# Questions tagged [observers]

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645 questions
102 views

### Clock in a sealed box at half the speed of light. How would it know it is moving?

Consider a clock enclosed in a sealed box here on earth syncronized with my wrist watch. I send the boxed clock around the galaxy a couple of times at half the speed of light (or the full speed of ...
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 ...
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### Interdependence of time, the speed of light and distance in General Relativity

In the relation $$\text{time}\cdot\text{speed of light} = \text{distance}\,,$$ by definition, the speed of light is constant. When bringing a clock, like in a GPS satellite, up into orbit, i.e. ...
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### Proper time vs time

I'm reading about relativity and I thought I understood things clearly about proper time / coordinates until I read the analysis of the Doppler effect. A and B are 2 fixed points, there is a clock in ...
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### How do we determine who is the “observer” when calculating time dilation? [duplicate]

I was reading about time dilation and I thought of a question. I really think that I am just misunderstanding something, so please bare with the question. But, I would appreciate if somebody could ...
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### Bekenstein Bound and frame of reference

According to Wikipedia, this is the formula for the Bekenstein bound $$S \le \frac{2\pi k RE}{\hbar c}$$ where $E$ is the total mass-energy. That seems to imply that the presence of a black hole in ...
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### Can matter actually cross the event horizon effectively falling into a Black hole? [duplicate]

The answers I have seen do not take into effect the growth of the radius of the event horizon as significant amounts of matter get close to the existing event horizon. I would have expected that ...
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### Problem in Special Relativity [closed]

"A space probe is sent to the vicinity of star Capella, which is $D = 42.2 \ light \ years$ from earth. The probe travels with a speed of 0.9930c. An astronaut recruit on board is 19 years old when ...
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### Twin paradox with one twin in orbit, one in radial free-fall

Many questions on Physics SE relate to the twin paradox, but I did not find any that ask this specific question. Suppose that object A is in a circular orbit around a spherically symmetric, non-...
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### What does it actually mean to say that time “speeds up” or “slows down”?

I was thinking lately about relativistic time dilation and time passing differently on other planets (which both, eventually turned out to be caused by the same principle). Now, quite naturally (as ...
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### Are black holes totally black and formed? [duplicate]

Good morning, I know some similar questions have been asked but I don't find a convincing answer in these. Excuse me if I have missed the answer. My question is: I have learned that some black holes ...
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### Observer in Many Worlds Interpretation [duplicate]

Something has been bothering me about the Many Worlds Interpretation. Proponents of it (e.g. Sean Carroll) often claim that it does away with the observer, or at least the paradox-inducing status the ...
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### Black Hole Formation Physics [duplicate]

Considering only Black Holes that form due to gravitational collapse since the big-bang, wouldn't it be true that that have not (in our reference frame), in fact, formed a singularity or even much of ...
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### How can our universe have black holes? [duplicate]

Assuming that we were never inside a blackhole at any point in the history of the universe, how can blackholes exist against Gravitational time dilation? For any point in space time in the known ...
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### Entropy and observers

Struggling to understand the stance Bousso takes in his paper, The Entropic Landscape, https://arxiv.org/abs/1001.1155 Is he saying that because only by observing an increase in available state we ...
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### Is it possible to speed up time using Special Relativity?

I know that with special relativity when having more velocity than a stationary observer your time relative to the observer slows down. Is it possible using special relativity to increase your time ...
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### Minimum separation from the spacetime interval

I've been working through invariant spacetime interval questions recently, and I came across a question in my lecture notes where; $$\Delta s^2=\Delta x^2 -(c\Delta t)^2 > 0$$ Now it is clear to ...
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### If the speed of light is a limit, is there a way to tell which direction we are moving through space and how fast?

Say two photons start at point A, and go in opposite directions. They will be traveling apart at 2x the speed of light correct? So if this limit is how fast you are able to travel through space (and ...
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### Why the speed of light is independent of the relative motion of observer? [duplicate]

As we use Lorentz transformation equation to relate velocity of particle measured by observer which is in frame s this frame is in relative motion having some velocity to that particle so Why the ...
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### Is relativity really relative?

Because of the relativity of linear motion, you can't tell the difference between whether your spaceship is moving and the stars are standing still or whether you're still and the stars are moving the ...
871 views

### Twin paradox in curved space time [duplicate]

In a flat space, where special relativity works, a travelling body can only return to the same point if we apply some kind of acceleration to the body. So twin paradox is not a paradox because a ...
101 views

### Doesn't the slowing of time when travelling at almost light speed create a paradox with an observer? [duplicate]

Disclaimer: I am extremely new to this and have no proper knowledge of this subject at all, this is just an idea that I had which I want to properly understand. I don't have any knowledge of necessary ...
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### Which observer measures proper time in the twins paradox?

I know that proper time is defined as the time which the clock moving relative to that observe shows. That is, a clock attached to observer A will always be As proper time. I also understand that this ...
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### How to calculate distance of earth when observer in space ship which travel with speed of light

Suppose we have two observer one is on earth and other is in space ship. If we launch the space craft and a light ray at the same time from earth, after 1 minute, what will be total distance travelled ...
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### Would a Schwarzschild black hole “appear” to be a sphere in all reference frames?

