I'm truly amazed by how you articulate, the way in which you describe subjects that seem to most to be hard, but yet you upload these videos that well explain supposedly difficult subjects, that some may not have been able to have learned if it had not been for you! Anyway, I'm currently researching Wormholes. I had already read both books on Holors and Tensors, already had seen a lecture series on Vectors, etc. It's safe to assume that I've done what's asked of me, in terms of having the ability to teach oneself.
Outstanding historical review. Just keep in mind a few caveats: The 1998 survey work on accelerating expansion has been recently questioned on the basis of statistical significance; The FLRW equations are a highly ideal toy model; the small positive cosmological constant is at odds with the Higgs mechanism and the Standard Model (predicted VEV). One can go on…
When you say the CC is at odds with the Higgs mechanism, are you talking about the "cosmological constant problem"? The thing that's off by 120 orders of magnitude?
I am convinced we are perceiving Acceleration due to Geometry. When I heard Super Massive Black Hole maybe Dark Energy. My theoretical mind instantly jumped to a Model where Super Black Holes due to expansion of space literally Bend Space in concentric fashion at edge of Universe. If the OG Stars were Black Holes driven. Then perhaps they are very even spaced at Rim of Universe literally bending space unto itself. That is why Light doesn't travel past that Point. Imagine a Model similar to Gamma Ray burst in Single Direction. That Burst/Direction is Big Bang it spreads out and bends back onto itself where the Burst itself is powering the Big Bang. I wish I good at drawing... A thought just crossed my mind writing this. There is no beginning. Any point in time can be the beginning. Maybe that is the reason for our Confusion. Like before SR people probably thought you could see a Standing Wave of Energy. But that is impossible time would stop.
I really appreciate your presentation and delivery. Your storytelling and supplemental illustrations make this extremely dense subject matter really engaging.
Great video! I'm pretty stuck on 27:17 "Motion relative to CMB". Can we give the CMB attributes of a universal rest frame? Against which all other frames can be judged? And thus "prove" objective motion? This sounds suspiciously like Aether theory stuff.
Yeah, I plan to address this in video 110d. There is such as thing as the "cosmic rest frame". It's the reference frame where you see no redshift or blueshift in the CMB. If this makes you raise your eyebrows and immediately think of Aether theory, that's not an unreasonable place for your mind to go. But the "cosmic rest frame" and the "aether" are not the same thing. The Aether was proposed for explaining why the speed of light in Maxwell's Equations was a universal constant. The idea was that the speed of light was "c" with respect to the special Aether frame, but was not "c" in other frames. And so the equations of electricity and magnetism are only valid in this "preferred" frame. Special and General Relativity reject this and say that the (local) speed of light is "c" in all reference frames (inertial or non-inertial), and Maxwell's Equations are valid in all reference frames (inertial or non-inertial). So SR & GR propose the laws of physics, including the speed of light and Maxwell's Equations, are the same in every single reference frame (whether inertial or non-inertial) as long as you write the equations using the correct 4D tensors. The "cosmic rest frame" isn't really about the laws of physics requiring a "preferred frame". It's more about observing that the matter and energy in the universe are distributed in a specific way. If you had a rocket and could move in any direction and any speed you liked, you could try measuring the CMB in different directions. In most frames you'd see redshift on one half of your ship and blueshift on the other half, but there is a specific unique frame where you see no redshift or blueshift, and the CMB frequencies are equal in all directions. This specific unique frame is the "cosmic rest frame". You could sort of do a similar trick with matter, where, if you looked at chunks of space millions of lighyears wide, you could make the matter in that chuck of space appear to have any velocity you like just pushing your rocket into a new frame. But there's a special unique frame where the average velocity of all the matter in that chunk of space is zero (loosely speaking, all the planets and stars whizzing around would have some other star or planet with the opposite velocity that would "cancel it out" when we average). The matter and energy in the universe didn't have to be arranged this way, but that's what we see. We can measure speeds "relative to the CMB", or "relative to the cosmic rest frame". But this frame is only special because of the way matter and energy are distributed. It's not special because it handles the laws of physics in some special way. Motion relative to the CMB isn't any more "objective" than motion relative to any other frame. Please let me know if you have more questions. I can try including my responses in the upcoming video so that others aren't confused.
@@eigenchris perfect response. I totally understand how this is different from Aether, and that it doesn't change SR/GR physics. None-the-less, it's a little jarring to come across after years of relativity being hammered in. Not sure if this is remotely true, but it seems like the existence of a CMB, specifically that one can be moved relative to, implies an early universe that is homogeneous and isotropic, as well as (and for lack of a better word) "static". Static in the sense that there seems to not be large rivers of plasma going this way and that. It was just a wonderful thermal equilibrium. Anyway, even thought it it doesn't change the physics, it just blows my mind that the universe started in some reference frame and we can detect that initial frame. It's bonkers.
@@DanSternofBeyer I don't see it as Initial Reference Frame. But LIGHT Initial Reference Frame. Something was happening but Information couldn't get to us. Like un-tinting a window slowly. What is behind that Tint? Timeless Quark Plasma?
Do I understand correctly, that galactic redshift should come from 2 "sources"?: 1: The actual velocity they have moving away from us. 2: The stretching of the light waves during their long travel period. (like you explained with the CMB) I guess point 2 would only be important for Galaxies that are very far away, but I usually don't see this mentioned.
I think you're describing 2 types of redshift. #1 is "doppler redshift", due to an object's particular velocity relative to the CMB. #2 is "cosmological redshift", due to the expansion of the universe. Both will contribute to the redshift we see. For example, the andromeda galaxy is actually blueshifted overall because it's moving towards us faster than the expansion of the universe can push it away.
6:08 After reading the paper, I don't think Einstein's first highlighted point here makes sense. When he talked about the relativity of inertia, he said that the momentum itself is something that should go to 0 as a point of mass gets moved to infinity. In our current view of relativity, the relativity of inertia only states that momentum can only be measured relative to another mass. Not that momentum must be 0 when there are no other masses. A choice of coordinates can let you measure the momentum as anything you want. The 4-momentum, however, has a length given by the rest mass, which is intrinsic to the mass and not relative. The second highlighted point is just saying "a finite universe will lose a lot of bodies over long periods of time, which will never be seen again". This isn't an impossibility in the theory. Just something Einstein didn't like about it. A metric that is dominated by a time-time component at infinity (as suggested in the A=0 or momentum necessarily goes to 0 at infinity idea), I believe would solve this from all momentum being converted to energy near infinity. This makes everything fall back into the potential energy well in which the universe exists.
