Ask a Physicist!

[Slenn]

Hello there!

I've seen a few of these threads before, and I was thinking: "I love explaining things! And I love science stuff, specifically physics!"

I have 4 semesters of experience in teaching calculus level physics. I am a 4th year graduate student in nuclear physics. And I've had many people outside of my field that say that I do a good job of making the subject of physics relatable or at their level of understanding. I don't like making the common layman out of the loop of what's currently in the field of science!

EDIT: I'm not too fluent with the philosophy of quantum mechanics or some of the interpretations. So questions regarding them may take some research for me to give a legitimate answer.

Questions I can answer regarding:
Anything conceptual
Classical Mechanics - Forces, motion, potential energy.
Thermodynamics - Heat, energy, entropy.
Electromagnetism - Electricity, magnetism, magnets, light, EM waves.
Quantum Mechanics - What is it and how it works.
Molecular physics
Nuclear physics
Some particle physics
Some astrophysics
New things in the field of physics
Forces of nature and how they operate.

I also love making postulations on how things in science fiction media might work. Like lightsabers or warp drives. I also like examining the physics of cartoons at times (as silly as that sounds), such as the summoning game in NGNL.

Questions I cannot answer or what I will not do
Anything to do with explosions. I'm not a Mythbuster (but I might be able to explain some of the stuff they did).
Your entire physics homework (but I will explain conceptual questions).
How to do your taxes
Why Samurai Pizza Cats is awesome

If you do ask me a question, I would like to know how much of a science background since it varies from person to person. This will let me gauge what words to use in an explanation.

Ask away! Remember, I love explaining things! It doesn't have to be in depth. It can be as simple as "why is the sky blue?"

 

[Zontar]

Is light a wave or particles? And if it's both, how does that work given the nature of waves and particle? This is something I never really managed to understand in my high school days and every now and again when I see it references in fiction (usually Futurama) it always bugs me a little.

 

[Asita]

What do you personally lean towards regarding the ultimate fate of the universe? The Big Crunch, Rip, Chill, Bounce, or something else?

Do any particular science fiction conventions - like, for the sake of example, sound in space, explosive decompression, lasers exploding on impact, teleporters, lack of heat convection, etc. - get on your nerves or do you not let it get to you? On a related note, how do you feel about the general presentation of Schrodinger's Cat?

Thoughts on the following videos?

Spoiler

 

[Slenn]

Is light a wave or particles? And if it's both, how does that work given the nature of waves and particle? This is something I never really managed to understand in my high school days and every now and again when I see it references in fiction (usually Futurama) it always bugs me a little.

How light is a wave:
The idea of light being a wave was pretty revolutionary for its time. This was because a lot of people were still subscribing to the idea that a propagating wave needs a medium to transfer its energy, like the air needed for sound waves. When Maxwell created his equations for electromagnetism in the mid 1800's, scientists dug through the math to see what might pop out of it. What they discovered was the idea of an oscillating electric field that was connected to a magnetic field that was also oscillating. But what does it mean for a "field" to oscillate?

An electric field is generated from electric charges. For a positive charge, like a proton, the electric field points away from the charge. And negative charges (like electrons) have fields that point towards them. We can't see these fields, but they are there. And we also know they are present even in the absence of any matter (a vacuum). Finally, from Maxwell's equations, we know that a changing electric field creates a magnetic field in response.

The idea of light being a wave comes from the idea of an electric field pointing back and forth in time. In response it creates a magnetic field that points back and forth. And lo the idea of light being an electromagnetic wave was born.

How light is a particle:
Just before Einstein came along, electromagnetism was pretty well ingrained into every scientist's head. But it wasn't until his work on the photoelectric effect that the idea of light being a particle would become popular. What he found was that ordinary light could be broken up into individual packets of energy that couldn't be broken up any further. This meant that a particular color of light couldn't have any other energy.

Sort of like how there's many things in Minecraft that can't be broken up any further.

This meant that the large amounts of light that we get from the sun are composed of billions upon billions of these individual pieces of light. And since it has energy, it has momentum. And since it has a finite speed, we like to think of these individual packets of moving energy as particles. We call those particles photons. They can be thought of the tiniest pieces of an EM wave you can find.

Does that help?

 

[Zontar]

Does that help?

Yes professor, and I promise I'll pass the test this time.

In all seriousness though, yeah this helped a lot. You explained it better then my science teachers ever did. Thanks.

