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Questions tagged [observers]

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Observer Watching a Ball Thrown Up on A Train [closed]

Let's suppose I'm on a train, moving with constant speed V1. At a time T1 I throw a ball up in the air, the ball do not accelerate but has constant velocity V2, and, in this hypotetical scenario, no ...
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1answer
83 views

Simultaneity in Newtonian mechanics

How would Newtonian mechanics answer the train and moving light question? The setup is: A train is moving in the positive x_axis with speed c/2. A person stands in the middle of the train. There ...
0
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1answer
114 views

Observers at the speed of light [duplicate]

Just a quick question to help me see if my reasoning is right. The speed of light is constant from all frames of reference. So does this mean that an observer travelling at the speed and taking into ...
0
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1answer
154 views

Inertial Frames/Inertial observer

I'm a toddler, please could you explain concepts as simply as possible, no words with more than 10 letters. Thanks. So I have begun work on Newton's Laws of Motion. I have also been told these laws ...
5
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2answers
354 views

Are my intuitions about special relativity right?

Suppose there is a bus which travels at constant speed $v=0.9c$, relative to my friend Eric who is standing still on the ground. I'm exactly in the middle of the bus, at distance $d$ form both ends, ...
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2answers
189 views

Special Relativity proper time confusion

Consider the question: Alice is in a spaceship moving at a velocity of 3/5c with respect to Bob on earth. When she passes earth both of their clocks read t = 0. a) When observers in Bob’s frame ...
1
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1answer
400 views

What's the observer in double slit experiment?

I watched a video on youtube about double slit and it said that when we try to observe electrons it starts to act like a particle but what are these observers that they use? they're shooting one ...
2
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0answers
51 views

Gravitational time dilation and the pace of time

If we are in empty space far from a black hole, at rest relative to the hole, we would look at a clock and a light source inside the gravitational field of the hole, then we would, according to the ...
1
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1answer
140 views

Special Relativity pothole paradox

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 ...
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3answers
90 views

Can light be stopped after being perceived?

Please note the following sentence before you answer the question: you may assume the light itself is not affected by the observer perceiving the light. Imagine the following schematic: A = Source B ...
3
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1answer
177 views

If relativity is symmetric, do the satellites in the GPS system see the earthbound clocks as running slow due to relativistic time dilation?

If relativity is symmetric, do the satellites in the GPS system see the earthbound clocks as running slow due to relativistic time dilation? How much slower does the satellite see the earthbound ...
1
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0answers
70 views

Time dilation on moving clocks [closed]

Three observers are standing on Earth's surface with identical indestructible clocks. One observer tosses his clock up such that it returns to its original height after a time T by that clock. The ...
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4answers
375 views

How does gravity slow a clock at a distance?

If you have two identical clocks that are synchronized and then you move one significantly closer to a black hole but still far from the black hole, the closer clock will mark time slower than the ...
20
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1answer
464 views

What is an observer in QFT?

In non-relativistic quantum mechanics, an observer can be roughly describe as a system with wavefunction $\vert \psi^O \rangle$ which, upon interaction with another system $\vert \psi^S\rangle$ (in ...
5
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3answers
541 views

Is there some theory which predicts that the speed of light in vacuum is constant for all observers?

Relativity theory takes this result for granted because it's one of its assumptions. Is there some theory which starts with some even more basic assumptions and then predicts that the speed of light ...
0
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1answer
120 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 ...
3
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2answers
189 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 ...
1
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1answer
190 views

If we see someone in a spacecraft and their clock is running slower than ours do they see ours running even slower?

From what I understand if we see someone's clock running slower than ours they will see ours running slower rather than theirs, but in my physics textbook they ask this question: A spacecraft ...
14
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2answers
2k views

Understanding Time Dilation at the Event Horizon

I was recently reading about the event horizon of black holes and came across the fact that, to a "stationary" observer, it takes forever for someone to fall into a black hole. The sources claim that ...
2
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3answers
253 views

Lorentz transformation appears to lead to a contradiction. Where is my mistake?

I recently bought a copy of Einstein's booklet Relativity, in which Einstein attempts to describe the basics of special relativity to laypersons. In Chapter 12, Einstein explains how the Lorentz ...
-1
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1answer
141 views

Are space mirrors that show our historical past past possible? [duplicate]

Say nothing new can be learned anymore about the past by mere historical deduction from the present. Can a giant mirror-like thing out there that's let's say 2000 light years away from earth (and has ...
0
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1answer
50 views

Black hole horizon encompassing massive particles from an outside observer's perspective?

I've seen a few posts on the internet which seem to suggest that an observer never seeing a particle crossing the horizon is an idealization for a massless particle. If the particle is massive some ...
0
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1answer
87 views

Time and gravity

am i right in saying that if you could raise the distance in the speed = distance/ time equation without altering the other parameters it would give the appearance that (to an outside observer) time ...
3
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1answer
136 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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3answers
329 views

If two observer don't agree about distance traveled and the Times it takes, why they agree about relative speed?

