The 4th Dimension in Relativity isn't Time - it's Space.

[@IndependentPhysics]

Unfortunately special relativity doesn't actually tell what (ontological) time really is, but rather how light behaves in time with respect to space.

[@santerisatama5409]

Ontological mathematical time is at least bidirectional. That's ontological necessity for mathematical physics. More generally, time is the flow in which all forms appear, endure and disappear.

[@realdragon]

In special relativity you can depict space-time as Minkowski space-time

[@landsgevaer]

So what does - or even can - 'ontologically' tell what time 'really' is? It sounds like philosophical sophistry.

[@luminousfractal420]

does time need to exist for 1+1 to equal 2? or is it just as it is.

[@darrennew8211]

@@luminousfractal420 1+1=2 is a description of some sorts of isomorphisms. One apple plus one apple equals two apples, but only if you ignore all the differences between those two apples. One apple plus one orange equals two fruits but only if you ignore they're different species of fruit. One heap plus one heap is still one heap, because heaps don't behave the same way as integers do. 1+1=2 is not in any way a universal law, except in the realm where it is.

[@erenakker3962]

I enjoy your videos; however, they often conclude with a cliffhanger that the subsequent video neglects to address.

[@dialectphilosophy]

We work on multiple tracks -- math education, philosophy exploration, and our own research. This means that the period between a video and its follow-up is generally pretty extended, spanning a range between 6-9 months. We understand this can be a frustrating experience as a viewer, but until our production team expands (sometime soon, hopefully!) this is the best we can pull off. So we do greatly appreciate your patience and continued viewership.

[@justaguy3518]

feels like a never-ending introduction

[@curtisharwood6626]

@@dialectphilosophy boooo

[@1.4142]

The video quality and audience varies too much. The Riemannian Geometry video had enough math and was very info dense, but this one is just repeating the same sentence different ways.

[@a.markuddr285]

​@dialectphilosophyQuestion: Has the twin paradox not been solved yet and you are waiting for solutions or are you still studying the subject and will post it on the channel?.

[@MrTheophilus71]

The dimensionality argument is bogus. We don't need to write the metric interval in terms of spatial units. It can also be represented entirely using temporal units. Another convention for the space-time interval is all time units so that DS squared equals DT squared minus dx squared divided by the speed of light squared for each spatial dimension. We then measure distances in units of years. Yes. That's the well known light years for distance. So using those units we could say space-time is really a time time continuum

[@dialectphilosophy]

Hey there, thanks for watching! As we've pointed out to several other commenters, you can indeed reframe the manifold in terms of entirely temporal units; however, the key issue is that, for whatever units we choose, each of the four dimensions of the manifold will always have the same units (either all space or all time.) This is because the manifold does not contain information about relations between space or time in any way, rather, as it is defined, it contains the relations between the spatial extension of measuring rods and the spatial displacement of a light beam (all quantities which we could express via temporal durations, if we wished). Since these relations are in turn determined by the physical behavior of light (namely its source-independent speed through space) the most important aspect of the manifold structure is the negative sign in the metric, which entails how physical processes relying on light-speed signal transmissions cannot progress as quickly if the constituent components of those processes are in motion relative to the propagation of signals. (This is most readily understood in the case where in a propagating light beam forms the hypotenuse of a right-triangle in a moving light clock.) The key takeaway is to not think of the spacetime manifold as a fabric or container of our reality, but rather as a data structure detailing the relationship between the motion of an observer's frame of reference and the causal propagation of light therein. Such an understanding allows us to see that what abstract quantities like spacetime curvature are actually telling us about, is merely only the relation between the strength of gravitational forces at any location about a mass and the effect of those gravitational forces on our physical measuring instruments (e.g. measuring rods and light clocks).

[@JH-le4sd]

Velocity*time isn't "space" in the way we usually think of it (i.e. it isn't *extension*), it's *displacement*. Problem is displacement and extension have the same units, and this had led to great confusion that they are thus the same thing. This video does a good job elucidating this.

[@garychap8384]

I'm having a problem understanding the distinction between 'extension' and 'displacement'. Surely every axis is 'displacement'... when considered perpendicular to the other three. No? Could you elaborate on the distinction?

[@Bubba_Grimm]

​@@garychap8384 ​ they seem to be opposite sides of a geometric surface, observer perspective dependant. Displacement from an opposing view is extension. I'm convinced that any conscious observer must operate in a higher dimension than that which it experiences. Hence dimensions themselves are relative! Possibly? Who knows!? But I think electromagnetism + gravity and 3 + 1 dimensions is not an insignificant pattern. I think we are describing our total experience, not the universe when we try to combine different ways of observing. Like there is only gravity in terms of electromagnetism, or electromagnetism in terms of gravity; space in terms of time or time in terms of space. Trying to combine them is like trying to force the two sides of a coin onto one side. But since our experience is an illusion of both sides of the coin simultaneously, we are tempted to assume everything must be contained to one side... 🤷

[@joshuastanlick6217]

Time only becomes spatial at relativistic speeds. The cross-section of all things outside your reference frame converge to zero at the speed of light. In dimensional terms, you cease to travel along the geodesic in the dimensions that make up length, width, and height, as they all collapse in to a temporal medium for the conveyance of information at relativistic speeds. This is explored from the topilogical perspective. Velocity, thusly mass-energy concentration, is not only key to spatially traversing any dimension, but is also the key to graduate to higher-dimensional reference frames; think of velocity as the scalar for the ratio of compression that the 3 (spatial at only non-relativistic speeds) dimensions encounter, where at the speed of light, all three dimensions converge into a non-spatial cross-section outside of your reference frame, and your cross section in time becomes infinite. Note that since velocity truly referes to the geometries of the condensate of space-time and your relative angular momentum in those geometries geodesics, you don't necessarily have to "go" at the speed of light. You can also be a black hole at Rsh, folding all spatial dimensions into a singularity, as mathematically, the spatial collapse is still achieved as the energy density at Rsh is such that the escape velocity is C for that object, while the object itself may have a relative angular momentum to you that is well below C. Still, for the sake of clarity, it may be more intuitive to imagine an object that is not a black hole, like an abstract space vessel traveling at C, to use as a framework for understanding. It is possible to alter the geodesic equation to incorporate temporal gradients that induce spacetime anisotropies, and to incorporate a temproal field that causes these inducements with something like Ag as a coupling constant when reconciling the stress-energy tensor. As well, think about how a photon at C perceives depth; the answer? It doesn't. Distance becomes as inpercievable at relativistic speeds as spatial traversal of time (from point A to point B, and point B to point A both forward and backward as opposed to mere incidental dilation) is to us at sublight speeds. The faster you go and the closer you get to the asymptote at C, the closer and closer you get to graduating to the native reference frame of causality (time), and only at 300KM/S, you will have the ability to traverse time as you would traverse the 3 dimensions at sublight speeds.

[@LukeLAMMan]

Thank you this is a useful conceptual distinction.

[@JH-le4sd]

@@garychap8384 Displacement involves motion, if I take a marble at position x and move it to position y, I have displaced it. But if I had a rod which had one end at x and the other at y, then the length of the rod is an extension. In both cases, the units would be those of distance, but they are not physically the same kind of objects/events.

[@ShimrraShai]

Also, 6:08ish - that is an interesting question as to whether it is about "spacetime" or about "dynamics" - indeed the whole point of the Lorentz symmetry is that it is a symmetry _of dynamics._ So we could ask whether dynamics defines spacetime or spacetime provides symmetries dynamics is required to respect. In fact, this is not trivial, because we could easily write up on paper laws for an imaginary universe where some laws follow Lorentz symmetry and some break it. (The thing as a whole would then break Lorentz symmetry; the point is that it is not illogical, just not so-far what the universe we live in seems to operate like.) Hence, this is a matter which has to be settled by experiment, i.e. to ask whether the real world actually works this way. And this brings up a crucial point, which is that where you say here about it having to do with "light" - indeed, that is an experimental contestation, i.e. does the behavior of these light clocks and the like translate to other matter? And in fact it does (so far), it translates to _every_ known physical law, not just light moving but electrons moving, even particles decaying - every "temporal" process seems to follow the Lorentz symmetry. And I would argue it is _this experimentation_ that is the best "buttress" for the "spacetime interpretation" of the Lorentz symmetry. But ultimately, yes, spacetime is a modeling entity, and there may be other/better/different ones. But still, there is nothing _wrong_ with it in any formal/logical or, so far, any empirical sense, even if it may make some sense to be aware of this.

[@chrimony]

One day I will meditate on this until I reach enlightenment. In the meantime, in the bookmarks it goes.

[@manueljohn456]

Nice. I've always had the intuition that physics and, well, metaphysics/spirituality meet at some points, and the likely candidates for me are always the points where physical theory breaks the brain. Singularities are also a good candidate, or the question of what happens if an observer approaches the speed of light (which is its own type of singularity I guess). Pro tip: pondering this while being pretty high

[@Astrophysics96]

Your description I think is attractive but at he sametime does not add something deeper or new to our understanding to nature or the theory it self. After all how you see it, is just a matter of taste. I believe your way of viewing it involves us with the philosophy of physics and the world views there are like realism, instrumentalism, anti realism...etc. That is how I understood it before, and that is why I see it fine: We are constantly traveling through time at the speed of light. By dealing with time as a kind of distance or space dimension, we can conceptualize and measure it more easily and fit into the equations. This approach allows us to incorporate it into equationstoavoid comlexity that would be faced if there was a different units in the equations. After all when we try to understand something, we often compare it to what we already familiar with, and look for patterns to make sense of and describe it. (Just like when gravity viewed as force it worked well, while inaccurate, and now we view it as curvature, its a deep precise description but its the way we view and see it not the true nature of it! curvature descrption seems to fit the behavior of gravity, it does not neccessarily means that is how it is! Gravity might be a particle that exhibits features that makes it look like curvature!). The point is we dont know. we onlycn describe according to our cognitive nature and categories we use to view the world. That is why I think viewing time as a space dimension to be fine and work very well. It makes time easier to deal with mathematically and conceptually. If we look at the challenge with quantum mechanics is similar: there are no direct analogies from everyday life to help us intuitively grasp and understand its phenomenon, which makes it harder and almost impossible to follow intuitively.

