ask a physicist

[Joker]

A common trope in science fiction is using a type of propulsion which "bends" spacetime. Some have the idea that a ship could move at normal speed while contracting the space in front of it. Effectively reducing the distance it has to travel.

Is this theoretically possible (with exotic matter)?

What would happen to any objects in or near the space being contracted?

Cheers.

 

[freyar]

Just going to start with a couple of the fast ones to get rolling, then I'll see if I can get back to this toward the end of the day. Keep the questions coming! These are great so far!

Can I affect the results of the 2-slit experiment* such that the photons favor one side?

*https://en.wikipedia.org/wiki/Double-slit_experiment

Sure. Cover one slit.

You may need to be a little more specific. Do you mean, "Can I look at the apparatus funny, and make the results different?" Or, "Can I affect the results with the power of my Mind?" Or, "If I make one slit narrower, are the results different?" Or what?

Pretty much this.

But I'll try to answer some of Umbran's specific questions. To get it out of the way, you're going to have to physically do something to the apparatus --- "looking at it funny" isn't going to do it. But, yes, making the slits different sizes is one way to do it. You have more options if you send electrons through the experiment rather than photons, since electrons are charged. For example, put a solenoid parallel to the slits between the slits and the screen. Then turn on the solenoid, so there's a magnetic field in it. That will shift the interference pattern around even if the electrons can't enter the region of the solenoid with the magnetic field in it. So, in other words, electrons are barred from entering the solenoid but can still "know" whether there's a magnetic field in the solenoid or not. This is known as the Aharonov-Bohm effect and is a classic example of electromagnetism being used in quantum mechanics. This wikipedia article is a bit technical but may be interesting.

 

[freyar]

One more fast one...

Has anybody ever told you that [someone like] Einstein was wrong, and they knew why?

This or something similar happens pretty much all the time, even now that I'm at a fairly small and not terribly prestigious school. I get several phone calls a year from people wanting help with their personal theory of physics, though sometimes they are too worried I'll steal it to tell me exactly what they think they've done. At my last job, there was one guy who came around the entire department wanting to explain how our understanding of light was wrong (security eventually had to ban him from the building). I once received a book of poetry in the mail from somewhere in Scandinavia (consider that I was a postdoctoral scientist, not even a professor, in North America at the time) which was billed in the cover letter as a theory of everything. Famous physicists tend to get stranger/more threatening stuff. My PhD supervisor once received someone's model of string theory, which was actually made of wooden balls and rubber bands -- yes, sent through the mail -- and another time was threatened with a law suit basically for once having worked with another famous physicist.

Anyway, this happens a lot. It requires pretty sensitive handling from the physicist, though the questioner isn't always respectful or may not realize how much time they're actually taking since they're typically more concerned about self-promotion than learning anything. But it does lead me to appreciate a forum like EN World, since the conversation here is respectful and genuinely curious.

 

[garnuk]

How does Einstein's theory of bent space for gravity work, when there is no "gravity" acting on the object to keep it in the well created by the bent space-fabric?

 

[Umbran]

OK. Electrons. Where do they get the energy to stay in orbit around the nucleus?

There are two answers to this. One is classical (Newtonian) mechanics, the other quantum mechanical. The classical answer is incorrect, but reveals what might be a bit of misunderstanding implicit in the question. The quantum mechanical answer is weird, and hard to understand and put in just text, because it is quantum mechanics.

So, I'll take a stab at both.

Classical:

Well, where does the Earth get the energy to stay in orbit around the Sun? Or the Moon around the Earth?

In classical mechanics, once you have a stable orbit, you don't need to continue to add energy to stay there. The orbiting object is pulled towards the center, but is moving fast enough sideways that it always misses the thing at the center. So long as nothing acts to slow down or speed up the sideways motion, it just keeps falling toward the center and missing. Very Douglas Adams - throwing itself at the ground and missing is the essence of being in orbit.

Quantum mechanics:

It isn't appropriate to think of the electron as a particle in an orbit. On the scale of an atom, the electron is a wave. Just for an idea, you can sort of think of it as a wave running in a circle around the nucleus, and it can be stable at distances that make the orbit an even number of wavelengths of the wave. This need to be an even number of wavelengths means that the electron can only sit at specific distances from the nucleus, so there are "energy levels" it can sit at. In order to become part of the atom, the electron sheds any excess energy, and then can just sit in a particular energy level.

