Questions tagged [observers]

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Observers at rest and simultaneity

Suppose we have two observers A and B and they are at rest. Observer A observes two objects falling from height H (A has same distance between the two objects). Does observer B will measure different ...
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To Be Forever Young [duplicate]

It is clear from the Lorentz Parameters, applied to Einstein's Equations, that as velocity, v approaches speed-of-light, c, the denominator (1 - [v/c]) tends to zero; when v=c, time, t=0: time stops?! ...
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Are longer lives more likely in Everettian multiverse?

There are some interesting statistical thought experiments related to observers and the Anthropic Principle such as the Doomsday Argument formulated by a physicist Brandon Carter and later improved by ...
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Reference systems in Special and General Relativity

I am enthralled by the notion of placing observers along with standard identical clocks in a line spaced from one another according to rods of standard length when place next to one another at the ...
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Sound wave and Principle of relativity

Principle of Relativity: "An experimenter doing an experiment in an inertial frame of reference can never measure the velocity of its own frame of reference. OR a law of physics is never expressed ...
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2answers
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Is it correct, according to quantum mechanics, to say that an observer can get entangled with the measured system?

Entanglement applies to any physical system, observers included. However, what one typically means by the observer being entangled with the measured system seems not to comply with the requirements ...
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1answer
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Some kind of slower time principle [duplicate]

I'm always trying to find underlying principles, like that the force is always directed toward a (locally) lower potential energy and alot of stuff like that. Recently I've begun to gain some layman ...
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2answers
79 views

Why can't we take space as universal frame of reference?

Suppose we have a ball filled half with water in space with nothing else around (nothing else in the whole space except the ball) and suddenly it accelerates for time t. obviously, there would be ...
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3answers
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The interference paradox

This is a thought experiment I created myself. Imagine two radio sources fixed at the ends of a rocket and they produce signal of same frequency such that they arrive at the center simultaneously and ...
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2answers
64 views

On local inertial frame and acceleration in general relativity

[Q1] In the general relativity, a local inertial frame is mentioned. The local inertial frame is a notion, which is related to (or represents) the equivalent principle. Here, I understand that, in ...
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0answers
55 views

Black Holes without Schwartzschild Radius condition? [duplicate]

Reading this comment makes me ask what does that actually mean that In fact, if you are close enough to $c$, the mass-equivalent of your kinetic energy may be enough to make you appear as a black ...
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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
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What's incorrect about this reasoning regarding special relativity?

Say two spaceships start at the same point and from the vantage point of your inertial reference frame $S$, Spaceship A travels at $.75c$ and Spaceship B travels at $.25c$, travelling in the same ...
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If I place a live video camera on another planet where time moves with a different speed

remember that movie "Interstellar"? Where the astronauts came to a planet where one hour here ment 7 years back on earth? Well, if you don't: Imagine a planet, far, far away where time moves in a ...
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Black Holes to an outside observer

If an object were to fall into a black hole, would the inability of light escaping cause that object to stand still from an outside observer's perspective? If not, then would the bent spacetime cause ...
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1answer
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Black Holes and Time [duplicate]

My understanding is that if I am outside a black hole and watch something fall in, the object will appear to slow down and will never actually get sucked in. So how can we ever get a black hole if ...
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How would an observer extremely close to light speed perceive (experience) the universe?

In the following we consider an Earth bound reference frame. For an observer very close (but under) the light speed, radiation from the universe would emanate from a single point in the direction of ...
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Minkowski diagram

I have not well understood the picture of geogebra regarding the angle of time (t') that is inclined compared to (t) of 26.57°angle . In the picture we see that the velocity is setted at 0.5c, for ...
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2answers
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Can a black hole event horizon form before the singularity for an outside observer?

This is not a duplicate, my question is not about whether BHs have or don't have a singularity, but my question is whether, because of time dilation, no matter has reached the center and formed a ...
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Rindler observers are at rest with respect to each other?

I'm studying a chapter about Rindler coordinates right now. In this they say that two Rindler observers at $x = 1/a_1$ and $x= 1/a_2$ will both have speed $v =0$ at $\tau = 0$ compared to the inertial ...
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Would an observer going faster than the speed of light in a vacuum be able to observe photons that they emit? [closed]

If I could exceed the speed of light in a vacuum, would I be able to see photons that I previously emitted? Would this be theoretically possible?
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Why does time slow down ONLY for objects travelling at high speeds?

Today in physics class, we learned about the theory of relativity and how it relates to time dilation. We were given the example of two photon clocks- one stationary relative to the Earth, and the ...
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1answer
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How do we know who is moving? [duplicate]

If I am in a free float environment and there is another object at rest relative to me, and then I start to accelerate towards this object, do I know I am the one who is moving, since I am the one who ...
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Thought experiment: if an observer can travel at the speed of light [closed]

What would the observer see if he could travel at the speed of light and shot a photon beam at the same time?
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1answer
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In QM does the observer really perturb the system to make a measurement?

