Ted Bunn’s Blog

Ken Ganga, a member of the Planck satellite collaboration, has some nice pictures of the launch on his blog.  (Not exactly breaking news, but I just found out about these pictures.)  There’s also a story about a potentially fatal problem with the satellite that was caught just barely before launch.

By the way, in addition to his Planck blog, Ken has a personal blog, mostly about funny things he’s found while living as an American expatriate in Paris.

Some time back in the’90’s I wrote a document explaining some things about black holes.  To my amazement, people still read it, and they occasionally send me questions as a result.  I’m happy to answer these when I can, and as long as I’m answering them anyway, I might as well post them here.

The latest is from Chris Warring:

My friend and I are having a debate over the question “If the Sun turned into a black hole, what would happen to the Earth’s orbit?”

I quoted from your article http://cosmology.berkeley.edu/Education/BHfaq.html  “What if the Sun *did* become a black hole for some reason? The Earth and the other planets would not get sucked into the black hole; they would keep on orbiting in exactly the same paths they follow right now….a black hole’s gravity is no stronger than that of any other object of the same mass.”

My friend argued that since astroids impact the Sun then they would also impact the black hole.  This would eventually increase the mass, increase the gravitational pull on the Earth, and place the Earth on a decaying orbit.

I have since read a little on Hawking Radiation, and that black holes evaporate.  I now wonder if the black hole that was our Sun would evaporate, losing gravitational effects on the Earth, and the Earth would end up drifting away from where our Sun use to be.

Here’s my answer:

First, let me say that all of the effects you mention are very small. They would alter the Earth’s orbit a little bit over very long times. When I wrote what I did about the Earth’s orbit, I wasn’t considering such tiny effects. But they’re fun to think about, so here goes.

It is true that, if the mass of the Sun (or black hole, whichever is at the center of the Solar System) goes up, then the Earth’s orbit will be affected. Specifically, it would move to a smaller orbit. And of course the reverse is true if the mass goes down.

First, let’s talk about what’s happening right now, and then consider what happens if the Sun turned into a black hole. Right now, things do crash into the Sun from time to time, increasing the mass of the Sun. On the other hand, there’s constant evaporation from the Sun’s atmosphere (as well as energy escaping in the form of sunlight, which translates into a mass loss via E = mc²). I’m pretty sure that the net effect right
now is that the Sun is gradually losing mass. Taken in isolation, this mass change would cause the Earth to drift gradually into a larger orbit.

That phrase “Taken in isolation” is important. There are other things that affect Earth’s orbit much more than this tiny mass loss rate. The main one is gravitational tugs from other planets, especially Jupiter. I
guess it must be true that the gradual mass loss of the Sun gradually makes all of the planets drift further out, although the details might be complicated.

There’s also the fact that the Earth is being bombarded by meteors. Those presumably slow the Earth down in its orbit. Taken in isolation, that effect would make the Earth spiral in towards the Sun.

I’ve never tried to work out the size of any of these effects. A lot is known about the effects of other planets’ gravitation on our orbit (the buzzword for this being Milankovich cycles). The other effects are much smaller.

Now, what would happen if the Sun became a black hole? Things like meteors would still get absorbed from time to time, but much less often than they do now. That may go against intuition, because we think of black holes as really good at sucking things in, but in fact the black hole has the same gravitational pull as the Sun on objects far away, and it’s a much smaller target, so fewer things actually hit it. So the rate
of mass increase due to stuff falling in will be less than it is now. On the other hand, stuff won’t be evaporating nearly as fast as it does now. (There would be Hawking radiation, but that’s incredibly small, much less than the rate at which atoms are boiling off the Sun now.) So the net effect would certainly be that the black hole would gradually go up in mass, whereas the Sun gradually goes down. The net result would be that the Earth would gradually get closer to the black hole.

But again, the key word is “gradually”: these are really really tiny effects. I’d bet that they’d be too small to have any noticeable effect even over the age of the Universe.

My friend Tim asked me this question:

What do you think are the chances that we’ll detect (not necessarily physically encounter, but detect) life on another planet by the end of the century?

I think the odds are quite good, actually.

