# Tagged Questions

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### If a black hole was near enough us to be seen by telescope (by eye), if it ate a star, how long would we be seeing the star being eaten? [duplicate]

Since we are external observers, an object falling in a black hole should take an infinite amount of time right? If so, what is the answer to my question in the title?
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### Does infalling matter ever cross the event horizon? [duplicate]

Assume you are immortal, then say if you fell towards a blackhole, to an observer far away you will appear to slow down as you approach event horizon and gradually come to stand still. However for ...
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### Should any matter form a singularity before crossing the event horizon?

I guess the destiny of any object that falling into a black hole will depend on its size and the density of the black hole it falling in. And from distant observer it should looks different. A small,...
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### How does a screen (computer moniter movie theater et cetera) produce the color black?

This might seem a dumb question, but here goes: I can't help but notice that a black screen on my computer looks more like black than the screen when the computer is turned off. Similarly a black ...
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### How can I slow down? Or, How can I speed up time? Reverse twin paradox

Everyone know the standard Twin Paradox. I have my clock synchronized here on Earth with my twin. I leave Earth, Travel for a time at 0.9c, turn around, come back at .9c and then my clock is slow ...
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### Can anyone clarify this example of Relativity of Simultaneity [closed]

While doing this course: http://www.worldscienceu.com/ I couldn't understand this example http://www.youtube.com/watch?v=53oAxycVhhg. (full example here https://www.youtube.com/watch?v=Iiugtmt18W4) ...
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### How to determine the three-velocity measured by a single observer?

In this question the issue of extracting the three-velocity of a test particle in some frame arose. The answer provided by @PaulT and in this paper (arXiv) seems to me to give rise to ambiguity when ...
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### With respect to what can't we travel at the speed of light? [closed]

According to theory of relativity the speed of light in vacuum is ultimate. But since objects move relative to each other, with respect to what can't we travel at the speed of light?
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### Extension of Schrödinger's cat thought experiment

My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But ...
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### Reciprocal Time Dialation in Special Relativity [duplicate]

I'm trying to understand theory of special relativity, but there is one thing that really makes me confused which is reciprocal time dilation in special relativity. In special relativity, the time ...
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### What do things move relative to?

When someone says that a spacecraft in otherwise empty space is traveling at a constant velocity of 10 km/h (for the sake of convenience) then what is the reference point for which this measurement ...
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### Deriving invariance of interval

I'm reading a book called A First Course In General Relativity, and when I got to the invariance of the interval, the book didn't derive it. Also, when the book derived time dilation and Lorentz ...
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### Is the age of the Universe relative to where you are within it? [duplicate]

The accepted age of the Universe is 13 billion years give or take. If you were a super space being and hypothetically had spent a large portion of this time sitting next to a black hole then your ...
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Here is the explanation to the Twin Paradox in Feynman's Lecture on Physics 16–2. we consider a famous so-called “paradox” of Peter and Paul, who are supposed to be twins, born at the same time. ...
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### Is the speed of light constant in all directions?

Consider the shape of the waveform emitted by a moving source: (with the horizontal axis "x" and the vertical axis "y"). According to the stationary observer, the wavefront of a short pulse ...
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### How to measure time in presence of a strong gravitational field? [duplicate]

I need an operative definition of "measuring time in general relativity" that takes in consideration also the presence of strong gravitational fields between me and clock, able to deviate the light ...
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### What happens when we manipulate the observer in the double slit experiment?

Consider the following alterations to the double slit experiment: turn the observer on and off rapidly move the observer further and further away, until the point at which it can no longer ...
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### “Measure of time in general relativity” [duplicate]

Suppose to be in an arbitrary gravitational field and you are moving in it arbitrarily with a clock in your hand. In this general situation I ask: if I read the positions of the hands of the clock, ...
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### Does photon experience time? [duplicate]

According to the special theory of relativity, for all observers the speed of light is c. Any observer travelling at the speed of light c does not experience time. Hence even protons shouldn't ...
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### Does anyone know how/where I could view the Double slit experiment in person? [closed]

I want to see the double slit experiment in person! The one where the observer effects the wave/particle state of an electron. Where would I be able to view this experiment? Is it on display in any ...
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### How can photon have wave properties if they travel at the speed of light? [duplicate]

I am not a physicist so please excuse me if this is a dumb question. As far as I understand Relativity, as observer (in this case a photon) travels at the speed of light, time stops. So how can the ...
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### Is wavefunction collapse “global”?

I have the feeling that the premises of this question may not be coherent (so to speak), but here goes: Suppose we have a system $X$ in a quantum superposition between states $0$ and $1$, say, with ...
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### How can we know that the speed of light is constant?

Science say that within a constant time frame the light travels a constant distance. But if time is relative how can it be possible to use it as a reference to say that something is constant?
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### What happens to objects sucked into a black hole after the black hole evaporates away?