It is commonly taught in introductory QM courses that in order to get to know the position or momentum of a particle, be it by "sending a photon" or similar experiments, the measurement necessarily ...
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1answer
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Is the M87 black hole really a black hole? [duplicate]

I apologize if this is a silly question, but... From what I've been told, new black holes cannot form, because time slows to nearly stopped as matter approaches the event horizon. Instead, a new ...
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2answers
156 views

Problem with speed of light

I constructed a thought experiment, can't figure the answer. Two long (say $1\ \rm{km}$ long) railway cars are on parallel east-west tracks in a zero gravity environment. Each car has a light source ...
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321 views

Definition of observer and time measured by different observers in general relativity

An observer in general relativity is defined as a future directed timelike worldline \begin{align*} \gamma:I \subset \mathbb R &\to M \\ \lambda &\mapsto \gamma(\lambda) \end{align*} together ...
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1answer
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The ladder paradox - with equal lengths

Take the usual ladder paradox in special relativity, but this time the ladder and the barn have the same length, at rest. Since each one of them contracts when viewed in the rest frame of the other, I ...
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1answer
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What would an external observer see happen to objects entering an evaporating black hole?

As I understand it, objects passing the event horizon of a black hole from their perspective will, from an external perspective, seem to keep approaching but never reach the event horizon. Given this, ...
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2answers
47 views

Is Einstein's train-lightening thought experiment consistent with Special Relativity? [closed]

Einstein’s train-lightening thought experiment contains only inertial motion so SR says we can ascribe relative motion and relative rest to either reference frame. Although it is not stated in the ...
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What creates the physical difference in ageing when travelling close to the speed of light? [duplicate]

I probably have a fundamental misunderstanding, so correct me if I'm wrong. When travelling close to the speed of light, movers experience time dilation, which is what results in the phenomenon of ...
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4answers
151 views

Communication near a black hole

One ship is in a stable orbit outside of the event horizon of a very large black hole so that there is significant time dilation. Another ship is in its own orbit around the same black hole, but at a ...
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2answers
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Relativity Confusion

In the time dilation formula $\Delta t=\gamma\Delta t_0$, I am confused about what $\Delta t$ and $\Delta t_0$ measure. If we have two people, Tom and Bob, moving at constant velocity relative to ...
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1answer
67 views

Recordings of journey traveling near speed of light

If you had two identical video cameras and started recording synchronized atomic clocks recording at the same speed and you put one on the spaceship and sent it out traveling at 99% of the speed of ...
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4answers
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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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1answer
48 views

Train and lightning bolts: why the time difference does not depend on the position of the moving person?

So imagine two lightning bolts hit the ground, simultaneously to a stationary observer. There is also a person on a train traveling to the right at a constant velocity. I know that if he started in ...
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4answers
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Is there a frame of reference in which I was born before I was conceived?

I'm struggling to understand the relativity of simultaneity and position. If my conception and birth are separated by time but not space, a frame of reference in which my birth and conception are ...
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1answer
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Why do external observers see LIGO results if an object falling into a black hole never reaches the event horizon? [duplicate]

If I throw a clock towards a black hole, its time slows down, it is redshifted, and according to many theories it never reaches the event horizon from my point of view. How is it then, that a star can ...
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2answers
82 views

Would a human age faster if these certain conditions are met?

So, a human would travel in a rocket in outer space. There is little to no gravity, and the pressure may or may not have an affect (?) on what i'm about to ask. It has been proven that when a human ...
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1answer
85 views

Do gravitational waves affect the flow rate of time?

Since LIGO has been able to detect the presence of gravitational waves, this implies that there is a density wave traveling through space with a high potential front and a low potential back side. ...
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Does absolute motion/stillness exist?

I am not a physicist :) I (think I) understand the idea that motion can be relative in the sense that, I'm standing still inside a moving train in regards to the moving train itself, but I'm moving ...
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What is the definition of coordinates of an observer? [duplicate]

I am confused about coordinates of an observer. Suppose we have an observer $A$ whose world line $\gamma$ is a geodesic and another observer whose world-line $\gamma$ is not. Since coordinates is just ...
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3answers
122 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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How are the predictions of the Schwarzschild solution consistent with our observations of black holes? [duplicate]

I am fully aware that there may be many misunderstandings in the following. However, in recent lectures we covered the Schwarzschild solution, and after some interesting mathematics we reached ...
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5answers
199 views

Is Relativity of simultaneity just a flaw in perception?

So I went through a couple of different books on special relativity ( including Einstein's own book) and I just can't seem to accept relativity of simultaneity as a real thing. I'll explain my ...
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1answer
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Do Events Conditional to Simultaneity Occur in Every Reference Frame?

Imagine a train with a table that contains a light on both ends (so that the line between the two sources of light is parallel with the line of motion of the train) and a light sensor in the perfect ...
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1answer
99 views

Einstein's Simultaneous Lighning and the Lorentz Transformation

I am reading Einstein's book, "Relativity: the Special and General Theory" translated by Robert W. Lawson, and I have some questions concerning Special Relativity -- specifically, the thought ...
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Biological aging interval equals Proper Time interval?

In special relativity does proper time interval equal biological aging interval in every inertial reference frame?
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3answers
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Method to observe clocks on spaceships moving near $c$?

I have read the explanation for clocks slowing down on a spaceship moving relative to a stationary observer -- something like a beam of light between two mirrors taking longer for the observer to ...