Jesse

But Bob K claims his "invariable time interval clocks" can be built today, using known physical principles. If he's talking about clocks which could prove his idea of a single universal "now" and eliminate the problem of simultaneity depending on one's (arbitrary) choice of reference frame, he is clearly mistaken about this.

wordsmyth

I'm rather ignorant of most physics and quantum mechanics, so can someone give me a simple laymans definition of "time" as it is used in the theory that it began after the big bang?

Shadowy Man

well wordsmyth: I wouldn't say that time began after the "big bang", so I can't help you out.

NumberTenOx

Of course no one knows, really.

But the theory of inflation says that the Universe could have been created from a patch of false vacuum about 10^-29 centimeters and weighing about a gram (with a density of a whopping 10^80 grams per cubic centimeter). From this small thing, the entire universe could have formed.

There are many variations of inflation. One includes the idea that there is a section of false vacuum that is eternally growing and creating universes, one of which is ours. This theory would allow for time to be eternal, both forward and backward.

Gotta go.

Jesse

I'm not sure what a "definition" of time would be, it's just what's measured by any physical clock that ticks at regular rate (and you sort of have to bootstrap up the notion of a 'regular rate' from the fact that various independent types of physical clocks have constant ratios between their respective ticks, I guess). The theory of general relativity says that if we extrapolate the current state of the universe back in time, eventually all the matter in the universe (including all clocks, of course) would reach a state of infinite density a finite number of ticks in the past, and it's impossible to extrapolate back further than that using known laws of physics. However, most physicists today believe general relativity gives incorrect predictions earlier than the Planck time 10^-43 seconds after the Big Bang, and that we need a theory of quantum gravity to say what was going on before that, so it's an open question whether this theory of quantum gravity would tell us that there was no time before the Big Bang.

The other important issue relating to time is the notion of "simultenaity", the question of how we decide whether spatially separated events happened at the "same time" or not. Special relativity says that observers in different inertial reference frames (ie observers who are not accelerating, but may be travelling at different constant velocities relative to each other) will have different definitions of simultaneity, and that physics works the same in all inertial reference frames (no 'preferred reference frame'), which means there is no absolute standard to determine if I'm standing still and you're moving or if you're standing still and I'm moving, and likewise no absolute standard to determine if two events were simultaneous or not.

Each observer judges whether two events are simultaneous by looking at the distance of the event from the observer and the time it took light to get from the event to the observer--if I see a star 100 light years away going nova in 2003, and then I see a star 101 light years away going nova in 2004, I would say both events happened simultaneously in 1903. But an observer moving at a high velocity relative to me may use the same technique and find that from his point of view, one nova happened years before the other. This is ultimately a consequence of the fact that in relativity, light appears to travel at the same velocity in all reference frames...in Newtonian physics, if I saw a light beam go by me at 186,000 miles/sec and I saw you riding a rocket at 185,000 miles/sec in the same direction, I'd expect you to see the light moving ahead of you by only 1,000 miles/sec, but relativity says you'll see it moving at 186,000 miles/sec too. Because of this, in Newtonian physics all observers who use the "backtracking light from an event" technique will agree on whether a given pair of events were simultaneous, but in relativistic physics it won't work the same way.

wordsmyth

As I said, I'm fairly ignorant of physics and quantum mechanics, so the following may seem like mad rambling.

That said, I've always viewed time as a stream, like a river, that allows things to flow from one state/position to another in a generally consecutive manner. Without this stream, nothing could change from one state/position to another. It would be like a river frozen solid. With that in mind, I have always thought that time must by necessity exist prior to the big bang or else the change in state from "no big bang" to "big bang" could not occur.

Jesse

Well, the common view that time is "flowing forwards" usually implies that there is a single true present, with things in the future not yet realized while things in the past are set. But the lack of a universal definition of simultaneity causes problems for this view...for me event A and event B might both be happening "now", while for another observer event A could be happening "now" while event B is in "the future" or "the past". This suggests something more like a block universe view where time is just a dimension in a larger block of spacetime, with every object's history as a sort of thread frozen in the block.