Yeah, I've found reading Einstein's old papers to be challenging. He's a figuring a lot of things out for the first time, and he sometimes makes mistakes or gets attached to ideas that aren't important. I find it hard to decide if Einstein is just wrong or if I don't understand his arguments because they're just buried under history and old ideas. Once you have the Friedmann Equations, the cause-and-effect relationship from mass/pressure/cosmological constant on the expansion of the universe becomes very clear and there's no need for the strange arcane arguments that Einstein was trying to use. I was mostly trying to cover the historical origins of cosmology according to GR in this video just to give people historical context on how the ideas developed.
@@eigenchris I prefer to think of it as "it's easier to write an equation than to understand all of its implications." Feynman struggled with implications of equations also. The situation is especially difficult with differential equations. My opinion is that there has been less mathematical progress understanding DE than other areas of math.
Einstein is especially concerned with the existence and meaning of inertia. He doesn't care too much about models of the Universe because quite frankly there's not much to care about. It is completely irrelevant to us whether the Universe is expanding, contracting, yoyoing or whatever, the only thing that matters is the Physics, it's a lot easier to explain inertia in the context of certain models which is why he preferred them above others.
7:34 Not all finite universes are positively curved. You can make them cube shaped (can be flat), dodecahedron shaped (can be different curvatures), or a couple of other options. A 3-sphere (4-sphere?) isn't the only one. All positively curved universes are finite though*. I think mass implied there was a positive curvature at that point or something. The trace of the stress energy tensor, which is equated to the Einstein tensor, (if an index is raised first, of course) will tell you the ricci scalar, which tells you the curvature of the universe at that point. If there's more mass-energy than there is pressure, you expect a positive ricci scalar and if there's more pressure than there is mass-energy, you expect a negative ricci scalar. The issue is that all 4-momentum should be timelike and travelling in the direction of itself, meaning there should be more mass-energy than pressure. More pressure than mass-energy implies that mass is moving through space hyperplanes more than they're moving through time hyperplanes**. *This is something I know from Riemannian geometry. Unsure whether this still holds in pseudo-Riemannian geometry. Also, I may have the wrong curvature. It may be the determinant of the ricci tensor rather than its trace... Actually no. It can't be the determinant because then that says the curvature of the hyperbolic plane is positive and the hyperbolic plane has a finite diameter. en.wikipedia.org/wiki/Myers%27s_theorem **What? Negative curvature without a cosmological constant implies most of the energy-momentum is carried by tachyons?
@@narfwhals7843 When I say "cube" I mean a torus extended to be a 3 dimensional manifold. You can imagine it as being a cube, but the top and bottom faces are glued together (as if there were a portal between them), and a similar arrangement for other opposite faces. The dodecahedron one works similarly, but the opposite faces need to be glued together with some rotation (since they don't actually line up without one). I'm not talking about surfaces of the 8-cell (hypercube) or the 120-cell (hyperdodecahedron) as 3-manifolds here. I'm talking about the inside of a cube and the inside of a dodecahedron (with some special glueing to handle the boundary points). The way the glueing happens can create an angle excess or deficit, which can be overcome by giving the manifold some curvature.
In my videos I'll be sticking to the "simply connected" (no holes) topologies for the universe. I don't plan on discussing the more exotic possibilities, other than maybe as an offhand remark.
When the Einstein field equations are derived, there is no cosmological constant, and as you described it was an ad hoc. Something I’ve always wondered is doesn’t that mean that the correct form of the Einstein field equations shouldn’t have a cosmological constant? If so, why do people still talk about it?
I tried to briefly explain why it's "allowed" at 8:40. Conservation of energy-momentum requires that the divergence of the energy-momentum tensor is zero (I explain this at the end of Relativity 107e). So the divergence of the left side of the EFE must also be zero (I show this in Relativity 107f). Including a constant term on the left side (multiplied by the metric) doesn't violate this, since the derivative of the covariant derivative of the metric is always zero by the "metric compatibility condition". It's similar to how you can differentiate "f(x)" or "f(x) + c" and get the same result. The constant doesn't matter. The cosmological constant is sort of like this constant. If you are familiar with Lagrangians, the EFE with the cosmological constant can be derived from the Einstein Hilbert action, so the cosmological constant term can follow from an action principle. Not sure if any of this convinces you, but it's the best understanding I have right now.
@@edwardlulofs444 Thanks. This particular video took a long time because of all the fact-checking I had to do and mistakes I needed to fix. But I hope it's worth it for people.
@@eigenchris a wonderful recap of the history, very worth it! it's great for this kind of information to be freely available and at such a high quality. thank you for putting it out there
I was waiting for this video series eagerly , chris. GR was just a milestone of work. Hope this cosmology series will b the same. I'll cover all these as well. Keep going bro. And thank you very much for everything. " let there be eigen "
One more : 6:25 - You state an assumption that the metric is flat and unchanging over time, thus leading to the conclusion that there is no mass. Could we instead think about this like being in the center of a star, or body of mass. The equal gravitational forces all around you create the illusion of ‘flat spacetime’. Said differently, can being in the homogeneous universe create the illusion of flatness, even in the presence of mass. We know the universe is expanding, thus the density is greater in the past. In the past the universe looked flat, and it looks flat now, but if you relate the two states they should have different qualities of the metric. Right? Due to the difference of density?
I think the catch is that, if matter has uniform distribution everywhere, and the universe is spatially flat, then we're forced to accept a universe that changes size overtime. If we assume uniform distribution of matter, spatially flat universe, AND a static universe, then we're forced to conclude the uniform distribution of matter is zero everywhere. The reasoning Einstein gives for this is pretty bizarre, and my reasoning might only be slightly better. It's much easier to understand if you look at the Friedmann Equations at 13:45. Matter/pressure is given by rho/p. A spatially flat universe means k= 0. A static universe means a-dot = 0 and a-double-dot = 0. You can play around with setting these parameters to different numbers, but if you set too many things to zero, then everything goes to zero. A forever-flat universe is just permanent special relativity... no curvature and no mass. We can get away with a SPATIALLY flat universe that has matter if the universe's size changes overtime (so SPACE is flat but SPACETIME is not flat).
