# Questions tagged [observers]

For questions related to observers in relativity or quantum mechanics.

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### Special relativity - what is the age of my twin after a one-way trip? [duplicate]

I have a twin who stays on Earth, and I will move to another planet 4 ly away. I will travel at a certain relativistic speed towards this planet. What I just described here is the first half of the ...
20 views

### The "observer effect": is this a basic law of every observation or measurement? [duplicate]

I would appreciate some clarification of the "observer effect": Is it generally true that every observation (or measurement) disturbs the object being observed? I understand the observer ...
105 views

### Would an object moving close to the speed of light appear colder to a stationary observer? [duplicate]

If an object moves close to the speed of light time will slow down in its reference frame as seen by a stationary observer. If my understanding is correct, this means that all movement - such as the ...
71 views

### "Rear clock ahead" effect in special relativity

I've read (pretty much) all the questions in the site regarding the topic, however I wasn't able to comprehend the author's solution to the problem in its totality. The problem is from David Morin's ...
27 views

### Two observer scenario

If I have two observers trying to determine the relative position of the same hydrogen electron and they have a device that allows them to make simultaneous measurements of the same electron (although ...
41 views

### Is it possible to detect your speed relative to light? [duplicate]

Let’s take two cases: (1) your velocity is constant and (2) you are accelerating. (1) isn’t the answer no, like that’s a core idea of Einstein’s relativity stuff? (2) I don’t know.
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### Does this special relativity question have enough information to solve it?

I recently encountered this question: How long would an someone need to spend on the ISS so that their biological clock would be one day younger than their twin who stayed on Earth? The ISS is ...
43 views

### Relationship between clock rate and speed

Let's consider example: The propagation of light from the middle of a moving train to its left and right ends. From the point of view of a train passenger, the light will reach the right and left ends ...
1 vote
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### Relativity of Simultaneity - Why doesn't the train/lightning example contradict the absolute speed of light?

I know similar questions have been asked here before, but none of them seem quite to address my particular confusion. I'm not afraid of math (I did well in calc III last semester, for example) but my ...
26 views

### Time dilation query 2 [duplicate]

The examples for the theory show that The clock run faster or slower and show time interval, not that the time itself does not differ in two frames one is in motion relative to other. Then, what is ...
77 views

### If, relative to us, objects never cross the event horizon, does this imply that we cannot observe a black hole grow? [duplicate]

From what I understand relativity predicts that outside observers would never observe external objects actually crossing into the event horizon, due to time dilation. How does that can reconciled with ...
1 vote
91 views

### How can two observers agree on the force between 2 moving charges?

I'm having a hard time figuring this out. This question has been asked before, I saw the answers, but I'm still struggling with this. So I decided to ask. I want to calculate the same force for both ...
87 views

### Does a train that moves at relativistic speed and then decelerates to zero speed, seem to expand to an observer at rest near the tracks? [duplicate]

A train is moving with a relativistic speed, according to an observer at rest near the tracks. To this observer, the train seems contracted in the direction of motion. Then, the train decelerates (...
1 vote
113 views

### What are the effects on a stationary observer at a specific distance from a Kerr Black Hole?

A Kerr Black Hole (BH) is a spinning BH. There is an Event Horizon (EH) which is $$r_H^\pm =\frac{r_{S} \pm \sqrt{r_{S}^2 - 4a^2}}{2},$$ where $a = \frac{J}{Mc}$ and $r_{S}$ is the Schwarzschild ...
90 views

### Details on analyzing relative motion

I have been viewing some videos and reading some articles about Relativity and relative motion. My search abilities seem to be rather lacking. I am looking for articles, questions, and/or forums ...
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### Does special relativity imply that there's such a thing as absolute time, or base time?

If time measured by one observer moving at a greater velocity than another observer is observed to be passing more slowly, does this imply that there's such a thing as "absolute time" or &...
71 views

### Why should our Knowledge of an Experiment Affect it's Outcome?

The "Delayed Choice Quantum Eraser" experiment shows that our knowledge of an experiment affects it's outcome. Why is this?
221 views

### If an observer at the middle of a spaceship shines laser in both directions and the ship changes on contact with laser, what will earth observer see?

