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In a TV ducumentary, I heard that time started with Big Bang, and it is meaningless to talk of "before". This was given without any further explanation, in the peculiar, irritating style of documentaries.

Now I wonder if this fact may be motivated by the means of motion. Time, afaik, is necessarily measured (or even defined) by motion. "Prior" to Big Bang, no motion, no time.

Is that correct?

NOTE: I have a degree in mathematics (BSc), but almost no knowledge of modern physics, so treat me as a layman when talking about it.

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    $\begingroup$ possible duplicate of How can something happen when time does not exist? $\endgroup$ – John Rennie Jul 24 '15 at 17:47
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    $\begingroup$ This has been addressed several times in various different ways. I've linked a duplicate that I think is closest to the spirit of your question, but see also this question, this question and this question. $\endgroup$ – John Rennie Jul 24 '15 at 17:49
  • $\begingroup$ For what's it worth, I would agree with you that: no change equals no time, but I would argue that is basically an opinion based question you are asking, due to lack of evidence about the full BB story. John's recommendation's pretty much sum up our understanding, in my opinion. $\endgroup$ – user81619 Jul 24 '15 at 18:01
  • $\begingroup$ I'm gonna read the posts you linked. Thanks a lot. $\endgroup$ – MadHatter Jul 24 '15 at 18:16
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The Universe we see around us today including the space was once concentrated to a size smaller than an atom. This infinitely dense object may have had a past before that state but no information would make it from that past into our universe so we may as well say that time began at the big bang.

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    $\begingroup$ Perdon me but if that size was smaller than an atom but greater than zero, the density of the matter within was not infinite. Furthermore, it may be incorrect to say that space was confined in such a small size, for afaik, the "space" could have been even infinite at the very instant of big bang, but shrinked in its tissue. The big bang didn't happened at a fixed point, but everywhere (see the typical example of the rubber balloon, just have care to not imagine it as immersed in a higher dimensional euclidean space). $\endgroup$ – MadHatter Jul 24 '15 at 18:14
  • $\begingroup$ Even if the universe was not a singularity and had some finite size, the density would still be infinite if there is an infinite amount of matter in the universe. $\endgroup$ – Alex Jul 24 '15 at 18:23
  • $\begingroup$ So the density of matter should be infinite even nowdays, at any point. Furthermore, note that singularity hypothesis is not in contrast with universe having a finite or infinite size. It just was in a "singular" state everywhere. $\endgroup$ – MadHatter Jul 24 '15 at 18:26
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    $\begingroup$ This is mostly misleading & partly speculation (cf. this post by Jimself, among many others). $\endgroup$ – Kyle Kanos Jul 24 '15 at 18:49

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