About the assumptions for the friedmann equations: why should the mass-energy density and pressure not vary over time? I understand being homogenous over space but being constant over time seems rather odd to me. Especially since there's a time dependant scale factor.
I wasn't trying to imply that they were constant overtime. I tried to use the word "uniform" instead of "constant". The idea is that they are evenly distributed over space but can also change overtime.
The real point is that this universe via gravity and other binding forces...forces clumping/gradient. This variation between energy states then drives all activity of universe. Neutron decay cosmology is inevitable. Conservation demands
Fantastic video! Such a great overview of history. I do have one conceptual question that I can't reconcile, which is basically why we can tell that the universe is expanding. I understand the redshift argument, but I'm confused as to why it causes a physical change. I think I can sum up my confusion by asking "what happens to a ruler as the universe expands?" My intuition is that you wouldn't notice: in some sense a meter has gotten longer but you can't tell, because your ruler is still a meter. But it seems like if light is red shifting etc, that means that the ruler would eventually break because it actually gets longer? Like there has to be a noticable effect on the ruler if there's a noticable effect on light right?
The universe expanding affects "free particles", and by that I mean it impacts particles that aren't exerting forces on each other. The parricles in a ruler are not free particles... they are bound together by electromagnetic forces. So a ruler would stay the same size over 1 billion years of universe expansion because electromagnetic forces are working against the expansion. If I were to use more fancy GR language... geodesics in an expanding universe will all separate from each other, but the particles in a ruler will not follow perfect geodesics; there will be 4-acceleration vectors due to electromagnetic forces that divert particles away from perfect geodesics.
@@eigenchris thanks! I think I maybe this helps me think of this more clearly. I think my first thought was that since space is expanding, *everything* is expanding so you'd just never be able to notice (because e.g. as space expands, your ruler would expand too, so you'd never notice that anything happened). Is it fair to say that instead, actual objects will basically "stay the same size" while space expands? So for example if we put a ruler between two free particles, and waited for a while, eventually the free particles would move away from the ruler but the ruler itself would stay the same length?
@@eigenchris But if the "force" due to the expansion of the universe became comparable to EM on that length scale, then the ruler would indeed change in length, right?
Hello Chris I've a very naive question, in Friedman equations a, a(dot) etc are being related to things like mass density etc, now if I consider expansion of Constant Mass universe then won't the mass density itself be variable? Pls explain, am I missing out something?
Yes, the density (rho) and pressure (p) can be functions of time. In an expanding universe, density will decrease overtime. But rho and p are not functions of space in a homogeneous universe, since we assume all spatial points are equivalent.
First time when Einstein introduce it, lamda wasn't negative but it take a value wich make Universe static so if the universe expand then lamda must be negative and if the universe shrink it's positive . all this for making a static univers
Regarding your explanation of Einstein's 1917 cosmology paper, all you are just saying is that the universe should collapse back on itself due to the mutual gravitation of all matter on itself. So Einstein added Lambda as a fudge factor term to keep the universe from collapsing.
In the energy-momentum tensor, the "x-pressure" component (Txx) tracks how much x- momentum is traveling through a surface of constant x. The Tyy and Tzz components do the same thing for y and z momentum. In cosmology think this is used less often for matter and more for energy (like the cosmic microwave background). The CMB radiation exists everywhere in space and travels in all directions, so there is "flow" of momentum in all directions. It's important to take the pressure terms into account when modelling the early universe, since our current modes say the early universe was dominated by energy.
"Once upon a time, there was nothing,then the nothing exploded and some of the shrapnel from the nothing exploding learned to play the violin but some was just a pebble or a fish " Yup ...makes perfect sense ... 🤣😅😂😅 So next time I want to blow shit up ,I'm going to need a bucket of nothing and some nothing to detonate it with . Got it , okey dokey ,right ,yes , great.
Close, don't forget that your bucket of nothing must also create time, space, and all equation of physics. Laughter is not a bad reaction. I prefer to be in awe of it.
The Big Bang theory is about the aftermath, not the beginning itself. So, the BBT never states that something came from nothing. It simply states that, since the universe is accelerating, it must’ve started at a point at time t = 0; a singularity. You grossly oversimplified years of scientific research and discoveries and then get surprised when it doesn’t make coherent sense.
(without having read the full paper, sorry)I think Einstein's objection at 6:05 is about Mach's Principle. In his mind it should be impossible to distinguish acceleration from rest as well in an empty universe and the fact that we can do this is induced by the gravitational background of "everything". So inertia should basically be a feature of the metric tensor field. There is no "true motion" without a reference, even identifying acceleration requires some prior knowledge. Dialect recently did a video about this and the "bucket experiment". ua-cam.com/video/Jz3mOlUOGoY/v-deo.html
I'm confused at the idea of "not being able to distinguish acceleration from rest in an empty universe". I'm actually going to change the word "acceleration" to "non-inertial", since I think that's a better term. Acceleration is a number in m/s^2 which is relative, even in Galilean relativity. But my understanding while making my series has been that non-inertial motion is objective and can be determined by an accelerometer in the observer's hand. (Where an accelerometer = spring-and-mass-system-in-a-rigid-box.) As long as you have 3 accelerometers positioned along 3 orthogonal axes, you should be able to detect non-inertial linear motion and non-inertial rotational motion without reference to anything else. I think dialect has made a couple great videos (the recent one about the metric in particular is fantastic). But many of their videos seem to fixate on this idea of not being able to detect non-inertial motion, and I don't really understand this.