I am wondering if all observers would agree that a non-rotating, non-charged black hole is spherical (i.e. there is no reference frame where one would measure it to be oblong in one dimension). I ...
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### Rindler Coordinates and homogeneous Gravity Field

I understood from the equivalence principle that an accelerated observer in free space is equivalent to a stationary observer in a gravitational field. As far as I understood further, this means to ...
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### What is an “observer” in quantum mechanics? [duplicate]

i read about the electrons double slit experiment, when both slits are open, electrons will form an interference pattern, but when there is a detector put to "observe" this weird behaviour, the ...
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### If we filmed what happens on an extremely time dilated planet in realtime

Say there was a planet that experienced insane time dilation due to orbiting a black hole, say 1hr on there is 7 years on Earth (something like that), and that it was close enough to Earth for light ...
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### Do you see the outside world from inside the black hole (can you see past the horizon from inside it)? [duplicate]

I have read these questions: If you fall in a black hole, when do you go past the event horizon? Can matter really fall through an event horizon? How can anything ever fall into a black hole as ...
245 views

### Is the relative speed of light really invariant, irrespective of the motion of the observer? [closed]

If 3 observers are on a planet which 100 light years from a star, and the star goes supernova, if one observer moves towards the star and one moves in the opposite direction, each observer will see ...
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### Does the position in universe of a person not matter regarding death?

I might be completely wrong, but watching a documentary about time, the Einstein theory of relativity came up. From their explanation, time "slows down" depending where you are in the universe. I ...
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### Can the horizon of a black hole move?

Because of time dilation we cannot observe a black hole forming in a finite amount of time. For the same reason I suppose we also cannot observe the horizon moving: everything happening on the horizon ...
73 views

### Doppler effect in light (Observer moving away from source)

I understand this intuitively and can picture it in my head, but when I do it on paper, the result is a sign difference that I cannot understand According to this diagram the wavelength = ct-vt = t(c-...
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### Clearing up a discrepancy when deriving the Lorentz transformation from length contraction

I've been working through the Feynman Lectures on Physics. I'm currently on lecture 15: The Special Theory of Relativity, specifically 15-5, the section on the deriving the Lorentz Transformation ...
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### Shortening of distances in Relativity Theory

I am studying relativity theory and I have understood why distances become shorter from the perspective of someone who is travelling close to the speed of light. However, later on I read that the ...
117 views

### How is it possible to enter a black hole, given time dilation?

This is a bit of a complex question - example scenario: X and Y (who happen to be immune from radiation and gravitation spaghettification, don't draw this into the equation) are some distance from a ...
261 views

### Consequences of infinite redshift in the event horizon of a Schwarzschild black hole

I am familiar with the usual derivation of the fact that it is impossible for someone to observe an object enter a black hole, because of the fact that light gets infinitely redshifted as its source ...
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### The speed of light and the increase of relativistic mass [duplicate]

The speed of light theory predicts that as things travel faster their mass increases, so I think we if we look at a plane accelerating from mach one to mach two and measure the relativistic mass of ...
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### Could you actually see the event horizon of a Black Hole?

For years I thought to myself that surely you'd be able to see the event horizon of a Black Hole when looking at it against the surrounding light. I thought that it would be similar to this: Where ...
54 views

### Would humans die of old age on an interstellar/intergalactic voyage? [duplicate]

It is often claimed that if we ever sent humans to colonize other stars, we would have to send them on generational ships. However, if the ship traveled at $99.9\cdots 9c$, wouldn't the passangers' ...
83 views

### If a massless observer is going at the speed of light, what speed will it measure of a light beam going parallel to it? [duplicate]

If a massless observer is going at the speed of light, what speed will it measure of a light beam going parallel to it?
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### Question on relative velocity [closed]

If a man is moving on a horizontal belt (with constant velocity w.r.t belt)which is also moving in the same direction with some velocity , then time w.r.t belt and w.r.t ground to travel some ...
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### How to draw a line between observation and minds? [duplicate]

When talking about quantum mechanics, a lot of people make the (pseudo-scientific?) argument that minds are required to collapse the wave function (and therefore, minds are important, we are back on ...
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### Does our planet's inorganic period negate the concept of creation through observation?

How can our physical world (only) manifest itself to us through observation when we know that our planet experienced a long inorganic phase that predated organic evolution by millions of years? In ...
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### Observer's effect and the Heisenberg choice

Quantum Mechanics postulates that the act of observation affects the behavior of the observed object.The most common example of this feature is the fact the unobserved object(e.g. a photon) behave ...
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### Does an object falling into a black hole appear to become flat for an outside observer?

In special relativity when an object moves close to the speed of light it will appear redshifted due to time dilation. In addition the object will also appear to be length contracted. But what about ...
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### Meaning of “stars being already dead when we look at them” in relativity

It is a very common statement that when we look at a star, it might already be dead because its light took time to arrive to us. But I am puzzled by this statement, since it seems to assume that we ...
104 views

### Wormholes and Time Machines

According this paper: Morris, Michael S. and Thorne, Kip S. and Yurtsever, Ulvi (1988) Wormholes, time machines, and the weak energy condition. Physical Review Letters, 61 (13). pp. 1446-1449. ...