Do measurement and collapse require an actual human? 

The word “measurement” is generally used for an action of some conscious entity which gives it knowledge of the value of some quantity that it sees as observable. So, although the observing entity doesn’t have to be human (as opposed to some intelligent animal, alien, or robot), it does have to be conscious, and so not every interaction with a another system necessarily counts as a measurement.

But if a quantum system that appears to some observer be in a pure state (which happens to be a superposition of eigenstates of some observable) interacts with a specially designed (and usually much more complex) system in a mixed state, then after the interaction it may be that the effective state of the original system ends up as a statistical mixture of eigenstates of that observable. This “decoherence” process is sometimes (incorrectly) identified as “collapse” of the wave packet, but the actual collapse to a specific eigenstate only occurs in whatever passes for the mind of the observer when that mind registers the information.

Note: The fact that we can demonstrate interference effects on a photon that has passed through a macroscopic optical system of prisms and mirrors is clear evidence that the claim that every interaction with a macroscopic system induces decoherence is balderdash.

Source: (64) Alan Cooper’s answer to In quantum mechanics, does a measurement include when a particle wave comes into contact with another system even if no human is there taking a measurement? In other words, is an actual human measurement necessary to cause a collapse of the wave? – Quora

Does Flat Imply Infinite?

There have been a couple of decent answers over the years but perhaps they could be expressed more briefly.

  1. The existence of an initial singularity in a solution of General Relativity does not require that the early universe was infinitesimally small. It could indeed have infinite spatial extent at every time on every possible worldline (with “expansion” corresponding to reduction of density rather than increase of “size”).
  2. Intrinsic flatness of space-time does not necessarily conflict with finiteness of space – related to the way a sheet of paper is intrinsically flat (in the sense of its own internal metric), even when rolled up into a cylindrical tube (which has finite “size” in the direction perpendicular to its axis).
  3. Some claims of flatness for finite universes are only about the limiting behaviour for large time.

Source: (1001) Alan Cooper’s answer to I understand that the Big Bang was not an explosion which many people erroneously believe, but it was the start of space and time and our universe. However, if it started as infinitesimally small how can physicist say space may be infinitely flat? – Quora

Are people in superposition states?

For any quantum system, every pure state is a superposition. The moment we have complete specification of one observable we lose complete specification of another. (In more technical language, whatever eigenstate we are looking at is a superposition of eigenstates of the complementary observable.)

But noone has ever seen a macroscopic object of any kind in a pure state, so the state of our knowledge is even weaker. Every system that is not completely isolated is in a statistically mixed state. Just the slightest thermal interaction with an external environment introduces classical uncertainty that precludes the identification of a pure state; and even for a completely isolated system that is anything close to the size of a human, the amount of information required to specify a pure state makes it out of the question.

So the answer to your first question is that quantum mechanics does suggest that anything you ever look at is in an unknown superposition.

But the second question is just totally meaningless.

Source: (1001) Alan Cooper’s answer to Does quantum mechanics actually suggest people in other rooms to you are in a superposition? Does it deny the physical processes happening within spatially separated observers? – Quora

Planck Length – Concept vs Implications

The concept of the Planck length can be fully understood on the basis of only a bit of dimensional analysis applied to the apparent fundamental constants of c, h, and G (whose existences had all been identified long before black holes and even photons were considered to be “real”). It’s implications are by no means yet fully understood, but it is obvious that they will have something to do with the relationship between quantum and gravitational phenomena – perhaps (but not necessarily) as a limit on the possible “sizes” of black holes and/or the wavelengths of “photons”.

Source: (1001) Alan Cooper’s answer to The concept of the Planck length cannot be fully understood without a deep understanding of black holes and photons. So, how did this misconception, that Planck length presented by Planck with vague evidence in his papers written years before that? – Quora

Simultaneity and Synchronization

Identical clocks in relative motion can only be synchronized from the point of view of an observer relative to whom they both have the same speed. To anyone else they seem to be progressing at different rates and so are always unsynchronized and there’s no “becoming” about it. There is a sense in which the relativity of simultaneity forces the clocks to be unsynchronized, but it’s generally easier to understand the other way around so I’ll look at that first.

