to count the Higgs boson as a fifth force rather than a new matter particle, but that’s just because I’m especially fastidious. The well-defined fermion/boson distinction is not precisely equivalent to the more casual stuff/force distinction, because relativity teaches us that the bosonic “force fields” are also sources for the forces themselves. But we think we know the difference between a force and the stuff that is acting as its source.
Anyway, that last paragraph got a bit out of control, but the point remains: you have stuff, and you have forces. And dark energy is definitely “stuff.” It’s not a new force. (There might be a force associated with it, if the dark energy is a light scalar field, but that force is so weak that it’s not been detected, and certainly isn’t responsible for the acceleration of the universe.) In fact, the relevant force is a pretty old one — gravity! Cosmologists consider all kinds of crazy ideas in their efforts to account for dark energy, but in all the sensible theories I’ve heard of, it’s gravity that is the operative force. The dark energy is causing a gravitational field, and an interesting kind of field that causes distant objects to appear to accelerate away from us rather than toward us, but it’s definitely gravity that is doing the forcing here.
Is this a distinction worth making, or just something to kvetch about while we pat ourselves on the back for being smart scientists, misunderstood once again by those hacks in the PR department? I think it is worth making. One of the big obstacles to successfully explaining modern physics to a broad audience is that the English language wasn’t made with physics in mind. How could it have been, when many of the physical concepts weren’t yet invented? Sometimes we invent brand new words to describe new ideas in science, but often we re-purpose existing words to describe concepts for which they originally weren’t intended. It’s understandably confusing, and it’s the least we can do to be careful about how we use the words. One person says “there are four forces of nature…” and another says “we’ve discovered a new force, dark energy…”, and you could hardly blame someone who is paying attention for turning around and asking “Does that mean we have five forces now?” And you’d have to explain “No, we didn’t mean that…” Why not just get it right the first time?
Sometimes the re-purposed meanings are so deeply embedded that we forget they could mean anything different. Anyone who has spoken about “energy” or “dimensions” to a non-specialist audience has come across this language barrier. Just recently it was finally beaten into me how bad “dark” is for describing “dark matter” and “dark energy.” What we mean by “dark” in these cases is “completely transparent to light.” To your average non-physicist, it turns out, “dark” might mean “completely absorbs light.” Which is the opposite! Who knew? That’s why I prefer calling it “smooth tension,” which sounds more Barry White than Public Enemy.
What I would really like to get rid of is any discussion of “negative pressure.” The important thing about dark energy is that it’s persistent — the density (energy per cubic centimeter) remains roughly constant, even as the universe expands. Therefore, according to general relativity, it imparts a perpetual impulse to the expansion of the universe, not one that gradually dilutes away. A constant density leads to a constant expansion rate, which means that the time it takes the universe to double in size is a constant. But if the universe doubles in size every ten billion years or so, what we see is distant galaxies acceleratating away — first they are X parsecs away, then they are 2X parsecs away, then 4X parsecs away, then 8X, etc. The distance grows faster and faster, which we observe as acceleration.
That all makes a sort of sense, and never once did we mention “negative pressure.” But it’s nevertheless true that, in general relativity, there is a relationship between the pressure of a substance and the rate at which its density dilutes away as the universe expands: the more (positive) pressure, the faster it dilutes away. To indulge in a bit of equationry, imagine that the energy density dilutes away as a function of the scale factor as R-n. So for matter, whose density just goes down as the volume goes up, n=3. For a cosmological constant, which doesn’t dilute away at all, n=0. Now let’s call the ratio of the pressure to the density w, so that matter (which has no pressure) has w=0 and the cosmological constant (with pressure equal and opposite to its density) has w=-1. In fact, there is a perfectly lockstep relation between the two quantities:
n = 3(w + 1).
Measuring, or putting limits on, one quantity is precisely equivalent to the other; it’s just a matter of your own preferences how you might want to cast your results.