[@ralfpaul4244]

Wenn Ihr das Problem so angeht seid ihr zum Scheitern verurteilt, denn das was wir als Dimensionen bezeichnen, sind genau genommen Multiplikatoren der Energie. Masse kg = 1D, Bewegung km/h oder m/s² = 2D, Raum km³ = 3D und Zeit h⁴ = 4D Wäre nach diesem Prinzip die Zeit nur eindimensional, müssten wir sie der Masse gleichstellen und addieren. Es gilt aber jede Energie kann nur eine Dimension besetzen und muss sich von anderen Energien unterscheiden um zum Multiplikator zu werden. Die einzige logische Erklärung wäre sich hier an Hawking zu orientieren, welcher Raum und Zeit als Funktionen der Energie betrachtete, welche vergleichbar gebunden sind wie Masse und Bewegung, da sie ihre Energie aus der gleichen Quelle, der Bewegung beziehen. "Wenn wir die Bewegung (km/h oder m/s²) der Massen (m) in einem bestimmten Bereich als geleistete Arbeit betrachten, dann ist der Raum (km³) die Ausdrucksform!" (Hawking) "Wenn wir uns allerdings die Längen (km) aus der Bewegung (km/h oder m/s²) leihen um den Raum (km³) zu beschreiben, muss das was "übrig" bleibt (h*h*h²) zwangsläufig die Zeit (h⁴) definieren. Somit hätte die 4D Raum-Zeit 7 Multiplikatoren, welche für ihre Krümmung einer Energie der 8.Ordnung (8 Multiplikatoren) benötigen würde, von der wir nicht Mal im Ansatz eine Vorstellung haben. Auf Einstein umgelegt wären wir dann bei Masse (m) * Umlauf^2 (c1)² * Rotation^2 (c2)² * Gravitation^2 (c3)² = Masse (m) * Raum (m) * Raum (V) * Zeit (t) oder m (kg) * c1² (km/h) * c2² (km/h) * c3² (km/h²) = m (kg) * V (km³) * t (h⁴) oder E=mc² bei c

[@JuliusBrainz]

What you say here is nothing new. It does not take 12 minutes to explain that c is there for dimensional reasons, that ct is the distance that a light beam travels. But just because it has a dimension of length does not mean it is space. Like a wavelength is not space, like the length of arc is not space, distance travelled by an object is not space. And it is not illiteracy to call ct the time dimension. Because we need to distinguish the nature of the dimensional. Spatial dimensions have the same nature, while the time dimension does not. So why should we confuse ourselves by calling space. I refuse to call a distance travelled by an object the Euclidean space which is what we usually call space in flat case.

[@TerranIV]

Time is treated as a literal fourth dimension in GA, whether you want to accept that as "real" or not is up to you, but the authors are not going off the rails in treating it literally. Of course the time dimension is different, but only in the respect that you can only move in a singular direction. There is no reason to think it is somehow "less real" other than your own personal opinion. While your opinion is fine, it is not science.

[@Blox117]

@@TerranIV its real in the same way that centripetal force is real. its brought about by a difference in position, which is what time is.

[@ralfpaul4244]

Energie ist die Möglichkeit der Veränderung aber nur die Begrenzung schafft die Notwendigkeit der Evolution, was auch für die Energie gilt, wobei LG eine der wichtigsten Rollen hat. LG ist nicht nur eine Geschwindigkeitsbegrenzung, sondern bestimmt auch die Größe von Objekten vom Atom bis zum SL. LG Beschreibt auch wie viel Energie Raum aufnehmen kann hoch*breit*tief=LG*LG*LG=Bum, der Raum kollabiert und zerfällt wieder in Masse und Bewegung, aus deren Zusammenwirken er ja entstanden ist, was einer der Garanten ist, dass dem Universum NIE der Baustoff ausgeht. (vereinfacht) MfG P.

[@Elrog3]

Your critique was good, but you stopped halfway to cling to the status quo. This extra dimension is just a mathematical object. That's it. Its not time.

[@ralfpaul4244]

@@Elrog3 Das Was wir als Dimension bezeichnen, sind physikalisch betrachtet nur Multiplikatoren der Energie. Die Regeln sind hier einfach. Die Energie muss sich unterscheiden um zum Multiplikator zu werden. Die Kräfte dürfen sich nicht ausschließen. Die Maßeinheiten (alle) müssen abgebildet und nachvollziehbar sein. Jede Ausdrucksform der Energie (Masse, Bewegung, Raum, Zeit) kann nur eine Dimension besetzen oder muss einer anderen Kraft gleichgestellt und addiert werden. Nach diesem einfachen Prinzip, lassen sich fast alle Theorien prüfen und zeigen auch die Probleme auf. Eine 4D T/Raum-Zeit dürfte nur 4 Multiplikatoren haben. Da der Raum (km³) aber schon 3 hat bleibt für die Zeit nur 1 Multiplikator und müsste mit der Masse addiert werden. Nach Hawking gilt aber die Zeit als die höchsten Energieform, weshalb nur die nächst höhere, also die 4.Dimension bleibt. Dann hat aber eine mögliche Raum-Zeit, welche von uns als Expansionsenergie bezeichnet wird, mit km³/h⁴ sieben Multiplikatoren. Sie sehe also das eine Theorie ohne Regeln nicht funktioniert. Masse (kg) * Umlauf^2 (km/h) * Rotation^2 (km/h) * Gravitation^2 (km/h²) = Masse (kg) * Raum (km³) * Zeit (h⁴) oder E=mc² bei c

[@Nobody_114]

There is 3 major flaws to Einstein's "light shined from the floor to a reflective mirror on the ceiling" thought experiment (ETE): 1) As you approach the speed of light (C), the light/laser does not return to its source on the floor. Why? Because the light wave is emitted in concentric circles at C independent of the observer's motion. In fact, it can be proven, that as you approach C, the light/laser will return to an offset position on the floor behind the source in the direction of motion (DoM). 2) For the same reason as above, the *distance* light/laser travels from the source to the ceiling increases before it hits the ceiling. This means that it is not time that is dilating, but distance increasing instead. To ensure the same distance travelled by the light/laser to calculate "time dilation" as ETE and that the light/laser returns to the same point as the source for the moving observer, you would need *vertical/perpendicular contraction* to the DoM in addition to the "Lorentz contraction" in the DoM. There are already papers published on this subject. 3) Unless one of the 2 observers accelerates or decelerates and meets the 2nd observer, none of the time dilation results can be verified, even for particles. This means, that the examples of particles taking longer to react to something (like muons coming to closer to earth's surface), or particles having a higher energy at velocities closer to C, can only be detected when the particles collide and react, at which point they are no longer moving close to C, which negates time dilation as the reason for the differences in reaction/collision energies while they were moving near C.

[@TerranIV]

I know that anyone can decide to call a lot of things a "dimension," but time is a literal dimension in GR (as in time is on the same level as the x, y, z dimensions). I think it is perfectly valid to say that it is a physical dimension, but it is NOT just the dimensions between the ends of the light clock. Remember, velocity is relative so you can't say which observer is "moving," all you can say is that one is moving relative to the other. The observers are displaced in TIME as well as distance. If you want to call the Time dimension "spatial" (i.e. "the fourth dimension") that is exactly what Einstein was getting at. To your point, ALL time may not be from motion in that dimension (i.e. a ticking of a local clock is not necessarily spatially displaced), but you could frame it that way, and most physicists would probably argue that time itself is caused by all matter moving at a constant c velocity in a direction of that spatial dimension.

[@se7964]

No, Einstein defined time as the measure of spatial displacement of a light beam in a given frame divided by the speed of light. You can read this directly in his 1905 paper. That means it’s just a mathematical construct, a relative measure of time and not the “real” or absolute time which we might consider a “dimension” within which we exist.

[@rondai4019]

Well, come back to relativity again after claiming it is wrong in two videos in 2024?....sad.....how could anyone going back instead of moving forward? Really sad....

[@richsalinas]

I don’t have a physics education or background so most of what dialect is explaining is beyond my comprehension. I do however enjoy when a new Dialect video pops up on my feed. 🍻 cheers to the team on another great vid. The animations keep getting better!

[@seneca983]

What if one would posit that, on the contrary, relativity actually has 4 dimensions of time and no space at all? On the time axis you have t and on the "space" axes you have x/c, y/x, and z/c which implies units of time for them as well. You might argue that you've only seen (ct, x, y, z) in textbooks, not (t, x/c, y/c, z/c). However, that's just an arbitrary convention and not any better than what I've used in this comment.

[@NightWanderer31415]

The video is quite frankly nonsense and/or semantics all the way through.

