Spacetime Definition and 1000 Threads

If we have come to realize that energy conservation is not the most general conservation law in our spacetime, isn't it odd that we don't have a simple name for the "real deal"? I bumped into this thought through Noether's theorem, which relates symmetries in fields to conservation of all kinds...

Do you have to have mater for the gravity field,or is gravity a thing that has all ways been.

Recently I was reading Stephen Hawking's brilliant book "A Brief History of Time" and I believe I can recall him writing multiple paragraphs about falling into a black hole. I think I remember him saying that if you were to fall in all of time begins to collapse and the singularity will always...

Can spacetime have an outside? I sometimes read stuff where it is mentioned. Is it true?

http://phys.org/news/2014-04-liquid-spacetime-slippery-superfluid.html What if spacetime were a kind of fluid? This is the question tackled by theoretical physicists working on quantum gravity by creating models attempting to reconcile gravity and quantum mechanics. Some of these models predict...

Homework Statement An observer on the bridge of a spaceship is undergoing a proper acceleration a, so that the observer’s worldline expressed in terms of the coordinates of an inertial reference frame S is given by t(\tau)=(c/a)\sinh(a\tau/c)x(\tau)=(c^2/a)\sinh(a\tau/c) (a) Draw on a spacetime...

Hi all, On the website of gravity waves by PhD comics: At minute 1 and 5 seconds to minute 1 minute 20 secs. "Now if the spacetime between you and me stretched or compressed, we wouldn't notice it if we had made marks on our metaphorical sheet...because these marks would also get...

Recently, I was thinking about the nature of reciprocity in some cases of natural phenomenon, for example, Newtons Thrid Law of Motion, or, Faraday's Law of Induction. Considering that spacetime distortion, due to a gravity field, leads to a change in the light path, or, a change in light...

...and what is the difference between true 4D and the minkowski space? To me, it would be much easier to see universe as a 4D and us humans just experiencing the dimension of time differently. In my mind i pictured the universe as a complete 4D structure which we humans experience in one...

Usually when gravitational lensing is discussed, the examples are those of matter bending spacetime into a positive curvature. https://commons.wikimedia.org/wiki/File:Gravitational_lens-full.jpg In these cases, distortion of light is clearly evident as images of galaxies from behind these...

In a lecture from a course in QM, it was mentioned that Shroedinger's equation is deterministic in one and two dimensions. But in third dimension it gives unstable solutions, loosing it's determinism. It was mentioned that "in space of D dimensions Gauss theorem leads to the conclusion that...

I've tried to imagine and draw the effect of a large object would have on space time in 3d but it just doesn't seem right. Would I be right to assume that maybe spacetime has more than 3 dimensions?

I have been wondering about space time and how it works like satillites putting corrections in there clocks by a few nano seconds a day because of gravity or lack of it! But I have also wondered about the effects of being super close to the sun and time that we observe there looking back at...

If there was no mass/energy would space time still exist? In other words, does on space time only exist because of an interaction between two points of energy?

I just want to share my paper I recently wrote. I am trying to submit for publication: [Reference to unpublished paper deleted]

Hello so if we have geodesic equation lagrange approximation solution: d/ds(mgμνdxν/ds)=m∂gμν∂xλdxμ/ds dxν/ds. So if we have schwarzschild metric (wich could be used to describe example sun) which is:ds2=(1-rs/r)dt2-(1-rs/r)-1dr2-r2[/SUP]-sin22. But that means that ∂gμν/∂xλ=0. So that means that...

I am neither adept at philosophy nor physics, but in the "what is spacetime" thread I got the distinct impression that some people were letting language conventions, which are human-reason based, take precedence over physics, which is experience based. Link-...

The Schwarzschild Metric (with ##c=1##), $$ds^2 = -\Big(1-\frac{2GM}{r}\Big)dt^2+\Big(1-\frac{2GM}{r}\Big)^{-1}dr^2+r^2d\Omega^2$$ can be adjusted to a form involving three rectangular coordinates ##x##, ##y##, and ##z##: $$ds^2 =...

Hello I am concered about way of solving geodesic equation. Is there a general solution to geodesic equation? How to calculate the Cristoffel symbol at the right side of the equation? Thanks for helping me out!

What kind of spacetime structure can make the lines of a soliton-like field propagate in higher-dimensional space-time that do not diverge as they converge on the target. Does General Relativity make this just completely impossible? Or is there a solution of General Relativity that allows this...

Bill_K submitted a new PF Insights post Tetrad Fields and Spacetime Continue reading the Original PF Insights Post.

If two particles are moving apart, how is it determined if they are moving through space or if the motion is due to the expansion of space-time? Especially if there was no other frame of reference.

What is the velocity of the photon through the fourth dimension x4? Photons are real, physical entities. The fourth dimension is a real, physical entity. Therefore, photons must have a relationship with the fourth dimension. They must have some velocity relative to it. What is the velocity...

I have these questions: 1) Why must light always move along a geodesic line? What is the principle behind that? 2) A second question about spacetime: We mostly depict or imagine spacetime as a net of flexible fiber that extends everywhere as a plane as we see it.. As we are looking it, what...

Hello, I was wondering if anyone knows about the the book "The Geometry of Spacetime: An Introduction to Special and General Relativity" by Callahan and what their opinions are. Thanks!

So we know that in GR electromagnetic waves have their trajectories effected by the gravity of stars and planets. But how about gravitational waves. Are their trajectories altered by gravity? If so, would this imply that gravitons are self-interacting if they exist?