To me, the parameter n describing how the density evolves is easy to understand and has a straightforward relationship to how the universe expands, which is what we are actually measuring. The parameter w describing the relationship of pressure to energy density is a bit abstract. Certainly, if you haven’t studied general relativity, it’s not at all clear why the pressure should have anything to do with how the universe expands. (Although it does, of course; we’re not debating right and wrong, just how to most clearly translate the physics into English.) But talking about negative pressure is a quick and dirty way to convey the illusion of understanding. The usual legerdemain goes like this: “Gravity feels both energy density and pressure. So negative pressure is kind of like anti-gravity, pushing things apart rather than pulling them together.” Which is completely true, as far as it goes. But if you think about it just a little bit, you start asking what the effect of a “negative pressure” should really be. Doesn’t ordinary positive pressure, after all, tend to push things apart? So shouldn’t negative pressure pull them together? Then you have to apologize and explain that the actual force of this negative pressure can’t be felt at all, since it’s equal in magnitude in every direction, and it’s only the indirect gravitational effect of the negative pressure that is being measured. All true, but not nearly as enlightening as leaving the concept behind altogether.
But I fear we are stuck with it. Cosmologists talk about negative pressure and w all the time, even though it’s confusing and ultimately not what we are measuring anyway. Once I put into motion my nefarious scheme to overthrow the scientific establishment and have myself crowned Emperor of Cosmology, rest assured that instituting a sensible system of nomenclature will be one of my very first acts as sovereign.
Jason,
I was trying to understand this description of light by Lawrence of light falling into a gravity well, from the perspective of an observer on the surface of the gravitational mass;
Using the model of a sonic boom, since light travels at the speed of light, it can’t emit photons that would precede it and be blueshifted by the light’s approach, all the waves across the spectrum would all arrive at once, hence white light, not blue or red.
I understand that redshift is proportional to distance, so that the further away a lightsource is, the faster it appears to recede. According to Big Bang Theory, the universe was expanding at a faster rate to begin with and has slowed, so the further away the source, the earlier in time the light was radiated and the faster the universe was expanding. Since the basic geometry of a conventional expansion would mean we are at the center of the universe, it was amended to space itself expanding. Yet from all I’ve read, it seems space is also, according to the equivalence principle, falling into gravitational wells.
So if space IS expanding, it doesn’t have to have been expanding at a greater rate in the past, since the further light travels, the more the effect is multipled, ie. light that is redshifted by crossing the initial space, has this effect further multiplied as the redshifted light is further redshifted by crossing more space. Eventually it crosses enough space that the source effectively appears to be receding at the speed of light and this creates a horizon line. I say “appears” because if space is effectively dynamically fluid, as it must be to expand and collapse, then the quantity of expansion is largely compensated for by the collapse of gravity. Since we only measure that light which has crossed space and not fallen into gravitational wells, or covered so much distance that it has been completely redshifted off the scale, we see what is flying away from us, not the compensating factors, ie. collapsing mass.
I realize I’ve been over this before, but I don’t think I’m really proposing anything which hasn’t been already accepted(expanding space/collapsing gravity), just putting them together in a larger cycle. In Sean’s last post, on the recent Edge question, he refers to a Paul Steinhardt (http://www.edge.org/q2008/q08_4.html#Steinhardt), who raises serious issues with making Inflation Theory work. So it’s not as though fundamental issues with BBT are resolved to the point of saying we are not missing anything.
What might actually cause space to expand, as opposed to reference points simply moving away from each other in stable space, is an open question. Is it some form of non-neutral vacuum fluctuation? Does light have some field effect we cannot yet measure? It appears to be some form of cosmological constant.
The fact is that these theories have only been developed over the last hundred years, so the astronomy on which it is based amounts to a moment in time, from one rather small spot in space. It wouldn’t be the first time in history a promising model had to be dropped because it simply didn’t answer the questions as well as other models that eventually replaced it.
Specialists are like gravity. Reality bends around their point of focus.
Generalists are like radiation. Weightless, but everywhere.
The sphere of dust particles around a central falling observer is not presumed to be moving faster than light. The distortion of this sphere is due to the Weyl tensor components of curvature and in a weak field limit is the source of tides here on Earth. The doppler shifting of light simply is anisotropic: It is blue shifted around an equator and redshifted for light emitted by particles at the antipodal points. Try to draw a picture.
The cosmological constant is thought to be due to the quantum vacuum of the universe. The momentum-energy tensor is
T^{ab} = (e + p)U^aU^b + pg^{ab}
e = energy density and p = pressure of the vacuum. If we equate this to a cosmological constant we get
/ = Trace(T) = (e + 3p).