Imagine that, when you pass by me at 86% of the speed of light, we set our clocks to both read, say, t=0. Then they will both read the same time at that event but will not be synchronized because each will see the other as running at just half speed.

and that I also use light signals to set a clock that is fixed in my frame one light year away (in the direction towards which you are headed) to also read t=0 at what seems to me to be the same time.

Then when you reach that clock it will read t=1/.86 years but your own clock will read just 0.5/.86 years (so your clock and my remote clock are not synchronized). I will attribute that difference to your clock running slow but from your point of view the time on your clock is correct because the distance which I saw as 1 light year appears to you to be only half of that.

But from your point of view it has seemed that my clocks were running slow. So according to you, the clock you see as reading 1/.86 years must have started at a time 2/.86 years ago which is long before the time when we passed one another. Or in other words the clock-setting event I thought was simultaneous with our meeting was not simultaneous according to you.

Now what I have shown here is just that relativity of simultaneity is a mathematical consequence of the symmetric nature of the asynchronization of the clocks. But despite the lack of “becoming” there is also a logical connection the other way.

Imagine that I send a message to reach you when I think you are 1 year away from reaching me (ie at a distance of 1/.86 light years when travelling at 0.86c) and ask you to set your clock at t=-1 at that time and that I set my own clock at t=-1 at the moment I think that message reaches you. But relativity of simultaneity says that you will think I made a mistake and that those two clock settings did not happen at the same time. You will actually think that the time when I asked you to set your clock was after I had set my own and this will cause you to expect that when you reach me my own clock would have got to t=1 if running at the same speed as yours but yours will just be at t=-0.5. And since my clock will actually be reading t=0 at our meeting this will lead you to conclude that it must be running slowly (and so that they cannot be synchronized).

Source: (1001) Alan Cooper’s answer to The simultaneity of relativity tells us that inertial observers in relative motion disagree on whether 2 spatially separated events are simultaneous, but how does this lead to the observers’ clocks becoming unsynchronised? – Quora

Black Hole Singularities

A Quoran asks: Do black holes have a singularity? (A point inside the event horizon which is infinitely dense)?
YES, black holes are theoretical objects in the theory of General Relativity which are usually associated with singularities of space-time. But NO such a singularity is NOT “a point inside the event horizon”. As a point in space-time the black hole’s singularity is actually an event rather than a particular point or location in space; and in fact for every point inside the event horizon, the singularity event is in the future of any particle that is ever at that point. In terms of the way it looks to an observer inside the event horizon, there is no longer any sense of any “centre” but oneself, and the theoretically predicted experience is not of falling inwards but rather of getting crushed by the collapse of everything around you. ….or something like that!

Source: (1001) Alan Cooper’s answer to Do black holes have a singularity? (A point inside the event horizon which is infinitely dense)? – Quora

Length Contraction Inherent in Maxwell Equations?

There are two senses in which length contraction is “inherent” in the Maxwell/Heaviside equations.

One sense is that in some cases length contraction follows from Maxwell’s equations. FitzGerald and Lorentz showed that if the structure of matter is determined by electromagnetic forces then the equilibrium spacing of stationary particles due to purely electrostatic forces would become contracted when they are in motion due to the presence of additional magnetic forces. [And one effect of this contraction, together with a related slowing and desynchronization of clocks, would be that moving observers (using their own contracted measuring rods and slowed clocks) would not notice any effect of their own motion on the apparent speed of light (or in fact on any electromagnetic phenomena).]

