Questions tagged [observers]

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10
votes
2answers
929 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?
10
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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 $E^2 = (pc)^2 + (mc^2)^2$ ...
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7answers
1k 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 ...
9
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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 ...
9
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4answers
932 views

With respect to what can't we travel at the speed of light? [closed]

According to theory of relativity the speed of light in vacuum is ultimate. But since objects move relative to each other, with respect to what can't we travel at the speed of light?
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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 ...
9
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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 ...
9
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3answers
805 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 ...
9
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2answers
944 views

What is the size of the world for a photon?

At relativistic speeds the distances contracts. What is the contraction ratio in the dimensions along the axis of travel between a static observer and a photon passing by?
9
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3answers
30k views

Is it possible to stop time? [duplicate]

Assuming the spacetime principle, if the space is modified the time does too. So if the velocity in the space is increase, does the time slow down? What happens if the speed is the speed of light, ...
9
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4answers
882 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 ...
9
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3answers
1k views

Is there a constant/baseline for how fast time passes?

Say we have an object completely unaffected by the effects of gravity/velocity. Is there a way to measure the passing of time for this object? Since time moves slightly slower for us on or near ...
9
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2answers
4k views

What is the essence of the Unruh-effect?

The essence of the Unruh effect is basically that coordinate-transformations lead to different excitations/occupation numbers of the quantum fields. Is that statement correct? So in QFT, while an ...
9
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3answers
695 views

Intensity of Hawking radiation for different observers relative to a black hole

Consider three observers in different states of motion relative to a black hole: Observer A is far away from the black hole and stationary relative to it; Observer B is suspended some distance ...
9
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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 ...
9
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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?
8
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2answers
569 views

Physical meaning of frames in general relativity

Despite studying the general theory for quite some time, this still eludes me. The geodesic equation can be cast in the form $$ m\frac{d^2x^\mu}{d\tau^2}=-m\Gamma^\mu_{\alpha\beta}\frac{dx^\alpha}{d\...
8
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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. ...
8
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1answer
270 views

Interpretation of Stress-Energy Tensor and Dust

MTW (chapter 5) and others state that $-T^a_b v^b $ should be interpreted as the four-momentum density in the reference frame of an observer with four-velocity $v^a$, where $T^a_b$ is the stress-...
8
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5answers
4k views

Does coordinate time have physical meaning?

I have always been a little confused by the meaning of the "$t$" which appears in spacetime intervals or metrics in general relativity. I concluded that $t$ was just a mathematical thing which allow ...
8
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2answers
265 views

Special Relativity, refractive index and catching up with a wave

Einstein was partially motivated by the following: With Maxwell's equations, a plane wave is a sinusoidal wave that varies in space in time and moving with speed $c$. These variations are linked by ...
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2answers
3k views

Time dilation at a black hole [duplicate]

According to the Wikipedia article on black holes: Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling ...
8
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1answer
268 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 ...
7
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5answers
1k views

What if two twins flew off in opposite directions and were reunited in a perfectly symmetric way, would they have aged same? [duplicate]

If No, well this can't be, as there is perfect symmetry, you can't tell one from the other. If Yes, they had relative velocities all along, then their times must have dilated and somehow they must ...
7
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2answers
3k views

Double slit experiment with animals as observers

I was searching about the double slit experiment, reading and watching videos, etc. If I understood correctly, when they measure the photon it behaves like a particle. On the Youtube video Tom ...
7
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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 ...
7
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5answers
386 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 ...
7
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5answers
329 views

Rapid (ac/de)celeration in relativity does what to inertial clocks?

EDIT (to clarify my question): I think some of the answers here are accounting for light travel time and telling me what I'd actually see on Earth's clock, so I've edited my first paragraph to ...
7
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4answers
898 views

If nothing ever falls into a black hole, why is there a puzzle about information?

From an outside perspective, nothing can ever pass the event horizon. It just scooches asymptotically close to the event horizon. So (from our perspective on earth), when a black hole reduces in mass,...
7
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2answers
621 views

Falling into a black hole – how can my perspective change from outside-observer to inside-observer?

