Questions tagged [observers]

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2answers
162 views

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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4answers
575 views

Is an event horizon absolute to all observers?

Recently I had discussion whether the event horizon of a black hole is absolute or relative to different (outside) observers. Does someone just 1m above the horizon (disregarding effects of tidal ...
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118 views

Would a Schwarzschild black hole “appear” to be a sphere in all reference frames?

I am wondering if all observers would agree that a non-rotating, non-charged black hole is spherical (i.e. there is no reference frame where one would measure it to be oblong in one dimension). I ...
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2answers
141 views

Planets close to a black hole

Like in the movie Interstellar, when they go near "gargantua" the big black hole that have a planet orbiting the black hole, they go on the planet and every minute equals X years in earth time. If ...
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1answer
165 views

How stellar black holes are formed? [duplicate]

This is a kind of follow-up of my previous Phys.SE question Can something (again) ever fall through the event horizon? that seems to have established that, for an outside observer, things never fall ...
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3answers
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Can Special Relativity hold even without motion?

I am having a bit of confusion, let us say there are two observers observing two simultaneous lightning strikes with a finite distance between them. Now one observer is located such that he is in ...
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3answers
3k views

A sees B's clock running slow and B sees A's clock running slow? [duplicate]

This paradox is very common it seems, in which A sees B's clock running slow and B sees A's clock running slow. Here is the question a little more concretely. Let's say B flies by A's spaceship. If B'...
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5answers
2k views

Twins Paradox - Does ageing depend on motion?

I am trying to understand the Twins Paradox (relativity) and its implications. Can we infer from the Twins Paradox that the ageing process (cell decay or other biological processes) depends on motion?...
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2answers
287 views

Consequences of infinite redshift in the event horizon of a Schwarzschild black hole

I am familiar with the usual derivation of the fact that it is impossible for someone to observe an object enter a black hole, because of the fact that light gets infinitely redshifted as its source ...
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3answers
390 views

Subjectivity of decoherence

I read that quantum decoherence is subjective, in the sense that two observers may not have the same "environment" and after each one has traced over those degrees of freedom they will end up with a ...
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6answers
297 views

In Special Relativity, is it allowed to ask 'How much time has elapsed in a second inertial frame at a particular moment in the first inertial frame'?

Or is it a meaningless question? For example, A and his friend B are the same age initially. B travels relative to A at a very high speed. A keeps observing B from his frame. At one moment, A ...
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2answers
142 views

Maximizing the time near a black hole

I just learned about the Schwarzchild solution, following Carrol's book. Now, there is a question that I want to answer: Consider an observer that starts at infinity with some velocity, comes close ...
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3answers
384 views

Shape Created by a Pile of Granular Objects Dropped Uniformly

I've observed many times that if you drop a lot of a 'granular' substance in one place and keep the nozzle out of which the substance flows, that the shape of the pile created very much resembles a ...
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2answers
25k views

Increase in Mass with Velocity [duplicate]

I just had a confusion. Does the mass of the body actually increase when it is moving with a certain velocity? Or does it only look like the mass has increase to another observer. How can the actual ...
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4answers
234 views

When we talk about speeds in relativity theory, where are they measured?

I recently asked a question here about if the direction we travel matters in relativity theory: Does it matter in which direction I travel in relativity theory? After I got answers and making more ...
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2answers
201 views

Is the existence of a photon relative? [duplicate]

If an observer passes an electron, in such a way that the observer is accelerating, the observer would see photons because accelerating charges induce electromagnetic waves. But from point of view of ...
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2answers
670 views

In the double slit experiment, what happens when there is more than one observer trying to see which slit the same electron goes through?

From all the demonstrations Iv been able to find of Heisenberg's double slit experiment, whenever an observer tries to "see" which slit an electron passes through it collapses the wave function. My ...
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2answers
493 views

How can a comoving observer be “privileged”?

Yesterday I went to my GR (undergraduate) lecture, and the professor exposed the concept of comoving frame of reference. Well, he said that in this particular frame an observer is a privileged ...
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2answers
157 views

Blinking flashlight infalling into the black hole, as frequency → ∞

This is my third question from a series were I progressively refine my thought experiment. The others are: Can something (again) ever fall through the event horizon? Time of flashlight crossing the ...
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1answer
3k views

Time dilation - why the observers see each other the slow one but then one of them is older or younger?

I'm in trouble with time dilation: Suppose that there's two people on the Earth (A,B), they are twins and each other has a clock. (So they are at the same reference frame). B travels in a spaceship ...
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4answers
1k views

How do I know which observer is running the time faster or slower?

Ok, I'm not a physicist, so I don't know if my question is silly (probably yes), but there is something in special relativity that I can't understand and I would really like it to be clarified. If ...
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3answers
230 views

Does Minkowski diagram show that time goes faster in “moving” frame of reference?

I have a question re the diagram that appears at: http://www1.phys.vt.edu/~takeuchi/relativity/notes/section12.html The diagram purports to show that the observer in each frame of reference will ...
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2answers
535 views

Relativistic spring

Imagine a cart at the base of the hill. The cart is next to a spring. You push the cart into the spring until the spring compresses $x$ meters. In your frame of reference, the cart has a potential ...
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5answers
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Observer effect, do this mean literally someone or just any interaction with other matter?

