Questions tagged [spacetime]

As a consequence of the Lorentz transformations, time and space transform into each other when changing reference frame. This calls for a unified description: Minkowski spacetime.

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How does the curvature of spacetime work in M-theory? [duplicate]

I understand that strings have a vibrational mode which produces a graviton. But how do the strings know to produce gravitons? Does matter cause constructive interference? I read that black holes are ...
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Space-time curvature

If the curvature of space-time decides the path for the motion of mass, then why does an object thrown vertically upward come down after some duration of time?
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What would it be like to go past the speed of light? [closed]

Question #1: I'm actually surprised that nobody has asked this question before, but theoretically speaking, what would it be like to go past the speed of light? This is actually only one of a few ...
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What's the inner ergosphere in a Kerr BH?

(My book uses the notation "ergosphere" as the hypersurface of static limit, "ergoregion" as the hypervolume within.) Studying the Kerr BH, I've come to the part about horizons and ...
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Universe and Anti-Universe pair

Two years ago Latham Boyle, Kieran Finn and Neil Turok, in their work CPT-Symmetric Universe, proposed that the pre and post bang eras constitute a universe/anti-universe pair. For Anti-universe in ...
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Understanding the proof for time contraction in special relativity

In this stack about deriving Lorentz transformations, the op writes an answer where he proves the relation between time in a moving frame and regular frame but I just can't understand what the logic ...
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Imaginary time & predictions

Is the imaginary time just a different convention to express the time evolution to make the calculations easier? Hawking also said that "It turns out that a mathematical model involving ...
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Replacing the $M$ coefficient in Schwarzschild metric with a distribution $M(r)$

Consider the Schwarzschild metric $$ds^2 = -f(r)dt^2 + \frac{1}{f(r)}dr^2+r^2 d\Omega^2,$$ where $f(r)=1-\frac{2M}{r}$. I take it that $M$, although not really the mass of a black hole, is the ...
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Why can gravitational acceleration not be detected by experiments

I was doing research into how $G$ forces are calculated, and I was told that they show all acceleration that can be "sensed" by humans (i.e. all non-gravitational acceleration). This ...
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Does spacetime topology have importance in physics?

Generally in textbooks they represent spacetime as $(M,\nabla,g,t)$ where $M$ is a Lorentzian manifold,$\nabla$ a torsion-free connection,$g$ a metric and $t$ a time orientation. But they do not talk ...
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What does a universe with a boundary look like?

Physically, what would it look like if we lived in a universe with a boundary at finite distance?
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Another form of Black Hole Information Paradox?

Consider the Penrose Diagram of Collapsing Gravitational matter :                           Any radial light ray (say P) originating from $\mathscr{I}^{-}$ is bound to end up in the Black Hole. The ...
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Determinant of a Jacobian of a space-time coordinate transformation

In Lewis Ryder's Quantum field theory's Classical field theory part, a certain variation of field is mentioned $$ \phi(x^\mu)\longrightarrow\phi(x^{\prime\mu})= \phi(x^\mu)+\delta\phi(x^\mu)$$ $$x^\mu\...
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Is it possible space could explain the dark energy in the universe? [closed]

I have kind of been thinking about it for a while now and this is what i came up with. in the beginning there was nothing but space, no matter, space has a charge though and it repels itself because ...
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Ladder Paradox and Understanding Space-time Diagrams

With the given details, An observer $O$ at rest relative to the garage with measured length $L_g$= 6.10 m. The garage is equipped with doors that can open or close instantaneously at a chosen time. ...
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Mass-energy equation and Einstein field equations

I don't know much about Einstein field equations, but I'm wondering are there any relationships between the Einstein field equation (or the vacuum field equation) and the Mass-Energy equation $E=mc^2$?...
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Could “distance” be an emergent property like temperature? [closed]

If you think about a pre-geometric theory of particles which might consist for example of only fermions, bosons and gravitons but no notion of space. Then the initial state and final state of an ...
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Swimming in spacetime: using your own gravity field?