Yet another SR "paradox". A spaceship is moving w.r.t someone on earth. From the POV of a ship observer standing in the middle, he shines a laser beam in both directions. Assume that the ...
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### Lorentz-like transformations of 1+1 dimensional Minkowski space

Alice lives in 1+1 dimensional Minkowski spacetime. Bob travels at a constant velocity $v$ with respect to Alice. When Alice assigns coordinates $(t,x)$ to an event, Bob assigns coordinates $F(t,x)$ ...
53 views

### The train-and-platform VS Causality and Simultaneity

for long time I was ruminating on the train and platform experiment and “what if” extension of this experiment… So, what if the light striking on either end triggers system (one on each end) with ...
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### Double slit experiment with detector and experimenter

In the double slit experiment we have particles which travel through two slits and then hit a final wall/detector. We can perform the experiment with or without an extra detector at the slits which ...
53 views

### Doppler shift of light as seen by a free falling observer towards a single star [duplicate]

Consider an observer who is free-falling towards a star with no angular momentum (radial movement only considered). The velocity of the observer equals the escape velocity at each distance, but in ...
1 vote
79 views

### Energy is the time component of 4-momentum in SR: Proof as per R. Wald's book

This is an excerpt fom R. Wald's book on General Relativity (page 61). I'm not able to understand how he deduces that $E$ must be the time component of $p^a$ with only the assertions made before this ...
1 vote
94 views

### Why do we See Black Holes?

The image of things falling into a Black Hole stays frozen on the event horizon forever. So how is it that we don't see the image of a dying star where a Black Hole formed? We see the black, with an ...
873 views

### Do observers agree on forces in special relativity?

If $\vec p$ is the spatial part of the four-momentum $p^{\mu} = m\gamma(c, \vec v)$, then in special relativity forces are defined such that $$\vec F = \frac{\textrm{d}\vec p}{\textrm{d}t},$$ where $t$...
1 vote
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### How can photons be emitted from a star, travel millions/billions of years in a "straight line", and be seen by two adjacent observers?

If photons are emitted by a star and travel millions/billions of years in a "straight" line, they can be seen by adjacent observers. This makes it seem like there must be an uncountable ...
70 views

### Simultaneity Question in Relativity [closed]

The topic of simultaneity is often described with an example using a “stationary” Observer 1 and a traveling vehicle (e.g. train) with an onboard passenger (Observer 2) passing each other at the start....
2k views

### What would a privileged/preferred reference frame look like if it existed?

So I'm still reading up on Einstein's theory of special relativity and what it means in regards to space-time and motion. One key feature and cornerstone being that there exists no such thing as a ...
30 views

### Can an inertial frame observe another frame moving faster than $c$? [duplicate]

Why is it impossible to observe someone moving faster than the speed of light? Suppose there is a frame A. frame A can observe something moving at the the speed $c/2$ right? So suppose there’s two ...
1 vote
191 views

### Would something falling into a black hole appear to be flattened to an outside observer?

At the event horizon, time essentially stops. An outside observer would never see you cross the event horizon, as my understanding goes. But in that case, how would this work out if I imagine the ...
69 views

### Help with understanding Einstein's thought experiment of the train [closed]

My understanding is that Einstein claimed that the aether model of light is inconsistent with the principle of relativity (that you cannot tell the difference between constant velocity and being ...
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### Is my understanding of the symmetry/reciprocity of time dilation in this example correct, and can order of events be reversed in reference frames? [duplicate]

Let us imagine a spaceship traveling to a planet around a star 4.0 light years away exactly at 99 percent the speed of light (Lorentz factor of around ~7) relative to earth. For simplicity let us ...
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### What is the ceiling on the perceived relative speeds of two bodies, from the point of view of a third observer? [duplicate]

What is the ceiling on the perceived relative speeds of two bodies, from the point of view of a third observer's reference frame? We all know the ceiling on perceived speed relative to oneself is $c$. ...
132 views

### What does Lorentz Factor really imply, if time Dilation is symmetric?

Let us imagine a spaceship traveling to a planet around a star 4.0 light years away exactly at 99 percent the speed of light (Lorentz factor of around ~7) relative to earth. For simplicity let us ...
1 vote
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### Does the angle of refraction depend on an observer?

