Dark Matter: Still Existing

I love telling the stories of Neptune and Vulcan. Not the Roman gods, the planets that were originally hypothesized to explain the mysterious motions of other planets. Neptune was propsed by Urbain Le Verrier in order to account for deviations from the predicted orbit of Uranus. After it was discovered, he tried to repeat the trick, suggesting a new inner planet, Vulcan, to account for the deviations of the orbit of Mercury. It didn’t work the second time; Einstein’s general relativity, not a new celestial body, was the ultimate explanation.

In other words, Neptune was dark matter, and it was eventually discovered. But for Mercury, the correct explanation was modified gravity.

We’re faced with the same choices today, with galaxies and clusters playing the role of the Solar System. Except that the question has basically been answered, by observations such as the Bullet Cluster. If you modify gravity, it’s fairly straightforward (although harder than you might guess, if you’re careful about it) to change the strength of gravity as a function of distance. So you can mock up “dark matter” by imagining that gravity at very large distances is just a bit stronger than Newton (or Einstein) would have predicted — as long as the hypothetical dark matter is in the same place as the ordinary matter is.

But it’s enormously more difficult to invent a theory of modified gravity in which the direction of the gravitational force points toward some place other than where the ordinary matter is. So the way to rule out the modified-gravity hypothesis is to find a system in which the dark matter and ordinary matter are located in separate places. If you see a gravitational force pointing at something other than the ordinary matter, dark matter remains the only reasonable explanation.

And that’s precisely what the Bullet Cluster gives you. Dark matter that has been dynamically separated from the ordinary matter, and indeed you measure the gravitational force (using weak lensing) and find that it points toward the dark matter, not toward the ordinary matter. So, we had an interesting question — dark matter or modified gravity? — and now we know the answer: dark matter. You might also have modified gravity, but one’s interest begins to wane, and we move on to trying to figure out what the dark matter actually is.

Dark Matter Motivational Poster

But some people don’t want to give up. A recent paper by Brownstein and Moffat claims to fit the Bullet Cluster using modified gravity rather than dark matter. If that were right, and the theory were in some sense reasonable, it would be an interesting and newsworthy result. So, you might think, the job of any self-respecting cosmologist should be to work carefully through this paper (it’s full of equations) and figure out what’s going on. Right?

I’m not going to bother. The dark matter hypothesis provides a simple and elegant fit to the Bullet Cluster, and for that matter fits a huge variety of other data. That doesn’t mean that it’s been proven within metaphysical certainty; but it does mean that there is a tremendous presumption that it is on the right track. The Bullet Cluster (and for that matter the microwave background) behave just as they should if there is dark matter, and not at all as you would expect if gravity were modified. Any theory of modified gravity must have the feature that essentially all of its predictions are exactly what dark matter would predict. So if you want to convince anyone to read your long and complicated paper arguing in favor of modified gravity, you have a barrier to overcome. These folks aren’t crackpots, but they still face the challenge laid out in the alternative science respectability checklist: “Understand, and make a good-faith effort to confront, the fundamental objections to your claims within established science.” Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source, and why your theory miraculously reproduces all of the predictions of the dark matter idea (which is, at heart, extraordinarily simple: there is some collisionless non-relativistic particle with a certain density).

And people just don’t do that. They want to believe in modified gravity, and are willing to jump through all sorts of hoops and bend into uncomfortable contortions to make it work. You might say that more mainstream people want to believe in dark matter, and are therefore just as prejudiced. But you’d be laboring under the handicap of being incorrect. Any of us would love to discover a modification of Einstein’s equations, and we talk about it all the time. As a personal preference, I think it would be immeasurably more interesting if cosmological dynamics could be explained by modifying gravity rather than inventing some dumb old particle.

