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Cosmic Antimatter Excess Confirmed

dmartin Re:Dark matter or antimatter? (113 comments)

Some dark matter candidates are, according to theory, their own anti-particle. The only reason it is not a more explode-y space is that dark matter interacts very weakly with other matter, including itself, and therefore has not been identified yet.

That would make dark matter very lonely. If it interacts weakly, wouldn't there need to be more of it to account for the effects that dark matter was invented to explain?

Gravity is incredibly weak for individual particles. The reason we notice it in everyday life is because there are a lot of particles in the Earth pulling us the same way and all those little bits add up. This is the bit that we rely on to explain the galactic rotation curves (and to explain the cold spots in the CMB). If the dark matter only interacted gravitationally then it would almost impossible for us to make any direct detection of this sort (but it is also difficult to explain how so much of it was produced).

The idea of the WIMP is that the dark matter, in addition to interacting gravitationally, also interacts via another force called the weak force. While these interactions would have to be somewhat small so that the dark matter did not all explode, or collide too much with itself, it would still be much much stronger than the gravitational interactions on a per particle basis -- but would not "add up" the same way. [As a very simple analogy, the electric forces between protons and electrons are very strong compared to their gravitational attraction, but on large distances matter is almost neutral because opposite charges attract]

This idea is appealing to physicists because
    1) if true, we have hope of detecting the dark matter and verifying its existence and
    2) it tells us (very broadly) that we would produce the right amount of dark matter as the universe was cooling (the so-called WIMP miracle)

more than 3 years ago

See a Supernova From Your Backyard

dmartin Re:Discovered within hours of its explosion? (182 comments)

I didn't make the unqualified statement that only that particular frame was the correct one. My qualified statement was that this was the correct statement in terms of reference frames. That is, if you wanted to use relativity to make the statement in the article correct this is how you would do it.

Since one of the two statements I am making is that the majority of people who are making comments about relativity don't understand it, the "particularly" refers to people thinking that picking a different origin significantly alters the time taken [for these purposes the relatively small GR effect of the Earth's gravitational well makes near no difference]. For the people who say that it has been around 21 million years (implicitly assuming a frame that is comoving with the Earth) are indeed correct, but unless they were mentioning relativity somewhere else in their post they have given no evidence that they do or do not understand how time dilation works.

more than 3 years ago

See a Supernova From Your Backyard

dmartin Re:Discovered within hours of its explosion? (182 comments)

And by the way, if you think I'm being pedantic, I'm not. I'm objecting to your implication that anyone who didn't make that specific useless statement obviously doesn't understand relativity.

That is not the statement that I am making. The statement that I am making is that of the people that make statements about reference frames, the majority are incorrect, particularly those that make the statement that it has only been a couple of hours in the Earth's frame.

I am ambivalent about people who took the statement at the intended reading "discovered within hours of the first light rays arriving at Earth" as they have not demonstrated a lack of knowledge about relativity.

more than 3 years ago

See a Supernova From Your Backyard

dmartin Re:Discovered within hours of its explosion? (182 comments)

Except we don't.

Many of the discussions are saying that an object 21 million years ago had to have blown up that long ago. There are even discussions of "there is no reference frame for time", suggesting that they are just using Galiean relativity. There are many other discussions pointing out incorrectly that in Earth's frame it was discovered within hours. The reason this is incorrect is that (to a good approximation) the star and Earth are not in relative motion, they are just separated by a considerable distance.

There have just been a couple of people pointing out the correct statement in terms of reference frames, namely that someone traveling at nearly the speed of light and following the beam would have experienced relatively little time. There are others that point out the statement actually means we detected the supernova within hours of the first rays of light reaching us.

I think the real reason people get infuriated with "nerds" is that they spend so much time insisting that they are right, being pedantic about the details while still harboring many misconceptions -- then getting defensive when these misconceptions are bought to their attention.

more than 3 years ago

Spammers Bribe Russian Officials

dmartin Re:So it's like America (83 comments)

America isn't a country on the globe?

