Physics Stack Exchange
Physics Stack Exchange

Questions tagged [spacetime]

Within relativity (both special and general), changes of reference frames can change both the notions of space and of time, with one depending on the other as well. As a consequence, it is necessary to treat both concepts in a unified manner. Hence the term spacetime.

Term introduced by Herman Minkowski to describe the ontological status of space and time in the theory of and subsequently also in and more modern theories.

Within relativity (both special and general), changes of reference frames can change both the notions of space and of time, with one depending on the other as well. As a consequence, it is necessary to treat both concepts in a unified manner. Hence the term spacetime.

The term has outgrown its original context and is now used in other theories.

4524 questions
170
votes
9 answers

Does someone falling into a black hole see the end of the universe?

This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event horizon the result is infinite. This implies that the…
John Rennie
  • 367,598
169
votes
7 answers

Why would spacetime curvature cause gravity?

It is fine to say that for an object flying past a massive object, the spacetime is curved by the massive object, and so the object flying past follows the curved path of the geodesic, so it "appears" to be experiencing gravitational acceleration.…
user1648764
  • 2,006
154
votes
2 answers

Does the Planck scale imply that spacetime is discrete?

On a quantum scale the smallest unit is the Planck scale, which is a discrete measure. There several question that come to mind: Does that mean that particles can only live in a discrete grid-like structure, i.e. have to "magically" jump from one…
vonjd
  • 3,801
144
votes
6 answers

If you view the Earth from far enough away can you observe its past?

From my understanding of light, you are always looking into the past based on how much time it takes the light to reach you from what you are observing. For example when you see a star burn out, if the star was 5 light years away then the star…
JD Isaacks
  • 1,863
130
votes
6 answers

What is known about the topological structure of spacetime?

General relativity says that spacetime is a Lorentzian 4-manifold $M$ whose metric satisfies Einstein's field equations. I have two questions: What topological restrictions do Einstein's equations put on the manifold? For instance, the existence…
Eric
  • 1,804
114
votes
7 answers

What do spacelike, timelike and lightlike spacetime interval really mean?

Suppose we have two events $(x_1,y_1,z_1,t_1)$ and $(x_2,y_2,z_2,t_2)$. Then we can define $$\Delta s^2 = -(c\Delta t)^2 + \Delta x^2 + \Delta y^2 + \Delta z^2,$$ which is called the spacetime interval. The first event occurs at the point with…
Gold
  • 38,087
  • 19
  • 112
  • 289
110
votes
3 answers

Why are 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 articles): The Big Bang never happened at all. The…
Janus Boffin
  • 1,438
  • 2
  • 13
  • 27
99
votes
9 answers

What is a manifold?

For complete dummies when it comes to space-time, what is a manifold and how can space-time be modelled using these concepts?
Richard971
  • 1,105
  • 1
  • 8
  • 5
94
votes
10 answers

Why do scientists think that all the laws of physics that apply in our galaxy apply in other galaxies?

I like watching different videos about space. I keep seeing all these videos saying scientists found so and so at 200 billion light years away or this happened 13 billion years ago. My question is why do scientists think that all the physics that…
andre chancellor
  • 845
87
votes
6 answers

Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed?

I have heard both that Planck length is the smallest length that there is in the universe (whatever this means) and that it is the smallest thing that can be observed because if we wanted to observe something smaller, it would require so much…
George Smyridis
  • 3,011
83
votes
7 answers

Does Coulomb's Law, with Gauss's Law, imply the existence of only three spatial dimensions?

Coulomb's Law states that the fall-off of the strength of the electrostatic force is inversely proportional to the distance squared of the charges. Gauss's law implies that the total flux through a surface completely enclosing a charge is…
Justin L.
  • 6,090
83
votes
13 answers

Turbulent spacetime from Einstein equation?

It is well known that the fluid equations (Euler equation, Navier-Stokes, ...), being non-linear, may have highly turbulent solutions. Of course, these solutions are non-analytical. The laminar flow solutions (Couette flow for example) may be…
Cham
  • 8,015
82
votes
17 answers

How exactly does curved space-time describe the force of gravity?

I understand that people explain (in layman's terms at least) that the presence of mass "warps" space-time geometry, and this causes gravity. I have also of course heard the analogy of a blanket or trampoline bending under an object, which causes…
Zac
  • 923
80
votes
6 answers

Does gravity CAUSE the bending of spacetime, or IS gravity the bending of spacetime?

In reading these discussions I often see these two different definitions assumed. Yet they are very different. Which is correct: Does gravity CAUSE the bending of spacetime, or IS gravity the bending of spacetime? Or do we not know? Or is it…
foolishmuse
  • 5,016
77
votes
10 answers

Is 3+1 spacetime as privileged as is claimed?

I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. There is a summary of arguments on Wikipedia. For…
Roman Starkov
  • 2,265
1
2 3
…
99 100