First, here’s something that I’m pretty confident is true: Within a few decades, we will have figured out how to measure the chemical composition of the atmospheres of other planets.  We’re moving fast in that direction right now, and while it’s a hard technical problem, I don’t see any show-stopping reasons why we can’t do it.  Basically, you have to have telescopes with sharp enough resolution to see the planet separately from its star, and then you just do spectroscopy.

I’ll be very surprised if we haven’t done this to hundreds and hundreds of planets within the next few decades.  We’ll know what molecules are in the atmospheres of those planets.  That means that we’ll detect life if a couple of conditions are satisfied:

1. Extraterrestrial life is not very rare.
2. Extraterrestrial life leaves identifiable chemical signatures in the atmospheres of host planets.

That’s as much as I can say with confidence.  From here on it’s guesswork.  Regarding #2, one important question is what would count as an identifiable signature.  People will naturally look at first for the chemicals that we find in our own atmosphere but that would not be there if there weren’t life.  I think that plain old oxygen (O₂) is one of the main ones here: the oxygen would all be in other forms such as CO₂ if it weren’t constantly replenished by biological processes.  I have no idea whether extraterrestrial life will be based on similar chemistry to ours, so maybe O₂ won’t be the signature we’ll see.  But it does seem likely to me that, if a planet has life on it, there’ll be molecules in its atmosphere that you wouldn’t expect to see in a dead planet, and once we get good at doing spectroscopy, we’ll find them if they’re there.  So I’m not too worried about #2.

#1 is the one nobody knows about.  Is extraterrestrial life found on lots of planets, or is it a one-in-a-trillion shot?  Here, you just have to make your best guess.  Personally, I don’t think it’s likely to be incredibly rare, so once we’re mass-producing spectroscopy of other planets, we’ve got a good shot at finding it.  But that claim is based on no data — it’s a Bayesian prior probability — so feel free to disbelieve me.

I think this life is far more likely to be simple microbes than big intelligent things.  I doubt we’ll be hearing messages from ET any time soon.  That doesn’t mean that I think searches for intelligent life like SETI are a bad idea, though: they’re quite cheap compared to lots of scientific research, and the payoff if they succeed is so huge that I think it’s worth throwing a little bit of resources their way, despite the long odds.

President Obama’s commission to examine possible options for the future of human space flight is getting ready to issue its final report.  They are apparently discussing seven different possible options, some that involve going to Mars, and some that don’t.  There was an interesting report in Nature last week about a recent public meeting held to discuss the various options. (Thanks to my brother Andy for pointing this out.)

Of course, I’m most interested in the implications for science, so this caught my eye:

The panel plans to cost out the scenarios by next week, and also to assess the benefits of each for 12 key areas.

One of those areas is the potential to gain scientific knowledge from each strategy, says panel member and astrophysicist Christopher Chyba, of Princeton University in New Jersey.

To that end, yesterday’s meeting was mostly devoted to presentations from scientists representing four communities supported by NASA: Earth sciences, space-borne biological and physical science, astrophysics and planetary science.

So what did the scientists have to say? Well, according to Nature one of them didn’t have much of a case to make:

Anthony Janetos, representing Earth sciences, was hard-pressed to find an example. The director of the Joint Global Change Research Institute in College Park, Maryland, Janetos hedged when panel member and former astronaut Leroy Chiao asked if the thousands of pictures he took during shuttle flights were really all that useful. Janetos said they were “marginally” useful.

The others seem to have more sanguine views of the potential for getting science from human space flight.  The astronomer Marcia Rieke naturally and correctly pointed to the Hubble Space Telescope, which has been incredibly productive and has always depended on humans in space for support.  Planetary scientist Steven Squyres says there’d be a big scientific payoff from sending humans to Mars, comparing a human mission to the Spirit and Opportunity rovers:

He said that astronauts on Mars could do in a minute what his rovers averaged in a day, and pointed out that Spirit and Opportunity had covered less ground during their entire mission than Apollo astronauts in a lunar rover were able to travel in a day.

Of course, the fair comparison is between what humans could do and what a robotic mission could do if it had the same budget as a human mission (i.e., thousands of times what was spent on Spirit and Opportunity).  I doubt very much that the humans would win out in that comparison.

