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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3answers
488 views

“Warp-drive” thought experiment

Why wouldn't this "warp-drive" work? Assuming we can build a flywheel that can survive near light speed and have an onboard power storage or beamed energy that can spin it up to that speed and back ...
0
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1answer
48 views

Coordinate Time and Proper Time/Simultaneity [closed]

At close to the speed of light, clock A is running slower than "stationary" clock B. Does clock A enter the future time reference of clock B as it slows down? Trying to understand where the forward ...
2
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2answers
108 views

Backwards in time?

A “Universe Today” article titled “Galaxies Are Moving Away From Us Faster than the Speed of Light” prompted the following question, which the author of the article was unable to answer: “Would such ...
0
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1answer
60 views

Why is spatial conformal infinity a point

One property of spatial infinity is that all spacelike geodesics end at it. Since spacelike geodesics can have different directions, I do not understand why spatial infinity is a point. It looks more ...
0
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1answer
183 views

Is the Universe infinite or it's just expanding to infinity?

According to the WMAP data in the past years we can say that our universe is considered flat or at least nearly flat and we also know that flat universe is allowed to be both infinite or finite in ...
0
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1answer
66 views

Special relativity, calculate velocity

Lets assume that a spaceship travels to a star 8 light-years away, in a time its crew considers to be 8 years. How do we calculate the speed of the ship? Is at as simple as saying: since the star is 8 ...
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1answer
54 views

According to some theories space is constantly expanding but at what granularity? [duplicate]

One idea from cosmology is that galaxies are all moving away from each other. Naively this could be taken to mean that they are expanding into something. Popularly this could be seen as them moving ...
0
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1answer
155 views

Space time diagrams and world lines

When you draw a space-time diagram and the associated world line for an object moving through space-time, the diagram will be dependent upon the reference frame you are observing from, right? Like for ...
0
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0answers
47 views

Confusion in Special Relativity: Rotating frame of reference

Suppose we are observing a rotating frame from an inertial frame, free from gravity, and try to measure the circumference of a circle drawn in the rotating frame. Since our measuring rod would be ...
0
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0answers
40 views

Calculating propagation time along the path of a photon

In this paper $\tau(t) := \int_{t}^{t_d}ds$ is defined as "the propagation time between the detection time, $t_d$, and $t$, as incrementally measured by a set of local observers along the light path". ...
1
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1answer
140 views

Is there a minimum distance?

I would imagine there is no limit to how small space can get. Is this correct? I am aware of planck's constant, but cannot objects be closer than Planck's constant is short? Perhaps this question is ...
1
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1answer
69 views

Gravity vs. Velocity time-dilation [duplicate]

Is there any difference in effects from time dilation of that due to gravity vs. that due to velocity?
0
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1answer
116 views

Time dilation: linear or exponential or other?

As you near the speed of light, time slows for you relative to the world around you. So you would be traveling at more than one hour per hour. Is the rate of time slowing for you linear or exponential ...
0
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0answers
37 views

Is the curvature formed due to mass/volume of spactime convex or concave?

I am really confused about the curvature of space-time. Does it bends in a concave shape in presence of mass/volume as shown by the example of rubber sheet and ball or does it bends in a convex shape? ...
1
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1answer
57 views

Difference between Distance and Space

So I take particle A and place it in space, then I place particle B 1,000,000 light years away from particle A. I strike a line between the two, and measure that line as having a length of 1,000,000 ...
1
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2answers
113 views

Do things move towards each other in space?

I watched a video once explaining space-time curvature, and it said that one of the things that made Einstein think of it is because objects in free fall move radially inward, so two objects falling ...
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1answer
137 views

proper time interval

We have two frames s and s' , s' moves with a constant velocity 'v' with respect to s in positive x direction, a particle B is thrown downwards from s' , and it collides with a particle A , the point ...
2
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2answers
97 views

Could time have tertiary directions?

