# Questions tagged [observers]

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624 questions
110 views

### Help me to understand how time is relative!

I'm a bit confused about how time can be relative. Example: if a cosmic event happened that could be observed from earth, multiple times, because light from this event reached earth at different times ...
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### Relativity and time

Einstein's relativity states that times goes slowly in a moving clock.That means if my friend moves at a speed of $v$ his time will go slowly. But I am also moving at a speed of $-v$ relative to him. ...
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### Trying to understand relativistic equation related to energy released from two particle colliding near speed of light

Here is what blogging my mind. For simplicity, let's assume that there are only three particles in the universe, particle A, B, and C. Particle A and B going on the head on collision at near speed ...
3k views

### Energy Conservation Dilemma

Assume that a man is travelling in a space ship at a certain relativistic speed with respect to a man at rest at some point in space, such that 3 minutes in the ship is equal to 5 minutes for the ...
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### Apparent discrepancy between two time contraction calculations

The problem I am dealing with is the following. We have two spaceships A and B s.t in $S_E$ their respective speeds are $v_A^E >0$ and $v_B^E<0$ along the x axis. At $t_0$ in $S_E$ their ...
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### Observers without point of reference can assume themselves to be at rest

We know the the speed of light/c is the same to all observers. But i cant grasp something. Observer is moving at $0.5c$ in relation to us. He doenst have any point of referece and everything around ...
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### Physical intuition for why proper time is an invariant quantity

Mathematically, I understand why proper time, $\tau$ is an invariant quantity, since it is defined in terms of the spacetime metric $d\tau=\sqrt{-ds^{2}}$ (using the signature $(-+++)$ and with $c=1$)....
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### How and why time stops inside a black hole?

I have recently learned that black holes are dead stars, who are collapsed, and attracted other objects towards it and this process results in a very very large gravity. So light is also unable to ...
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### Relativity and the age of the Universe

I put my assistant in a spaceship and accelerate it to near the speed of light. 100 years from now (in my time), my assistant is travelling with speed $0.99c$. At that time I put up a super ...
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### 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 ...
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### What is the size of the world for a photon?

At relativistic speeds the distances contracts. What is the contraction ratio in the dimensions along the axis of travel between a static observer and a photon passing by?
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### Do quantum events depend on the presence of observer?

Similar to that in the Schrodinger's cat thought experiment the cat remains in superposition unless observed by an observer, also in the double slit experiment an electron is passing through both the ...
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### An experimentation on time dilation [duplicate]

In the video https://www.youtube.com/watch?v=ev9zrt__lec from 03:00 to 05:00 I hope that both the clocks would read the same equal amount ,say,1 revolution, in measuring the time elapsed for the ...
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### Traveling close to the speed of light, would a person (or anything else) have a longer existence or would the existence be passing in slow motion?

If something, let's say, an electron or a person, travels at some speed close to the speed of light, time would slow down, right? But would it be passing in slow motion or would it have a bigger ...
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### The nature of the observer in special relativity [closed]

In special relativity, is it silently assumed that the observer is only a “measuring machine” without an innate conception of space and time, so what he thinks about motion is exactly what he measures,...
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### Can two entangled electrons can get disentagled in Nature, withous us making a measurement?

Imagine we have two free, entangled [by their spins, which are in this case non-separable, while $x$ (or $p$) electrons are clearly separable]. In an experimental setup, we can measure (and observe) ...
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### What's stopping two independent observers from measuring the speed and position at the same time, separately? [duplicate]

From http://www.hawking.org.uk/the-beginning-of-time.html This means, it doesn't take into account, the Uncertainty Principle of Quantum Mechanics, which says that an object can not have both a ...
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### Given Einstein's Theory of General Relativity, would a sample of plants grow to bloom quicker in space than on Earth?

I watched a program the other day on Einstein's General Relativity. It was fascinating. As I understand it GR is the best model of the (large) Universe. The program stated that time literally passes ...
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### Teleportation and the observers effect

I'm a layman, and I was wondering, if teleportation is to ever exist, I'd assume it'd be in the sense that you would take the object you are trying to teleport, get it's exact makeup, then send that ...
361 views

### Lorentz Transformation in Different Reference Frames

TO ALL: Thanks to the help of many dedicated forum members, I have learned that this problem can be explained by understanding Minkowski diagrams. Here is a consolidated list of helpful links: https://...
3k views

### Do echo-locating bats experience Terrell effect?

At relativistic speeds there is an optical effect called Terrell rotation causing objects passed by to seemingly rotate. As bats use sound rather than light when echo-locating, at what degree would ...
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### 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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### 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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### 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 ...
116 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 ...
187 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 ...
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### 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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### 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 ...
428 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 ...
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### 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 ...
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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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### 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 ...
186 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 ...
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### 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 ...
429 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 ...
478 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 ...
544 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 ...
146 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 ...
193 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 ...
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### 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 ...
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 ...
272 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 ...
144 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 ...
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### 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 ...
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### 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 ...
138 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 ...
368 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 ...
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### 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 ...