@@eigenchris I used to agree with you about Dialects fixation in the twin paradox videos. But the latest one showed where that comes from and Einstein was massively influenced by Mach's view. Maybe I'm making a mistake, though, and the principle is about rotation only? en.wikipedia.org/wiki/Absolute_rotation#General_relativity en.wikipedia.org/wiki/Mach%27s_principle#Mach's_principle_in_general_relativity These all mention rotation specifically, but I think that is only because that _should_ be an obvious form of non-inertial motion. The main point, I believe, is that inertia is defined with respect to local geodesics, which in principle contain information about the whole past-causally connected universe. And that is what Einstein means by "inertia depends upon the g". An accelerometer tries to follow a local geodesic, which is defined by the rest of the universe, so it is not really an indicator of "true motion". I don't think this is a fully resolved issue in modern physics. We can just accept that acceleration is special, but I think that goes against what Einstein set out to do.
I guess I don't understand what you mean by "true motion". In my mind there is "inertial motion" (following a geodesic) and "non-inertial motion" (not following a geodesic), and an accelerometer can pick out the difference between the two. What does "true motion" mean? I'm not clear on why the history of the universe or other matter in the universe is relevant. I'd figure the concept of "inertial" vs "non-inertial" is a completely local concept anyway.
@@eigenchris If I understand correctly true motion would be motion that an observer can detect on themselves. It would mean there is an experiment the traveling twin can do to determine that they are truly the one that is moving while earth twin remains stationary. According to newton this experiment can be done by rotating a bucket of water or a spring with weights on it because inertia will distinguish the rotating state from the non-rotating one. But according to Mach, who influenced Einstein immensely, that inertia is induced by the rest of the matter in the universe and so needs a reference. According to Mach, without this reference you can not distinguish inertial motion from non-inertial motion without _some_ reference and the calibration of the accelerometer counts as a reference to the outside world. I think this argument is sound but very subtle. And in GR this reference are the local geodesics, which carry information about the rest of the universe. I wasn't trying to question any of your assumptions, btw. I just wanted to maybe help you understand Einsteins argument. I think your way of distinguishing inertial from non-inertial is perfectly fine for the context of your videos. From the wiki: However, Einstein was convinced that a valid theory of gravity would necessarily have to include the relativity of inertia: So strongly did Einstein believe at that time in the relativity of inertia that in 1918 he stated as being on an equal footing three principles on which a satisfactory theory of gravitation should rest: The principle of relativity as expressed by general covariance. The principle of equivalence. Mach's principle (the first time this term entered the literature): … that the gµν are completely determined by the mass of bodies, more generally by Tµν. In 1922, Einstein noted that others were satisfied to proceed without this [third] criterion and added, "This contentedness will appear incomprehensible to a later generation however." It must be said that, as far as I can see, to this day, Mach's principle has not brought physics decisively farther. It must also be said that the origin of inertia is and remains the most obscure subject in the theory of particles and fields. Mach's principle may therefore have a future - but not without the quantum theory. - Abraham Pais, in Subtle is the Lord: the Science and the Life of Albert Einstein (Oxford University Press, 2005), pp. 287-288.
I find the 1st 5 minutes of this video a bit mindblowing because am i right in saying that if you have: Earth light years 🌎 ------------------------------------------>------------------------------------INFINITE AMOUNTS OF MASS then the gravitational pull towards the INFINITE AMOUNTS OF MASS would be immensely strong even though we're a long way away? I think it must be true because Fraser Cain said if you have 2 atoms a gazillion light years apart then eventually they will be gravitationally drawn to each other. & so I think maybe if you have an infinity times that atom then the curvature must be infinity times stronger & so if you have infinite mass either side ------------------- infinite mass then space is like a tight guitar string. Like we're getting pulled in every direction infinitely like we're in a BH
When I studied the history of math and saw so many famous mathematicians make mistakes reasoning about infinity, I decided to never make conclusions about infinity.
Neutron decay Cosmology Neutrons which contact event horizons become the vacuum energy for one single Planck second then reemerge in lowest density points of space where they decay into amorphous atomic hydrogen. As neutrons do. The decay from neutron to proton, electron, neutrino, from near point particle to one cubic meter is a volume increase of 10^45. Expansion. Dark energy. The decay product, amorphous atomic hydrogen, doesn't have stable orbital electron so can't emit or absorb photons. Dark matter. In time the hydrogen stabilizes and follows usual evolution pathway from gas to filament to proto star to star to neutron star until in the distant future it is again about to contact an event horizon. The universe is steady state evolving locally.. Events horizons acting as energy pressure release valves venting from highest energy pressure conditions to lowest energy points of space. From aggregated singularity to distributed dispersed diffuse. This is done using topology.. Neutron decay Cosmology There physical process, path of least action, solution to black hole paradoxes, dark energy, dark matter and critical density maintenance.
well he did teach it. The only part he didn’t understand was Einstein’s paper, but he didn’t teach that part; he taught something else. Since he didn’t understand it, he taught something else as a replacement. He literally did as you said.
This channel is such a hidden gem. Thanks for the great content
I'm truly amazed by how you articulate, the way in which you describe subjects that seem to most to be hard, but yet you upload these videos that well explain supposedly difficult subjects, that some may not have been able to have learned if it had not been for you!
Anyway, I'm currently researching Wormholes. I had already read both books on Holors and Tensors, already had seen a lecture series on Vectors, etc. It's safe to assume that I've done what's asked of me, in terms of having the ability to teach oneself.
Seriously, he is one of the best teachers I have seen
14:17
Friedmann - Hmm, it looks like according to your theory, the universe can expand or shrink over time
Einstein - Not sure man, that looks sus
Outstanding historical review. Just keep in mind a few caveats: The 1998 survey work on accelerating expansion has been recently questioned on the basis of statistical significance; The FLRW equations are a highly ideal toy model; the small positive cosmological constant is at odds with the Higgs mechanism and the Standard Model (predicted VEV). One can go on…
When you say the CC is at odds with the Higgs mechanism, are you talking about the "cosmological constant problem"? The thing that's off by 120 orders of magnitude?
@@eigenchris yes
I am convinced we are perceiving Acceleration due to Geometry. When I heard Super Massive Black Hole maybe Dark Energy. My theoretical mind instantly jumped to a Model where Super Black Holes due to expansion of space literally Bend Space in concentric fashion at edge of Universe. If the OG Stars were Black Holes driven. Then perhaps they are very even spaced at Rim of Universe literally bending space unto itself. That is why Light doesn't travel past that Point.
