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Questions tagged [observers]

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92
votes
14answers
22k 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-...
49
votes
8answers
141k 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 ...
47
votes
7answers
11k 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)?
58
votes
10answers
8k 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 ...
33
votes
7answers
5k 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 ...
31
votes
7answers
11k 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, ...
20
votes
4answers
4k 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. ...
120
votes
9answers
25k 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 ...
34
votes
9answers
8k 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 "...
82
votes
8answers
9k 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 ...
21
votes
2answers
1k 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 ...
21
votes
10answers
6k 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 ...
11
votes
2answers
1k views

Black hole formation as seen by a distant observer [duplicate]

Possible Duplicate: How can anything ever fall into a black hole as seen from an outside observer? Is black hole formation observable for a distant observer in finite amount of time? Specifically,...
10
votes
2answers
1k views

Does non-mass-energy generate a gravitational field?

At a very basic level I know that gravity isn't generated by mass but rather the stress-energy tensor and when I wave my hands a lot it seems like that implies that energy in $E^2 = (pc)^2 + (mc^2)^2$ ...
15
votes
3answers
2k views

The reference frame of $c$

I don't have a lot of knowledge of special relativity and associated topics; some of the few things I know are that "all motion is relative" (that is, there is no 'stationary reference frame'), and ...
12
votes
3answers
3k 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 ...
12
votes
9answers
1k views

How do we know a quantum state isn't just an unknown classical state?

When an observer causes the wave function of a particle to collapse, how can we know that the wave function was not collapsed already before the measurement? Suppose we measure the z-component of the ...
25
votes
2answers
2k 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? ...
9
votes
3answers
774 views

Is energy content of a system different depending on the observer?

For the sake of simplicity, let's imagine that the entire universe is empty except for a single lump of (classical) matter with mass $m$. In its center of momentum frame, it is clear that the total ...
28
votes
5answers
4k 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 ...
18
votes
2answers
1k 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/...
18
votes
3answers
3k 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 ...
9
votes
5answers
2k views

Time Dilation - How does it know which Frame of Reference to age slower?

Okay, I'm asking a question similar to this one here: Time Dilation - what happens when you bring the observers back together?. Specifically, I am curious about a specific angle on the second part of ...
11
votes
2answers
538 views

Can something (again) ever fall through the event horizon?

Since I am more confused by the answers given in this site to the many variants and duplicates of this question, with some arguing that from the point of view of the falling observer, it happens in ...
4
votes
4answers
1k views

How do I know which observer is running the time faster or slower?

Ok, I'm not a physicist, so I don't know if my question is silly (probably yes), but there is something in special relativity that I can't understand and I would really like it to be clarified. If ...
3
votes
3answers
834 views

Uncertainty principle and multiple observers

My understanding is that an observer can measure the precise location of a particle so long as the corresponding uncertainty in momentum measurement is not an issue and vice-versa. Say there is ...
12
votes
3answers
1k views

Which clock is the fastest inside an accelerating body?

The picture shows an accelerating spaceship with two clocks inside it. It is so far away from all other bodys that gravity is of no importance. Will the bottommost clock be slower than the topmost ...
2
votes
3answers
717 views

Watching something fall into a black hole from far away

I am observing (theoretically) an object falling into a black hole from a safe distance away. My understanding is that from far away it appears as if the body will asymptotically approach the event ...
48
votes
1answer
4k 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 ...
13
votes
10answers
8k views

Hidden observers in Double Slit experiments - Do they matter?

I'm still struggling a bit with some ideas around double slit experiments. One that keeps cropping up for me is the role of observers. Imagine a classic double slit experiment with a hidden observer ...
3
votes
2answers
2k views

How long does it take for a black hole to form?

