Marcel Popescu

I've long believed that time travel is impossible. One of the many reasons I have for it is: it violates the conservation of mass/energy (something disappears from the universe at moment A and something appears in the universe at moment B). Recently, I've had to reconsider this problem. I started with an observation: the conservation of mass/energy is not violated when something moves. Formalizing this statement, an object changing its X, Y and/or Z coordinates does not violate the conservation of mass/energy. Now, if I consider that the universe is actually a 4-coordinate system, it can be inferred by analogy that an object changing its X, Y, Z and/or T coordinates is not violating conservation either. (The fact that we don't currently know any such objects is irrelevant; there was a time when we didn't have airplanes or cars.) Of course, this isn't proof , but I wasn't proving a theorem in the first case; all I had was a thought experiment, an apparent c...

Assumptions Behind every indirect fact (like a calculation) there are a number of assumptions; some of them of minor import, some of them vital. Most people ignore this aspect so much that they don't even realize the assumptions are there. (To paraphrase Ayn Rand, not only don't they "check their premises", they don't even know what those premises are.) Here's an example: estimating the speed of a moving car. Let's say the car leaves town A at 3pm and reaches town B at 6pm. We can see on a map that the distance between the two towns is 150 miles, therefore we calculate that the car had an average speed of 50 miles per hour (150 miles divided by 3 hours). What assumptions have we made, and how do they impact our result? For one thing, it is unlikely that the car has moved at a constant speed. It might have slowed down when passing through populated areas and sped up when in between; it might have even stopped for refueling. The actual speed was almos...

When arguing about something, the verb "is" should be avoided as much as possible because of its ambiguity. "A is B" can represent: Identity: life is life. Membership to a class: a car is an object. An attribute: this apple is green. A relationship: 5 is greater than 3. A modification of a verb: John is walking. ... probably other meanings I'm forgetting now. Of course, in colloquial language "is" cannot be avoided and is not normally a problem.