 

[Slenn]

Snip

The Hindu religion is one of the only religions that we know of that subscribes to the idea of a cycling universe. They noticed that the seasons change following a yearly cycle. So they postulated that the universe will cycle through creation and death. And I like that idea. I also like to think that the universe will stop expanding at some point reaching the maximum entropy possible and somehow there's something magical that brings it all back together again. Sort of like Isaac Asimov's "The Last Question", which is a read any science fiction fan should dive into, because it goes into the fate of entropy and power consumption.

In Hollywood, we've actually seen both sound and no sound in space. Like 2001 Space Odyssey. I personally really like Star Wars and the scope of the movies. The point of the movies isn't supposed to be a scientifically accurate setting. It's supposed to convey drama. The others you mentioned don't really bother me either. What is a red line for me is when the story attempts to give an accurate explanation for what's going on, but ends up not doing so well. The more something to the right balance of explanation to imagination, the better. We don't want the audience to be left in the dark, but we don't to make a fool of ourselves.

RE: Cat. You mean the thought experiment? I think it generally is alright as long as the teacher explains the conceptual difference between classical and quantum mechanics. What the cat explains is multiple states on top of each other. In a classical system, you'd know exactly what state the cat is in. But quantum only lets you predict the probability of what state the cat is in.

RE: First vid. Pretty dang amusing. No doubt some of the laws we've discovered will remain unchanged as our research proceeds into the future.

RE: Second vid. See my paragraph on my red line earlier.

 

[Chessrook44]

How many crowbars do you own?

OK now that that silliness is out of the way... what would you say is your opinion of the existence or nonexistance of parallel dimensions or alternate universes within our understanding of physics, and whether there is any possibility of visitation in either direction?

 

[Thaluikhain]

Oh, are there any misconceptions about science that particularly annoy you, or things you'd wish movies would stop doing?

 

[Slenn]

Oh, are there any misconceptions about science that particularly annoy you, or things you'd wish movies would stop doing?

That science is out to "get" or "disprove" something. During scientific research you only use the term "disprove" if you're wanting to eliminate a long list of possibilities. And even when writing a paper, you don't use the word "disprove" when describing to the audience about your elimination process. Bottom line, science may have an initial goal or a subject of study, but more often than not the results are going to be unexpected. The biggest subject of controversy among pop culture these days is the global warming debate. For scientists, there's no controversy. But for many people outside of science, they have this idea that some of these scientists are falsifying data in order to convince the populace. They jump to the conclusion that the scientists must be wrong, before asking any questions.

I'm also not a big fan of people labeling science as something that's restricted to "smart people." Anybody can learn it. It's a process of eliminating ignorance through examining the universe. Scientists are not rigid machines, they are living breathing organic beings. They follow a philosophy that's only restricted to the tools that are available to them. Science may seem limited, but the answers uncovered always lead to more questions.

Regarding the movies, I'm going to take one step back and talk about one of my favorite shows called CSI. The show was a great drama featuring lots of drugs, violence, and lust in a place that looked flashy and colorful. Its heroes were actually some of the best well rounded and multi-dimensional characters out there. And for the longest time that group was headed by Gil Grissom, one of the best fictional scientists I've seen out there. What the show does fall flat on is how fast the research goes. Most studies will take days, not hours or minutes. For the only show that I regularly watched on Spike TV, it was a small price to pay for one of the best dramas that I've seen on TV.

I guess I could say that more movies and TV shows need to pace things out more slowly. That and not restrict scientists as side characters.

 

[Slenn]

How many crowbars do you own?

OK now that that silliness is out of the way... what would you say is your opinion of the existence or nonexistance of parallel dimensions or alternate universes within our understanding of physics, and whether there is any possibility of visitation in either direction?

Well, a good question to ask would be: How would we know in the first place that it would be another universe? What would be expected when we first look through that portal? Our universe is so vast and crazy with trillions of galaxies, that we might not be able to tell the difference. It may need an understanding of the universe that is inaccessible to human imagination. Our minds are pretty vast places, but they can only do so much.

What about the repercussions of having connected universes? If we had two universes and they both shared similar laws, what would happen if there was some way of translating objects and energy between the two? In a closed universe, we assume that there's a given set of rules, much like a soccer game. The only difference being that everyone knows all the rules, and there are consequences for breaking them. But we don't know all the rules for our universe, consequently we can't declare something new as "breaking the laws of nature" because it's probably another unknown part of our universe.

