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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39 views

Does gravity cause the gravity of another mass to curve?

For example say star 1 is at position x, however star 2 causes gravitational lensing on star 1's light that is traveling toward you and you now see star 1 at position y. Will star 1's gravity move you ...
0
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4answers
47 views

Does the universe expand in every direction evenly? [duplicate]

I've heard that the universe is expanding constantly and that galaxies are moving further and further away from each other because of this. However, does the universe expand in every direction evenly ...
-1
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1answer
59 views

Is there energy output when mass moves between two spacetimes [on hold]

Is there energy output when mass $m$ moves between two spacetimes? Say, it starts in a flat spacetime and then falls into a black hole (other examples don't come to mind, but this doesn't mean they ...
1
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2answers
54 views

How do we know singularities/black holes actually bend time?

Recently I read up on some articles about singularities / black holes, how they form and how they affect their surroundings. Just to clarify, I am no physicist and I found those articles on the ...
1
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2answers
61 views

Why is the possibility of the universe having a centre around which everything orbits not considered feasible?

I'm sure there is some kind of reason as to why this isn't considered as a legitimate theory but upon giving it though it seemed to make sense to me. I figure if all gravitational orbits are ...
2
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1answer
63 views

How can the size of the universe change over time when time is part of the universe?

Bit of an awkward way to phrase it, but basically: Time in relativity is just one of four dimensions of space-time. Nothing really special about it. Yet the universe was once smaller than it is now ...
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0answers
44 views

The expansion of the universe and its edge

I have understood that the universe is expanding at an accelerated rate. I have also thought, perhaps mistakenly, that this expansion means not that the masses in the universe are moving away from ...
2
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1answer
82 views

GR says that time and space are aspects of the same thing, yet there is no observable for time in QM

I understand that the topic of a time operator in quantum mechanics has come up more than a few times so forgive me if this is a repeat question but I couldn't find anything specific to my question. ...
19
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2answers
1k views

Chasing someone who has fallen into a black hole

Assume that my friend and I decided to explore a black hole. I parked the spaceship in a circular orbit safely away from the horizon. He puts on his spacesuit with a jet pack and carefully travels ...
1
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3answers
51 views

What is the measure of distance in higher dimensions?

In our world we are using kilometers to measure distance. What measurement is used to measure distance in higher dimensions?
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0answers
22 views

Gravity of Masses in 4-D [on hold]

I have developed a fairly stable understanding of the curvature of spacetime caused by a massive object (meaning with mass, not necessarily a lot of mass), though I have been wondering about a new ...
3
votes
2answers
98 views

How does critical density affect the expansion of the universe if gravity is the curvature of space-time?

From what I know there are three scenarios about the end and expansion of the universe that all depend on the concept of critical density: If the matter of our universe is above critical density, ...
3
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1answer
40 views

Would order 1AU metric expansion of space begin if the solar system were not inside a galaxy?

In this question I describe the >30 years of laser ranging between lasers on Earth and the retroreflector arrays on the Moon. Amazingly, after comparing this data to simulation of all of the orbital ...
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0answers
25 views

Killing Horizon for a space-time metric

How many Killing Horizon (KH) are possible for a particular S.T. metric. Because there is a surface gravity associated to each KH, which is constant over KH surface. Do a Black Hole have many surface ...
-5
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0answers
50 views

Is any physical quantity continuous? [closed]

As the title says: "Is any physical quantity continuous?"
1
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1answer
102 views

Why not $\rho^{2}=T^{\mu\nu}T_{\mu\nu}$ as an effective mass density (squared ) in general relativity?

Why not $\rho^{2}=T^{\mu\nu}T_{\mu\nu}$ as an effective mass density (squared) in general relativity? It's covariant, and as far as I can tell is zero for any electromagnetic field tensor. \begin{...
1
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1answer
38 views

Does the Komar mass density act like a four density?

