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.

learn more… | top users | synonyms

12
votes
3answers
1k views

How does gravity truly work? [duplicate]

I Am only 12 years old and I'm constantly wondering and trying understand how gravity really works. On YouTube everyone always talks about objects wrapping space time around themselves and uses the ...
1
vote
1answer
63 views

Questions about how the curvature of universe is described with a single number

I often read of physics talking about "the" curvature of the universe, giving it some letter $K$, and claiming that we have seen that $K = 0$ within such and such certainty. I have three questions: ...
-1
votes
1answer
66 views

Wouldn't it take an infinite amount of time to create an infinite universe? [duplicate]

I have heard many people say that the universe is possible infinite. But, if it takes time and energy to create bodies such as planets and stars and the universe has only existed for 13.7 billion ...
1
vote
1answer
42 views

Could the rest mass be somehow considered as a very exotic kind of kinetic energy?

In its rest frame a free system of invariant mass $m$ has energy $E=mc^{2}$. That frame is moving in spacetime with a four-velocity whose magnitude is $c$. Inside my brain something asks: Would ...
1
vote
0answers
80 views

Absolute direction of time [closed]

As Universe is expanding into emptyness, we think that flow of time is forward. This means that expansion is directed forward in time. Even though we don't know for sure whether this expansion will be ...
6
votes
4answers
238 views

Is light amplitude spatial?

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 ...
1
vote
1answer
76 views

Meaning of $R=0$, $R_{ab}=0$. $R_{abcd}=0$

First let me state some definition The Einstein tensor is given by \begin{align} G_{\mu\nu} = R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R \end{align} and note that \begin{align} G^{\mu}_{\phantom{\mu} ...
-1
votes
2answers
448 views

Space-time hospitality

Are there any reasons to assume that organic processes cannot arise in relatively flat spacetime environments? An argument for the existence of a critial value or window.
3
votes
0answers
26 views

Parity and Time reversal when the number of space or time dimensions is even

There's a side remark in the middle of section 2.6 of Weinberg I that I find a bit unclear. Suppose that $L(p)$ is a boost that carries the four momentum $k^\mu=(0,0,0,M)$ to $p^\mu$, and that ${\bf ...
0
votes
1answer
59 views

Does mass compress space-time?

My understanding of relativity explains that the presence of mass warps space-time so that light travelling through the warp follows at straight line but the warp itself is curved and therefore the ...
0
votes
2answers
64 views

Don't rulers expand at the same rate as the universe they measure? [duplicate]

If you placed a giant ruler between the sun and our nearest star for example, wouldn't the divisions on the ruler expand at the same rate as the universe - therefore maintaining a constant distance? ...
-1
votes
1answer
45 views

Since Alcubierre drive moves a flat region of space, does it mean that space acts lika a liquid? [closed]

The Alcubierre drive would move a patch of flat space through space and, leaving exotic matter aside, is mathematically correct. Does that means that space-time is not a "solid" lattice i.e. not ...
5
votes
1answer
113 views

How do we choose how to discretise spacetime in loop quantum gravity?

In LQG we choose a two-complex (or other) on which to discretise GR before quantising. I'm a little uncertain about the nature of this choice. Is the two-complex an a priori property of the universe ...
2
votes
2answers
56 views

Could it be possible to bend space in more than one “way”?

Ever since the announcement of the discovery of gravitational waves, I have found myself pondering related notions of space. Recently, I was thinking about an analogy I've heard to explain how mass ...
1
vote
0answers
93 views

What is the justification for continuing to model space-time as a manifold in string theory?

Background The question I have arises from taking seriously the idea that, in a physical theory, one should first define all physical quantities via (thought) experiment. Justification and ...
0
votes
1answer
61 views

Black hole's singularity: Does it has to be multi-dimensional?

