I don't think we will ever know the answers to fundamental philosophical questions such as why or how the universe exists.
However, dark matter is measurable physics. I'm sure if technology and knowledge keep getting better we'll be able to solve this one, but it won't help with the aforementioned fundamental philosophical questions.
Certainly one of the more interesting questions about the universe. Are there properties of the universe that are unknowable? Who knows, maybe a whole class of interactions are so rare or have such low energy we may only ever receive the slightest of hints that they exist.
As a programmer, another interesting question involves dimensionless physical constants, such as the fine structure constant. Are these numbers rational? Irrational? Uncomputable? In computing theory, an uncomputable number is one that no finite algorithm given infinite time and storage can produce. Things like Pi and e are computable. Any randomly chosen real number is almost certain to be uncomputable. We may never know if the physical constants of the universe are computable, but I think it would tell us something very fundamental about our universe.
That's why I tried to frame the question in regard to properties of the universe itself. There are a ton of things that are unknowable, such as what is beyond the cosmic horizon.
Specifically, "what is it?" is a weird question. People have been answering it in increasing detail for a long time and it still frustrates us. First it's elements, then atoms, then a stew of protons and neutrons, which happen to made of sub-atomic particles themselves, until we reach the new "fundamental". Someday, a particularly crafty scientist will be able to solve the quark parity problem or manage to crush one into bits and then we'll have a whole new layer of stuff beneath quarks. Or maybe they won't, but we'll still wonder what a quark is and why God or nature made that the smallest unit, and we'll always want to split it into two.
The why of it goes back to Feynman's explanation of magnetism to me.
At some point we should be able to reach a set of fundamental laws that may be themselves be a manifestation of an unmeasurable set of fundamental laws. The takeaway I had from quantum mechanics was that, given infinite time and resources, we will reach a point where the answer is: "we can't know" or "that's the most basic truth we can ever show"
Why time is unidirectional may be answered, but questions about why there is time in the first place or what came "before" it will forever be fundamentally beyond the grasp of our science.
That seems prematurely pessimistic to me. We are only in the science and physics business for a relatively short period of time, even if you take ancient developments into account. And if you focus on modern science and physics, it is more like a blink of an eye. Only a century ago we realized how wrong we were about the fundamental things because we only had experiences at human scales before.
Who is to say that there will not be another revolutionary change in our understanding of the universe just like relativity and quantum mechanics? Or two or five. In ten years or a hundred years or a million years?
Maybe it comes from a totally unexpected direction. Maybe we next learn how minds work and this makes us discover that something build deep into it obstructs or view of reality. It wouldn't be the first time that our natural intuition about the world lead us astray for millennia.
Even if it is a question of physics that doesn't mean it's an answerable one. There may be impenetrable barriers between us and any supposed higher-order multiverse.
Even in this universe, it's possible to be entirely cut off from regions. What does a black hole look like inside its event horizon? We have models, but it's impossible to measure. On a larger scale, assuming the continue expansion of the universe, the furthest galaxies will eventually be receding from us faster than the speed of light, making them un-observable.
I don't think the fundamental question is how these branes exist.
The fundamental question is: what's the most basic truths that we can ever know?
If there are hyperdimensional branes the likelihood is that we wont be able to learn much about their origins because of their relationship to our universe. An acceptable answer then becomes "We can't know how they exist..."
I was watching a documentary on an early christian sect (forget which one) the other day who believed that the earth/physical universe wasn't created intentionally but the result of a great cataclysm on a higher realm of existence.
Brane theory was what came to mind immediately for me.
Most people like to think that science is all about sharing information and that the popular theories are as far as mankind has come to understand the universe. Sadly this is not true. For every scientific discovery which has the potential to be used as a weapon, there will be forces trying to keep it hidden.
Instead of looking forward, it may sometimes be more productive to look backwards - especially pre-Einstein. James Clerk Maxwell and his luminiferous aether is as good as theory, if not better, than any modern explanation about what's out there. He even provided us with the math describing it (though is was removed later on).
The years after, towards the end of the nineteen century, is full of scientists claiming incredible discoveries - like anti-gravity, free energy, wireless transfer of information as well as power, apparatus for remote viewing and more. Nikola Tesla being the most famous I guess. Think how different the last hundred years would have been if any of it had reached the public domain.
On the other hand; making an atom bomb isn't really that complicated (the difficult part is surviving doing it). If every person you know had one, that would probably not work out. My point being that there are arguments for keeping information restricted. Put capital in the mix and it's bound to be some collusion.
Imagine you had a grid of antennas and that you could adjust the phase and frequency of each as to get the signals from all of them to be in-phase at a specific point. Everyone visiting this site could probably think of a handful of ways to do exactly that. Then imagine you had a lot of antennas and a lot of juice - you could more or less focus an insane amount of energy wherever you wanted on this planet without anyone being any wiser. What would you do with a power like that? Would you share?
> James Clerk Maxwell and his luminiferous aether is as good as theory, if not better, than any modern explanation about what's out there
No, it's a disproven theory that has been superseded by the far more elegant notion of Lorentz symmetry.
> The years after, towards the end of the nineteen century, is full of scientists claiming incredible discoveries
Cranks are a constant presence.
> Imagine you had a grid of antennas and that you could adjust the phase and frequency of each as to get the signals from all of them to be in-phase at a specific point.
What in the world does all that have to do with the hunt for dark matter? No one has any reason to think dark matter might be usable as a weapon. Any indication that it might would surely become general knowledge long before we could put it to use. Just like e=mc^2
My favorite pet theory (head-canon may be a more appropriate term) is that Dark Matter is a set of matter that's just as rich and interactive as ours - made up of particles that follow the analogue of our laws, with weak, strong and electromagnetic forces - but slightly different, so that that parallel universe is full of intelligent life, investigating "shadow matter" that only interacts via gravity, wondering how sad and empty and dark it must be for those particles that make up only 10% of the universe.
If it interacted with itself in ways other than just gravity, it would make clumps. The problem is, it doesn't make clumps. Its distribution is very clearly non-clumpy.
Also, if this was gravity leaking between parallel universes, I believe it would produce measurable artifacts, and therefore could be tested.
It's useful to add to this that "interacting with itself" in a way that allows clumping implies radiative dissipation of momentum or elastic collision or both. Without that it takes much longer to ditch enough momentum to fall into an orbit closer to the centre of mass around which it's moving.
That is, the non-clumpiness implies that if there is dark chemistry, the reactions must take a long time compared to the present age of the universe.
Can we say it doesn't clump or can we only say it doesn't clump on the galactic scale? Could it clump but then go into it's equivalent of a super nova with very small clumps?
There is a limit to the changes in the momentum of cold dark matter before it produces visible effects in galaxies of different ages. Explosive processes in the dark matter sector must be rare, whatever their mechanism and whatever their daughter products (even if it quickly decays to cold dark matter again, but on a different trajectory).
