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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Spacetime geometry around two black holes

For space-time geometry we all know images like he one below. But if I were to take a neutron star and put it right next to but not touching a black hole what effect would that have on the geometry of ...
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28 views

About a *center of the universe* [duplicate]

I understand that the history of our universe began with an explosion - the Big Bang. Now, I saw a comment in some old post, that "the universe has no center". Is that true? If there was initially a ...
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1answer
47 views

How would an observer distinguish between space and time in a “Lineland” universe?

Note: "Lineland" here is borrowed from the Flatland novella. If you were placed as an observer inside a "Lineland" universe, i.e. a Minkowski spacetime that has one space dimension and one time ...
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How come some people are claiming that the Big Bang never happened?

A news story is going viral on social media networks claiming that two physicists have found a way to eliminate the Big Bang singularity, or in layman's terms (as claimed by many sensationalist news ...
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Is it better to define the observable universe as the universe?

This is somewhat triggered by this question: It's established that universal energy is not constant. But is the net change positive or negative? When I say entire universe, is that equivalent to ...
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26 views

Could the gravitional pull of all the travelling photons “relativistic mass” in our galaxy explain “dark matter”? [duplicate]

Does that make sense mathematically? If we could sum up all the photons in-transit in our galaxy and calculate the total gravitational pull, could it provide an alternate explanation instead of ...
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1answer
88 views

What is the role of the spacetime algebra?

For Minkowski space $M^4=\mathbb{R}^{1,3}$ the Clifford algebra $Cl_{1,3}$ (Dirac algebra) with $\{\gamma^\mu, \gamma^\nu \}=2 g^{\mu \nu}$ is sometimes called "spacetime algebra". What is its ...
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61 views

Space-Time and Entropic gravity [closed]

Gravity warps space-time, and Gravity can be thought of as Entropy. (Entropic gravity) So, as spacetime expands, how is space-time Entropy (or the inverse of gravity) effected? (If this is a noob ...
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21 views

Measuring speed of an object relative to what? [duplicate]

I understand that everything in the universe is moving through spacetime and the spacetime vector is equal for all the objects. So if If something is moving at speed of light, the mass of it is zero ...
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2answers
187 views

What does it mean to divide space and time?

Goldstein's mechanics book, on the chapter on relativistic mechanics says that "We cannot assume that all observers make the same division into time and space in the same way." What does it mean to ...
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4answers
500 views

Why do we theorize that the Big Bang created space? [duplicate]

I just asked this astronomy question about how far away the light is that would show us the beginning of time, the Big Bang. Some answers claim that the light I ask about doesn't exist, because "The ...
5
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2answers
183 views

Is a spinor in some sense connected to space?

Spinors transform under the representation of $SL(2,\mathbb{C})$ which is the double cover of the Lorentz group $SO(1,3)$ - or in the non-relativistic case under $SU(2)$, the double cover of $SO(3)$. ...
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91 views

Theories that predict the number of space-time dimensions

My impression in that most theories assume three spatial dimensions and one temporal dimension, though could in principle be formulated in others numbers of dimensions without inconsistencies. I know, ...
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2answers
99 views

Gravity: Is there curved space besides curved spacetime?

Wikipedia: Curved spaces play an essential role in General Relativity where gravity is often visualized as curved space. Is the Wikipedia article "curved space" talking about curved space or ...
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0answers
46 views

Does the curvature of spacetime by gravity affect homogeneity and isotropy of the space of the universe?

The FLRW metric starts with the assumption of homogeneity and isotropy of space.(Wikipedia) FLRW metrics of the universe have no or only very weak curvature - It is curved space. In contrast, ...
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2answers
102 views

Why isn't the Heisenberg uncertainty principle stated in terms of spacetime?

As I understand it, there are two "versions" of the Heisenberg uncertainty principle: Position-Momentum uncertainty \begin{equation} \sigma_x \sigma_p \geq \frac{\hbar}{2} \end{equation} where ...
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0answers
37 views

What does the phrase 'muons are brought to rest' mean?

