Wtf?!?

[su_liam]

Could someone explain this?

The Height of a Giraffe

[Jaxilon]

Well, I'm not feeling like reading all of that right now and the math is beyond my current skills.

From the bit I read however I think they are determining the largest size an organism could be based on the laws of physics.

Just like a deep sea creature explodes when you pull them to the surface, or how you would be crushed to death on a high gravity planet I think they are trying to say that based on the components of the universe, ie, atoms, there is a limit to how big a living being can be and still function. Steel, wood, and cement, all used in construction have limits to how much they can hold and my guess is they are saying there is a limit to how much a carbon based life form can take and still run? Thus, I guess a limit to how tall a Giraffe can be?

That's my gut reaction after reading a paragraph and is likely totally wrong. Maybe one of our resident Physicists will come along and open our eyes.

What I really wonder is how much money they got for doing this study and how you came upon it

[su_liam]

The previous estimate was 2.6cm. Strangely this was dated August 3rd, 2009. This seems about four months and two days late.

I found it while trying to suck the internet dry of planetological knowledge. Whilst simultaneously trying to find a job. I'm not sure which endeavor this resulted from...

[SereneParadox]

Those equations are so cool looking. I aspire to be able to translate that to English within 4 years.

[tilt]

giraffe measurer is a job - isn't it
and that was a lot of equations in one text just about passed out starting to read it *lol*, but I believe Jax is right, and yes, I also always wonder where people get funding for their studies... like the guy who studied bra-sizes... which board actually sat down and said: "yep, sounds like a good study - have some money" ... of course the board meeting could have been in a strip club with lots of drinks, who knows? *lol*

[Ascension]

Oh, cool. I've always wanted to study dancing girls and how a culture defines their sexiness. Gimme some money Or I could study how tall I could stack a single chain of atoms one atop another, like this guy is sort of theorizing about.

[ravells]

This stuff will come in handy when we start to bio-engineer new life-forms on a commercial scale and we want to build bio-mechs so we can that that cool SF world we were always promised. It's a bit disappointing that the bio-mechs cannot be more than 3.6 metres tall.

[Steel General]

Looking at all those formulas gave me a headache.

[torstan]

So what's the point of this research? Well, firstly this isn't the primary research of this person so they didn't go to a funding body and ask for funding for this work. They have a position at a university and this is something they decided to use their time to think about. It's a good thing that people have freedom to choose a certain amount of their own research topics.

The second point is that this is actually quite important research from a particular point of view. It's to do with the anthropic principle. So first I'll see if I can cover what the anthropic principle is:

Consider the following. There are very few planets with atmospheres of oxygen and large bodies of water. There are very few planets with life (we only know of one). The chance of either is very low so the chance of both must be extremely low. Say for example that the odds of a planet having a breathable atmosphere were 1 in 10 and the odds of a planet having life was 1 in 10. Then the odds of finding a planet with both a breathable atmosphere and life would be 1 in 100?

The logic here is clearly wrong. Life requires a breathable atmosphere so the two are not independent statements. We don't spend a lot of time and money trying to figure out why we are on earth rather than Mars. We observe a planet with oxygen and water and a reasonable temperature because we need to have those conditions to exist in the first place. This is an example of a very rare observation (a planet with oxygen and water) that we don't need to worry about explaining. This is the anthropic principle. It says that there are some things that have to occur for us to exist and observe them. So the fact that we observe them is no surprise, no matter how rarely they might occur.

Now let's roll this back to physics. There are many things in physics that look to be very finely balanced. There are weird patterns and coincidences. So for example it was a strange coincidence that the number that determines how hard you are to push around (your inertial mass) was exactly the same as the number that determined how hard you are pulled towards the center of the earth (your gravitational mass). This coincidence turned out to be caused by a deep connection between gravity and acceleration that forms the basis of general relativity. So a seeming coincidence is explained by a deep underlying principle. Physics progresses by such steps. We see a coincidence and rather than accepting that it just happens to be the case that two numbers are the same, instead we ask why they are the same. We propose a law that relates them and then we test it. So if we see a coincidence, then we go looking for an underlying law.

How does this relate to the anthropic principle? To answer this we need to talk about a very bright guy called Weinberg. There's a number called the cosmological constant. It's a term in Einstein's field equations that sets out how fast the universe expands. Now it's pretty small, very small in fact. We also know that it's very unlikely indeed for it to be very small. In the early universe it should get big quickly - so it needs to start off almost unimaginably tiny to end up being the size it is today. There's no reason yet why that should have happened. This of course has lead to all sorts of theories about how the cosmological constant could arise from deeper principles that require it to be small. These are certainly valid avenues of research. However Weinberg proposed an alternative solution. He asked what would happen if the cosmological constant did what the theories say it should do. What if it starts off non-zero and then as the universe evolves it gets really large? In that case the universe expands enormously quickly, molecules don't form and large scale objects never exist. In that case observers wouldn't exist and there would be no-one to see the universe. So the cosmological constant has to be small for us to measure it.