This is a late night thought, but it seems interesting enough to ask: Has anyone considered the possibility that time might have more directions that forward and backward? Could time go sideways?
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2answers
94 views

Are fundamental particles just a trajectory in spacetime? [closed]

Long time ago Wheeler proposed an idea that there is only one electron, some kind of a closed(?) four dimensional knot in spacetime, and in one particular time we slice this knot with a plane and ...
0
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0answers
23 views

how to read the space time continuum plot

The space time continuum is shown a lot via a 3d plot with what looks like balls on a curved surface. How is it that all events in 4 dimensions can be drawn as just a curved surface? Is it because ...
0
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1answer
64 views

What are White Holes?

What are white holes? How do they affect spacetime? A black hole has extreme gravity which bends the spacetime fabric, but what does a white hole do? Also, what are they made of?
2
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1answer
320 views

Does the Michelson-Morley experiment disprove gravitational waves?

I may be misunderstanding gravitational waves, but doesn't the idea of a gravitational wave imply that spacetime is a field or "medium" through which waves propagate at speed $c$? Was not this idea ...
3
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1answer
139 views

How can a photon “stop”? What does its world line look like? [duplicate]

Einstein famously made a thought experiment: what would he see if he sat on a beam of light? His answer was -- it's impossible. Owing to him being a body with mass, he can never ever reach light speed ...
3
votes
1answer
82 views

Is each succeeding dimension time? [closed]

I have been researching but I have found nothing on this topic (at least after basic google searching and some deeper searching on this site), I'm not sure if this is a common sense thing or it is ...
0
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0answers
54 views

Outer Space & Universe Expansion: [duplicate]

Experts agree that the stuff within the universe is moving farther apart from all the other stuff in the universe--much like the dots marked on the outside of a balloon that is being inflated. We ...
4
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1answer
630 views

Why do people put exponentials there

In his book, Sean Carroll, says p. 194 chapter 5: To impose spherical symmetry, we begin b writing the metric of Minkowski space in polar coordinates $x^{\mu}=(t,r, \theta, \phi)$: $$ ...
0
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2answers
162 views

Would the speed of an object keep increasing if it keeps falling infinitely? Please check details [closed]

Assuming "A" and "B" to be two wormholes (Einstein-Rosen Bridge) inside the Earth's atmosphere, and Warmhole B be the opening to Warmhole A, would the speed of an object keep increasing if it enters ...
2
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3answers
377 views

Relative Time from a far place

I have read that whenever we see the sun we are seeing the sun as it was before 8 minutes ago. Meaning if the sun were to somehow go dark we would not know until 8 minutes after. Now I became ...
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1answer
65 views

It's a Hypothesis about the Beginning of the universe [closed]

Let's take a piece of paper and plot a couple of dots in it. If we shrink the paper infinitely, at some point it would reach a state of infinite density, that is; a Singularity. But even at that ...
0
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0answers
79 views

3D-representation of space-time

When I read something about GR, I nearly always see some pictures that look like trampolines, like this one. I know that the curvature of space-time is described by the Riemann-Tensor $R$. I was ...
5
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1answer
163 views

How to measure Torsion and Non-metricity?

In General Relativity, we most often work with the Levi-Civita connection (metric and torsion-free). What kind of experiment can we make to be sure that our physical space-time indeed is torsion-free ...
43
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3answers
6k views

Why should the Planck constant be a constant throughout all space?

Our value for the Planck constant $h$ can be found on experiments on Earth, but how do we know that the Planck constant doesn't change throughout space, for instance it depends weakly upon the ...
1
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2answers
123 views

Falling with same acceleration and meaning of gravity

My question is what does falling with same acceleration has to do with what Einstein concluded concerning the gravity in terms of the curvature?
-1
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2answers
80 views

Big Bang: Initial velocity of all matter in four dimensions? [closed]

Consider this hypothesis: Two objects A and B, are able to maintain their spatial distance. Object A is near a neutron star, and thus it clock is running at a different speed than object B, and both ...
0
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0answers
28 views

Minimum amount of space [duplicate]

I always believed that there were no limits in the world and then I learned that a minimum temperature exists, and maybe a maximum speed. And now I am wondering about the boundaries of the world. Is ...
0
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0answers
19 views

Resources on the discreteness of space and time

Does anyone know any books or other sources of information on the discreteness of space and time? The only ones I've managed to find require graduate level physics knowledge, which sadly I do not ...
5
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4answers
254 views

Is there a geometric interpretation of the spacetime interval?

