Tagged Questions

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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Question about Gravitational Deflection of Light

I was wondering if deflecting light around two black holes or more would cause light to come back to the original spot. Such as if light from today traveled near a black hole so it was deflected 50° ...
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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 ...
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Gravity: Why Do things fall to Earth? [duplicate]

If gravity is in reality spacetime geometry why when I drop an object on the surface of the Earth does it fall to the ground? Does spacetime push it?
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I ask this question again in a simpler, shorter form. Maxwell's equations can be derived with Special relativity starting from the Coulomb's law. Therefore all the phenomena of classical ...
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How does “curved space” explain gravitational attraction? [duplicate]

They say that gravity is technically not a real force and that it's caused by objects traveling a straight path through curved space, and that space becomes curved by mass, giving the illusion of a ...
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How does the curvature of spacetime induce gravitational attraction?

I don't know how to ask this more clearly than in the title.
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Recommended book for beginners on advanced science topics [duplicate]

I have a background in engineering so I have some familiarity with basic math and science. I've recently been reading about other topics such as Einstein's relativity and have become interested in ...
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What would be seen when a ship is accelerated to relativistic speeds and back down? [duplicate]

Consider this scenario: Two identical space ships, the SS Observer and the SS Accelerator. Each ship is indestructible and contains an indestructible clock. They begin positioned 1 light-day away ...
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Spacetime curvature effect on chemistry

Do current chemistry / astrophysics / stellar chemistry calculations include the effects of the curvature of spacetime on chemical reactions? For example, the heat transfer from a point closer to the ...
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Questions arising from the presentation of “Schild's ladder” in “Gravitation” (Misner, Thorne, Wheeler)

MTW, p. 248, presents a description of "Schild's ladder" construction which I quote here extensively enough for referencing some subsequent questions. A. Transport any sufficiently short stretch ...
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Could fast vibrations cause us to travel forward in time

Assuming it's possible to vibrate a human at near light speed without harming him, would a few minutes of this from his point of view be much longer from a stationary observer's point of view? In ...
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Fixed background theories which feature lorentz invariance?

Are there fixed background theories that also display lorentz invariance? Could be with lorentz invariance being true only approximately at some limit... Realized we can still arrive at time-dilation ...
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What would be seen if a firing laser is accelerated to relativistic speeds then back?

Consider this scenario: Two identical space ships, the SS Observer and the SS Accelerator. Each ship is spherical with a band of mirrored material around the outside and contains a green laser. They ...
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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 ...
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How does curved spacetime cause motion revisited

There was a previous question titled "Why would spacetime curvature cause gravity?" asked March 10, 2014. The answer given was essentially that since the time component of an object in curved space ...
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Geodesic equation (free particle)

How to find a coordinate system whose geodesic equation does not have the "Christoffel symbol" term? (i.e. free particle - generalized Newton's second law.)
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Minkowski geometry definition

The general relativity is based on Minkowski geometry definition with its special properties. The general relativity cant be approved wihout Minkowski geometry definition. Why Minkowski geometry is ...
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Is the popular explanation given for gravity in General Relativity misleading? [duplicate]

In most popular explanations of General Relativity, both in print and film/television, gravity is demonstrated using an example of a 2 dimensional plane being flat, then when putting a heavy object in ...
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Does limiting to infinity violate laws of physics?

It is noteworthy that one cannot simply divide any length more than the Planck-length. If so, one cannot simply divide any volume more than the $(Planck-length)^3$. So if I want to find the limit of ...
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Why is it useful to study spatially inhomogeneous spacetimes? [closed]

There are many scholars who have dedicated their efforts towards finding solutions to the Einstein Field Equations that are spatially inhomogeneous. Current cosmological observations indicate that on ...
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Are there continuous spacetime events?

What is the formal definition of an event? According to Wikipedia, "an event is a point in spacetime (that is, a specific place and time) and the physical situation or occurrence associated with it." ...
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Do multiple dimensions exist? [duplicate]

I came up with this line somewhere: To properly experience $n$ dimensions the retina of the organism should be at least of $n-1$ dimensions. And human eye has $2$ dimensional retina, so we ...
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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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Can we actually prove with 100% accuracy that there are more than 3 dimensions plus time (t,x,y,z)? [duplicate]

Can we actually prove with 100% accuracy that there are more than 3 dimensions plus time? In the news there are always articles like "there are probably 7, 11 or more dimensions", "time is actually ...
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Is spacetime elastic or inelastic [closed]

If spacetime is nonelastic and the zero mass curvature is zero than local perturbations of spacetime by mass describable as circumference -2r(pi)is less than zero must be balanced by areas of ...
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Numerical relativity in causally pathological spacetimes

To perform numerical relativity simulations one almost universally adopts the so called "3+1" approach: spacetime is divided up into spacelike slices, each representing a "moment in time". After some ...
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Can blue light encounter slower clock time over red?