Now to complete the argument you need one more thing. You need there to be lots of universes (as predicted by string theory). If you have lots of universes then you have lots of different cosmological constants. In some (very small) fraction of them, the cosmological constant is small enough to allow matter as we know it to form. Now only those universes will have the possibility of having observers in them. Therefore all observers must measure a small cosmological constant. Therefore there is no mystery as to why the constant is measured by us to be small. This is exactly parallel to the argument about why we see a planet around us with a breathable atmosphere. These unlikely circumstances are precisely required for us to exist - so we shouldn't be surprised when we observe them. Instead this is taken by some to be a validation of string theory and its prediction of many (10^1000) different universes that all have different values for the physical constants such as the cosmological constant. Now I'd say that's overstretching things more than a little, but that's the argument.

Okay, so what does this have to do with giraffes?

Well, the argument in that paper derives the height of the giraffe in terms of the proton mass and the ratio of the strength of forces. At the end of the paper he runs the argument in reverse. Say we take our observations of large scale molecular life and ask what this requires the fundamental constants of physics to be. Then he claims that we can pin down many of the fundamental parameters. If they were other than that, then large scale life could not exist. If we couple this with string theories roulette ball of universes where these numbers are different in every universe, only those universes where all these numbers take values in these ranges can accommodate large scale observers like ourselves. An advocate of the anthropic principle would then say that there's no point in looking for a deeper understanding of the root of these values - instead just accept that they are the toss of the dice required for us to measure them.

In my opinion, this is certainly an interesting argument. It's important to have a good idea of what is and is not required for use to exist. For example, some people have tried to claim that the current measurement of the amount of dark matter can be explained by the anthropic principle - but it's not required for the existence of stars, galaxies and planets so there's no way we can explain that number away on these grounds. Therefore it clearly tells us that we should look for a deeper understanding of the dark matter density. However I'd also say that we shouldn't discard a question just because there might be an anthropic explanation. Yes, the cosmological constant might be the way it is due to pure chance. However there may still be a deeper explanation, and if we solve that then we will gain a vastly improved understanding of the fundamental force that drives the evolution of the universe, how it began and how it might end.

So I think that the research into the connection between the height of a giraffe and the mass of a proton is very relevant to what questions we can and should ask in physics but I don't agree with the anthropic principle or the multi-universe explanation due to string theory that it relates to. I hope that helps a little with the context? Throw me any questions you have about it and I'll see if I can answer them.

[torstan]

Okay, I'll take a shot. Firstly - how on earth did you dig this up? It's a hep-th paper - so it's a theoretical physics paper. That area of the arxiv is usually about string theory. And indeed this paper is relevant for string theory. I'll see if I can explain why.

Firstly, clearly the numbers should not be taken to be exact. We know that t-rex ran and it was over 3.6m tall. But that's not a counter to the paper as they explicitly state that they're being fast and loose with their factors when doing the derivation.

There are two things going on here. First is an estimate of the height of a living organism from basic fundamental properties. Second is a question about how much those properties could change and still result in large living organisms. The second is pretty hidden but is very important to some arguments that are raging at the heart of theoretical physics. It's a light hearted paper, but there's a serious point underneath it.

So the arguments appear to be the following:

1. creatures are made of molecules.
2. they breathe an evolved atmosphere
3. they can run without overheating.

The argument that they are made of molecules allows an estimate of the local temperature. We know the binding energy typical of molecules. If the local temperature is too high it overwhelms this binding energy and you won't get large scale structures of bound molecules. So if it's too hot you don't have cellular life. Equally, if it's too cold then energy transfer between molecules won't work so multicellular life won't be going anywhere fast.

In 2. we get an estimate for the size of the planet life exists on. It has to be heavy enough to gravitationally trap gas molecules. It can't be too heavy or the atmosphere would be all hydrogen. So that allows us to estimate the mass and size of the planet - which also tells us the acceleration due to gravity on the surface of that planet.

In 3. we look at how a creature might run. When we run we need to leave the ground and return to it. We expend energy that depends upon the force of gravity, and we disipate heat across the surface area of our bodies. This depends upon the ambient temperature. So by using the local temperature gained from 1. and the acceleration due to gravity in 2. along with some assumptions about the efficiency with which we lose heat across the surface area of our bodies we can get an idea of the size of the largest living organism that can run.

So they get the size of creatures to some order of magnitude. Clearly the numbers should not be taken to be precise. Their estimates are broad and they freely admit that they're ignoring factors of 2 or 3 in many places. However it does map out a proof that there is a maximum size for multicellular running organisms that cool themselves by dissipating heat from their skin. Now you can accuse them of the sin of lack of imagination by not considering wheeled creatures, or creatures with large fanned arrays on their backs to increase their cooling area, but the overall logic seems good.

Now there's the question of what the point is of the research in the first place. This is a thornier issue. I'll tackle it in the next post.