But the data say otherwise. So most of us suck it up and get on with our lives. Don’t get me wrong: I’m happy that some people are continuing to work on a long-shot possibility such as replacing dark matter with modified gravity. But it’s really a long shot at this point. There is a tremendous presumption against it, and you would have to have a correspondingly tremendous theory to get people interested in the possibility. I don’t think it’s worth writing news stories about, in particular: it gives people who don’t have the background to know any better the idea that more or less everything is still up for grabs. But we do learn things and make progress, and at this point it’s completely respectable to say that we’ve learned that dark matter exists. Not what all of us were rooting for, but the universe is notoriously uninterested in adapting its behavior to conform to our wishes.

147 Comments

147 thoughts on “Dark Matter: Still Existing”

  1. In Tegmark’s mathematical multiverse, MOND would still be the correct theory in some sector. So, even if it turns out not to be relevant for our universe, one still has to study it in order to understand why we don’t find ourselves living in the MOND universe.

  2. Let’s get real: if the universe really was just “mathematical” (and one among all the possible descriptions etc.), then anything at all could be true. With no inherent underlying principle, we could just as easily find ourselves in a world with the gravitational attraction being according to 1/r^2.001 as 1/r^2, etc. or even that changed with time in all kinds of ways, or even just arbitrary patterns (since they can all be logically “described,” albeit in ever more complicated ways.) A mathiverse gives no basis for predicting anything at all, despite the misleading intuitive implications.

  3. John Merryman,

    Photons don’t radiate. They have no electric charge.

    Count Iblis and Neil B.,

    If Tegmark’s mathematical universe is correct, then that doesn’t mean you can arbitrarily change constants or equations. Firstly, any set of equations that we can observe must have originated from some basic mathematics which are capable of generating new universes. Second, among those sets of mathematical setups which can generate new universes, we must exist in one capable of generating life. These place tremendous constraints upon what sorts of universes we can observe, and so if we knew how to connect fundamental theories (like string theory) to ideas like MOND as well as our own universe, then perhaps we would be capable of saying that there is a universe governed by MOND instead of dark matter. But I just don’t think that very hard work has yet been done for any truly fundamental theory (as I think string theory is the only such theory we have at the moment).

  4. Hi Sean,

    I was about to ask the same as Brian in #37: what is your take on the situation in Abell 520?

    The Bullet cluster was a spectacular result, because it beautifully confirmed our assumptions about how dark matter, gas, and galaxies behave, [but] Abell 520 does the complete opposite,” comments Julianne Dalcanton of the University of Washington in Seattle.

    Separation Anxiety: Cosmic collision may shed light on dark matter
    By Ron Cowen

    Best,

    B.

  5. Neil, the underlying principle for reality (including this universe) is that it needs to be self-consistent. Otherwise it cannot exist. That puts tremendous constraint on what can exist.

    “A mathiverse gives no basis for predicting anything at all, despite the misleading intuitive implications. ”

    How can this be true? Only a mathiverse would allow one to do calculations and predictions? That is what physics is all about.

    There seems to be no alternatives to a mathiverse. Or maybe you can say what is the alternative?

  6. Jason,

    Photons don’t radiate. They have no electric charge.

    Maybe you could read what I asked, before responding; “we understand how photons gravitate, but is that the same as how lightwaves radiate? Radiation and gravitation would seem to be opposite sides of some larger cycle of expansion and contraction, so when we define light in terms of a particle, it conforms to the contraction side of the equation, but redshifting is a function of the expansion side of the equation. Particle or wave, but not both at the same time.”

    Matter doesn’t radiate. Energy does. Is the photon the initial consolidation of mass out of energy?

  7. John,

    Well, I had assumed you were talking about EM radiation, which photons do not emit. Are you talking about some other form of radiation, such as gravitational?

  8. John, radiation (like EM) carries energy, but energy as such does not radiate generally. Charged matter radiates EM when accelerated, and neutral matter is supposed to radiate gravitational waves when accelerated (difficult to measure though).

  9. B and Brian, I don’t know much about Abell 520. It looks like a mess to me, and my strong suspicion is that we’ll need better data to figure out what is going on. The nice thing about Bullet cluster is that it’s pretty clear what is going on.