This goes into the Einstein field equation
R_{ab} – 1/2Rg_{ab} = -(8piG/c^4)/g_{ab}.
This is a part of the canonical model here. There are some funny elements to this. In particular a Ricci curvature with R_{ab} = Kg_{ab} (K = constant) defines an Einstein space with some nonzero Ricci curvature in source free case. Yet here we have defined a momentum-energy tensor according to a source, the vacuum, and then turned this into an Einstein spacetime.
Steinhardt is the author of the quintessence theory and a cosmological idea called ekpyrotic (if I remember the spelling). The quintessence idea I think is a certain state of the vacuum, maybe related to dark matter, which is a space and time variation in the inflaton (Higgs-like) field in the universe. The dark energy vacuum I think is the dominant phase, and there is the final phase called phantom energy. These are I think related to each other in a way analogous to the onset of the “heavy destruction” of a Landau electron fluid phase in solid state physics. But that gets into some technical issues.
Lawrence B. Crowell
Okay, first of all, the change in the rate of expansion is really irrelevant to this point. The expansion rate could be completely constant in time, and we’d still observe objects further away having larger redshifts (this is, in fact, a very good approximation for the nearby universe).
Okay, I think I see what you’re saying now. Yes, there will probably be some biasing of measured redshifts due to the local environment of the source. However, there are two points to make:
1. Since matter falls into potential wells, looking far away, into the past, the universe was more uniform, thus the effect is more important locally, and less important far away (meaning it should cause a slight underestimation of the expansion rate).
2. This is, however, a minuscule effect. If we travel just 10,000 light years from the center of a 10^13 solar mass object (this is far more dense than any galaxy, I believe), then our gravitational redshift will only be around 10^-5, a value that is so small as to be negligible for the systems in question.
Lawrence,
The question is what are the effects on light falling into a gravitational well? Is it stretched antipodally and squeezed equatorially? What would be the effects of it climbing out of this well? Does it squeeze antipodally and stretch equatorially? Is there some residual effect that would explain whatever magnification results?
Jason,
Think in terms of the description of gravity as a bowling ball on a rubber sheet. The ball creates a gravity well. What determines whether the sheet is actually flat where there is no ball? Think in terms of hills and valleys. Where does the valley end and the hill begin? This is my feeling of expanding space, that it is the hills around these gravity wells and if you could bulldoze the hills of expanding space into the wells of collapsing space, the result would be flat (Euclidian) space.
As I’m proposing it, it’s not simply that gravity bends space one way, but that radiation bends it the other way. Gravity doesn’t scatter light, mostly it bends its path around the source of gravity. So if radiation, which emanates from gravitational objects in the first place, has the opposite effect, so that it expands space, rather then collapsing it, it wouldn’t scatter light either, nor would it even bend it around, because there isn’t the gravitational point of attraction. It would simply cause space to effectively expand, thus redshifting light crossing it, as a local, but extremely minute effect that would only be apparent at great distances. In this way, gravity and radiation would be opposite sides of a cycle of expanding energy and collapsing mass.
The weakness of the gravitational bending of light is realized when you consider that the Abel galaxy cluster is about 1 billion light years out and it lenses a quasar billions of light years behind it, and the angle of view and lensing is subtended by a few second of arc. Gravity is a very weak interaction.
The energy of light is Planck’s constant of action times the frequency E = hf. If it proceeds radially into or out of a Schwarzschild gravity well (a static spacetime solution) then the energy of the photon is
E = hf(1 – sgn*2GM/rc^2),
where sgn = + for the photon climbiing out and sgn = – for it falling in. For sgn = + the detector is at r —> infinity and the photon leaves a surface of radius r, and for sgn = – the photon comes in from infinity and is detected at the surface of the gravitating body at radius r.
I don’t know really what else to say about this. Don’t confuse this with the statements about an infalling shell of material and the antipodal expansion and equitorial contraction, which results in the observer at the center of the shell seeing an anisotropy of Doppler shift. To explicitely show this requires some analysis with the Weyl curvature or a general form of the geodesic deviation equation. I think I will avoid going there for that will involve some detailed mathematics.