I study molecular biology; my skills in maths aren't the best – so I'm asking for answers that aren't purely mathematical, if possible. I ask you to elaborate on formulas or diagrams, if included in ...
7
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5answers
332 views

Can the event horizon save conservation laws for black holes?

How reasonable it it to conclude that, from a remote observer’s frame, matter falling towards a black hole never crosses the event horizon, because ∆ t → 0 as v → c (according to the Lorentz transform)...
6
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4answers
1k views

Formal definition of an observer?

What is the formal definition of an observer in special relativity? I have seen a few: The actual coordinate system. The collection of synchronised clocks that cover the coordinate system. A well ...
6
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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 ...
6
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3answers
267 views

The interference paradox

This is a thought experiment I created myself. Imagine two radio sources fixed at the ends of a rocket and they produce signal of same frequency such that they arrive at the center simultaneously and ...
6
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2answers
322 views

Definition of observer and time measured by different observers in general relativity

An observer in general relativity is defined as a future directed timelike worldline \begin{align*} \gamma:I \subset \mathbb R &\to M \\ \lambda &\mapsto \gamma(\lambda) \end{align*} together ...
6
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6answers
698 views

What does the accelerated twin see in the twin paradox?

I am referring to the usual twin "paradox" where one twin remains on Earth while the other one takes a round trip journey on a rocket. I understand how the experience of the twins is not symmetrical, ...
6
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4answers
3k views

What does observation mean in two-slit electron diffraction experiment? [duplicate]

My question is clear, that I ask: What do we mean by "observation" in 2-slit experiment for electrons (or any other wave-particle)? You know, we say that :"if we observe the electron, it shows a ...
6
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1answer
154 views

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-...
6
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1answer
184 views

Hubble flow and inertial reference frames

Looks like different questions, but they are basically the same: 1) Let us imagine a mostly empty expanding universe with constant $H$. Two observers (Ginger and Fred) that are separated by a ...
5
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2answers
3k views

How can one prove that the number of images formed by the reflecting surfaces of two plane mirrors at right angles to each other is 3?

How can one prove that the number of images formed by two plane mirrors at right angles to each other is 3? Is there a mathematical proof for the same?
5
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3answers
551 views

Is there some theory which predicts that the speed of light in vacuum is constant for all observers?

Relativity theory takes this result for granted because it's one of its assumptions. Is there some theory which starts with some even more basic assumptions and then predicts that the speed of light ...
5
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6answers
296 views

In Special Relativity, is it allowed to ask 'How much time has elapsed in a second inertial frame at a particular moment in the first inertial frame'?

Or is it a meaningless question? For example, A and his friend B are the same age initially. B travels relative to A at a very high speed. A keeps observing B from his frame. At one moment, A ...
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2answers
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 ...
5
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1answer
177 views

When looking for a black hole, will we always find a collapsing star instead?

I know that from the point of view of an outside observer, the collapse of a star into a black hole never finishes, i.e. any observer will always see a (red-shifted) version of the star before its ...
5
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4answers
933 views

How can a mechanical clock tick slower due to time dilation?

I was about to watch Interstellar for the third time when I decided to do some research on its phenomena. Time dilation is the one that struck me most. I have read and read about it and understand -- ...
5
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1answer
676 views

Would dense matter around a black hole event horizon eventually form a secondary black hole? [duplicate]

Possible Duplicate: Black hole formation as seen by a distant observer Given that matter can never cross the event horizon of a black hole (from an external observer point of view), if a black ...
5
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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. ...
5
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2answers
365 views

Are my intuitions about special relativity right?

Suppose there is a bus which travels at constant speed $v=0.9c$, relative to my friend Eric who is standing still on the ground. I'm exactly in the middle of the bus, at distance $d$ form both ends, ...
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2answers
1k views

Relativity of simultaneity - An example

I am trying to understand the relativity of simultaneity in different frames, and I am trying to work out an example. Suppose along the x-axis there are two points 2000m apart. Event A happens at t=0 ...
5
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2answers
133 views

Time contraction. Is there a way for time to pass faster, instead of slower?

Well.. we all know about time dilation, ie time running slower. So, if you are too fast, or if you are near a blackhole. This happens. But is it possible to do the opposite? How? We can think in the ...