But now let's connect the two universes. What would that imply? This would mean that conservation of energy would most likely be in error. Because now energy can flow between the two. This might explain things like magic in fiction, which can supposedly violate conservation of energy. But this would only mean that we would have to expand our definition of "universe" if energy is conserved between two or more universes. Something to ponder about.

 

[Rosiv]

What is electricity, and do we have electricity in our body or brains? I know we have cells and those cells have gradients of different concentrations of ions. And somehow that makes a "charge"? I never really understood the whole concept of charges and electricity i guess.

 

[Slenn]

What is electricity, and do we have electricity in our body or brains? I know we have cells and those cells have gradients of different concentrations of ions. And somehow that makes a "charge"? I never really understood the whole concept of charges and electricity i guess.

To answer the first question, yes we do have electricity in our body and in our brains. It may not be the same current as electricity through wires, but it is there. To understand this better we need to take a look at the proton and the electron, two of the central components of an atom. The proton has positive electrical charge, while the electron has negative charge. Their opposites attract, keeping them together.

But how does something get ionized? When someone combs their hair, atoms in the brush bump against the atoms in the hair. As a consequence, electrons will get kicked off and create a clump of negative charge on one object (say the comb). The absence of electrons create a positive charge on the hair because the atoms in the hair now have more sources of positive charge than negative charge.

In our body, when we speak of positive ions, they're talking about atoms that have lost electrons. It's been a while since I took a biology course, but our bodies make use of ions to send signals. Positive ions will attract electrons so that they can become electrically neutral. If they take electrons from the atom next to it, that next atom will become a positive ion. And that atom will seek an electron to fill its gap. And the cycle continues.

I do hope that helps. Please ask more questions if I didn't quite answer it.

 

[one squirrel]

Darn, during my study, more than once I've thought to myself "Hmm, I'd really like to ask a physicist on this", and now that I have the opportunity to do that, I can't remember my questions.

The ones i can come up with right now are (hope I can phrase them to even make some sense):

1) Is it right that the faster something moves, the slower the time passes for that thing? So if you have two objects, which one is the fast one and which one is the slow one, isn't that just a matter choosing a coordinate system?

2) I've never had a really good understanding of the principle of virtual work. Do you have an intuitively accessible explanation for that?

3) Do you know something about the symmetry of the stress tensor, and if so, how do you prove symmetry?

 

[Slenn]

Darn, during my study, more than once I've thought to myself "Hmm, I'd really like to ask a physicist on this", and now that I have the opportunity to do that, I can't remember my questions.

The ones i can come up with right now are (hope I can phrase them to even make some sense):

1) Is it right that the faster something moves, the slower the time passes for that thing? So if you have two objects, which one is the fast one and which one is the slow one, isn't that just a matter choosing a coordinate system?

2) I've never had a really good understanding of the principle of virtual work. Do you have an intuitively accessible explanation for that?

3) Do you know something about the symmetry of the stress tensor, and if so, how do you prove symmetry?

1. Yes indeed that does happen. This has been observed in real world scenarios with atomic clocks on flying jets. The jet engine's clock will tick more slowly than the clocks that are stationary with respect to observers on Earth. And which object is "faster" does indeed depend on your frame of reference. If you're an observer on the jet, its clock will appear to tick normally, while the clocks on Earth will appear to tick slowly. This concept is explored more deeply in special relativity, general relativity's much much much simpler brother.

2. The total amount of work done on an object depends on the path taken. A good example is a person dragging a box across the ground. Since the force of friction is always pointing in the opposite direction of travel, the work will always be greater the more distance is traveled. So a curved path from A to B will require more work than a straight path from A to B. The idea of virtual work follows the same thought experiment. Ignoring friction for now, if a particle or object has many possible paths of motion to take, it will take the path that will require the least amount of work. The reason why it's called "virtual" is because the mathematics concerning it follow a "what if" scenario, as in "what if the object went down these other paths?"

3. Admittedly, this is more of an engineering question, so I'm not all too familiar with how it works. But I do know what each of the components represent. The tensor gathers all the information about stress being delivered on the object. The numbers along the diagonal of the tensor represent the stress that's along the coordinate axis (x,y,z). The off-diagonal members represent sheer stress, which is stress that's delivered on the surface of the object

I'm not sure what you mean by "prove symmetry." But a symmetric matrix is one whose components are reflective across the diagonal:
1 3 5
3 0 2
5 2 0
That's how you can tell when a matrix or tensor is symmetric.

