Questions tagged [observers]

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1answer
114 views

If “time is relative”, what does “the universe is 13 billion years old” mean? [duplicate]

Physcists say that the universe is approx. 13 billion years old. However, the amount of time that has elapsed since the big bang differs per object, dwpending on the trajectory that that object has ...
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2answers
448 views

Derivative of proper time w.r.t. time?

On page 86 of Theoretical Minimum (part 3 on special relativity) by Susskind, he writes: $$\frac{d\tau}{dt}=\sqrt{1-v^2}$$ Where $v$ is the velocity of a moving reference frame relative to the ...
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1answer
77 views

Observer problem in Relativity [closed]

Which clock or observer conclude that time dilation has occurred? Is it the stationary clock/observer or the moving one? (Please bear in mind that each observer/clock independently concludes that one ...
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4answers
170 views

In the twin paradox, are some events lost to the observer turning around?

From what I understand of the twin paradox, the time difference in time can be accounted for when the twin on a rocket ship turns around/switches reference frames. Does that imply that, events that ...
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4answers
211 views

Clarification of proper time and inertial frame of reference

“Standing beside railroad tracks, we are suddenly startled by a relativistic boxcar traveling past us as shown in the figure. Inside, a well-equipped hobo fires a laser pulse from the front of the ...
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1answer
154 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
326 views

Einstein's train experiment with clocks

Referring to Einstein's train hit by lightning bolts thought experiment. What if, in the middle of the train, together with the moving observer there is a stopwatch, with 2 displays. One display ...
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2answers
183 views

Young and Freedman Proof: No Observer Can Travel at the Speed of Light

I have been reading the section on Relativity in the ninth edition of University Physics by Young and Freedman. They include the following proof that no observer can move at the speed of light. ...
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1answer
204 views

What would happen if we put Schrödinger inside the box and the cat opened the box door? [closed]

Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen ...
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2answers
219 views

Solving the ladder paradox: Minkowski distance leads to a contradiction

Consider the ladder paradox; Let the length of the barn in its fixed frame be $80m$, and consider the ladder's reference frame which is moving toward the barn with a speed $v$, and in ladder's frame ...
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1answer
173 views

Question on Painlevé Gullstrand coordinates

A known set of coordinates used for the Schwarzschild metric is the Painlevé-Gullstrand coordinates. They consist in performing a change from coordinate time $t$ to the proper time $T$ of radially ...
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3answers
134 views

Special Relativity-Simultaneity

If two events are simultaneous in one reference frame, are they simultaneous in all reference frames? Please provide thorough qualitative explanation If blue light is going at speed $c$, with red ...
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1answer
313 views

Length contraction in GR

In general relativity, $\sqrt{1-v_{e}^2}$ (with $v_e$ the escape velocity as a fraction of the speed of light) is the factor by which time is dilated in a gravitational field. Is it also the factor by ...
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2answers
733 views

Determining the rest frame

Sorry if this has been asked before in some way, I did try looking through the special relativity tag and didn't really see a good answer to my question. I am having trouble determining which frame ...
0
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1answer
92 views

What if you pointed a laser away from a black hole while falling into it?

Often this is asked if it was pointed towards a black hole. so I thought, if a scientist decided to sacrifice himself and jumped in a black hole, released some dry-ice mist and pointed a laser pointer ...
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6answers
623 views

What does the accelerated twin see in the twin paradox?

I am referring to the usual twin "paradox" where one twin remains on Earth while the other one takes a round trip journey on a rocket. I understand how the experience of the twins is not symmetrical, ...
0
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1answer
68 views

Observer independent quantities in special relativity

I have been thinking about what things about a point particle do all observers agree about? And I thought trajectories of particles must be the same for all observers, right? But, clearly it is not. ...
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2answers
136 views

Length contraction of a train

Suppose a train of length $3x~\rm m$ and a tunnel of length $x~\rm m$. When the velocity of train is $282842712.5~\rm m/s$ (from length contraction equation), then its length contracts to $x~\rm m$. ...
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0answers
88 views

Indeterminacy in Quantum Mechanics

Heisenberg's uncertainty principle tells us that there is a lower limit on the accuracy with which we can determine both the momentum and the position of a particle simultaneously. $\Delta$$x$ $\...
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2answers
112 views

Is it possible to change the frequency of the light by magnetic fields?

I do not recall where, but I have read someting about the light approaching a blackhole. In the writing, it was said that an object would gain kinetical energy due to the blackhole's gravity, and ...
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1answer
384 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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0answers
44 views

Thoughts on Black Holes [duplicate]

Given that time slows to a stand-still at the event horizon of a Black Hole, how long would it take a black hole to form, from the perspective of an outside observer? It would seem, as a massive star ...
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3answers
142 views

Inside a box moving with constant velocity

Suppose You were put into a box with all the instruments you might require. The box moves with constant velocity on the surface of the earth. We are asked to detect inside the box whether we are ...
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4answers
363 views

How Lorentz transformation is connected to reality and physics?

I don't understand whats the meaning of Lorentz transformation in Special relativity. In physics, the Lorentz transformations (or transformation) are coordinate transformations between two ...
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8answers
9k views

How can time dilation be symmetric?

Suppose we have two twins travelling away from each other, each twin moving at some speed $v$: Twin $A$ observes twin $B$’s time to be dilated so his clock runs faster than twin $B$’s clock. But twin ...
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1answer
83 views

How does a distant observer see matters that “form” an initial black hole?