Do you mean that America is not special in being like Russia in this regard?

more than 3 years ago

Has LHC Seen a Hint of the Higgs?

dmartin Re:Electromagnetic and electroweak (96 comments)

The electromagnetic and weak forces are combined into the electroweak force, but the theory predicts that in order to combine them (and be compatible with experiments) that you have to have an extra particle or particles to break the electroweak symmetry. That is the role of the Higgs. So our current theory of an electroweak description is accurate assuming that the Higgs or something that plays its role exists.

If there was no Higgs (or replacement) then we have a theory that still works phenomologically, but does not combine the electric and weak forces. Instead it has a bnuch of couplings that take very specific values *as if* the forces were combined.

more than 3 years ago

Running Your Own Ghost Investigation?

dmartin Re:wow (810 comments)

Just to bolster the point that it does not do to be too narrow-minded, alchemy is in fact real. It is just that nowadays it goes under the much more respectable name "nuclear physics".

Although in fairness the people studying nuclear physics were interested in the pursuit of things unknown and ended up discovering that you could change one element into another, rather than being the (N+1)-th group to attempt to change one element into another by mixing various elements.

about 4 years ago

Mathematics As the Most Misunderstood Subject

dmartin Re:he's right (680 comments)

A Ph.D. tells you nothing except that the holder did some original research at an early point in their career.

There is also little if any correlation in being able to research, and being able to teach. Culturally, "everyone knows" the purpose of a phd is to become a professor and teach university students while collecting a $100K+ salary. The upper 50% to 10% cream of the crop actually get hired to do that. So, pretty much by definition, as a general cross section of the population, they are in the bottom of the barrel of teaching ability. So I'd be expecting, unless they're education phds, they're almost by definition probably not going to be good teachers.

[Emphasis added]

Sorry, by definition of what exactly? You assert that the upper 50% to 10% cream of the crop actually get hired [to teach university students while collecting $100K+]. Unless you are claiming that your criteria for determining the cream of the crop includes poor teaching ability (which I doubt you are) there is nothing by definition about your assertion that these people must be poorer than average teachers. I am guessing that you implicitly meant that cream of the crop referred to the research ability, and are then asserting (without proof or references) that research ability and teaching ability have a strong negative correlation. Which is particularly odd given your first sentence: the claim that there is little if any correlation between the two!

To put the argument more formally
    Axiom #1) A Ph. D. tells you nothing except the holder did some original research [quoted from parent]
    Axiom #2) There is little if any correlation between research ability and teaching ability [your first sentence]
Your argument is that by definition [sic] we can put "someone has done research" together with "weak to no correlation between research and teaching" to conclude that Ph. D.s have the "bottom of the barrel teaching ability".

Your argument is contradictory. Your claim (Axiom #2) would lead us to say that we know nothing about the teaching ability of PhDs solely based on the fact they can do research. Maybe you meant that there was a correlation but that it was negative? In which case all you are doing is asserting a very strong (and certainly not by definition) statement into your argument.

If you are actually interested in looking at the correlation between research strength and teaching strength (something which is not captured soley by the definition, which is why they do research on it rather than just a formal proof) you could do worse than starting here:

more than 4 years ago

Satellites Spy On Black Friday Shoppers

dmartin Re:One more reason (171 comments)


If you just go to the first part of what he said - you can get more stuff for the same amount of money if you buy on a date that is slightly later than the date everyone else uses - it is completely rational. There is artificial demand in the market because everyone is attempting to do the same thing. It is perfectly rational to delay a few days to get more stuff -- or to spend less on Christmas and more on the kids on small things for the rest of the year. The *only* reason that the 25th is special is because religious people celebrate that particular day. It doesn't (and arguably shouldn't) have to occur on the same day as "present day". I see this as a reason to be mildly envious of his kids: they are learning rational behavior and budgeting skills.

