AMA about Physics

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Burning
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AMA about Physics

Postby Burning » Fri Nov 07, 2014 6:00 pm

Who am I, and why am I answering questions about physics?

I have a PhD in physics, specializing in high energy astrophysics.
I taught for two years as a visiting professor at a small college, before getting out of academia.
While I'm not connected with current research in any personal way, but I try to keep informed.
Physics is a big subject, and I don't pretend to know it all. I'm strongest on astrophysics and weakest on advanced particle physics. I promise you that I'll give a stab at anything.



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Re: AMA about Physics

Postby BadMrMojo » Fri Nov 07, 2014 6:15 pm

Thanks for taking up the challenge, Burning.


Do you have any input about, insight into, or opinions on the possibility of the quantization of time - the idea that time is actually digital (working in discrete, indivisible units) rather than infinitely analog (working on a spectrum: no matter how small a unit of time may be, it can always be half that span of time)?

I've heard mention of it only as a fringe theory and claims that it could make some sense in rationalizing some of the "limits" of the standard model.

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Re: AMA about Physics

Postby Citizen Joe » Fri Nov 07, 2014 7:39 pm

Could you explain the Nemesis theory (Brown Dwarf in the Oort Cloud) and what the current research is saying about it?

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Re: AMA about Physics

Postby McNutcase » Fri Nov 07, 2014 11:24 pm

What's the current consensus on the age of the universe, its size, and whether it's open or closed?
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Re: AMA about Physics

Postby zircher » Sat Nov 08, 2014 7:30 am

(Feeling a little silly this morning, but these are legit in that I do want to hear what your thoughts on them are.)

In your humble opinion, which games (video and RPG) get the physics right?

What's your plan to drag the world of today into the world of tomorrow using science? :-)

Going with a physics theme, what are your super villain powers? [You know, the ones that totally defy physics while seemingly working with them.]

And bonus question that you may totally ignore, Are God and Science compatible - can they exist in the same heart?
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Re: AMA about Physics

Postby lostclause » Sat Nov 08, 2014 9:45 am

What are some good resources for learning physics above the high school level? Assuming we're going for concepts over practical uses.

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Re: AMA about Physics

Postby Burning » Sat Nov 08, 2014 10:33 am

BadMrMojo wrote:Do you have any input about, insight into, or opinions on the possibility of the quantization of time - the idea that time is actually digital (working in discrete, indivisible units) rather than infinitely analog (working on a spectrum: no matter how small a unit of time may be, it can always be half that span of time)?

I've heard mention of it only as a fringe theory and claims that it could make some sense in rationalizing some of the "limits" of the standard model.


A thorny question right out of the gate. :) What I'm about to tell you is based mostly on what I've been able to find on the web. Any mistakes are my own. I'll try to keep the hand-wavium to a minimum.

The concept of quantized time is one that as far as I can tell has been kicked around from the early days of quantum mechanics. It might have somewhat stronger status than "fringe theory," but only because most physicists that seem prepared to comment on it aren't willing to say that evidence rules it out. There's really not a good way so far to tell the difference experimentally between a universe with quantized time and one with continuous time. The mainstream models certainly don't make use of it.

One of the big motivators for trying to make a theory with quantized time is to avoid some of the infinities that can crop up in quantum mechanics. This is related to the Heisenberg Uncertainty principle. Normally, physical quantities do not have precise values; instead there's a range of values that the quantity could have if you took the trouble to measure it. A lot of these physical quantities are paired with another one such that the less quantum fuzziness there is in the first the greater the range of possibilities for the second.

The uncertainty partner for time is energy.

So if we have a quantum event that has a very short duration, that implies that it must be a very high energy event. (You can't have a large quantum uncertainty in the energy without letting large values for the energy into the picture.) The logical end point of this is that any event that is truly instantaneous must involve infinite energies.

