AMA about Physics

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Citizen Joe
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Re: AMA about Physics

Postby Citizen Joe » Sat Nov 08, 2014 6:59 pm

Conceding the point about space itself expanding, A) is the rate of expansion increasing or decreasing? B) has there been an observation of a space reduction or annihilation?

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Re: AMA about Physics

Postby Burning » Sat Nov 08, 2014 7:17 pm

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


Tricky. Even with the proviso of concepts over practicality, it kind of depends on how deep you want to dive and what you are most interested in.

While I've read a fair number of popular accounts of physics, a lot of them are heavy on the hand-wavium. Part of the problem is that physics is highly mathematical, and it is tricky to describe some of the concepts without the math. The other problem is a general problem experts in all fields have; they tend to forget that even they had to learn the basics at one time and leave out details because they assume them to be common knowledge.

I haven't seen a lot of good resources for "classical" physics. If you are feeling ambitious and are comfortable with math, you might try The Feynman Lectures on Physics. My impression from reading it was that Feynman was probably an uneven teacher, but when makes himself dig into the basics, he's brilliant. It's in three volumes, and they aren't cheap, even in paperback, so I would recommend seeing if you can get the first volume from the library if at all possible to give it a test drive.

"Modern" physics (since it began more than a century ago, I question the validity of the term) gets a lot more popular accounts. I honestly haven't looked at any published in the past decade, so I might be missing some gems.

The best account I've seen of the general strangeness and philosophical implications of quantum mechanics is Where Does the Weirdness Go? by David Lindley.

For astronomy and cosmology, Timothy Ferris is pretty sound, but will be missing recent developments at least in the ones I have read. I particularly recommend Coming of Age in the Milky Way and The Whole Shebang.

I've personally never seen a high energy particle physics book that I would recommend without reservation. I feel that The Elegant Universe by Brian Greene is probably the best I've personally read, but in the later portions of the book he slips increasingly into "take my word for it, I'm smart" territory.

For relativity, I would actually recommend another text book: Spacetime Physics by Edwin Taylor and John Archibald Wheeler, although with the proviso that only if you can get it from the library or find a cheap used copy. It's good, but not good enough to pay the price Amazon is charging for it. The treatment is very conceptual, only going into deep mathematical details in a few chapters. Even the some of the homework assignments are useful for understanding some of the stranger wrinkles of relativity.
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Re: AMA about Physics

Postby Johann » Mon Nov 10, 2014 1:37 pm

I ask because you are a PhD, so the assumption is that you have taught or do teach:

I have a daughter that is vehemently anti-science/ anti-math.

I would like for her to have a reasonable grounding in both subjects.

From your perspective, how would you suggest:

a) Deciding what is "important" to teach/learn for someone not intending to pursue a science focus.

b) How to convince a person not interesting in science and math that it may have some practical use in their life.

Disclosure: My kids are homeschooled, and Mom (primary teacher) is also of the liberal-arts persuasion. I'm not, but have very little time to get my point across as to the importance of those subjects.

I don't expect you to solve my childbearing and marital woes, but I am always interested in the opinions of other science types when it comes to this particular issue.

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Re: AMA about Physics

Postby tombombodil » Mon Nov 10, 2014 2:01 pm

All the answers you have given involve a sub-question I've always been curious about. How do physicists "observe" things like the CMBR? What instruments and methods do you use to judge the Age of the universe? How certain are we (and how are we certain) that these measurements/observations are accurate or that we're even interpreting them correctly?
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Re: AMA about Physics

Postby Burning » Mon Nov 10, 2014 8:28 pm

Citizen Joe wrote:Conceding the point about space itself expanding, A) is the rate of expansion increasing or decreasing? B) has there been an observation of a space reduction or annihilation?


The best evidence is that the rate of expansion is increasing.

The evidence for expansion in general is that light from distant object is red-shifted (i.e. shifted to a longer wavelength and lower energy than it was leaving the source) and that the more distant the object, the greater the red shift. This observation requires two things.

First you need to be able to measure the atomic spectrum of the object. You have to be able to identify the emission lines from the different elements by their positions relative to one another on the spectrum, so that the fact that they are all shifted in wavelength doesn't confuse matters.