[@nHans]

Sure. But that fourth "spatial" dimension - obtained by multiplying _t_ with _c_ - is not like the other three. In fact, when I studied Special Relativity - admittedly a long time ago - the fourth dimension was represented by _w_ = _ict,_ where _i_ = √(-1). So while _x, y,_ and _z_ represented "real" 3D space, _w_ was "imaginary" space. The math worked out just fine. The Lorentz transformations - space contraction, time dilation etc. - came out naturally and beautifully as rotations in this 4D space comprised of _x, y, z,_ and _w_ = _ict._

[@dialectphilosophy]

Yes, the fourth spatial dimension requires a different sign because it encodes the spherical limit in which light can propagate over any small time interval dt. This is most easily understood by recognizing that the distance light travels in a moving frame forms the hypotenuse of a triangle with the other spatial dimensions. Thus, from the pythagorean theorem, a moving observer's "proper time" cdt-prime (the distance between horizontal ends of their light clock) is related to the spatial distance traversed by a light beam over a small time period, cdt, via (c*dt-prime)^2 = (cdt)^2 - dx^2.

[@-_Nuke_-]

What I understood from this video is that we have 3 dimension of space, 1 dimension of time and a "4D" manifold that plots an observers 3 dimensions of space plus how close to the speed of light he is traveling as the 4th "dimension". Just like a dimension that maps heat... For example... That is very interesting; I don't know if that's what Einstein had in mind, but still a very interesting idea... This would then mean, that we have no intuition to what is happening, we simply accept reality as we measure it and then we simply plot it in this manifold... Which is... Exactly how science works... So nothing surprising there! Could this give us any real insight? This is literally equivalent to the electric or magnetic fields... That are just us plotting what we measured on a manifold... This is crucial science, but has zero intuition about what magnetism or electricity are... Just like we get no intuition by Newton's gravitational field... Again, Im not surprised by that - I have said it many times - science doesn't care about why things work, only how; Im most excited to find out more! Dialect is maybe the BEST channel on the philosophy of relativity right now in the ENTIRE internet. Thank you guys, keep blowing our minds away! 😂

[@thetelescreen372]

something i think you are tripping up on in a lot of your videos is representations of structure vs structure itself. We use minkowski space as it is a representation of a symmetry we have observed in nature (lorenz invariance). All other representations that preserve this fundemental structure are ontologically valid until we get more data that precludes some of them. If we believe reality to be real and physical in any meaningful sense, then we should be primarily concerned with its structure, not this or that representation.

[@thetelescreen372]

said another way, relativity is about what transformations am I allowed to do to the state of the world that leaves the evolution of that state unchanged. what transformations, therefore, leave the laws of physics invariant. I'm inclined to agree with Einstein that whether you view the geometric interpretation as physically real is a matter of personal preference, what matters is the invariance, as that is the underlying structure.

[@rongarret1]

You have quit a talent for obfuscating simple ideas. You have also missed the point about the time dimension. It is not the *units* that make the time dimension privileged, it's the *sign* . Space and time are interchangeable via the constant c just as mass and energy are equivalent via the constant c^2. But equivalence via a multiplicative constant does not mean that they are the *same* . It's pretty easy to distinguish mass from energy, just as it's pretty easy to distinguish space from time, and this *is* reflected in the math: the time dimension has the opposite sign from all the space dimensions in the relativistic invariant.

[@dialectphilosophy]

You're incorrect here. The sign does not "convert" a spatial dimension into a time dimension. Rather, it indicates that the "space" of one observers fourth dimension (the rod laid along the length of their light clock) is related to the displacement of light between endpoints of that light clock from another observer's frame of reference via the pythagorean relation. In most succinct terms, the negative in the metric indicates that no causal information can propagate faster than the speed of light. The essence of stupidity lies in the inability to make the appropriate distinctions in the appropriate places. When we say a word like "time", we precisely mean something which does not share in the features of space. Simply because we can choose to represent relative measures of time via spatial displacements does not mean time and space are the same thing/interchangeable. Indeed, if we are to advocate for such conflations, then certainly we cannot evade charges of mental deficiency! See our video "Einstein was Wrong About Time" for more on this subject!

[@VectorTwelve]

​@@dialectphilosophy You really ought to take better care in how you respond as someone trying to produce comment. After reading that response, I literally don't care about the information or it's accuracy. Sometimes silence is the best option.

[@zohn-yq6wx]

If you want to visit and see the space-time manifold(the fabric of space). Then (don't) don't do this , E=MC^2 . That only gets you to 99% of C. Because +E light has photons, photons have mass and nothing with mass can travel faster than C. You must do this instead. If you want to visit and see the space-time manifold(the fabric of space). Look +E🔼(-E)=(-M)>C^2 you see, you see. Remember nothing with mass(M) can travel faster than C. You must do this +E🔼(-E). The 🔼= delta, a science term (to change to convert something). We must convert change +E to (-E), and surround a space-ship with a negative (-E) bubble negative energy. And, the ship's mass(-M) and weight drop to zero or below zero(negative). As a result, a massless weightless ship can travel faster than C. And you can visit and see the space-time manifold(the fabric of space). And what would you see ? Maybe, the flat road of space and time for all eternity, as long as you travel above C you see.

[@EricKolotyluk]

It's been over a century that Einstein published his theory of General Relativity, but few people grok what it means. However, the explanations of General Relativity continue to evolve... those people evolving those explanations are working to create a world where more and more people can appreciate such beauty of nature, and they should be applauded for trying to build such a beautiful world of knowledge and understanding.

[@99foxlord]

I grok this.

[@dariuslegacy3406]

I think some people have taken the whole "physics is the language of the universe" as gospel and forget that a lot our models are simplified representations of things and their relationships as we measure them, not as they actually are. It's clearer with the Copenhagen convention but even Newtownian and Einsteinian Physics ultimately rests on conventions for interpreting equations. The Post-modern in me wants to say the only things we can meaningfully say exist are those things that are in the domain of our experience, any concept of time outside what we can try to measure with a co-moving clock (psychological time) is arbitrary. Though I do find the 4 dimensions of Space-space as odd yet unique in its framing of relativity, which can only help deepen our understanding.

[@3rdPartyIntervener]

"physics is the language of the universe" - wrong "physics is the language of modeling the universe" - right

[@noavailablenamesatall]

@@3rdPartyIntervener In the limit, this is one and the same, because purely formally there must exist a model that exactly corresponds to our universe.

[@NightWanderer31415]

This video is extremely misleading and suggests that the author doesn't understand Relativity at all. The distinction of 'space' and 'time' dimensions isn't a matter of units: it's intrinsic to the theory and related to the signature of the metric tensor g (-+++ in the usual convention). In Minkowski spacetime, g=-c^2dt^2+dx^2+dy^2+dz^2. In general, a four-vector v is spacelike if g(v,v)>0, timelike if g(v,v)

[@se7964]

The metric just says that the fourth dimension (the spatial displacement of a light beam) is related to the other dimensions (the measuring rods) by the Pythagorean relation, which is most poignantly exhibited when one examines the distance traversed by a light beam in a transversely-oriented light clock. To the contrary of what you’ve written, I’d say this author understands relativity better than most people, the majority of whom conflate mathematical language models for reality itself. Einstein himself greatly disliked the purely geometric interpretations of relativity, as he thought they were not realistic. So yes, the fourth dimension is different than the other as specified by the metric, but this is because it defined as such, that is, defined via the displacement of a light beam. What the signature of the metric this tells us is that a light beam can only traverse so much distance in so much time, so any relative measures of space or time constructed from that light beam are therefore similarly confined.

[@NightWanderer31415]

@@se7964 "So yes, the fourth dimension is different than the other" (quoting directly from your reply) = the end of this guy's video. Time and space are different. Einstein disliked the geometric interpretation of Special Relativity at first, but then fully embraced it later when he developed General Relativity. The whole point of Einstein's theory is that gravity IS geometry; to not understand this is to not understand Relativity.

[@landsgevaer]

You can call that dimension space, but then it is a special kind of spatial dimension that is different from the other three. And that different type of dimension we call time. Aaaand we're back.

[@dialectphilosophy]

As stated in the video, the important distinction here is that this last dimension of "space" (more truthfully, displacement) is merely being used to *represent* an observer's psychological and subjective notion of time. Representation is not reality and indeed, the essence of stupidity and confusion lies in the inability to draw the appropriate distinctions in their appropriate places; hence if we fail to recognize that an observer's psychological notion of time is not what we mean when we say "time itself", then certainly we cannot evade being charged with such deficiencies!

[@NightWanderer31415]

@@dialectphilosophy you seem to confuse "time" as understood in the context of Relativity with the perception of the passage of time, which is related to entropy and is not part of Eintein's theory. The closest concept to this in Relativity would be the proper time measured by an observer; this is a geometric property of the observer's worldline and it has nothing to do with Psychology or conciousness at all. The "different spatial dimension" isn't merely used to represent an observer's subjective notion of time: it's intrinsic to the causal structure of the spacetime manifold, which in turn comes from the signature of the metric tensor and allows you to clearly idenfity which vectors are timelike, spacelike and null, i.e. to clearly distinguish 'space' from 'time'.

[@Kalumbatsch]

@@dialectphilosophy It's not "psychological and subjective", it's objective and measurable with great precison and accuracy. The rest is meaningless word salad. Try again.

