# Tag Info

20

Your ear is an effective Fourier transformer. An ear contains many small hair cells. The hair cells differ in length, tension, and thickness, and therefore respond to different frequencies. Different hair cells are mechanically linked to ion channels in different neurons, so different neurons in the brain get activated depending on the Fourier transform ...

15

I see that two examples in optics have been mentioned, a diffraction grating by Mark Eichenlaub, and a lens by sigoldberg1. I would like to elaborate a bit, because there is a subtle difference between the two. On the one hand, a diffraction grating separates out light of different frequencies, i.e. colors, transforming them into different positions. This ...

12

The reason of a more modest version of your statement (your big claim is not right) is that the sum $$\sum_{n=-\infty}^{\infty} |a_n|^2$$ has to converge. That's because this sum is proportional to $$\int_0^{2\pi} |f(x)|^2 dx$$ which converges for bounded functions (a basic insight about Fourier expansions and Hilbert spaces of periodic functions). ...

11

This doesn't make much sense: light year is in any case a unit of distance. What is common is to use "reduced units", for examples units where $c=1$ (speed of light) or $h=2\pi$. But in these cases the opposite would happen: you would say "year" to mean a distance. Or for example you say "has a mass of xyz MeV" instead of "$MeV/c^2$". About the Fourier ...

10

Dear user1602, yes, $\psi(x)$ and $\tilde\psi(p)$ are Fourier transforms of one another. This answers the only real question you have asked. So if one knows the exact wave function as a function of position, one also knows the wave function as a function of momentum, and vice versa. In particular, there is no "wave function" that would depend both on $x$ ...

10

Yes, it happens in reality too, nicely demonstrating that the Fourier analysis predictions are confirmed. An easy way to see this is to take an electrical sine wave signal, which is nice and monochromatic, and pulse it on and off. If you examine the spectrum of the pulsed wave on a spectrum analyser, you will see the spread of frequencies about the centre ...

9

Before answering the question more or less directly, I'd like to point out that this is a good question that provides an object lesson and opens a foray into the topics of singular integral equations, analytic continuation and dispersion relations. Here are some references of these more advanced topics: Muskhelishvili, Singular Integral Equations; Courant ...

8

Remember the double slit experiment? The interference pattern is the Fourier transform of the hole(s). This boggled my mind when I first learned it. In the limit where the screen is far from the mask, the rays of light actually physically compute the Fourier transform (see Fraunhofer diffraction).

8

This has been extensively studied in linguistics and acoustics. Humans and other primates predict speaker gender through a combination of fundamental frequency $F_0$ ("pitch") and Vocal-Tract-Length estimates ($VTL$) which are a proxy for body size. Sometimes "formant dispersion" is used for $VTL$. It is usually defined as ...

8

Your running into circles will stop once you commit yourself to a choice. What to regard as postulate is always a matter of choice (by you or by whoever writes an exposition of the basics). One starts from a point where the development is in some sense simplest. And one may motivate the postulates by analogies or whatever. The CCR are a simple ...

8

The sound that reaches your ear is just air pressure fluctuating over time. You can use a transducer of some sort to convert the value of air pressure to some other form - for example: to the depth of a groove being cut into a helical track on a layer of wax on a rotating drum to the depth of a groove being cut into a spiral track on a circular disc of ...

8

It's sloppy language that is confusing you here. A Jacobian is not a transformation. The Jacobian of a transformation measures by how much the transformation expands or shrinks volume(/area/length/hypervolume/whatever) elements. Example: let $x' = 2x$. Then $dx = dx'/2$. The Jacobian is the $1/2$, meaning nothing more than "a unit of the $x'$ scale has a ...

8

No. Consider any state with a momentum wavefunction symmetric about zero. It's position-space and momentum-space norm-squared probability distributions are not changed by time-reversal, even though the wavefunction clearly is. Here is an explicit example. Take the four Gaussian wavepacket of mean positions $x_0$ or $-x_0$, mean momenta $p_0$ or $-p_0$, ...

7

Those things are surely not enough to find the inner product $\langle q|p\rangle$ uniquely. For example, starting with the conventional $Q,P$, you may redefine them by a canonical transformation, for example by $$Q\to Q'=Q, \quad P\to P'= P + Q^3$$ Then $P', Q'$ obey all the four conditions in the same way as $P,Q$. They also have eigenstates and ...

7

Your visual range includes roughly one octave as compared to roughly twelve in your aural range. Further your visual system uses only four types of light sensors each with limited frequency discrimination, while your hearing has fine frequency discrimination. So while light spectra could have harmonic structure your visual apparatus is ill equipped to ...