The Komar mass is a means of measuring gravitational mass in spacetime. Via Wikipedia (https://en.wikipedia.org/wiki/Komar_mass) it is stated as (for a stationary metric): $$m=\int\rho d\mathrm{vol}=\...
3
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0answers
73 views

Clarification on the 11 dimension in the M-theory [closed]

In the book the Grand Design by Stephen Hawking, he wrote about the M-theory, and how in that theory it has 11 dimensions. I do not quite get that, so can someone explain it to me a little bit. I ...
0
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2answers
155 views

Have scientists ever experimentally proved the Andromeda paradox-like events? [closed]

We all know that a light year is of the order of 1016 metres and all the objects in the space are beyond a light year. So the Andromeda paradox should give considerable difference in time while ...
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0answers
38 views

Frames of reference vs Coordinate systems and the meaning of “inertial” [closed]

Wikipedia provides the following definition for inertial frames:(the reference given is to Landau's Mechanics book) In physics, an inertial frame of reference (also inertial reference frame or ...
-1
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0answers
45 views

Why do strings need 9+1=10 spacetime dimensions to exist? [duplicate]

Why do strings need 9+1=10 spacetime dimensions according to previous theory, or equivalently, 11 dimensions as per M-theory? Why cannot they exist in the 4-dimensional world?
2
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2answers
76 views

Expanding Universe [closed]

So an expanding universe has the metric $$ds^2 = g_{\mu \nu} dx^\mu dx^\nu = -dt^2 + a(t) ( dr^2+r^2 d\Omega^2)$$ where $d\Omega^2 = d\theta^2 + \sin^2\theta d\varphi^2$ as the usual spherical ...
0
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1answer
67 views

If the universe stopped expanding, when would we realize it?

Suppose in this moment the universe stopped expanding, how long would if take and how would we register the change? Since the bodies are still and it is space that is expanding in between, we wouldn'...
0
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0answers
15 views

Flat Universe and Evolving Geometry [duplicate]

I have heard in multiple locations that in the case of a flat universe, the universe is infinite. This seems to present multiple issues: a) If a flat universe were infinite, would would the space of ...
26
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5answers
6k views

If particles are points, then aren't atoms empty space?

Zero dimensional points do not take up space, so then wouldn't everything in the universe be literally empty? Or is there something that I'm missing?
-1
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3answers
302 views

About time and time dilation [closed]

This question is related to this answer of John Rennie. He says: The length of the red line is the same in both figure 1 and figure 2 I guess his meaning of red line is the space-time distance ...
4
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4answers
120 views

Visualizing gravity

When you see graphics trying to help people visualize what gravity in Einstein's relativity "looks" like it will often be a mostly two-dimensional plane with a concave warp where a massive object is ...
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0answers
18 views

Theory of relativity [duplicate]

I’ve been reading about Einstein’s Theory of Relativity, specifically the segment on gravity being explained as consequence of spacetime being curved by mass. I can understand the concept of object ...
-1
votes
1answer
89 views

In Einstein's “relativity of simultaneity” thought experiment, would not the passenger on the train see a dimmer signal? [closed]

This is the updated, more precise question--is this a paradox?: Suppose a rocket traveling close to the velocity of light which emits a single photon from its midpoint at point A, illustrated ...
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3answers
75 views

Is there any experimental evidence that Universe has more than 3 spatial dimensions?

Someone asked about proof of higher dimensions. But discounting time as a dimension, along with the hyper-dimensional GUTs/Supersymmetry theories, has this possibility been tested and what did the ...
1
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0answers
28 views

Capacitors in space [closed]

I'm new here and I have a question for all of you. What happens to a capacitor in open space? There are different kind of them, which one of these is better to use there? And... We know that if we ...
3
votes
1answer
46 views

Can an inhabitant of a spacetime region measure its curvature tensor?

So, lets say that I am an ant living on a 2-D spherical surface that is stretching to the equator...like half a sphere. I can not describe this surface in terms of the outside coordinates only someone ...
0
votes
2answers
292 views

Atomic Weight and Time Dilation

So, this might sound kind of ridiculous but I was thinking about Relativity and since Gravity is a warping of Space-Time, or Time Dilation, why don't we measure Atomic Mass in Units of Time Dilation? ...
1
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1answer
44 views

Can molecules/atoms/any subatomic particle cause space time curvature?

