Hot answers tagged

83

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, ...


36

One good piece of evidence that all particles of a given type are identical is the exchange interaction. The exchange symmetry (that one can exchange any two electrons and leave the Hamiltonian unchanged) results in the Pauli exclusion principle for fermions. It also is responsible for all sorts of particle statistics effects (particles following the ...


15

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. :)


13

I think the best answer to your question is simply "because that's all we can see when we do experiments." That is, no matter how hard anyone tries or how much energy they toss into the processes, electrons and quarks show no signs of any appendages, surfaces, hair-like structures, bumps, volume, whatever. When you model them mathematically as points, the ...


13

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 ...


12

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 ...


12

That's a great question! Unfortunately, the only honest answer is "that's what we see in nature, with great precision and complete reproducibility." There is no deep theoretical understanding. The more exotic form of your question is phrased in terms the self-energy of an electron, and it's a question that plagued Nobel Laureate Richard Feynman his entire ...


11

The anti-particle corresponding to a neutron is an anti neutron! The neutron is made up of one up quark and two down quarks. The anti-neutron is made up of an anti-up quark and two anti-down quarks. Both have zero charge because the charges of the quarks within them balance out. You are correct that elementary particles with no charge are often their own ...


10

As a useable heuristic I would go with something along the lines of the intermolecular forces between the surface molecules of the bodies are comparable to the scale of one-to-one intermolecular forces between nearby{*} molecules due to other components of the same body You could make it a little more strict by replacing "comparable to" with ...


9

Long answer: Any Chemistry textbook. Short answer: The number of electrons of an atom is the same as the number of protons in the nucleus. This number of electrons (Identical to the position number in PSE!) defines all the chemistry of that atom.


8

Short answer: the strong nuclear force. The strong nuclear force binds nucleons (protons and neutrons) together. It is a very short-range force, which is why it only acts over distances on the scale of atomic nuclei. There is repulsion between the protons, which is why, as the number of protons goes up, more and more neutrons are required to stabilize the ...


8

In contrast with the previous incorrect answers that I hadn't noticed, there isn't any ambiguity or confusion about the Bose-Einstein or Fermi-Dirac statistics for composite systems such as atoms. A particle – elementary or composite particles – that contains an even number of elementary (or other) fermions is a boson; if it contains an odd number, it is a ...


8

The photon couples to all particles with electric charge or magnetic moment. This includes all of the quarks, the charged leptons $e,\mu,\tau$, and their antiparticles. It also includes particles composed of quarks and charged leptons: the proton and neutron (though the neutron only magnetically), the charged mesons, etc. Many electrically neutral mesons, ...


7

From: NobelPrize.org "Her continued systematic studies of the various chemical compounds gave the surprising result that the strength of the radiation did not depend on the compound that was being studied. It depended only on the amount of uranium or thorium. Chemical compounds of the same element generally have very different chemical and physical ...


7

The nuclei of heavy elements (lead, gold, ...) approach the asymptotic density of extended nuclear matter (and therefore the density of neutron stars). The lighter elements do not. That said, it would be an error to refer to nuclei as "miniature neutron stars" because the binding force and dynamics are different. Nor are nuclei protected, shielded or held ...


7

The new particles are baryons, and baryons are composite particles made up from three quarks. So the new particles aren't fundamental in the way that the Higgs is. To make an analogy, suppose physicists discover a new element. That's interesting, but like all atoms it's made of electrons, neutrons and protons. So the new element is just another way of ...


7

Photons mediate the electromagnetic force. Atoms are not necessary for photons to exist. You just need charged particles (electrons, protons, etc) to interact with each other from a distance. There are many ways for a photon to be created and destroyed. Depending upon its wavelength, as it propagates in free space , it could "disappear" and a pair ...


6

There are several different things that need to be explained / explored here. First - the speed of light in vacuum is independent of frequency / wavelength. The same is not necessarily true for light in any medium other than vacuum: this is why we can see rainbows! Second - not all objects emit "white" light. The emission spectrum of a star depends, among ...


5

There are two different issues here. (1) Is it a good approximation to describe a particular composite system as a boson or a fermion? (2) If so, which is it? Question #2 is the easy one. The spin-statistics theorem tells us that iff the spin is an integer, the object is a boson. For an atom or ion, this is determined by whether the total number of ...


5

This is confusing enough to make me want to scribble down a few equations. This is a creation operator for deuterium: $$D_{\rho}^{\dagger}\left(k\right)=\sum_{\alpha\beta\gamma}\int\mathrm{d}^{3}l\mathrm{d}^{3}p\mathrm{d}^{3}q\delta^{\left(3\right)}\left(l+p+q-k\right)\ ...


5

An elementary particle is not like a billiard ball at a very small scale. You yourself state i know sometimes it behaviors like a wave, but it sometimes can be seen as a particle. This statement does not apply to macroscopic particles, it applies to microscopic quantum mechanical entities when the dimensions become equal or smaller than a billionth ...


5

Does the fact that protons and neutrons have larger mass than electrons mean they're bigger in size? No. The electron and muon are both believed to be "point-like" (which really means smaller than we can measure" despite having $\frac{m_\mu}{m_e} \approx 200$. That is not to say the proton isn't bigger---it is---but that mass does not imply size in any ...


5

The term "element" is reserved for atoms that have a nucleus that is a combinations of at least one proton and optionally one or more neutrons. Also, only a difference in the number of protons makes a nucleus considered that of a different element. Changing just the number of neutrons only makes a different isotope. Changing the number of electrons is ...


5

I can't give a complete answer because it seems there is still some research ongoing. Unlike what most people have been taught, water is not colorless. At least, large masses of water will be seen blue, such as the sea or a swimming pool. (Left: tube if filled with (light) water. Right: empty tube.) The fact is that water absorbs mostly the red ...


5

Yes. For the phrasing of this question, the neutron qualifies. Neutrons have been slowed and collected, which are diverted from nuclear reactors via beamports. The methods for doing this are quite complicated, but in the final state, they are confined within a box where the "walls" present a nuclear barrier to the neutrons. The neutrons have a wavelength ...


5

At the Large Hadron Collider we have studied matter down to a length scale of about $10^{-19}$ metres, which is about a billion times smaller than an atom. All the results so far confirm our existing theories. So it seems very unlikely that an undiscovered class of small atoms exists. The size of an atom depends on well understood physical principles. At ...


4

This is very legitimate question for something we usually take for granted. I think it would be possible to define macroscopically touching as the situation, in which the total force between two electrically neutral rigid bodies is larger than pure gravitational (for some measurable value). The difference is of course the normal component of the surface ...


4

Your day to day experience of the material world is governed by chemistry. This is at some level the science of atoms and groups of atoms. Things like hardness, colour, toxicity and others are all largely determined by the interaction of atoms. In particular the outer coating of atoms, the electrons. Obviously the details of why element or compound A is ...


4

Maybe one should add to the analysis of @QEntanglement and the nice electron probability clouds in the illustrations in the other answers, that also the space between the nucleus and the electrons is teaming with the exchange of virtual particles between the electrons and the nucleus, necessary to create the potential which determines the energy levels of ...



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