From what I understand, Last Thursdayism is the belief that the universe was created last thursday.

Supposing Last Thursdayism is true, if you consider going at a really fast speed when Thursday begins, around 85% the speed of light for a week, time dilation when you get back will be a tiny bit above 1 week, which means that everyone will be created last Thursday, but when you when you stop speeding, you will have been created two Thursdays ago, which means that not the entire universe was created last Thursday.

This is because your "Last thursday" isn't everyone's "Last Thursday". But if this happens, then last thurdayism should be false, and also means that it can be proved wrong.

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    Bizarre though it is, this belief can not be "disproved" mathematically even if we assume that mathematics of special relativity is strictly true. The belief presupposes absolute now (otherwise "last Thursday" is not a well-defined expression for the entire universe), and such a thing is consistent with mathematics of relativity, as Lorentz's alternative interpretation of it shows (it amounts to picking some privileged reference frame). Such absolute now is not, however, experimentally detectable in principle. – Conifold Dec 7 '17 at 3:42
  • Nice idea but it is not possible to falsify the idea that existence is momentary and evanescent, as is claimed by many. No test will work. Your time-dilation experiment would not reveal anything, as virmajor notes. – user20253 Dec 7 '17 at 10:56
  • I just want to point out that this idea isn't new; it was proposed by the father of Edmund Gosse who wrote about it in his memoir Father & Son; he wanted to reconcile the science & the bible; unfortunately for him, both sides simply thought it was a ridiculous notion. – Mozibur Ullah Dec 7 '17 at 11:55
  • '"How can I tell," said the man, "that the past isn't a fiction designed to account for the discrepancy between my immediate physical sensations and my state of mind?"' is a variant of "last Thursdayism" that might be clearer. – user935 Dec 8 '17 at 5:05

As virmalor pointed out, last Thursdayism is not that literal. It is the assertion that the universe came into existence in a preconfigured state, at some point in time, as opposed to developing from an actual big bang, etc.

Unfortunately, there is no way to tell the difference between the two, so long as the preconfiguration is "correct." There is no difference, from an internal perspective, between a 14 billion year old universe and a universe that is only 10 days old but was preconfigured as if it were "running" for 14 billion years.

But suppose part of the universe appeared to be about 14 billion years old, while another part appeared to be only 7 billion years old, because the preconfiguration was not consistent with the physics of the universe from that point on, then we could surmise that something funky is going on in the universe and we might say that it was preconfigured.

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I think you're misunderstanding the idea behind "Last Thursdayism" on two fronts.

First, as can be seen from the selection of "Thursday", the main point of the posit is to point out a problem in proving things that we can only observe indirectly by effects. Or to word it another way, the observer only has access to what they are observing and everything else could be otherwise explicable or at a minimum doesn't prove that such events took place at the times they appear to. (Here, the point is that it's a skeptical thesis -- not that you can't quibble with it on what can / cannot be seen).

Consequently, the counter example completely misses the mark in two ways. On one level, the Last Thursday bit is not meant to be take literally -- but rather to point out the degree to which we cannot "know" that things weren't just invented last Thursday. On a second level, what you are observing remains what you are observing, so the person in the time-dilation from traveling near the speed of light won't be able to prove that in fact two weeks have transpired, they will only have the impression and evidence that makes it appear to them that two weeks have transpired...

There's of course a trick in that this "know" is the know that most thinkers after Plato have given up on and not something like JTB.

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