Lawrence B. Crowell
I figured I’d illuminate a little more on this subject of light rays in curved spacetime. If you have a set of light rays then at an time they define a wave front. A spacetime with curvatures or gravity wave or what ever will distort this wave front the red or blue shift the light in the wave front. There are a set of equations which describe this called the Raychadhuri and Sach’s equations. I am not going to write in any depth on these for they are fairly complicated objects, yet one can look them up. They are pretty standard stuff in general relativity from the 1950s. They also apply to distributions of matter on timelike curves as well.
Lawrence B. Crowell
Lawrence,
Gravity’s effect on light does seem to be minor, except at the extreme. The nature of gravity is contraction and the nature of light is expansion. Of course, as a source of thermal energy, light can also cause mass to expand, to an extremely minor degree.
John,
Well, it doesn’t work that way. On average, the universe is expanding. Thus, though on average space appears to be flat, space-time is most definitely not flat.
Nope. First of all, gravity doesn’t bend space: gravity is the bending of space. It is energy and pressure caused by various forms of matter that bend space-time.
And, during the current epoch, the bending of space-time by radiation is completely negligible: the density is just far too low to have any significant effect.
Not in the least. The effect of normal matter and radiation is far from symmetric. Radiation has vastly lower energy density, for one, which means it doesn’t have anywhere close to the same effect on gravity. Furthermore, even if they did have the same energy density, their effects still wouldn’t be symmetric, because the behavior of radiation is categorically different (radiation has pressure, while normal matter, on large scales, does not).
The bending (curving is a preferrable term) of spacetime by gravity does exist. The Lagrangian for a Yang-Mills field theory is L = (-1/4)F^{ab}F_{ab} for F^{ab} a covariant tensor with field components. The momentum-energy tensor is
T^{ab} = &L/g_{ab} – g^{ab}L,
and it is not hard to get the momentum-energy tensor which feeds into the Einstein field equation
R^{ab} – 1/2Rg^{ab} = -8piG/c^4 T^{ab}.
Yet if you know or look it up the gravitational constant G is small, and c is fairly large and you are dividing by c^4 in the coupling constant 8piG/c^4. So it takes HUGE electromagnetic field densities (or some other Yang-Mills gauge field such as QCD) to induce spacetime curvatures.
A star might pump out what appears to be lots of photon energy, but this is nowhere near what is needed to induce spacetime curvatures. In fact such spacetime curvatures induced by EM radiation or other gauge fields likely only plays a major astrophysical role in the very early universe, or maybe in high energy interactions between elementary particles in tiny regions, such as an exceedingly high energy (near GKZ limit) cosmic rays impacting an oxygen atom in the upper atmosphere, or maybe with the gold heavy ion collisions performed at RHIC — at least maybe there are some signatures of spacetime physics (black hole-like structures) in the context of string theory or other quantum gravity theories.
Lawrence B. Crowell
High energy cosmic rays are themselves so small in number, though, that they don’t gravitate appreciably.
Jason,
Remember I’m not even on board when it comes to describing time as basis for motion, as opposed to consequence of it. If every clock is its own dimension of time and the only temporal absolute is the absence of all motion, then the question is whether the positive of expanding energy is balanced by the negative of contracting mass. Where does the energy for radiation come from, if not from the mass accreted to gravitational bodies in the first place? Does all energy above the level of the CMBR eventually coalesce back into mass? I realize that from your perspective, I’m just stupid and misguided, but for me, it is a basic pattern that does at least as effective a job of organizing the facts(redshift of galaxies, gravitational attraction) as the device of trying to describe how the universe originated from a point. An idea originally proposed by a theologian intent on justifying Genesis.
Radiation is also energy and pressure.
So it would only be over intergalactic distances that the effect might even be noticeable. Especially since there is even less gravitational counteraction.
Wouldn’t the fact that gravitational bodies occupy a vastly smaller area then irradiated space be a countervailing factor?
So if this pressure is causing space to expand, but the entire volume of the universe doesn’t, possibly because it is infinite to begin with, then this pressure would exert itself on already collapsing gravitational fields, causing a greater inward force then can be explained only by gravity, thus removing the need for dark matter, as well as explaining the Pioneer effect.