[@michaelstark2832]

@@dialectphilosophy So, we are inserting psychological and subjective notions into our attempts to describe reality? let's see...where does that take us back to...certainly before the Scientific Revolution and probably before Classical Greek geometry

[@ralfpaul4244]

Nein Jede Dimension ist in der Physik Energie und muss sich unterscheiden, damit sie zum Multiplikator werden kann. In der Geometrie haben wir Längen (km) welche wir normalerweise addieren, denn erst durch ihre Zusatzfunktion als hoch*breit*tief, werden sie zu Multiplikatoren. Genau aus diesem Grund ist Masse auch 3D (geometrisch) so da man mehr wie eine Bewegungsform an sie binden kann. Mit (Umlauf

[@ShimrraShai]

This is an interesting way to look at it, but why exactly _must_ we understand the 4th (or 0th, actually in typical formalism) component of the space-time manifold as this spatial distance you have described? Why _can't_ it be time? NB. you don't need to imagine adding extra physical rods into the clock either. Simply saying ct is the summed distance of the bouncing light beam is sufficient - it's the same way as interpreting what the odometer measure on a car means if the car were repeatedly driven back and forth between the same two spots over and over. There is no need for stacking up roads to make it clear. It's not that you can't think of it that way, it's that I don't understand what the benefit is you are seeking in this regard and you will need to explain that. Another NB: you can indeed do it as (t, x, y, z) where t is literally time with units seconds and (x, y, z) have units meters. No "ct". It is just that when you write down the proper distance or proper time you have to insert the "c" factors where needed, e.g. dtau^2 = dt^2 - 1/c^2 (dx^2 + dy^2 + dz^2) (proper time) dsigma^2 = (dx^2 + dy^2 + dz^2) - (c dt)^2 (proper distance) and note "c dt" is just the infinitesimal increment of ct, so there is no predictive difference. But note - also we have s/c _hidden_ in the _first_ equation, by taking ds^2 := dx^2 + dy^2 + dz^2 (*) so that dtau^2 = dt^2 - (ds/c)^2. Hence your logic here would also suggest we should try to ask what _space_ means in terms of _time_ measurements - i.e. argue what you are arguing here but think not "what's up with ct?" but "what's up with s/c?" - and I'm sure we could do that too, but that only further serves to bolster the interchangeability picture at work here, not scuttle it. (*) perhaps confusingly, most(?) authors seem to use ds to refer to dsigma above; I tend to feel this way makes more sense by keeping things distinct, i.e. spacetime distance-as-space sigma vs. spatial distance s only.

[@minotaurbison]

1:42 MOOOOMMM the C fell off my axis again!

[@samuelirungu6454]

😂

[@dougr.2398]

The metric is neglected here. It is a pivotal part of the dimensionality and how it combines with the spatial dimension. Sometimes the metric is represented by i, the square root of minus one. That makes the axis i • c • t. Or ict.

[@dougr.2398]

Note also that every click actually measures a distance. Each tick of a metronome or pendulum records the distance travelled by the support rod tip

[@shpensive]

Dialect is the uploader I get most excited about. Tackling these questions that are so important and neglected, with the rigor to get it right, the bravery to echew convention, and a beautiful, thoughtful presentation!

[@kamartaylor2902]

I swear General Relativity is harder to understand than Quantum Physics.

[@Rhaxin]

@@SpacetimeTraveler-p4e You can visually picture the concepts so someone with only high school maths can understand them, but the maths of GR are very complicated. So complicated Einstein needed help with it and few who ever lived fully grasp it, even the simplest parts are considered very hard. I strongly doubt you are capable of fully comprehend the maths of GR, even if you're sitting on a Harvard diploma in physics.

[@reedie2000]

This is mainly special relativity. There are many interpretations of quantum mechanics, whereas we’re just supposed to accept relativity. Dialekt is essentially exploring the interpretation of what relativity means on a deeper physical level.

[@davidwuhrer6704]

​@@eigenvector123The bra-ket notation is just a different way of writing Bayes' theorem.

[@0xPulcra]

That 4th param in this may be density of some kind - we have here instead a "density field" of light's transport medium, not some "flow speed" field. Light has to do more to progress as the density increases toward the massive object. In this take, the bending of light is just refraction.

[@dialectphilosophy]

Yes indeed, that is known as the "variable speed of light" interpretation of GR, and it is certainly a completely valid one! While we prefer the River Model for reasons we'll get into down the line, adopting either the River Model or Variable Speed Model is infinitely preferable to the conventional geometric view, since the former models both offer causal, physical explanations for gravitational phenomena, while the latter one does not.

[@0xPulcra]

@dialectphilosophy looking forward to it. The quality of and clearly the effort you put in to author these videos, in terms of content as well as presentation, is impressive.

[@adrianantico3750]

Density of space per unit of distance. Space bunches up for an outside observer when gravity is present and conversely, space separates to observers who are in gravity observing space not in gravity. I suspect the bunching is directly proportional to the volume of space that's displaced by matter. Why it spreads out across space the way it does is likely a design decision bt the architect.

[@ar4203]

​@@dialectphilosophyI enjoy your videos, but it confuses me that someone who is so willing to question the established scientific paradigm, is still so stuck in a materialistic view of the universe/reality? All of your confusion about the physics and relativity(which surely there ARE genuine questions about the philosophic interpretations) actually seem to be rooted in a number of biases based on a physicalist/ materialistic philosophy which ends up causing problems when science is ultimatley coming in every direction towards a subjective/observer based reality. Relativity, quatum physics, all of it breaks down when you exclude conscioussness. Correct me if im wrong, but your problem with relativity seems entirely rooted in materialism. There is a fundamentality of fractal Self recursive universe of conscious being

[@DomainedeFontvieille]

Wrong dear. d=Distance t=Time c=Velocity c=d/t So c*t=d then = (d/t)*t=d, so (4m/2s)*2s=4m so ok, at 1:51 you said c=1 so we have d/t=1 then (d/t)*t=1(d/t) * t then (d/t)*t=1d. simplified then we have d=1d or c*t=1d and not as you say c*t=t . At 1:57 you delete time unit and time unit and after at 1:58 you replace 1d with 1t. It's not correct, wrong. You have to write c*t=1d (spatial units) but not as you say c*t=1t(spatial unit). Regards 🙂

[@jayorag]

When I thought I understood space-time manifold at last.... a new Dialect video appears in my yt feed 😮

[@TheLethalDomain]

And the further you venture through these videos, you're going to understand it even less. There's a common denominator here. Be critical of this person, not just relativity.

[@JZ-uo8cg]

@@TheLethalDomainI’m glad people are finally realizing that about this guys videos. The first once’s I saw were full of glazing comments

[@AlHearn]

@@JZ-uo8cgThroughout history, those who proposed new theories have been criticized. Sometimes they were right; sometimes they were wrong.

[@RaviShankar-1028]

Total nonsense, space & time dimensions behave completely different and to talk of time as part of space dimensions is totally ludicrous. For instance in the presence of mass, space dimensions contract while time dimension expands i.e. time ticks slower. Hence the -ve sign for time as opposed to the positive +sign for space dimensions when calculating the space-time interval "s" between events. Also in General Relativity's Field equation, space & time components are expressed as different entries in the metric tensor, stress-energy tensor, Ricci curvature tensor, etc., due to the intrinsic difference in their properties and behaviour. Dialect keeps making these senseless dumb videos to confuse everyone.

[@The.Renovator]

"...the 4th dimension of our spacetime manifold isn't a temporal dimension at all, it's another dimension of space," but this "spatial" dimension is still *defined* by time, therefore it's a temporal dimension. This seems like circular reasoning. You explained that this 4th dimension defined by the *ct* axis gives us a spatial distance, which isn't incorrect, but we already use this to measure distances _IN 3D-SPACE,_ it's a called a light-year. I'm not exactly sure what the insight here is.

[@nicholasparkin6054]

Hello Dialect, I am very excited for the next installment with the return of the river model (aether is the river?)! I have been considering a lot recently a thought experiment of one looking down at beings on a moving body of water that they themselves existed within but did not experience (a 2d planar body of water with 2d inhabitants), and then learning that these beings had decided that they were moving, not because their space (the water) was moving, but because their space time is curved... This would seem rather amusing to the knowing outside observer. Of course, in some sense they wouldn't be wrong. Take any nD Euclidean space with a flowing space and then can construct a curved "spacetime" that curves in the direction of flow. Equivalent mathematically, but philisophically one has a clear advantage over the other.

[@dialectphilosophy]

Yes indeed, if you have a river that converges towards a sink, since inertial paths along that river will always converge, you can interpret or describe those paths via geodesics on a curved space-and-time manifold. Non-euclidean/abstract geometries in that sense then become very useful for describing such systems.

[@nicholasparkin6054]

@@dialectphilosophy I responded to your reply on the other video which covers this questions as well. I will ust add that this preference for the Parmenidean Spacetime curvature viewpoint over the Heraclitean flow viewpoint has lead many physicist claiming that time is an illusion because it just like another dimension of spacetime. Of course with a flow viewpoint you wouldn't jump to this conclusion. The lack of philisophical thinking in physics today is dire. It's bad in relativity, it's much worse in Quantumn Mechanics.

[@realdragon]

Semantics. And no, general relativity isn't the most abstract thing in physics

[@forTodaysAdventure]

so then what does it mean when i fall through an event horizon and the sign changes on ct?

[@ronrice1931]

The weak nuclear force violates time-reversal symmetry in certain processes, such as a Charge Parity violation, and this asymmetry introduces an arrow of time at the level of particle interactions. There are, however, no known fundamental physical interactions that violate spatial-inversion symmetry, i.e., parity symmetry. That is what makes time something other than simply a fourth dimension of space, and that is why the term 'spacetime' is meaningful whereas the term 'spacespace' is not.

