Questions tagged [observers]

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6
votes
6answers
697 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, ...
1
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3answers
105 views

Einstein's train, considering the thunder

From Einstein's "easy" explanation https://www.bartleby.com/173/9.html (Yes, I have reviewed 8 other answers to similar questions. Please bear with me.) M is on the platform, T (aka M') is on the ...
-5
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0answers
61 views

What is exactly meant by “time is relative”?

Let's say we are traveling to the galaxy X - which will take 4 years for us to reach. However, it will take 100 years for the observer on the earth. How come is it possible? How 4 years can be ...
0
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2answers
96 views

Basic Special Relativity Question [on hold]

I am trying to understand the implications of c being relative to the frame of reference. Is the following analysis correct? N is on a slow train moving 3 m/s and M on the platform next to N both ...
0
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0answers
16 views

Which frame to consider for time dilation? [duplicate]

Clearly for constant motion we cannot define absolute rest or motion. So ,when dealing with time dilation problems for example twin paradox , how do we define which frame is in motion? If we ...
-1
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1answer
2k views

Two Trains Relative Motion [closed]

I am confused about finding the solution to this relative motion problem: You are on a train traveling east at speed of 20 m/s with respect to the ground. Your friend is sitting on another train ...
3
votes
3answers
721 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=\...
5
votes
1answer
177 views

When looking for a black hole, will we always find a collapsing star instead?

I know that from the point of view of an outside observer, the collapse of a star into a black hole never finishes, i.e. any observer will always see a (red-shifted) version of the star before its ...
5
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2answers
3k views

Which observer's time is proper time?

I just have a quick question about time dilation/proper time because my physics book makes it a little confusing. Let's say we have an observer on Earth, and then an observer on a space ship. The ...
3
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3answers
838 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 ...
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1answer
72 views

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 ...
2
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4answers
224 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 ...
0
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0answers
22 views

Electromagnetic waves & observer effect [duplicate]

So I’ve been doing some research & realised that they done the double slit experiment with large molecules and the molecule behaved as a wave , so does this mean that all matter is really just a ...
0
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1answer
69 views

Components of speed of light

I'd like to start off by saying this may be totally wrong; thus I would like some help clarifying this. Suppose you are in a vacuum, and a beam of light is travelling at, say, a N$45°$E bearing. The ...
0
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1answer
188 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 ...
1
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1answer
127 views

Velocity in different time frames

A satellite orbiting the earth experiences time faster than a person on earth as its in a weaker gravitational field (which has a greater effect than the slowing down of time caused by its motion) ...
0
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1answer
36 views

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 ...
0
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1answer
75 views

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?! ...
0
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1answer
68 views

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 ...
1
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2answers
70 views

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 ...
1
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2answers
103 views

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 ...
0
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1answer
181 views

To an observer in a black hole, would light be stationary? Would it be as if the black hole is accelerating faster than the speed of light?

Taking into account that we know that gravity and acceleration are fundamentally indistinguishable from each other, Would an observer in a black hole believe that he's accelerating upwards faster than ...
0
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2answers
166 views

Are quantum measurement of a system only occurring with human interaction?

In quantum physics, any functional interaction like measurement/observation forces particles down to a single state. Yet when plants do their photosynthesis it's been discovered that they actual ...
1
vote
1answer
56 views

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 ...
2
votes
1answer
210 views

Wave particle duality/double slit experiment

In double slit experiment, if flux of electrons are very low, then.. if one "observes" the electrons before it has reached the slit, it "behaves" as a particle and this is due to the interaction (...
6
votes
3answers
267 views

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 ...
1
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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 ...
1
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2answers
68 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 ...
0
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1answer
193 views

Special Relativity pothole paradox [duplicate]

Consider a man who is 1 metre in width and is running (at relativistic speeds) towards a pothole in the ground which is also 1 meter in width. In the reference frame of the man it appears as though ...
5
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6answers
296 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 ...
1
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0answers
56 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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1answer
175 views

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. ...
2
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2answers
78 views

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 ...
-1
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2answers
61 views

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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0answers
48 views

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 ...
2
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5answers
727 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, ...
0
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1answer
51 views

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 ...
0
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0answers
40 views

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 ...
3
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2answers
508 views

Video of Double Slit Experiment Observer Effect

I have been watching a lot of videos of the double slit experiment and trying to wrap my head around it. I found examples of where the experiment has been done, however the videos cut out and only ...
3
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2answers
71 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 ...
6
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1answer
154 views

Twin paradox with one twin in orbit, one in radial free-fall

Many questions on Physics SE relate to the twin paradox, but I did not find any that ask this specific question. Suppose that object A is in a circular orbit around a spherically symmetric, non-...
0
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1answer
47 views

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 ...
60
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10answers
8k views

Can black holes form in a finite amount of time?

One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
2
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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 ...
0
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0answers
19 views

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 ...
0
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1answer
67 views

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?
5
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4answers
567 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 ...
1
vote
1answer
223 views

If the faster I go, the slower time becomes for me, then does that mean the slower I go the faster time becomes?

Disclaimer - I’m not a trained physicist. I just have an interest in the subject so this question is probably going to come across really confusing but I’m trying my hardest to explain. So firstly, ...
0
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1answer
144 views

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 ...