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Questions tagged [observers]

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132 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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528 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
199 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 ...
3
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1answer
139 views

Understanding of derivation of time dilation

While deriving time dilation using the two postulates of Special theory of relativity we imagine that a light pulse is traveling back and forth between 2 mirrors. In the rest frame (when the observer ...
3
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3answers
403 views

The observer detail of double slit experiment

I have been searching internet for a while and I couldn't find any detailed information about the observer type in double slit experiment of electrons. For example is the observer a camera or ...
3
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2answers
376 views

Time Slowing Down [duplicate]

I'd like to ask a question if I may about the relativity. When travelling at the speed of light(which is impossible for particles with masses) or very close to it, would it be me, myself, seeing my ...
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3answers
874 views

Uncertainty principle and multiple observers

My understanding is that an observer can measure the precise location of a particle so long as the corresponding uncertainty in momentum measurement is not an issue and vice-versa. Say there is ...
3
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2answers
105 views

Why physics should be the same in all inertial frame? [closed]

One of the postulates of special relativity is that physics should be the same in all inertial frame. Suppose we have two observers $A$ and $B$ suppose that $A$ is accelerated. Now suppose that we ...
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3answers
143 views

Doubt of speed of light

I have a question about how an observer sees the speed of light or the distances when at rest. Suppose two events occur simultaneously in $A$ and $B$. If someone in $X$ is watching, they will see ...
3
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1answer
137 views

Will stationary charge radiate E.M waves if observer is accelerating?

For ex. if my frame of reference is oscillating then will i see E.M waves produced from a charge which is not oscillating with any other or ground frame of reference? Can i get a really clear answer ....
3
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1answer
147 views

Escape from black hole for different observers

When a particle falls through the event horizon, it cannot escape the black hole anymore, as doing so would need a speed greater than c. Is the above statement valid for all observers? For an ...
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2answers
245 views

What is the “reference frame of a particle” in General Relativity?

In Special Relativity, when one talks about "the reference frame of a particle" it is quite clear what they mean. First of all: one starts by setting up cartesian coordinates on the flat spacetime. ...
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4answers
1k views

Relativistic electromagnetism and electromagnetic forces on 2 protons

The question I have about it is how we can get the same result of net force acting on the individual protons if we judge the system from 2 different reference frames. One using more of the magnetic ...
3
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1answer
126 views

Time dilation of pulsar period in GR

Suppose I have a pulsar at some radius $r_p$ from a black hole and an observer at a radius $r_o$ where $r_o \gg r_p$. Assume that both the pulsar and observer are stationary. The pulsar 'pulses' at $...
3
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1answer
164 views

How fast do you have to be traveling in order to travel one light year in one year due to relativistic effects?

My apologies if my understanding is incorrect, but I believe that as you approach relativistic speeds you experience time dilation as compared to an outside observer. So taking into account this ...
3
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2answers
547 views

Decoherence when no one is looking?

I understand that in the single-electron-at-a-time double slit experiment, if a detector is placed before the slit, the interference pattern vanishes. Suppose I left the detector on, but put a bag ...
3
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4answers
160 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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5answers
172 views

Is relativity really relative?

Because of the relativity of linear motion, you can't tell the difference between whether your spaceship is moving and the stars are standing still or whether you're still and the stars are moving the ...
3
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1answer
264 views

Doppler effect differs when the entity moving is different

Why is the doppler shift different when whether the source is moving or the observer? Although they are traveling at the same speed. Noticing that the difference can't be neglected when the speed is a ...
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2answers
145 views

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?

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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1answer
404 views

Special Relativity in water

If there are 2 observers in water moving relative to each other as well as to water, they measure different speeds of light. So does time dilation occur for them? (since Time dilation is based on ...
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2answers
2k views

Schutz's geometrical proof that spacetime interval is invariant

I'm trying to understand the proof that spacetime interval is invariant under for any two inertial observers. I know it's easy to arrive at the result using Lorentz transformation but I'm trying to ...
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3answers
860 views

Perceived direction of light emitted in moving reference frame

I was thinking the other day about the simple example used to demonstrate time dilation effects and to derive the Lorrentz factor - where the time it takes for a light pulse to be emitted, bounce of a ...
3
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1answer
130 views

Why is the camera not the culprit? [duplicate]

Perhaps I am completely wrong, but as I understand it our observation of a system can affect the outcome. The example I remember is the double slit experiment where electrons behave as a wave at ...
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2answers
76 views

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 ...
3
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1answer
126 views

In special relativity, is there a prescription to find the coordinate transformation to a general (for instance accelerated) observer? And in GR?

Suppose we are in an inertial frame, so that the metric in our coordinates is just the Minkowski metric, \begin{align} \text{d}s^2 = -\text{d}t^2 + \text{d}x^2 + \text{d}y^2 + \text{d}z^2. \end{align} ...
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5answers
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Twin Paradox: Why does't the traveling twin measure faster-than-light photons on Earth?

