Spacetime Definition and 1000 Threads

In flat spacetime what we say that something (energy, information, charge, whatever) is conserved we take some region of space at moment t1, check the amount of that something, then we count the amount of the same thing at t2. What is ‘at moment t1’? It means that we cut spacetime using 2...

I've been informed that when you move through spacetime: If you move through time with the same space-time curvature, your effective "speed" through time is c. Does anyone have a link that explains this?

I understand that accordingt to GR mass curves the spacetime (I'm not referring to spatial curvature k), so that the universe globally considered is a manifold with constant curvature, is this right? If so, is this curvature positive or negative in the current cosmological model?

I've tried proving the invariance of the spacetime interval from Lorentz transformations 3 times now, but every time I end up with two extra terms that don't cancel! Could I have some help?

"Topography" of spacetime over time. How has spacetime curvature changed since the early universe? In the beginning, (close to the beginning that is), everything was hydrogen (sort of). That hydrogen was more or less homogeneous save a few little places, that would, after billions of years...

We get the idea of space-time from Albert Einstein linking the two together. Can somebody give some details on this subject?

We know that matter is discrete, and energy is quantized, (and more quantized things I don't know about.) There is also Planck length? and Planck time? Is spacetime continuous? If we don't know yet, I sure hope it is, because I much prefer to imagine discrete matter and energy interacting in...

Lets think about how spacetime curvature has changed since the early universe. In the beginning, (close to the beginning that is), everything was hydrogen (sort of). That hydrogen was more or less homogenous save a few little places, that would, after billions of years, would become stars and...

Dear all, In flat conformal space-time, e.g. \quad g_{\alpha \beta} = e^{4\kappa} \eta_{\alpha \beta} where \kappa is some function of space-time coordinates. What sort of paths do photons and massive particles follow? Could anyone describe their paths with some analagy or a...

How in the heck do physicists know that spacetime was created at the big bang just by observing galaxies moving away from each other? Couldn't that mean that that's when the MASS was created?

As a complete amature, I have a question about gravity and spacetime. Gravity is seen as a distortion of spacetime and we have the traditional view of a mass resting on a rubber sheet, distorting the space around it. But most masses are moving in space - usually fast or very fast. Is there a...

Homework Statement http://www.maths.ox.ac.uk/system/files/private/active/0/b07.2_c7.209.pdf Q1 (Also in Q2, what is the definition of 4 momentum for a massless particle?) The Attempt at a Solution I can do the entire question if I first derive the Lorentz transformation but I...

Since to travel in a straight line through space time requires the subject in question to have zero change to its state of motion, ie maintain constant velocity at all times. Does this mean that the only entity to ever have and that ever will achieve a straight line through space time is light?

Hi, I'm trying to learn stuff for my relativity module, and it goes on about spacetime and the 4-dimensional continuum and four-vectors and all that business. I am having trouble imagining whatever it is I am supposed to picture when people talk about this. Take this thing, i believe its...

Is it possible to define unambiguously retarded and advanced Green's function in spacetime without timelike Killing vector. Most often e.g. retarded Green function G_R(t,\vec{x},t',\vec{x}') is defined to be 0 unless t'<t but maybe one can express this condition using only casual structure...

Has the interlinking of space and time been proven? If so, how? Is there any hope left for the graviton?

This is driving me crazy. Consider a two-dimensional spacetime, with coordinates (t,x). If this is a flat spacetime, we can just imagine a regular-old two-dimensional plane. On that plane I could just as easily map a Cartesian/Euclidean coordinate system as a hyperbolic system of coordinates...

How does LQG work? I know there are gravitons in it but how do they interact with spacetime? Why is it a theory of quantum gravity but not everything? Thanks, Cosmolojosh

The wikipedia article titled "Geodesics & General Relativity" opens with: "the path of a planet orbiting around a star is the projection of a geodesic of the curved 4-D spacetime geometry around the star onto 3-D space." Uhhhh... "Real" spacetime has four dimensions - 3 space; 1 time...

I answer many questions thanklessly in IT forums so thank you in advance. I am trying to understand spacetime. My question is: Is it right to think of spacetime like whales in an ocean. The way they displace water is the way matter and energy displace spacetime? Or am I thinking in the...

Do Einstein's field equations explicitly show that energy alone can curve the metric of spacetime? True, energy is included in the stress-energy tensor, but is it assumed that energy in of itself curves spacetime? Or, is it possible that only energy "embedded" in mass contributes to...

When Riemann tensor = 0, spacetime is flat. Is the geometry of this flat spacetime that of special relativity?

Does it have a sense to ask what is Spacetime made of? Because we know: Spacetime can be bent, which results in gravity and gravitational lensing effects etc. If it was made of "nothing" it could'be bent, right? Spacetime "flows" into a black hole and at the event horizon the speed of...

Suppose you have an question like: "In the t-x spacetime diagram of O, draw the basis vectors \vec{e}_0 and \vec{e}_1 Draw the corresponding basis vectors of \bar{O} , who moves with speed 0.6 in the positive x direction relative to O. Draw the corresponding basis vectors of...

I've recently heard for the first time about the topics of Emergent Gravity and Emergent Spacetime. But I cannot find any resource that explains these ideas in a way that lay people can understand. If you know of one, please post the link, or write an explanation in simple terms. I've...

so what exactly is warping spacetime and a parallel universe? and what kind of physicist studies that? is this cosmology or something else?

