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Questions tagged [observers]

For questions related to observers in relativity or quantum mechanics.

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163 votes
9 answers
40k views

Does someone falling into a black hole see the end of the universe?

This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event ...
John Rennie's user avatar
131 votes
15 answers
35k views

How can anything ever fall into a black hole as seen from an outside observer?

The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a far-away-...
Matt Luckham's user avatar
  • 1,707
107 votes
12 answers
15k views

How can time dilation be symmetric?

Suppose we have two twins travelling away from each other, each twin moving at some speed $v$: Twin $A$ observes twin $B$’s time to be dilated so his clock runs faster than twin $B$’s clock. But twin ...
84 votes
10 answers
13k views

Can black holes form in a finite amount of time?

One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
Itai Bar-Natan's user avatar
67 votes
10 answers
172k views

Would time freeze if you could travel at the speed of light?

I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit. So, if I were to travel in a spacecraft at (practically) the ...
Question Overflow's user avatar
58 votes
2 answers
6k 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 ...
veronika's user avatar
  • 2,726
55 votes
10 answers
18k views

Is Schrodinger's Cat itself an observer?

In Schrodinger's Cat thought experiment, why doesn't the cat itself qualify as an observer? Reading through the replies there seem to be two suggestions for what can take the role of observer: any ...
Malcolm Storey's user avatar
52 votes
8 answers
14k views

How is the classical twin paradox resolved?

I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
vonjd's user avatar
  • 3,701
50 votes
5 answers
12k 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 ...
Augs's user avatar
  • 621
46 votes
9 answers
7k views

Extension of Schrödinger's cat thought experiment

My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But ...
Loreno Heer's user avatar
44 votes
8 answers
19k views

What is an observer in quantum mechanics?

My question is not about (pseudo) philosophical debate; it concerns mathematical operations and experimental facts. What is an observer? What are the conditions required to be qualified of observer, ...
Isaac's user avatar
  • 2,880
41 votes
10 answers
12k views

Does any particle ever reach any singularity inside the black hole?

I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and "...
user avatar
39 votes
14 answers
9k views

How could quantum effects occur in the early universe without an observer?

In inflationary cosmology, primordial quantum fluctuations in the process of inflation are considered responsible for the asymmetry and lumpiness of the universe that was shaped. However, according to ...
Alex L's user avatar
  • 1,145
39 votes
7 answers
7k views

Isn't the uncertainty principle just non-fundamental limitations in our current technology that could be removed in a more advanced civilization?

From what I understand, the uncertainty principle states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on ...
user avatar
36 votes
3 answers
9k views

Is there a frame of reference in which I was born before I was conceived?

I'm struggling to understand the relativity of simultaneity and position. If my conception and birth are separated by time but not space, a frame of reference in which my birth and conception are ...
IchVerlore's user avatar
36 votes
7 answers
13k views

Conservation of energy and Doppler effect?

From what I understand, the frequency of light coming from a source moving towards an observer increases. From $ E=h\nu $ , this implies an increase in the energy of each photon. What really is ...
S L's user avatar
  • 1,541
33 votes
4 answers
8k views

How does a photon experience space and time?

To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere. ...
Mozibur Ullah's user avatar
33 votes
3 answers
3k views

The age of the universe

Many times I have read statements like, "the age of the universe is 14 billion years" . For example this wikipedia page Big Bang. Now, my question is, which observers' are these time intervals? ...
Yossarian's user avatar
  • 6,047
30 votes
5 answers
3k views

What do you really see on a line of clocks as you pass by them at high speed?

According to my understanding of SR, if I travel at 0.8c relative to a line of clocks, I should see the clocks in front of me going 3 times faster than my own, and those behind me going 3 times slower ...
m4r35n357's user avatar
  • 1,903
29 votes
6 answers
11k views

Practically, how does an 'observer' collapse a wave function?