Second, you need to know intrinsically how bright your source is, so you can deduce how far away it is based on how bright the light that reaches your detector is. Not all astronomical bodies are good for this. Most types of stars, for example, have a brightness that could depend on a lot of different factors, such as age and mass, that we aren't necessarily able to figure out without knowing the brightness in the first place.

Fortunately, there are objects that meet both of these criteria. A good one is what's called a type 1A Supernova. This is white dwarf star that has exploded due to enough matter accreting on it (probably from a companion star) that its gravity increases enough to re-trigger fusion. These have a very distinctive spectrum ("type 1A" is originally a classification of the spectrum) and also are very consistent in brightness (since the total mass needed to restart fusion is also very consistent). Not only is the brightness consistent, but it's very high, so you can actually detect them in distant galaxies.

To first approximation, the relationship between the distance and the red-shift is linear. That is to say, if you double the distance you also double the red-shift. If the rate of expansion of the universe were constant over time, this linear relationship would be an absolute, not an approximation.

You tell what's happening to the rate of expansion by comparing the actual relationship to the linear model. Since light travels at a finite speed, the greater the distance to the object, the earlier in the history of the universe you're looking. So if in the distant past the expansion rate of the universe was greater, then the most distant objects should have a greater red-shift than from the linear model. (So, just to make up numbers, doubling the distance might increase the red-shift by 2.1.) Contrariwise, if the rate of expansion was smaller in the past, so will the red-shift of the distant objects be less than from the linear model. (Maybe doubling the distance only increase the red-shift by a factor of 1.9.)

It was only in the late nineties that measurements of sufficient precision were made to tell that it wasn't straight linear expansion. Pretty much everyone expected that the rate of expansion would be greater in the past, but instead it was seen to be less in the past.

There haven't been any observations of spatial contraction. Even on the large scale, the random movements of objects puts noise on the signal of the expansion of the universe. I think (though I may be mistaken) that in principle, a small area of the universe could have, locally, a sufficiently greater than average concentration of mass for space to contract in that region, but it's unlikely that we would be able to pick the effect out.

If at some point the expansion of the universe does come to a halt and reverse, I believe that we would see the effect of that first in the nearer objects. The red-shifts would decrease and then change to blue-shifts. However, changes like this happen on very long time-scales. As in humans might not be around to see it kind of time-scales.
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Re: AMA about Physics

Postby Burning » Tue Nov 11, 2014 9:58 pm

Johann wrote:I ask because you are a PhD, so the assumption is that you have taught or do teach:

I have a daughter that is vehemently anti-science/ anti-math.

I would like for her to have a reasonable grounding in both subjects.

From your perspective, how would you suggest:

a) Deciding what is "important" to teach/learn for someone not intending to pursue a science focus.

b) How to convince a person not interesting in science and math that it may have some practical use in their life.

Disclosure: My kids are homeschooled, and Mom (primary teacher) is also of the liberal-arts persuasion. I'm not, but have very little time to get my point across as to the importance of those subjects.

I don't expect you to solve my childbearing and marital woes, but I am always interested in the opinions of other science types when it comes to this particular issue.



For someone not going into some sort of technical field, the most important things people can learn from math and science is the mental toolkit. How to evaluate evidence, do a back-of-the-envelope estimate, identify other people's hidden assumptions, identify your own hidden assumptions, take complex problems and link them to simple ones you understand, etc. Even if you are not in a technical field (perhaps especially if) the modern world is a complex, technical place. The ability to look at it skeptically and analytically is, in my opinion, more important than any specific fact or set of facts that you might learn in science or math class.

The problem is that, at least traditionally, almost no one actually tried to teach these skills deliberately. They concentrated on teaching facts and trusted that those students that were suited for science or math would pick up the analytic skills along the way. It doesn't really end up making much sense as an approach, even if you believe that the information is more important than the skills, because the students that don't pick up the skills don't really learn the information.

There was, when I was still in academia, a growing segment of the physics community that was looking at changing this. A lot of them were probably most interested in communicating physics effectively to the students. They didn't necessarily have the skills as an end goal, but they recognized that exercises and techniques that made the students practice the general skills were necessary for effective learning of the material.

While there was a depressing amount of resistance from the rest of the physics community, I have reason to hope that it has taken hold enough to start trickling down to the grade and secondary schools. Certainly, my son's math and science classes seem better than anything I experienced as a child.

None of this is to dismiss the importance of being well-informed on math or science. I just see those as secondary benefits.

