A Question About Geometrical Optics
In a conventional diagram of the geometrical optics of any lens or mirror, the line through the source parallel to the axis of the optics does not necessarily represent an actual ray, but rather is included as a way of easily identifying where the image is located. It is used because the optics is assumed to have the property that all rays from the source to any part of the lens (or mirror) are bent in such a way as to converge on some other point (the image) and the mathematical form of the rule describing how this bending occurs makes sense for all points in the plane of the lens (or mirror) regardless of whether or not they are actually on the lens or mirror (and so regardless of whether or not the deflection actually occurs).
It is actually possible to calculate the angle of deflection for each ray (either by an algebraic formula or a geometric construction), and so to just use rays which pass through the lens (such as those through T and B, which I have shown as solid lines in the picture) without any reference to the imaginary ray (shown as a dashed line in the picture) which would bend at H if the lens was big enough; but the formula or construction is simplest for that imaginary ray and it works just as well as any other for determining the image point.
Block Universe Nonsense
A Quora question asks: Is it true that the past still exists and the future is predetermined? Is the ‘’block universe’’ correct, since Special relativity is scientifically proven? – Quora
No. It’s not true that in the context of Special Relativity “the past still exists”.
In fact, according to Special Relativity (which does appear to be correct for situations where the effects of gravity are negligible – though nothing outside of mathematics is ever “scientifically proven”), the whole idea of the (unique) past is meaningless since there are events that you consider past that are not in the past for a remote relatively moving observer who is at an event which you consider simultaneous with your present.
Also, again according to Special (and maybe* General) Relativity, the word “still” is also meaningless as its use at any time refers to events that are simultaneous with the statement, but that criterion will correspond to different sets of events for different observers.
*- I have added the word “maybe” since Viktor Toth# has mentioned that in General (but not Special) Relativity it is possible to identify a preferred set of inertial frames (in which the redshift due to expansion is the same in all directions), and these do share a common concept of simultaneity.
But of course, even in a Galilean universe, there is also the question of what you mean by saying that the past “exists”. Yes it exists as the past (and even in SR the past of my worldline exists as my past) but that does not mean it is happening now. And the same applies to the future. Even in a non-deterministic universe the future “exists”, as what will turn out to have happened, regardless of the fact that we have no way of predicting it.
#PS I am not linking to Viktor Toth’s answer because, despite having some good bits, I feel that it is marred by the inclusion of some very misleading (and quite unnecessary) statements about Quantum Mechanics.
Another Quora Question
A Quora question asks: “How does quantum physics know that if a system were not measured it would be in multiple possible states (without measuring it), and that when measuring it collapses into one definite state?”
Quantum theory doesn’t “know” anything. All it does is describe what we know and makes predictions about what we may find out in future. What we call the “state” of a system is just a summary of what we know about it, and a system on which we can gain no new information without losing some of what we already have is said to be in a “pure” state.
An example is the case of a single electron whose position we are ignoring (so it can be considered fixed) and whose only measurable property is the direction of its spin. If we first measure the spin component in the vertical direction (say that of the z-axis), then we will always find that the spin is pointing straight up or straight down; and if we repeat that measurement we will see that the direction is unchanged. But if we then follow that with a measurement in any perpendicular direction then we have a 50% chance of finding that the spin is now pointing in that new direction and 50% chance of its opposite. And if we now return to the original direction we find equal chances for pointing up or down.
Here, the spin up state is a pure state because we cannot determine the sideways component without losing information about the vertical component, but being in the up state is not the same as being in both left and right states at the same time, and it is also not the same as being in a statistical mixture (ie in one or other of those two states but we just don’t know which).
P.S. In either quantum or classical physics, a system that is not measured or which we have only measured incompletely may be in any one of several pure states with different probabilities, and we call the resulting state a statistical mixture; but that is not the same as being in multiple pure states at once.
Bergson vs Einstein
After reading this article twice, and yet again the paragraph where the author purports to show that “it’s wrong to think that Bergson’s idea of duration can be assimilated into the idea of psychological time”,
I am still unable to find any explanation of the difference between our internally experienced psychological time (which, by the way can not necessarily always be “aligned with external clock time”) and “the first-person experience of (Bergson’s unmeasurable) duration” (which they appear to identify as the “lived time” in terms of which “An hour in the dentist’s chair is very different from an hour over a glass of wine with friends”).
On the other hand Steven Savitt’s “solution” does not address the subjective nature of duration and appears to just identify it with the non-subjective proper time associated with a possible observer’s world line – which seems to be just giving up on the idea of any special “philosophical” time as this has always been the only kind of time that is ever discussed in relativistic physics.
Source: Who really won when Bergson and Einstein debated time? | Aeon Essays
Reflections on Handedness
Is Heat Radiation Just Infrared?