  10. Brian Lacki —

    Picking up on the sub-thread (strand?) in #29-35-38-41 above:

    Yes, it has seemed to me that it’s hard to find a simple, true statement about the role of pressure, as distinct from mass-energy, as being a source of gravity, since it seems to behave that way in some examples but not others. I have personally put this question directly to a number of certified Big Brains, including at least one CV’er (not Sean), and not yet gotten a sensible answer. Perhaps we can attract Sean’s attention here?

    Your reading in Peacock sounds quite to the point, but I can’t comment on it until you tell me whether rho is mass density or energy density (the latter would be expected, but the factor of 1/c^2 in (rho + 3p/c^2) suggests the former).
    When was the book published?

    Hoping for enlightenment,

    Paul Stankus

  11. Thanks, Sean. My question in post #37 wasn’t really “fair”, because all I think we can do right now about Abell 520 is speculate. I just took a fling – just wanted to hear what you would say.

  12. Paul,

    In what example does pressure not act as a source of gravity? Because that would pretty much falsify general relativity.

  13. The only “simple, true statement” about the role of pressure is Einstein’s equation: G_uv = 8 pi G T_uv. Spacetime curvature on the left, stress-energy tensor on the right. Everything else is just words that we attach to specific solutions to give us a warm and fuzzy feeling — for example, “negative pressure causes the universe to accelerate.” Einstein’s equation relates tensors, not scalars, so there is no simple “source of gravity” in the Newtonian sense. But the equation itself is completely unambiguous; there aren’t any unsolved problems here.

  14. Paul,

    Here’s a reasonably simple way to think about pressure’s role in gravitational physics.

    What matters in GR is the stress-energy tensor, T^{ij}, which tells you about the density of the i-th component of energy-momentum in a 3-volume orthogonal to the j-direction. So T^{tt} tells you the density of energy in a spacelike 3-volume orthogonal to the t direction, and (T^{xt}, T^{yt}, T^{zt}) gives you the density of the momentum 3-vector, again in a spacelike 3-volume orthogonal to the t direction. When we set j equal to a spacelike direction, say x, the 3-volume orthogonal to it includes time as one of its directions, so the “density” becomes a rate of change with time of energy or momentum per unit area. The rate of change with time of momentum is force, and force per unit area is pressure, so a component such as T^{xx} is pressure in the x-direction.

    The stress-energy tensor of a blob of pressure-free matter is just rho u^i u^j, where rho is the mass density measured in the matter’s rest frame, and u is the matter’s 4-velocity. If you took a gas and computed the stress-energy tensor molecule by molecule this way (ignoring fiddly details of molecular structure such as bond potential energies, vibrations, etc.) and added up all the single-molecule stress-energy tensors, you would get the correct stress-energy tensor for the gas as a whole; because of the different directions for u for the different molecules, diagonal pressure terms like T^{xx} would appear automatically in the total, when written in a coordinate frame in which the gas was at rest. There is no extra term you need to add to T to account for the pressure.

    The thing is, though, we don’t usually do the calculation that way: adding up rho u^i u^j for billions of molecules. Instead, we have macroscopic quantities describing the gas: its macroscopic density and its pressure, measured not in the rest frame of one molecule, but in the rest frame of the gas. If we want to construct T that way, we need to make sure that the density we use is the complete mass-energy density (i.e. it must take account of the kinetic energy of the molecules, not just their rest mass), and we need to explicitly make use of the pressure. We then get a tensor, in the rest frame of the gas, of the form diag(rho,P,P,P).

    Similarly, when we’re dealing with an electromagnetic field we need to use the appropriate stress-energy tensor for that, which will generally contain both pressure and tension terms.

    But however we end up constructing T, if there’s pressure or tension around there will be diagonal terms in T, and when T goes into Einstein’s equation as the source of spacetime curvature, those terms will make a difference.