I have been reading/learning about the double slit experiment, its implications in quantum theory, and how it explained that “particles” can behave as both waves and particles. I know that the wave ...
Runeaway3's user avatar
  • 460
27 votes
6 answers
6k views

Is time dilation due to the travel time of light?

I'm trying to think about special relativity without "spoiling" it by looking up the answer; I hope someone can offer some insight - or at least tell me I'm wrong. Suppose I have an ordinary clock ...
Michael Lee's user avatar
  • 1,365
27 votes
5 answers
5k views

Hole-and-nail paradox in special relativity

Yesterday we started relativity on our physics class, and my professor taught us a few concepts. We did some examples on how things changed by looking them from different reference systems, and a ...
Abrahamlure's user avatar
26 votes
8 answers
9k views

In the twin paradox does the returning twin also come back permanently length contracted flatter than the twin on Earth?

This video from Brian Greene suggests this is so: https://www.youtube.com/watch?v=2sZUNud6rRw&list=PLj6DWzIvBi4PFDXCCV1bNhVUgDLTwVbFc&index=60 It shows if you stop a pole in the barn (...
ralfcis's user avatar
  • 460
26 votes
3 answers
3k views

So Black Holes Actually Merge! In 1/5th of a Second - How?

I've read a lot of conflicting answers in these forums. However, today saw the awesome announcement of gravitational waves. Two black holes merged: http://www.slate.com/blogs/bad_astronomy/2016/02/11/...
Bhagwad Jal Park's user avatar
25 votes
8 answers
5k views

In the theory of special relativity speed is relative so who decides which observer’s time moves slower? [duplicate]

If for example we have 2 people one moving in speed v relative to the other, according to special relativity the time passing for the moving person is slower than for the stationary person. However ...
שון אלמליח's user avatar
25 votes
2 answers
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 ...
vidstige's user avatar
  • 358
24 votes
10 answers
7k views

What are the mechanics by which Time Dilation and Length Contraction occur?

What are the mechanics of time dilation and length contraction? Going beyond the mathematical equations involving light and the "speed limit of the universe", what is observed is merely a phenomenon ...
Nathan Barry's user avatar
24 votes
5 answers
5k views

Can someone conceptually explain time dilation?

Can someone intuitively explain how physically time dilation happens? Please don't explain about the invariant speed of light and the mathematical background, I am familiar with that. I just can't ...
Pekov's user avatar
  • 449
24 votes
1 answer
3k views

Does light really travel more slowly near a massive body?

It is a routine problem for beginners in general relativity to calculate the coordinate velocity of light for the Schwarzschild metric. Starting from the metric: $$ ds^2 = -\left(1-\frac{r_s}{r}\...
John Rennie's user avatar
23 votes
5 answers
3k views

What is physically different about a moving vs still object in space?

If I have two asteroids. One dead still in space and one whizzing by at 10,000mph. What is the difference between the two, physically? If I freeze time and look at the two of them - what differences ...
Jack's user avatar
  • 341
22 votes
2 answers
2k views

Is the uncertainty principle just saying something about what an observer can know or is it a fundamental property of nature?

I ask this question because I have read two different quotes on the uncertainty principle that don't seem to match very well. There are similar questions around here but I would like an explanation ...
Asciiom's user avatar
  • 613
20 votes
3 answers
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 ...
Prashan Shan's user avatar
20 votes
9 answers
30k views

Special Relativity - two beams of light in opposite direction

I just want to first say that I'm aware I am asking a question due to my own confusion and ignorance and not because of anything to do with special relativity. I hope that's alright. What I'm ...
Lucy Meadow's user avatar
20 votes
1 answer
837 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 ...
Slereah's user avatar
  • 16.4k
19 votes
8 answers
8k views

Could speed of light be variable and time be absolute?

I get my "demonstration" of time dilation from the textbook thought experiment. A laser is mounted on a cart with a reflective ceiling. At $t=0$ the cart starts moving and the laser is fired....
Three Diag's user avatar
19 votes
5 answers
5k views

If our solar system and galaxy are moving why do we not see differences in speed of light depending on direction?