Interesting, but it'd need to interact in ways we don't understand. If it interacted and formed life like we know it, it would need to clump, which it doesn't. For a dark matter candidate to behave correctly, particles need to fly right past each other, heedless of one another except for gravitational effects. Never even collecting into clouds as interstellar gas does.
Who knows though, the interactions between them may not alter their trajectory, but may be so complex that they can support self replicating patterns.
Yeah - perhaps they don't need to clump together as much to develop intelligent life. We need stars and planets; they could work on much smaller scales.
If I were writing a science fiction novel, I'd love to talk to enough scientists to posit a plausible mechanism.
You might enjoy Stephen Baxter's "Xeelee" books. I started with https://en.wikipedia.org/wiki/Vacuum_Diagrams and enjoyed it, but could see its various stories coming across as too jumbled up and disconnected for some.
He paints a grand vision of almost unparalleled scale and scope. I really liked his Xeelee cycle, even though the writing is quite arid. But holy cow, this is a writer who thinks BIG.
Unfortunately he takes a few liberties with physics. Dark matter could never produce and sustain life. It's just too poor in terms of interactions. You need richly interacting matter for that, producing the most richly interacting atom of all - carbon - in order for life to appear and evolve.
Asimov's The Gods Themselves runs along those lines:
The main plot-line is a project by aliens who inhabit a parallel universe (the para-Universe) with different physical laws from this one. By exchanging matter with Earth, they seek to exploit these differences in physical laws. The exchange of matter provides an alternative source of energy in their dying universe.
I've heard ideas thrown around explaining both the relative weakness of gravity (compared to other forces) and parallel universes. Gravity would be strong, but prone to "leaking" along a hidden dimension, and the gravitational sum of those invisible worlds would explain the blobby, non-interacting nature of dark matter.
I don't know of any scientists actually giving this consideration, but it does make for a fun head-canon.
I'm not thinking "parallel worlds" in the Sliders sense, just in the "dark matter" sense. If there were "enough" to blur it out, each one would likely not have enough mass to really have a complete universe.
On the other hand, if we only make up 15% of the known mass of the universe, that's room enough for 5 more parallel shadow universes living beside us!
Or maybe dark matter just doesn't need as much mass for intelligent life to develop. Who says they need stars and planets?
Parallel as in further "up" and "down" along some unperceived Nth-dimensional axis sorta thing, as I'm imagining it. We'd be some percentage of dark matter to every other "known universe", ourselves, in a cosmological principle sense, not particularly unique or special.
If the gravitational force also falls off along such an axis, assumptions about what percentage each parallel universe consists of becomes more complicated. The nearer ones could be more massive than otherwise expected. Each successive parallel universe would also contribute a smaller amount to the whole.
Yes, up to now there's nothing directly detected, but we should also not forget that just two years ago a pessimistic article could have been written about the detection of the gravitational waves, this is how it could have looked like: "predicted 100 years ago, never detected, even if there was some investment in the detectors and specific scientific work for 40 years, and in spite of some indirect evidence." It is also easy imaginable somebody claiming that LIGO shouldn't be funded only to "measure the change of a 4 km length in a tiny fraction of the size of a single proton."
But then... hundred years after the prediction, the magnificent thing did happen:
So, it seems like it's still an open possibility, and even better, it might be resolvable one way or the other in the near future -- at least for the primordial black hole hypothesis.
Having just finished reading Lisa Randall's "Dark Matter and the Dinosaurs," I was a bit disappointed to not see any mention of her new "dark disk" hypothesis. Maybe her research is a bit more "fringe," but it seems like all of these experiments are pretty far out there.
The "dark disk" hypothesis basically says that there are actually multiple types of dark matter particles, and there's a disk made of heavier or more strongly interacting dark matter particles in a disk shape overlapping with the milky way disk.
Another promising theory is that dark matter isn't even necessary, we only assume its present due to its gravitation effect, and that can be explained by other phenomenon.
Gravity decreases in proportion to distance squared between objects, but its not observed beyond a certain distance. To compensate, that space was assumed to be filled with dark matter. This theory suggests that gravity switches from a inverse-square law to a different set of rules after a certain distance.
> Gravity decreases in proportion to distance squared between objects, but its not observed beyond a certain distance.
This isn't true. F = GMmr^-2 has infinite range. This is true from a theoretical standpoint, and it's also borne out in observations. The Andromeda Galaxy, presently 2.5 million light-years away, is being pulled toward our galaxy and will eventually collide with it. Observations on an even larger scale support the idea that the equation has infinite range.
The above theoretical construct could in principle be contradicted by observations, and if it were, someone would win a Nobel Prize, so there's every incentive to find persuasive evidence that contradicts it.
The Andromeda Galaxy, presently 2.5 million light-years away, is being pulled toward our galaxy and will eventually collide with it.
To support an inverse square law, we'd need to know the mass of our galaxy, and the mass of the Andromeda galaxy. We have the speed, computed from various blueshift methods. We'd also need to know the rate at which the speed changes.
Do we know any of those three? The mass of the Andromeda galaxy is very different if you assume its filled with dark matter, vs. only has visible matter. Which mass is used to confirm the inverse square law? If it includes dark matter, you have circular reasoning: dark matter is hypothesized to make the inverse square law work within the Andromeda galaxy. If it doesn't include dark matter, then why does the dark matter in the Andromeda galaxy contribute to its rotational motion, but not the attraction to the Milky Way?
Ah. I took the comment to say the inverse square isn't observed beyond a certain distance, not that gravity wasn't observed at all beyond a certain distance.
sort of OT of the post, but on topic of all the cool discussions about physics we've had here, I want to share one my favorite channels on youtube[1]. Eugene Khutoryansky has put together an amazing collection of very intuitive explanations of all kinds of physics, including cosmology. I also recommend his series on math and electromagnetics.
Dark matter as particles, rather than some undiscovered gravitational effect at galactic scale, seems like a difficult case to make. If it is particles, the distribution of dark matter in a galaxy has to be highly dependent on the distribution of ordinary matter, or vica versa, and yet there is no interaction. I'd bet on gravity waves and/or quantum mechanical gravity effects leading to an explanation, rather than the discovery of a new class of matter.
Dark matter as particles seems like a natural case to me:
1. We know that some particles don't see some forces, e.g. leptons don't experience the strong nuclear force. So it's natural to expect particles that don't see the electromagnetic force.
2. Dark matter and ordinary matter do interact through gravity, which is the force that governs cosmic structures. So the distribution of dark and normal matter will be naturally related.
> If it is particles, the distribution of dark matter in a galaxy has to be highly dependent on the distribution of ordinary matter, or vica versa, and yet there is no interaction.
Incorrect. The distributions do appear to be interdependent, in exactly the way you'd expect if the two kinds of "matter" interacted only gravitationally.
Yes, that's true. But are there arrangements of dark matter that are not holding visible matter galaxies together? Does dark matter appear anywhere else?
Yes, that's how we'd infer the presence of an overdensity of dark matter in the absence of visible matter : a background being lensed by an invisible foreground.