  15. Jason, Carl,

    My initial understanding, aquired in thirty years ago, is that light is either waves or particles, but not both at the same time. That the process of measuring physically collapses the wave to particles, not that measuring simply discovers the particles contained within the waves and that they are actually both particle and wave at the same time. A possible analogy being that a lightning bolt collapses the energy in a cloud to a specific spot on the ground. Carver Meade wrote some interesting stuff that sounded similar.
    A thread I tried starting in the recent god discussion, about the nature of time, ties into this. If time is a physical dimension along which physical reality travels from past events to future ones, then reality is both particle and wave because there is nothing to specifically define one from the other, but if time is a process by which motion in space creates events, then time is the wave of future potential collapsing into the particle of information that recedes into the past, as various fields interact, much like the interaction of the fields of sky and ground connect as lightning bolt, or tree. Both of which accrete energy to a specific point at the interface.
    Admittedly my understand of physics is diluted by interaction with various other fields of study.

  16. Sean,

    MOG will not disappear until the day a press release is announced lauding the direct detection of dark matter particles. The discovery of dark matter is crucial, easily as much as the discovery of the CMB, for establishing the epistemological soundness of cosmology. Right now, anti-Big Bang folks use our lack of understanding of dark matter as yet another piece of evidence that we are really no further along in our understanding of the cosmos than we were generations ago. For them, dark matter is the 21st century version of the ether. Trust me, few events would be more devestating to an anti-Big Banger than the direct detection of dark matter particles. If the amount of dark matter directly detected matches the abundance implied by lensing, CMB, supernovae, and galaxy clusters, this would be one of the greatest scientific achievements of humanity. Until this happens, if it does, there will always be that discomforting lingering doubt that perhaps dark matter is a cosmic mirage after all.

  17. I suppose it’s far too early to say, with the nature of dark matter still up in the air, but are there any respectable models in which the net proportion of dark matter is declining over time?

    Could that explain the apparent acceleration of the Universe’s expansion, if dark matter acts as a gravitational brake but is gradually turning to some other form of mass-energy less effective as such (if that is physically possible)?

    I guess in that case one would expect to see anomalous rotation more pronounced in galaxies the more distant they are. They would tend to be smaller though, which might muddy the waters somewhat.

    On a related note, nobody has mentioned an intriguing idea mentioned a year or two ago, in New Scientist I think or maybe Scientific American, that dark matter is a kind of inverted and magnified (and perhaps time-shifted?) “image” of mass projected from the interiors of black holes. That was the gist of the proposal as I recall.

  18. John Merryman,

    Nope, not at all. All quantum mechanical particles, photons included, behave in a manner that makes them appear to be both particles and waves at the same time. This is basically just another way of saying that quantum mechanics is weird, and its behavior doesn’t actually coincide with anything we experience in the macroscopic world. The “particle” and “wave” behavior are really just analogies of the true behavior.

    And wave function collapse, by the way, is merely an illusion. Representing the full wave function of the particle and its environment shows us that decoherence caused by interactions with the environment cause the different eigenstates of the particle to stop interfering with one another, causing the appearance of wavefunction collapse:
    http://en.wikipedia.org/wiki/Quantum_decoherence

    spaceman,

    Sure, but where the science is concerned, big bang doubters really aren’t worth listening to. There’s more than enough evidence for dark matter, and the big bang theory in general.

    John R. Ramsden,

    I believe this is a general feature of dark matter theories. But they’re typically stable enough that it’s a very small effect. After all, if the annihilation of dark matter was too rapid, either it’d all be gone by now, or we’d be able to see the resultant radiation. There are a number of theorists who have proposed methods of detecting dark matter annihilation as a means to either test for interesting new physics, or just to detect the dark matter.

    As for using galaxy rotation curves to test for this, well, there’s just far too much uncertainty in galaxy formation and structure for that to be useful at the current time.

  19. Sean — (reply to #88 re: the role of pressure)

    I’m not claiming that there are any unsolved problems or ambiguities in GR. What I’m asking about is more a question of pedagogy, ie what kinds of simpler staements can we reasonably make, short of the full Einstein equations, tha will help build up our intuition?