[@Milesian2003]

Of course there is a "real" time--it is the evolution of the Cosmos--its endless chain of causes and effects. We measure Cosmic evolution, we mark it, with a clock. A clock is anything with a highly regular cause-effect physical mechanism. Minkowski's (it wasn't Einstein's!) "space-time" is altogether different---an artificial construction that represents nothing real. It serves only as an abstract representation of the observer's "rod and clock" measurements of space and time based upon the nonsensical premises that space is nothing and light moves at c in every observer's frame.

[@ronrice1931]

@Milesian2003 Thank you for that highly unscientific explanation.

[@Milesian2003]

@@ronrice1931 Define "Time". Isn't a good definition necessary to all scientific and philosophical endeavors?

[@ronrice1931]

@@Milesian2003 I suggest you ask a physicist.

[@Elrog3]

@@ronrice1931 I suggest you don't. Physicists are horrible at communication.

[@jonathandawson3091]

I am sorry, but this is the worse video I have ever seen from Dialect. Your argument is that (ct) appears in the proper time? Really? See the 4th dimension is special, because it has an opposite sign in the metric. That is why it is not same as the other 3 dimensions. So it is very naturally different from space. You cannot call it space. You may as well call it time. This is, in fact, how you can define time in relativity. They are flippable, that is true - you can call the 4th dimension space, but then you'll have to call the 3 dimensions time.

[@basinbeatbox66]

not the argument, space and time in relativity are argued to be non-ontological, so the only difference between the dimensions is mathematical, and since mathematics makes no difference between dimension and space the temporal dimension can be equally referred to as a space, just with different navigation laws. that is to say that if it is valid to call the spatial dimensions spaces it is also valid to the temporal dimension space.

[@idegteke]

What is the definition of “dimension” in the context of this video? I suspect that - like in the case of “life”, “matter” etc. - we don’t really have a solid definition. The scope or reach of any structure of consistent theories (AKA science, unless you have a better definition of it) is ALWAYS limited by the solidity/validity of definitions of it’s core concepts: a chain is exactly as strong as it’s weakest link, as we all know…

[@brianedmonston239]

Yes, the flow model (and the variable speed of light model) is/are very helpful in understanding what is truly happening in special relativity. Thanks for your great videos.

[@thebiggorp1623]

I watched this video and then thought how does this explain light bending around massive objects? The answer I came to was that it doesn’t rather the objects and observer have expanded and the flowing spacetime contracts our observations of it making the light seem bent relative to the newly observed massive object. Apparent gravity is due to the expansion of massive objects, more massive objects expand faster. Spacetime appears to flow to counteract the expansion of massive objects, creating gravitational curvature or tidal forces. Scenario: two planets are separated by a star, and the send a light beam near directly between each other. The light beam not experiencing any time and being arranged to facilitate the interaction travels into the future of one planet from the past of another, the star is not in the light beam’s way because half way into the future it has not expanded fast enough to intercept it. The light beam reaches the other planet which now also receives light from the star which was emitted half way between its prior size and its current so to the second planet it looks as though it must have intercepted, but it hasn’t so instead the second planet perceives the light as having bent around the star. Now since stars actually have volume there is a certain angle at which the light will intercept it but this angle is not just dependent on the star’s radius it is also dependent on its rate of expansion and thus mass.

[@OneLine122]

It's the other way around. Massive objects contract (or try to) and spacetime expand to counteract. So when light comes close, it takes longer to go through it. Since we generally consider time as constant, then we see the light as contracting instead, so it lenses. If we could follow the light instead and keep the distance constant, we would see it as going slower instead and no lensing.

[@ffs55]

SOLID. editing, voice, script, visuals all A+

[@Brockbrooks]

Another idiotic take. Stop pretending you understand what you're talking about. You don't.

[@Bob-Fields]

"Where the heck is this this extra rod supposed go?" LOL

[@-_Nuke_-]

;) A "hole" new dimension mate!

[@zbe8]

ct is used on time axis so that all axes are in same units. You don't need 12 minutes of philosophy to convey that, lol.

[@rudyberkvens-be]

The only really wise comment thus far, including mine.

[@zbe8]

@@SpacetimeTraveler-p4e You can. Just math, graphs, etc. won't be so pretty.

[@dorozi8202]

Cope

[@gregoryallen0001]

y'all are nerds esp the original commentor

[@zbe8]

@@SpacetimeTraveler-p4e Sorry, what? You would usually make c = 1, but you can define it as any number you want.

[@exoyt7575]

With the cube, can you have color without temperature and vice versa? If so would that make it 5D?

[@Voidapparate]

You can add as many dimensions as the number of different information you want to represent.

[@alphalunamare]

High priced printers have many colour channels, some up to 12 or 13. It's just a fact of simple arithmetic. nothing special. All dimensions are not empirical.

[@alphalunamare]

@@SpacetimeTraveler-p4e Thus far, this is true :-).

[@Thedmljd]

Technically speaking C is the limit of the universe and not necessarily the speed of light. Its not that light travels at the speed of light but that light cant go any faster by the limitations of space time. 6:26 from my understanding light is a constant and is the same for all observers so everyone will see the same light travelling at the same speed regardless of movement. There is no rest because rest is relative too, everything is moving. To the person moving with the light clock their clock will appear stationary and the stationary one will appear to be moving there is no true or ultimate frame of reference. meaning that the stationary perspective and the moving perspective are both right.

[@WhatWhy42]

So if time is just the distance of you and what you're looking at then does time go slower if you stare off in the distance? I don't think were solving the mystery of time/distance/relativity until we find the center of the universe.

[@alexdevisscher6784]

Clever way of leading into the Gullstrand-Painlevé metric! I have to admit, for most of the video, I had no idea where you were going with this.

[@soopergoof232]

Exactly. He's giving a prequel to what the cryptic "curvature of space" actually codes for -- it's the *rate of acceleration* of flowing space, aka the 'strength' or FORCE of gravity.

[@The.Renovator]

@@soopergoof232 But this isn't really anything new? Anyone who has studied relativity knows that time and what we perceive as gravity are inextricably linked, and that mass-density has a direct influence on the flow of time.

[@PaulPassarelli]

Nope. A color or a temperature is a field, not a dimension!

[@orbitalshawn0625]

If we divide everything by c-squared it'd be a time-time manifold then-- I don't find that reasoning very compelling. The confinement plus light as a truer equivalent of time is an interesting idea. It feels like that is more than just an interpretation though. The mirror/confinement theory has to be described in detail to suggest that light plus confinement is a superior notion to time. Maxwell equations is basically all I can imagine and that already has time ...

[@Kowzorz]

One issue with "dividing everything by c-squared" is that those values don't have analogous measurement values. In the space-space manifold, dimensional values relate directly to the actual length-of-rod value we observe. When we divide by c-squared, we lose that model-reality parity without due justification. Regarding Maxwell's equations, the reason the em-wave solution is uncanny is because it drops the time value as a variable -- it sort of solves for it, generating C. I think it's myopic to just gloss over that ability as merely "already having time" since it's the existence of this C value and the facts generated from such existence which give us relativity in the first place. Personally, I find the idea of "matter is confined light" somewhat compelling, so keeping the literals of spacetime theory to bouncing light rays keeps an uncanny parallel between that hypothesized reality of the small and the theory we already use to describe the large.

[@orbitalshawn0625]

@Kowzorz The spatial components become the time dx/c it takes for light to traverse a distance dx, when you divide by c-squared. Distance measurement can be replaced by clocks and just as it was suggested time can be replaced by distance. In Maxwell equation time is explicitly in the formulation. The vacuum solution is the wave equation-- a time-dependent PDE. The solution involves right and left moving functions, f(z+-ct).

[@orbitalshawn0625]

@Kowzorz Also "matter as confined light" cannot explain fermions. Consider the rotation of spinors.

[@dialectphilosophy]

One can certainly divide by c-squared to arrive a "time-time" manifold, but the key importance is that all the units always remain the same (either all displacement/distance or all temporal intervals). Meaning, that to claim that one unit can somehow become the other (e.g. that space can somehow become time or be mixed with time) requires additional philosophical assumptions which are not justified within the scope of the mathematical definition of a manifold itself. Meaning in turn that to justify such a conflation of space and time, one has to make outside epistemological or ontological appeals (such as by making an argument that how an observer "experiences" time somehow must be "true" time) which themselves are tenuous at best, given particularly that when we say something like "time" we precisely mean something which does not share in the features/characteristics of space in the first place. Hence, we cannot say the spacetime manifold represents our reality, because indeed it does not contain three dimensions of space and one dimension of time, but rather it contains all dimensions of space or all dimensions of time. Indeed the ultimate idea here is to not get hung up on the idea that the manifold is some mystical fabric underpinning our reality, but rather, just like any other mathematical manifold, is a formalistic data structure which preserves the relations between observed gravitational phenomena in a convenient fashion.

[@orbitalshawn0625]

@dialectphilosophy I agree that GR is mathematical (and predictive) and interpretations are permitted as long as they are consistent. I do not think looking at the metric's units and term-dependence is a compelling argument for tensors living on a Riemannian manifold with a diffeomorphism property to be simply data structures. In every addition in physics, units must be the same. I think you'd have to look deeper than the metric for mathematical justification. Lorentz transformations should be analyzed and/or the comparison of SO(3) for time-like to SO(1,1) for light-like. Diffeomorphism also means my metric could look like dudv +dy^2 +dz^2 and we could rescale or even choose proper time over length. I think there is a better way to argue what you've said.

