## Friday, July 22, 2011

### "Speed Of Gravity Is 9.8 m/s^2"

OK, if you think I was nitpicking when I criticized the use of the phrase "rate of speed", you'll have an aneurysm with this one! So look away now! :)

I was driving into work this morning, and I happen to be listening to a local station here - 93.9 Lite FM (I know, it makes me sound kinda lame!). Still, the show Valentine in the Morning can be quite funny at times when they discuss certain topics. But this morning, following up on their geography quiz, they had a science quiz with a set of questions related to science. One of the questions that was asked was "What is the speed of gravity?"

Immediately, the host of the show (Valentine) answered "9.8 m/s^2!" And he was deemed by the person asking the question to be CORRECT!

Now, I think I don't need to explain why this is NOT correct, do I? I do? Oh brother!

1. Just looking at the units alone, one can tell something isn't kosher. Speed (and velocity) has units of length/time. The units given "m/s^2" is length/time^2, which is acceleration.

2. 9.8 m/s^2 is the acceleration due to gravity at Earth's sea level. It's a value at a specific location. It means that it isn't a constant, and certainly changes when one is elsewhere. g is different at higher elevation, and even more different on the surface of the moon.

3. As far as we know (and this has yet to be explicitly verified experimentally), the "speed of gravity" is equal to the speed of light. If sun suddenly disappears, we'll now about it not instantaneously, but at the same time that our sky goes dark a few minutes later.

The issue here isn't that the answer is wrong. More importantly, is that while people remember certain information, the CONTEXT of what they remember can be faulty. Facts are useless if they dangle in mid-air and without understanding what they mean and how they are used. Seeing something like this kinda let me to believe that there's a lot more people who think that speed of gravity is 9/8 m/s^2. Could this be that this is another example where what you say (or teach) is not what they understand?

Zz.

IngridJ said...

That´s the big question in education: How do we know what our students learn? By explaining and discussing in different ways? By asking more open questions så the students must explain the central point? But we can never know, can we?
I quote what you ended with in another post: "As we can see here, what goes in does not necessarily be the same thing that sticks and stays in".

Kris said...

As a layman I enjoy reading your blog and learn alot. The one thing from school that stuck with me is that there is no such thing as a dumb question. Let's see how dumb a question this might be. I'm going to ask about the speed of gravity.

Why is it thought to be 'speed of light' and not instantaneous? Imagine an empty region of space with a planet (or station or ship) 30 light minutes away. In the middle of the space a white dwarf suddenly appears. The viewers would not see the light of this star until 30 minutes later. But wouldn't the surrounding space warp immediately due to the star's mass and wouldn't that gravity be felt by the viewers before they see the star?

If this is dumb, please be kind. ;-D Thanks for a great blog and insights!

ZapperZ said...

But the question is, why would such disturbance traveled "instantaneously", even it it is a matter of space-time fluctuating?

There are initial indications that gravity travels at around c. See here:

http://physicsworld.com/cws/article/news/16644

While more confirmation is needed, even if we allow for a large degree of uncertainty, it definitely ruled out anything "instantaneous" here.

Zz.