    One example you will often read is “An object’s gravity is proportional to its energy density plus three times the pressure.” This general statement then leads to the conclusion that a universe filled with positive pressure stuff will decelerate more strongly than one with zero pressure; and from there it’s not a big leap to believe that negative pressure can result in negative deceleration, ie acceleration, in the universe. So this simplified statement about the role of pressure can help lead the non-expert in the right direction.

    Do you, as an expert pedagog, approve of the statement “An object’s gravity is proportional to its energy density plus three times the pressure”? as a good, if inexact, general rule to keep in mind? It clearly has value; but as per the counterexample described in comment #38 above I think it’s wrong, and not just a little but but really flat wrong. A localized object can have its internal pressure raised — think of an explosion — even by very large amounts with absolutely _no_ effect on its gravity as defined by its pull on distant objects. What, if anything, am I missing here?

    So I think there is a serious pedagogy question here, and I’m not just talking about casual writing. The statement “An object’s gravity is proportional to its energy density plus three times the pressure” appears verbatim in a sidebar to the much-read article “From Slowdown to Speedup” by Adam Riess and Michael Turner in the Feb 04 Sci Am. Pesonally I was startled to read a statement that I think is flat wrong being presented authoritatively in Scientific American. Am I off base, or missing something? What do you think?

    Paul

  20. Paul– Right, I know what you mean and I am precisely objecting to statements of the form “An object’s gravity is proportional to its energy density plus three times the pressure.” Or at least, not to the statements themselves, but the temptation to take them too literally. In general relativity, there is no single scalar quantity that gives rise to “an object’s gravity.” It’s a suggestive thing to keep in mind, to drive home that pressure plays a role in GR that it doesn’t in Newtonian gravity, but if you then try to apply it to circumstances beyond those the original authors had in mind, you’re likely to run into problems. So I tend to avoid statements like that altogether.

  21. Speaking of subtle issues in defining gravitational attraction in GR, and re the issue of gravity around a pencil of light etc. raised above: Consider interpenetrating pencils of light each with power P, moving past each other in opposite directions (say x, -x.) There is an energy density per unit length, given as P/c, and thus combined effective mass per unit length density 2P/c^3. Assume correctness of above referenced “…Tolman’s derivation (Phys. Rev. 37, p. 602, 1931) that a mass will (gravitationally) accelerate towards a pencil of radiation with *twice* the magnitude of acceleration as it would towards the equivalent mass density source of gravity.” That gives a gravitational field of g = 4G*lambda/r, where lambda is equivalent mass density.

    Well, we should also consider the “gravi-magnetism” (which I wish they’d called gravitism for short) from this beam. If the light was all moving one way, there’d be such a field in lab frame. It would show up as an alteration of the “force” on a piece of matter moving along the beam, in like manner to a charge moving past a line of charge. (Note: the effective force (as for work done) on moving matter is mg, but with m being the relativistic mass.) The gravi-magnetic fields of the light beams should cancel out (from symmetry, regardless of just what formula determines them.)

    However, consider a bullet zipping along the beam relative to “us” for which the beams are of equal intensity. If the field is simple (no net gravimagnetic field), then the proper acceleration of the bullet should be gamma squared more than normal (because it has to hit at the same time as the stationary mass, as classically, and demanded by the equivalence principle – floor coming up to meet those particles.) That makes sense, since the proper force of gamma squared is reduced for us by lateral transformation to be gamma, which is right for the increased mass.

    Yet this isn’t right from the bullet’s point of view. It sees one beam with Doppler squared intensity combined with another with the inverse intensity of that. (The Doppler formula gives the frequency change, and thus energy per photon, but there are more energy hits per unit time/more photons.) At say, 0.6c, the ratio of that over the mass-energy density we see is going to be (4 + 0.25)/(1 + 1) = 2.125. That does not comply with the expected attraction being gamma squared the lab value, which would be 1.5625 in this case. (Remember that for the bullet, it is sitting there and no gravimagnetic fields should apply, just (presumably) the combined fields from the two pencils of light. Those pencils now being of unequal power shouldn’t change the additive principle.)