Lawrence,
The gravitational field for a star only extends out a few lightyears, yet the photons radiated travel for ten+ billion lightyears. When you consider the differences in not only distance but volume covered, plus the fact that every point in space is washed by light from nearly every star within that 10 billion lightyear radius, while the gravitational effect of any star is essentially local to its own area, that gravity would be a much, much stronger factor in its given field, is understandable. It would seem that radiation, as expanding factor, predominates in volume, while gravity, as contracting factor, prevails in density. They play to their strengths.
I know. And you’re wrong. The description of space and time as space-time indicates a particular symmetry: that transformations between reference frames of different velocities can be mathematically described as a sort of rotation. This is an accurate description whether you like it or not.
That’s not really a question. It’s just energy conservation. This in no way means that matter and radiation have symmetric behavior under gravity: they don’t.
What do you mean “above the level of the CMBR?” But I am aware of no mechanism that could cause the radiation that exists in the current universe to be turned into massive particles (the energy is too low).
It didn’t originate from a point. Singularities of that sort almost certainly cannot exist, as they are mathematical nonsense.
No, radiation has energy and pressure. Big difference.
No. Other effects completely dominate the curvature due to radiation, specifically the gravitation due to normal matter, dark matter, and whatever dark energy is.
The average density of the normal matter and dark matter is still vastly larger today than the average density of radiation, so it’s not enough.
No, it wouldn’t. First of all, you have to do a hell of a lot better than just wave your hands and say, “this is the effect.” Secondly, the pressure doesn’t “cause space to expand.” It actually slows the expansion down (during the radiation dominated era, the expansion rate slowed more rapidly than during the matter-dominated era, and this was due to the pressure that radiation has that normal and dark matter lack).
The only significant effect that radiation has on the collapse of gravitating bodies is to act as sort of an accountant: it carries off the energy that is lost when gravitating bodies interact and move to lower orbits. The effect of radiation pressure is well understood, and does not grow in strength with distance compared to gravity.
The gravitational field of any body extends to infinity. It has the exact same falloff as the radiation emitted.
Jason,
That it can be mathematically modeled to a high degree of accuracy doesn’t mean that model is reality. History is a model of time as a particular dimension and it is very effective, but the fact is that the material energy doesn’t manifest all of time for all time. It is a process of flux that creates time, not one based on it. Time is a measure, like temperature. Temperature can also be measured to a high degree and it can affect our ability to measure other dimensions in very precise ways, but no one claims temperature is the basis of motion, rather then a measure of it.(Not to start the argument over again, just giving Lawrence a heads up on this particular point of discussion.)
Obviously. Radiation expands. It’s matter that contracts.
It’s a long way from radiation to massive particles and we don’t really know what does occupy that space. If anything, photons would be the initial condension out of a energy field. How about quantum fluctuations out of a quantum field? Be interesting to see what the LHC comes up with.
Agreed there.
Hmm.. I don’t suppose radiation without energy and pressure would make any sense?
And dark energy is? It doesn’t take much dark energy to add up to 70% of everything, given the spaces involved. Okay, let’s say it isn’t regular radiation. Let’s say that it’s a positive vacuum fluctuation. It would still be falling into gravity wells, as well as increasing pressure on them. Expanding space between galaxies that is then falling into them.
A big chunk of that is the dark energy and if some part of the spin rate of galaxies attributed to this gravitational attraction were due to external pressure, the balance would be more equal.
Is that pressure, or friction? My impression is that this earlier era would presumably be more dense, whereas matter has condensed out of this radiation. You are saying that matter can no longer condense out of radiation? Is that faith or knowledge?
Yep. Expands as gravity contracts.
No, that’s why it’s pervasive, while gravity is focal. Even expansion.
Since gravity mostly affects mass, it’s a matter of proximity, size and density that determines real effects. So the effect is overwhelmed as soon as another field is stronger. While radiation really does travel pretty damn far.
Jason – you’re a hero. John Merryman – have you ever spent time learning General Relativity? Your every comment screams that you have not, but I thought I’d ask. If not, then you need to take an introductory course (or carefully read through an introductory book) if your preparation is sufficient. You cannot just try to make these “arguments” without understanding why people have come to this understanding of nature.
Your comments about gravity “mostly affecting mass” and many others indicate you have an elementary misunderstanding of GR.