We assume that there is an infinite density at the center of a black hole. But we also know that if it was really infinite, it would apply an infinite gravitational force to masses even if they were ...
0
votes
3answers
218 views

Two “devil's advocate” questions related to LIGO measurement results interpretation

"If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things." Rene Descartes Laymen like me typically refers to Wikipedia ...
0
votes
2answers
84 views

How do you know what kind of space(time) you have when solving the Einstein Field Equations?

I'm experimenting with the EFE, and I ''invented'' a metric; a diagonal non-zero metric, and I discovered that the Riemann tensors are equal to zero which implies the Einstein tensor $G_{mn}$ equals ...
0
votes
3answers
94 views

How is Space-Time produced? [closed]

The parable of the ant walking on the surface of a balloon tells us that as the balloon expands, more of surface is created, hence more place for the ant to walk. Space-Time is also in the same ...
0
votes
1answer
55 views

What dictates the rate in which time passes? [closed]

I understand that special relativity tells us that our perception of the passage of time can change depending on reference frames. But why does time pass the rate at which it does in my reference ...
0
votes
0answers
124 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", ...
3
votes
1answer
52 views

Is the cosmological horizon expanding faster than space itself?

I read that the rate of expansion of the universe is still a mystery. But if it's actually decelerating, wouldn't it mean that our cosmological horizon is expanding faster than space, and that one day ...
1
vote
0answers
22 views

Time in the early universe [duplicate]

When it is said, for example, that grand unification took place "Between 10−43 second and 10−36 second after the Big Bang", how are these figures of time derived? Moreover, do these figures have any ...
4
votes
5answers
160 views

Is the universe bounded?

As I understand it nobody can pinpoint an objective "center" of the universe nor "where" the Big Bang happened. It seems the observable universe is limited by our event horizon at some 14 billion ...
0
votes
3answers
98 views

Thinking about the properties of 'nothing' [closed]

If a certain identifiable part of space that has no type of measurable energy fields manifesting 'in it' for a given duration ; is such a totally empty space the same as 'nothing'? Anything with any ...
4
votes
2answers
432 views

Age of the universe and the singularity at the Big Bang

Using the standard model of cosmology we calculate the Hubble time to obtain an estimate of the age of the universe. This model assumes a beginning of time in the past. But that point is a true ...
18
votes
1answer
762 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 ...
0
votes
0answers
55 views

Scale factor of the universe

Do we impose that the scale factor $a(t)$ of the Universe is a continuous function? Or there is a physical meaning? Usually in physics we define functions to be continuous, such as the velocity of a ...
1
vote
1answer
114 views

What are gravitational waves relative to? [closed]

I have been having trouble picturing what kind of waves say Sun and Earth would make. Looking from top perspective Sun is in the middle and denting space while Earth is moving which is also denting ...
-4
votes
1answer
94 views

Time slows down as you approach the speed of light, travelers observe speeds faster than light travel? [closed]

If I was on a spaceship traveling almost at the speed of light, my time would slow down relative to the time for a stationary external observer. My question is, since we measure distances between ...
2
votes
4answers
137 views

Does light in vacuum actually travel at the speed of light? [duplicate]

I know my question sounds like a joke (and I suppose on some level it is) but I'm confounded by the following: As the thought experiment goes, if I'm in a spaceship flying rapidly the people on earth ...
4
votes
0answers
86 views

Aren't black holes required to exist forever in our frame of reference?

I know that for an observer far away, nothing ever crosses a black hole horizon (due to time dilation), while in the frame of reference of a falling observer the horizon is nothing special on its way ...
1
vote
1answer
71 views

Is it possible to travel faster than the speed of light under extreme warping of spacetime?

The event horizon of a black-hole is defined as the point at which light itself cannot escape, supposedly due to a black-hole's immense gravity and effect on spacetime. However, would this not require ...
2
votes
3answers
185 views

Does it make sense to say that objects moving at relativistic velocities appear in space-time cross-section?