7

The route to the uncertainty principle went something like this: In Heisenberg's brilliant 1925 paper [1], he addresses the problem of line spectra caused by atomic transitions. Starting with the known $$\omega(n, n-\alpha) = \frac{1}{\hbar}\{W(n)-W(n-\alpha) \}$$ where $\omega$ are the angular frequencies, $W$ are the energies and $n, \alpha$ are integer ...

7

The wavefunction vector $|\Psi (t) \rangle$ is supposed to be a function of time only. When you write $| \Psi (t) \rangle$ you are not considering the projection of the wavefunction nor on the position neither on the momentum space, but just the state of the system at time $t$, which is nothing but a postulate of Quantum Mechanics. You will have the ...

6

Let's look at frequency instead of notes. Let's say the string has a natural frequency of $100 Hz$ and that harmonics are present when you pluck it. Then, the frequency content of the sound will be of the form: $a_1 \cdot 100 Hz + a_2 \cdot 200 Hz + a_3 \cdot 300 Hz + ...$ Now, let's say you fret this string halfway such that the natural frequency ...

6

You can either accept it as a postulate (in which case it is often more convenient to postulate the CCR and CAR for creation and annihilation operators) or you can derive the relation in the position basis with $$\hat x = x \wedge \hat p = -i \hbar \nabla \Rightarrow [ \hat x , \hat p ] = - i \hbar x \nabla + i \hbar + i \hbar x \nabla$$ as you have to ...

5

1) One may define the Fourier coefficients $a_n$ and $b_n$ (or the complex Fourier coefficients $c_n$) of a periodic function $f(\theta)=f(\theta +2\pi)$ if $f\in {\cal L}^1(\mathbb{R}/ 2\pi\mathbb{Z})$. 2) Here ${\cal L}^1$ is an example of an ${\cal L}^p$ space, where $p\geq 1$ is a power. By definition, a function $f$ belongs to a ${\cal L}^p$ space if ...

5

Yes, the Schaden and Spruch interpretation is correct. The interpretation is not used much because it's not as well connected into how experiments are run. In the usual text books, the Fourier transform is taken over position and time $(\vec{x},t)$ to get energy and momentum $(E,\vec{p})$. This is done by four integrations, one each getting rid of one of ...

5

Given that leftaroundabout and vonjd have addressed the fundamental place of the Fourier transform in the formalism, let me talk a little about an experimental application. What is the shape and size of a atomic nucleus? From Rutherford we learned that the nucleus is rather a lot smaller than the atom as a whole. Now, electron microscopy can just about ...

5

If you integrate $$\int f(x)\textrm{d}x = F$$ then $F$ has the units of $f$ times the units of $x$. Similarly if you differentiate, $$\frac{\textrm{d}f}{\textrm{d}x}$$ has units of $f$ divided by units of $x$. If you look at the simple example of integrating and differentiating with respect to time to go between position, velocity, and acceleration, you'll ...

5

No, the definition of light year is "the distance that light travels in a year in vacuum" http://www.google.co.uk/search?sourceid=chrome&ie=UTF-8&q=define+light+year Short of redefining English, the statement is incorrect! By changing the Units, you would simply change the numeric value of a l.y., but not its definition.

5

If you have a state $|\psi\rangle$, the position wave function is: $$\langle \vec{x}\mid \psi\rangle$$ and the momentum wave function is: $$\langle \vec{p}\mid \psi\rangle = \int\;d\vec{x}\; \langle\vec{p}\mid\vec{x}\rangle\langle \vec{x}\mid\psi\rangle$$ The reason why the two expressions are ...

5

This relates to one of the coolest examples ever - lenses and how you see. Very, very, roughly, when light interacts with a macroscopic object the result is the Fourier transform of the shape of the object, contained in the scattered light. A lens basically computes the Fourier transform of some of the scattered light. The Fourier transform is its own ...

5

His response: Nevermind, for the first time I accept I was wrong. BUT using lightyears to measure time is possible. It's possible if you rape physics, definitions, and just organize things so that you actually, in the end, obtain time from dimensional analysis. The idea would go along this line of reasoning: I know a man walks 5 km/h, hence ...

5

The origin of your problem was already explained in the previous answers, let me just do so in a bit more detail. It is better to think of some normalizable wave function rather than the $\delta$-function itself. As you probably know, you can get arbitrarily close to a $\delta$-function by making a wave packet narrow and taking a suitable limit (see below ...

5

Frequency is just a way of analyzing a time dependent motion. Consider plucking a string by first pulling one point on the string away from its equilibrium. The string shape will be like a triangle, two straight bits of string coming away from where your finger is holding the string, but meeting at a slight angle where your finger holds the string. That ...

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