[@sudokode]

The 4th dimension isn't time or space. It's turtles all the way down

[@bart-v]

yeah, but also with some elephants

[@dwaynemarsh6159]

😂😂 yow 😂😂😂 you got me I swear😂😂

[@damnmexican90]

Atoms formed. Lol

[@audiodead7302]

Turdles all the way down.

[@davejo6741]

And a new type of dimension-- down only, because turtles only go down 😅😅😅😅

[@kenfrontline]

This is a partial answer,\It assumes a beam of light as in a focused travel of direction. If or when the light is turned on. It is instantaneously traveling in all directions at once therefore this hypothesis gives only a single answer of a focused beam, not the answer of an omi-directional light which presumes light as infinite. A source beyond light created the light and thus the light to travel instataneous in all directions focused or un focused .

[@santerisatama5409]

Cool, a new Dialect video! The mathematical timespace can be defined by reinterpreting the temporal degrees of freedom as duration that can increase outwards < > and decrease inwards >

[@tamalchakraborty791]

sorry but you just CANNOT plant a number(scalar) alongside (x,y,z) and call it fourth dimension (the example you used with color &temp). the overall coordinate transformation has to follow some predetermined rules . those predetermined rules of coordinate transformation(or diffeomorphism) have to be followed when you're taking an overall 4 dimensions (i.e. 3 dims of space and 1 of another variable)

[@Kowzorz]

There is a context to "dimension" wholly outside the Linear Algebra/Other Maths space that doesn't rely on dimensional unit consistency to prevent the operational rules from breaking. Hell, in computer science, programmers often deal with heterogeneous multidimensional objects containing heterogeneous multidimensional objects.

[@tamalchakraborty791]

@Kowzorz here the context is the 4 dimensional space-time/"space-space" manifold , and in order to understand manifolds deeply in STR/GR you need concepts of diffeomorphisms and other things. So yes, the context matters and this video used one context to explain another.

[@alphalunamare]

What you should have done is to have acted like a String Theorist and demanded that a Colour Cube requires 7 dimensions not 4 , but X,Y,Z & C,M,Y and K. Vector Analysis 101 might placate your confusion.

[@tamalchakraborty791]

@@alphalunamare YOU are the string theorist here, not me.

[@alphalunamare]

@ I just googled 'diffeomorphism' and its nothing but a simple relationship between manifolds of different supposed kinds. It has stuff all to do with your linear algebraic assumptions. So, no need for the 'big' words. As for me being a string theorist, well that is quite funny. I once knitted an 8ft scarf for Cardiff City Football Club (that's not Wrexham btw), it was so long because I hadn't learnt how to cast off. Besides, x, y and z were already scalars so it was a bit mean of you to refuse a fourth for no good reason. Do you have a second perchance?

[@carly09et]

9:07 thanks, I now understand my objections to "proper time" - the neural nominal language switch.

[@pratiksaren6575]

Even though you said that the fourth dimension is defined in a particular way to make things clear and precise, I just wanted to let you know beforehand that because definitions like this are analogical inductive statements, and because we don't have the knowledge of how this concepts are grounded, there is always a mix of certainty and uncertainty, like the problem of the one and the many. So, science definitely has room for infinite growth, as opposed to people trying to think of ultimate theory of reality, which is stupid because of circularity or infinite regress.

[@paulmichaelfreedman8334]

Actually the first dimension is time, and the second, third and fourth are space.

[@luudest]

lol, but true!

[@elmaruchiha6641]

Actually those are mathematically equivalent ,because you can represent each time t from the time-axis with c*t from the 4. space-axis like mensioned in the video. (In physics you do not may add times with distances,but c*times with distances.Math does not care about units.)

[@santerisatama5409]

All dimensions are time, drawing a line and computing resolution is a temporal construction both in ideal ontology and pixelated phenomenology.

[@maha-madpedo-gayphukumber1533]

Right

[@santerisatama5409]

@@SpacetimeTraveler-p4e That makes me wonder where Einstein thought he was living in if not in his thoughts and other senses....

[@markpmar0356]

Even the fine graphic was able to convey the apparent expansion of the mass of the earth outward as flowing space approaches the surface of the earth.

[@mackensieswanholm4996]

if space flows and changes are also not just measuring our perceived distance with a measuring rod and extrapolating it out to the whole universe like we did with time ?

[@The27thS]

The speed of light always struck me as an incongruous physical reference for something as fundamental as the geometry of space itself. It is not intuitively obvious what light has to do with anything and makes about as much intuitive sense as invoking the speed of water to describe why lines are straight. Is it purely empirical that light happens to have the least impeded movement, or would any first principles always derive it's speed? What does speed even mean if this all reduces to distance anyway?

[@alphalunamare]

I am sure one day a cave dwelling craw fish will establish the relationship between the speed of water and the straight line of an eel's attack.

[@minimo3631]

Think of it moreso as the speed of causality, for me it makes more sense then

[@alphalunamare]

@minimo3631 Very true but that just begs another question :-)

[@hareecionelson5875]

you are correct that this is arbitrary, and before Maxwell and Faraday, there was no reason to pick the speed of light as being related to causality. However, Maxwell published his wave equation for light in 1865. he only is that the speed of a wave is given with respect to the medium. The medium for most materials can be picked up and moved at a constant speed (the atmosphere is being dragged along by the Earth) The medium of light the electric and magnetic field. These are not materials you can boost to some constant speed, their magnitudes and rates of change are absolute values for an observer. Maxwell's equation violates Galileo's principle of relativity, unless the speed of light really is constant for all observers. by the same logic, a photon is not an observer, it experiences no time and no distance, you are not allowed to place yourself in the experience of a photon, otherwise you would say your own speed is zero, which violates Maxwell's law. combine that with the fact that nothing with mass can travel at the speed of light, the light is the maximum speed that information can travel at. Gravitational waves also travel at 'c' , so 'c' is very much the speed of sound in the fabric of the universe

[@alphalunamare]

@ Still doesn't explain 'Causality'. It is always explained as a consequence of Special Relativity when in fact it is nothing of the sort. It is a euphemism for 'why is light so fast'.

[@jack.d7873]

Thank you. You're finally getting Special Relativity. No more trolling. But you're still missing the "what time is" part. You're using measuring rods within a 3D space. Whereas spacetime is a 4-dimensional space. You asked where the t-axis should align in a x,y,z coordinate system. Every space dimension is at right angles to it's lower dimension. t Therefore, time being the 4th spatial dimension, is at right angles to our 3D intuitive spatial dimension.

[@midas-holysmoke7642]

If the mirrors in the gravitational field are separated by a distance d perpendicularly to the gravitational field, this distance will remain unchanged for all observers whatever the position in the potential field. However, the rate at which the beam of light goes up and down will be different depending on the position, thus leading to different local speed of light c. However, each observer will define its unit of time as (for instance) the time required to travel the distance between the mirrors, and get the same value c in their unit system. Therefore the curvature of spacetime is not an ontological curvature but rather the curvature associated to the different unit systems depending on the positions and speed. Aether and river model makes a lot of sense with this interpretation

[@petevenuti7355]

Nice operational definition for the purposes of a relativity video. Totally side steps such questions as why does _anything_ happen (time must exist as a something more or nothing would happen, and when nothing happens and there's no interaction of anything it's essentially the same as nothing) And Why is time a constrained dimension? (it always progresses, and in the same direction, it can't take an arbitrary direction like choosing x y or z in three-dimensional space, it can only point to "next") I like the way you were explaining the time dimension as a space dimension, though in my internal model of the way things work I see all 4 dimensions as time but the one we call time is constrained.

[@soopergoof232]

Here's an excerpted comment from another vid about flowing-space -- OK it appears you're amenable, at least provisionally, to the Flowing-Space model of gravity, right? And that's great. That would assume that what's flowing is the "ether", right? But here's the issue -- how does this 19th-century, tenuous, almost-nothing "substance", by the act of flowing, constrain the sun into a stable sphere against the thermonuclear-powered expansion pressure of the core? And how does it power extreme core-collapse events like supernovae and hypernovae? Further, how does this same flowing mechanism power far more energetic and _sustained_ gravitational processes like quasars? Clearly, what's causing gravity by flowing is *pressure-driven*, and it's something other than the 19th century fluffy pseudo-stuff 'ether'. WORDS HAVE MEANING. To communicate in present-day etymology, a new description/definition of the space medium is needed. Maybe something like 'hyperdense subPlanckian Plenum'. And THAT would be the 'stuff' whose accelerating flow is the cause of gravity. Heckamighty, it would also be 'dark matter', its 'granuarity' residing below our sensory and EM resolution, hence 'dark' to our perception.

[@zachariemelanson485]

I don't understand the river model. If you imagine a photon traveling out against the "current" you would calculate that it take the same amount of time as if you imagined simply that time moves slower closer to the mass. But if you calculate the 2 way photon trip, the way towards the mass would be significantly faster as the photon is moving with the current, but the real 2 way time is 2 times the time it takes to go "against the current". Would love to see a video on it in kore details.

[@KaiVieira-jj7di]

Time is the length along a matter world-line, and the rate is a constant, always. There is the Schawzschild-Droste world-time (global time coordiante) but we're in the Gullstrand-Painleve coordinate map where no such world-time is defined. You are correct that the ingoing and outgoing photon speed is different, e.g. at the horizon the ingoing speed in the background coordinates is 2c and the outgoing speed is 0.