  22. While I now agree that there’s no simple scalar defining how much gravity an object produces, I think Paul Stankus’ scenario does involve a question about a scalar. Here’s an example of a scenario I believe Paul is trying to ask about:

    Suppose there is a stationary non-rotating star (which I’ll define as an object in hydrostatic equilibrium) that is spherically symmetric and composed of a perfect fluid. Initially it will have a certain mass density and negligible pressure. So, inside the star, in the star’s rest frame, all terms of T_uv are zero except T_tt (where we’ll use Schwarzschild coordinates). Also let’s say that there are no other objects that contribute to T_uv, so that outside of the star, we have a Schwarzschild metric.

    We (being observers at infinity at rest with respect to the star) observe a test particle in orbit around the star. The particle orbits very far from the star, so that its path is well described by Newtonian mechanics. We measure the period P_i of the particle and its semimajor axis a_i, and define a quantity M_i = a_i^3 / P_i^2.

    Now the star collapses to a degenerate object, without any ejection of matter or radiation. The degenerate object is still spherically symmetric, composed of a perfect fluid, non-rotating, stationary, in hydrostatic equilibrium, and so on. All non-diagonal terms of T_uv remain zero. The exterior metric is still some Schwarzschild solution. The degeneracy pressure of the object, however, is much larger, such that the star’s particles are ultrarelativistic and p = e / 3, where e is the energy density and p is the pressure.

    After letting the object settle into hydrostatic equilibrium, we again measure the test particle’s period P_f and its semimajor axis a_f, and define M_f = a_f^3 / P_f^2.

    Which of the following is true?

    1. M_f = M_i
    2. M_f > M_i

    If (1), then why hasn’t the appearance of pressure in the star changed M?

    If (2), then what condition of Birkhoff’s theorem does not apply, so that M can change (if Birkhoff’s theorem prevents that), and why doesn’t that produce the forbidden monopole radiation?

    In the Schwarzschild metric, there is a scalar parameterizing the metric, M. Furthermore, what we observationally measure is a scalar, M_i or M_f. So, Paul’s question is one about the scalar M in the already given Schwarzschild metric, which describes some aspect about the metric that presumably depends on rho and p.

    The impression I’m getting is that (2) is right. From what I read, Birkhoff’s theorem applies to vacuum solutions. But, of course, the star itself is not a vacuum, and T_uv is changing inside the star. We must have boundary conditions at the star’s old surface, which presumably change to reflect the addition of pressure. The only free parameter at the boundary would be M. Usually, M doesn’t change — any pressure terms, for example, would be negligible in a Cepheid, or even a realistic typical supernova. In that case, the exterior metric could not possibly change, since the one free parameter is constant. So, usually, it’s correct to say that the exterior field is static. But, if I’m understanding this correctly, the appearance of pressure can change the effective M, and then the metric is not static. I don’t know whether (2) would still cause monopole radiation, though, since I know very little about that.

    In retrospect, I have to wonder how broadly Birkhoff’s theorem can be applied. Clearly it can’t be that once a sphere of mass M, always a sphere of mass M. Suppose you have a spherically symmetrical ball of gas contained in a spherical vessel of negligible mass floating in a vacuum. By Birkhoff’s theorem, the exterior metric must be Schwarzschild. For all we know it’ll always be that way. But it’s not necessarily the case that the metric will remain symmetrical. Just open a hole in the container at one pole, and the gas will vent out end, establishing a preferred axis and breaking the spherical symmetry.

    As for the neutron stars, I now realize that pressure can be much less than energy density even at the critical radius. A billion solar masses of gas, at a density of water and nearly zero pressure, will collapse into a black hole. That’s probably why a 1.5 solar mass stellar core will become a 1.5 solar mass neutron star, and why the degeneracy pressure doesn’t mess up Kepler’s Third Law for pulsar planets — even in a neutron star, it’s still probably not very big. Also, anything that goes on in the event horizon after it collapses doesn’t affect things outside the event horizon, so I suppose it doesn’t matter if the star’s pressure becomes very high inside the horizon.