Another sense is that length contraction is necessary to preserve the form of Maxwell’s equations. Poincare had already noted that if the coordinates used by a moving observer were related to those of one who is stationary by a standard Galilean transformation (ie by just a progressive shift of position with no change of length and time scales) and if Maxwell’s equations applied to (say) the stationary one then they would have to be modified in order to make correct predictions for the other. And he showed that the only kinds of coordinate transformation that leave the form of Maxwell’s equations unchanged are those involving length contraction and time dilation.

[Einstein then noted that if all the laws of physics, written in terms of the natural coordinates of an observer, have a form that is independent of the state of motion of the observer, then there is in fact no way to tell which of two relatively moving observers is “truly” stationary and whose clocks are “truly” synchronized. He therefore sought to express all the laws of mechanics (eventually also including gravity) in a form which used only the actually measured coordinates of each observer rather than those of some assumed fixed “rest”(or “aether”) frame (which would have been more complicated to do if possible, but was not actually possible to do properly because no observers could actually tell whether or not they were actually in motion).]

Source: (1001) Alan Cooper’s answer to Is length contraction inherent in the Maxwell/Heaviside equations? They do have some asymmetry. I remember reading something to this effect but can’t find it and could be wrong. – Quora

Why is photon neutral?

If a gauge invariant quantum field theory (QFT) has a Lie group gauge invariance, the matter fields are defined in the fundamental representation of the group, and the gauge fields are defined in the adjoint representation of the group. If both representations are non trivial, the quantum fields in both representations carry the gauge charges. In the case of the U(1) group the adjoint representation is trivial, therefore the gauge field (photon) is also defined in the fundamental representation. Here only the matter field (electron) carries gauge charge (coulomb charge) and the gauge field does not carry gauge charge. In other words the photon is electrically neutral.

Source: (1001) Sanjay Sood’s answer to Why in quantum electrodynamics the photon is electrically neutral? – Quora

Why is photon electrically neutral?

 If a gauge invariant quantum field theory (QFT) has a Lie group gauge invariance, the matter fields are defined in the fundamental representation of the group, and the gauge fields are defined in the adjoint representation of the group. If both representations are non trivial, the quantum fields in both representations carry the gauge charges. In the case of the U(1) group the adjoint representation is trivial, therefore the gauge field (photon) is also defined in the fundamental representation. Here only the matter field (electron) carries gauge charge (coulomb charge) and the gauge field does not carry gauge charge. In other words the photon is electrically neutral.

Source: (1001) Sanjay Sood’s answer to Why in quantum electrodynamics the photon is electrically neutral? – Quora

Superposition Independent of Space and Time

The spin observables for an elementary particle can be studied without any reference to the particle’s position. They are then modeled as operators on a finite dimensional Hilbert space of spin states that is essentially independent of the infinite dimensional Hilbert space of “wave” functions on position space (which identify those aspects of the particle’s state that are related to its position observables). And in this context for example the eigenstates for any spin component of a spin 1/2 particle are superpositions of eigenstates for other components.

Now you might (correctly) think that this talk of spin components means that we must still be thinking about directions in physical position space. But in fact the study of any two-valued observable (with values “Yes” and “No” or “True” and “False”) forces us to also consider other observables whose eigenstates are superpositions of the Yes and No eigenstates and whose relationship to the original observable are mathematically equivalent to those between spin components in different directions despite not actually having any connection with directions in physical space. And in quantum computing, although spin directions for a single particle are often used as a conceptual model, the choice of Yes/No observable might in practice be something different.

Of course, to study the progress of a quantum calculation, while disregarding space we still need to consider the time evolution of the system; but some aspects of the relationships between possible outcomes can be studied without reference to time in a similar way to how the time-independent Schrodinger equation can be used to study stationary states and energy levels of an atom or molecule. And in these types of analysis the question of whether and how some states are superpositions of other states is still relevant even though there is no reference to either space or time involved.

Source: (1001) Alan Cooper’s answer to Would quantum superpositions function in the absence of time and space? – Quora