According to Fermat's principle, light should take the least time between two points. Therefore, is it correct to say that the angle of refraction is solely dependent on the difference between speed ...
1 vote
72 views

### How long until light reaches me if I travel away from a point? [closed]

Let's assume that: I'm 1 light year away from some point in space I'm travelling away from that point at 0.5c (or whatever fraction of the speed of light it takes for relativity to play a big role) A ...
80 views

### Do all observers see the same curved spacetime -- or would that be a (prohibited) preferred inertial frame? [duplicate]

My question -- pardon if not asked in the most incisive scientific prose Do all observers see the same curved-space time? Let me clarify: Given that objects traveling at different relative speeds will ...
222 views

Consider a distant observer traveling at .867 c ( $\gamma=2$ ) relative to the solar system along the line that is collinear with the sun's axis of rotation. As the clockwork solar system spins ...
29 views

### Why is the energy of a particle with $4$-momentum $p^a = mu^a$ measured by an observer with $4$-velocity $v^a$ given by $E=-p_av^a$? [duplicate]

On page 61 of his book “General Relativity”, Robert Wald says that the energy of a particle with $4$-momentum $p^a = mu^a$ measured by an observer with $4$-velocity $v^a$ is given by $E=-p_av^a$. How ...
468 views

### Effects of acceleration and gravity on proper time

Let's consider the effects of gravity and acceleration on proper time of an observer (yes, I want to know how the equivalence principle works). First situation: a spaceship with rocket engine stays ...
1 vote
101 views

### Is the Lorentz boost boosting me or the frame?

I am starting to dive deeper into Special Relativity, but something that always confuses me is whether the Lorentz boost is the increase in my own velocity to catch up with moving objects or the ...
1k views

### What does it mean that a falling mass in space doesn't sense any force?

I heard someone explain that if you were falling in space (i.e, a rooftop) you wouldn't sense gravity as a force acting upon you. But it would accelerate (≈ 9.8 m/s^2) , which to me suggest that a ...
254 views

### How are electrons distributed on the screen in the double-slit experiment with an observer?

As I understand it, when both slits are open, the electrons spread out in an interference pattern, and if one of the slits is closed then the electrons are spread out according to a single-slit ...
1 vote
251 views

### Would someone falling into or orbiting a black hole see themselves?

Spacetime is extremely curved in the vicinity of a black hole. When a light ray is targeted close to a black hole, it is curved to a significant angle. This angle increases as the target is pointed ...
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### Can we observe a more recent space?

The space we are viewing now is their distant past as their light has only reached our eye or telescope after travelling a long distance at the speed of light. However, for lights that are still on ...
452 views

### Can an observer sitting at rest at infinity, simultaneously measure the proper time and proper distance of a particle travelling in any geodesic?

In the book General Relativity by M.P Hoson, the author tries to calculate the velocity of a particle radially infalling in Schwarzschild spacetime. The velocity is measured by the observer sitting at ...
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### Relative speed of a spaceship question

We are just starting to learn about relativity and special relativity in class and I feel like I am having a really hard time wrapping my brain around it, especially when it comes to spaceship ...
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### Has the Twin Paradox Really Been Resolved? Interesting peer-reviewed paper states that it hasn't been resolved [closed]

Has the Twin Paradox really been resolved? Here is an interesting peer-reviewed paper Has the Twin Paradox Really Been Resolved? (RG) which states that it hasn't been resolved: Of particular interest ...
### Time dilation and understanding which is $\Delta t$ and which is proper time $\tau$
In the textbook that we are using, the definition for proper time $\tau$ is the interval between two events, as measured by an observer who is at rest with the two events. The definition for $\Delta t$...