There are searches for this which are possible because Einstein rings are well understood, and those appear in particular alignments such that it would be hard to miss the foreground source if it were at all visible. Other lensing structures are subject to tracing error; that is, you lose the ability to distinguish between effects from alignment and effects from the distribution of foreground mass and the distance to the background visible objects.
One major problem is that dark matter overdensities need not be in exact alignments, or produce readily detectable Einstein rings. Moreover, the overdensities must not be that "over" or ordinary matter would have been drawn in, and we could see that. (Which would be neat as it would offer evidence on how early galaxies formed in the first place, and narrow out of contention lots of modified gravity proposals.)
There is a debate over the large scale structure of dark matter, with lots of ideas provoked by the X-ray clusters Abell 222 & 223 among others. There may be a "web-like" structure to dark matter where the filaments can be very large compared to the scales of individual galaxies but still quite narrow compared to the less dense spaces between them. These structures appear pretty commonly in simulations grounded in the standard cosmology, although it's not wholly clear why (they may be an artifact of numerical methods, for example).
There are ground- and space-based observatories in the pipeline that will over the next few years narrow the possibilities for various kinds of large scale structure of extra-galactic/extra-galaxy-cluster dark matter.
Of course we will or rather we will understand what causes the phenomenon which we attribute to dark matter.
I don't really see the failures mentioned in the article as failures. We had an idea about how nature works, tested it and showed that we were wrong. It's just the scientific method at work.
1. Dark matter is the "gravity echo" from the past. Gravity is the structural bending of spacetime by mass. The effect of the bending is supposed to travel at the speed of light. What if the effect has been delayed and is added to the present effect?
2. When a mass is moving through spacetime, the bending of spacetime is asymmetric in the leading and trailing directions, where the leading direction forms a leading crust and trailing forms a flatter tail, just like the blue-shift and red-shift effects on light. The leading gravity wave crust adds to the strength of the gravity.
Personally I think that "dark matter" is not matter at all, but ripples that form valleys and hills in the space-time membrane like a plastic bag that has been stretched (by the big bang) and crumpled slightly back.
But I am not a physicist so it is purely speculation from my part and I have no math to back it up. It's just the way I picture it in my head :)
I think it comes off as a little crazy to hypothesize about these things if your level of experience is basically reading wikipedia (as mine is).
I think you can gain a lot of understanding that way (plus enjoyment), but I don't think that kind of surface level understanding can be easily extended into new theories. In order to generate new theories, I would imagine that you need a background in the math behind all this stuff.
Anyway, as to your last point: my understanding is that entanglement in no way implies "close to instant communication." The speed of light is the still the upper limit on transferring information [0].
Your theory seems to be based on pop-science understanding of concepts like entanglement. A physicist listening to you would just smile and nod their head. There is no need for particles to pass through a single point for entanglement to be a thing. Both the standard model and quantum theory tell us that such an event isn't possible. And as far as I understand, you don't have matter until 10^-12s, well after cosmic inflation.
Quantum entanglement is not a force, does not cause particles to do things, and cannot be used to build a quantum internet. It merely refers to the fact that although quantum states cannot be known till measured, even distantly separated particles will satisfy quantum relations (opposite spin for two particles, etc). They satisfy them in such a way that they either somehow communicated, knew all along, or other possible strange options discussed in multiverse theory. Bell's inequalities prove that they did not know all along.
Dark matter can't be ordinary matter in a second universe because it doesn't interact with itself. It'd have to be dark matter in another universe. Which would be indistinguishable from dark matter in this universe.
Nothing in physics seeks to do anything. The idea that matter would get together and plan to form black holes so that it can transition into some other state is just silly.
You are trying to solve problems that don't exist, with physics that don't exist. Please, read up on:
I've taken the criticism received and edited my comment, but after reading your comment again I think you are being overly critical.
"There is no need for particles to pass through a single point for entanglement to be a thing."
I never said there was, merely proposed it as a possibility.
"Quantum entanglement is not a force, does not cause particles to do things, and cannot be used to build a quantum internet."
Not with classical bits, but theoretically building a quantum internet is possible if we ever figure out how to transmit data via qubits. (as per articles linked)
"Dark matter can't be ordinary matter in a second universe because it doesn't interact with itself."
I don't follow your reasoning here, is there a law that more precisely backs up this statement?
"You are trying to solve problems that don't exist"
On the contrary, I am just using my imagination to attempt to grasp a possible version of grand unified theory, which as far as I know is still something we are trying to work on.
"with physics that don't exist."
How do you know that if you don't know the answer to what dark matter is in the first place? Science isn't just about what we already know, it's about creating falsifiable and testable hypothesis outside the current knowledge-set to expand the currently accepted knowledge set. To criticise hypothesis on content and methodology is valid, but to criticise it simply because it doesn't conform to the current knowledge set is short sighted imho.
> I am just using my imagination to attempt to grasp a possible version of grand unified theory, which as far as I know is still something we are trying to work on.
You're attempting to write a super-duper machine learning giant app without having bothered previously to even learn how to code. Start by learning how to turn on the computer first, and go from there.
It's easy to kick back and dream about grand theories. It's also easy to forget the steep road of learning that would allow you to even approach that realm.
You can transport a qubit via quantum entanglement, yes. That's totally a thing. This might be useful for things like quantum encryption. If that's all you are saying, great. You seemed to be implying that it can replace the current internet and we no longer need fiber optics, wires, or other forms of classical communication.
First, to transport a qubit, you need to send an entangled particle first. This is usually done with fiber optics. You could also send a bunch of particles in traps on a truck, or over the, air, whatever, but you still need to exchange particles for quantum teleportation. Once you've exchanged entangled particles, you can use them to transmit the quantum state of another particle. This might be useful for quantum encryption, but you could just use the originally exchanged particles instead.
Second, you've already stated this, you cannot transmit classical bits. You'd still need all the original Internet infrastructure to transmit classical information. Things like, the British are coming, a funny cat gif, any message really. In fact, entangled states are actually useless without transmitting classical information about the partner particle. Without that, it's impossible to even detect that the particle is part of an entangled pair, much less use the information for anything. The quantum eraser experiment is a great example of this. Without the information about the other entangled particle, all you get is a screen showing that the particle always went through a single slit. Once you know the which way information about each particle, you can separate them out into two interference patterns that cancel out.
Now, there may be something that connects particles quantumly that can be used for communication. Such a thing is not currently part of quantum mechanics and isn't quantum entanglement, such an effect would need some new name.
Regular matter clumps together to form gas clouds, stars, black holes, planets, etc. In general, matter collapses. This happens because particle interactions and collisions generate heat that gets radiated away. Without a mechanism to lose energy, gravitational collapse isn't possible. When we look at galaxies, the stars, gas clouds, etc, tend to fall inward as they lose energy and more matter gets towards the middle than the outside. In spiral galaxies, you see a huge bulge of stars towards the middle. When we look at how fast they rotate, we measure that matter is distributed more or less evenly across the entire plane of the disk. This measurement is really what kicked off our current understanding of dark matter followed by many other discoveries regarding matter distribution.
