# Tag Info

121

For organic matter, such as bread and human skin, cutting is a straightforward process because cells/tissues/proteins/etc can be broken apart with relatively little energy. This is because organic matter is much more flexible and the molecules bind through weak intermolecular interactions such as hydrogen bonding and van der Waals forces. For inorganic ...

65

Wow, this one has been over-answered already, I know... but it is such a fun question! So, here's an answer that hasn't been, um, "touched" on yet... :) You sir, whatever your age may be (anyone with kids will know what I mean), have asked for an answer to one of the deepest questions of quantum mechanics. In the quantum physics dialect of High Nerdese, ...

48

You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is accelerating continuously and would thus radiate away its energy and fall into the nucleus. One of the reasons for "inventing" quantum mechanics was exactly this conundrum. The Bohr model was proposed to ...

38

Neither of those statements are true. It's an easy approximation to make: a neutron star has all of that 'space' removed from between nucleons --- so we just need to know how big a neutron star of mass equal to the solar system would be. Well, the only significant mass is the sun (jupiter is about 1% the mass of the sun---negligible). If the sun were ...

30

Groups in Seattle, Colorado, and perhaps others managed to measure and verify Newton's inverse-square law at submillimeter distances comparable to 0.1 millimeters, see e.g. Sub-millimeter tests of the gravitational inverse-square law: A search for "large" extra dimensions Motivated by higher-dimensional theories that predict new effects, we ...

28

Fundamental particles are identical. If you have two electrons, one from the big bang and the other freshly minted from the LHC, there is no experiment you can do to determine which one is which. And if there was an experiment (even in principle) that could distinguish the electrons then they would actually behave differently. Electrons tend to the lowest ...

22

I don't think that this is a physics restriction, but one of current engineering capability. As you link points out, using 12 atoms allowed the information to be retained without effecting the information stored next to it. You will also need enough data-mass to allow for the reading and writing of the information without affecting the data next to the one ...

20

Let's talk about the balloon first because it provides a pretty good model for the expanding universe. It's true that if you draw a big circle then it will quickly expand as you blow into the balloon. Actually, the apparent speed with which two of the points on the circle in a distance $D$ of each other would move relative to each other will be $v = H_0 D$ ...

19

There is a rigorous formal analysis which lets you do this. The true problem, of course allows both the proton and the electron to move. The corresponding Schrödinger equation thus has the coordinates of both as variables. To simplify things, one usually transforms those variables to the relative separation and the centre-of-mass position. It turns out that ...

19

Measure the gravitational attraction between two atoms? Heavens no. That's such a tiny, tiny attraction. The atoms will be attracted to themselves gravitationally, but only minutely. They'll be attracted gravitationally much more strongly to the Earth, to the lab setup and measuring equipment, to the buildings around the measuring equipment, and even to the ...

17

No, there aren't any holes like that in the EM spectrum. There are other ways of creating photons than by having electrons bound in atoms transition from one level to another. (For example, you can create pretty much any frequency of photon you want by accelerating a free electron.)

15

Yes, quantum mechanics – even non-relativistic quantum mechanics for several electrons orbiting nuclei – fully, quantitatively, and comprehensively explains all of chemistry (including biochemistry and, in fact, biology). This fact has been known since the late 1920s. To understand the periodic character of the properties of the elements, one must realize ...

14

This is quite far from a silly thought although this is not apparent at first sight. Apart from a couple of details which are well understood and have firm physics behind them - such as the fact that deuterium and tritium exist in some proportion and the hyperfine-structure distinction between ortho- and parahydrogen, as far as we can tell all hydrogen atoms ...

14

It depends on what's being cut. When metal is cut, what happens is that, on a small or not so small scale, it shears. That means layers slide over each other. The mechanism by which they slide over each other is that there are imperfections in the crystal structure called dislocations, and the crystal layers can move by making the dislocations move in the ...

13

Common sense of touching can be expressed in "scientific means" as an event when exchange-repulsion interaction between 2 objects (you and the geek) extends some arbitrary value, say 1meV. I leave finding an agreeable threshold which is easy to measure to later discussion. :)

12

Do not confuse mass with charge. Although the proton is more massive, the magnitude of its positive charge is equal to the magnitude of the electron's negative charge. Hence, neutral atoms!