Sorry if this question has been asked, but it was difficult to search through given the volume of Twin Paradox questions. Anyway. My question isn't with the twin paradox per se; it's with what the ...
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0answers
77 views

Can a black hole actually grow, from the point of view of a distant observer? [duplicate]

Possible Duplicate: Black hole formation as seen by a distant observer I've read in several places that from the PoV of a distant observer it will take an infinite amount of time for new matter ...
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5answers
762 views

Explanation for a much simpler version of the twin paradox?

I have seen the classical twin paradox before. It uses a twin stationary on Earth and the other traveling away and back. I have seen many contradictory solutions for it, some use general relativity, ...
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4answers
269 views

What counts as an observer and memory states in quantum interpretations?

The Everett interpretation has memory robots. Copenhagen requires observer memory states. Consistent histories has its IGUSes. Decoherence has its existential interpretation. All of them refer to ...
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3answers
746 views

A question in Special Relativity

In books the equation for length contraction is derived by supposing that the velocity of the spacecraft is the same for both observers. So the question is that, is the velocity really the same for ...
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3answers
2k views

How does time dilation work without a privileged reference frame?

As I understand special relativity, light travels at the same speed in all reference frames. What I fail to understand is why time dilation would occur in one reference frame, but not by an equal ...
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5answers
709 views

What is the exact mechanism by which time dilates?

What is the exact mechanism by which time dilates for a fast moving object? Can the time dilation be explained by any theory other than relativity?
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2answers
6k views

Electron behavior changes when observed?

I saw this video of the double slit experiment by Dr. Quantum on youtube. Later in the video he says, the behavior of the electrons changes to produce double bars effect as if it knows that it is ...
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1answer
125 views

If we are in the car, we see the trees on the street are moving backwards. Do the trees have kinetic energy?

I copied this question from the internet. I thought about it but I can't make out the meaning of the statement.
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1answer
3k views

Does the increase of (relativistic) mass, while flying near speed of light, has any impact on astronauts? [duplicate]

Possible Duplicate: Would travelling at relativistic speeds have any impact on human biology? I am asking myself this question for a few days. What is the correct answer on: Does the increase of (...
2
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4answers
142 views

Time of flashlight crossing the event horizon as seen from an external observer

This is a follow-up to my previous question: Can something (again) ever fall through the event horizon? Consider the following thought experiment: I am again on my rocket at height $h$ over a black-...
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2answers
582 views

Photons moving relative to photons [duplicate]

I know we can't move at the speed of light, but if we were to travel on a photon how fast would we see other photons going? The speed of light is constant... So do photons see other photons moving at ...
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3answers
675 views

If time slows down when you move faster, does this give you an “effective speed” different from your proper speed?

Suppose you're travelling at 0.9c toward the sun, and you tag past the Earth and start a clock. Would Lorentz contraction/time dilation cause you to get to the sun faster than (about) 9 minutes (...
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2answers
473 views

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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5answers
186 views

Is there anything we are aware of that can't be slowed by time dilation?

While thinking about the ambitions of this post: Can radioactivity be slowed through time dilation? I was asking myself, is there even anything that is dependent on time and can't be slowed through ...
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4answers
212 views

How much could one slow down aging using existing spacecraft?

Imagine that I board a spacecraft and head away from the Earth at top speed until I've aged twenty years, then I turn around and come back. How much younger will I be than the unexciting folk who hung ...
2
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2answers
90 views

Is it ok to have two events $A$ and $B$ so that for one person $A$ occurs before $B$ but for another $B$ preceds $A$

Imagine two laser beams A and B are released at the same moment to bounce between two mirrors, A was moving and B was at rest, doing the calculations I found that for a person at rest B would reach ...
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3answers
236 views

Using Einstein's Relativity: Who is younger? [duplicate]

Suppose we have a person A and a person B. Person B travels very close to speed of light and never returns. He's constant in speed. Then, we can say two things: B is younger than A. A is younger ...
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2answers
412 views

Rindler Coordinates and homogeneous Gravity Field

I understood from the equivalence principle that an accelerated observer in free space is equivalent to a stationary observer in a gravitational field. As far as I understood further, this means to ...
2
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1answer
172 views

How to say “speed of light is constant in all frames” in QFT?

In classical mechanics, if you assume the speed of light is constant, it's easy using the mirror and photon thought experiment to conclude that given observers $U,V$ with $U,V$ travelling at speeds $u,...
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1answer
129 views

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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1answer
92 views

Bug riding on a ball moving at almost the speed of light [closed]

A ball with a bug on it is thrown at almost the speed of light. The bug looks back and observes the thrower throwing the ball. In the context of special relativity, what is the weight and the height ...
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3answers
862 views

Would a craft travelling increasingly close to the speed of light appear to be decelerating?

I've been pondering the implications of time dilation. Is it right to assume that if a craft was travelling at a speed very close to the speed of light (>= 0.9999c for example), that to an external ...