If dark matter formed stars, planets, gas clouds, etc, like normal matter, it would clump towards the center of galaxies, just like normal matter.
Anyone can theorize whatever they want about the universe, and should not be discourage from doing so. But if your theory makes claims based on current understanding and observations, but the knowledge of that current understanding and observation is incorrect, it seems logical to help that person along. If you want to base your new theories on what is currently understood, it will benefit you greatly to learn as much about current understanding as possible.
There is no evidence that dark matter even exists other than the fact that our universe is not expanding at the rate we think it should, based on our current theory of gravity and how much mass we can account for based on the radiation currently reaching us (light). I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence just to counter-balance our 1) wrong equations and/or 2) wrong observations. So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture.
> There is no evidence that dark matter even exists other than...
Most of our theories about the universe are based on evidence from secondary, tertiary, etc effects. It's often the best we can do.
I don't think you'd find anyone who would object to the idea that "our theory is incomplete" ("all models are wrong", etc), so it seems like your main objection seems to be that we've given a name to a family of theories that attempt to explain the phenomenon we've grouped under "dark matter".
> I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence just to counter-balance our 1) wrong equations and/or 2) wrong observations
It's rather human to make an estimate of likelihood based on how long ago something was conjectured to exist :) Moreover "wrong equations" and/or "wrong observations" are part of the very theories attempting to explain our current state of knowledge.
> So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture.
Actually, usually the issue with these discussions are with scientifically literate folks who read a Scientific American article on dark matter in 1998 and have engaged with that as a strawman ever since. Somewhat tongue-in-cheek, but it is odd to me how many people are convinced they're bringing light to the darkness with these kinds of comments.
> There is no evidence that dark matter even exists other than the fact that our universe is not expanding at the rate we think it should ...
You're thinking of dark energy, the net effect of which is to change the overall expansion profile of the universe as a whole. Dark Matter was first proposed when galactic rotation profiles failed to meet theoretical expectations -- matter far from the center of each galaxy had a higher velocity than it would if visible matter had been the only factor.
> I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence
Occam's razor (the simplest explanation tends to be the right one) suggests that a new unobserved particle is more likely than abandonment of F = GMmr^-2 . This doesn't mean the equation must be correct, it's a question of reaching for the low-hanging fruit.
> So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture.
First, nothing is ever proven true in science, only false. Second, dark matter is more than pure conjecture, since there is observational evidence. A pure conjecture would be an idea about reality having no observational support at all (unicorns, Bigfoot). Dark matter is a hypothesis crafted to explain observations, but so far there's no persuasive theory to explain it, and no observations of candidate particles either.
I should have been clear that I'm not making a distinction between energy and matter. I'm referring to the postulated "dark" forms of both. Both of them are pure speculation, and is nothing but the equivalent of taking whatever observable "error" there is in our formulas and labeling it "dark". The only reason there's two kinds of "dark" is because both the space aspect (matter) of our math is wrong AND the time aspect (energy) is wrong. GR and SR are correct but incomplete, for representing spacetime. Just like Newtonian rules are correct but incomplete.
And as for your last sentence, trust me I understand the scientific method, and how proof, evidence, and fact interrelate with knowledge. Nice philosophical observations, but having nothing to do with this discussion.
> The only reason there's two kinds of "dark" is because both the space aspect (matter) of our math is wrong AND the time aspect (energy) is wrong.
You're pairing space and matter, then time and energy, and comparing them as though these entities are naturally paired in current theory. They aren't. Space and time are elements of spacetime, matter and energy are interchangeable by way of a rather well-known equation, but these things don't arrange themselves as you're trying to do.
> And as for your last sentence, trust me I understand the scientific method, and how proof, evidence, and fact interrelate with knowledge.
So you didn't say, "So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture."
Nothing is ever proven in science (falsified, yes, proven, no). And dark matter and dark energy are both more than "pure conjecture," a domain reserved to notions lacking observational evidence.
I also use the words "space" and "time" independently as if they were separate things, when I know full well every last detail about SR/GR (being an engineer myself). So yes, only in the context of discussing "dark stuff" I will lazily interchange matter and energy. Only on HackerNews do I ever encounter the type of ass-holes who will parse sentences intentionally wrong as to yield the incorrect conclusion. You are one such person.
> Only on HackerNews do I ever encounter the type of ass-holes who will parse sentences intentionally wrong as to yield the incorrect conclusion. You are one such person.
Troll alert. Space and time are elements of spacetime, they are an integrated whole, a fact first pointed out by Einstein's math teacher Minkowski. When Einstein first read what Minkowski had written, he said, “Since the mathematicians have invaded the theory of relativity, I do not understand it myself anymore.”
Only later, after learning tensor calculus and beginning work on GR, did Einstein understand what Minkowski was going on about. But in those days people were interested only in getting it right, not posturing as right even when they're wrong.
It's not name-calling. This person really is a troll, a term with an unambiguous definition: http://www.urbandictionary.com/define.php?term=troll . He tries so hard to earn the label that it seems unjust to withhold it.
In the days of Usenet, before there was an Internet, this sort of language was regarded as neutral and informative -- it's not abusive when it's accurate. But the Politically Correct movement in social media is seeing a revival, such that even accurate use of these terms is regarded as counterproductive.
In the Usenet era, some individuals would strive to earn the label, and applying it would save people a lot of time trying to engage in constructive conversations with people who were manifestly unable to rise to the occasion.
Bottom line -- it's possible to take PC to a pointless extreme. And I'm hardly the first to make this point.
Fair enough, but this begs the question of what constitutes incivility. If present trends continue, telling someone that they're wrong will be regarded as uncivil behavior. To avoid censure it will only be possible to assert that they've posted "alternative facts."
I happen to agree that incivility represents a real problem in social media, and we've seen many sites abandon their discussion groups because of uncivil posts and people. But I think the argument can be made that definitions have changed as well as behavior.
> That goes back to Usenet as well.
Not really. Having posted there for many years, I can tell you from direct experience that the perceived threshold of incivility has changed completely. One need only review posts from that era to see the point that definitions and standards have changed.
But this is now, and an argument about what was once acceptable doesn't seem particularly persuasive even to me, especially now that we have an embodiment of incivility running the country.
Just to keep score: You called me a troll, because I called someone an a-hole, for calling me a liar. That's what just happened. What a totally PC Snowflake Soap-Opera. Of course that's based on the assumption that the 'Sock Puppet' burner accounts aren't BOTH YOURS to begin with. hahahahaha. omfg that would be hilarious. In case it doesn't even matter, i'm using my REAL identity.
Nice dissertation on 2+2=4. I've known relativity for 30years. The point i was making (as i'm sure you genuinely DO actually know, despite pretending once again to need to correct me), is that even physics professors in the middle of physics lectures will say "space" or "time", depending on context. Learn the fact that English and all languages have syntactical nuances. Oh,and thanks for the warning that you are a Troll, but I don't mind. I'm biting the hook. Nothing thrills me more than debating physics. Thus the 30yrs.
the same thing could have been said about the discovery of Neptune [1]: "there's no evidence a planet is there, it's pure speculation. The changes in Uranus' orbit must be the result of mathematical error and not another planet."
