Reports of a physicist “taking on gravity” have recieved a bit of attention recently, with a New York Times article outlining Erik Verlinde’s idea that gravity is an emergent property of thermodynamics.
I think it’s great that the piece was written — even though apparently it hasn’t excited any physicists since the start of the year. Regardless of how current or accepted the idea is, the topic is interesting enough that it’s making lots of people read about some pretty complex scientific ideas. And that’s always good!
But one thing that does kind of irk me about the story is the seeming philosophical naivety of both the journalist, and the physicist (assuming the physicist hasn’t been completely misquoted). And working through the philosophical problem reveals some really interesting issues, I think. Let me explain.
The piece starts off as follows:
It’s hard to imagine a more fundamental and ubiquitous aspect of life on the Earth than gravity, from the moment you first took a step and fell on your diapered bottom to the slow terminal sagging of flesh and dreams.
But what if it’s all an illusion, a sort of cosmic frill, or a side effect of something else going on at deeper levels of reality?
And then it continues later, quoting Verlinde:
“For me gravity doesn’t exist,” said Dr. Verlinde, who was recently in the United States to explain himself.
And once more:
“We’ve known for a long time gravity doesn’t exist,” Dr. Verlinde said, “It’s time to yell it.”
‘Not fundamental’ is not the same as ‘doesn’t exist’
Let’s get something straight: saying that something is an emergent property, or not a fundamental property of the world, is not the same as saying that the property does not exist! Not even close!
If those two things were the same, then most philosophers and scientists wouldn’t think that very much existed at all. Perhaps all that they would think exists is energy or information or whatever they think is the most fundamental stuff in the universe.
Now, there is a philosophical position that you can take about certain things (properties, objects etc) called “eliminativism”. To be an eliminativist about, say, unicorns, is to say that they don’t exist. You are “eliminating” them from your ontology (that is, the list of things you think exist).
There have been some interesting eliminativist ideas in philosophy of science. Bertrand Russell (political dissident, mathematician, philosopher, total hero) once argued that causality doesn’t exist — he was an eliminativist about causation, arguing that the word should stop being used in serious, scientific discussions. (He was wrong, by the way.)
But there’s a problem…
But hang on. Verlinde might think he’s an eliminativist for the following reason: Everyone thinks that gravity is this fundamental force in the world. But they’re wrong! What THEY THINK GRAVITY IS, doesn’t exist!
If what we meant originally by “gravity” is “a fundamental force”, then this physicist’s theory that says that it’s an emergent force is in fact saying that gravity doesn’t exist.
So what’s the answer?
At this point then, it might seem like a definitional problem. We should just decide what we meant by “gravity” to start with and move one.
And that’s almost right. But there is an answer: Verlinde is wrong to say he thinks gravity doesn’t exist. (And it’s worth noting that he doesn’t say it in the paper, he only says it to the journalists.)
If Verlinde were right, and his poisition was an eliminativist one, then the following absurd conclusion is true: every theory that has ever changed the way we think about anything was an eliminativist one. But that’s obviously false.
Consider the Copernican revelation: the earth goes around the sun, and not vice versa. Copernicus did not wake up one morning and announce “My god! The earth does not exist!” Instead, he woke up one morning and said “My god! The earth is not the centre of the universe!”
Of course, you could insist that what you meant by “earth” was the thing we live on, which is in the centre of the universe. And then, Coperincus certainly did show that what you’re calling “earth” does not exist.
But a much more helpful way of viewing the matter is that Copernicus showed us that the earth is not exactly what we thought it was.
Similarly, Verlinde’s claim that gravity is an emergent property is not the claim that gravity does not exist. Rather, if he’s right, he’s shown us that gravity isn’t exactly what we used to think it was.
[For those particularly interested in this issue, there are some fascinating borderline cases of theories that may have shown us new facts about old things, or shown us that the old things don't exist. The most famous of these is phlogiston.]
Erik P. Verlinde (2010). On the Origin of Gravity and the Laws of Newton arxiv.org arXiv: 1001.0785v1