Jason is being too patient. I do not wish to insult you, but you need an introductory graduate student understanding of GR to buy into a discussion like this and it truly does not seem like you have one.
Thanks, Mark.
John,
You keep saying this, but it really isn’t an accurate description of what is going on. Rather, as matter collapses, radiation escapes. It’s a rather different idea.
To (maybe) better understand this, consider what would happen if we had an interacting, self-gravitating cloud of gas and dust that did not emit any photons (i.e. no Bremsstrahlung radiation, nothing but elastic collisions). What would happen is that as the particles in the cloud interact, and transfer momentum between one another, some of the particles will achieve a momentum that gives that particle escape velocity: the particle will escape the potential well. Depending upon a particle’s mass, it will need some specific amount of momentum to reach this escape velocity, so lighter particles will escape more easily. Due to energy conservation, every time such a particle escapes, the average action of the cloud will be to collapse inward on itself ever so slightly.
The only difference with photons is that they always have enough momentum to reach escape velocity, because they always travel at the speed of light (no black holes in this thought experiment…they’re negligible in this sort of situation anyway).
This is the physical picture of what’s going on. And it’s just not accurately described by your exceedingly simplistic “matter contracts while radiation expands.”
The same is the case with radiation. Both follow 1/r^2 laws. This is just down to the fact that we live in 3+1 dimensional space-time. Unless there’s something weird going on with gravity at large distances that we don’t yet know about, gravity falls off at exactly the same rate as radiation, all the way out to infinity.
It’s an energy question. When the average energy of the particles in your system are greater than the rest mass of the lightest particles, then matter/anti-matter pairs of those particles will make up part of the “radiation” energy density. For example, when the energy of individual photons is above the rest mass of the electron (~0.5MeV), you could have two photons collide and produce an electron/positron pair. Once the temperature drops below a certain point, however, the number of collisions producing new electron/positron pairs is outnumbered by the number of annihilations of electron/positron pairs, and in a relatively short time all of the electron/positron pairs disappear, leaving behind only the few that don’t have a pair (it’s another interesting question as to why the early universe had a slight overabundance of normal matter).
So, sure, it can happen today whenever you have a situation where you have a plasma that is hot enough to produce, at the very least, electron/positron pairs (the matter/anti-matter pair that requires the lowest temperature to be produced in large numbers). And so you might well get this sort of thing happening in, say, neutron star collisions or in supernovae. But most of the time the temperatures are just far too low for electron/positron pairs to be produced, so there is no possibility of cooling so that normal electrons can condense out of the mass (note: the production of the imbalance in matter and antimatter can only happen at still higher temperatures, higher than we’ve yet pushed in our most powerful particle accelerators, so any electrons that condense out of such a state today would have existed before that region got hot enough for them to condense in the first place).
Mark,
I’m reasonably aware of my own ignorance and I commend Jason for his patience. My simple minded point is that much of the processes in the world I live in can be understood in terms of a convection cycle of expanding heat and condensing particles/order. Not just the climate and the geology, but many aspects of psychology, physiology, politics, economics, sociology, etc. So given the extent to which physics is dominated by energies that expand and ordered systems that contract, I think it reasonable to assume there is some grand cycle that we can only perceive bits and pieces of.
Jason,
Got to work, return to argue this evening…
Uh, what you appear to be describing sounds rather like thermodynamics, which is quite well-understood in these situations. But it’s not a “cycle” by any reasonable meaning of the word.
My mention of cosmic rays was to indicate that maybe these generate “blobs” that have some small amplitude for a black hole. This idea is not original completely to me, but is a part of some physics out there by Nimi and Randall. This would be a quantum system with some black hole amplitude on the scale of a nucleon. So obviously this is nothing large or that has a gravity field of any extent. But if you shove enough mass-energy into a small enough of a region, such as with a high energy scattering, you might get some probability for a black hole. In this case there is a possiblity for a tiny quantum black hole amplitude to exist in small region for some very brief period of time. The RHIC experiements might in fact be detecting these.
Of course some think that these black holes will gobble up the Earth, but of course we are talking about a small amplitude for a quantum black hole that decays very quickly. So such a scenario is not possible. It is similar to concerns early on that a nuclear chain reaction might cause the Earth to become engulfed in a nuclear cauldron and burn like a star.