As for advice on how to convince someone of the value of math and science, I'm not sure I know. I can't honestly say that I was a very good teacher back when I still taught. The best thing I can think of suggesting is to look for a connection to something she does care about. Look at how she can apply the mental skills to something that does interest her.

Good luck. I really wish I could be more help on this one.
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Re: AMA about Physics

Postby Burning » Wed Nov 12, 2014 11:10 pm

tombombodil wrote:All the answers you have given involve a sub-question I've always been curious about. How do physicists "observe" things like the CMBR? What instruments and methods do you use to judge the Age of the universe?


So the big thing to acknowledge is that virtually all observations in physics are made through instruments, and its been that way for decades at least. The further back you go, the more physics was performed with direct observation with the human senses, but even then instruments have been important to observation for centuries. Nowadays, there is very little on the forefront of research that can even be perceived with human senses, and even when it can be, it is highly rare that people actually do so. I'll get more into the implications of that in relation to the later parts of your question.

Limiting ourselves for purposes of discussion to astrophysics, the bulk of observations are of electromagnetic radiation (radio, microwave, infrared, visible light, ultraviolet, x-rays, or gamma rays). The light can be characterized in terms of spectrum (what wavelengths are present), brightness (how much of each wavelength is coming at us per second), and polarization (how is the electric field in the electromagnetic way oriented). And of course we look at the variations of each of these from point to point on the sky and from moment to moment.

Depending on what you are observing, some of these are more important than others. For the CMBR, spectrum and polarization vary for different points on the sky. Brightness doesn't vary that much by direction, and none of them really vary over time.

Different instruments have different strengths. There's often some sort of trade-off between the different properties you might want to study in designing an instrument. For example, to get good spectral resolution, you might need to give up on some of the angular resolution (eyesight analogy: someone with excellent color sense but blurry vision).

Usually, observations are made with a variety of instruments. That way you can have some observations that are extremely precise in one way or another, combined with others that are less precise in any single area but are of decent precision in all areas. You can use these different observations to build up a fairly coherent picture of whatever you're studying.

Astrophysical observations are divided into surveys and targeted observations. Surveys sweep over the whole sky or a significant portion of it. They're generally less precise overall but give a good view of the big picture. Targeted observations spend more time on a single portion of the sky than is possible in a survey (assuming you ever want your survey to be completed). This gives the most precise picture possible of the areas you study, at the cost that you are only studying a small fraction of the sky.

I believe most of the observations of the CMBR are part of all sky surveys, since the variation over the whole sky is one of the most important pieces of information in understanding the early universe. However, there are almost certainly some long targeted observations of some parts of the sky to try to get a more detailed picture.

I'm not really knowledgeable on microwave detector physics. I worked at the opposite end of the spectrum. If you're interested in more specifics for microwave observations, I can do some research, but I figured that some generalities would do for a start.

As for how this information is used to calculate things like the age of the universe, that is of course very highly dependent on the theories that are used to interpret the data in the first place. The broad strokes for this particular question are that cosmologists take the distance/red-shift relationship that they observe and interpret it as indicating that the universe is expanding. They then run the clock backwards; based on the inferred motions, there was a time in the past when everything we can see was scrunched down into a very small volume. How long ago was that?


How certain are we (and how are we certain) that these measurements/observations are accurate or that we're even interpreting them correctly?


When the scientists give an uncertainty in a calculation, like the ones I quoted for the age of the universe, those uncertainties are all based on the quality of the measurements. There is always the understanding that these are under the assumption that the underlying theory is correct. It has to be that way, because there is really no way to quantify the possibility that the theory is wrong.

The scientists usually have an extremely good idea of how well they can trust their measurements. In the construction of any instrument (at least when done correctly) there is a lot of testing to characterize the behavior of the instrument before it ever sees "first light." You calibrate the instrument against sources you understand very well, so that you know what the data will mean when you use it to observe the sources you seek to understand. You also collect data with it blocked off from any source at all, so you understand the instrument noise that could obscure a signal.

There's always the possibility of something going wrong, of course. That's why science is so big on repetition of observation. It has to be acknowledged that the picture that gets presented in grade school science class that every observation gets repeated by another scientist and every result confirmed is just not true. But interesting results lead to more observations as people try to understand them better. There will generally be a steady supply of eyes on a problem that generates interest.