Quantized time is only one way of getting around this problem, and it carries with it some strange consequences. For example, to introduce quantized time to current quantum physics without rewriting it from the ground up would suggest that energy is periodic. That means that there would be some very large quantity of energy that, if you added it to a quantum system, the system's energy would for all practical purposes be unchanged. Sort of the energy equivalent of setting your clock forward 12 hours.

This doesn't disprove quantized time. Maybe energy really does behave in that weird way. Maybe we really do need to rewrite quantum physics from the ground up. But you can probably see why it's not considered the course of first resort by a lot of physicists.

A less extreme possibility is to still suppose some minimum possible time period, but to not have event durations confined to multiples of that time period. This is somewhat related to the concept of the Planck time.

The Planck time is the shortest meaningful period of time under the laws of physics as we understand them. It's important to understand that this does not mean that no event could take place in less than the Planck time. It means that we have no theory that can describe such an event. Quantum gravity might if anyone ever works it out.

As a practical matter, we are a long way away from experimentally probing Planck time scales. According to Wikipedia, the record shortest measured event in 2010 was about a quintillionth of a second long (short scale), and that around 26 orders of magnitude greater than the Planck time.

On a purely personal level, the concept of quantized time is even harder to get my head around than most of quantum physics (and that's saying a lot).
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Re: AMA about Physics

Postby Burning » Sat Nov 08, 2014 12:51 pm

Citizen Joe wrote:Could you explain the Nemesis theory (Brown Dwarf in the Oort Cloud) and what the current research is saying about it?


Been a while since I've heard that talked about. I'll confess to resorting to Wikipedia for this, although I've dug into some of the references, so I'll try to unpack some of the stuff that is presented rather vaguely. Anyway, it looks like the status of the Nemesis hypothesis kind of depends on who you talk to.

The origin of Nemesis was an observation that major extinction events seemed to have occurred at more or less regular intervals. This part of the whole thing, by the way, seems to have held up under continued analysis, although the details have been refined. Since there was evidence for two extinction events being caused by comet or meteorite impact, scientists started speculating on a general extraterrestrial cause for the events.

The Oort cloud was already known about at the time, so that provided a convenient source for objects to smash into Earth. The fact that it only happens every 26 or 27 million years suggests that something is disturbing the Oort cloud on that time period. Gravity is pretty much the only thing that could do that. The periodic nature suggests something that is in a highly elliptical orbit, so that most of the time it's too far off to have much influence, but near perihelion it's close enough to give comets in the cloud a sufficient tug to disrupt their orbits. The period gives us the mean size of the object's orbit. You can then fiddle with the parameters of the object's mass and the orbit's eccentricity to get something with the right level of disruption. It turns out you can get an object of plausible size.

Realistically under this scenario, it's not just one comet that's getting thrown into the inner solar system, it's a boat load. We need enough of them that it's a near certainty that at least one will have a catastrophic encounter with Earth. That means that sometimes there will likely be more than one crash in a single cycle. That's not necessarily a problem, as long as the period where comets are slicing through the inner solar system is short enough that we couldn't easily distinguish multiple catastrophes from a single one in the fossil record.

The biggest negative point against Nemesis is that we can't find it. The whole sky has been thoroughly surveyed in the infrared spectrum, multiple times actually. A brown dwarf close enough to be Nemesis should have been detected. I haven't seen this said in so many words, but I surmise that there is not a good candidate for an object that has sufficient mass but would be cold enough to be dark in infrared.

The second point against it seems to be that the extinction events seem to actually be too regular. The problem is that for Nemesis to have a sufficiently elliptical orbit it needs to be far enough away from Sun that nearby stars should be perturbing its orbit, meaning that over the entire span of time that the extinction events are observed, the period of Nemesis's orbit should have changed measurably.

On the side for supporting Nemesis, there's another reason to believe that there's something big out there that has gravitational influence over the outer solar system. The planetoid Sedna has an extremely elliptical orbit, and given its circumstances that's difficult to achieve.