As Roland went on to say, they found after trying this that the presents didn't actually matter. If that is a decision they have reached collectively then why should they care what some random stranger such as yourself may think? Not to be rude, but I think that I would worry about your kids more, feeling they have to please and conform.

more than 4 years ago

NASA Announces Discovery of 30-Year-Old Black Hole

dmartin Re:Bad Astronomy? (195 comments)

It is correct to say that an accretion disk can form around a neutron star as well.

The distinguishing characteristic is that a neutron star bigger than its Schwarzschild radius. Not just a little bit bigger, but at least 11% bigger [see the Buchdahl-Bondi limit; this a theoretical limit for any perfect fluid spheres -- actual neutron stars don't come close to saturating that bound]. This means that the accretion of charged particles that is spiraling inward will eventually hit the surface, stopping the charged particles very rapidly. The radiation from suddenly stopping charged particles (Bremsstrahlung) is fairly distinctive, and is not seen here.

By contrast, an accretion disk around a black hole loses energy and eventually passes through the horizon. There is no sudden breaking and hence no Bremsstrahlung radiation It is the accretion disk and the lack of Bremsstrahlung that convinces us that the most likely candidate is a black hole.

[The reason the size limit was important is that as you get close to the horizon, redshifting makes things harder to see anyway. The point of the Buchdahl-Bondi theorem is that any perfect fluid sphere has to be about 11% bigger than the size of a black hole of equivalent mass. This limits the total redshift due to the object to a modest factor of 2, ensuring for a large class of matter (including neutron stars and all known matter to date) that the collision with the surface if it existed would be visible. This does not prevent unknown matter with exotic properties having s surface that is beyond the event horizon but close enough in the we would not see the Bremsstrahlung radition, but it is very difficult to construct "reasonable" solutions.]

more than 4 years ago

School Swaps Math Textbooks For iPads

dmartin Re:Expensive (439 comments)

I agree that you cannot simply throw money at a problem and hope that it just goes away. However I don't think that comparing public and private schools demonstrates the superiority of the private sector to redistribute wealth, at least not in the way that you think it does.

First, private schools are expensive. The parents sending their children to private school do not get to opt out of the taxes that support the public education system. Instead they are paying the government and the private sector. This shows these parents are willing to support their children's education; and having a home environment that understands the value of an education and is supportive of it is a really big piece of the puzzle.

Secondly the private schools are allowed to be more selective. You could argue, correctly, that this is a demonstration of the private system being more efficient: they will not accept known troublemakers, and discipline can be enforced more rigorously. The public school system is tasked with providing an education for the entire populace on the assumption that a society in which everyone is educated to at least a certain standard is beneficial to all. For example you want people who are voting for president to have the ability to think through the consequences and implications of the policies that person is advocating. We can argue the degree to which the public school system fails at its task, but it is quite different from that of the public school system. If we wanted a fair comparison we could look at research universities (both government and private) and compare them, as in both cases the institutions are trying to produce the best results possible and do not have to try a one-size-fits-all model.

I am certainly not debating that this particular initiative is successful, or that the education system is okay as it stands. However I do want to live in a society of people that have a basic knowledge of history, logic, mathematics and literacy. I think that is worth paying for. In particular your comment

Governments breed waste, inefficiency and tyranny and can never lead to a net gain for society when compared to a private institution.

does not seem to ring true. There is a reason why firefighters are not private institutions, after all!

more than 4 years ago

Quantum Physics For Everybody

dmartin Re:No mathematical background? (145 comments)

But the fact that we can state the Uncertainty principle in a finite dimensional Hilbert space (as you point out) shows that the Uncertainty principle does rely on properties of infinity. It fact in the finite dimensional case it becomes somewhat easier to understand what is going on. Take the spin-1/2 system which is two dimensional. The eigenvectors any of the operators s_x, s_y or s_z form a basis for the state, however each operator's eigenbasis is not parallel to any other operator's eigenbasis. A vector which "lines up" like (1,0) and (0,1) in one basis cannot in the other two which we can draw on a sheet of paper (and remind students about breaking things into components).