In Euclidean space, the invariant $s^2 = x^2+ y^2+ z^2$ is equal to the length square of the position vector $r$. This is easily understood and can be represented geometrically in a graph. On the ...
4
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3answers
124 views

Can space and time separately be curved?

How can I imagine curved time, if it is not a part of four dimensional spacetime? Similarly for space. What are the measurable, observable consequences of these two phenomena in a laboratory or in ...
1
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1answer
99 views

How does the expansion of the Universe affect Black Holes?

As know our Universe is expanding, which I have always visualised as some stretching of the metric of space-time, which "flattens" it. In the same time. our theoretical perception of Black Holes ...
1
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3answers
205 views

Is Bekenstein entropy limit inconsistent with universal continuity?

It is unknown whether the universe is discrete or continuous in its intricate quantum level structure. See for example: Can universal continuity be experimentally falsified? Is the universe finite ...
0
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1answer
85 views

Could this be a valid reason for why the universe could be infinite?

So, special relativity says that every frame is as good as any other frame, and there is no absolute frame of reference. All good. Suppose there is a finite universe (let's just assume the universe ...
0
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2answers
118 views

Can less gravity speed up time? [closed]

We know more gravity slows down time per Einsteins Theory of Relativity. However will a significantly less gravitational pull speed up time? Or is "our time" aka earth standard of time the fastest ...
2
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2answers
129 views

How fast does space/time travel?

It is said that before the big bang there was no before, as space/time was created in the big bang. The big bang creates a rapid expansion of space/time... so how fast is rapid?
0
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0answers
21 views

Given the intervals of a punctured flat spacetime how to reconstruct the intervals of the complete flat spacetime?

A set $\mathcal S$ of events can be characterized as a flat unbounded spacetime through a function $s^2 : \mathcal S \times \mathcal S \rightarrow \mathbb R$ which assigns spacetime interval values to ...
0
votes
1answer
68 views

Einstein space - proper definition [closed]

Excuse, this is my first question at this forum, I try to be clear and short. What is the exact definition of Einstein spaces? It's enough to say $$ G^{\mu\nu}_{;\mu}=0~? $$ Where $$ ...
0
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0answers
36 views

Is there a bound on spacelike intervals between the trajectories of a pair of chronometrically rigid participants in flat spacetime?

Considering, in flat spacetime, two participants, $A$ and $B$, who were "chronometrically rigid" with respect to each other (i.e. $A$ found constant ping duration with respect to $B$, and $B$ found ...
1
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1answer
101 views

According to general relativiy, why does earth revolve around the sun? Does the earth follow geodesics in spacetime caused by sun?

I know according to classical theory, its centrifugal force. Also, I need to assure that mass curves spacetime, not space. Now, if mass curves spacetime, how can an apple which follows geodesic in ...
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2answers
142 views

How do we know we live in 3 dimensions?

How do we know that we are really living in 3 dimensional world? We can draw 3D in a paper that is one dimension. So, maybe the world around us just looks 3D! how can we prove that we're living in 3 ...
-3
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1answer
49 views

Explanation and quantum gravity [closed]

Is spacetime accelerating? Is it accelerating within an acceleration ?Or is it a constant? (are different objects or particles influenced by other rates in spacetime compared to one-another and if ...
4
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3answers
492 views

Physically what does warping (of space-time) mean?

So there's general relativity and Einstein's field equations that tell us "mass(or equivalently energy) warps space-time, and the warping tells mass how to move", but I'm still having trouble ...