View Full Version : Avoid generalities

2012-Apr-27, 07:56 PM
I think experts are dishonest to use terns such as big, small, fast, slow etc instead of giving us a number with units. I suppose they do this as they can usually show that in some perverted way they are correct if someone challenges them. The alternative is to say all but the smallest galaxies are bigger in volume than the most massive super massive black holes, both of which are big compared to most anything else other than the known universe or a galactic group. That illustrates that we either need numbers or highly qualified statements = qualified generalities. What is the radius of the largest known super massive black hole, event horizon? Neil

2012-Apr-27, 09:52 PM
Do you say "The 1000 cubic metre room" or "The large room"? Do you object that a large room can be smaller than a large house?

Huge numbers are largely meaningless to people - if I said to you the 7e11m star would you intuitively know if that was a large star? How large? These generalities establish a relative size scale within a group of objects. That is all.

2012-Apr-27, 11:45 PM
I'm far from a math whiz, but I picture 10 meters by 10 meters by 10 meters if you tell me 1000 cubic meters. That is an unusually high ceiling, but we can imagine one or both of the other dimentions longer to compensate for the lower ceiling. The main room in the VAB = vehicle assembly building is about 100 times bigger than the 1000 cubic meters. I don't think the largest house is that large, so yes I object.
700 billion meters = 700 million kilometers = I don't think we have found any with that large a diameter about 5 AU, but maybe. Neil

2012-Apr-28, 05:19 AM
Betelgeuse = radius of ~1180 solar radii = 8.2e11m. Which highlights my point. The number was meaningless to you. "a huge star" would have made more sense.

Note - can be. Not is always. Allows the opposite - my point was that that was large for one object can be small for another and we accept that in every day life. When we are talking about things it is easier to use descriptive terms, when we are trying to calculate or do anything involving precision we use the numbers - what is the issue? You seem to want scientists to walk around saying "The 7e8km radius star has a 2.4e6m radius planet, then 2 6e6m radius planets, a 3.4e6 radius planet, then a 7e7 radios one..." - or you could say "The fairly small star has a small rocky planet, then 3 larger rocky planets followed by a huge gas giant..."

You actually need to know more about a class of object to decode numbers into things like large/small. They actually make understanding something harder.

2012-Apr-28, 06:04 AM
What is the radius of the largest known super massive black hole, event horizon? Neil
i) Sgr A* has been measured at ~ 3.7+1.6-1 Rsch(3σ).
ii) 1 Rsch = 0.1 AU, assuming a circular gaussian model, or about ~ 4 solar masses. (Rsch=2GMBH/c2).
iii) The mass density is in excess of 9 x 1022 solar masses per cubic parsec.

(Source: Imaging an Event Horizon submm VLBI of a Supermassive Black Hole, Doeleman, Agol, Backer et al)

Because of its proximity and estimated mass, Sgr A* at the centre of the Milky Way, presents the largest apparent event horizon size of any black hole candidate in the Universe.

Hope this helps.


2012-Apr-28, 02:59 PM

Is this a thread on the language of numbers (in which case it probably belongs in S&T or OTB) or is this a thread on Astronomy? I am unclear about the OP.

2012-Apr-28, 03:25 PM
I usually prefer relative to absolute scaling. 10^16 mass solar provides an immediate scale reference; 10^42 grams is just a big number. Relative numbers also automatically rescale when we figure out we have over or underestimate the mass or volume of everything.

2012-Apr-28, 06:52 PM
There are several gaps in my understanding of the Selfsim example, but 8.2e11 divided by 1.18e3 = 7e8 meters = 700,000 kilometers which is not exactly the solar radius, but it does indicate that the inputed numbers are likely within 10%, which is close enough for most purposes= I can check your numbers for ball park accuracy. Large errors are not unusual on the internet. Neil

2012-Apr-28, 09:51 PM
So far we have been about 80% astronomy, but the discussion could go to advertising which is an even worse offender than astronomy in my opinion. This is not really a fault of astronomers as astronomers have to deal with plank size to billions of light years.
I'm unsure what to do with Jerry's post: Yes, meters, kilograms and seconds are sometimes inconvenient. Most disciplines have dropped grams, but 100,000 units in common use somewhere is even more inconvenient. In my posts, I often translate to two different units, in hopes that one of them is better adapted to the reader's preference. Generalities don't have units and that is my complaint. Neil

2012-Apr-29, 06:32 AM
OK here is a question: is it better to use a generality or to make up numbers which may not be correct, thus giving you a false sense of accuracy that allows you to make a series of increasingly inaccurate predictions and come to a wrong conclusion?