Let object A move at relativistic velocity relative to a frame O. In 4D space-time (Minkowski diagram) the space view of O at any given moment of its own time is a space-like hypersurface (hyperplane, ...
0
votes
1answer
19 views

Energy to move in space-time

Disclaimer : I am quite a beginner in physics, so very sorry if this is too much basic of a question but I have never read anything about what I am going to ask now. Or maybe I have but did not ...
1
vote
1answer
63 views

Is it correct to think about a point in time as the set of positions of all “things”?

Is it correct to think about a point in time as the set of positions of all "things" (photons, electrons, etc) that exist in the universe at that moment, despite the fact that simultaneity is ...
1
vote
0answers
38 views

Do relativistic frames imply different realities? [duplicate]

From Einstein's principle of relativity space-time and mass are relative to the frame from which they are being observed. Now would that mean a ball moving at the speed of light wrt to earth would ...
10
votes
4answers
5k views

Does velocity or acceleration cause time dilation?

What causes time dilation? Acceleration or velocity? I've seen multiple comments on this forum that assert velocity is the cause, but that doesn't seem right to me. You can't have velocity without ...
1
vote
2answers
87 views

Is Time Dilation in GR Simply a Consequence of Curved Space?

My understanding of time dilation in General Relativity is that it is the consequence of particles traveling at the speed of causality following longer, curved paths. Since it takes longer for two ...
4
votes
3answers
219 views

Gravitational self-interaction

Today, someone asked me why "the warped space-time warps itself" (he read it in Kip Thorne's: The Science of Interstellar). I guess this is related to the gravitational self-interaction. But I don't ...
1
vote
0answers
39 views

Measuring distance in a spaceship moving with 0.5c [closed]

If there are a spaceship moving at a speed of 0.5c that sent a photon when an asteroid was exactly $3\times 10^8$ m away, will the distance between the spaceship and the asteroid when the photon hits ...
0
votes
2answers
74 views

How is the universe flat?

I have real trouble visualising what is meant by the descriptor 'flat' when referred to the shape of the observable universe. Which one of the below is more accurate? a) It is flat in a 2D way, like ...
1
vote
3answers
84 views

Time in spaceships near the speed of light

I know that the time on a moving object moves slowly compared to a stationery one, but if someone on a spaceship that moves at V=0.5c and a person is driving it. Will he notice that the clock on his ...
4
votes
2answers
101 views

Gravitational waves in other dimensions

I know this question is purely speculative, as we don't know if more dimensions do exist and also we do not know if gravity is indeed stronger in other dimensions (if they were to exist). But, one of ...
2
votes
1answer
262 views

Do Gravitational Waves Actually Repel Spacetime? [closed]

I read something explaining that g-waves actually repel space-time. It described the following. Mass bends space-time creating gravity. A massive object like the Sun, if it suddenly disappeared the ...
3
votes
1answer
177 views

Quantization on Minkowski/Schwarzschild spacetimes based on unusual surface

I'm reading the book of Wald "Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics", and I'm pondering on this problem: In Minkowski spacetime, we usually quantize our fields with ...
1
vote
1answer
55 views

Turbulence in gravitational waves

I want to understand physical meaning and possible implications of few terms often used in physics specially with regards to gravitational waves / space-time fabric Space-time fabric is disturbed by ...
2
votes
2answers
76 views

How does gravitational wave compress space time?

My question came from the talk of how gravitational wave stretches and compresses space time. Say there are two protons that are 1 centimeters apart, as a G-wave passes through them, would the ...
0
votes
1answer
27 views

How is light used to measure the change in time caused by gravity?

Is the following analogy correct, if not- what changes are necessary to correct it? Let's consider a guy riding on a photon with a clock, and every time his clock ticks he paints a dot. Let's also ...
0
votes
1answer
105 views

How can gravitational waves be detected when spacetime bends the instruments? [duplicate]

Apologies for the naive question, hoping someone with a proper physics background can explain this in layman's terms (as opposed to the more detailed answers at LIGO flawed by the identical expansion ...