I did see Can black holes form in a finite amount of time? but it does not seem to discuss how a distant observer would see evolution of collapsing matters that form a black hole. Does it view these ...
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3answers
185 views

Schrödinger's cat question

what would happen if a CC Camera or videocorder is put along with Schrödinger's cat inside the sealed box? Would it influence the outcome?
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3answers
216 views

Why does gamma show up in the relativistic velocity equations in the $y$ and $z$ terms?

I understand the addition of velocities in relativity in terms of the $x$ direction is $$u_x=\frac{v+u_x'}{1+vu_x'/c^2}$$ What I don't understand is why, when calculating for the $y$ and $z$ ...
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1answer
66 views

What (if any) significance exists in the analogy between the speed of light limit and the Schwartzchild radius of a black hole

I have always been intrigued by what seems to be very similar characteristic of the phenomena of traveling very, very fast, (close to the speed of light , and the Schwartzchild radius. AS a body gets ...
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1answer
102 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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1answer
67 views

Time travel (Velocity and Mass)

I learned that if I move on a high velocity, if my watch shows 12:00 and a my home clock shows 12:00 before the trip, when I come back, my watch will be like 12:05 while the clock will be 12:10. I ...
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5answers
170 views

Two objects are thrown into a black hole. The first crosses the event horizon at time's end, so when does the second one cross?

An observer throws an object towards a black hole, and then an arbitrary amount of time later, throws a second object towards the black hole. Disregarding Hawking radiation and assuming the Black Hole ...
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2answers
211 views

Special Relativity: Time dilation paradox [duplicate]

Two spaceships, A and B, travelling rapidly through space, pass each other on a parallel course. According to special relativity: A observes B speeding past, and perceives that time on B's ship is ...
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3answers
102 views

Can two simultaneous independent measurements circumvent uncertainty? [duplicate]

Without understanding the mathematics, I have learned that the uncertainty principle precludes being able to precisely measure the position and velocity of a particle at the same time. So my question ...
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3answers
219 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 ...
1
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1answer
648 views

Why is the buoyant force on an object the same for two different observers? [closed]

Situation: (self-made) A (green) block is partially immersed in a (blue) liquid and the vessel is accelerating upwards with an acceleration $a$. The block is observed by (stick-figure) observers $O_1$...
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4answers
2k views

Double Slit Experiment. What effect does the detector actually cause?

When a quantum of light arrives at a double slit, it passes through both slits as a wave and arrives upon a second screen with the interference pattern of a single wave that has been split into two ...
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1answer
63 views

How looks a wheel browsing 299000km in 1 second from its own frame?

Imagine a wheel with a 299km perimeter turning at 1000 round per second going 299000km (the speed of the wheel is 299000km/s relative to the starting frame). How does it look in the frame of a pilot ...
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1answer
70 views

Is Hawking Radiation compatible with a GR solution with an Infalling Observer Crossing the Event Horizon?

EDIT: See my answer. Question is malformed as it was based on the assumption that something like Hawking radiation (i.e. mass draining) occurred prior to the formation of the event horizon based ...
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2answers
96 views

General Relativity in the absence of observers

I see that just as in quantum mechanics, General Relativity is depending on observations taking place, hence they are depending on the existence of an information carrier which are photons, virtual or ...
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1answer
155 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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2answers
139 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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2answers
228 views

Is the dynamics of spacetime observer-dependent?

Consider de Sitter spacetime in static coordinates: $$ ds^2 = \Big(1- (H_\Lambda r)^2 \Big) dt^2 - \frac{dr^2}{1- (H_\Lambda r)^2} -r^2 d\Omega^2, \qquad r\lt H_\Lambda^{-1} \,. $$ This metric ...
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1answer
69 views

Black holes and red light

A friend of mine is reading the book "Black Holes: The Reith lectures" from Stephen Hawking. He is reading the French version, so I cannot give you the exact line, but on page 38, it is said that if ...
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2answers
342 views

Special relativity and spinning tires

I apologize for the subsequent headache. There is a person who claims quite adamantly that Einstein is wrong, using the following reasoning: A clock on a train slows down the faster the train moves....
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1answer
163 views

How would kinetic bombardment look like on impact to an observer on Earth's surface? [closed]

To be clear, I do want to know this because on the Worldbuilding Stack Exchange's chat room and on the Discord servers I am in, I have been told conflicting accounts about how such bombardment would ...
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1answer
249 views

Does Lorentz transformation apply to a non-inertial frame? Why?

Two space travellers, S1 and S2, head away from a point in an inertial frame S. The first travels along the trajectory $x = ct/4$ and the second along $x = \beta t^2$. Sketch their trajectories on a ...
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3answers
213 views

What does “same location” mean in special relativity (definition of proper time), when true length does not exist?

The following statement combined with the application of time dilation principle confused me on how proper length and time are measured and what they represent. "The speed at which electrons traverse ...
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2answers
206 views

Is relativity of simultaneity just a convention?

Lorentz transformations are well known to imply time dilation, length contraction, and relativity of simultaneity. This is prominently featured in any course on Special Relativity (SR), e.g. in ...
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3answers
340 views

Special Relativity Paradox based on Einstein's explaination [closed]

Einstein explained the special relativity (roughly) like this: He first assumed the speed of light would not change regardless the speed of its source. Then he described a person on a moving train was ...