Imagine a Model similar to Gamma Ray burst in Single Direction.
That Burst/Direction is Big Bang it spreads out and bends back onto itself where the Burst itself is powering the Big Bang. I wish I good at drawing... A thought just crossed my mind writing this. There is no beginning. Any point in time can be the beginning. Maybe that is the reason for our Confusion. Like before SR people probably thought you could see a Standing Wave of Energy. But that is impossible time would stop.
I really appreciate your presentation and delivery. Your storytelling and supplemental illustrations make this extremely dense subject matter really engaging.
Maravilloso. Gracias por tu esfuerzo y tu generosidad.
Thanks for your great lecturers, making such a useful lecturers on most technical topics. Really useful and enjoyed kindly keep it up make more
Great video! I'm pretty stuck on 27:17 "Motion relative to CMB". Can we give the CMB attributes of a universal rest frame? Against which all other frames can be judged? And thus "prove" objective motion?
This sounds suspiciously like Aether theory stuff.
Yeah, I plan to address this in video 110d. There is such as thing as the "cosmic rest frame". It's the reference frame where you see no redshift or blueshift in the CMB. If this makes you raise your eyebrows and immediately think of Aether theory, that's not an unreasonable place for your mind to go. But the "cosmic rest frame" and the "aether" are not the same thing.
The Aether was proposed for explaining why the speed of light in Maxwell's Equations was a universal constant. The idea was that the speed of light was "c" with respect to the special Aether frame, but was not "c" in other frames. And so the equations of electricity and magnetism are only valid in this "preferred" frame. Special and General Relativity reject this and say that the (local) speed of light is "c" in all reference frames (inertial or non-inertial), and Maxwell's Equations are valid in all reference frames (inertial or non-inertial). So SR & GR propose the laws of physics, including the speed of light and Maxwell's Equations, are the same in every single reference frame (whether inertial or non-inertial) as long as you write the equations using the correct 4D tensors.
The "cosmic rest frame" isn't really about the laws of physics requiring a "preferred frame". It's more about observing that the matter and energy in the universe are distributed in a specific way. If you had a rocket and could move in any direction and any speed you liked, you could try measuring the CMB in different directions. In most frames you'd see redshift on one half of your ship and blueshift on the other half, but there is a specific unique frame where you see no redshift or blueshift, and the CMB frequencies are equal in all directions. This specific unique frame is the "cosmic rest frame". You could sort of do a similar trick with matter, where, if you looked at chunks of space millions of lighyears wide, you could make the matter in that chuck of space appear to have any velocity you like just pushing your rocket into a new frame. But there's a special unique frame where the average velocity of all the matter in that chunk of space is zero (loosely speaking, all the planets and stars whizzing around would have some other star or planet with the opposite velocity that would "cancel it out" when we average). The matter and energy in the universe didn't have to be arranged this way, but that's what we see. We can measure speeds "relative to the CMB", or "relative to the cosmic rest frame". But this frame is only special because of the way matter and energy are distributed. It's not special because it handles the laws of physics in some special way. Motion relative to the CMB isn't any more "objective" than motion relative to any other frame.
Please let me know if you have more questions. I can try including my responses in the upcoming video so that others aren't confused.
@@eigenchris perfect response. I totally understand how this is different from Aether, and that it doesn't change SR/GR physics. None-the-less, it's a little jarring to come across after years of relativity being hammered in.
Not sure if this is remotely true, but it seems like the existence of a CMB, specifically that one can be moved relative to, implies an early universe that is homogeneous and isotropic, as well as (and for lack of a better word) "static". Static in the sense that there seems to not be large rivers of plasma going this way and that. It was just a wonderful thermal equilibrium.
Anyway, even thought it it doesn't change the physics, it just blows my mind that the universe started in some reference frame and we can detect that initial frame. It's bonkers.
@@DanSternofBeyer I don't see it as Initial Reference Frame. But LIGHT Initial Reference Frame. Something was happening but Information couldn't get to us. Like un-tinting a window slowly. What is behind that Tint? Timeless Quark Plasma?
Do I understand correctly, that galactic redshift should come from 2 "sources"?:
1: The actual velocity they have moving away from us.
2: The stretching of the light waves during their long travel period. (like you explained with the CMB)
I guess point 2 would only be important for Galaxies that are very far away, but I usually don't see this mentioned.
I think you're describing 2 types of redshift. #1 is "doppler redshift", due to an object's particular velocity relative to the CMB. #2 is "cosmological redshift", due to the expansion of the universe. Both will contribute to the redshift we see. For example, the andromeda galaxy is actually blueshifted overall because it's moving towards us faster than the expansion of the universe can push it away.
6:08 After reading the paper, I don't think Einstein's first highlighted point here makes sense. When he talked about the relativity of inertia, he said that the momentum itself is something that should go to 0 as a point of mass gets moved to infinity. In our current view of relativity, the relativity of inertia only states that momentum can only be measured relative to another mass. Not that momentum must be 0 when there are no other masses. A choice of coordinates can let you measure the momentum as anything you want. The 4-momentum, however, has a length given by the rest mass, which is intrinsic to the mass and not relative.
The second highlighted point is just saying "a finite universe will lose a lot of bodies over long periods of time, which will never be seen again". This isn't an impossibility in the theory. Just something Einstein didn't like about it. A metric that is dominated by a time-time component at infinity (as suggested in the A=0 or momentum necessarily goes to 0 at infinity idea), I believe would solve this from all momentum being converted to energy near infinity. This makes everything fall back into the potential energy well in which the universe exists.
Yeah, I've found reading Einstein's old papers to be challenging. He's a figuring a lot of things out for the first time, and he sometimes makes mistakes or gets attached to ideas that aren't important. I find it hard to decide if Einstein is just wrong or if I don't understand his arguments because they're just buried under history and old ideas. Once you have the Friedmann Equations, the cause-and-effect relationship from mass/pressure/cosmological constant on the expansion of the universe becomes very clear and there's no need for the strange arcane arguments that Einstein was trying to use. I was mostly trying to cover the historical origins of cosmology according to GR in this video just to give people historical context on how the ideas developed.
