Ask a physicist anything.

[PhilosophicalBluster]

WC, please explain to me Schrodinger's cat.

 

[shinbits]

In its simplest form - on short length scales, energy must be treated as a system of discrete particles, not as a continuous stream.

that's useful. thanx.

 

[Cabal]

that's useful. thanx.

On reconsideration that's probably as much a summation based on the history and order in which things were discovered as much as anything else - however I do believe it's still an effective description as many quantities are quantised on the small scale - charge, fields, sound waves, light, and according to some hypotheses, even space and time itself.

 

[Cabal]

WC, please explain to me Schrodinger's cat.

I'm not WC, but I'll have a go....

It's a thought experiment to show how crazy quantum mechanics seems when applied to our everyday experience.

A quantum statistical system, such as radioactive decay, cannot be described exactly, but it can only be described probabilistically. We cannot say for certain whether an atom of radioisotope will decay or not at any given time.

The outcome of a radioactive decay can be described fairly simply using two quantum states:

|decay> or |not decay>

However, due to the way quantum mechanics is formulated, because we cannot say for sure whether or not the atom with decay or not the only way to describe the system is to do so in terms of weighted probabilities, i.e:

|wavefunction> = 1/(sqrt2)(|decay> + |not decay>)

(The probability of a given outcome occuring is given by the square of its coefficient, in this case, both outcomes are equiprobable and thus have a square coefficient of 1/2, that's why there's the 1/sqrt(2) there).

If we perform some kind of measurement on the system, to see whether it's decayed or not, we have an equal chance of seeing the atom decay or not decay. But the important point is, we can never predict exactly which, we can only give the average rates of how often we will see either outcome (a bit like tossing a coin) and we will only find out the actual outcome for any one atom until we do a measurement it, not before.

The Schrodinger's Cat thought experiment extends this scenario to the fate of a cat, isolated from observation in a sealed box. Inside the box is a radioactive atom connected to a counter. The counter is connected to a vial of poison that will open if a decay is registered, releasing the poison which assuredly kills the cat.

The "quantum weirdness" that this is supposed to illustrate is - until we actually open the box to observe the outcome of the decay, the cat is technically in a state similar to the one before, something like:

|wavefunction> = 1/(sqrt2)(|dead kitteh> + |not dead kitteh>)

Schrodinger's cat, therefore, is in a weird limbo of being in a superposition of both alive and dead by extension of the quantum properties of the radioactive atom - until we make our measurement, i.e. opening the box.

(Hmmm, that was wordier and mathsier than I intended, maybe I should have left WC to answer this.... )

 

[Wiccan_Child]

In the most concise way possible, how would you define quantum physics?

It's an update to classical physics. Classically, particles are discrete points with well-defined positions, momenta, energy, and they can have any energy. Quantum mechanically, position (et al) aren't as well-defined, and they can only have certain energies (in an atom, at least; a free electron can have whatever energy it likes).

That's why it's called quantum mechanics: particles in a bound system can only take certain quanta of energy.

In essence, QM says things are described by wavefunctions, which implies various properties are more fuzzy than we're used to. This fuzziness becomes unnoticeable at our scales, but is quite important at small scales.

Sorry, that wasn't concise at all!

WC, please explain to me Schrodinger's cat.

Quantum mechanically, a particle exists as the sum of all possible states. Schrödinger's cat is a way to illustrate this: if we don't know what state the cat is in, then the cat must exist as the sum of all possible states. In this case, there are two states: dead and alive.

It's a little more complex than that, though. The list of possible states isn't dictated by what we humans know, but more generally by how the system has been interacted with in the past. The 'cat in a box' is, in reality, far more complex than a simple atomic system, so the cat isn't really in a superposition of being both dead and alive.

 

[Doveaman]

And because I love it so so much:

Cats in Space: Leo Major and Leo Minor.

This would explain why there are so many anomalies in the space-time continuum.

Upsetting, isn’t it?

 

[AV1611VET]

Also, if you don't align the corners, it falls down the hole!

I don't think so.

A manhole cover rests on top of a flange, which is an inner ledge that "protrudes" into the width of the hole.

This means that the cover is wider than the hole it is covering.

Notice the flange in this picture?

​

I would assume square manholes are the same.

 

[catzrfluffy]

Ah, but the diagonal of a square is longer than the breadth or height, so it could still fall down that way.

Whereas a circle is the same diameter all the way round.

Though, if they made the flange big enough, I suppose that wouldn't happen.

 

[AV1611VET]

Ah, but the diagonal of a square is longer than the breadth or height, so it could still fall down that way.

If it was a box or a crate --- yes, the top could fall in; but a box or crate usually uses a hinge.

If a square manhole can get past the flange and fall into the hole itself, that would be due to poor design.

 

[catzrfluffy]

But why are there no square ones?

EDIT:

nevermind

 

[TheReasoner]

If it was a box or a crate --- yes, the top could fall in; but a box or crate usually uses a hinge.

If a square manhole can get past the flange and fall into the hole itself, that would be due to poor design.

That's why we always say: K.I.S.S.
Keep It Simple, Stupid.
Or, "An engineer knows he has acchieved perfection not when there is othing left to add, but when there is nothing left to take away."

It's simpler and easier to make round manholes. Hence that is what you do. You can use hinges, or you can make a flange (is that the word?) large enough to prevent it from falling in. But this would be hopelessly complex and inefficient. A round manhole is simply a much more elegant and efficient solution.

So, ¡Viva la ∏!

 

[AV1611VET]

But why are there no square ones?

EDIT:

nevermind

Can I hire you to work for our city? You ask good questions!

 

[catzrfluffy]

The Bermuda Rectangle? Where is that? 0_0

 

[Wiccan_Child]

But why are there no square ones?

I'm sure this has been answered already, but square manholes can fall down if orientated improperly. Round ones, no matter what the orientation, never fall down.

 

[catzrfluffy]

Why is the one three posts above made of triangles?

 

[Wiccan_Child]

The Bermuda Rectangle? Where is that? 0_0

In Bermuda, of course .

Interestingly, the Bermuda Triangle is no more dangerous than anywhere else in the world: the rate of disasters is not above average, despite what popular authors would have you believe.

 

[Wiccan_Child]

The Bermuda Rectangle? Where is that? 0_0

Easier to take out, I suppose. It also means that you have two light pieces, instead of one massive piece.

On the other hand, it also means that it's easier to drop down...

EDIT: Here's a non-cut-up-square-man-hole-cover for you:

I'm guessing it stands for west-side New York?

 

[catzrfluffy]

pointy...

 

[catzrfluffy]

Whats the lightest object you can drop from, say, the Empire State Building, and still make a dent in the pavement?

 

[Chesterton]

Here's a ancient manhole excavated in Rome, I guess:

ETA: I thought I was making a joke, but I just learned that SPQR is still the motto of the modern city of Rome. Didn't know that. Better than the city motto of Rome, New York: "At least we're not Utica."