It is known that in a curved spacetime, due to the center of mass not beeing clearly defined object can change position using contortions. However an object generates its own gravity and changes in ...
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In Schwarzschild space we can calculate the space/time distortion due to one point mass, how do we do it for multiple point masses

How do we determine the distortion of space/time at a point that is being influenced by multiple masses?
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If there were no matter or energy, would space-time be an affine vector space? [duplicate]

When we fill a balloon, if we view it from the two dimensions of its surface, it is not possible to determine an origin. However, when looking at it in 3 dimensions, the origin is clear. It makes me ...
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How can spacetime change with time?

As far as I understand, spacetime fuses the 3 dimensions of space with the dimension of time into a single entity. If there's a big curvature, things will experience a change in how they experience ...
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What is the velocity of an object sitting on the surface of the Earth in General Relativity?

In General Relativity an object on the surface of the Earth feels a force from the Earth due to the surface of the Earth's constant acceleration through space or space-time. From what I've heard time ...
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Can we represent 4D graphically? [closed]

Actually I know that axes are always perpendicular but after three axes we cannot draw any other axis that is perpendicular to all the other three axes. can any one say how can we draw another axes ...
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Confused with proper times in accelerated system of reference

Im told in an exercise that Alice gets in a spaceship and from a the base's inertial reference frame her worldline is: $x^0 = ct$ and: $$x^1 = L \left(1-\cos\left(\frac{t}{T}\right)\right)$$ In the ...
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Doubt on the need for Topological Manifolds [closed]

I happen to be trying to motivate physically and intuitively the need to use topological spaces. So without using emerging concepts such as differentiable manifolds or extremely formal concepts as ...
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Quantum Fluctuations of Light-Cones

We typically draw light-cones to study causal relation between two space-time points. Source : Wikipedia Space-Time is manifest as the metric in various calculations. If one considers that something ...
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If the Universe is already infinite and is expanding, how could it be expanding to more than infinity?

There are arguers that say the Universe is streching about every its point but also tell it is infinite. How it can possibly be?Maybe we should reinterprete what infinity means.
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Einstein-Rosen bridge

The paper in question is Ref. 1. This is a traversable Einstein-Rosen bridge. However, this paper calculates that an in-falling observer will cross the throat of the wormhole in finite proper time but ...
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Gravity bends space, so how can space have only 3 dimensions? [duplicate]

Due to the laws of gravity and electromagnetic attraction (decreases with $\frac{1}{r^2}$) we know that space should be limited to 3 dimensions. At the same time we know that gravity bends space. All ...
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Is S-matrix theory a subset of modern quantum field theory?

As a graduate student of physics in the early 2000's, our particle physics classes started with quantum field theory, since QCD had long been established as a good model of the nuclear strong force. ...
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If we consider a hypothetic universal time/clock independent from the laws of physics, can we say the time won't be relative if that clock is used? [closed]

If we have such a clock and we use it to travel to a back hole and stay there for say 24 universal hours and we come back to the earth, is the passed time exactly the same on earth according to that ...
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Why can't we take a limiting relative frequency over nested spheres in relativistic spacetime?

The Measure Problem of eternal inflation cosmology is to determine the correct probability measure over the theoretically infinitely many individuals (or civilizations or worlds) for self-locating ...
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Can time be just another spatial dimension which the universe travels through with a particular direction and constant velocity?