If Sedna was formed with our solar system, it had to start in a near circular orbit; that's the only way things can form. Planets and planetoids form by gravitational collapse of matter. Two objects in very similar circular orbits will stay close enough to each other for mutual gravitational attraction to pull them together. In elliptical orbits, on the other hand, small differences have huge effects. Either the orbital periods are significantly different, so one lump soon leaves the other behind, or the objects aren't travelling in the same direction when they pass each other by.

So either Sedna was captured from outside the solar system (almost certainly how we acquired Nemesis, if it exists, by the way) or something disrupted its orbit after formation. Capture is not impossible, but it requires extremely long odds. On the other hand, if Senda did form in the solar system, the neighboring stars that we can see just don't have the pull needed to disrupt a circular orbit that much. So we need something provide the necessary gravitational influence. The big problem is that Sedna doesn't necessarily need Nemesis; it could have been influenced by a one time passer-by or another distant planet/planetoid that wouldn't have the right influence on the comets to match the extinction events.

I think Nemesis is looking a bit rocky but not necessarily down for the count.
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Re: AMA about Physics

Postby Burning » Sat Nov 08, 2014 2:42 pm

McNutcase wrote:What's the current consensus on the age of the universe, its size, and whether it's open or closed?


Age is the easiest to answer. Consensus is pretty strong on that point, giving a value of about 13.8 billion years with uncertainties on the order of a few tens of millions of years. This result gets arrived at independently from a number of different methods, so it's probably pretty sound.

As a side note, you may have heard at various times the "scandal" that when astrophysicists try to calculate the age of the oldest stars, they get an answer older than the age of the universe. Of course no astrophysicist believes that the universe formed with old stars in it, so this has always been taken to mean that either cosmological or stellar models (or both) needed correction. So it's not viewed in astrophysics as a scandal, but instead business as usual in an evolving field of study. It just sounds more embarrassing than some other discrepancies that science has had to deal with.

Anyway, the good news is that, while stellar models seem to still be giving higher numbers, the two numbers now have overlapping uncertainty ranges (oldest stars calculated as about 14.5 billion years with an uncertainty of about .8 billion years). Obviously, the ideal would be to get the models refined to the point where the numbers are consistent and have smaller uncertainties, so even the best estimate of the stellar ages was less than the age of the universe. Regardless, overlapping uncertainties is a win.


Size depends on whether you are talking about the visible universe or the whole universe.

The size of the visible universe is the distance to the source of the cosmic microwave background (CMBR to its close friends and family). The CMBR comes from the time when the universe had cooled enough that electrons and nuclei were able to combine into atoms. At that point, the opacity of the universe dropped abruptly. The photons in the CMBR have been travelling unimpeded ever since. Cosmologists put the distance to the "surface of last scattering" at 46 billion light years.

That distance may sound goofy on the surface, given that the universe is supposed to be 13.8 billion years old. A light year is the distance light travels in one year, so it would seem at first blush that the universe should be 13.8 billion light years in radius. The extra distance is due to the expansion of the universe. The space in between where the light came from and here got bigger in the intervening time.

Rather bizarrely, the actual size of the universe could be somewhat smaller than that. This is under the assumption that the universe is curved to such a great extent that a straight line path curves back on itself (or comes close). The linked paper is academic and technical but has islands of lucidity in it. :) Basically, the authors used the observations of the CMBR, considered the possibility that you might be able to look through the same part of space in two directions due to this curvature, and then figured how tightly the universe could be curved before it would introduce discrepancies (basically that it would become more obvious than it is that you're looking at the same thing in two different directions.) I really love this result.

All that said, I believe that the majority of cosmologists believe that the universe is, if not infinite, large compared to the visible universe. This ties into the last point. Whether the universe is open or closed is essentially the same question as to whether the universe is infinite or finite. A closed universe is, in the jargon of the biz, finite but unbounded. It curves back on itself. This is the type of universe that will eventually stop expanding and collapse back down. An open universe is infinite and will expand forever.