I have a physics question in a text sheet. I am currently doing work on relativity where calculations of rest frames and laboratory frames are being completed. The formula: $t(\text{laboratory frame}) ...
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1answer
39 views

Can invisible galaxies exist? [duplicate]

Is it possible that there are newly formed galaxies which are moving so fast that we will can't see them, even though they exist "inside" our visible universe? I've been wondering if this could be a ...
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2answers
646 views

What is space made of?

General Relativity posits that matter curves spacetime, such that geodesics point towards the object in question, hence, gravity. Now, how does matter do this? What is spacetime "made of", anyway, ...
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1answer
27 views

Is the term “Lagrangian density” specific to spacetime?

Wikipedia talks about Lagrangian densities here. But they never actually say whether they're just applying the concept to spacetime or that Lagrangian density is the analog for Lagrangians but for ...
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0answers
48 views

With the new time cloak technology could it be possible to stop time for everything except one person

So I was just reading up on some new research on national geographic from the Cornell University scientists and it seems that they've managed to bend waves of light around something to hide it from ...
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1answer
47 views

Is a black hole really a hole in space? [closed]

What if when a supernova occurs, instead of it condensing into a singularity it creates enough force to tear a hole into the fabric of space? Is a black hole just what is sounds like, a hole in space? ...
5
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2answers
126 views

Lorentz transformation paradox

I got to solve a simple exercise about special relativity when I kinda came to the conclusion it doesn't stroke with my intuition. The homework-problem The problem was about two people Sam and ...
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1answer
116 views

Are we living in a Universe with 3 time dimensions and 1 spatial dimension? [duplicate]

If I said that we are living in a Universe with 3 time dimensions and 1 spatial dimension, would I be wrong? By that I mean if the spacetime interval was evaluated as ...
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2answers
2k views

Why isn't the center of the galaxy “younger” than the outer parts?

I understand that time is relative for all but as I understand it, time flows at a slower rate for objects that are either moving faster or objects that are near larger masses than for those that are ...
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2answers
847 views

Can a non-Euclidean space be descripted through an Euclidean space of higher dimension? So why use non-Euclidean?

If you draw a big triangle in Earth 2D surface you will have an approximated spherical triangle, this will be a non euclidean geometry. but from a 3D perspective, for example the same triangle from ...
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2answers
104 views

Infinite Universe and Big Bang

A common view now in the physics community is that the Universe is infinite in extent (which makes Hubble sphere an infinitely small part of the Universe). On the other hand, there is the Big Bang ...
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1answer
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Does the payload of an Alcubierre drive have to be on board during the preparation phase?

I was thinking about the practical aspect of using an Alcubierre drive, assuming one existed. I'm no expert, but my understanding is that, since the destination has to be in the forward light-cone of ...
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3answers
92 views

How strong must gravity be to stretch time?

I'm not sure if this is true or not but I heard that gravity has the ability to stretch time, and I was wandering if this is true. If so, how intense/powerful does the gravitational force have to be ...
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1answer
305 views

Our Universe Can't be Looped? [duplicate]

With reference to the Twin-Paradox (I am new with this), now information of who has actually aged comes from the fact that one of the twins felt some acceleration. So if universe was like a loop, and ...
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3answers
151 views

Twin Paradox speed of light

Twin Paradox Can anyone clarify and or correct the following for me? A space ship is flying at speed v equal to 0.8 times the speed of light. Within the ship are three stations, a transmitter at ...
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0answers
318 views

Twin Paradox Without Acceleration

So I've been doing a lot of reading about the twin paradox and have encountered several different explanations that strive to resolve it. First off let me start by saying general relativity is not an ...
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0answers
64 views

Gravitational time dilation in changing curved space time

Imagine a portion of spacetime which is changing its spacetime curvature because of an object with great mass travelling nearby. For instance, before it was flatter, and after the object passes it ...
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5answers
83 views