Lorentz factor depend on speed, but to measure the speed, we need to know the distance traveled and the times it takes to get the ratio. But according to special relativity theory, for two equivalent ...
5
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1answer
144 views

How stellar black holes are formed? [duplicate]

This is a kind of follow-up of my previous Phys.SE question Can something (again) ever fall through the event horizon? that seems to have established that, for an outside observer, things never fall ...
1
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1answer
40 views

Differences in calculations should be due to differences in environment?

Lisa Randall in her book Warped Passages writes, "A very reasonable thing to expect from physical laws is that they should be the same for everyone. No one could blame us for questioning their ...
3
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2answers
179 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. ...
2
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3answers
84 views

Special relativity: why moving refrence frames?

The idea that one refrence frame(RF) is moving relative to another is extremely counter-intuitive for me. since I always think if one thing moves and the other is stationary then they share a common ...
2
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2answers
123 views

What should I consider as an observer to measure the speed of cosmic objects?

I mean for example if earth is the observer, then there might be entire galaxies travelling faster than the speed of light relative to earth. So according to Einstein relativity this shouldn't be ...
4
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2answers
185 views

Is the existence of a photon relative? [duplicate]

If an observer passes an electron, in such a way that the observer is accelerating, the observer would see photons because accelerating charges induce electromagnetic waves. But from point of view of ...
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2answers
168 views

In diagrams which illustrate special relativity, can an observer “know” that he is moving?

Or is it always assumed that an observer in an inertial frame of reference thinks that he or she is at rest, and that it is only others - observers and frames of reference - that are moving? In ...
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1answer
46 views

Speed of light - quantum unobserved

Although I am sorry that I don't understand it very well at all, I understand that the slit-lamp experiment is affected by whether it is being observed. How do we know that the speed of light is not ...
3
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1answer
239 views

How a reference frame relates to observers and charts?

Recently I've been watching the General Relativity lectures from the "International Winter School on Gravity and Light" by Frederic Schuller. In those lectures he made the following two definitions: ...
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0answers
50 views

If you hit the light barrier what would it look like? [duplicate]

Is it correct to say that if you hit the speed of light you would travel at the same speed as a single image in time would? And if you were to (if possible) travel faster than light, you would ...
0
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1answer
150 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, ...
1
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1answer
403 views

Is my interpretation of $dt/ d\tau $ correct?

$dt/ d\tau $ = Lorentz factor Imagine inertial frame A is situated on a spaceship moving near the speed of light relative to inertial frame B, which is an astronaut in space. The Lorentz factor = $\...
7
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1answer
157 views

Hubble flow and inertial reference frames

Looks like different questions, but they are basically the same: 1) Let us imagine a mostly empty expanding universe with constant $H$. Two observers (Ginger and Fred) that are separated by a ...
5
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2answers
136 views

Planets close to a black hole

Like in the movie Interstellar, when they go near "gargantua" the big black hole that have a planet orbiting the black hole, they go on the planet and every minute equals X years in earth time. If ...
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4answers
176 views

How is it possible for an observer to measure the speed of light as it moves AWAY from him?

I often come across statements such as this one, by N. David Mermin, a well-known physicist: "Let P be a valid procedure for carrying out the time and distance measurements that allow one to ...
2
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1answer
81 views

What is an observer that is not a whole living human person? [duplicate]

In current quantum models, name one or more objects that collapse an entangled waveform into a single classical particle other than a complete living human person. The "duplicate" I just read has an ...
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1answer
300 views

Problem with Minkowski diagram for twin paradox

I always assumed that you could explain the twin paradox in special relativity using Minkowski diagrams, such as the one shown in wikipedia. In that diagram it is shown how a change in reference ...
2
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1answer
118 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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0answers
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Is the Hubble parameter different for each observer?

As the universe expands, galaxies disappear beyond the observable universe. So we could calculate the rate of expansion of the universe (the Hubble parameter) by estimating the rate of disappearance. ...
3
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2answers
115 views

Is the age of the universe different for a returning space traveller?

When an astronaut returns to the planet he came from, he would then count the exact same number of galaxies as an observer on that planet, but they could have had experienced wildly different amounts ...
48
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1answer
4k views

Does entropy depend on the observer?

Entropy as it is explained on this site is a Lorentz invariant. But, we can define it as a measure of information hidden from an observer in a physical system. In that sense, is entropy a relative ...
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2answers
378 views

Will moving observer see time dilation?

Please look at this animation. The green dots and red dots in the animation represent spaceships. The ships of the green fleet have no velocity relative to each other, so for the clocks onboard of ...
3
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5answers
281 views

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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2answers
475 views

The special theory of relativity angle contradiction

I always see this picture for proof of the time relation between frames that moves relatively constant speed in the special theory of relativity. The time for the observer who is in the box which ...
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2answers
144 views

Does a particle falling into a black hole ever reach the center?

Once an object or particle passes the event horizon, will it ever reach the center of the blackhole. I ask this from my elementary understand of relativity. This is my understanding: if an object ...