If man had discovered Gravitational Lensing BEFORE Einstein had formulated General Relativity we would have given some name to the effect, and perhaps even considered it a characteristic relationship between stars and light. The relationship happens to be an INDIRECT one. Star mass bends space, and the light merely "appears" to bend, when it traverses that space. It doesn't actually bend.
What I'm saying about Dark energy/matter, is that there is also an INDIRECT relationship there. It's not just a special invisible mass and invisible energy. It's a fundamental misunderstanding about what spacetime is. I think the "dark" quantities are every bit as much an illusion as the "light bending" illusion created by stars.
The light is traveling in a straight line ALWAYS, but merely appears (to us) to bend, because the space it's traveling thru (in a straight line) is itself bent. The space itself is bent. The light goes straight.
That's the distinction i was attempting to point out, when I said if mankind had visually noticed star-induced 'lensing' (before Einstein explaining what to expect) we would have ASSUMED the light itself was bending, and that space was 'flat' (unbent). Thinking space is flat and light is bent (the opposite of what is true), would have been the same kind of blunder we are making today believing that Dark matter/energy is actually real.
> The light is traveling in a straight line ALWAYS, but merely appears (to us) to bend, because the space it's traveling thru (in a straight line) is itself bent. The space itself is bent. The light goes straight.
Space is curved, and the light passing through it is also curved. That was the point of my link to the Einstein Ring page -- to show that light is in fact curved along with space.
The first important confirmation of GR was an experiment conducted in 1919 that showed curved light paths of starlight passing near the sun, observed during an eclipse.
If you happened to be located near a black hole, at 1.5 times the radius of the event horizon, by looking along a tangential path, you would see the back of your own head, regardless of which direction you looked. The reason? Light is curved along with space.
It's not accurate to say, as you are doing, that light always follows straight paths. It is accurate to say that light follows the curvature of the space through which it passes.
> ... would have been the same kind of blunder we are making today believing that Dark matter/energy is actually real.
Try to avoid moving ahead of the evidence. The present evidence is that dark matter and energy are real, again following Occam's razor. But I can't say these things are real as a matter of concluded fact, and you can't say they aren't. No one knows, and science requires us to wait for observation and theory to sort it out. Science doesn't progress by proclamation, but by way of theories that resist sincere efforts at falsification.
I do understand why you think what you do. Most physics articles (and even the Wikipedia page on Gravitational Lensing) are explaining it wrong. They are saying that the light bent. That's not what's really happening. The light doesn't bend relative to the space it's flowing thru. It flows in a perfectly straight line. Gravity has no direct effect whatsoever on light, because light is massless. Gravity only can effect the SHAPE of the spacetime density field. The fact that the spacetime is bent makes it look to us, as if the light changed direction. It didn't. Light passing thru an ordinary optical glass lense DOES bend, but light passing by a massive object in space absolutely does not bend. It merely "looks" like it did.
> Most physics articles (and even the Wikipedia page on Gravitational Lensing) are explaining it wrong.
Ah, the encyclopedias are wrong. That may be true, but only if you meet your burden of evidence. You cannot meet your burden of evidence, and you show no sign of even trying.
Oh, I can prove it with evidence. The only thing that can change the direction of motion of a particle is a force. Photons have zero mass and zero charge, therefore no force (including gravity) can act on light (strong and weak interactions are not applicable here). Therefore light will always travel in a straight line. When light appears to 'bend' it is not because the light itself changed direction in its reference frame, but because the space the light is traveling thru in is warped. Any physics professor will understand precisely what i'm saying, and all agree. Someone who has only read a few articles online will not. I've understood this since 1986. I'm very old and wise you little child.
> The only thing that can change the direction of motion of a particle is a force.
Quite false. You're overlooking the fact that photons are the carrier particle of the electromagnetic field, which takes the form of waves in space -- waves that change direction without the application of forces. An optical lens changes the direction of photons without exerting a force. So does curved spacetime. These are examples of hundreds of things about physics that contradict your outlook.
Again, at 1.5 times the radius of a black hole, looking tangentially, you would see the back of your own head. So even in this local frame of reference, light has taken a curved path along with curved spacetime. In other frames of reference, light is obviously not traveling in straight lines. In fact, it can be argued that light never travels in straight lines -- that would be true only in a universe without any mass at all.
You could argue that water always travels along straight lines inside a pipe and never changes direction, and in the case of the pipe itself changing direction, you could argue that the water is always traveling in a straight line from its own perspective inside the curved pipe, but having said that, people would see your ideas for what they are.
> Any physics professor will understand precisely what i'm saying, and all agree.
You appear to have forgotten I have already disproven this with my John Wheeler quote: "Mass tells space-time how to curve, and space-time tells mass how to move." As with masses, so with photons. If masses could travel at c, they would take the exact path photons do -- curved ones.
We've banned that account—for obvious reasons, given how it behaved in this thread.
Unfortunately, several of your comments were also uncivil. Please err on the side of civility when posting here.
Also, please don't engage in flamewars on HN. A good-faith discussion about how someone is wrong is fine, but at the point when good faith dries up, such discussions become tedious tit-for-tats, which amounts to mutual trolling. We definitely don't want those kinds of threads here.
What about the evidence from the bullet cluster? The gravitational center of mass is offset from the the visible center of mass. So something must be gravitating that is not visible - i.e. dark matter.
Matter isn't the only thing that produces gravity. Energy also produces gravity. Given that the bullet cluster is also incredibly hot, it seems entirely plausible that its unusual gravitational morphology is somehow related to its energy content.
You'd expect it to be incredibly hot where the gas is, no? And the interesting thing about the bullet cluster is that the excess of matter is found where there is no gas and stars. Plus, if the gas were super hot, it would emit lots of light, including x-rays.
My understanding was that it is in fact incredibly hot where the gas is, and it is emitting lots of x-rays. And you're jumping ahead of yourself to say that it is matter, we just know that there is gravitational lensing.
My main issue is that the bullet is anomalous in multiple ways, and dark matter doesn't explain the heat issue. If it did, then it wouldn't be useful as an explanation for the rotation curve problem in other galaxies. I feel that a more parsimonious explanation would cover both the lensing and the heat.
You're right. Collisional matter that dissipates radiatively completely explains the heat issue: many of the individual components of the dust and gas in the galaxy clusters came close enough to each other to interact electromagnetically, trading off their momentum-energy for momentum-energy in photons and other products of scatterings. Ordinary intergalactic gas and dust from the two clusters got squashed together and the squashing made it hot enough to glow.
In the standard cosmology (\Lambda-CDM), CDM is cold dark matter that is collisionless, does not dissipate radiatively and does not feel electromagnetism even in very close quarters. So it does not heat up, and its momentum is only influenced the gravitation sourced by the components of the colliding galaxy clusters (which includes the dark matter itself), with the result that the "clouds" of dark matter in each cluster sail right through one another and past all the ordinary matter, with effectively no deflections of their trajectories.