Suppose an object falls into a black hole that's so massive that it wouldn't get torn apart at the event horizon. What happens to it after the black hole evaporates away? According to the theory ...
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### At a center of Gödel's universe

A few quick questions clarifying a picture about Gödel's universe, they bug me badly! Taken from here. So Gödel's universe is made out of dust particles. All of them have angular velocity. Do this ...
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### Traveling slower by traveling in opposite direction as the Earth rotates

I know that if I were to travel fast, the time would pass by fast for me. But if I were to travel fast in the opposite direction of the earths rotation while I'm still on earth, would the time pass by ...
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### Pole and Barn Paradox w/ Spacetime Interval

I'm having trouble with a pole and barn paradox problem. The problem is as follows: A pole vaulter is running with a pole at $v=\frac{\sqrt3}{2}c$. Her pole has a proper length of $L$. She runs ...
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### Does curved spacetime arise from inhomogeneity of gravitational field?

In general relativity textbooks such as Sean Carroll's Spacetime and Geometry, there is often a line of reasoning that goes like this: Strong equivalence principle states that free falling frames ...
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### Can the fact that the vacuum energy in curved spacetime is not boost invariant be explained without mathematics?

I read in an answer to a question if Hawking radiation can be explained without too much mathematics that this is impossible insofar the vacuum energy is not boost invariant in a curved spacetime. ...
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### Can there exist an observer able to observe a collapse of a star into a black hole?

We know that an observer at infinity cannot see a star forming into a black hole as the matter will take progressively longer and longer time to compress (from this observer's point of view). Is ...
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### Twin paradox in special relativity: length contraction

Can the concept of twin paradox be applied to length contraction as well? meaning that the twin which is in spaceship will have its meter rod "actually" contracted while he will see his brother's ...
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### How is time dilation consistent between both observer?

There is a special variant of time dilation paradox bugs me. Imagine there are 2 small spaceships moving toward each other in constant speed. Each ship has an observer on-board. According to both ...
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### Synchronization of clocks

I have read the book "Introduction to special relativity " by Robert Resnick In the book I have found that "If the distance between the clocks is L, one observer will see the other clock lag his by ...
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### Meaning and logic of Einstein's train thought experiment

This seems to be a recurrent topic but I wasn't able to find any satisfactory discussion about this tought experiment. I will quote the original story from Einstein's and put my doubts inside: Up ...
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### Black holes shouldn't really exist? [duplicate]

General relativity states that for an observer sufficiently far from the gravitational field of a blackhole, the space time geodesic nearr the event horizon is so long that we should never observe an ...
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### Geiger counter in the Schrodinger's cat experiment

Inside the Schrodinger's cat's box, the moment the radiation is detected by the counter, doesn't this mean the system already has a fixed eigenstate (a collapsed wave function, or is decoherent, ...
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### Observer in the double slit experiment with photons

In the double slit experiment with photons, the interacting observer is an instrument, detector… If you replace the detector with a piece of metal with the same mass as the mass of the detector, the ...
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### Value of $E_x$ through an $X-\textrm{boost}$

Suppose I'm standing at the (0,0,0) point of a coordinate system and I see an electric field where $E_x=f(t,x,y,z)=\sin(x\cdot y\cdot z\cdot t)$ and at time $t_0=0$ I start moving on the x-axis at ...
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### how long will take for an astronal travel in near lightspeed in his point of view

I always had a question about relativity. Suppose an astronaut in orbit Saturn just must reach earth fast as possible. Your ship can travel at any speed between 0 and the speed of light. the distance ...
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### Regarding relativity of simultaneity

I recently started studying Special Relativity an my book discusses the following: Say I have synchronized two separated clocks in a reference frame S, if then an observer in another reference frame ...
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### Special Relativity and Composition Law for Velocities with respect to an observer watching two objects moving away from a central point

So I understand that special relativity is all about the frame of reference and there is a lot to do with time dilation and how space-time is warped at velocities near $c$. So my question is what ...
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### Is acceleration relative in relativity?

Suppose a box A is moving relative to a Box B, then by time dilation equation if I take 1 sec passed for an observer in A then for an observer in B will be little longer. Now if I suppose that the box ...
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### Rapid (ac/de)celeration in relativity does what to inertial clocks?

EDIT (to clarify my question): I think some of the answers here are accounting for light travel time and telling me what I'd actually see on Earth's clock, so I've edited my first paragraph to ...
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### When calc. time elapsed for an inertial reference frame during travel between two points, are length contraction and time dilation taken into account? [closed]

When calculating the amount of time elapsed for an inertial reference frame over the course of its travel at constant velocity between two points, are the effects of both length contraction and time ...
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### Relativity of simultaneity: two light sources

A. Suppose a moving train. The train has two wheels(front and back) and each wheel is connected to light source inside the train. The light source is triggered(light is emitted) when train passes ...
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### Lorentz contraction explanation correct? [closed]

I was having a heated SE chat debate with someone re relativity. One of us believes the explanation above is correct, while the other believes it is not. Who is correct, and, if the explanation is ...
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### Does travelling towards something at relativistic speed cause it to appear to speed up?

If one were to travel towards a giant TV floating in space, at a fraction of the speed of light, would a video playing on the TV appear to play faster? I'm guessing the answer is no. From what I ...
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### Twin Paradox From Neutral observers perspective

Suppose two planets A & B, with B moving away from A at velocity v. A spaceship departs from A in the direction of B at velocity v (putting it in the same inertial frame as B), and remains that ...