12

I assume you're talking of the hydrogen atom; the hamiltonian of the nucleus + electron system is $$H = \frac{p_e^2}{2 m _e} + \frac{p_n^2}{2 m _n} - \frac{e^2}{|r_e - r_n|}.$$ You can do a change of coordinates (center of mass coordinates) $$\vec{R} = \frac{m_e \vec{r}_e + m_n \vec{r}_n}{m_e+m_n} \\ \vec{r} = r_e -r_n$$ and find the conjugate momenta to ...

11

It is a very interesting question that allows to point out the differences between a Neutron Star and Nuclei. Although the dedicated article in Wikipedia Neutron Star fully covers the information, it is relevant to summarize here the elements. Nuclei are essentially different to Neutron Stars and some reasons are: Different bounding force: while Nuclei ...

11

For any kind of magnetic data storage you need a magnetic state that is stable over time. The magnetic moment of an isolated single atom does not have any preferred direction, therefore the energy states are degenerated. The 12 atoms used in this experiment is not a lower limit, in principle it can also work with 2 atoms given the right magnetic ...

11

In any proper quantum mechanical understanding of the atom, a bound electron does not have a position and follow a path (i.e. have a time-varying position) in the sense that it would have in a classical or semi-classical theory. Instead the electron "has a state" or "occupies an orbital" (an orbital not a orbit!), and because there is not a path there is ...

10

I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus. Even if the electron actually orbits the nucleus, wouldn't that orbit eventually decay? Yes. What you've given is a proof that the classical, planetary model of the atom fails. I can't reconcile the rapidly moving electrons required by the ...

10

First you say It's easy to visualise and comprehend the excited states of electrons, because they exist on discrete energy levels that orbit the nucleus By way of preparation, I'll note that in introductory course work you never attempt to handle the multi-electron atom in detail. The reason is the complexity of the problem: the inter-electron effects ...

10

If you could compress the mass into that small a space, it would collapse into a black hole, at which point the notion of "size" becomes harder to define, with space-time being so warped. The "event horizon" radius would be about 3 km, if I get the formula correctly. The idea of "there's a lot of space in atoms" comes from computations which state that the ...

10

Does position count as a characteristic? If so, then yes, atoms do have unique characteristics. Edit: Perhaps I should rephrase this. If position counts as a characteristic, then yes, atoms do have unique characteristics.

10

In particle physics, you have to be specific about what you mean by breaking down a particle: A neutron in an atom can decay into a proton, an electron, and an electron-antineutrino. But this does not mean that a neutron is made of a proton, an electron and an electron-antineutrino. What the neutron is made of are three quarks (one up and two down). If by ...

10

Carbon-14 makes up about 1 part per trillion of the carbon atoms around us, and this proportion remains roughly constant due to continual production of carbon-14 from cosmic rays. The half life of carbon-14 is about 5,700 years, so if we measure the proportion of C-14 in a sample and discover it's half a part per trillion, i.e. half the original level, we ...

10

Short answer: The space between the nucleus and the electron is not empty space, it is filled with an electron cloud. (You will understand this answer better if you read the long answer) Long answer: Firstly, physics is a description of what we can observe. Depending on the scale of which you are describing, physicists, over the years, have different ...

10

Its due to the WP-Rayleigh–Taylor instability: it is an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid. Hot hair raises and colder goes down. A Mushroom cloud formed by hot wet air : google for images or videos of 'Rayleigh–Taylor'

10

In addition to dmckee's answer this link summarizes the experiments of antiprotons catching protons and neutrons, creating temporary nuclei. It is the symmetric state to the one in the question : an anti-proton-neutron nucleus that lasts for a bit (fig 5.7). Anti-protons can be made in abundance and controlled experimentally because they are charged, ...

10

This answer I once gave for What does it mean for two objects to "touch"? discusses what touching even means. It's not a direct answer to your question, but I think it may help you view the issue in a different way. Warning: It's one of my long, talky answers that some people love and others hate. The physics in it is accurate (and for many folks, ...

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