The well-known fable of an astronaut sending signals out to an external observer while falling toward an event horizon states that the time lapse between such signals becomes greater even if in the ...
9
votes
8answers
4k views

What exactly is the 'observer' in physics and/or quantum mechanics? [duplicate]

Possible Duplicate: nature of an observer For instance, in the double slit experiment, what is exactly defined as an observer? I remember from somewhere, light is also an observer?
2
votes
4answers
2k views

Double Slit Experiment. What effect does the detector actually cause?

When a quantum of light arrives at a double slit, it passes through both slits as a wave and arrives upon a second screen with the interference pattern of a single wave that has been split into two ...
10
votes
2answers
921 views

What is the highest speed time dilation has been tested?

What is the highest speed time dilation has been tested? How close to the Special Relativity prediction did it get?
6
votes
2answers
2k views

What do you feel when crossing the event horizon?

I have heard the claim over and over that you won't feel anything when crossing the event horizon as the curvature is not very large. But the fundamental fact remains that information cannot pass ...
3
votes
5answers
34k views

The bigger the mass, the more time slows down. Why is this?

If I were to stand by a pyramid, which weighs about 20 million tons, I would slow down by a trillion million million million of second. Don't know if that's exactly right, but you get the point. Also, ...
8
votes
5answers
1k views

Falling into a black hole

I've heard it mentioned many times that "nothing special" happens for an infalling observer who crosses the event horizon of a black hole, but I've never been completely satisfied with that statement. ...
4
votes
1answer
3k views

Which observer's time is proper time?

I just have a quick question about time dilation/proper time because my physics book makes it a little confusing. Let's say we have an observer on Earth, and then an observer on a space ship. The ...
2
votes
2answers
2k views

Schutz's geometrical proof that spacetime interval is invariant

I'm trying to understand the proof that spacetime interval is invariant under for any two inertial observers. I know it's easy to arrive at the result using Lorentz transformation but I'm trying to ...
1
vote
2answers
234 views

Age of universe?

Well relativity teaches us that time interval between two events is a frame dependent quantity, then how can we say that our universe is 13.8 billion years old? Should it not depend on who is asking ...
14
votes
6answers
23k views

How can time be relative?

I don't understand how time can be relative to different observers, and I think my confusion is around how I understand what time is. I have always been told (and thought) that time is basically a ...
7
votes
4answers
3k views

Why is time-order invariant in timelike interval?

Why do two observers measure the same order of events if we are inside the light cone? (e.g. if $ds^2 > 0$ time-order is preserved according to the classical mechanics book I am reading, but it ...
13
votes
5answers
1k views

Numerical simulation of the double-slit experiment including watching the electrons

The double-slit "thought experiment" described by Feynman in Lectures on Physics Volume 3 section I-6 Watching the electrons consists in firing electrons through a double-slit to observe the ...
18
votes
3answers
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 ...
16
votes
9answers
13k 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 ...
9
votes
7answers
3k views

Twin Paradox: Still a Paradox?

Alright, so David Griffiths in his "Introduction to Electrodynamics" states that the Twin Paradox is not a paradox at all since the traveling twin returns to Earth. By returning to Earth, the twin had ...
7
votes
5answers
368 views

Does an expanding event horizon “swallow” nearby objects?

In a view of a remote observer, an object falling into a black hole is "hanging" at the horizon (slowly falling with a deceleration). Around this moment, the event horizon expands for some reason that ...
5
votes
2answers
1k views

Why don't two observers' clocks measure the same time between the same events?

Person A in reference frame A watches person B travel from Star 1 to Star 2 (a distance of d). Of course, from person B's reference frame, he is at rest and is watching Star 2 traveling to him. ...
3
votes
1answer
153 views

At what moment will matter falling into a black hole affect its size?

Does the mass of matter falling into a black hole affect the size of an event horizon the moment it passes through it, or when it has been incorporated into the singularity?
8
votes
1answer
267 views

Is it possible for a black hole to form for an observer at spatial infinity? [duplicate]

To my knowledge if you calculate the coordinate time (time experienced by an observer at spatial infinity) it takes an infinite amount of time for an object to fall past the horizon of a Schwarzschild ...