Theoretical cosmologists tend to favor a universe that is "flat" i.e. right on the borderline between being open and closed. It will expand forever, but the expansion rate will approach zero as time goes by. The big reason for preferring this is that we have reason to believe that if the universe isn't flat, it has to be extremely close. A small deviation from flatness in the closed direction should have been enough for the universe to have collapsed long ago. A small deviation in the open direction would have caused it to expand so quickly that no structure (i.e. things like galaxies, stars, planets, little things like that) should have been able to form. If the universe isn't precisely flat (with an accompanying good physical reason for being so), then it begins to look like we were just extremely lucky. Scientists hate answers like that.

It's all still something of an open question, however. There's good observational evidence that, at least right now, the expansion of the universe is accelerating. The name that you may have heard for the cause of this expansion is "Dark Energy," but that's really at this point still a case of "let's give the thing we're ignorant about a name." So a flat cosmology is still attractive, but that means that eventually the Dark Energy can't continue to make expansion accelerate. The closed universe may at first glance seem to be out of the running, but if dark energy can lose enough steam for the universe to be flat, it raises the possibility that it could even lose steam a little bit faster.

Basically, the only thing I think people are sure of is that physical cosmology will remain an exciting field of study for the foreseeable future.
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Re: AMA about Physics

Postby Burning » Sat Nov 08, 2014 4:35 pm

zircher wrote:In your humble opinion, which games (video and RPG) get the physics right?


Do you mean getting it right in the story/setting or getting it right in the simulation? I guess either way, I would say Kerbal Space Program. :D

The thing of it is, for the story and setting aspect, I'm not a big fan of hard science fiction. I like straight out fantasy, science fantasy (by which I personally mean mixing actual magic with high tech), pulp science fiction (there's lasers, spaceships, and robots; and hang how the whole thing works), and semi-hard science fiction (there's made up science, but the implications matter). None of those are "getting the physics" right in the areas that make the world interesting.

In video games, pulp sci-fi is usually about the hardest I play. For table top, I'll give a shout out to Traveler for having communication at the speed of travel. I think the decision not to have something like Star Trek's subspace radio really adds to the feel of the world.

For the simulation, unless you are playing something like KSP, I really think that the physics just needs to be realistic enough to not jerk you out of your suspension of disbelief, but gameplay should otherwise trump out. There was a "boardgame" in a kids science magazine I got back when I was in grade school that had you racing rockets with realistic rules for acceleration in outer space. I wouldn't mind a really well designed tabletop game with that mechanic. I think in a videogame it would give me motion sickness.

What's your plan to drag the world of today into the world of tomorrow using science? :-)


Wait 24 hours. :rimshot:

More seriously, I would love for the general public to get excited about the space program just as an endeavor for expanding human knowledge. I don't really know how to achieve that. Realistically speaking, even at the height of the space programs popularity, the majority were into it for reasons of national prestige. I actually do believe that funding general research has a great practical return on investment, but it seems hard to convince people that this isn't just a lie to get them to give you money. It's going to take someone smarter than me to figure that one out.

Going with a physics theme, what are your super villain powers? [You know, the ones that totally defy physics while seemingly working with them.]


I'd say telepathy is the one that I could give the best hand-wavey justification for. I mean its just electric signals in the brain, there's going to be EM radiation that a good receiver can pick up on, right? ;)

The one I really want is teleportation. That's harder to justify, at least while keeping a straight face.

And bonus question that you may totally ignore, Are God and Science compatible - can they exist in the same heart?


I'm not going to go deep into this, but I'm going to say that yes they can, and that they in fact do in many people. I'm not sure they typically dwell in the same part of the heart, but they can be close neighbors. My sense of wonder at the natural world does inform some of my personal sense of the divine. I would recommend The Sacred Depths of Nature by Ursula Goodenough as a fascinating and often moving description of one woman's exploration into these matters.
"Space is blue, and birds fly through it." - Werner Heisenberg

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