In reality, we don't think of its position in its orbit, so much as the probability of finding the electron at a particular point near the nucleus. The electron is sort of smeared out in a cloud (sometimes called the "electron cloud") around the nucleus, until we directly observe whether it was at a given place. The equation to calculate the probability distribution is a differential equation, so it requires some calculus to get it right, but it falls out that the "orbitals" that result have some interesting shapes - they aren't just circles. These shapes give rise to the shapes of molecules.

https://en.wikipedia.org/wiki/Atomic_orbital#Orbitals_table

 

[Janx]

Sure. Cover one slit.

You may need to be a little more specific. Do you mean, "Can I look at the apparatus funny, and make the results different?" Or, "Can I affect the results with the power of my Mind?" Or, "If I make one slit narrower, are the results different?" Or what?

I would say all those but covering one slit as that is no longer the 2 slit experiment.

I was mainly thinking if there were any kind of field or something I could use to weight it. What if I ran the slits horizontally instead of vertically (thus not left/right, but top/bottom)? Would gravity pull some photons toward the bottom? Stuff like that but more sciency.

 

[AlexM]

I get it. But since it's a wave, where does it get the energy to stay in the wave state? Photons have characteristics of a particle and a wave. I'm just trying to understand gravity I guess. Is there gravity at the atomic level or just energy states?

 

[tomBitonti]

I thought an electron stayed "in orbit" because its location can't be fixed. That is, the electron wants to zip into the nucleus of the atom, but that fixes the location and the momentum too precisely to satisfy the uncertainty relationship. What ends up is an electron buzzing around the nucleus in a manner which minimizes its energy. The closer the electron gets, the less potential energy it has, but the greater kinetic energy it has, because, as the electron's position becomes more certain, the less certain the electron's momentum must become. A balance is reached which minimizes the sum of these energies.

Feynman covers this in his lecture notes. See:

2–4 The size of an atom

http://www.feynmanlectures.caltech.edu/III_02.html#Ch2-S4

We now consider another application of the uncertainty relation, Eq. (2.3). It must not be taken too seriously; the idea is right but the analysis is not very accurate. The idea has to do with the determination of the size of atoms, and the fact that, classically, the electrons would radiate light and spiral in until they settle down right on top of the nucleus. But that cannot be right quantum-mechanically because then we would know where each electron was and how fast it was moving.

Suppose we have a hydrogen atom, and measure the position of the electron; we must not be able to predict exactly where the electron will be, or the momentum spread will then turn out to be infinite. Every time we look at the electron, it is somewhere, but it has an amplitude to be in different places so there is a probability of it being found in different places. These places cannot all be at the nucleus; we shall suppose there is a spread in position of order a. That is, the distance of the electron from the nucleus is usually about a. We shall determine a by minimizing the total energy of the atom.

The notes are wonderful to read, especially for learning about quantum mechanics.

Thx!

TomB

 

[Umbran]

I get it. But since it's a wave, where does it get the energy to stay in the wave state?

Being a wave isn't a state that requires extra energy to maintain. It is just how the thing is, really all the time. We think of it as a particle or a wave based on what math is more convenient to describe what's going on.

Photons have characteristics of a particle and a wave.

*Everything* has characteristics of a particle and a wave - including you. The larger (really, more massive) the object, the shorter its "wavelength". So, for the stuff we think of as objects - humans, baseballs, and so on, have such a short wavelength we can't notice it. Electrons have much, much less mass than you or I, so it's wave nature is far more important to understanding its behavior.

I'm just trying to understand gravity I guess. Is there gravity at the atomic level or just energy states?

The force holding an electron in an atom isn't gravity. It is electromagnetic - the nucleus as a positive electric charge, the electron has a negative charge, so they are attracted to one another.

Now, technically, the nucleus and the electrons have mass, so there should be gravity - but they have so *little* mass, and the force of gravity between them is so weak, that it might as well not be there. For our intents and purposes, we can ignore it.

In any case, this is a Q & A thread, not a discussion thread - if it requires more discussion, we should break it out to elsewhere.

 

[freyar]

OK. Electrons. Where do they get the energy to stay in orbit around the nucleus?

I'm just going to say that Umbran's given some really nice answers to your questions regarding this and emphasize one point that's appeared implicitly, which is that the electron actually has less energy than one that is separated from a proton.

And as Umbran has said in response (to AlexM and tomBitonti), it would be great to move any further discussion of this question to a new thread in the Misc. Geek Talk forum to keep this thread a bit easier to navigate. Thanks!