In infinite dimensional cases things are more complicated because there are various subtitles that can arise. But these subtitles are not at the core of the uncertainty principle, merely a technical distraction that needs to be addressed.

And I really don't understand this statement:
you need the features of Hilbert spaces that are unlike Euclidean spaces.
All finite dimensional Euclidean spaces, for which we have a reasonable intuition, are Hilbert spaces. In the infinite dimensional case Hilbert spaces are defined to carry over the properties of Euclidean space while eliminating some of the perverse things that can happen in infinite cases (i.e. ensuring Cauchy sequences have limits in the space).

more than 4 years ago

Metrics Mania and the Countless Counting Problem

dmartin Re:Technically (138 comments)

It's an allusion!

("Final countdown" starts playing in the background)

more than 4 years ago

Interstellar Hydrogen Prevents Light-Speed Travel?

dmartin Re:Damn it, now they tell me (546 comments)

It really depends on what you think is relevant. For example if the purpose is to do research for people on Earth, you probably are interested (at least in part) in the time taken for the round trip, and how long people on Earth have to wait to see the benefits of their investment. If you are looking at colonization then you are probably more interested in the amount of time as experienced by the people travelling on the ship. In this case the difference between 99.9% of the speed of light, and 99.99% of the speed of light is significant.

To make the example concrete, let us take your example of Alpha Centuri:
Distance: ~ 4 light-years.

  • 99.9% of the speed of light:
    Time (Earth observer): 4 years and 1.5 days
    Gamma factor*: 22.4
    Time (Ship observer): 65 days
  • 99.99% of the speed of light:
    Time (Earth observer): 4 years and part of a day.
    Gamma factor*: 70.7
    Time (Ship observer): 20.5 days.

So from the point of view of the *crew* the journey takes about a third the time, although from Earth you are correct in stating they are essentially the same.

* The gamma factor, or time dilation factor (or length contraction factor), is given by special relativity. If you speed is v and the speed of light is c then
Gamma factor = 1/sqrt(1-(v/c)^2)

more than 4 years ago

Best estimate of monthly spending on food:shelter

dmartin Re:1:1 (582 comments)

Zero is not a number, it is a concept.

We could argue about whether or not numbers are anything other than concepts, but zero is a number. It is as much of a number as 1, 2, 3, ......

One of the big hangups in early mathematics was that they were confused about the very thing you are: namely there is a difference between 0 and non-existence. I can ask what speed an object at rest has, and the answer is 0 (in whichever units you want). The answer is not "it does not have one" or that the speed does not exist. If you try doing velocity addition, or momentum conservation etc on an object at rest you will see that you do need to stick in '0' for the velocity.

(You may object -- there is no difference between an object not having momentum and thus not counting it, or having zero momentum. The easiest way to see a difference is to go to a difference reference frame -- then the momentums all transform. It would not make sense for a non-existant momentum to transform).

The real problem with division by zero is that
    0 x = 0
for any x. If we just have access to the RHS, we have no information about x, so asking "what do we need to multiply 0 by in order to get zero" the answer is "anything you want".

As to the question of whether or not infinity is a number, or simply a limiting process depends. There are not just one set of numbers, and you can actually define infinity to be a point, or simply a limit. (There are even multiple ways of "adding infinity" the most common of which in complex anaylsis is to add a "point at infinity" in which case -inf and +inf are the same thing.)

But it is true the most common way of dealing with numbers is to treat infinity as a limit.

about 5 years ago

The LHC, Black Holes, and the Law

dmartin Re:US LAW ? (467 comments)

Who cares what the American law says ? Its built by CERN, its in the France-Switzerland border ...

Followed by

Whoa there bucko. Sweden is next to France?!

No, but Switzerland is .......

Map of France/Switzerland
Wikipedia entry on Sweden.
Wikipedia entry on Switzerland.

about 5 years ago

Girl Gamers More Hardcore Than Guys

dmartin Re:REJECTED!!! (284 comments)

True, but your argument then is "by definition" (of a gamer girl) and your experience is not relevant.

