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Questions tagged [observers]

For questions related to observers in relativity or quantum mechanics.

An observer in relativity defines a reference frame in which observations or measurements are made.

An observer in quantum mechanics is anything that makes a quantum measurement, but there is no consensus on a precise definition.

This tag is not normally used for reference to:

  • Astronomical, meteorological, or other observers where understanding the observer is not an essential part of the physics.
  • Instrumentation "observer" effects where experimental apparatus affects the quantity being measured, e.g. inserting a thermometer changes the temperature of system, or adding a multimeter changes a circuit.
  • Human observer biases that may affect scientific results.
1503 questions
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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 horizon the result is infinite. This implies that the…
John Rennie
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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-observer. If this is the case how can anything…
Matt Luckham
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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 $B$ observes twin $A$’s time to be dilated so…
John Rennie
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88
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10 answers

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 infinite amount of time for the object to reach…
Itai Bar-Natan
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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 speed of light, would I freeze and stop moving?…
Question Overflow
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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 quantity depending on the computation, measurement…
veronika
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Is Schrodinger's Cat itself an observer?

In Schrodinger's Cat thought experiment, why doesn't the cat itself qualify as an observer? Reading through the replies there seem to be two suggestions for what can take the role of observer: any "large" body any "living" thing (or should that…
Malcolm Storey
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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)?
vonjd
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Could I, within my lifetime, reach any star I wanted if I went fast enough?

Disclamer: I'm not talking about FTL travel here. I'm also not talking about any weird space warping mechanics like wormholes and such. I've always thought that if a star was 4 light years away, then it would be impossible to reach it with less than…
Augs
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Extension of Schrödinger's cat thought experiment

My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But wouldn't then the scientist in turn be in a…
Loreno Heer
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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, both mathematically and experimentally?
Isaac
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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 "weird things happen". Now to the question at…
user1549
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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 equipment but just a natural phenomenon. However, isn't…
user8791
39
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14 answers

How could quantum effects occur in the early universe without an observer?

In inflationary cosmology, primordial quantum fluctuations in the process of inflation are considered responsible for the asymmetry and lumpiness of the universe that was shaped. However, according to the Copenhagen interpretation, any random…
Alex L
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Conservation of energy and Doppler effect?

From what I understand, the frequency of light coming from a source moving towards an observer increases. From $ E=h\nu $ , this implies an increase in the energy of each photon. What really is confusing, is where does that extra energy come from?…
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