As for how we can be certain that the interpretation is correct, here we start to get into philosophy of science. Another lie from grade school science class is that a theory can eventually be proven and become a law. Theories can not be proven, if by that we mean that we have eliminated any possibility of error. We can only say that only say that a theory has been tested and not proven false. (As a side note, no, laws aren't proven either. Also, laws are actually components of theories. Saying that a theory may someday become a law is something like saying your house may someday become a floor.)

If we for a moment take skepticism to a ridiculous extreme, there has never been an observation of the cosmic microwave background. People have built instruments, and those instruments are observed to behave a certain way under certain conditions. To assert even that microwaves exist, let alone that microwave radiation permeates the entire universe and that radiation has its source in the early history of the universe, is to assert knowledge of something that is really only an interpretation of the observed behavior of our instruments.

Of course you can't do science this way. You probably can't even live this way. This level of skepticism is a short step from leading you to pure solipsism. At a certain point, although we must concede the logical possibility that a future observation could overturn our theories at any level, some of our interpretations have stood up to sufficient scrutiny that we just believe them.

We often use the word "theory" as if it denoted one simple proposition and its consequences, but real theories aren't really so tidy. Modern cosmological theory is a synthesis of a lot of different concepts and observations of varying levels of maturity.

We gain confidence in the interpretations of science through their repeated success. Predictions based on the interpretation are confirmed when someone performs the observations to check. Seemingly different lines of inquiry converge on the same result. The game of theoretical science is to try to come up with explanations that get more out of the theory than was put into it and to do so in a way that can be objectively tested.

To take this out of the abstract, let's look at the status of some of the concepts in cosmology I've already talked about.

Dark energy is, as I said, primarily a shorthand name for ignorance at this point. There are trial explanations that people are working on, but so far they haven't resulted in much testable.

We hope that one day Dark Energy will be a shorthand name for a good explanation of he observations that expansion is accelerating. Right now, these observations are in their second decade now, and have been confirmed by multiple groups. They have a fairly solid status. No one expects that they will be overturned, but they're probably young enough that were they overturned it would be a surprise but not a shock.

The general phenomenon that the universe is expanding is probably not in any doubt by any professional physicist. It is based on a wide variety of independent measurements. To cast much doubt on the observations, we would need to cast doubt on other mature areas of inquiry: the Doppler effect, atomic spectra, stellar models. It also has the benefit of being predicted by general relativity, a theory that has been successfully tested on much smaller scales than the cosmological.

The cosmic microwave background is one of the big triumphs of Big Bang cosmology. It was predicted before it was ever detected. If the universe was hot and dense in the distant past, there had to be thermal radiation still around from that time. The one-time rival explanation for an expanding universe, the steady state model, does not predict the CMBR, so its existence becomes a vexing problem under that model. Furthermore, steady state cosmology can't provide a similar successful prediction. As a result, the number of working physicists that pursue steady state instead of big bang cosmology, if not zero has become vanishingly small.

Science in general is not a set of isolated hypotheses that stand or fall on their own. Instead, things are interconnected. Many questions are far from settled, but whatever the answer, it will be tied to a web of other results
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Re: AMA about Physics

Postby Burning » Wed Nov 12, 2014 11:11 pm

I'm not good at brevity. :lol:
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Re: AMA about Physics

Postby tombombodil » Wed Nov 12, 2014 11:46 pm

Burning wrote:I'm not good at brevity. :lol:


Neither am I, and I get more out of long-winded explanations anyway. Thanks a ton for that post! It's given me a lot to chew on :thumbup:

Another thing I thought of (in terms of theories and skepticism etc) is that something can be far from "set in stone" (the way I would consider Newtonian physics or the Pythagorean theorem to be "set in stone") can still be experimentally valid and more importantly, practically useful. For example non-real numbers are pretty esoteric to most people and didn't even get accepted as worth consideration by academics until a few hundred years ago, but mechanical and civil engineers use them all the time to allow them to build physical things that we walk on/in.

It could follow that a theory/model that later proves to be off the mark, could in the present be somewhat useful for a number of practical applications, not to mention being a necessary stepping stone to a more accurate theory.
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Re: AMA about Physics

Postby Citizen Joe » Thu Nov 13, 2014 7:56 am

You are a science nerd and somebody asked about your passion. Brevity would be like a supermodel asking a gamer about his 17th level paladin and expecting him to not go off on some mind boggling gamer story.


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