In general, physicists don’t bother talking about things at all unless they’ve been doped out mathematically. We’d love to have observational evidence for or against ideas, too, but for most of the things in this thread, that sadly isn’t possible.
It doesn’t seem to me to be a philosophical question first and foremost. In general relativity the time any observer experiences (their proper time) between any two events is a line integral (along their worldine). This corresponds to the ‘length’ of their path in spacetime. That’s is simply how the physical predictions of the theory relate to the mathematics of the theory.
It is very easy to show, from the assumptions of big bang cosmology, at some finite point in the past the Universe was in a singular state. Of course you bring up the fair point that physically GR breaks down at the singularity (mathematically speaking the singularity is not a point in spacetime). This is often ignored when casually talking about a nice neat singularity like the big bang singularity which can be described fairly accurately as a “place” or “point in time” (not all singularities are amenable to this sort of description).
We needn’t consider the singularity itself though to show that the line integral of an observer (and hence the amount of time they experience) is bounded in the past, indeed the bounded nature of the proper time for some (and in fact all) observers is precisely what defines the existence of the singularity. It just so happens that, using our current best model, the proper time for comoving isotropic observers is bounded at about 13.77 billion years in the past from the present day. For any other class of observers this bound would be less.
If you disagree with any of that then I will offer cites.
The problem is we’re talking about something that I don’t think can be reduced in an obvious way. The basic idea is to take spacetime and view it as a substructure of some larger structure (i.e. complexified spacetime). Some of the maths relating to quantum gravity are actually easier to tackle in another substructure of complexified spacetime called the Euclidean section. Once you have the solutions in the Euclidean section you can then analytically continue them in to normal spacetime (the Lorentzian section). My understanding is that this also has the added bonus of making singularities disappear.
What I am saying is that clearly the worldline of an observer at the point of the big bang doesn’t continue in to normal spacetime (if it did there would be no singualrity). So in a mathematical structure of which normal spacetime is a substructure, but where there is no singularity, it must continue in to the parts of the larger structure which isn’t normal spacetime. I don’t think there is any explicit physical interpretation for those parts of the larger structure.
The answer is 42
If you tried to find the amount of time elapsed for a tachyon between any two events, you would get a quantity that you would either interpret as a length, “e.g. 5 miles”, or as an imaginary period of time, e.g. “2i minutes”. The problem is that there isn’t a clear way to interpret this and reversing the direction of time is not equivalent to transforming in to a tachyonic frame of reference.
The worldlines of tachyons are spacelike curves in spacetime and it’s worth noting too that not all spacelike curves are bounded in the same way that non-spacelike curves are in big bang cosmology. I.e. they can have infinite proper length in both directions
I think what may be throwing you is the idea that, for somebody viewing someone falling into a black hole, it seems as if they (the infalling observer) take forever to get there (and this is perfectly right). By analogy, it then seems that you can never get to the big bang, either. But for the infalling observer, it actually takes a finite (typically short) time to cross the event horizon and hit the singularity.
Hawking pursues a specific approach to quantum gravity, which involves a mathematical relationship known as Wick duality between our usual, Lorentzian spacetime with 3 spatial and 1 temporal dimension, and an Euclidean spacetime with 4 spatial dimensions. It is this Euclidean spacetime to which the ‘north of the northpole’ analogy applies; basically, it is simply a four dimensional shape that is smooth everywhere. What exactly this means with respect to our own Lorentzian spacetime I’m not exactly clear about.
I posted a thread on this that promptly died, but maybe there will be some takers here. One of the few people on the planet who can go toe to toe (sorry bad mental image there) with Hawking and win, is Lenny Susskind and he just happens to have written the perfect book for us physics groupies - The Theoretical Minimum: What You Need to Know to Start Doing Physics.
edit: I just got it yesterday and it’s a lovely little hard cover book.
To expand on this:
*There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
*[RIGHT]from The Restaurant at the End of the Universe[/RIGHT]
Have you seen google doodle today?
In the beginning the Universe was created.
This has made a lot of people very angry and has been widely regarded as a bad move.
I’m glad that this question has somewhat obtained a degree of respectability because for years ISTM people have been talking at cross-purposes.
When the average person talks about “before” the big bang, it’s really a metaphysical question and it’s not specifically about just this space-time brane or whatever. The word “before” is just used because it’s really hard to word this stuff.
And anyway the “No such thing as ‘before’ the Big Bang” retort seems to get trotted out even when no-one is speaking temporally e.g. when people just talk about an explanatory gap.
So, IMO, to dismiss the question as being nonsensical and “like asking what is North of the North pole” is akin to saying “I know there is no Multiverse”, which I am sure few physicists would say.
I guess the biggest problem we face is that time is one-dimensional. No one (afaik) has examined time as being anything more than before-now-after. Because I guess we have no other practical way to view it. If we can measure a linear distance in a 3-dimensional space, who is to say that we are not observing a one-dimensional slice of multi-dimensional time?
Thanks Delta, I’ve posted in your other thread.
The space-time equations, maybe?
I read in a semi-popular physics book once that in a space-time of N dimensions, physics makes sense only with 1 or n-1 time-like dimensions. No I can’t give you the book name, nor do I really know what it means, but I’ve always been curious as to what it meant.
Can’t say that I understand it but I do know that there have been serious papers that do propose more than one time dimension.
You can recast 4-dimensional space-time as 3 time-like dimensions and 1 space-like dimension, but that’s just playing with the numbers. As for the proposals for 2 or more actual time-like dimensions, they are not to my knowledge well accepted, but even if they were the statement “If we can measure a linear distance in a 3-dimensional space, who is to say that we are not observing a one-dimensional slice of multi-dimensional time?” would remain meaningless. Time is not like the familiar everyday spatial dimensions. We would not observe those additional time-like dimensions.
I remember that as a Johnny Carson joke, not sure if it came from a Carnac routine or something else.