I am a layman and was wondering, the quantum observer effect. The regular notion to laymen seems to be literally "if you look at it", but as I am coming to understand the world I live in better I feel ...
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1answer
242 views

Formation of a black hole and Hawking radiation

From the perspective of an outside observer it takes infinitely long for the black hole to form. But if the black hole is no extremal black hole, it emits Hawking radiation. So the outside observer ...
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2answers
210 views

Effect of gravity at near-lightspeeds

Let's say I'm in a space station, hurtling towards our galaxy nearly close to the speed of light. From my reference frame, I see the galaxy coming towards my ship at the same speed. I pass the Sun, ...
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1answer
367 views

How a reference frame relates to observers and charts?

Recently I've been watching the General Relativity lectures from the "International Winter School on Gravity and Light" by Frederic Schuller. In those lectures he made the following two definitions: ...
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3answers
344 views

Is the wavefunction unique to the observer?

I know this may be beating a dead-horse, but I'm still puzzled by this topic even after reading so many other related SE questions/answers and online articles. I will be using the Copenhagen ...
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2answers
212 views

Help understanding why can't we measure absolute velocity?

(This is probably a clone of some other question, but I haven't been able to find a satisfactory answer, so I hope this is okay.) So, I know that in special relativity, there's no such thing as a ...
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3answers
267 views

For a giving metric in GR, how do we learn which observer the metric refer to?

For example, I have been told the Schwarzschild observer is far away from blackhole and events,(namely, I think, the observer is static at infinity of the coordinate.) And the second example,the ...
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379 views

Are the authors saying that the observer effect plays no role in Bohr's thought experiment of the Heisenberg uncertainty principle?

Here is an excerpt from Eisberg & Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. Here is introducing Bohr's though experiment to establish a physical origin for the ...
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Why can't we accept that there's no black hole? [duplicate]

According to a distant observer, any mass never falls into the event horizon, they just appear to be moving slower towards it. So, why can't we tell people this : there are black holes, but it's just ...
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0answers
126 views

Role of the observer in Gödel's universe

I am here to clarify myself about the role of the observer in Gödel's solution (1949) of Einstein's field equations. The Universe we are dealing with is anisotropic, since the axis of rotation ...
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What does it mean 'the observer' in Quantum Physics? [duplicate]

Is it only necessary a human consciousness? A measurement device? Can it be said that any of them cause the wave function collapse?
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1answer
556 views

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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4answers
111 views

Present vs Past in Cosmological time

My question is in regard to “seeing back in time” when observing objects in space. I understand the concept that light takes a certain amount of time to reach the earth and we can determine the ...
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4answers
1k views

What is the universal speed limit relative to? [duplicate]

If all speeds are relative, then what "governing" force is that speed limit relative to? Is there some sort of fixed or absolute grid with locations everything is compared to? Does this also mean ...
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5answers
35k views

The bigger the mass, the more time slows down. Why is this?

If I were to stand by a pyramid, which weighs about 20 million tons, I would slow down by a trillion million million million of second. Don't know if that's exactly right, but you get the point. Also, ...
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2answers
126 views

Does event horizon expand instantly or does it take time for large black holes?

When something falls into a black hole, its mass increases and its event horizon expands a little to reflect the new mass. In case of very large supermassive black holes, with event horizon spanning ...
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1answer
164 views

At what moment will matter falling into a black hole affect its size?

Does the mass of matter falling into a black hole affect the size of an event horizon the moment it passes through it, or when it has been incorporated into the singularity?
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3answers
741 views

Relationship between Coordinate Time and Proper Time

While I was reading Ta-Pei Cheng's book on relativity, I was unable to derive the correct relationship between coordinate time $dt$ (the book defined it as the time measured by a clock located at $r=\...
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2answers
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How long does it take for a black hole to form?

The well-known fable of an astronaut sending signals out to an external observer while falling toward an event horizon states that the time lapse between such signals becomes greater even if in the ...
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1answer
411 views

Derivation of Relativity of Simultaneity

Suppose you have a train moving forward relative to an inertial observer at velocity $v$. Suppose you have a clock 1 at the front of the train and a clock 2 at the back, and, in the frame of the train,...
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2answers
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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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4answers
11k views

If traveling at the speed of lights stops time, why does it take light 8 minutes to reach Earth?

I just learned that, according to Einstein's relativity theory, time reaches zero for an observer (light) when traveling at the speed of light, so everything is supposed to be at the same place in the ...
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4answers
594 views

Traveling between two planets at rest to one another [duplicate]

If I travel at relativistic speed from planet A to planet B which are at rest relative to one another, I will be younger than people on A or B when I arrive. However how does this mesh with the fact ...
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3answers
128 views

What exactly does the observer's coordinate system mean in special relativity?

This seems like a very basic question but I'm having a hard time understanding it, more precisely hard time visualizing it. Let say there is an observer, and we wish to somewhat formalize his ...
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1answer
48 views

Bekenstein Bound and frame of reference

According to Wikipedia, this is the formula for the Bekenstein bound $$ S \le \frac{2\pi k RE}{\hbar c}$$ where $E$ is the total mass-energy. That seems to imply that the presence of a black hole in ...
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3answers
526 views

Schwarzschild geometry, what is physical meaning of coordinates?

A past exam has a question: For the Schwarzschild metric external to a non-spinning spherical mass, what is the physical significance to the coordinates $t,r,\theta,\phi$? Not sure how to answer ...
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2answers
198 views

Confusion with relativity of simultaneity

I know variations of these have probably been asked numerous times before, but I'm having trouble with this specific scenario. Imagine the classic Train Paradox, except instead of lighting strikes ...