The result is that gravity (measured by lensing of background objects) points to a hot cloud of dust left in the middle of the collision, the stars and other denser visible ahead of that debris, and two mostly-transparent regions leading the way ahead of the visible matter.
> the bullet is anomalous in multiple ways
It's a bit more accurate to say that there are questions still unanswerable by observation of the bullet cluster alone, and that searches for other cluster collisions are likely to provide further partial answers.
In particular, further observations will favour or disfavour different numerical simulations of large scale structure formation under the standard cosmology. However the variables most directly tested align roughly with whether cold dark matter is almost wholly particles similar to heavy sterile neutrinos or almost wholly particles like axions, rather than whether particle cold dark matter is there at all. Even TeVeS proponents aren't especially optimistic on that latter point (e.g. Angus and Diaferio, two prolific TeVeS-as-relativistic-MOND researchers, in their 2012 paper https://arxiv.org/pdf/1206.6231.pdf starting at the bottom of page 23).
Ah, ok, I was going to say, if the excess energy in heat was sufficient to explain the gravitational lensing, you'd have several problems. You're just pointing out that there are other significant anonymous things going on we don't understand. Until they are also explained, the solution is not a complete one.
You've confused the logic for inferring dark energy or some other force at work, with dark matter which is involved in lensing experiments, galaxy rotation curves, etc.
No one knows why space is expanding. The dark energy is just the 'label' they give to whatever is causing it to expand. Both dark matter and dark energy are postulated to be whatever shape of puzzle piece fits into the puzzle slot where the error in our formulas exists. What i'm saying is that it's more likely that the formula itself is wrong than it is that something magically exists to conveniently fit that slot.
> The dark energy is just the 'label' they give to whatever is causing it to expand
This depends on how you look at it.
In the standard cosmology we define a preferred frame wherein an observer will see the matter (that's in the most general sense of "not the gravitational field", so it includes atoms and their components, photons, and various types of dark matter (e.g. neutrinos, which are "hot" dark matter, since they move relativistically and do not experience electromagnetism)) content of the universe as homogeneous and isotropic. This is physically reasonable since along every unobscured line of sight we see a lot of galaxies of various shapes, "tilts", sizes, surface brightnesses, and spectral lines. Observations also lead us to conclude that there is a relationship between redshifting of the spectral lines of common types of galaxies (and common radiative occurrences within them, like type A supernovas), and the change of the other observables (angular size on the sky, luminosity, etc.) that correlate with greater distance. This in turn led to the discovery of the Hubble "constant", and provoked ever deeper field telescopic studies to prove its value.
So if we assume that along every line of sight, including obscured ones, we have much the same view of many many galaxies at a variety of distances, we can make a variation on the Friedmann equation that parameterize several things that would lead to the observables of galaxies when we model their known (and unknown) components as a set of perfect fluids.
We can take the Hubble "constant" and put it into a Robertson-Walker vacuum spacetime. RW spacetimes can be grokked by dimensional reduction. Consider a cylinder that we slice (foliate) along its axis into a set of infinitesimally thin circles stacked on top of each other. We describe the radius of each circle with a function r(h) where h is the height of the circle from the base of the cylinder. Where r(h) is constant, we have a cylinder, but if r(h) increases with h, then we have something like a cone balancing on its apex; r(h) can describe a wide variety of shapes. For an 3+1 RW spacetime that is similar to our universe, we foliate on the timelike axis and define a function a(t) where t_0 == now with the spacelike coordinates set on a chosen observer (us here on Earth, for example). "t" counts upwards as we go into the past from t_0, and a(t) goes to zero as t increases. "t" is the lookback time and a(t) is the scale factor. The chosen observer is the special observer mentioned above, who sees the matter of the universe as isotropic and homogeneous at the largest scales. The most useful coordinates on such a spacetime are comoving, that is each gravitationally bound galaxy cluster stays at the same coordinates at every time t.
If we mix together the Friedmann equations, the Lemaître idea of spacetime having a zero radius at some large lookback time, and the RW spacetime that can model that, we get the FLRW model of the standard cosmology. We take the RW case where there is no extrinsic curvature, that is, when we foliate on the timelike axis each spacelike hypersurface is spatially flat; that is similar to saying that when we slice up our dimensionally reduced solid along its height, we get a set of circles of the same radius (i.e. a cylinder rather than a cone). We absorb the expansion parameter into a(t) as another of the fluids.
We then consider two types of fluid: those that dilute away as t -> t_0 -> future and those that do not dilute away. The former is "matter", including dark matter; the latter is "dark energy".
When we consider them as components of an action, diluting-away fluids are attractive and non-diluting fluids are repuslive. When we consider them in terms of the matter tensor T in General Relativity, the diluting-away fluids have positive pressure and the non-diluting fluids have negative pressure.
It's important to return to the point that this model has a preferred frame, and that translating the non-diluting fluid into "the same for all observers in all frames of reference" physics leads one to assume that dark energy is just a feature of the Lorentz-invariant vacuum. So dark energy arises in the cosmological model but corresponds to the ground state of the empty-of-matter spacetime in frames of reference other than the preferred one picked out by the cosmological model.
That is, the statement that "dark energy drives the expansion (via negative pressure or repulsion)" is frame-dependent, and thus observer-dependent, and with a change of frames of reference (and even a change of coordinates on the preferred frame), the statement becomes untrue. What is true in all frames is that there is an intrinsic property of space in an expanding spacetime that has a constant energy-density no matter how large a volume of space is considered.
The exact equation of state of dark energy is an area of active research and also tests to make sure that the assumptions that inevitably lead to it (isotropy at huge scales, homogeneity, spatial flatness, redshift-distance relations and other things implying expansion) are not blown up by evidence from ever finer observations.
So it's not so much a 'label' as a phenomenon whose microscopic details have yet to be discovered.
There are lots of those in physics, and we've had a good century of probing the microscopic details of phenomena discovered at the end of the 19th century and since, so this shouldn't really be causing anyone sleepless nights.
in which Schrödinger (in 1918!) describes, "matter in the large essentially as a compressible fluid of constant density at rest under a constant spatially isotropic tension which ... must be equal to 1/3 of the rest density of energy".
The notation is old-fashioned, but check out Schrödinger's paragraph at the bottom of Harvey's page 5 !
Your statement that there's "an expanding space-time that has a constant energy-density", gets to the nub of it for sure, but in reality no one knows if any of the fundamental field strengths are indeed constant. To me the need for "dark" stuff to be postulated, is more likely to indicate that gravity strength, or light speed, are more likely to NOT be constant over large space-time ranges, than the likelihood that there is a whole class of 'dark' particles and waves that currently don't interfere with any other matter in a provable way.
So if someone asked me what's my 'evidence' that gravity or light-speed are not universally constant, my answer is simply: "The evidence for dark-energy/matter, is that exact same evidence". There should be some wave-function (possibly resulting from what we call a big bang), in terms of G and C, that once integrated out over the current life of the universe, will yield precisely that positions and velocities of the galaxies that we currently observe. To me it seems far less likely that there's an entirely new set of particles we cannot see, despite the Standard Model of particles being proven correct out to 40 decimal places.
