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Questions tagged [observers]

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9
votes
4answers
840 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 ...
1
vote
2answers
96 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 ...
9
votes
3answers
3k views

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 ...
-2
votes
2answers
44 views

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 ...
5
votes
2answers
108 views

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 ...
2
votes
2answers
160 views

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 ...
0
votes
1answer
74 views

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 ...
0
votes
2answers
83 views

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 ...
1
vote
2answers
153 views

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 ...
1
vote
6answers
227 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 ...
-3
votes
2answers
69 views

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 ...
2
votes
2answers
149 views

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 ...
1
vote
1answer
59 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-...
0
votes
2answers
59 views

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 ...
0
votes
1answer
68 views

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 ...
2
votes
1answer
113 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 ...
4
votes
2answers
198 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 ...
0
votes
2answers
111 views

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 ...
2
votes
0answers
71 views

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 ...
0
votes
1answer
51 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' ...
-3
votes
2answers
77 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?
0
votes
2answers
153 views

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 ...
1
vote
1answer
45 views

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 ...
-2
votes
2answers
63 views

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 ...
0
votes
2answers
87 views

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 ...
1
vote
1answer
44 views

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 ...
1
vote
1answer
69 views

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 ...
1
vote
2answers
91 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. ...
0
votes
1answer
27 views

The contact binary star system in Cygnus

Astronomers are predicting that they will combine in 2020. Thus creating a red nova that will be visible to the naked eye here on earth. The stars are about 1,800 light years from earth. My question ...
1
vote
2answers
117 views

Does a graviton in vacuum have a rest frame?

I have read these questions: Does a photon in vacuum have a rest frame? Based on dmckee's answer, the answer is no to a photon's rest frame. In the modern view each particle has one and only one ...
1
vote
4answers
161 views

Relative motion and time

When someone reaches to a speed which is close to the speed of light with respect to earth, will he see the things actually moving faster than when he is in low speeds?
0
votes
1answer
109 views

Lorentz Transformation Exercise confusion

So there is this very simple situation in one of my exercices: In the earth's frame of reference a tree is at the origin and a pole is at $x=20$km. Lightning strikes at both the tree and the pole ...
-2
votes
3answers
105 views

Can I speed up time by slowing down universe? [closed]

if this is a silly question, I apologize in advance. baseline facts: ...
1
vote
1answer
96 views

In Algebraic QFT, is the state observer dependent?

In the usual approach to QFT presented e.g., in Weinberg's book, the state of a system is dependent on the observer. Quoting this book, in page 109 we have: Notice how this definition is framed. To ...
1
vote
4answers
231 views

Does time really slow down in relativity, or is it an illusion caused by the Doppler shifting of light? [duplicate]

Does time really dilate, or is it only how we perceive time, which is in the form of light waves. When an object moves away from another object at very high speeds, the light takes longer to reach it ...
7
votes
5answers
292 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 ...
0
votes
4answers
122 views

What is the proof that the speed of light in vacuum is independent of relative motion? [duplicate]

Most people know that the speed of light in vacuum is independent of relative motion but I want to know the proof of it.
1
vote
0answers
70 views

Confusion about expression for length contraction of rod [closed]

My physics book states that in the construction in the figure, if the event C has coordinates x'= 0 and t' = l in the primed frame, then event C has x -coordinate in the unprimed frame: $$x_C = \...
0
votes
0answers
48 views

How can two people on opposite sides of the Earth both be accelerated? [duplicate]

As I understand it, then when I am standing on Earth I feel a force on my feet and is accelerated through spacetime. But what if a person is standing on the other side of Earth. That person is also ...
11
votes
7answers
792 views

Is the second postulate of Einstein's special relativity an axiom? [closed]

I'm trying to grasp Einstein's special relativity theory and can't seem to find a clear answer as to why Einstein concluded that the speed of light is constant to observers in different inertial ...
-1
votes
1answer
118 views

Doubt on relativity

I have a doubt: if in Classical physics the maximum speed is limited to the speed of light, will simultaneity be relative even in Classical Physics? Also, two observers in separate inertial frames, ...
0
votes
2answers
190 views

The role of the “observer” in quantum theory?

I've been reading about different ideas of getting the observer out of quantum theory. One Sean Carroll seems to like is Everett's many worlds concept. I had a thought that I need someone here to ...
4
votes
2answers
339 views

How can a comoving observer be “privileged”?

Yesterday I went to my GR (undergraduate) lecture, and the professor exposed the concept of comoving frame of reference. Well, he said that in this particular frame an observer is a privileged ...
0
votes
2answers
115 views

What will the observer observe?

Consider the case where observer $A$ is at rest, and observer $ B$ is moving with speed $\frac{c}{2}$ (where $c$ is the speed of light) propagating a wave with wave speed $c$. So my question is what ...
1
vote
0answers
36 views

Does a black hole evaporate by the time an observer reaches the singularity? [duplicate]

Assume you are an invincible observer who falls into a blackhole... Your time runs slower than the universe outside, as you get closer and closer to the singularity, the ratio between your time and ...
9
votes
4answers
2k views

Is time sped up when orbiting a black hole? Why? What does that mean?

I was watching a very interesting short documentary in which the author said that while it takes 8 minutes in space to loop around the black hole, an observer from Earth observes that it takes 16 ...
0
votes
2answers
110 views

Mistake in simple calculation in special relativity [closed]

I am just learning a little bit of special relativity on my own and am obviously still quite confused. I would appreciate it if someone could point out the error in the following calculation: Let A ...
-1
votes
2answers
131 views

Can someone explain Einstein's light clock?

The explanations for time dilation that I have seen all use the thought experiment of a photon bouncing between two parallel mirrors, which are themselves moving in a direction perpendicular to the ...
1
vote
1answer
309 views

Does one age quicker at higher altitudes?

See title: Does one age quicker at higher altitudes? A few years ago I heard that you would age slightly faster (or slower) at higher altitudes. Is this true? What is the theory or evidence for ...
-3
votes
2answers
69 views

If The Flash ran a 100m race at 99.9% the speed of light, in what time would he finish the race according to himself and a stationary observer? [closed]

According to the time = distance/velocity formula it would take him 0.33 microseconds to complete. t = 100/(0.999 * 299 792 458) = 100/299 492 665.54 = 0.000000334 seconds (0.33 microseconds) My ...