    Am I understanding this correctly?

  23. jason,

    Nope, not at all. All quantum mechanical particles, photons included, behave in a manner that makes them appear to be both particles and waves at the same time. This is basically just another way of saying that quantum mechanics is weird, and its behavior doesn’t actually coincide with anything we experience in the macroscopic world. The “particle” and “wave” behavior are really just analogies of the true behavior.

    Thank you for taking the time to engage me on this.(The arm was getting tired.)

    Let me approach it from a different angle. Say the relationship between waves and particles is similar to that of nodes and networks. As a further example, lets say the network is a forest and the nodes are trees. Now obviously they do exist at the same time. (The original understanding was from thirty years ago.)
    So what is the real difference between trees and forest? One is form, trees and the other is function, the genetic propogation of tree DNA. As a larger organism, the forest is constantly passing through generations of trees, as individual examples sprout, grow, die and fall. Just as trees(and people), as multicellular organisms, consist of generations off cells being created and shed. So as process/function goes from past forms to future forms, these forms go from being in the future to being in the past. Just as the rotation of the earth proceeds through the series of time units called days, as individual days go from being in the future to being in the past. As our own lives go from being in the future to being in the past ,even as we live them from past events to future ones.

    Now assuming you understand what I just said about time going in both directions, depending on whether it is form or function, how does this apply to cosmology. Do we agree that energy radiates out, ie. expands and matter gravitationally contracts? If so, then compare it to the relationship between form and function, with matter as form and energy as function. Energy is constantly radiating away from older structure, such as stars and galaxies. Then it eventually condenses into or joins up with other structure and expands the total mass of that form. Meanwhile this structure is formed, grows as it absorbs more energy, eventually breaks down and radiates away all its constituent energy.

    So energy is like the forest. It is constantly moving on to the next generation of trees/stars as it leaves the old ones, so it is going from past events to future ones. Meanwhile, form, the individual trees, start in the future, sprout, grow old and die and are left in the past, just as stars congeal out of the energy of the galaxies, ignite and radiate away their energy.

    So does this pattern apply to particles and waves as well? Actually a more accurate term would be fields, rather then waves. Well, in the forest, anytime you want to measure something, it’s usually a tree, so it’s a bit of a catch 22 in describing the forest. To connect to the network, you cause a node. So what is the field?

    If reality is simply a matter of energy/motion in space and time is a function of this motion, as events are formed and recede into past, rather then a fundamental dimension on which physical reality proceeds from past events to future ones, then reality is a field effect that goes from past events to future ones, while the information created is the structure, like the individual trees, stars and particles, that start in the future as potential and recede as their constituent energy passes back out into the field and on to other structure.

    So the “quantum nature” of systems” is the field that proceeds into the future, while the “coherence” caused by that apparent wave function collapse is the appearance of form(the tree) that once it stops gaining energy and starts losing it, decoheres completely and recedes into the past. Energy goes past to future, while form/information goes future to past. It is impossible to have energy without information, just as it is impossible to have information without energy. These two directions of time are what defines the difference between the two.

    Better stop here and see if anything is communicated….

    Spaceman,

    Better add finding dark energy as well. There could logically be quite a lot of mass out there that isn’t gravitationally dense enough to seriously start radiating, so some dark matter is a logical possibility. 70% dark energy to explain why the redshift doesn’t correspond to BBT is a much larger problem.

  24. Fantastic thread, thanks to everyone who has contributed so far. The explicit discussion of the relationship between pressure and gravity is especially interesting.

    If any of the CV bloggers are up for it, I’d love to see a specific discussion of Tegmark’s most recent “mathiverse” paper (that was mentioned here) in its own thread. Reactions from various thinkers I respect range from “oh, he’s obviously right, thank goodness someone has finally shown the math” to an eye-rolling “how very silly and meaningless, what a waste of time.”

    In general I’m predisposed against anything that rings of Platonism, but that’s just a metaphysical or philosophical position & therefore one that can be overridden by factual evidence.

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