You already have the neutrino which interacts only via weak force and gravity, They show up much the same in bubble chambers as dark matter does in our telescopes; they don't. We only see where they interact with more observable matter, is it really that far fetched to imagine a particle that interacts through gravity only?
I'm not sure what you're trying to argue here, or even why. I'll guess that you're interested in physical cosmology and would like a set of brief and not-too-technical reactions to your ideas (which I take to be implied questions) from someone who knows the \Lambda-CDM cosmology reasonably well.
Dark energy it is a property of empty space that does not dilute away as more space appears in an expanding universe. The only way to abolish dark energy is to eliminate the expansion of the universe. It only has a "fundamental field strength" in particular chosen frames of reference in which one can represent it as a field with a constant energy-density. The comoving frame of the standard cosmology is one such frame of reference. However, in most other frames this energy density vanishes, and when that happens a general relativist will decide that the energy density was an artifact of the choice of frame of reference or system of coordinates and ditch the word "fundamental".
Now, there are lots of ways we can complicate the action S_{\Lambda-CDM} in a Lagrangian formulation of the standard cosmology by introducing further repulsive terms into it beyond constant * \Lambda (as in the Einstein-Hilbert action or an expansion of it) or L_{repulsivematter} in a parameterization, and indeed there have been numerous attempts to do so. However, short of eliminating the expansion of space at all times and in all reference frames there is no way to get rid of a repulsive, non-diluting, non-concentrating (when time-reversed or if the critical density of the universe turns out to lead to a contraction of space in the future) term.
But in the standard cosmology, we have \Lambda, which is just the cosmological constant, i.e., dark energy is a property of the ground state, which is the vacuum. When there's more vacuum, there's more dark energy. Mathematically, we start with an action that leads to the standard write-down of the EFEs. Physically, this matches observational tests at large scales, which is convenient because the standard write-down of the EFEs is almost the only way to match observational tests within our solar system (c.f. the parameterized post-Newtonian formalism).
> gravity or light speed are not universally constant
What exactly do you mean by "gravity ... not universally constant"? In particular, what do you mean by gravity?
In General Relativity speeds are something that are extremely hard to talk about except in the local neighbourhood around a single point. In fact, many general relativists would argue that talking about "universal" speeds violates the spirit of general relativity. However, if our universe continues to be modellable as a smooth manifold with a Lorentz metric, we get a sort of quasi-universality of "c" in that at every point one can construct an infinitesimal region of spacetime in which "c" is a parameter in the action of matter and takes on the same _locally measured_ value in each such region for an observer in that region. There is ample evidence that favours this up to energy scales accessible to us on Earth and visible with observational platforms on and near our planet.
There are certainly metric theories other than GR that allow for different sources to couple to different metrics, but most of these have to undergo a phase change to an effective single metric with universal coupling in the early universe or we would see clear evidence for them (in particular, the distribution of heat in the early dense phase of the universe still has to produce the Standard Model at lab energies and also stars and galaxies and labs). Some productive and well-regarded physical cosmologists have proposed these types of theories, even recently (e.g. Afshordi & Magueijo), and they explictly reject a universal value of "c" (in particular c approaches infinity in their model's extremely early universe).
But in the standard cosmology we have General Relativity, and we don't vary the value of G or of c when grinding through the Einstein Field Equation; we take those values as given to us by nature, and have no evidence for them varying in the observable universe (and a pretty substantial amount of evidence against such variations, in particular including petabytes of spectroscopic data from objects in the sky).
> "There should be some wave-function (possibly resulting from what we call a big bang), in terms of G and C, that once integrated out...
I'm sorry, I don't understand this. Could you explain further?
> To me it seems far less likely that there's an entirely new set of particles we cannot see, despite the Standard Model of particles being proven correct out to 40 decimal places.
Dark energy is not particles. As I said above, it's a feature of the vacuum, and the most fundamental feature of the vacuum is that it is empty of particles. A tl;dr here is that we don't know what creates more space rather than less space, but we know that when more space is created there's more dark energy in the comoving frame but not more particles.
Are you thinking of dark matter here, rather than dark energy? If so, we already have hot dark matter in the form of neutrinos. Neutrinos are "hot" because they move relativistically, and thus are prone to carry momentum far away from galaxies very quickly. Cold dark matter is "cold" because it moves non-relativistically and so the momentum CDM carries lingers in place in and around galaxy clusters. While it would be convenient if CDM were like heavy neutrinos, there is no reason in the standard cosmology that CDM has to be particles at all, or even just one species; the only requirement is that it be almost entirely collisionless and non-radiative so that we don't see it and so that it doesn't release its trapped momentum (e.g. by converting some of it into hot dark matter or light or standard model particles).
Finally, the Standard Model is mute on gravity, and yet you -- made up of Standard Model particles -- are feeling it right now. So I'm not sure how your argument about the correctness of the Standard Model fits with your argument.
A bit off-topic, but not totally: I am so sick of nautil.us spamming my back button into oblivion that I won't click a nautil.us link never more, until they fix that and someone here say it is fixed.
I normally just open things in a new tab, avoids that problem entirely.
But just for you, I clicked the link normally, waited for it to load, hit back, and ended right back up at the HN page. So either an adblocker fixes the problem, or they have.
They seem to be adding history items when you scroll. Try scrolling down the page some, then going back. It took me about ten clicks to get back when I tried it.
So much of what you said is hilariously wrong. Probably the most acute example is your claim "Standard Model is mute on gravity". Are you kidding me? Page one of any book of the Standard Model discusses the 4 fundamental forces. Guess what one of them is: GRAVITY.
You conducted yourself so nastily in this thread that we've banned this account. This sort of flamewar is exactly what we're hoping to avoid on this site.
Really? So what's the gravity term in the Standard Model Lagrangian? [1]
> Page one of any book of the Standard Model discusses the 4 fundamental forces. Guess what one of them is: GRAVITY.
Well, let's just test this assertion of yours.
Right at hand I have Halzen & Martin [2] and "gravity" appears in the index (on p. 389) pointing to pp 27 and 348. Section 1.8 (pp 27-28) explains why gravity is not addressed in the book, and at p. 348 there is a brief discussion following the Weinberg-Salam unification scale at eq 15.58 about whether, given it is large, the gravitational interaction can still be neglected. The treatment there is unsurprisingly fully classical.
Maybe you don't like this particular textbook.
How about Cottingham & Greenwood [3]? This is an excellent book aimed at grad students, and has the advantage of having its introductory chapter online:
"The Standard Model excludes from consideration the gravitational field."
Well, at least that's on page one.
Who next? How about Griffiths [4]? In the middle of page 50 we find:
"This is all adding up to an embarrassingly large number of supposedly 'elementary' particles: 12 leptons, 36 quarks, 12 mediators (I won't count the graviton, since gravity is not included in the Standard Model)."