Does GR use spacetime mainly in order to include SR? Couldn't gravity be explained just with mass curving space and not spacetime? Why can't we explain the trajectories by field lines in regular space and choose the warp the structure of space itself,which is a really tricky business? How can...

I have a question. Please excuse me if this seems naive as I am not a physicist. This thing dark energy has been proposed to explain the acceleration of the expansion of the universe. From what I understand, it is everywhere. Could it be possible that dark energy really does not exist, but the...

I set up a Twin Paradox scenario and accompanying spacetime diagram to help better understand the resolution, but I had a question about the diagram I was hoping someone here could help answer. Please excuse the hastily drawn diagram! (Note: the ' frame corresponds to the outbound trip, the...

How can the age of the universe be determined when there is no absolute measure of time or distance in the universe. Every other place in the universe has another rate of the passing of time. Since time expands with the expansion of space, trying to use expanded time to measure time is circular...

Hi, As I know we now think that time translation is not a symmetry of spacetime because of the Big Bang, so we cannot say that our physical laws are applicable at every point in time. But then isn't the developing of the Big Bang theory against this asymmetry?

Is there any experimental evidence that pure energy (massless) can curve spacetime?

A new entry for Physics Forums Insights has been posted https://www.physicsforums.com/insights/spacetime-diagrams-light-clocks/ For now, please use this thread to comment.

It is customary, when discussing a particle's motion through spacetime, to talk about its path x^{\mu}(\lambda) , where x^{\mu} are the the spacetime coordinates of the particle in some frame, and \lambda is some parameter. I have a doubt regarding this parameter. Everywhere I've looked...

Hello every one . Simple question , What happens to the ended spacetime fabric in the singularity poing of the black hole if the black hole starts to move in spacetime ? Would the dead spacetime fabric at that point return to the regular shape of space time fabric ? ( as we know spacetime at...

Once I heard a fellow-layman on QM say that he imagined two entangled particles never been separated in the conceptual sense, that is: the two particles are conceptually 'one', only they connect two parts of spacetime, and are so able to exhibit correlations over spacetime with respect to the...

Hi, First: I'm pretty sure my question has been asked numerous times, so I'm absolutely happy with links to other threads. I've used search but it didn't come up with satisfying responses, probably mainly because I don't really know what search terms to use. So the question is: How can a...

Since a black hole goes to a singularity, theoretically wouldn't added mass to that point decrease the spacetime curvature by increasing of the circumference, and then not have a loss in information.

I am not a physicist and don't understand a lot of it, I can't do maths, but in all other areas I consider myself an above average educated civilian. I had a strange dream and it got me thinking. Has anyone ever done a synchronized atomic clock test with a superconducting element? Like placing...

Hi, I got interested in physics last night and there are a few philosophical questions that came into my mind. I have no formal education in this field but nonetheless, maybe you can answer in a simplistic manner. 1. Particles that has mass affect spacetime and are also affected by spacetime...

In my post graduate course, several years now, our professor in field theory have mentioned that in field theory the fields of mass-energy seem to be space and time themselves, like electric and magnetic fields in ElectroMagnetism. Specificaly he said that "the problem is that in...

Hi everyone, I've been trying to buy a copy of the first edition of the textbook "Spacetime Physics" by Taylor and Wheeler in my country, but I haven't been able to get my hands on a copy of it. Moreover, the e-books available online are poorly scanned with a bad font. I was able to download...

Homework Statement Say we have two manifolds N(dim d) and M(dim d-1). Let Φ: M →N be a diffeomorphism where Σ = Φ[M] is hypersurface in N. Let n be unit normal field (say timelike) on Σ and ⊥ projector (in N) defined by: ⊥^a_b = \delta^a_b + n^a n_b Where acting on (s, 0) tensor projection...

Homework Statement Consider a 2D spacetime with: ## ds^2 = g_{tt}dt^2 + g_{tx}(dtdx + dxdt)## where: ##g_{tt} = -x## and ##g_{tx}=3## Draw a spacetime diagram showing the null geodesics, including one which passes through the origin. Then show that for a massive object, when ##x## is positive...

I was chatting to someone recently about the motion of objects and whether they propagate through spacetime or not. They were/are convinced that motion through spacetime is simply not possible arguing something along the lines of the following: "Objects move through space. If you depict an...

I'm having a difficult time researching the answer to my question about the speed of light. Now obviously it is a speed not only reserved for light but also all other massless particles/waves. It's obviously a constant property of our Spacetime since we can manipulate th speeds of different...

I think I may have finally had my eureka moment in understanding how curved spacetime could cause an object at rest to be attracted to another very massive object due to gravity. Could someone please confirm whether what I've written below is correct please? The unit vectors associated with a...

Putting aside gravity, if there were some way to stretch and twist spacetime, what effect would that have on the matter inside the volume? Say we were talking about a part of spacetime that enclosed the earth, would the Earth itself be stretched and twisted? Also, wouldn't a ruler I used be...

I recently had someone ask me why we use 4-vectors in special relativity and what is the motivation for introducing them in the first place. This is the response I gave: From Einstein's postulates( i.e. 1. the principle of relativity - the laws of physics are identical (invariant) in all...

I have just read an article http://phys.org/news/2016-05-space-three-dimensional.html about the following paper: http://arxiv.org/pdf/1502.01843v2.pdf and wonder what specialist think about it, resp. says their intuition.