I am aware that matter with mass experiences dilation and photon is massless. there is no inertial frame where photon is at rest. However, wave form of light has momentum and energy. Blue light wave ...
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Are the principles of space-time homogeneity and Isotropy independent of one another?

Einstein in deriving the Lorentz transformations, used the principles of space-time homogeneity and Isotropy. Does space-time isotropy follow from space-time homogeneity or are they completely ...
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Are only measurements registered as events?

When we measure an electron or some other system behaving quantum mechanically, we observe something. Now, this can be called as an event in relativity terms meaning if we live in 1-D spacetime, only ...
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Why did the big bang happen when it did?

Was there time before the big bang? I assume there wasn't, so is it even logical to ask if there was anything before the big bang? If there was time before the beginning of the universe, what happened ...
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Why are objects at rest in motion through spacetime at the speed of light?

I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
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Calculate the relativity of time of an event occurred in different frames

Imagine we have a train which is moving with a velocity and there's an observer inside of that train and there is an observer that is outside of the train observing the train moving. There's a ball ...
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Thought experiment on space curvature due to gravity

Let's say you built a huge, straight rigid beam out in space, far from the sun (outside the orbit of Pluto). It is very long, say 200,000 miles long, but can be very narrow. Then you move it to the ...
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How to explain (pedagogically) why there is 4 spacetime dimensions while we see only the 3 spatial dimesions?

I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple ...
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Is there proof gravity bends space or is it just the most convenient explanation?

I have read this sentence in an article: The theory [of general relativity] holds that gravity is geometry: particles are deflected when they pass near a massive object not because they feel a ...
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Gravity's force on space

Here is a quote from Brian Greene, The Fabric of the Cosmos: Space, Time, and the Texture of Reality, p. 273: “The early universe provided an arena in which gravity exerted its repulsive side with ...
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Why does string theory require 9 dimensions of space and one dimension of time?

String theorists say that there are many more dimensions out there, but they are too small to be detected. However, I do not understand why there are ten dimensions and not just any other number? ...
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Time in general relativity

A physical quantity is introduced by its operational definition. In general relativity we use a differential manifold to describe the 4-dimensional space-time and, to identify a point in it, we use a ...
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Is presuming that any linear uniform motion is transformed into another one sufficient to assume that Lorentz transformation must be linear?

I've read this condition in a script: The Lorentz transformation must be linear because any uniform motion must be a uniform motion in any inertial frame. All other proofs that I've read so far need ...
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Do all black holes have the same mass density? [duplicate]

I have been reading about black holes in Stephan Hawking's 'A Brief History of Time'. Then I was reading more about it on Wikipedia; I am making a school project on it. I do not really remember what ...
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Can space have angular momentum that can be transfered to/from physical objects?

This is related to another question that discusses whether space can have momentum and energy. Apparently gravitational waves or ripples in spacetime is how space stores and transfers energy/momentum. ...
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Spacelike interval and length contraction

I am having a serious trouble finding an agreement between the two facts in Special Relativity: (1) given two events $A$ and $B$ in spacetime separeted by a spacelike interval $s$, the distance in ...
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Infinite Universe and Big Bang

A common view now in the physics community is that the Universe is infinite in extent (which makes Hubble sphere an infinitely small part of the Universe). On the other hand, there is the Big Bang ...
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What is the 'resolution' of the reality in pixel terms?

What resolution should a TV screen have so that its image were so faithful as reality as if the TV were a window? Also what would happen if Physics could reproduce a 'pixel' of the size $l_{p}^{2}$ ...
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Is there a model of the universe with the transfinite (space)time?

In mathematics there is a concept of ordinal numbers where one can count to infinity and beyond. For example the least number that is greater than all the finite numbers is denoted by $\omega$. Such ...
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If Alice is moving at relatavistic speeds compared to Bob, is collapse still simultaneous?

I have read some papers about experimental proof of non-locality involving a laser that goes through a beam-splitter and then each "half" goes to an observer (traditionally "Alice" and "Bob"). It has ...
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Effect of a photon on curvature of spacetime [duplicate]

Since light has no rest mass, is it reasonable to assume that the curvature of spacetime is not affected (distorted) by a single photon? Sure, a photon is attracted due to gravity. But could be ...
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How to derive the relation satisfied by “gravitational magnetic field” from an equation of the Weyl tensor?

Let us call the spacetime $M$ with a metric $g_{ab}$. There is a unit spacelike vector field $\eta^a$ orthogonal to a hypersurface. So that we can define the so-called gravitational electric and ...
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 ...