I doubt that you can ever justify sending humans into space on scientific grounds.  But that’s not and never has been the reason we send humans into space.  If we send humans to Mars, it’ll be for the intrinsic awesomeness of the achievement.  Personally, I don’t think that awesomeness is worth the price at the moment.  I think if we’re going to spend upwards of $10¹¹ on engineering and R&D, it should be on a massive investment in energy technology for Earth.  If we do send humans to Mars, of course, I reserve the right to think it’s awesome and to be excited about it.

By the way, one of the Augustine panel’s seven options is particularly baffling to me:  “what the members called the “flexible path,” which would avoid the “deep gravity wells” of the Moon and Mars, saving the time and cost of developing landers to carry astronauts to the surfaces of those bodies.”

A flyby of the moon might be followed by more distant trips to so-called Lagrange points, first to the location where the gravity of the Moon and the Earth gravity cancel each other out, then to where the gravity of the Earth and Sun cancel out. There could also be visits to asteroids or flybys of Mars leading to landings on one or both of the low-gravity moons of Deimos and Phobos.

This seems to me to have most of the disadvantages of human space flight but to cut way, way back on the advantages, i.e., both the scientific payoff and the intrinsic awesomeness.

Our department just took delivery of a new 14″ telescope, to be used for classes, student projects, and public observing nights:

As you can see, it’s not in the  best possible observing location at the moment.  Plans are in motion to give it a permanent home on the roof of our building.

Thanks a lot to Dean Newcomb for buying us this!

Successful launch yesterday.

Like most academics, I obsessively keep track of who’s citing my work.  As a result, this paper caught my eye today.  (If that link doesn’t work, try this one.)  The lead author is a UR alumnus and winner of both of the physics departments main awards in his senior year.  During my first year here, I taught him in an independent study course on relativity.  He went off to graduate school in mathematics, but he later saw the light and came back to physics.

I haven’t read the paper in detail yet, but from the abstract it looks like a very nice piece of work (in addition to having the good taste to cite me). Congratulations, Andrew!

NASA successfully launched the Kepler satellite, which will spend 3-4 years surveying nearby stars to look for Earthlike planets.  We’ve discovered lots of giant planets so far, but we know relatively little about how common smaller planets like ours are.  Assuming that life elsewhere is most likely to have evolved in environments similar to our own (a reasonable guess, although it’s important to bear in mind that we don’t really know it’s right), this is obviously a really important piece of information to acquire.

Alicia Soderberg is the winner of the 2009 Annie Jump Cannon award.  This very prestigious award is given by the American Astronomical Society to a female astronomer, at most 5 years post-Ph.D., for “outstanding research and promise for future research.”

I taught Alicia when she was an undergraduate at Bates College.  In fact, I think I was nominally the advisor for her senior thesis or something.  I say “nominally,” because I really had little or nothing to do with it: her thesis work was with a group at Harvard, and she  really worked with them, not me.   (Even so, I’m a bit embarrassed that I can’t remember for sure whether I was her advisor: I don’t think that there are any other students about whom I’m unsure whether I was  their research advisor!)

By the way, if you don’t know who Annie Jump Cannon was, you should read about her.  She did very important work in astronomy, figuring out the classification of stars that is still used today, at a time when women were largely excluded from science. Despite the importance of hr work, she was denied full recognition, only receiving a proper academic appointment very late in her career.

This news is a few days old, but in case you haven’t heard, John McCain has apparently been making fun of various earmarks in the current spending  bill on his Twitter feed.  One recent tweet:

$2 million “for the promotion of astronomy” in Hawaii - because nothing says new jobs for average Americans like investing in astronomy.

I agree that that earmarks are a lousy way to allocate funding.  But it really bothers me that complaints about earmarks so often take the form of contemptuous mocking of science.  For the record, astronomy jobs are jobs, and astronomy is a significant industry in Hawaii.  Yes, the average American doesn’t work in the astronomy industry, but the average American doesn’t work in, say, road construction either. 

This post from the Cosmic Variance blog (a very good science blog, by the way) says it all much better than I can.