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I understand that before the Big Bang time (as well as dimensions) didn't exist. But how could this be? If there was no time then nothing could change, and so time itself couldn't come into existence.

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    $\begingroup$ Nobody has a clue. "If there was no time then nothing could change, and so time itself couldn't come into existence." That's a great thought - but it could go either way. You could say "time didn't exist before time existed, so it could have started 'any time' or 'every time'" :O $\endgroup$ – Fattie Oct 31 '18 at 13:13
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    $\begingroup$ Time is a relative concept. There might be time before the big bang. But, we set time = 0 at the Big Bang. Also, note that the existence of the Big Bang is suggested by theories and observations. Although how much we love the Big Bang (pun intended), it may or may not be correct. $\endgroup$ – Kornpob Bhirombhakdi Oct 31 '18 at 13:54
  • $\begingroup$ Probably better for Physics. I've heard it said several times that time began with the big bang. A more detailed explanation of that statement might be helpful, if it can be put in layman's terms. $\endgroup$ – userLTK Nov 1 '18 at 5:56
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    $\begingroup$ I like to think of "what happened before the big bang?" as analagous to "what is to the North of the North pole?". $\endgroup$ – Steve Linton May 25 '19 at 7:43
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    $\begingroup$ Kind of involves semantics of "time". $\endgroup$ – Inertial Ignorance Sep 28 '19 at 22:48
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This is both a physical and philosophical question because it depends what you mean by "time".

From a physical perspective, time doesn't really exist. We know that things change, and we measure a rate of change in relation to other things that also change (like clocks) but how we define time as an absolute concept is not something we have a firm answer to. It may not exist at all. To understand the details fo this you need to learn a whole lot of quantum mechanics, string theory and theoretical physics in general, which will take about a decade, two degrees and a doctorate. Alternatively, I've been reading a great book called "The Order of Time" by Carlo Rovelli. He is a theoretical physicist who has worked for decades to understand this stuff and then written all about it with hardly any maths and some drawings of Smurfs. I would highly recommend it.

Also, from a more philosophical perspective, if the Big Bang started with a singularity in which time and space and all matter were compressed into one single point with no dimensions, then that point contained no information about what was before it. If we can never know what was before the singularity (or even if there was anything before), then there is very little point in discussing it, so we can choose to set that as the zero point of time and call it the beginning because anything that happened before that is irrelevant. Like starting a stopwatch to time a race when the cars set off - there was time before, but that's not important for the race so we call the start of the race a time of zero.

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Here's another way to look at it: winding back the age of the Universe towards $t=0$ is a little bit like the reverse of watching (from a distance) an object fall into a black hole.

In the case of a black hole, we can (theoretically) see an object accelerating towards the event horizon. From our distant frame of reference, as it gets closer, relativistic effects result in time appearing to slow down. The closer it gets to the event horizon, the faster it falls but the slower this appears to us, until the object appears to "freeze" right on the event horizon.

Except, our models suggest that's not what actually happens. Instead, it takes an infinite amount of time for the object to arrive at the event horizon, and since we can't ever be at $t=\infty$ (other than in our mathematical modelling), in practical terms we can never see the object actually reach the event horizon.

That's just an example of how time can be considered as asymptotic: as distance approaches 0, time approaches infinity. There's also an asymptotic function in the other time direction: in the Big Bang for example, as time approaches 0, density approaches infinity. To put it somewhat simplistically, some models of physics treat one or both of these situations as mathematical singularities.

Since our cosmological models don't quite work as we wind back the Universe to the earliest moments of the Big Bang – there's no current theory that describes what was happening before $t_P$ or Planck time, at $10^{-43}$ seconds – we simply cannot say what state the Universe was in at $t=0$.

One possibility is that $t=0$ only exists mathematically, just like $t=\infty$ for an object falling into a black hole, and that in our mundane real Universe, it simply isn't possible to wind back time to an actual singularity. But there are other models that treat singularities as real, and this whole area is a very active and highly contested field of research. I'm simply offering one way to conceptualise a possible Universe with no "start" time.

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