Is it possible that time is not a 'time-like' dimension, but a proper spatial dimension just like the others but we only have access to a 3D cross-section of it? I did some research about the internet ...
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Distinction between different points at Big Bang Singularity

As per the Big Bang model of Cosmology roughly 13.8 billion years ago a singularity exploded exponentially to eventually become the present universe. At the present time (basically current time-slice) ...
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Reference frame as a $(1,1)$-tensor field $R$ with $R^2=R$

In a paper about the splitting of the space-time [for reference "THE STRUCTURE OF SPACE–TIME: RELATIVITY GROUPS" (International Journal of Geometric Methods in Modern Physics Vol. 3, No. 3 (...
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Solving the Euler-Lagrange equations for a complex scalar field in which the time derivatives and gradient are separate

This is found at the bottom of page 9 of David Tong's QFT lectures. The Euler-Lagrange equations for the complex scalar field: $$\mathcal L=\frac{i}{2}(\psi^*\dot\psi-\dot{\psi^*}\psi)-\nabla\psi^*\...
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Are the relativistic effects of gravity strictly due to spacetime 'curvature'?

I happen to live on an infinite flat Earth. It has a constant uniform gravitational field extending above it. Don’t I still observe relativistic effects such as light not traveling in straight lines,...
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If mass curves spacetime, why do planets in a vacuum follow curved paths?

Earth orbits the Sun because the Sun's mass curves spacetime. But the Sun is 150 million kilometers away from here; how can mass curve spacetime that it's not actually in? Is that a form of action at ...
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Are the fermions in 4D Euclidean $\mathcal{N}=4$ SYM Majorana? If not, what are they?

It is stated in this paper (as well as in many other) that the fermions of $4$D Euclidean $\mathcal{N}=4$ Super Yang-Mills (SYM) are Majorana fermions (see eq. (42) and (43)). However it is stated in ...
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Representing bound geodesics in Penrose diagrams

I recently started reading about conformal (Penrose) diagrams and have since been faced with a couple of conceptual doubts. Based on the coordinate transformations, null curves in Penrose diagrams are ...
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Any 3D model that shows how Earth rotates around the Sun and How Space-time curvature looks like?

I understand that Gravity will effect on space-time and create a curvature, which tells the mass how to move, therefore, the SUN can rotate smaller objects because it has stronger gravitational pull. ...
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Can we say that Galilean Spacetime has the signature $(0,0,0,+)$?

Minkowski Spacetime can be treated as either having the signature $(+,-,-,-)$ or more commonly $(+,+,+,-)$ as in Minkowski Spacetime the spacetime interval between two events is the quantity that is ...
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Negative mass as a property of space-time [closed]

What if negative mass is a property of space-time itself? We know that dark energy works against gravity and is currently expanding universe, while this expansion is increasing. Can this be because ...
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Are fields projections in spacetime of higher dimesions?

I am initiating my learning on QFT and my question might be really basic, but I am having difficulties finding an answer, I would appreciate any help to find this in the literature. Here is my ...
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Can we travel to everywhere in two years? [duplicate]

I know that technology is far away from this but it is only a thought experiment. Assume that we could constantly accelerate with 1g (starting outside of the earth). Lightspeed is around 300 000 000 ...
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Does it violate energy conservation? (What are the possible flaws in my reasoning?) [closed]

The universe has been expanding since the Big Bang. This expansion of the spacetime fabric can be estimated by the Hubble’s constant, and is usually compared (analogously) to a ballon being blown up, ...
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When did the Big Bang happen?

Did the Big Bang happen at a point? goes through the fact that the Big Bang happened everywhere at the same time. John Rennie's answer explains this as being a consequence of all points in space ...
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What determines intensity + range of a gravity field?

Related questions.. How does the Sun's gravity extend out millions of miles to influence the far reaches of our solar system? Reference: movie - "Interstellar", 2014 Nolan) / How does a ship ...
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Why does a phase shift in a light pulse imply a non-commutative structure of space (which implies gravity has a quantum structure)?

In the news report Physicists propose test of quantum gravity using current technology (Lisa Zyga, Phys.org, 27 October 2017), a test is proposed to determine if gravity has a quantum structure. From ...
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Why does combining background independence and diffeomorphism invariance force you to conclude that spacetime must be quantized?

I was recently watching a fermilab video on loop quantum gravity and at about 4:20 it is stated that, when combined, background independence and diffeomorphism invariance force you to conclude that ...

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