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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1answer
947 views

What is the 4th dimension? [closed]

I have heard before that the 4th dimension is time, however, another theory makes a lot more sense to me. This is that the 4th dimension is the third dimension stacked on top of each other in a ...
6
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3answers
633 views

The Big Bang in an infinite universe

If the universe is spatially infinite (and assuming, if it makes a difference, that we don't have eternal inflation), what actually happened 13.7 billion years ago? Was the energy density infinite (or ...
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2answers
895 views

How could spacetime become discretised at the Planck scale?

I didn't have much luck getting a response to this question before so I have tried to reword and expand it a little: In early 2010 I attended this inaugural lecture by string theorist- Prof. ...
4
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1answer
318 views

What are the current (popular(ish)) approaches to modelling the quantum nature of spacetime at the Planck scale?

My guess at a list of them would be: spin foams, casual sets, non-commutative geometry, Machian theories, twistor theory or strings and membranes existing in some higher-dimensional geometry... ...
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7answers
3k views

Experimental evidence of a fourth spatial dimension?

As human beings, we observe the world in which we live in three dimensions. However, it is certainly theoretically possible that more dimensions exist. Is there any direct or indirect evidence ...
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4answers
1k views

Can spacetime be non-orientable?

This question asks what constraints there are on the global topology of spacetime from the Einstein equations. It seems to me the quotient of any global solution can in turn be a global solution. In ...
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10answers
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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 ...
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2answers
1k views

What does “foliation” mean in the context of a “foliation of spacetime?”

I've seen foliation used in the context of "foliation of spacetime" here and elsewhere in papers and such. Generally defined in reference to a "sequence of spatial hypersurfaces." But I don't know ...
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6answers
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What is our location relative to the Big Bang?

Given what we know about space, time and the movement of galaxies, have we or can we determine what our position is in relation to the projected location of the Big Bang? I've read some introductory ...
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3answers
4k views

Example of space-like intervals in spacetime

From wikipedia: When a space-like interval separates two events, not enough time passes between their occurrences for there to exist a causal relationship crossing the spatial distance ...
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2answers
2k views

What does it mean that the universe is “infinite”?

This question is about cosmology and general relativity. I understand the difference between the universe and the observable universe. What I am not really clear about is what is meant when I read ...
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3answers
2k views

If the observable universe were compressed into a super massive black hole, how big would it be?

I understand only a little of general relativity, but that's why I'm here! :) Consider the hypothetical situation of some extra-terrestrial intelligence pushing all the mass in the observable ...
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6answers
3k views

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 ...
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4answers
2k views

Measuring the speed of light and defining the metre - absolute or relative?

If the metre is now defined as the distance light travels in vacuum in 1⁄299,792,458th of a second and the speed of light is accepted to be ...
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1answer
153 views

Four-dimensionalism vs energy economy

Four-dimensionalism claims that the universe is basically one huge space-time worm and that everything exists at once (however you want to say that since "internal time" is then just another ...
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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
637 views

Why does moving through space at the speed of light automatically limit us from moving through time? [closed]

If I'm moving through MY reference frame at the speed of light, isn't time still passing by normally for me? Help me think about this fourth dimension- space-time. I want to intuitively understand ...
3
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1answer
541 views

Introductions to discrete space-time

It's comparatively easy (cum grano salis) to grasp the following concepts: Euclidean space-time (continous space and continuous time) classical mechanics (discretely distributed matter in continous ...
24
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1answer
1k views

How does classical GR concept of space-time emerge from string theory?

First, I'll state some background that lead me to the question. I was thinking about quantization of space-time on and off for a long time but I never really looked into it any deeper (mainly because ...
14
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3answers
886 views

Swimming in Spacetime - apparent conserved quantity violation

My question is about the article Swimming in Spacetime. My gut reaction on first reading it was "this violates conservation of momentum, doesn't it?". I now realize, however, that this doesn't allow ...
4
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3answers
900 views

What are some approaches to discrete space-time used in modern physics?

This thought gave rise to some new questions in my mind. What are the consequences for: How would it affect duality i.e. particle, wave property of photons? How does this statement affect the ...
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5answers
3k views

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 a the total flux through a ...