# 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.

3,263 questions
Filter by
Sorted by
Tagged with
32 views

### Are tachyons a mathematical impossibility (not just physical)?

I recently learned from a helpful SE user that, in general relativity, the "law of geodesic motion" is actually a mathematical law, not a physical one. That is, a "test particle" (...
18 views

### What is the cardinality of all points in space time? [closed]

Since the universe isn't a cartesian grid, and can bend and warp, what is the cardinality of all points in space time?
64 views

### Does Absolute Rest Imply Absolute Simultaneity in General Relativity?

In Special Relativity, absolute rest implies absolute simultaneity. In other words, although simultaneity is standardly taken to be relative to an inertial reference frame, if you think there are ...
397 views

### How can we perceive light if light doesn't move through time?

From my understanding in space-time everything moves at the speed of light $c$ in some direction of this four dimensional space. Light itself moves only through space, so it doesn't move at all in ...
30 views

### Is spacetime equivalent to changes in index of refraction of aether? [closed]

If space is made of aether, and it is denser in regions of mass concentrations and have higher index of refraction in those regions, if light goes where index of refraction is high number, it will ...
53 views

### Should time be a loop or a line?

It's interesting to see that torus was so popular in physics, that it worked so well. There was famous Gauss–Bonnet theorem which stated basically $$\oint_S K dS= 2\pi \chi(S)$$ where K was the ...
813 views

### What's incorrect with using the two formulas for Lorentz contraction and time dilation at the same time?

Lorentz contraction: $L = L_0 / \gamma$ Time dilation: $t = \tau \gamma$ Using the above two formulas at the same time leads to contradiction to the principle of the constancy of light velocity. What’...
75 views

### How equivalence principle explains curvature of space?

We know that the equivalence principle explains the slowing down of time, since, with respect to an inertial observer the time difference between sending two successive light pulses at the head of the ...
84 views

### Objective direction of time in general relativity

In general relativity, the coordinates $x^\mu$ we put on the manifold are arbitrary and need not have any physical interpretation. For this reason, it is said there is no objective notion of time in ...
44 views

### On the derivation of Rindler metric

I'm trying to reproduce the derivation of the Rindler metric in this wikipedia article but I came at a point which I cannot undertand. In the Rindler coordinates section, $\alpha$ is the proper ...
40 views

### Can spacetime be curved without matter inside it? [duplicate]

I can imagine space to be curved without matter in it. But can this happen in reality and what does it mean? A black hole is said to be a vacuum solution but doesn't it require matter to form a black ...
92 views

### Curvature of space without mass or energy [duplicate]

Are there solutions of Einstein's equation $$R_{i,k}=0$$ which imply there are 'curvature' without matter or energy? For example, space is curved but there are no matter or any energy there so an ...
73 views

### Warped space shouldn't cause movement in Gravity so what does? [duplicate]

I'm trying to get an intuitive understanding for how gravity works and have been reading various explanations from answers here to Sean Carroll and others' youtube videos. I have a set of university ...
66 views

### Another slices of the universe than space? [closed]

We observe a particular kind of "slices" of our universe: the slice that is called "space" (in the special sense of our 3 dimensional physical space). Are other kinds of slices in ...
40 views

### Anti de Sitter & de Sitter Spacetime [duplicate]

Can anyone please explain what are the Anti de Sitter and de Sitter spacetime and what is special about them? I am learning general relativity and I stumbled upon them a few times, even on the subject ...
61 views

### Is inertial mass actually reverse-time gravity reaching back from the distant future and pushing?

James Woodward of Cal State Fullerton says so. He say it's just like an advanced wave in QED. He says that gravity accounts for inertial reaction forces as long as the interaction of local stuff with ...
144 views

### On the dependence of time in transformation between reference frame in Special relativity

Suppose we have a reference frame S with (position,time) variable as $(x,t)$ and in another frame$S'$ we have another set of coordinates $(x',t')$ moving at a speed of $v$ with respect to $S$ then ...
65 views

### A coordinate-free understanding of the space-time manifold

I study dynamics and continuum mechanics. Over the years I've gotten used to the coordinate-free, or geometric, way of thinking. A velocity vector, for example, is a tensor. It is the same object when ...
24 views

### What was the volume of the universe a short time after the big bang? [duplicate]

I assume this question is somehow ill posed, but I do not know in which way. I think it is not a difference here whether the universe expanded into space, or space expanded itself. Seen from the ...
44 views

### Is there a Maximum Data Density of a Physical Volume?

Is there an established or proven relationship concerning how much data can fit in a given volume of space / spacetime?
288 views

### How is spacetime curvature generated in string field theory?

In String Theory, the graviton originates from a mode of vibration of the fundamental strings and spacetime curvature itself is somehow generated by the strings themselves. In String Field Theory, ...
69 views

### An apparent paradox in General relativity using a binary black hole

Note: Quantum gravity effects are ignored in this question. Imagine 2 black holes each with mass $m$ approaching each other and after some time the Event Horizons (EHs) of both black holes touch each ...