[@zachariemelanson485]

@KaiVieira-jj7di Right, I forgot that Gullstrand-Painlevé coordinates is sometimes called "the river model". Both the name and it's visual representation like in this video I find to be very misleading to intuition. It might help in some cases with some of math but a change in coordinate doesn't change what observers "see", and in my opinion almost always makes thing less intuitive than the "simple" Schwarzschild coordinates.

[@KaiVieira-jj7di]

@@zachariemelanson485 I prefer the Gullstrand-Painleve coordinates, at least for now and I'll take a wait-and-see stance until I see how novices completely screw it up. The Schwarzschild coordinates have been a disaster with "time slowing down" or worse stopping on horizon (the coordinates aren't defined on the horizon) so I have higher hopes for these kinds of rain-frame coordinate systems. Ultimately what's most important is lost, i.e. that coordinate maps are purely fictional representations (spacetimes) of the physical field, the gravitational field. Taking any coordinate representation as a the physical system is where novices go dreadfully wrong.

[@Inductica]

Excellent, thought provoking video as always! A few questions and comments: Couldn’t one argue that the expression ct is one that modifies the time units to make them comparable to spatial units? And that these lengths that you point out are still units of time in principle, but that they are lengths which represent those units of time. Time is a measurement of change. One kind of change is a change in position, so one way to measure time with a distance: the distance that light travels (assuming light always travels at c) in one second. I tend to think that we should simply start from scratch to understand gravitation instead of trying to understand how the flawed 4D Einstein/Minkowski theory relates to reality; it was never meant to relate to reality to begin with, it was only meant as an abstract framework for reproducing measurements. “[the lorentz transformations and the spacetime manifold] aren’t really telling us about the behavior of space or time itself, but rather are telling us about the behavior of light and its interactions with matter and space in different physical situations” Fantastic! What I’m taking from this is that existence is not really a 4diemensional object, rather the 4D math we use in GR is just a data structure to keep track of how the durations of internal processes (as measured by a light clock) change when an object moves (with respect to an observer) or is in a gravitational field. Am I understanding you correctly? I agree with your attitude toward dimensions, that they are exclusive measurements. My own definition of a “dimension” is “linearly independent quantitative characteristic.” Another note: I’m not sure if I agree with the conceptualizations you put forward at 9:02. Is there any reason to think that an idealized clock, one that is not affected by motion or gravitation, could exist? We may need to define time differently (though still objectively) as simply a relationship between events, that when two events are able to influence one another, they are said to be simultaneous, and that when a first event is able to influence a second event, but only through a series of intermediate events, that first event is said to be before the second event. I think something like this might be a more objective conceptualization of time.

[@Inductica]

@@SpacetimeTraveler-p4e I don't follow at all.

[@Inductica]

@ We come from very different sets of assumptions.

[@Milesian2003]

I think was can produce a better definition of "idealized clock". I would suggest we specify an atomic clock that runs at the fastest time--that has no redshift of its frequencies. That clock must be at rest in space and not held stationary in a gravitational field. We can use it as our standard time---in fact was already do. The more an atomic clock slows relative to that clock, the faster it is moving in space--and that applies to gravity also as Dialect asserts. Atomic clocks are space speedometers!

[@KaliFissure]

Interesting premise....🤔 I suggest that time is a compactified dimension one single Planck second in size. The moment of computation in a wolfram model This creates/ forces limits, of action. Lambda and event horizon. And the gradient between these limits we call gravity. But..... truly classically closed and compact, creating a modular form, the limits also connect at a catastrophic point where maxima becomes minima. Free neutron decay. In at event horizon Take EinsteinRosen bridge Out in deep void Decay into amorphous monatomic hydrogen in a Rydberg state. Dark matter. Expand from neutron 0.6fm³ to 1m³ of amorphous hydrogen gas. Expansion. Dark energy . Then stabilize and coalesce and fall, towards an event horizon

[@brunonikodemski2420]

Was not this demonstrated years ago, via the Einstein Probe satellite, where orbiting clocks (high speed) were measured against stationary (almost) clocks on earth? Lorentz equations pretty much validated. Those probes were definitely "local observers" so it should mean that Time can "vary" anywhere in the Universe, depending on the observer. Given that, our place in the Universe has its "own" time right now, but other parts of a Minkowski pathway may have other time "rates".

[@KaiVieira-jj7di]

Lorentz equations would not even apply, except that they'd be totally wrong.

[@KaiVieira-jj7di]

I wonder... if one day Dialect comes to understand relativity, will these videos still be made available to the public? It would be wonderful if they so students can learn about where thinking goes wrong and even deepen the students understanding of relativity, what it is and what it is not.

[@isthattrue1083]

Yeah, it's linear and non-linear space/time. That's how something could move through time by moving through the 4th spatial dimension like we walk across a room. This is actually described in ghost phenomenon over and over across millennia. Some ghosts appear to teleport instantly. They are not, they are moving in a direction that allows it to seem like, to 3 dimensional beings, that it is moving across a space instantly when they are really moving through non-linear time/space. They would be able to see us moving through linear time visually outside of normal time relative to us basically. Time behaves differently in that extra dimension as it folds over itself rather than being spread over all space.

[@k_dankov]

Congratulation for this video! It is an obvious fact and nice somebody finally said it. Space-Time doesn't incorporate time into a 4-D manifold it incorporate the properties of the observer's moving refference frame compared to the real rest frame, which nobody can determine but which is there after all. I will wait with great interest your next chapter hopefully on General Relativity, which is even more interesting in terms of understanding the real properties of 4D-Space and its relation to Energy!

[@fz5lb]

a description that dosent come down to the idea that its all timey wimey we can talk about that later, and scratches the surface on the math at play good job

[@Writer-hex]

Time is just the space direction in which we are moving at the speed of light. Length contraction leads to distance being infinitely small in that dimension. That's all.

[@ClickToLearnQuantumComputers]

Great explanation!

[@TerranIV]

No, we are moving in a 4th dimension at c (i.e. the Time dimension). Light/energy moves in spatial dimensions at c, but does not move at all in in the Time dimension. From a light beam's perspective it does not move in space either, as all space is infinitely small along its path. We also experience a similar length contraction along the Time dimension, which is why there is no measurable space "between" events in spacetime.

[@msmith323]

If there is no time, there can be no speed of light

[@AnaishYumit]

Not Time and nor Space, but Direction. It is Direction within Space. Space and Time are both consequences of Motion, merely different aspects of Motion. Space in human terms, consists of height, depth, width or something similar ( I have heard and seen numerous versions of this) but the critical element of Direction is always missing.

[@jojodi]

Excellent video! Can I ask, what is your background in this material? Classical academic path in Physics, professional, self-taught, etc. ?

[@zemm9003]

​@@SpacetimeTraveler-p4e Relativity is so poorly taught at University that in a sense all Physicists must be self taught if they ever hope to understand the concepts deeply.

[@Mujahed0001]

Ah yes, the classic move-let’s focus on the person rather than the ideas they present. Nothing sparks creativity and progress like making discussions about credentials instead of concepts. I don’t know the background of this individual, but I follow their logic, and it makes sense to me so far. If you have alternative suggestions or a different perspective, please don’t keep us waiting.

[@opiesmith9270]

This ends is the aether being a real physical entity, rather than fields like a gravitational field. A flowing substrate. Guaranteed. And if thats the case, that’ll be a disappointment.

[@darrennew8211]

Wonderful as always! Now I wonder whether a freely-falling clock in high gravity would appear to tick slower at the same rate as if it was sitting still in the same field.

[@FrankChiang-s3z]

Then d(ct) can be defined by 2 points in the 3 dimensional space?

[@4ltimit1]

Finally. I've been looking for anything similar to the model I've been working on. With how many thousands of years humans have been around, I figured someone must have had a similar thought. The river theory is similar to my explanation that space is quantized and not an endless volume. Gravity is not a fundamental force - space "flows" toward objects with mass - gravity is a byproduct of mass consuming space. We observe that space is expanding and the rate of expansion is increasing; space is being created. I explain to people that space not only is created but is also destroyed. Some form of energy, dark energy in this case, is necessary for the creation of space; space is made of energy and stores that energy. Similarly, when space is consumed by massed particles the energy used to make the space is freed to be used by the particles which consumed it. This is what allows fundamental forces to play out. Gravity isn't a fundamental force. Gravity is the byproduct of fueling the three fundamental forces we have observed.

[@The.Renovator]

This doesn't make sense though, the river theory is still hinged on the concept of time--in that nothing can "flow" if time isn't present.

[@4ltimit1]

The video and I didn't say anything about removing time from the table but it is an important thing to talk about. The fourth dimension is a spatial dimension, not time, which is why experienced time passes at different rates in different places. Our time isn't a fundamental property. Further, our expression of time is based on rate of particle interaction not a seeming fundamental time beneath that; which I imagine you are reckoning to despite our current lack of tools to measure it. Time as we measure it is experienced but it is dependent on deeper, more fundamental, interactions which is why it measurably passes at different rates in different places (Earth's surface and GPS satellites in orbit for example). This is comparable to how temperature also isn't a fundamental; the entropy and pressure of a substance determine it's temperature. Reducing the pressure on a fluid reduces it's temperature without removing any heat flux. If you focus in on a single atom and attempt to determine it's temperature it doesn't have one but that doesn't mean it's jiggle doesn't impact the surrounding atoms. Just as our macro scale concepts of physicality don't extend to the very small our sensed time doesn't extend to the very small either. This is why electrons exist as a probability distribution instead of having a fixed position; where they interact with your instruments only matters if they interact at some point along their journey and your instrument is part of that path. Electrons effectively get more turns to act in comparison to larger objects like protons. It's especially interesting when you look at the anti-protons which were successfully stored in helium electron shells, displacing an electron in the process, and the subsequently less chaotic movement expressed by the atoms. Maybe we have talked about my ideas before and I've previously said things about time regarding this. However, this time I didn't mention time once in my explanation so I find it odd for a stranger to reply as you have; it must be because the video brought up time. Time is still involved but it is not the fourth dimension in general relativity.