I hope that helped! Please let me know if I left out anything you'd like to know more about.

 

[FalloutJack]

Help me, Professor Kokonoe!

Time for a bunch of space questions.

{1} Recently, there had been the passing of an asteroid which - according to record - would appear as two main masses attached thinly in the middle. I described it as a peanut, and then remembered another news report (both of these made it to the Escapist) in which a scientist made the statement that asteroids were not solid but a series of weakly-gathered space rocks and debris that were maybe as solid as, say, a rice crispy treat. (I apologize for the food analogies if you are hungry.) Balking at this, because it runs contrary to what I've read about physics and objects in space, I determined that if you can have a peanut-shaped asteroid - and it looked pretty solid to me - then asteroids were, in general - solid rocks, not loosely-associated space matter. What say you?

{2} I have heard plenty on the subject of dark matter (which seems to make up a great deal of curious black material in space, denser and thinner in accordance with the general setup of space itself. (To wit, there is a halo of it around our solar system that appears to aid in keeping the system together with its density.) However, I have also heard of dark energy, which I have NOT found as much information on. If you have more, I would like to know some things about it. Like...there was a blurb somewhere stating that it may aid universal expansion. Interesting, and why would that be when the dark matter seems to keep things from spinning out of control?

{3} It has been stated - due to their content and make-up - Europa and Pluto could be housing aquatic life underneath a dense ice layer. However, to confirm that, we would have to be able to send a probe down and get through the ice. I cannot fathom a probe impacting the ice with sufficient force to break through and surviving, nor would I believe that a probe could land on the surface and drill through it all. I have it in my head that whatever orbiting device is delivering the probe must also fire a railgun slug or something along similar veings to get the desired penetration and make a hole for a probe to drop into. My questions regarding that are... Would that even work and/or be possible to lug there and operate remotely before accurately dropping said probe? About how badly might such an act impact what could very well be a sealed environment like that? Got any alternate ideas?

{4} So, CERN has been able to produce and examine anti-matter, quark matter, and the Higgs-Boson itself. What's next on the list? Any other difficult materials we can collect and collate data on?

{5} Professor Stephen Hawking - as you know - suffers from a parapeligic condition which has claimed the lives of other younger sufferers, yet he has managed to keep on living and contribute as he has. Putting aside the emotional part of it and general willpower (because I know he tolerates it well enough due to his considerable intelligence and desire to think on things), what is actually keeping him alive that hasn't done so for others?

 

[davidmc1158_v1legacy]

I do have a question. Can the concept of imaginary time be described in laymen's terms? And, if so, could you please explain it in a fashion that won't cause my neurons to scream like a Lovecraftian protagonist?

 

[Thaluikhain]

What is electricity, and do we have electricity in our body or brains? I know we have cells and those cells have gradients of different concentrations of ions. And somehow that makes a "charge"? I never really understood the whole concept of charges and electricity i guess.

To answer the first question, yes we do have electricity in our body and in our brains. It may not be the same current as electricity through wires, but it is there.

As an aside, certain animals can detect this in others. The playtpus, for example, can hunt prey it can't see by feeling the effects on the electromagnetic field from the target's nervous system. It can almost literally hunt by hearing the heartbeats of its victims.

 

[Lightspeaker]

In our body, when we speak of positive ions, they're talking about atoms that have lost electrons. It's been a while since I took a biology course, but our bodies make use of ions to send signals. Positive ions will attract electrons so that they can become electrically neutral. If they take electrons from the atom next to it, that next atom will become a positive ion. And that atom will seek an electron to fill its gap. And the cycle continues.

Its been a little while since I studied nerves but as I recall: its to do with electrochemical gradients across cell membranes being passed along neuronal fibres and then transmitted via chemicals across gap junctions. Those gradients are formed by ionic pumps that actively shift specific types of ions to one side or the other of the membrane and the impulse is down to the opening of ionic channels which allows them to flow back.

I'd have to look it up if you wanted more specifics.

Source: Me, I'm a doctor of molecular biology.


Good for you, anyway. My dad is a nuclear physicist himself.

 

[Slenn]

{1} Recently, there had been the passing of an asteroid which - according to record - would appear as two main masses attached thinly in the middle. I described it as a peanut, and then remembered another news report (both of these made it to the Escapist) in which a scientist made the statement that asteroids were not solid but a series of weakly-gathered space rocks and debris that were maybe as solid as, say, a rice crispy treat. (I apologize for the food analogies if you are hungry.) Balking at this, because it runs contrary to what I've read about physics and objects in space, I determined that if you can have a peanut-shaped asteroid - and it looked pretty solid to me - then asteroids were, in general - solid rocks, not loosely-associated space matter. What say you?

