Space-time Definition and 419 Threads

I remember when I learned some basic continuum mechanics, Lagrangian is just a integral of lagrangian density over space, which is quite easy to accept because it's just a continuous version of L=T-U. Now I'm trying to start a bit QFT and notice that Lagrangian is an integral over space-time...

(First: to the moderator: I am not sure if this should go here or in general relativity, because it spans both topics.) Google "non-differentiable space-time" and one gets lots of interesting papers. But I am missing one essential idea that I am sure should be obvious: these papers continue to...

The 4th derivative of a sine wave is itself. This, of course, not only is a fact, but can be used to define the sine wave. My question is, are there any theories relating the abundance of sine waves in QM to the fact that the largest dimensions of space-time are together 4 dimensional? For...

Just curious. How come we say that at the moment of the bb space and time started to expand, and within this BB contained all the matter of the universe. But we see matter as creating curves in space time creating gravity. Wouldnt this imply that matter came after space time as it is changing...

I'm just going to start typing and hope I'm able to explain this in words. I'll start by defining the universe. Basically, I'm introducing this universe that is a 3 dimensional surface of a 4 dimensional hypersphere, and I'll be talking about the "dumbed down" versions of this, meaning, the 2...

What would the units be on the curvature of spacetime? G(curvature)=8πGT/c^4

Is it meaningful to talk of relative velocity between two moving points at a distance in curved space-time? This interesting issue came up in the course of discussion of the thread "Curved Space-time and the Speed of Light".I remember Dalespam giving some formidable logic with a very good...

Can two objects overlap in space-time? Why or Why not? If so, what circumstances must exist? If not, by what mechanism are two objects deemed separate?

If you have read my other threads, i am having trouble understanding special relativity. The issue seems to be my understanding of space-time. Space-time infers to me that two events are not separated by only a length in three dimensions, but also time, with time being essentially...

Please read and critique this argument for me please, any help is appreciated. Imagine a geodesic, and a matter wave that traverses this geodesic. The action of this matter wave determines the motion of the matter wave along this geodesic over a given space-time interval, and is specified...

Please read and critique this argument for me please, any help is appreciated. Imagine a geodesic, and a matter wave that traverses this geodesic. The action of this matter wave determines the motion of the matter wave along this geodesic over a given space-time interval, and is specified...

The speed of light in vacuum is expected to be a universal constant. Now let us review the fact in the light of the following situation: We consider three different points A,B and C in curved space-time(in vacuum).Observers stationed at A and B see a light ray flashing...

I have a question... I would like to generically describe a 4-vector locally in space-time. Would i go about that by simply taking a 4-vector and multiplying it by a metric? like u^{\alpha}=u_{\beta}g^{\alpha\beta} with u^{\alpha} the new 4-vector in the space specified by the metric?

Hi, I'm new here. I want post a specific question that's been rattling around in my head. Basically, if you consider the curvature of 3 dimensional space into a 4'th dimension due to gravitational field, has anyone considered the 'direction' of that curvature ? If you think about the...

The existence of entropy in gravity implies that there are microscopic degrees of freedom in space that carries the entropy. This implies space is discrete. Discrete space breaks lorentz invariance, which has been strongly constrained by both FERMI and thought experiments. String theory...

Hi. I don't mean to sound stupid but I'm quite new to learning about space but I'm real hooked on understanding how it all works. One thing that I come across a lot is Space-Time...I'm really not sure that I understand how it in itself works...How something can move in a different time if...

Hello everyone, I like studying space-time related subjects and want to do something relating to those after graduated. My university offers 2 majors which I think both have sth to do with space-time: Cosmology and High Energy Particle Physics. Both seems equally interesting so I'm having...

I just joined this forum, so forgive me if this is an inappropriate question for this section. Also, I have not studied physics to any meaningful degree; I am just a curious person who likes to think about the universe. So, on to my question... The premise: I was watching a documentary...

Hi there, What is the difference between Space-time Supersymmetry and Supersymmetry? Is Space-time Supersymmetry the same thing as Supergravity? What is Supergravity... All these terms make me very confused... Thanks a lot!

Hi there, I have a few queries and they are as follows: (i) What is the difference between the Einstein metric and the Space-time metric? (ii) What does the curvature of the metric really mean? Does one calculate the Riemann curvature tensor, but what does that really tell you if it...

Why did Einstein combine time and space? Does this mean when we observe something we don't know where it is as opposed to when it is? Is a object 1 light year away or 1 light year ago. How do we separate the 2 to know where something actually is?

Hi, I have a question about a fact that I did't understand, and all the world takes this as it is: How can the space-time distortion, generated by gravitational waves, can be measured? All scientist say that the space is stretched and squeezed, but how much means that, because 1 meter stretched...

General Relativity explains that every single piece of mass alters the space-time fabric, creating a curve. But since we live on a three-dimensional universe shouldn't the curvature alter the fabric in every dimension, creating some kind of weird closed curve that might look as a bubble around...

I read on a website (http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html): "The world we live in consists of four dimensions, the three space dimensions and one that is not exactly time but is related to time (it is in fact time multiplied by the square root of -1)." In my...

hi this is anom. i am new in physics forum. i have come up with some ideas and would love to share it with you. any compliments and constructive criticism will be appreciated.