[@Peter-o9n6p]

A busty milkmaid from Nantucket Used the milk to explore Newton's bucket But when kicked by a bull Faith in Mach's Principle Went to Hell when the animal struck it

[@audiodead7302]

I feel like you missed an opportunity with the final line of the poem.

[@vilexross]

The is no time expect photon clocks in gr. ~Dailecstein

[@JrgenMonkerud-go5lg]

Time is what you make it. Relative times are just what tgey are and only comparable locally in an empirical sense, but tgere remains a lot of freedome to choose description.

[@JrgenMonkerud-go5lg]

@SpacetimeTraveler-p4e yeah, this is just one way to lay out units. But so is any other way to lay out coordinates

[@JrgenMonkerud-go5lg]

@SpacetimeTraveler-p4e who told you that was "real time". Buddy that is just as bad a definition of objective time as any.

[@JrgenMonkerud-go5lg]

@SpacetimeTraveler-p4e the thing you have just blurted out is one way of of infinite ways to define time, in gr you can take a measure like that, and you can say it is what clocks measure and so on, you can say things about empiricist definitions of what is real vs what is abstraction, but it is nonsense, what if you had a situation where one type of clocks measured according to one coordinate system and the clocks you are used to, in accordance with your experience of time and what the definition you put forward would call a second, then there would be no way to recover the neat emiricist definition at all. There is no objective measures of time deducible by observation and logic, only abstractions applied to measured relative rates, that can be measured locally. Saying stuff like, time is what clocks measure, is just an operational definition with no ontological significance.

[@JrgenMonkerud-go5lg]

@SpacetimeTraveler-p4e so the curvature, time and space measures in GR are only really what we think spacetime is like fro. Straightforward assumptions based on our measurments, but there are no objective deductions to be made from that basis about geometry or scales of time and space and their derivatives, just the derivatives as we see them in accordance with the relative change between whatever we are talking about and rhe nature of rhe measures we utilize to make up such conventions.

[@nkchenjx]

makes sense. That is why we can jump back in time in this model because really it is jump back space at a later time showing in the model an earlier time.

[@sistajoseph]

The story is not over. Some clarity is still needed, thanks.

[@sluggo206]

What is a light clock? I can think of starting and stopping a stopwatch and how far light travels between those moments, but that doesn't tell me anything useful. The light I care about is not one ray that's linear or bouncing: it's a lot of rays going every which way and allowing me to relate to everything in my environment. I think we're back to "Time is what a clock measures", and reinterpreting the fourth dimension as space doesn't really clarify things in the day-to-day world.

[@dmytrooleinichenko9865]

If space is flowing - has a speed of flow, so we can define some sort of absolute speed of space motion? But what is this global frame of reference?

[@ThunderBassistJay]

Exactly what I've been saying for over 30 years. Forget absolute time. Duration can be expressed as distance light in a vacuum.

[@WilliamRoosa-h3f]

Instead of thinking of "space" flowing (it is not a vector) think of it being a "density" (scaler). Light moves slower through denser substances right. So if "space" is denser the lower down in the gravity well you would expect light to move through it (time) at a slower rate. My intuition would lead me to believe that if "space" was flowing into the mass from every direction than if would start accumulating there. That does not make sense to me. It can't just keep building up there forever. But a scaler "density" does not have to "flow into the mass" it can just be that scaler value like a temperature or color. Thoughts?

[@tomgould3475]

If a stationary object has no outside reference to compare it to for measurement of distance: How do we measure time? Time still exists, but there is no measurement of space. As a thought experiment, let's say that it's possible to rest in space without spacial move: Are we speaking of time as impermanence?

[@davidwuhrer6704]

The speed of light is not about light, it's about causality. Light in a vacuum travels at the speed of light, and can help illuminate the concept, pun intended, but as soon as you bend or break light, or have difffusion, the analogy doesn't work. Proper time (Eigenzeit) is not psycholgical, it is very much ontological. The psychological perception of time is much weirder.

[@pauldruhg2992]

Awesome! How do you explain redshift?

[@timmygilbert4102]

Isn't light a EM wave? Therefore is a sphere around the event? Beam if light just being a wavefront?

[@timmygilbert4102]

@SpacetimeTraveler-p4e i mean light is always a sphere, and a sphere is emitted every time it interact with an electron, so a beam of light is just superposed direction from multi sphere allowing a single em wave energy to not be diluted on the background. Not just from a single point of emitter, but from all electron interacting

[@se7964]

Really great work! Thank you for clarifying these important issues so accessibly and with such neat animations. Love what this channel continues to produce! ❤

[@SandipChitale]

Actually to get a 45 degree line as it is shown in the spacetime diagrams I always thought the the Y axis needs to be normalized to t/c. t/c | / | / | / | / |/ 45 degrees +-------------------------------- units of distance if we do not do that and use the standard units for time (seconds) and distance (meters/seconds) the trajectory of light (given the very large velocity of light) will be very close to the x axis i.e. the past and future light cones will leave an extremely narrow wedge around x axis - which in our experience seems to coincide with x axis i.e. we perceive nearby events as simultaneous. ALso to treat time dimension same as space is absurd. One cannot go back in time. Also the negative sign in Minkowsi metric is also an indication of that difference.

[@sjzara]

I don’t agree with this. The time dimension doesn’t work like a space dimension. It’s got properties of an imaginary number, and works so as to support causality in a way that the pure space dimensions don’t.

[@aleksandarmilenkovic5861]

You could be absolutely right. I'm not sure if it's only me who sees the full implications of i^2=-1. The Imaginary unit comprises in itself the superposition of two opposite polarities and it can not be squared retaining the same polarity two times in a row. i^2 is always -1x+1, or +1x-1, and never +1x+1 or -1x-1.. So, the imaginary unit is the unit of switching or flipping of opposite polarities. It's an oscillation or puls that can be used as a unit of time since it represents an actual occasion with its own temporal thickness and occurs at absolute speed c.

[@elindauer]

Beautiful visualizations and insightful commentary as always.

[@wealleuropean]

I found the video’s take on the fourth dimension really interesting! It’s cool to think of the fourth dimension as another spatial dimension instead of time. The idea that it represents the space a light beam travels over a set time makes sense in a way. I agree that viewing the fourth dimension as a mathematical tool can help explain complex behaviors of light and matter. It definitely made me rethink the traditional spacetime concept. Would love to see more discussions like this that challenge the usual perspectives in relativity!

[@goid314]

Wow, this was surely an eye-opening experience. I feel like some cosmological scientists are really need to see this too, with their bizarre theories

[@rhcpmorley]

The word ‘Space’ also has several (related) meanings. At its core, Space literally means ‘emptiness between’ i.e. 'nothingness'. The word Space is also used to refers to the dimension of spatial position, i.e. the three vectors of xyz-axis (which is how physics always explains Space). The xyz-axis simply references relative spatial position. Or Space might mean ‘everything not on earth’ as in ‘outer space’. And sometimes Space refers to the entire known universe....a vast, vague collective noun! But science is so often not specific when it uses the word. It liberally, lazily conflates these different meaning of the word space In the context of dimension, the xyz-axis, space is the dimension of relative spatial position. That is the three sub-dimension of length, breadth and height (xyz-axis), calibrated in standard units, allow us to overlay an abstract framework that calibrates and indexes position, and relative spatial position. But the xyz-axis is a STATIC reference, it has no vector for change. Space (in the context of xyz-axis) references static spatial position. Enter Time, the dimension of change. Bingo, you have spacetime the dimension of relative changing spatial position, i.e. relative motion. So Spacetime is an abstract framework (dimension) for referencing relative motion. But, again, it is reference frame specific. The ‘warping‘ of spacetime is due to the erroneous imposition of a linear universal Time dimension, rather than a reference-fame specific Time dimension (motion cases the reference frame to change continually). Einstein was wrong to say that Space and Time are ‘the same thing’. Space and Time ONLY interact in the context of motion. [OR maybe Science is using the words Time and Space bot in their context as collective nouns - Time being the non-specific collective noun for change, and Space the non-specific collective of 'stuff' then Spacetime is the vast (non-specific) collective noun for all relative motion of all 'stuff', ever. ] Take your pick. Dimensions and collective nouns are both abstract nouns, they only exist in your mind. And if Space references ‘stuff’ or ‘existence’ i,e. the position of a physical reality (mass). And Time references change, and all change is caused by energy differential. Then, fundamentally, Spacetime simply references the fundamentals of mass and energy (differential).

[@aleksandarmilenkovic5861]

Instead of spacetime manyfold one should call it an extensive continuum, as A.N. Whitehead did.

[@aleksandarmilenkovic5861]

Ah, yes, it's an actual entity that comes into being in reverse order. It ends before it begins and unfolds only in retrospection of its history.

[@TemperedWambat]

What about phenomena not directly mediated by light (e.g: gravity, nuclear forces)?

[@dialectphilosophy]

Great question. In this case, although light is not the signal-carrier, we would infer that whatever does serve as the signal carrier likewise travels at the speed-of-light in a wave-fashion, as the speed of light is ultimately the speed limit for causal information, not necessarily just the speed limit for light.