Electromagnetic radiation and gravity force fall off as 1/r^2 for a similar reason. In the case of EM radiation if we have some number, call it N, of photons that are emitted at one time from a point then N is conserved. Now think of an imaginary sphere (Gaussian surface) enclosing this point at a distance r. This sphere has an area 4pi r^2. No matter how one changes the radius there will always be N photons which cross this sphere. So for p = energy per unit area we then have that the total energy E = N*e, e = energy of each photon, is equal to p times the area it crosses:
E = int dA p = 4pi r^2 p.
Now we agree that N is conserved as well as e, and of course we are assuming that the photons all have the same energy. It is then apparent that for E to be a constant that p = k/r^2, for k = a constant.
In the case of gravity it is similar in that if the force is determined by lines of force which are preserved in much the same way the number of photons are above then the force of gravity must also vary as ~ 1/r^2. Newton actually equated the centripetal force F = mr(omega^2) to some force F = kr^n and arrived at n = -2 as the case which obeyed Kepler’s second law.
The reason for the 1/r^2 fall off for radiation and gravity as analogous, but are not physically identical. The electric field falls off as 1/r^2, which is a reason for the fall off with radiation, and is associated ultimately with the masslessness of the photon. This gets into some other details, and H. Yukawa propsed a force that varied as
F = -k e^{-ar}/r^2,
for k and a constants. This was a potential for the nuclear interaction mediated by a massive particle, the meson.
I always find it amusing to see people take flash pictures at night of distant objects. The 1/r^2 makes pretty short work of that.
I do advise that people study the physics they don’t understand, and in the case of JM it is a bit clear that he not only does not understand relativity, but a lot of basic physics as well. Even just basic Halliday & Resnik level physics can tell you a whole lot about how the world works and clear up some of the confusion that is apparent.
Lawrence B. Crowell
Jason,
It can be countervailing forces, but convection is a thermodynamic cycle, as specific material absorbs heat, whether from radiation, pressure, etc. causing it to expand and as Lawrence points out, spead this energy over an expanded area, causing it to cool and contract.Call it whatever you want. Agood example is the current political revolution bubbling to the surface, as the structure of the old guard looses is sources of energy that are expanding out the cracks in the system.
Lawrence,
When we are young, we can choose what we want to study. When we get old, the lessons are what get shoved in our face. You might say I’ve had enough personal experience to know how and why structures crumble and what amount of energy it takes to keep them moving in a forward direction. Maybe it didn’t come from text books, but the fact is that reality is the territory and the math is just one more map. I’ve seen enough come and go to know only children think it’s all a straight line from start to finish. Wisdom is having been around enough to know those cycles are not just issues to overcome, they are the reality.
Not meaning to get snippy, but it has been a long day. Thank you both for your efforts and attempts to lift my brain up a few steps.
John,
Thermodynamics is not a cycle. Overall, it describes a one-way, irreversible process: the increase of entropy. Of course, there are cycles that exist in nature, but they’re never completely adiabatic: there is always some increase in entropy.
Jason,
Entropy applies to a closed system and closed systems loose energy, but where does the energy go? While I realize cosmology describes the entire universe as one singular unit, going from start to finish and even if I agree with that description, it still begs the question of where the energy of the universe came from, prior to the singularity and where does it go, after the fadeout.
Gravity sucks. Eventually something gets spit back out. Even if it’s another closed set, going from beginning to end, physics just hasn’t tied up all the loose end to say there isn’t some attractive element that pulled it all together to begin with.
Unless you propose some supernatural, one off event, nature requires explanations, as well as descriptions.
By definition, closed systems do not lose energy. Increase in entropy isn’t about loss of energy. It’s about systems tending towards more probable states.
And yes, you can still talk about the entropy of open systems. That’s not a problem in the least. It’s just that the “entropy always increases” 2nd law of thermodynamics doesn’t always apply (granted, it doesn’t always apply in closed systems either, as it’s only a probabilistic law, as we now know from its derivation from statistical mechanics).
The 2nd law does, however, appear to apply to an expanding universe, as there is no ordered inflow our outflow of energy that is required for entropy to decrease in an open system.
