Questions tagged [observers]

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How does an Object at event horizon stop to the observer?

I have read about an object, that once at the even horizon the object would be seen as stopped to the observer. So my question is in regards to the light reflection off of the object. If the object ...
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Does time really slow near the speed of light or does it only seem that way to an outside observer? [duplicate]

I was lying awake last night struggling to sleep as one does, thinking about time and Einstein's concepts of relativity and having some fun with thought experiments similar to the one Einstein ...
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At what constant speed should I travel one light-second to make my time and a stationary person's time 1 second off?

at what constant speed should I travel one light-second to make my time and a stationary person's time 1 second off?
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Length Contraction & Accelerating Observers

A particle undergoing constant proper acceleration $\alpha$ will appear to have a hyperbolic worldline in the Minkowski spacetime $(ct,x)$ of an inertial frame given by: x=\frac{c}{\alpha}(\sqrt{c^2+...
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"Moving" clocks

In regard to relativity of simultaneity, why are clocks in the "moving" frame of reference unsynchronized (i.e., in the direction of motion, they run behind toward the front and ahead toward ...
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Can one record a speed faster than $c$?

A ship goes to the moon at 0,5 c, the captain will record on his clock about 2 seconds and a half, right? Now he increases the speed to 0,999 c (or more), thanks to relativity he should record on his ...
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Observing attraction between parallel wires from two perspectives

Two parallel wires through which there flows a current in the same direction. While the electrons are moving, obviously the wires attract each other (the right hand rule for magnetism and electricity)....
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Can't understand a statement about motion

From the book where I am studying motion, It says Motion is a combined property of the object under study and the observer. There is no meaning of rest or motion without the viewer. I know that, for ...
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For someone staying on Earth, what is the minimum possible time to send a spaceship to Alpha Centauri and have it back?

Suppose we want to send a spaceship to Alpha Centauri and calculate the minimum possible time it will take for us here on Earth to have the spaceship back. We are not interested in how fast the time ...
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How fast would a clock falling into a blackhole tick relative to the reflection of a clock stationed far away?

Lets say you have two clocks, you attach one to a mirror and drop in into a black hole, while you keep the other next to you, point it towards the black hole and view it's reflection on the mirror. ...
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Why the proper time of photon is zero? [duplicate]

I'm currently taking classes on General Relativity, and whilst working on Geodesis equation: Find out that for mass particle q is equivalent to proper time but in case of photon the proper time of ...
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A question about relativity and time dilation from a lay-person

Observer A is in a spacecraft flying toward earth Observer B is standing on earth A is traveling at near light-speed and will reach the earth in 1 week. When B looks at A's clock, he sees it moving ...
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What does and observer inside of a collapsing shell observe?

What does and observer inside of a collapsing shell observe? Lets say we have a shell of matter collapsing to a black hole. What would observers near the center see? How would the rest of the universe ...
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Ok, so I understand why an electron would change while being observed. But does it change exponentially when being observed by more than one person?

Ok, so I understand why an electron would change while being observed. But does it change exponentially when being observed by more than one person? Say having 10 or 100 observers.
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How to represent a pair of inertial frames in relativity?

Can two Cartesian inertial frames, in general relative motion, be drawn parallel to each other? A picture in Wikipedia suggests this, but some answers on this site appear to differ. Consider observers ...
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Simple resolution to the twin paradox?

There are a ton of videos that cover the twin paradox. They're all quite involved and don't really get to the heart of the matter, which is the fundamental asymmetry between the two observers (as ...
1 vote
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How do you define present from physics point of view [closed]

So how would you exactly define present, aside from fact that time is an illusion; is it the age of the universe in this sector that we experience subjectively as present, does it have to do anything ...
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Observing from a reference frame moving with high speed

Suppose at a distance $x$ from earths surface,a rocket comes with $v$ velocity which is comparable with the speed of light. Suppose lifetime of the rocket is $T$. We want to know whether the rocket ...
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Relativity of simultaneity in special relativity

I’ve read a lot of article about the topic and I think I understood it : according to the theory, two observers in two different frames of reference can disagree on the order of two events. For ...
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I hope I can convey my question in a reasonable way: I know that if Alice is moving towards Bob at $0.75\,c$ and Bob does the same towards Alice, then to calculate how each one of them measures their ...
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Is the proper time invariant while going from stationery frame to freely falling frame in Genral Relativity?

While reading GTR, I found the following calculation: Consider an arbitrary gravitational field and let us take $x^{\mu}$ as the stationery/lab frame and $\xi^{\mu}$ as the freely falling frame, where ...
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Consider this setup: The black circle is the observer, who has a $360^{\circ}$ range of observation around it. In its orbit are other circles, orbiting at relativistic speeds. They are all orbiting ...
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Why should the speed of light necessarily be the same for all observers in train thought experiment?

I am learning about train thought experiment that explains relativity of simultaneity , but this picture confuses my mind. It seems to me even if the speed of light is not the same for all observers,...
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I was watching this video about length contraction on youtube. It takes up an example where a rocket is moving towards a planet with velocity $v$ with respect to the earth. From that example, it ...
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Could I, within my lifetime, reach any star I wanted if I went fast enough?

Disclamer: I'm not talking about FTL travel here. I'm also not talking about any weird space warping mechanics like wormholes and such. I've always thought that if a star was 4 light years away, then ...
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Accelerating with $9.81m/s^2$ for one year [duplicate]

I know, for many this might be a really basic problem... When I multiply $9.81m/s^2$ with 1 year I get just above 1c (1.032c as per wolfram alpha) as a result which seems like a nice coincidence. I ...
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Why doesn't the classical addition of velocities apply to light?

Recently, relativistic physics has sparked interest in me. I read in one of my textbooks that the classical addition of velocities does not apply to light but the explanation given in the book is ...
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What does length contraction actually mean? (special relativity)

I am little bit confused about the idea of length contraction based on my textbook which did not elaborate this topic much. lets say there are two guys named A and B. A is at rest and B is in motion ...
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Two trains ABC and PQR are moving with equal speeds in opposite directions relative to a platform observer. Special relativity problem [closed]

Observers B and Q stand at the midpoints of their trains. A flash of light E1 is emitted when A and P coincide and another E2 when C and R coincide. The platform observer sees both flashes at the same ...
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Why doesn't time contract?

I'm tutoring a Year 12 (high school) physics subject which requires me to understand special relativity, in particular, time dilation and length contraction. I have only studied 1 semester of 1st year ...
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When an object interacts with the environment, does the wave function collapse? [duplicate]

When an object interacts with the environment, does the wave function collapse? So macro-world objects and humans cannot accidentally experience quantum events even if they wait for an infinite amount ...
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