Student Questions and Scientist Responses:

1. Devin
2. George
3. Brian
4. Emilio
5. Brianna
   

Question 1:
Hello Professor Bob Mathieu,
My name is Devin ----. Two years ago you answered several questions sent to you by Mr. Battagliese's seventh-grade science class in Memorial Middle School, in South Portland, Maine. This year a select five (me being one) were picked to chose one of those questions and teach the class about it. The question I picked was: If a black hole is impossible to see, how do astronomers find them? I read through the answers that astronomers sent, and
thought yours was a very good, understandable, response. That is why I decided to e-mail you and expand
on some ideas. Here are some questions I have:

• Is gravitational lensing another way to find a black hole?
  If it is an alternative to find black holes, how does it work?
• How high can the velocity of a star orbiting a black hole
  be?
• What else could the object be?

If you could answer some of these questions it would be very helpful to my project. There is no pressure if you choose not to answer these questions. Thanks so much for reading!

Sincerely,
Devin ----

Answer

Hello Devin,

Good to hear from you. I'll try to answer your excellent questions below, and if you have any more or want to follow up, feel free. Good luck on your project!

Bob Mathieu

Is gravitational lensing another way to find a black hole? If it is an alternative to find black holes, how does it work?

Yes indeed, although it is a bit difficult to explain. Einstein's General Theory of Relativity said that the existence of a mass (say, the Sun) would curve the space around it; one manifestation of such curved space would be every-day gravity. More profoundly, Einstein predicted that a light ray would follow the curved path of space around a mass, whereas Newtonian physics would have predicted that a light ray would follow a straight path regardless of the presence of mass. Our observations of gravitationally lensed galaxies is one of the neatest proofs that Einstein was right. In answer to your question, if a black hole happens to pass between us and a distant star, the light from the star follows the steep curvature of space in the vicinity of the black hole, and thus the trajectory of a light ray is curved around the black hole.

That's the easy part! The harder part to explain is that it turns out that from our vantage point the total light from the star that we see increases ... in other words, the star appears to brighten as the black hole passes near to our line of sight to the star. Remember, the star itself does not actually brighten - it only appears to do so because we see more light from the star then we would in the absence of the black hole.

There have been several major observational projects, for example one called MACHO and one called OGLE, which continually monitor the brightnesses of millions of stars in selected fields of view in our galaxy or in the Magellanic Clouds. Put simply, they are looking for events of stars brightening (only once per star!) that would be the signature of a black hole passing near our line of sight to a star. The number of such events (which these projects *have* found) allows us to place a limit on the number of free-floating black holes in our galaxy, and thus their fractional contribution to the dark matter that we know our galaxy contains. (Turns out to be small - the dark matter is not black holes - but nonetheless it is exciting that they are out there free-floating throughout the galaxy.)

How high can the velocity of a star orbiting a black hole be?

Many hundreds of kilometers per second.

What else could the object be if it is not a black hole?

Well, that's really the point. Since we don't know of anything else that could be as massive but not emit light, we are left with the identification of the object as a black hole. The existence of X-ray emission from gas very rapidly orbiting the black hole and heating to temperatures of millions of degrees due to "friction" also points to the objects being black holes.
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Question #2

Dear Dr. -----,
I was hoping you could help me with my astronomy project. One of the questions originally asked of you was 'What are some of the arguments regarding the existence of aliens?' and I will ask you to expand on your response.

1. You said the universe is infinite. If the universe is infinite, how can it be expanding? If it is infinite, can it not get any bigger?
2. You mentioned that aliens may exist outside our mutual horizons. What do you mean
   by 'mutual horizons'? Do you mean other dimensions or parallel universes?
3. Do you think aliens ( if they exist) would ever feel the need to visit Earth?

Your response is appreciated.
Sincerely,
George -----

Answer

Some musings ...

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Question #3

Dear Dr. Saffer

Hello, my name is Brian -----. I am from Memorial Middle School in Maine and wanted to ask you a question for my science project on the Big Bang.

I wanted you to further explain your response on #2 of the survey, which was "What Caused the Explosion known as the Big Bang".