@@eigenchris I prefer to think of it as "it's easier to write an equation than to understand all of its implications." Feynman struggled with implications of equations also. The situation is especially difficult with differential equations. My opinion is that there has been less mathematical progress understanding DE than other areas of math.
Einstein is especially concerned with the existence and meaning of inertia. He doesn't care too much about models of the Universe because quite frankly there's not much to care about. It is completely irrelevant to us whether the Universe is expanding, contracting, yoyoing or whatever, the only thing that matters is the Physics, it's a lot easier to explain inertia in the context of certain models which is why he preferred them above others.
7:34 Not all finite universes are positively curved. You can make them cube shaped (can be flat), dodecahedron shaped (can be different curvatures), or a couple of other options. A 3-sphere (4-sphere?) isn't the only one.
All positively curved universes are finite though*. I think mass implied there was a positive curvature at that point or something. The trace of the stress energy tensor, which is equated to the Einstein tensor, (if an index is raised first, of course) will tell you the ricci scalar, which tells you the curvature of the universe at that point. If there's more mass-energy than there is pressure, you expect a positive ricci scalar and if there's more pressure than there is mass-energy, you expect a negative ricci scalar. The issue is that all 4-momentum should be timelike and travelling in the direction of itself, meaning there should be more mass-energy than pressure. More pressure than mass-energy implies that mass is moving through space hyperplanes more than they're moving through time hyperplanes**.
*This is something I know from Riemannian geometry. Unsure whether this still holds in pseudo-Riemannian geometry. Also, I may have the wrong curvature. It may be the determinant of the ricci tensor rather than its trace... Actually no. It can't be the determinant because then that says the curvature of the hyperbolic plane is positive and the hyperbolic plane has a finite diameter.
en.wikipedia.org/wiki/Myers%27s_theorem
**What? Negative curvature without a cosmological constant implies most of the energy-momentum is carried by tachyons?
But cubes or other polygons are not differentiable manifolds, are they? So we can't do Riemannian or pseudo riemannian geometry on them.
@@narfwhals7843 When I say "cube" I mean a torus extended to be a 3 dimensional manifold. You can imagine it as being a cube, but the top and bottom faces are glued together (as if there were a portal between them), and a similar arrangement for other opposite faces. The dodecahedron one works similarly, but the opposite faces need to be glued together with some rotation (since they don't actually line up without one).
I'm not talking about surfaces of the 8-cell (hypercube) or the 120-cell (hyperdodecahedron) as 3-manifolds here. I'm talking about the inside of a cube and the inside of a dodecahedron (with some special glueing to handle the boundary points).
The way the glueing happens can create an angle excess or deficit, which can be overcome by giving the manifold some curvature.
In my videos I'll be sticking to the "simply connected" (no holes) topologies for the universe. I don't plan on discussing the more exotic possibilities, other than maybe as an offhand remark.
@@neopalm2050 Ahhh I see. Thank you. I'm still very much in the process(beginning) of understanding topology and manifolds.
When the Einstein field equations are derived, there is no cosmological constant, and as you described it was an ad hoc. Something I’ve always wondered is doesn’t that mean that the correct form of the Einstein field equations shouldn’t have a cosmological constant? If so, why do people still talk about it?
I tried to briefly explain why it's "allowed" at 8:40. Conservation of energy-momentum requires that the divergence of the energy-momentum tensor is zero (I explain this at the end of Relativity 107e). So the divergence of the left side of the EFE must also be zero (I show this in Relativity 107f). Including a constant term on the left side (multiplied by the metric) doesn't violate this, since the derivative of the covariant derivative of the metric is always zero by the "metric compatibility condition". It's similar to how you can differentiate "f(x)" or "f(x) + c" and get the same result. The constant doesn't matter. The cosmological constant is sort of like this constant. If you are familiar with Lagrangians, the EFE with the cosmological constant can be derived from the Einstein Hilbert action, so the cosmological constant term can follow from an action principle. Not sure if any of this convinces you, but it's the best understanding I have right now.
Thank you Chris for the vids.
When will you upload 108-d video about Kruskal ... coordinate?
Probably in a couple months. I plan on finishing the 110 videos first.
@@eigenchris From the subject and quality of the videos, you must spend a large amount of time making. Very high quality. Thanks.
@@edwardlulofs444 Thanks. This particular video took a long time because of all the fact-checking I had to do and mistakes I needed to fix. But I hope it's worth it for people.
@@eigenchris a wonderful recap of the history, very worth it! it's great for this kind of information to be freely available and at such a high quality. thank you for putting it out there
The Hubble Doppler is this curvature of metric as one goes outwards as there is more mass the further one goes.
I was waiting for this video series eagerly , chris. GR was just a milestone of work. Hope this cosmology series will b the same. I'll cover all these as well. Keep going bro. And thank you very much for everything. " let there be eigen "
Great video, took GR back in uni and never understood origin of cosmological constant
One more :
6:25 - You state an assumption that the metric is flat and unchanging over time, thus leading to the conclusion that there is no mass.
Could we instead think about this like being in the center of a star, or body of mass. The equal gravitational forces all around you create the illusion of ‘flat spacetime’. Said differently, can being in the homogeneous universe create the illusion of flatness, even in the presence of mass.
We know the universe is expanding, thus the density is greater in the past. In the past the universe looked flat, and it looks flat now, but if you relate the two states they should have different qualities of the metric. Right? Due to the difference of density?
I think the catch is that, if matter has uniform distribution everywhere, and the universe is spatially flat, then we're forced to accept a universe that changes size overtime. If we assume uniform distribution of matter, spatially flat universe, AND a static universe, then we're forced to conclude the uniform distribution of matter is zero everywhere. The reasoning Einstein gives for this is pretty bizarre, and my reasoning might only be slightly better. It's much easier to understand if you look at the Friedmann Equations at 13:45. Matter/pressure is given by rho/p. A spatially flat universe means k= 0. A static universe means a-dot = 0 and a-double-dot = 0. You can play around with setting these parameters to different numbers, but if you set too many things to zero, then everything goes to zero. A forever-flat universe is just permanent special relativity... no curvature and no mass. We can get away with a SPATIALLY flat universe that has matter if the universe's size changes overtime (so SPACE is flat but SPACETIME is not flat).