Above are three standard textbooks introducing the Standard Model, and they all support my assertion and not yours.
"The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains well how these forces act on all of the matter particles. However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model, as fitting gravity comfortably into this framework has proved to be a difficult challenge"
I would be very keen on any evidence that supports your claim that the Standard Model is not mute on gravity.
I'd also be keen on what else you believe I was "hillariously wrong" about. I'd be happy to expand upon, back, or source most of the statements as I do here, if you particularize your complaints and are reasonable and polite about it.
And finally, what really are you trying to accomplish here?
[1] here's Cottingham & Greenwood's[3] write-down of the SM Lagrangian:
You can google the phrase "Standard Model" or look it up in any physics book you want, and there will always be a table labeled "Standard Model" that shows all the particles, which relate to the 4 fundamental forces (carriers), charges, spins, masses, etc. I'm sure you're familiar with this chart. The graviton is on that chart and is specifically describing gravity based on each mass value of each particle (hardly "silence" there). I mean literally every particle type has a mass value (even when zero) right? Is that silence? The general laymans meaning of "Standard Model" means everything in physics that is referenced on that chart (including gravity), and that's how I meant it.
However, what YOU apparently mean by "silent on gravity" is referencing the fact that the Standard Model is incomplete for gravity, because General Relativity is not yet accounted for. I should've realized you might be referring to the lack of any unified field theory, but I didn't. If you had said "doesn't fully explain" rather than "is silent on", it would've sounded perfectly fine to me.
No, what I mean is that there is no gravitational term in any formalism of the Standard Model, and never has been. Its Lagrangian does not include a kinetic or interaction term whereby the spectrum of the Standard Model can include a graviton. Gravity is simply absent from the mathematics of the Standard Model, which is a specific quantum field theory.
You can certainly describe gravitation using a massless spin-2 graviton, and perturbative quantum gravity -- another specific quantum field theory -- does exactly that, and is a perfectly fine effective field theory that entirely matches General Relativity absent superposed sources and outside of strong gravity (which is defined by the EFT's renormalization group flow).
You can add this graviton to a particle zoo. But then it's not the particle zoo of the Standard Model, any more than writing down an "action of everything"[1] gives the action of the Standard Model.
> The general laymans meaning of "Standard Model" means everything in physics that is referenced on that chart (including gravity), and that's how I meant it.
is only correct if you underline the "how I [mean] it" part and make that central to evaluating the sentence's truth. It's hard to disprove solipsistic physics, though, and rarely interesting.
Additionally,
> which relate to the 4 fundamental forces (carriers) ... I'm sure you're familiar with this chart.
Sure, but there are several such charts, and they're not all the same. Here's one from Fermilab and SLAC, institutions that are pretty familiar with the Standard Model: http://www.symmetrymagazine.org/standard-model/ (There's no graviton in it).
Fundamental forces come from fundamental interactions, but there is no force police force enforcing a law that there are four of them or that the carriers are gauge bosons in a particle zoo. The Higgs interaction can be treated as a force[2] and its mediator is a scalar boson rather than a gauge boson, so does the Standard Model have four fundamental forces? One can take differing views of the electroweak interaction above 80-90 GeV: does the Standard Model have only two fundamental forces?
One can certainly treat Einstein gravity as at least a d'Alembert force arising from the affine connection; but alternatively one could take the position that d'Alembert forces generically depend on a choice of frame of reference and on that basis they cannot be fundamental. Is gravity a force? You get a different answer from Newton (yes) than you do from either Einstein (yes, but it's fictitious) or Misner, Thorne & Wheeler (mu, there is only spacetime geometry).
So an argument that as there are four fundamental forces there must be four force carriers is on shaky ground to start with. Even if we were to accept that, it does not follow that all the forces in question relate to the local symmetries of the Standard Model.
Most people consider the 4 fundamental forces to be 'part of' the Standard Model, even if the field equations are not complete. But like I said, i did understand what you meant by 'silent on gravity' after your first clarification.
I wish nautil.us would fix their broken page design so it doesn't fill one's browser history with duplicate links and keep one from leaving the site. This isn't a design choice, it's parasitic.
We know what it is. It is a socially constructed abstract concept, like "god" or "afterlife" or "pure substances" of Spinoza.
BTW, the "substance propositions" Spinoza has been developed are perfectly sound, except one small flaw - no such phenomena as pure substance ever existed. Dark Matter and related crap is of exactly the same nature - much more sophisticated phantoms of the mind supported by a sect of believers.
I've downvoted your comment because your tone isn't constructive and you're making a rather illogical jump to the philosophical arguments of Spinoza in comparison to questions around modern measurable/observable phenomena in science.
If you'd like to make a less hostile post with a clearer justification of your reasoning I'd be more than happy to debate the subject with you.
I do not have anything to add. As a student of philosophy I have seen way to many socially constructed chimeras and memes to not notice modern ones.
The only criteria for establishing a scientific truth is still a replicable experiment which proves an implementation according to some established law. Any simulation does not account for such kind of experiment, being mere a "cartoon" made according to some abstract model. The difference between experiment and simulation is of the same kind as between a cartoon and actual reality.
> " As a student of philosophy I have seen way to many socially constructed chimeras and memes to not notice modern ones."
There's a willful ignorance in this sentence that belies your youth and intelligence. I suspect you don't have an adequate background to understand the observational evidence for our expectation of dark matter (I barely do to be quite honest). I don't think you can fairly dismiss offhand the conclusions drawn by hundreds of expert physicists based on your experience.
I've known more than one physicist that were also well-studied students of philosophy. If you have reached a brilliant and well-justified conclusion that they failed to see you should clearly write it to justify your case.
>"The only criteria for establishing a scientific truth is still a replicable experiment which proves an implementation according to some established law. Any simulation does not account for such kind of experiment, being mere a "cartoon" made according to some abstract model. The difference between experiment and simulation is of the same kind as between a cartoon and actual reality."
Science (and philosophy for that matter) are defined from consistency of observation. I fail to see your point about an experiment measuring a "cartoon" when that is the reality in which we intrinsically exist and for which prediction will provide the most value.
I'm not asking you to send me a resource that has led you to your current conclusions.
I'm asking that you formulate an argument that I can read that will make me believe that your arguments are well-formulated here. You made a very strong statement and I think that if you could logically justify it in written form it would be a valuable contribution to the thread.
I could only try to clarify the same principles a bit more. Non-falsifiable and non empirically verified hypothesis does not amount for a science and constitutes a set of beliefs similar to religion among a community of its followers. As long as there is absolutely no way to reliably test or even measure, leave alone empirically validate anything, this set of beliefs is technically is a non-science and should be explicitly named as mere speculation.
My first comment was the claim that what we deal with is a socially constructed phenomena, similar to a religious belief or a mass-hysteria because it is a) cannot be tested and b) constructed by a certain social group. Technically, I do not see any contradiction in my claim.
However, dark matter is measurable physics. I'm sure if technology and knowledge keep getting better we'll be able to solve this one, but it won't help with the aforementioned fundamental philosophical questions.