No, cosmology approximates our region of the universe by assuming that it extends infinitely in all directions. There’s no reason to believe this is actually the case, but there are good reasons to believe that it doesn’t matter for any inferences we make from the theory. We typically don’t try to describe all of existence as one unit, and often don’t even try to describe all of existence in the first place, just the region of the universe that we can observe, either directly or indirectly.
As for where the energy came from, that’s not really an interesting question: energy in matter fields isn’t conserved in an expanding universe anyway. Educate yourself a bit here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html
What’s interesting, rather, is where the very low entropy state from which our region of the universe evolved came from. I can’t believe you’ve been on Sean’s blog this long and are still ignorant of what he’s said on this topic.
What are you babbling about?
Duh. But it’s also important that those explanations/descriptions be correct, and yours clearly are not.
A few comments:
Some mention was made of the Pioneer anamoly. This is not likely anything due to gravitation of some unknown physics. At least it can’t be due to the cosmological constant, for that is too small. Further given there are three other such spacecraft crawling away from the solar system one might expect a similar effect with them as well. One might suggest that a clump of dark matter is gravitationally tugging at the Pioneer. Maybe, but this would suggest there is a fair amount of it out there and this would perturb the Oort cloud of small planetoids (Pluto has been demoted from being a planet) and would likely have made this unstable. Yet clearly the Oort cloud is as old as the 5 billion years of the solar system. So there is not likely much dark matter haunting the outer reaches of the solar system. Then again, though unlikely, we just got very lucky and sent the craft near a clump that just happened by.
The most likely explanation is that there is some sort of outgassing from the craft. A leaky tank or something of that sort might be shooting of a bit of reaction mass with some velocity that is nudging it off the expected course.
Thermodynamics plays a role in astrophysics, such as the area of a black hole is proportional to its entropy, or the collapse of matter in a gravitationally imploding star is thermodynamically similar to the compression of a gas with a “gas law” or some equation of state. Yet most of what we have been talking about here does not involve a temperature (thermodynamics = heat-motion) and so appealing to thermodynamics here is not on the mark.
Lawrence B. Crowell
Jason,
Possibly then that falls under the category of Sephen Jay Gould’s ‘Punctuated Equilibrium,’ a rewording of the old catastrophism. Which is that systems don’t so much evolve, as they settle into an equilibrium, until such time as something disrupts or destroys that stable state, at which time different forms develop to take advantage of the changed circumstances and eventually settle into a new equilibrium, until the process repeats itself. Evolution vs. revolution.
So we know the age of the universe is finite, because redshift assures us it is expanding, but other then that other factors say that space is flat as far as we can tell? What about redshift? Isn’t that evidence of curvature? Why would we need models that assume a flat space, if redshift is incontrovertible proof? It alway seemed to me that Inflation theory was an attempt to shoehorn a spatially infinite universe into a finite cosmological model. If space is expanding, but the universe isn’t,our location may be radiating infinitely in all directions, but it is also absorbing energy from the same infinite sources.
I realize you don’t accept my ideas, but from my perspective, I have followed the evolution of Big Bang theory for several decades now and just don’t buy into it anymore, so from my perspective it’s like trying to explain epi-cycles to me and insisting I’m just too stupid to understand all the complexities involved. Honestly, I would make more effort if I felt it was on the right track, but I see it that science settled on a model several decades ago and has put every effort possible into fitting all information into that model. Thus we have a finite universe that is curved according to redshift, but flat according to measurements of CMBR. One that is expanding space, but stable speed of light. As well as having begun as an expansion of space that was faster then the speed of light, but not really, because it carried light along with it. Although it appears bigger because it takes light more time to cross this expanded space, that was supposedly carrying the light along with it. Did I also mention that the energy content of the universe is 96% invisible, because observations and theory don’t quite match, so since theory must be right, there is just lots of stuff we don’t see. I’m truly sorry that not only does the emperor appear naked to me, but fat, old and ugly as well.
To the people whom I live around, I tend to be a mediator. Of course that does require me to figure out who is full of it and who is being rational.
Old sixties joke/bumpersticker; “There is no gravity. The earth sucks.”
Yours are? Isn’t that faith? The question isn’t, “What is correct?” The question is, “What is logical?”