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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-3 votes
3 answers
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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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1 answer
107 views

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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0 votes
1 answer
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In what coordinates does a non-inertial observer make measurements?

This question is related to my answer here: Why is proper time equated to $ds$? I use the $(-,+,+,+)$ signature. For an observer whose proper time is $\tau$, the line element can be written as $$ds^2=-...
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2 answers
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Specific version of the observer effect

I have a practical experiment in mind which would test whether human minds can function as quantum mechanical observers. My question is then: has a similar experiment been performed, and if so what ...
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2 answers
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Observer effect with humans [duplicate]

According to the observer effect, physical systems can behave differently when there is an ‘observer’. This has been demonstrated in cases where the ‘observer’ is a machine. It seems logical that if ...
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Chart transition maps in special relativity

To my understanding, Lorentz transformations are the chart transition maps between charts of different observers, but what would be the name of chart transition maps of a single observer? Naturally he ...
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6 votes
1 answer
442 views

Schwarzschild metric in terms of a falling observer's coordinates

The Schwarzschild metric tensor $\textbf{g}(r)$ in terms of a distant observer's coordinates $(t,r,\Omega)$, can be written as: $$ds^2=-\left(1-\frac{r_s}{r}\right)dt^2 + \left(1-\frac{r_s}{r}\right)^{...
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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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4 votes
6 answers
1k views

"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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-8 votes
3 answers
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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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1 vote
1 answer
54 views

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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4 votes
5 answers
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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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0 votes
2 answers
60 views

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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4 votes
1 answer
153 views

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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1 answer
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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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5 answers
77 views

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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1 vote
1 answer
55 views

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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1 answer
74 views

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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4 answers
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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 ...
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1 vote
0 answers
51 views

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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1 vote
3 answers
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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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2 answers
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Layman question about relativistic motion

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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2 answers
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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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Paradoxes of length contraction: does the radius shrink? [duplicate]

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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0 votes
1 answer
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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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1 answer
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Confused about length contraction

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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48 votes
4 answers
10k views

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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0 answers
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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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2 answers
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Scenarios for objects moving 99.99% the speed of light. How long will it take them?

My understanding: It's my understanding that when travelling close to the speed of light, that time dilation occurs. I've seen different formulae I found this formula: $\Delta t' = \frac{\Delta t}{\...
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2 votes
4 answers
94 views

How fast can clocks go?

We know that clocks can move slower when near a massive object, however, how FAST can clocks go? Can clocks go as fast as they want?
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0 votes
0 answers
14 views

How do you resolve this inconsistency? [duplicate]

So, I just ran into an inconsistency about energy which I don't know how to resolve. I might just be doing something really stupid, I'm not sure. So, if someone in the rest frame sees an object ...
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0 votes
2 answers
91 views

On a misleading intepretation of the train paradox in Special Relativity [duplicate]

I begin by saying that I'm a math student who has recently come across the study of special relativity, so I apologize in advance for my lack of physical insight. In the famous train paradox, just to ...
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1 vote
2 answers
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Time slows down with speed compared to what reference point?

So as far as I understand time dilation it means that time slows down as an object approaches lightspeed. This is an issue even with for example satellites around earth compared to people on earth (...
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0 answers
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Special Relativity, thought experiment

Here's a small paragraph from Carl Sagan's "Cosmos" describing what a person travelling near speed of light would observe(as a thought experiment): "...As your speed increases, you ...
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3 votes
6 answers
203 views

Special relativity: what can be evaluated on the basis of the Minkowski metric alone?

I have edited the question: I've come to realize that there is an ambiguity that I should have recognized, but I didn't. My edit of the question is towards removing the ambiguity. Please check the ...
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1 vote
2 answers
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How much Earth time passes over a 1000 lightyear journey?

Ok, so I'm writing a sci-fi novel featuring a lot of interstellar travel. I was wondering how much time would pass, from an Earth-based perspective, if I were to travel 1000 lightyears from Earth, and ...
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-1 votes
5 answers
126 views

What is preventing this statement to be true: the twin brother who accelerated away is really decelerating

To summarize the twin paradox: the brother who was in the rocket and flew away is the one having a slower time, because he "accelerated", decelerated and then accelerated again and came back ...
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-1 votes
2 answers
121 views

Why does light move? [closed]

According to Einstein's theory of special relativity, the phenomenon known as time dilation occurs when an object is moving faster than another. The closer to the speed of light this object gets, the ...
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4 votes
1 answer
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Does the count of particles depend on the observer?

The Lagrangian and Action, say in QED, is invariant to Lorentz boosts and independent of observers. (It’s possible to convert from a Lagrangian to the Hamiltonian view via a Legendre transform $H=vp-L$...
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2 votes
1 answer
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Geometric Derivation of Lorentz Transformation

In Schutz (2nd edition) on page 22 at the top it states: $t'= α(t − vx)$ and $x'= σ(x − vt)$ Fig. 1.4, see below, gives us one other bit of information: events $(t' = 0, x' = a)$ and $(t' = a, x'= 0)$ ...
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6 answers
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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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1 vote
3 answers
142 views

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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0 votes
1 answer
53 views

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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7 votes
5 answers
2k views

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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0 answers
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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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1 vote
4 answers
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Interpretation of proper time in Carroll's Spacetime and Geometry

At page 9 in Sean Carroll's book Spacetime and Geometry he states: The proper time between two events measures the time elapsed as seen by an observer moving on a straight path between the events. ...
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1 answer
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How can we speed up time using relativity?

This is a thought experiment question on time dilation/ relativity etc. 'Jack McCormick' leaves the Earth at 99.99% the speed of light. Later, he looks at his wrist watch and his wrist watch tells him ...
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