# Questions tagged [observers]

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620 questions
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### 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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### 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 ...
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### Observer effect, do this mean literally someone or just any interaction with other matter?

I am a layman and was wondering, the quantum observer effect. The regular notion to laymen seems to be literally "if you look at it", but as I am coming to understand the world I live in better I feel ...
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### Special Relativity - speed of light question

Just a basic question: I know that if you are traveling at $x$ speed the time will pass for you slower than to an observer that is relatively stopped. That's all just because a photon released at the ...
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### 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 ...
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### Does the observer or the camera collapse the wave function in the double slit experiment?

Ok so if we setup a camera before the slit we will find a single photon and will follow through accordingly, likewise by having a camera setup after the slit, we can retroactivly collapse the wave ...
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### Wavefront emitted by bodies at traveling near the velocity of light

I studied that no body can travel with the velocity of light. But, assuming that when a body moves nearly velocity of light, will it obey length contraction law of Einstein or will it emit the same ...
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### How do we know space is expanding when we are part of space? [duplicate]

From what I understand space itself is expanding, and the Big Bang attempts to describe this expansion at the very early stages of the universe. This is usually described in a visual way as 2 dots on ...
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### How do photons experience time? [duplicate]

I know that as velocity approaches the speed of light the time dilation shoots to infinity as shown below. 1)So I want to know how time is perceived from the point of view of the photon? 2)Since ...
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### 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?
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### An infalling object in a black hole looks “paused” for a far away observer, for how long?

As I understand, to an observer well outside a black hole, anything going towards it will appear to slow down, and eventually come to a halt, never even touching the event horizon. What happens if ...
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### 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 ...
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### 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. ...
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### If time slows down when you move faster, does this give you an “effective speed” different from your proper speed?

Suppose you're travelling at 0.9c toward the sun, and you tag past the Earth and start a clock. Would Lorentz contraction/time dilation cause you to get to the sun faster than (about) 9 minutes (...
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### What is the maximum time dilation between two objects, if one is standing still and the other is moving at $c$?

What is the maximum ratio in the rate of change in time in reference to object $A$ which is standing still and object $B$ which is moving at the speed of light?
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### Do scientists literally believe the Schröedinger's cat though experiment?

I've heard the Schrödinger's cat "paradox" (although there's nothing particularly paradoxical about it, just counterintuitive), but I've never been clear on whether or not it's meant to be taken with ...
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### 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, ...
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### Does the increase of (relativistic) mass, while flying near speed of light, has any impact on astronauts? [duplicate]

Possible Duplicate: Would travelling at relativistic speeds have any impact on human biology? I am asking myself this question for a few days. What is the correct answer on: Does the increase of (...
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### Does the future already exist? If so, which one?

In the NOVA Fabric of the Cosmos program, Brian Greene explains a theory in which there is no "now", or more specifically, now is relative. He describes an alien riding a bicycle on a far off planet ...
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### What is an “observer” really? [duplicate]

Possible Duplicate: What is an observer in quantum mechanics? I'm sick of quantum physics explanations which term experiments where the outcome depends on "if you observe it or not". For example, ...
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### Does photon possesses no time to cover any arbitrary distance?

Photon travel 8 minutes (with speed $c$) from the sun to reach the earth. Any particle (or space-ship) with velocity $0.99 c$ covers the same distance (93 millions km) within less than 2 minutes (...
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### 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 ...
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### Can a black hole actually grow, from the point of view of a distant observer? [duplicate]

Possible Duplicate: Black hole formation as seen by a distant observer I've read in several places that from the PoV of a distant observer it will take an infinite amount of time for new matter ...
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### In general relativity (GR), does time stop at the event horizon or in the central singularity of a black hole?

I was reading through this question on time and big bang, and @John Rennie's answer surprised me. In the immediate environment of a black hole, where does time stop ticking if one were to follow a '...
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### 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,...
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### 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 ...
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### Am I faster than my shadow?

I guess that everything that happens to me (or I do), happens before my shadow "records" the information. Is that right? Is it the same for any observer?
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### Formation of a black hole and Hawking radiation

From the perspective of an outside observer it takes infinitely long for the black hole to form. But if the black hole is no extremal black hole, it emits Hawking radiation. So the outside observer ...
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### 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 ...
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### 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?
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### Time slowing down problem

When someone moves, time slows down for him. Let, a man standing still and another moving very very very fast, this happens for an hour (as measured by the standing man). Time has moved slower for the ...
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### 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 ...
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### Time dilation - why the observers see each other the slow one but then one of them is older or younger?

I'm in trouble with time dilation: Suppose that there's two people on the Earth (A,B), they are twins and each other has a clock. (So they are at the same reference frame). B travels in a spaceship ...
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### What is the speed for an object that travel close to the speed of light?

I have some questions regarding Einsteins theory of Relativity that should be fairly easy to answer. Lets say we make an experiment where we have a rocket (with an astronaut inside) that travels very ...
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### What counts as an observer and memory states in quantum interpretations?

The Everett interpretation has memory robots. Copenhagen requires observer memory states. Consistent histories has its IGUSes. Decoherence has its existential interpretation. All of them refer to ...
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### 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. ...
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### Does the Lorentz transformation not apply to light?

Since you would know that light always travels at the constant velocity with respect to all frame of reference, according to relativity, whenever we are traveling at the speed of light, our time with ...
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### Why are subjects upside down when I look through my door peephole?

There is a convex lens 3/4" on the outside and 1 1/2" convex lens on the inside. Each lens has one flat side facing out and one convex side facing in. What can be done to correct this image?
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### Using Einstein's Relativity: Who is younger? [duplicate]

Suppose we have a person A and a person B. Person B travels very close to speed of light and never returns. He's constant in speed. Then, we can say two things: B is younger than A. A is younger ...
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### How does a strobe lamp stop a fast moving object?

A strobe lamp can be used to seemingly stop a fast moving object when calibrated. Commonly used in quality assurance during production to inspect otherwise non-observable assembly line activity. What ...
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### Does a complete theory of quantum gravity require anthropic post-selection?

Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least ...
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### What is the difference between observer, frame of reference, and gauge?

It seems to me that there is considerable relationship between the three concepts: frame of reference, observer, and gauge. How do they overlap? My current understanding is that an observer with a ...
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### Time dilation at the speed of light

Does a massless particle travelling at the speed on light in a vacuum (c) experience an infinite time dilation effect? i.e. Would the time dilation extend from the perspective of the massless ...
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### 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 ...
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### Why is matter drawn into a black hole condensed into a single point within the singularity? [duplicate]

Possible Duplicate: Why is matter drawn into a black hole not condensed into a single point within the singularity? When we speak of black holes and their associated singularity, why is matter ...
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### Effect of gravity at near-lightspeeds

Let's say I'm in a space station, hurtling towards our galaxy nearly close to the speed of light. From my reference frame, I see the galaxy coming towards my ship at the same speed. I pass the Sun, ...
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### Why can I never see any stars in the night sky?

I have always lived near a large city. There is a stark contrast between the picture linked below for example, and what I see with the naked eye. Sometimes I can see a few stars here and there, but ...
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### 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 ...