It’s really no different than with any other kind of energy. Heat is basically just energy that is disorganized ie distributed in a way that is too complicated for us to keep track of – like the random motion of molecules in a gas as opposed to bulk motion or pressure waves in that same gas.
With regard to electromagnetic radiation, every hot body emits a broad spectrum of disorganized electromagnetic radiation at all frequencies – not just infrared (though that is the dominant range at the temperatures we normally experience); but it is also possible to produce organized electromagnetic radiation (eg from electronic circuits and lasers) and this is not “heat” even when it is infrared (though it can be used to create heat when absorbed by a body – just as organized mechanical energy can be converted to heat by friction).
Why is KE quadratic?
Conservation of Momentum (or equivalently Newton’s Law of action and reaction) tells us that when two particles interact just with one another, the acceleration of each multiplied by its mass (which we refer to as “Forces”) are equal and opposite. For any motion, the accumulated product of acceleration during a time interval times distance travelled over that interval is equal is equal to half of the change in the square of the speed in the direction of acceleration.
(This is just the calculus identity [math]\int{x’’ dx}=\int{x’’ x’ dt}=\int{x’ x’’ dt}=\int{x’ dx’}=\Delta(x’^2/2)[/math]; but if that’s not familiar to you, then a more elementary version is the fact that for constant acceleration with [math]v=at[/math] at time [math]t[/math], the average speed from time [math]0[/math] to time [math]t[/math] is [math]\frac{1}{2}at[/math] , so the distance travelled is given by [math]x=(\frac{1}{2}at)t=at^2/2[/math] , and acceleration times distance is [math]ax=a(at^2/2)=(at)^2/2=v^2/2[/math].)
So the quantity that is increased by applying force through a distance (to do “work” on a particle) is quadratic in its speed.
But why is this important enough to give it a special name? That is because it allows us to define a quantity that is conserved throughout the evolution of any physical system in which the Forces between particles depend on their relative displacement only(*), and have no other dependence on time. This ensures that the speed lost when moving against forces (such as when a projectile moving away from a planet slows down) can be recovered if the motion is later reversed. So in the motion of any such system of particles, the sum of [math]mv^2/2[/math] for all the particles plus the net work done against forces is a constant. We call this the “Energy” of the system and identify the part involving the speeds as the “kinetic” part of that energy – and the work done against forces (which includes an arbitrary constant depending on what we take as the starting point) is a called “potential” energy since it could in principle be returned to the system in future interactions.
(*)- If we allow velocity-dependent forces such as friction then the process might not be reversible and we might have to include also other kinds of energy such as heat in order to still have a conserved quantity.
Bertlmann’s Gloves (yet again on Quora)
Can quantum entanglement can be looked at as a example of glove manufacturing? They are produced in pairs, and if without looking we send them to different parts of universe, once opened, one will always be left and other right? If not why not?
NO. The idea that every electron has a spin direction of its own, which does not become apparent until we view it but is always there (like the handedness of a glove), IS compatible with what we observe if we only ever measure spin components along one axis, but it is NOT compatible with what we observe when we measure spins in directions that are not either parallel or perpendicular.
There is nothing “spooky” about the fact that measuring the spin of one electron from a pair immediately tells us (but not a distant observer of the other electron) what the distant observer will see IF they measure the spin in the same direction. But what IS “spooky” is that it allows us to predict the result of a measurement of the remote electron at say 45 degrees to the one we measured with greater confidence than would be possible with ANY pre-assigned set of spin values in all directions.
See this famous paper by John Bell for a colourful illustration of these ideas.
Gas with Temperature Gradient
The answer to the posed question is indeed a simple “no”, but to establish this does not require any analysis of internal gas dynamics.
If there is ever an overall pressure gradient, then in the absence of external forces the container will accelerate. (By conservation of momentum the gas within will accelerate in the opposite direction until the pressure gradient is eliminated and eventually reversed, and in the absence of friction this would result in an oscillation, but in any case it won’t be an equilibrium until the forces on the walls are in balance.)
In the equilibrium situation the constant pressure means that the density will (in the ideal gas approximation) be inversely proportional to temperature, and I think that any student familiar with the gas laws would accept that each layer of gas can thus remain in balance with its neighbours (one being cooler and denser and the other hotter and more rarified).
The analysis of how this is derived from kinetic theory (with molecules not being confined to layers etc) is more interesting, but does not appear necessary for an answer to the original question.
With regard to what was probably the really intended question, namely how to reconcile the $n \sqrt{T}$ flux out of each layer with the constancy of $nT$, it might be sufficient to tell a student who is not ready for the full analysis just that the flux of those incoming molecules which interact with the layer does not come just from the neighbouring layers but is a mix from various distances which turns out to give zero net flux when $nT$ is constant.
Crookes radiometer – Wikipedia
Thermal transpiration – Wikipedia
Light Mills | The n-Category Café
On Stresses in Rarified Gases Arising from Inequalities of Temperature