Questions tagged [observers]

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3answers
76 views

Is the slowing of time with speed exactly proportional to the increase of mass with speed?

This article on time dilation explains how time slows and mass increases with speed. Are they proportional? And is time dilation with a specific speed-mass equal to the time dilation with a ...
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8answers
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In the twin paradox does the returning twin also come back permanently length contracted flatter than the twin on Earth?

This video from Brian Greene suggests this is so: https://www.youtube.com/watch?v=2sZUNud6rRw&list=PLj6DWzIvBi4PFDXCCV1bNhVUgDLTwVbFc&index=60 It shows if you stop a pole in the barn (...
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4answers
546 views

Very Massive Relativistic Body [duplicate]

You're observing a massive object (probably a neutron star), and it is moving at a significant fraction of the speed of light relative to you. The mass of the object is just below the mass necessary ...
52
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11answers
14k views

Is Schrodinger's Cat itself an observer?

In Schrodinger's Cat thought experiment, why doesn't the cat itself qualify as an observer? Reading through the replies there seem to be two suggestions for what can take the role of observer: any ...
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2answers
118 views

Does the relative speed of time mean there is less energy where time is slower?

Time runs relatively slower near a planet than in outer space. Does this mean that there is less energy near the planet? Is there a relationship between energy and the speed of time? If so, this ...
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4answers
145 views

Why don’t I see proper time?

Say we have my friend blasting off in a spaceship to Planet Physics and I’m sitting here on earth. I think it’s quite obvious that I will measure the proper length between the planets (because I could ...
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3answers
153 views

Using time-dilation to our advantage?

Assume we have a super-computer working on an important problem, and we estimate it will take about $200$ years to solve. But, we need the answer in $20$ years. Could we ship the super-computer into ...
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0answers
32 views

Relativity thought experiment [duplicate]

I have two doors that I can close arbitrarily quickly, and I want to shoot a long rod at relativistic speed through both doors. When measured at rest relative to the doors, the rod is longer than the ...
2
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1answer
76 views

Relativistic rockets paradox [closed]

The next problem is, as I understand, a spin on the ladder paradox which is extensively documented everywhere. There are two rockets ($A$ and $B$), of length $L_p=1$m which are traveling in opposite ...
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0answers
54 views

Derivative of energy in General Relativity

I have found this equality that is the derivative of the energy for a local observer in GR, with energy defined as $E=-p^\mu u_\mu$: $$dE/d\tau = -p^\mu p^\nu\nabla_{(\mu}u_{\nu)}$$ trying to derive ...
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0answers
123 views

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 ...
5
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2answers
164 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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0answers
24 views

Curved space relative motion

Suppose I live in a 2d curved space. I can describe intrinsically the motion of a particle which moves in my 2d manifold using time "t" as a parameter. Now suppose there is a second observer (let's ...
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1answer
56 views

Spacetime dilation Moon vs Earth [closed]

As I understand it, time will pass slightly faster on the surface of the moon compared to the earth This is due to the difference in velocity and gravitational fields of the two planets. How could I ...
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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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4answers
145 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 ...
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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 ...
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2answers
96 views

Basic Special Relativity Question [closed]

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 ...
6
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1answer
191 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 ...
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0answers
23 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 ...
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4answers
110 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 ...
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1answer
38 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 ...
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1answer
78 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?! ...
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1answer
74 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 ...
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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 ...
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2answers
73 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 ...
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2answers
114 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 ...
1
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1answer
57 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 ...
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2answers
82 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 ...
6
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3answers
277 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
80 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
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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6answers
299 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 ...
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 ...
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2answers
64 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
50 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 ...
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1answer
53 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 ...
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0answers
41 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 ...
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1answer
53 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 ...
3
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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 ...
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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
70 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?
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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 ...
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1answer
62 views

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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1answer
75 views

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
87 views

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 ...
1
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1answer
181 views

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
165 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 ...
6
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2answers
326 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
79 views

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 ...