About the assumptions for the friedmann equations: why should the mass-energy density and pressure not vary over time? I understand being homogenous over space but being constant over time seems rather odd to me. Especially since there's a time dependant scale factor.
I wasn't trying to imply that they were constant overtime. I tried to use the word "uniform" instead of "constant". The idea is that they are evenly distributed over space but can also change overtime.
amazing, the world's most famous scientist was humble enough to admit he was wrong.
To say a region of space is statistically uniform is to ignore that the is no gradient free point in the universe. Everything is falling all the time.
Yes, the statistically uniform idea is controversial. There is evidence for and against it. There are many unanswered questions.
The real point is that this universe via gravity and other binding forces...forces clumping/gradient. This variation between energy states then drives all activity of universe.
Neutron decay cosmology is inevitable. Conservation demands
Fantastic video! Such a great overview of history.
I do have one conceptual question that I can't reconcile, which is basically why we can tell that the universe is expanding. I understand the redshift argument, but I'm confused as to why it causes a physical change.
I think I can sum up my confusion by asking "what happens to a ruler as the universe expands?" My intuition is that you wouldn't notice: in some sense a meter has gotten longer but you can't tell, because your ruler is still a meter. But it seems like if light is red shifting etc, that means that the ruler would eventually break because it actually gets longer? Like there has to be a noticable effect on the ruler if there's a noticable effect on light right?
The universe expanding affects "free particles", and by that I mean it impacts particles that aren't exerting forces on each other. The parricles in a ruler are not free particles... they are bound together by electromagnetic forces. So a ruler would stay the same size over 1 billion years of universe expansion because electromagnetic forces are working against the expansion. If I were to use more fancy GR language... geodesics in an expanding universe will all separate from each other, but the particles in a ruler will not follow perfect geodesics; there will be 4-acceleration vectors due to electromagnetic forces that divert particles away from perfect geodesics.
@@eigenchris thanks! I think I maybe this helps me think of this more clearly.
I think my first thought was that since space is expanding, *everything* is expanding so you'd just never be able to notice (because e.g. as space expands, your ruler would expand too, so you'd never notice that anything happened).
Is it fair to say that instead, actual objects will basically "stay the same size" while space expands? So for example if we put a ruler between two free particles, and waited for a while, eventually the free particles would move away from the ruler but the ruler itself would stay the same length?
@@eigenchris But if the "force" due to the expansion of the universe became comparable to EM on that length scale, then the ruler would indeed change in length, right?
@@DavidPumpernickel Yes, that's my understanding.
@@DavidPumpernickel Yes, infact that's the concept of the universe ending with the "Big Rip" as far as ik.
Amazing clarity!
Hello Chris
I've a very naive question, in Friedman equations a, a(dot) etc are being related to things like mass density etc, now if I consider expansion of Constant Mass universe then won't the mass density itself be variable?
Pls explain, am I missing out something?
Yes, the density (rho) and pressure (p) can be functions of time. In an expanding universe, density will decrease overtime. But rho and p are not functions of space in a homogeneous universe, since we assume all spatial points are equivalent.
Thank you my Friend!
First time when Einstein introduce it, lamda wasn't negative but it take a value wich make Universe static so if the universe expand then lamda must be negative and if the universe shrink it's positive .
all this for making a static univers
Regarding your explanation of Einstein's 1917 cosmology paper, all you are just saying is that the universe should collapse back on itself due to the mutual gravitation of all matter on itself. So Einstein added Lambda as a fudge factor term to keep the universe from collapsing.
the Lambda is there due to the math, and he gave it a nonzero number to keep the universe from collapsing (so i heard)
Excuse me Sir! What is the difference between metric and metric affine?
I don't know what a "metric affine" is. Could you link me to an example of where someone talks about this?
@@eigenchris ua-cam.com/video/hs24WVl_v50/v-deo.html
what does pressure mean in cosmology?
In the energy-momentum tensor, the "x-pressure" component (Txx) tracks how much x- momentum is traveling through a surface of constant x. The Tyy and Tzz components do the same thing for y and z momentum. In cosmology think this is used less often for matter and more for energy (like the cosmic microwave background). The CMB radiation exists everywhere in space and travels in all directions, so there is "flow" of momentum in all directions. It's important to take the pressure terms into account when modelling the early universe, since our current modes say the early universe was dominated by energy.
Typo at 26:41 2.725K
what do you use for the graphics?
Microsoft Powerpoint.
"Once upon a time, there was nothing,then the nothing exploded and some of the shrapnel from the nothing exploding learned to play the violin but some was just a pebble or a fish "
Yup ...makes perfect sense ...
🤣😅😂😅
So next time I want to blow shit up ,I'm going to need a bucket of nothing and some nothing to detonate it with .
Got it , okey dokey ,right ,yes ,
great.
Close, don't forget that your bucket of nothing must also create time, space, and all equation of physics. Laughter is not a bad reaction. I prefer to be in awe of it.
@@edwardlulofs444
I can only imagine what it must be like to live in awe of charlatans like that vacuous ,idiotic clown Einstein...
May God help you.
wtf is this comment
@@dertechl6628
It's called ridiculing the masonic lies from twats like Newton, Einstein ,Darwin and the rest of the bullshit artists.
The Big Bang theory is about the aftermath, not the beginning itself. So, the BBT never states that something came from nothing. It simply states that, since the universe is accelerating, it must’ve started at a point at time t = 0; a singularity. You grossly oversimplified years of scientific research and discoveries and then get surprised when it doesn’t make coherent sense.
Ah yessss, thanks.
First! Like by default, I know I'm gonna love it
Get ahead of the curve and integrate neutron decay cosmology now! :)
Good work
Just Awsome
(without having read the full paper, sorry)I think Einstein's objection at 6:05 is about Mach's Principle. In his mind it should be impossible to distinguish acceleration from rest as well in an empty universe and the fact that we can do this is induced by the gravitational background of "everything". So inertia should basically be a feature of the metric tensor field. There is no "true motion" without a reference, even identifying acceleration requires some prior knowledge.
