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Twins on Video Call

Each would see the other seemingly slowed down while they are moving apart and speeded up while they are coming together. But the one who turns around will see the speed-up for exactly half of his or her travel time while the ones back home will only start seeing the speed up after seeing the turn around (which will be more than half way to the reunion as it will take some time for the signal from the turning traveller to get back to Earth).

Of course both are aware of the signal taking some time to travel between them, so neither would think that what they were seeing was actually happen exactly when they saw it. But after correcting for signal travel time both would infer that the other was ageing more slowly throughout both departure and return with the only difference being that the traveller would infer that the homie had speeded up (or suddenly jumped ahead) during the period of turn-around.

Source: (1002) Alan Cooper’s answer to What would happen if the twins in the Twin Paradox were theoretically able to video call each other during the duration of the trip? – Quora

Why Three Quarks? 

Viktor Toth’s answer correctly describes how our current theory is constructed so as to have the kind of symmetry that leads to three quarks being required to make a proton, but says nothing about why the theory had to be constructed that way and even less about why physics should actually follow such a theory. The first question (of why our theory had to be built that way) is answered by looking at various patterns of relationships between the scattering cross sections for different interaction and decay processes of elementary particles (though the details are beyond my capacity to explain in a Quora answer). But the second question (of why physics actually is that way) is, so far as I am aware, something that no-one yet has an answer to. (It may turn out that the number three is for some reason the only one consistent with some more fundamental properties that we think physics should have, but I have not yet seen any such argument.)

Source: (1002) Alan Cooper’s answer to It takes three quarks to make a proton, not two and not four. Why? – Quora

Best Colour for Radiator

The best colour for a visible “radiator” is whatever fits in best with your decor – since its reflectivity in the visible spectrum has no effect on its utility as a heating (or cooling) device.

Actually, most home heating (or car cooling) “radiators” transfer most of their heat by conduction (to the air) and convection, but even the efficiency of direct thermal radiation depends only on the “blackness” in the infrared part of the spectrum – which can be high even when the radiator is perfectly white in the visible part.

Confusingly, home heaters that rely primarily on radiation are generally not called “radiators” but “radiant heaters” and they typically consist of a heating element and a shiny reflector (which is definitely NOT black!). The heating element (which actually does radiate) is typically some kind of metal coil on which any paint would quickly burn off but whose efficiency may be somewhat enhanced by the fact that it usually has a dark oxidized surface layer (which probably reduces its reflectivity and so enhances its emitivity in the IR as well as the visible part of the spectrum).

Source: (1002) Alan Cooper’s answer to Why is black the best colour for a radiator? – Quora

Does being black really make things get warmer? 

The colour black does not, by itself, “make things get warmer”, and in fact it sometimes makes them get colder. But it does increase the rate at which they absorb energy from visible light and so increases the rate at which they will get warmed up by sunlight. The other side of the coin, however, is that if the blackness of an object extends into the infrared part of the spectrum then the object will get cold more quickly if exposed on a clear night. This is because the way physics works is that there is a universal connection between the rates at which an object absorbs and emits radiation at any given frequency, but the frequency distribution that it emits depends on its temperature. (The only reason we don’t normally see more light from a black body than a white one is that what we see from the white is not emission but reflection, and when the object is hot enough to emit visible radiation then in fact the “black” one will glow brighter.)

Source: (1002) Alan Cooper’s answer to Does the color black really make things get warmer? Why or why not? – Quora

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