You'd have to point me to the article. Embarrassingly, I've never heard that about their density, but it makes sense. All objects exert gravity on each other, even space debris left over from the creation of the solar system. The planets have high density because we had the immense good fortune of building up a large amount of mass, which in turn creates a lot of gravity that crushes everything inward. Asteroids that build in size due to gravity (or lucky collisions) do not have enough gravity to crush the mass into a sphere.

{2} I have heard plenty on the subject of dark matter (which seems to make up a great deal of curious black material in space, denser and thinner in accordance with the general setup of space itself. (To wit, there is a halo of it around our solar system that appears to aid in keeping the system together with its density.) However, I have also heard of dark energy, which I have NOT found as much information on. If you have more, I would like to know some things about it. Like...there was a blurb somewhere stating that it may aid universal expansion. Interesting, and why would that be when the dark matter seems to keep things from spinning out of control?

So, to start off, I'll go into dark matter. When astrophysicists were examining the motion of the galaxies through time lapse photography and computer simulations, they noticed something peculiar. According to our understanding of classical mechanics, the objects near the edge of the galaxy were moving much faster than expected. This would imply that unless there's more mass around the center to create more gravity, those objects would be flying off into space from their speed.

Dark energy on the other hand was proposed from something much bigger. Hubble et al, found that not only are the galaxies moving away from one another, but they're accelerating. Why would they be accelerating? The only thing that could fit this was an invisible energy source known as dark energy.

A debate that's yet to be settled is whether or not there's enough dark matter or mass in the universe to eventually counteract this acceleration. Until our instruments are of finer quality, this debate will be unsettled.

{3} It has been stated - due to their content and make-up - Europa and Pluto could be housing aquatic life underneath a dense ice layer. However, to confirm that, we would have to be able to send a probe down and get through the ice. I cannot fathom a probe impacting the ice with sufficient force to break through and surviving, nor would I believe that a probe could land on the surface and drill through it all. I have it in my head that whatever orbiting device is delivering the probe must also fire a railgun slug or something along similar beings to get the desired penetration and make a hole for a probe to drop into. My questions regarding that are... Would that even work and/or be possible to lug there and operate remotely before accurately dropping said probe? About how badly might such an act impact what could very well be a sealed environment like that? Got any alternate ideas?

You're probably thinking that the ice layers are probably too thick to probe through. That may be, we really won't know unless we actually try and drill. Voyager 1 and 2 had small nuclear reactors to have them going for decades. So it's within the realm of imagination that there'll be a drill with a power source long enough to punch through an ice layer. Funding for a space program gets complicated because it takes months to get to Jupiter, and years to get to Pluto. The more mass you send with a rocket requires more fuel, which requires more money. The thing that requires the least amount of money is a robotic probe, because they don't need as many things as a human does.

{4} So, CERN has been able to produce and examine anti-matter, quark matter, and the Higgs-Boson itself. What's next on the list? Any other difficult materials we can collect and collate data on?

Most recently there was the tetra-quark, which is a hadron composed of 4 quarks (mesons have 2, baryons have 3). There's always studies on neutrino physics. Some people are proposing a new type of neutrino to accommodate for some of its missing mass. This would make anti-matter just another type of matter. And people are also searching for a mechanism responsible for the neutrino's mass. This is because the standard model predicts massless neutrinos, which is why people are looking for what's creating it.

{5} Professor Stephen Hawking - as you know - suffers from a paraplegic condition which has claimed the lives of other younger sufferers, yet he has managed to keep on living and contribute as he has. Putting aside the emotional part of it and general willpower (because I know he tolerates it well enough due to his considerable intelligence and desire to think on things), what is actually keeping him alive that hasn't done so for others?

He's got the Philosopher's Stone? *shrug*.

 

[Frezzato]

Suppose there are two people at opposite ends of a rotating cylindrical section in a space station that is, oh, 100 feet in diameter. This section is meant to provide artificial gravity, far from the earth's pull, generating the equivalent of 1G. What do you think would happen to a baseball as it is thrown from one person to the other, across the empty gap in the rotating section? I'm not talking about just velocity, but would the ball travel in a straight path or would the other person already be past the initial point of aim?