Dialect recently did a video about this and the "bucket experiment". ua-cam.com/video/Jz3mOlUOGoY/v-deo.html
I'm confused at the idea of "not being able to distinguish acceleration from rest in an empty universe". I'm actually going to change the word "acceleration" to "non-inertial", since I think that's a better term. Acceleration is a number in m/s^2 which is relative, even in Galilean relativity. But my understanding while making my series has been that non-inertial motion is objective and can be determined by an accelerometer in the observer's hand. (Where an accelerometer = spring-and-mass-system-in-a-rigid-box.) As long as you have 3 accelerometers positioned along 3 orthogonal axes, you should be able to detect non-inertial linear motion and non-inertial rotational motion without reference to anything else. I think dialect has made a couple great videos (the recent one about the metric in particular is fantastic). But many of their videos seem to fixate on this idea of not being able to detect non-inertial motion, and I don't really understand this.
@@eigenchris I used to agree with you about Dialects fixation in the twin paradox videos. But the latest one showed where that comes from and Einstein was massively influenced by Mach's view. Maybe I'm making a mistake, though, and the principle is about rotation only? en.wikipedia.org/wiki/Absolute_rotation#General_relativity
en.wikipedia.org/wiki/Mach%27s_principle#Mach's_principle_in_general_relativity
These all mention rotation specifically, but I think that is only because that _should_ be an obvious form of non-inertial motion.
The main point, I believe, is that inertia is defined with respect to local geodesics, which in principle contain information about the whole past-causally connected universe. And that is what Einstein means by "inertia depends upon the g".
An accelerometer tries to follow a local geodesic, which is defined by the rest of the universe, so it is not really an indicator of "true motion". I don't think this is a fully resolved issue in modern physics.
We can just accept that acceleration is special, but I think that goes against what Einstein set out to do.
I guess I don't understand what you mean by "true motion". In my mind there is "inertial motion" (following a geodesic) and "non-inertial motion" (not following a geodesic), and an accelerometer can pick out the difference between the two. What does "true motion" mean? I'm not clear on why the history of the universe or other matter in the universe is relevant. I'd figure the concept of "inertial" vs "non-inertial" is a completely local concept anyway.
@@eigenchris If I understand correctly true motion would be motion that an observer can detect on themselves. It would mean there is an experiment the traveling twin can do to determine that they are truly the one that is moving while earth twin remains stationary.
According to newton this experiment can be done by rotating a bucket of water or a spring with weights on it because inertia will distinguish the rotating state from the non-rotating one.
But according to Mach, who influenced Einstein immensely, that inertia is induced by the rest of the matter in the universe and so needs a reference.
According to Mach, without this reference you can not distinguish inertial motion from non-inertial motion without _some_ reference and the calibration of the accelerometer counts as a reference to the outside world.
I think this argument is sound but very subtle.
And in GR this reference are the local geodesics, which carry information about the rest of the universe.
I wasn't trying to question any of your assumptions, btw. I just wanted to maybe help you understand Einsteins argument.
I think your way of distinguishing inertial from non-inertial is perfectly fine for the context of your videos.
From the wiki:
However, Einstein was convinced that a valid theory of gravity would necessarily have to include the relativity of inertia:
So strongly did Einstein believe at that time in the relativity of inertia that in 1918 he stated as being on an equal footing three principles on which a satisfactory theory of gravitation should rest:
The principle of relativity as expressed by general covariance.
The principle of equivalence.
Mach's principle (the first time this term entered the literature): … that the gµν are completely determined by the mass of bodies, more generally by Tµν.
In 1922, Einstein noted that others were satisfied to proceed without this [third] criterion and added, "This contentedness will appear incomprehensible to a later generation however."
It must be said that, as far as I can see, to this day, Mach's principle has not brought physics decisively farther. It must also be said that the origin of inertia is and remains the most obscure subject in the theory of particles and fields. Mach's principle may therefore have a future - but not without the quantum theory.
- Abraham Pais, in Subtle is the Lord: the Science and the Life of Albert Einstein (Oxford University Press, 2005), pp. 287-288.
Very good 👍.
I find the 1st 5 minutes of this video a bit mindblowing because am i right in saying that if you have:
Earth light years
🌎 ------------------------------------------>------------------------------------INFINITE AMOUNTS OF MASS
then the gravitational pull towards the INFINITE AMOUNTS OF MASS would be immensely strong even though we're a long way away?
I think it must be true because Fraser Cain said if you have 2 atoms a gazillion light years apart then eventually they will be gravitationally drawn to each other.
& so I think maybe if you have an infinity times that atom then the curvature must be infinity times stronger
& so if you have infinite mass either side ------------------- infinite mass then space is like a tight guitar string.
Like we're getting pulled in every direction infinitely like we're in a BH
When I studied the history of math and saw so many famous mathematicians make mistakes reasoning about infinity, I decided to never make conclusions about infinity.
5:54 lmao fair enough
Finally
Imagine winning a Nobel prize by an accident
That's the stuff 😁👏🏻
Neutron decay Cosmology
Neutrons which contact event horizons become the vacuum energy for one single Planck second then reemerge in lowest density points of space where they decay into amorphous atomic hydrogen. As neutrons do.
The decay from neutron to proton, electron, neutrino, from near point particle to one cubic meter is a volume increase of 10^45. Expansion. Dark energy.
The decay product, amorphous atomic hydrogen, doesn't have stable orbital electron so can't emit or absorb photons. Dark matter.
In time the hydrogen stabilizes and follows usual evolution pathway from gas to filament to proto star to star to neutron star until in the distant future it is again about to contact an event horizon.
The universe is steady state evolving locally..
Events horizons acting as energy pressure release valves venting from highest energy pressure conditions to lowest energy points of space.
From aggregated singularity to distributed dispersed diffuse.
This is done using topology..
Neutron decay Cosmology
There physical process, path of least action, solution to black hole paradoxes, dark energy, dark matter and critical density maintenance.
First
Maybe try understanding it before teaching it....
Is there a part of this video you find not satisfactory?
well he did teach it. The only part he didn’t understand was Einstein’s paper, but he didn’t teach that part; he taught something else. Since he didn’t understand it, he taught something else as a replacement. He literally did as you said.
@Cre Henge you don't seem to be very understanding of his understanding