Sunday, December 07, 2008

PS on "The Block Universe"

In May, I wrote a post on “The Block Universe,” a commentary on a paper by Vesselin Petkov that I thought might serve as a useful basis to explain why Special Relativity is not incompatible with presentism, the idea that it is only the “now” that “exists”. Note speech marks, for both are undefined terms. That lacking definition is essentially the point of my argument.

Now, some days ago, author of said paper contacted me to let me know I allegedly “completely failed to understand” what he wrote, and urged me to publish my writing in a journal so he could react to it, followed by “friendly warnings”. If I come around to converting my post appropriately, I might indeed send it somewhere, but it admittedly isn't very high on my list of priorities. As far as I am concerned, I said what I had to say, and the eternal reinterpretation of Special Relativity isn't a topic I am particularly keen on engaging in to begin with. It followed some back and forth of emails that did not amount to anything.

Such reminded that my statement might have confused some people, I thought it would be worthwhile to briefly summarize the point of my argument. It really isn't so hard to understand. Special Relativity is based on these axioms:
  1. Space and time form a 4-dimensional, flat, differentiable manifold with Lorentzian signature.

  2. The laws of physics are tensor equations over this manifold.
If you take out the word “flat” you get General Relativity, but that's a different story and shell be told another time.

My argument is now simply, from these axioms you can not follow what “existence” means, thus you can not outrule presentism for it builds on this notion. You can then go ahead and produce a lot of fog by talking about clocks and rods and rulers and twins etc, but all of this is completely irrelevant. If you introduce additional assumptions about what constitutes a physical object then you might be able to come to conclusions. But these assumptions do not follow from Special Relativity and should be named explicitly.

As a concrete counter example for the claim that shows why this talk is empty, I suggested the following: Introduce an arbitrary time-like slicing in Minkowski-space and name it “existence slicing” to mean what is on the same slice “exists” together. This is not incompatible with Special Relativity.

No, there isn't anything more to this example, that was already it. I've shown one can define a notion of “existence” only on 3-dimensional submanifolds and this is compatible with Special Relativity. Now you can say you don't like this definition of “existence”. Fine. Nobody said you have to.

If you want you can understand “together” in the above example as “simultaneous”. This is not the notion of simultaneity commonly used in Special Relativity (based on inertial observers), but nobody forbids you to introduce a different one as you please. Again, you might not like that, but that was not a requirement I aimed to fulfill. For more precise statements, please see mentioned earlier post.

Just to make it clear, I personally don't even believe in presentism, I am just trying to say that the claim it is outruled through Special Relativity is logically false. If you want to come to any conclusions, you will need additional assumptions for what you mean with “existence”.


  1. "Friendly warnings"? What a maroon!

  2. I really like the way you present the fallacies and I support rigorous definitions when conclusions are to be derived. Now, about the quote at the end of your post.
    Have you ever tried to explain to your grandmother what "manifold" or a "Lorentzian signature" are?

    "One man's rigor is another man's mortis" (Bohren and Albrecht 1998).

  3. Hi JuanPi,

    My one grandmother I explained a lot about black holes and Hawking radiation (it was the topic both of my Diploma as well as my PhD thesis), and I recall having had a lengthy discussion with my other grandmother about Special Relativity, that however was before I studied physics, and left me very confused. I don't think the notion of a manifold specifically came up. I usually try to avoid technical expressions unless I really need them. Best,


  4. If a coin dated 30 BC is an obvious fraud, why not one dated 2006? A 2009 coin is legitimate (a month's debate is decided normal to content).

    Only the present exists. Nothing justifies education, production, or exertion. Tomorrow will arrive unbidden and unchanged. The European Middle Ages had God Himself enforcing inerrant unchanging universal wisdom.

    Dentistry. Test of faith!

  5. Hey Bee, can you help me clarify what I'm trying to say here:



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