Hi, From what I know, science is the study of the observable world, its theories are supported by evidence. Now GR is a theory, and it informs that mass curves the 'fabric' of space and time. The thing I don't understand is that there is no evidence of mass curving spacetime, then how is...

I have been working through Schutz's A First Course in General Relativity and was a little confused by how he presents the space time interval: \Delta \overline{s}^2 = \sum_{\alpha = 0}^{3} \sum_{\beta = 0}^{3} M_{\alpha \beta} (\Delta x^{\alpha})(\Delta x^{\beta}) for some...

The following is a paraphrase of an argument for the discreteness of spacetime, made by Smolin in his popular-level book Three Roads to Quantum Gravity. The Bekenstein bound says there's a limit on how much information can be stored within a given region of space. If spacetime could be described...

In his latest paper below, he discusses the microscopic degrees of spacetime and entropy, and how the emergent thermodynamic features of spacetime do not require details of the microscopic degrees of freedom. Still, I wonder what sort of microscopic degrees of freedom of space time would be...

Hey everyone, I am new to the forum and have a question on the logic of General Relativity. Correct me if I am wrong but my understanding is that according to Einstein's equivalence principle: 1) Freefall in gravity is the same as weightlessness in sufficiently small areas (so an...

STUPID QUESTION ALERT! Is spacetime an absolute reference frame?

How one can define a spin in Qunatum Filed Theory in curved spacetime. If the space is flat it's invarainat under Poincare group - so in particular it's invariant under SO(3). Spin operators are simply generators of SO(3). If the space isn't flat we cannot define spin in this way. I know that...

hello, quick question, If i am interested in the shape of space time, am i interested in the curvature tensor, or the metric? Thanks

Is EM theory in curved spacetime the same as "unification"? I am wanting to learn about classical EM theory in curved spacetime (just curious) and I found this old thread containing some references https://www.physicsforums.com/showthread.php?t=3950 Is simply talking about the EM field in...

What are the effects of Spacetime "Drag"? I have learned that general relativity predicts that a rotating body (such as earth) produces not only a curve in the spacetime continuum, but it also 'drags' spacetime in the direction of rotation. 1) How is this possible? 2) What effects does...

So, multiply connected spacetime may exist or even does already according to current knowledge, right? And in which of the thousand of millions of books about GR, which every physicist seems to have to have written about at least once, ever you read about it, authors claim that, although it's in...

Would someone please provide me with some papers or sources about the tensor gravitation theories on the Minkowski spacetime? They are field theories wherein the matter is supposed to not affect the geometry of spacetime and thus leaving the background spacetime unchangend with respect to the...

I'm a lay person reading Three Roads to Quantum Gravity. I like the book very much but do have trouble understanding parts of it. Hopefully I can find clarification on a point of confusion for me. I thought loop quantum gravity was based on relational spacetime - that spacetime wasn't...

I was just wondering. I have been looking all over for an answer to this question but I can't seem to find any. I read a biography about Einstein recently and it said that when quantum mechanics first came about he said (something like): "I will not deny its usefulness, only the conclusions...

since the first moment I've started studying the theory of relativity i thought that the minkowski metric represents a flat spacetime (a 4D euclidean space) but while I was surfing the WWW , I arrived to an interactive applet the helps you visualise the idea of spacetime curvature is GR , here...

Homework Statement I am given a static spacetime line element which has the property that the metric is time independent. I am asked to calculate some of the Christoffel symbols, which I have done. The question asks to show that for an observer at rest, the four-velocity is given by V^a =...

I was just thinking about teleportation and wondered if spacetime could be entangled. Or if you could entangle just selected dimensions. Then that would flip the teleportation problem on it's head... Any thoughts?

Michio Kaku stated that current physics does not rule out the possibility of time travel even tho it would take comprimable measures to achieve it, such as the energy of a supernovae, my question is even if you can bend spacetime to that extreme to get to a planet with life on it, how are you...

Homework Statement on a spacetime plot (y-axis) = t (x-axis) = x find the spacetime interval between points (0,0) and (300,700) Homework Equations Interval = (c^2(del t)^2 - (del r)^2)^1/2 The Attempt at a Solution So I can see clearly my del t is 700 (700 - 0 = 700 ) but for the...

After a while of trying to understand this, it's still kind of confusing. I get how the planets orbit around the sun because of the sun's spacetime curve, but if one were to drop something, how does spacetime curvature cause it to fall? How can everything in the universe have a gravitational...

Can spacetime be created ? if yes how if not why ? I understand that spacetime is not physical and more of geometry. Still bec it exist virtualy ,as gravity depends on warping of spacetime. So can we ever create this spacetime , does it existing before big bang ??

I've tried several hours to understand Lorentz transformations(for space and for time)...it simply dosn't make any sense...I've posted here,on math section,because I need a better mathematical view over it... whitout this I can not understand much out of the restricted theory of relativity,thus...

Hi guys, simple question I have: Do particles without a rest mass (including EM radiation) cause spacetime to bend? Or only those with a rest mass have gravity?

What are the facts by which we believe that it's spacetime itself which is expanding since the big bang and not that it's only the matter-energy in the universe which expands in a pre-existing unchanging spacetime canvas?