Does spacetime have a “preferred” speed/metric? [duplicate]

I read recently that spacetime has a preferred speed, c, and that all observers would agree on the speed of anything moving at c. In particular, there is nothing unique about light when talking about ...
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1answer
51 views

Empty Space - Could we exist without it? [closed]

I have been giving empty space much thought. I imagined reaching the edge of the Universe and thought of 2 scenarios(focusing on 1): There is no more empty space 1.1 Would space extend with me if ...
2
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2answers
100 views

Curvature gravity and a falling apple? [duplicate]

I know very little of physics after Einstein. I am aware of that Einstein's gravity theory says that the existence of matters creates curvature of a space-time, so that our Earth orbits our Sun. I ...
6
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2answers
2k views

What does it mean for objects to follow the curvature of space?

In science documentaries that touch on general relativity, it is often said that gravitational pull isn't an actual a pull (as described by classical physics), but rather one body travelling in a ...
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4answers
93 views

Is light amplitude spacial?

In diagrams I often see light waves depicted as little sine waves that travel through space. And often when describing polarizers, the explainer will angle their hand to show the angle of ...
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9answers
4k views

According to General Relativity, Does The Past “Exist”?

I'm curious about just what is meant by time being another dimension, like the three (observable) spatial dimensions. Does this imply, according to General Relativity, that the past and the future ...
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6answers
189 views

Mass without Matter?

My cursory studies of general relativity had introduced me to the concept of how gravity is associated with the warping of spacetime and that the presence of matter in space warps space and this ...
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1k views

Can spacetime be non-orientable?

This question asks what constraints there are on the global topology of spacetime from the Einstein equations. It seems to me the quotient of any global solution can in turn be a global solution. In ...
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3answers
118 views

Is a metric tensor field the same thing as $ds² = -dt² + dx²+ dy² + dz²$?

I am having trouble understanding the nature of the metric tensor field on spacetime manifolds. In particular, a Riemannian manifold $(M,g)$ is defined as a real smooth manifold $M$ equipped with an ...
4
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3answers
68 views

How do we expect distance measurements to compare inside and outside the event horizon of a black hole?

I've read that as one approaches the event horizon of a black hole, time is dilated relative to time measured farther away from the event horizon (clocks tick slower near the event horizon). I've ...
2
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1answer
65 views

Does a spacetime manifold have the structure of a metric space?

My first introduction to spacetime physics was Wheeler and Taylor's book Spacetime Physics. This book gave me an appreciation for how important the spacetime interval was for giving the distortions to ...
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1answer
106 views

General relativity: is curvature of spacetime really required or just a convenient representation?

I'm not really far into the general theory of relativity but already have an important question: are there formulations that can do without spacetime curvature and describe the general theory of ...
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320 views

How does a rotating object cause frame dragging?

Frame dragging is a consequence of general relativity. But I don't really understand it. Of course I can find metaphors like the "honey metaphor" where stirring a honey can move the specks even if ...
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2answers
751 views

Is space-time a property of mass?

Imagine looking at a quasar millions of light years away. From your perspective, a photon emitted by the quasar has spent millions of years travelling through space, and its trajectory has been ...
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3answers
2k views

If the observable universe were compressed into a super massive black hole, how big would it be?

I understand only a little of general relativity, but that's why I'm here! :) Consider the hypothetical situation of some extra-terrestrial intelligence pushing all the mass in the observable ...
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Is a non-accelerating object far from a gravity source moving at the speed of light through time?

I'm trying to understand the Minkowski spacetime better. If an object is not undergoing acceleration, and is far from any large mass, does it travel maximally "fast" through time? Can we calculate a ...
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2answers
202 views

Curvature of spacetime as a real thing?

I get the curvature tensor in General Relativity, it is “just” math. Does space-time REALLY curves as a tangible thing, or is Einstein proposing a mathematical abstraction? More naively, please ...