May be a silly and simple question, but I've been wondering if: The speed of light is constant, and When we're moving in the same direction (where both the emitter and the receiver move with the ...
Gensys LTD's user avatar
19 votes
3 answers
5k views

What does a sphere moving close to the speed of light look like?

What shape does the viewer in a reference frame with $v=0$ perceive? I suppose that since the sphere moves in one direction only (oX only, not oY) its section would change into an ellipse, where the ...
menislici's user avatar
  • 378
18 votes
5 answers
6k views

Does special relativity imply that I can reach a star 100 light years away in less than 100 years? [duplicate]

I am just starting familiarizing myself with special relativity, and want to understand if the following is a correct thought process: Consider a star that is 100 light years away from Earth, from my ...
Ishamael's user avatar
  • 291
18 votes
3 answers
3k views

Is it a postulate or a well proven fact that speed of light remains constant w.r.t any observer?

We usually heard that speed of light in vacuum $c$ remains same no matter how observer is moving? I am wondering whether is it taken as a postulate or a proven phenomenon that $c$ is constant ...
kaka's user avatar
  • 543
18 votes
5 answers
7k 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 ...
dts's user avatar
  • 954
18 votes
3 answers
6k views

What will the universe look like for anyone falling into a black hole?

I've heard that, from the perspective of an external observer, something falling into a black hole will eventually look "frozen": light waves will move to the infrared and further into lower ...
user avatar
18 votes
6 answers
2k views

A paradox of length contraction

Suppose the proper length of a train is longer than that of a bridge, and the bridge can't bear the total weight of the train but can bear it partially. As the train goes very fast, it becomes shorter ...
Shen's user avatar
  • 1,643
18 votes
3 answers
3k views

How one uses the definition of observers in General Relativity?

I believe the best way to sum up the idea of observers used in almost all treatments of standard Special Relativity is what Schutz says in his General Relativity book: It is important to realize ...
Gold's user avatar
  • 36.3k
17 votes
4 answers
13k views

Why does everyone say that the faster you move through space, the slower you move through time, when that's not the case?

Suppose, I'm on earth and my brother is moving away from earth at a constant speed, $v=0.8c$. Now, if 5 seconds $(t_0)$ pass for me, the amount of time that will pass for my brother according to me ...
tryingtobeastoic's user avatar
17 votes
4 answers
5k views

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 $c=1$)....
user35305's user avatar
  • 3,207
17 votes
2 answers
2k views

Relativity of Simultaneity

Relativity of Simultaneity seems to be about OBSERVING two events simultaneously (please correct me if I am wrong). However, as long as the two events are separated by a distance (any distance) then ...
Physics n00b's user avatar
16 votes
13 answers
8k views

If objects in motion experience time differently, how does my body stay synced when I move my legs or arms?

If I move my swing my arm really fast, the matter in my arm should experience time slower than the matter in my body. So how does my body still sync with each other? And a more general question that ...
user2218544's user avatar
16 votes
12 answers
14k views

What happens if a super fast rotating ball accelerates near speed of light?

Assume we have a ball with diameter 1 meter and mass 1 kg rotating at 99,5% speed of light (...
CoffeDeveloper's user avatar
15 votes
11 answers
8k views

Can you actually stand inside a rainbow? I see people claiming you can

I have been thinking about this for ages and it has been driving me crazy. I was convinced that rainbows are always complete circles that appear to be arcs because they cross the horizon. Then, I saw ...
Jake's user avatar
  • 183
15 votes
9 answers
5k views

Since the speed of light is constant and also the speed limit; would you, in your reference frame, have no upper bound on your speed? [closed]

Let us imagine you are in a vacuum and after having maintained a speed of 0 km/s (standing still) you accelerate to 297,000 km/s (99%). You know this is now your speed because you have a speedometer ...
Michael King's user avatar

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