Space-time curvature is caused by huge objects in space like black holes, merging black holes, or planets. This curvature is what causes gravity. Can molecules cause at least a really tiny curve ?
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3answers
65 views

Is there any place in the universe where time passes more quickly than in the milky way?

I also understand that due to the relative orbital velocity, time on Earth is dilated compared to the centers of the solar system and galaxy. Is there a place that the Milky Way rotates around, and ...
1
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0answers
67 views

How is the Universe expanding faster than the speed of light? [duplicate]

I was watching a youtube video about the expansion of the universe. So the video said that the universe is expanding at a speed faster than the speed of light. I have 2 questions about that. My first ...
-1
votes
1answer
72 views

What do things move relative to?

When someone says that a spacecraft in otherwise empty space is traveling at a constant velocity of 10 km/h (for the sake of convenience) then what is the reference point for which this measurement ...
-1
votes
3answers
62 views

How can we look back at our Universe 13Billion years ago? [duplicate]

I read this article on Space.com and I quote "As far as we know, that's as far [back] as we can see — we get an image of the universe as it was when it was about 389,000 years old," So what I wanna ...
21
votes
5answers
2k views

Does curved spacetime change the volume of the space?

Mass (which can here be considered equivalent to energy) curves spacetime, so a body with mass makes the spacetime around it curved. But we live in 3 spatial dimensions, so this curving could only be ...
-6
votes
0answers
41 views

The origin of the universe and its current movement [duplicate]

Our universe started with the big bang, which was not within our universe, since it didn't exist. Let's assume it was the result of a quantum fluctuation in time. I have 2 questions: The common ...
2
votes
1answer
68 views

Is the age of the Universe relative to where you are within it? [duplicate]

The accepted age of the Universe is 13 billion years give or take. If you were a super space being and hypothetically had spent a large portion of this time sitting next to a black hole then your ...
0
votes
1answer
48 views

How can I send a message to myself in the past? [duplicate]

It is easy to send a message to myself in the future: I can put a letter in the post and will receive it tomorrow. Can you think about a way to send a message to myself in the past; it does not need ...
0
votes
1answer
32 views

What is the fate of a hyper-sphere universe?

This PBS video postulates a positively curved universe which is in the shape of something the narrator calls a "hypersphere". http://youtu.be/AwwIFcdUFrE?t=6m33s Such a universe would have no edge, ...
0
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0answers
30 views

Can orbiting a black hole 99.99% speed of light make other object with relatively fast clock travel faster than light, relative to me?

If i orbit a black hole, 99.9% speed of light, time for me is moving slowly, relative to me, planet earth is aging fast, and i have traveled 10 years into the future(relatively) in 1 second, suppose ...
2
votes
1answer
44 views

How to construct a space which is translation invariant but not rotation invariant

I am just confused by the following idea. Consider a 3-dimensional translation invariant space, we now have 3 translation generators. Then let us start with a point, the full 3-dimensional space ...
8
votes
3answers
134 views

What is “a general covariant formulation of newtonian mechanics”?

I am a little confused: I read that there are general covariant formulations of Newtonian mechanics (e.g. here). I always thought: 1) A theory is covariant with respect to a group of transformations ...
0
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0answers
79 views

What would a universe with 2 time dimensions be like? [duplicate]

In a universe with 2 dimensions of time, and 3 spatial dimensions, the equation for finding the change in time is: $\Delta T= \sqrt{(\Delta \alpha_1)^2+(\Delta \alpha_2)^2} \geq 0$ Where $T$ is the ...
6
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1answer
249 views

Emergence of space from quantum mechanics

Once talking to a visiting professor at my institute, I heard about some simple model that captures the emergence of space coordinates as the eigenvalues of some infinite-dimensional quantum ...
1
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1answer
87 views

How is Riemann tensor related to the curvature in the coordinates?

I came across statements such as "the acceleration observed in a weak gravitational field is mainly due to curvature in the time coordinate. " I want to know how we can explicitly find the curvature ...
0
votes
1answer
40 views

Homogenity and Isotropicity of space

In school it is given that law of conservation of momentum is a result of homogeneity of space and law of conservation of angular momentum is a result of isotropicity of space but what is isotropicity ...