The only thing you know "from experience" is that there is a gamer girl may in fact want you to wake up at 6 am for raids, etc.

(Right conclusion, wrong argument)

more than 5 years ago

"Lawful Spying" Price Lists Leaked

dmartin Re:Tempest in a tea cup (245 comments)

The really interesting part about National Security Letters is that they're fairly obviously unconsitutional, but were designed in such a way that the judiciary would never rule on their constitutionality. By making it a crime to reveal that you've received an NSL, you make it impossible for anyone to demonstrate that it existed in the first place, and thus prevent anyone who was targeted by them to establish standing to sue. So if someone tries to challenge it, the executive branch can argue correctly "You can't prove an NSL existed, therefor you can't prove you were harmed by NSLs, therefor you have no reason to sue".

It seems really easy to sidestep this. Take the NSL to a judge, or use it as evidence to sue. If they come after you for revealing the existence of an NSL there is your proof that it has impacted you and you have standing. If the courts rule that states secrets are justified, and that your action was indeed illegal then you are basically in trouble -- you have admitted blatantly violating the law and will probably be imprisoned. But if you should win and you can have it ruled the law was unconstitutional then the law you violated has no power anyway (the constitution in the US granting the government limited powers). So challenging it is risky, and would take someone with very strong principles (and a strong stomach) to see it through, but it is not the Catch-22 you make it out to be.

more than 5 years ago

German President Refuses To Sign Censorship Law

dmartin Re:What the? (272 comments)

which itself (unlike the electors in the US) is NOT elected by the people, but nominated by parties in the Parliament (Bundestag)

Actually the president is technically elected by the electoral college. [snip]

Right, which is exactly what the other poster said: the electors in the US system are directly elected by the people, the electors in the German system are not.

more than 5 years ago

German President Refuses To Sign Censorship Law

dmartin Re:What the? (272 comments)

I don't know about Germany, but in New Zealand we have a very similar way of voting in our members of Parliament so I will take a stab at why you would not want your president to be able to veto a law thing.

Unlike the US, we do not directly elect our head of state (I presume this is also true for Germany as well). Instead we vote for the party that we want to be in power, based on their policies and the party appoints a PM. This is actually quite common in many places, and it means that the Prime Minister can change inside a term. e.g. The United Kingdom had a PM change from Blair to Brown without an election. In the US if the president was to resign, the VP would become the president, it cannot be reassigned based on party politics. In practice the choice for PM is announced before the election, so many people do vote based on who they want their prime minister to be.

[The US is even stranger here, as you get the right to vote for your sentators, representatives and your electoral college member, but that is a whole different digression.]

The prime minister does have a fair amount of power, and does a bunch of figure head stuff (negotiate treaties, etc). But as it is not an elected position, the PM has fairly limited legislative power. The idea of one person vetoing a law that the other democratically elected MPs voted for would not be accepted, the PM already has a fair amount of unofficial power in the form of increased media time, and influence over the majority collation at the time. The fact that Germany has a system where the PM can overrule a law that violates the constitution is, in my opinion, a good thing.

[The closest NZ has to this is the governer general -- as a member of the commonwealth our official head of state is the Queen of the Commonwealth. She appoints the GG who then approves laws in her place. The GG could, in principle, turn down any law for any reason but that would quickly turn public opinion against being part of the commonwealth and would probably make NZ reconsider its position within the commonwealth.]

In contrast, ignoring the issue of the congressional college, the US populace votes directly for the position of president on the understanding that this one position will have a lot of legislative power in the form of vetos. Whether that is too much of a concentration of power for a single individual is up to you to decide, but at least it is an elected position. Ignoring our governor general (who theoretically has a lot of power, but would lose it is she ever tried to yield it) our system does not have as much power with a single person, and our elections for (psuedo-)head of state tend to be much more civil that the USA counterparts.

Hope this helped explain the origins / reasons for the differences!

more than 5 years ago


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