You stated that the universe could have started as supercooled liquid. How can stars emerge from frozen matter? Also, do astronomers believe remnants of the supercooled liquid still exist in its original form? I have one last question. How much space would this supercooled liquid have taken up, compared to our universe today?
Thanks,
Brian -----

Answer

Hi Brian,

Good questions!

The universe could have started as supercooled liquid... How can stars emerge from frozen matter?

My statement was meant as an analogy - this is like that... - not as an exact description. Here's how the analogy goes:

You can take a sample of extremely pure water, and VERY SLOWLY cool it down until it is actually below the freezing point, and it can remain in the liquid state. It turns out that the freezing process usually starts at a place in the water where there is an impurity, or an imperfection, or a disturbance. In this case, a speck of dust would do the trick, or even if you reached in and tapped or stirred the water. The water would then freeze very rapidly, proceeding outward from the impurity or disturbance. We also refer to this as a "change of state." When water freezes, it releases a great deal of heat, this is called the latent heat of crystalization.

The state of the universe just prior to the Big Bang might have been LIKE that, but not EXACTLY that. It might have been in a state of energy like that of the supercooled water, below that where space and time would appear as we see them today, but not yet "frozen out". A disturbance of some kind, what kind we do not know, could have initiated the "state change" from the universe's previous characteristics to the ones that existed after the Big Bang.

Just keep in mind, it's a lot more complicated than I have described above, and the water analogy is just something we use to get a general mental picture of the process.

Also, do astronomers believe remnants of the supercooled liquid still exist in its original form?

Actually, we do think there might be "pieces" of this previous state in the universe today, but no one knows what they might look like. We certainly haven't run into any of them to date.

I have one last question. How much space would this supercooled liquid have taken up, compared to our universe today?

I don't know. Consider that right after the Big Bang, the whole universe was smaller than the size of an electron - MUCH, MUCH smaller.

Best wishes,
Dr. Saffer
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Question #4

Dr. -----,
How is light affected as it passes through a black hole? Are the other forms of electromagnetic radiation affected in the same way?

Sincerely,
Emilio -----

No Response
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Question #5

Dear Mr. Davis,
Several years ago you answered a survy called "Our We Alone". On question ten, "If scientist
discovered te existence (or past existence) how will our lives change", you provided a link to the S.E.T.I website. My teacher and I have read through the site you gave us and read about your work on the NSS. I have to teach my peers about the subject and was wondering how much more advanced our technology would have to get before we can discovery intelligent life beyond our planet? Also, in the site it say that we have not yet gained evidence but what about the media and stories? How does this effect us?
Thank you for your time,
Brianna -----

Answer

Hi Brianna -

Good questions! Let me answer them in reverse order. You're right; we've not detected intelligent life beyond earth. Stories about UFO's and aliens may come on occasion from well-intentioned persons, but they fail to survive critical questioning, which is an
important part of scientific research.

As to your question about present day SETI technology, it's pretty good stuff, but not perfect. Just as computers of ten years ago seem pretty slow today, detectors of that era are also feeling their age. We just retired the detectors first used about ten years ago, in favor of the more recent NSS (New Search System) detectors.

This could lead a pessimist to suggest that we shouldn't do any SETI observing -- just wait for
detector technology to be perfected before starting. There are at least two things wrong with that suggestion. To begin with, detectors DON'T get better by just sitting around -- we learn a lot about how to make them better by actually using them. And secondly, there may never be a perfect detector -- that's like waiting for the perfect PC before writing your term paper. The technology is 'good enough' (just as the PC is 'good enough) to do a lot of useful observing that hasn't been done before, and which MIGHT turn up evidence of extraterrestrial technology.

Most of us think that detection may someday happen, but we have only very rough ideas of how hard it will be -- it could take centuries, or it could happen tomorrow. My personal feeling is that each engineering generation should take its best shot at doing a creditable, cost-effective search, and pass those results, positive or negative, on to the next generation. In this way the detection will happen as early in human history as possible.

Mike Davis

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