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Comment by retube | Hacker News Reader

The question doesn't really mean anything. Space (and time) and attributes/characteristics/features of our universe. So there is no "outside" (multiverse models aside). Likewise you cannot ask what was "before" the big bang, as time only came in to existence with the big bang.

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Likewise you cannot ask what was "before" the big bang, as time only came in to existence with the big bang.

If there were ever a reason not to like cosmology, it's statements like that.

Definitely have to disagree. I think this sort of understanding is what makes physics and cosmology awesome. It's hard to imagine as we experience 3 spatial dimensions and 1 of time (and can only experience that time dimension in one direction). There could be universes with no time, or a dozen space dimensions, or 2 time dimensions. How would you define "before" for a universe with multiple time dimensions?

Edit: the human perception of time is an interesting subject. Physics (GR) holds that there is a 4D space-time. What we experience as separate constructs is really just too sides of the same coin. Although thermodynamics seems to define a "direction" to time, there's no real reason why you couldn't move in both time directions. For example, the production of an anti-particle in a particle accelerator (e.g a positron) is really just a particle (electron) travelling back in time to the production point. You can get around cause and effect issues if you assume the universe is in fact a 4D static environment, where time travel (forward or backward) has been predetermined by the static layout created during the big bang. This does of course have implications for free will... However, the multi-verse interpretation of QM gets you out of this.

Whoa whoa, when you say: "the production of an anti-particle in a particle accelerator (e.g a positron) is really just a particle (electron) travelling back in time to the production point." Do you mean the particle actually traveled back in time, or that you are able to create a model with bi-directional time, and the model's calculations work with the observed results in the accelerator, or is this the type of question I'm not allowed to ask?

According to some philosophies of physics, there isn't a difference between the two things you just said. Traveling backwards through time is a valid, experimentally consistent, and self-consistent interpretation of a physical phenomenon. If you're not satisfied that means that it's "really" happening, then I think you'll need to be more specific about what you think "truth" means. Spoiler: physics makes your brain hurt. For example, I hear that that are small enough time/distance scales where causality because mathematically ill-defined.

sure you can ask it! and it's a good question, and similar to many which cause heated debate in QM (essentially - the model works beautifully, but what does it _mean_?). Basically I don't have the answer. All I can say is that when an anti-particle is created, that is IDENTICAL to - per our model - a particle travelling back in time and being destroyed at the production point. Now we know the model works extremely well. It describes the system to a very high degree of accuracy, and can be used to make predictions about how that system evolves. So - if the model is correct (let's assume it is), and anti-particle production -> moving forward in time == particle moving backwards -> destruction, which is it that we are observing?

Basically, I think most physicists would say these two scenarios are the same thing, just expressed (or observed) in two different ways.

Edit: actually this highlights an important distinction between the quantum and macro-worlds. Things can only happen in one direction in the macro-world thanks to entropy (everything -> disorder). Whereas in the quantum world there's no such restriction, and things can happen in either direction.

> time only came in to existence with the big bang

My brains segfaults imagining that there was a time where time didn't exist...

Think of it this way: We don't have any way of measuring time that doesn't also involve space. Whether it's pendulums, or atoms vibrating, or photons bouncing around, any way we measure time is linked to (and relative to) whatever space is around it. If space is being deformed by a large mass being there, time will be affected (slowed down). Some theories of time say that what we think of as 'time' is really just a byproduct of space.

So if space didn't exist (in any meaningful way that we would understand today) before the big bang, time couldn't exist either.

For anyone who hasn't seen it, this lecture by Bill Phillips (Nobel Laureate, of NIST, given at the Perimeter Institute) on time, relativity, cooling and the Bose-Einstein condensate is probably the most entertaining real science lecture you're ever going to see:

http://feeds.tvo.org/~r/tvobigideasVideo/~5/QOpqIcl2UDk/BI_F...

It's long, but it won't merit a tl;dw unless you have no curiousity, imagination, sense of humour or innate geekiness.

That's like trying to imagine what being dead "is like" for materialist atheists. If you don't believe in an afterlife, then your consciousness simply ceases. There is no experience of being dead.

I imagine being dead as being what my life was like before I was born.

What was that like?

Exactly

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