Hello. This is my first post on the forum. Without a long introduction I'm just an average guy with a huge amount of curiosity but know nothing. I have a couple questions (very theoretical I'll warn you) Space and time is interweaved, like a fabric (thus spacetime). You move through it...

From what I understand of GR, time dilates due to the gravity of massive bodies. I like to think of it as the three coordinates of space drag, or lag, through time at a slower rate resulting in distortion of space-time. Correct me if my logic is flawed. But my question is this; why do bodies...

When deriving the conserved quantity in the case of space-time symmetry, a line in my notes goes from: \int{dt.(1+\epsilon\dot{\xi}).L[q(t+\epsilon\xi)+{\delta}q(t+\epsilon\xi)]} - \int{dt.L[q(t)+{\delta}q(t)]} where L is the Lagrangian and \xi is a function of time and both integrals are...

I was just wondering are you always going at a constant velocity through space-time since, as your through space increases you time slows down and vice versa. So does this mean you are maintaining a constant velocity, and if so what is that velocity.

Can gravitons bend space-time? How can it be shown experimentally? Ideas?

Suppose A travels in a spaceship and B is on earth. At the time of launch they both synchronise their clocks. When 30 secs have passed by in A's clock he reads the time in B's clock to be 40 secs.But when 30 secs have passed in B's clock A's clock shows 40 seconds. Isn't this a contradiction to...

Hi, I was wondering what happens if one has a photon that is so energetic that it gets trapped in its own gravitational field. Imagine a mass m orbitting a large mass M in a circular orbit with radius r and with transverse velocity v. By equating the gravitational force on m with its...

Hi all, I m not a physicist but a biological oceanographer. I would like to know how should I consider space-time variations of tiny cells in the ocean? Usually people deal with space and time separately but do not compute the data over a space-time scale (which I believe is important...

if the time component of space-time is not a constant as assumed, i think it could explain the old question of why our spacecraft don't move at the expected speed/acceleration. http://www.dailygalaxy.com/my_weblog/2009/11/rosetta-space-mystery-could-be-clue-to-a-new-physics.html#more

Been enjoying Physics Forums and this is my first question so before I ask I should I applaud everyone for the helpful friendly atmosphere. I'm trying to understand the concepts of continuity and connectedness. I have no set theory background nor physics backgrounds so I just read online...

I don't understand how the space-time fabric works. I've seen videos where the sun is in the space-time fabric and it makes a ripple in it and the Earth revolves around this. This works fine if our universe was 2D, but what if there was something above the sun in that diagram? Basically, I'm...

When looking through a few books, I ran into one by someone named Jack Armel. It was a theory he developed called "Entropic Space-Time" (also the title of the book). I've never heard (or remember hearing) this theory before...I'm presuming it did not carry out too far (it'd be helpful if...

I have just started a class that deals with relativity. My professor uses space-time diagrams all the time but has never bothered to fully explain them. Every time I have questions about my homework problems, people tell me to work it out by drawing a space-time diagram. Could anyone explain...

I am wondering if anyone knows of research being done, or theories that are related to the actual structure of space-time as they are related to physical constants such as the speed of light, Planck's constant, and the cosmological constant. I'm far from being a expert, but I've been playing...

This is not a homework question. I'm 41 and have recently become interested in physics but have no background in the field. My question is this: Does General Relativity account for tension in Space-Time? That is, when space-time becomes extremely vast, where there is very little matter...

is it possible to compare Einstein 4D space-time to a fluid ?

Ok, here me out on this one. I got thinking a while back after I read some very amazing books quantum physics and theory of relativity and and a thought just came to me. I might just be crazy though. My friend and I were discussing light speed and how gravity seems to "bend" light. We all...

Simple question (I think). According to GR, is gravity no longer thought of as a force of attraction, but simply a curvature of space-time induced by mass? The earth, for example, creates some kind of "space-time well" that keeps us in our seats as we type on our keypads? (That's poorly...

Good morning, We are trying to understand Special Relativity through internet, and we would like to ask a couple of questions about the subject. We plan to post more questions as more doubts arise. Both doubts concern the Minkowski space-time diagram. First doubt: in the temporal dimension...

"Swimming" in space-time On the main forums page, there is a link in the "Scientific American" section to an article http://www.scientificamerican.com/article.cfm?id=surprises-from-general-relativity". I've had a look at the article and I don't even begin to believe it. Even if you...

I've heard a lot about how an extremely powerful gravitational force can distort space and time. I was wondering (hypothetically) if it is possible to distort space-time enough by focusing a somehow artificial gravitational field into a single tiny point to pop a hole in space-time and travel...

Hey all, Thought I'd post this here as it is a little tricky trying to get a straight answer from the boys and girls of theory over at the GR/SR section without causing havoc of some kind. It seems to be fairly well accepted that space-time is curved. So maybe someone here can answer the...

I've been wondering if there is a relationship between the mass of an object and how much it "curves" space-time. I can't seem to find an equation or connection, I have looked at four-momentum but am not certain what it actually calculates. Maybe I just don't know it and there is yet an equation...

Many people believe that spacetime is discrete but what evidences have they ?? in one of issues of 'Scientific of American' explained that if space time was discrete then depending of the energy of light and since spacetime would be discrete the 'effective' space of light would not the same...

One thing that I really never understood is why space-time is connected. I have not taken general relativity yet, so I don't know the mathematics yet, but I want to know the answer. Why does an object have to move through space-time? Why can't an object move only through space or only through...