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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Infinite halving of a distance

If an object is, say, 100 cm. from a wall, and I move the object halfway to the wall and stop, then the distance is reduced to 50 cm. If I continually move the object by one half of the remaining ...
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2answers
70 views

Gravity and the Speed of Light

Let's assume that I am on an airplane that is at about 4,000 altitude and now let's also assume that I am standing on one of the wings with a light torch, if I point the light torch below to the ...
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2answers
125 views

How does the warping of time behave as a fourth dimension?

My problem is that I have a hard time grasping something if I can't conceptualize it, though maybe this can not be conceptualized, just accepted, in which case I thank you for your patience. The ...
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2answers
187 views

Proper length in GR

What meaning/use is associated with the notion of proper length in general relativity? Do you know an example of any quantity that depends on it? I have so far found statements like "the length ...
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0answers
21 views

Higher dimension world possibility [duplicate]

Could it be possible that all of the unexplained phenomena (like uncertainty) in our 4D world might be originated from a higher dimension universe? If so, then of course we'll never observe the ...
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1answer
100 views

How can I use Einstein's field equations? [duplicate]

Every time I try to find the answer to this question I get redirected to different pages that ultimately do not end up answering my question. I have some understanding of Riemannian geometry but have ...
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0answers
529 views

Does the existence of instantons imply non-trivial cohomology of spacetime?

Gauge theories are considered to live on $G$-principal bundles $P$ over the spacetime $\Sigma$. For convenience, the usual text often either compactify $\Sigma$ or assume it is already compact. An ...
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3answers
6k views

Is it possible to stop time? [duplicate]

Assuming the spacetime principle, if the space is modified the time does too. So if the velocity in the space is increase, does the time slow down? What happens if the speed is the speed of light, ...
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3answers
193 views

Why doesn't $ds^2 = 0$ imply two distinct points $p$ and $p'$ on a manifold are the same point?

Let's suppose I have a spacetime manifold $M$. Let $p$ be a point on my manifold. Now I move from $p$ to some other point $p'$. Presumably I should have moved some "distance" right? How can I speak of ...
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5answers
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Does the curvature of spacetime theory assume gravity?

Whenever I read about the curvature of spacetime as an explanation for gravity, I see pictures of a sheet (spacetime) with various masses indenting the sheet to form "gravity wells." Objects which are ...
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0answers
105 views

Questions about “geodesic path” and “arc length of a geodesic path” in the context of GTR and “Lorentzian manifolds”

I'd like to check my understanding of the notions "geodesic path" and "arc length of a geodesic path" in the context of GTR and "Lorentzian manifolds". Considering a set of "spacetime events", ...
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1answer
49 views

Quantum physics and constructable numbers [duplicate]

I do not know much about quantum physics. However, I do know it believes the world is discrete ( has quanta). This seems to contradicts the fact that we can create an object of length root 2 since you ...
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1answer
116 views

Are all galaxies we see really our Milky Way? Any possibility?

There is a possibility for our universe to be the surface volume of a (higher-dimensional) hyperspace. So if this possibility is true,then is there also a possibility that the other galaxies we see ...
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1answer
109 views

Why is space (almost) flat? Is it because masses are approximately homogeneously distributed? [duplicate]

The question I have is: Why is space (almost perfectly) flat in our neighbourhood? (I am disregarding the deviations due to the sun and the planets.) Is it correct to say that space is (almost) flat ...
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0answers
101 views

How does Einstein's gravity work? [duplicate]

I'm a chemistry student interested in physics. Hope the question doesn't sound funny. As opposed to Newton's gravity, which doesn't explain how gravity works, Einstein explained gravity as a result ...
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1answer
201 views

Is my diagram of spacetime curvature valid (relatively)?

I've been wracking my brain trying to understand what "curved spacetime" really is, and I think replacing one dimension with the time dimension then drawing the world-lines through time was the "aha!" ...
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0answers
162 views

What evidence is there of a universe older than 13.8 billion years

I've read an analogy that finding iron-rich galaxies just 900 years after the Big Bang is like finding an old man in a crib in a nursery. We just recently found a supermassive black hole 12 billion ...
12
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5answers
2k views

Hubble's law and conservation of energy

If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...
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3answers
154 views

Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?

IIs it valid to apply Einstein's Relativity to scenarios involving expansion of space? For a practical example of this: Is it legitimate to speak of distant red-shift galaxies as experiencing time ...
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235 views

Can a sufficiently large black hole be singularity-free?

This came to me after reading that a black-hole that has the mass of the observable universe will also have an event horizon that covers the observable universe. Since the definition of a black hole ...
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1answer
61 views

Even the universe is infinite, it could keep expanding? [closed]

the study showed Cantor, that there are infinite types, the universe would be expanding as well as the possibility of Stephen Hawking. http://phys.org/news/2015-02-big-quantum-equation-universe.html
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2answers
146 views

Why does the thought experiment of a photon bouncing of a mirror imply anything for other objects?

Okay, so I am reading a book, "The Elegant Universe" by Brian Greene, which talks about motion and its effect on time. Greene makes the point that time changes with motion by saying that if you have ...
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1answer
163 views

How can the universe by infinite when it has known start and finite time since? [duplicate]

Just watched the 'How Big is the Universe?' TV program and it indicated that the universe is believed to be infinite in size. They used an approach involving measuring the internal angles of ...
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1answer
86 views

Is the Big Bang instant considered a beginning of all the coordinates of our universe? [duplicate]

Is the Big Bang instant considered a beginning of all the coordinates of our universe? If we consider the BB to be the Beginning of of our universe, can we call this the Origin of all of the 3 ...
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4answers
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What is Minkowski spacetime?

I was browsing through an article on spacetime when I caught the words Minkowski Spacetime. A Wikipedia search brought me an article too complex for me to totally understand. So what is Minkowski ...
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Why is spacetime not Riemannian?

I apologize if this is a naïve question. I'm a mathematician with, essentially, no upper-level physics knowledge. From the little I've read, it seems that spacetime is Lorentzian. Unfortunately, the ...
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1answer
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Can surrounding masses influence the speed of time (excluding the time dilation due to gravity)?

https://www.youtube.com/watch?v=ev9zrt__lec @6:22-7:22 If you removed the middle and front spaceships, will the time of the last spaceship flow differently? As this is what it appears to suggest.
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98 views

Why is space 3 dimensional and could it be different? [duplicate]

Is there any particular physical law or property which caused space to be 3 dimensional, and is there theoretical possibility of having more or less dimensions in physical space?
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0answers
120 views

Do past and future exist? [closed]

Ok, first thing I want to say is that I'm a layman. No high school math background whatsoever, so I'd be greatful if you could provide an explanation without equations. Now I'm quite interested in ...
2
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2answers
813 views

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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0answers
33 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 ...
2
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1answer
53 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 ...
94
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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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1answer
127 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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1answer
94 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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2answers
210 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 ...
2
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4answers
881 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 ...
7
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2answers
249 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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1answer
127 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
135 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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80 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, ...
4
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2answers
149 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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55 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}) ...
1
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1answer
48 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 ...
2
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1answer
30 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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67 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
60 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? ...
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2answers
144 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 ...
0
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1answer
274 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 ...