What does classical physics, if anything, tell us about what happened before the big bang? Was there a time dimension before the big bang, or did it really occur at t=0 and predate anything else?
Also, are there negative distances?
I argue that the time dimension, like the three other visible ones, did not exist until the big bang, and that therefore there was no time “before” t=0 - which I concede may be wrong. And also that there are no negative distances, only distances with direction which could arbitrarily be negative, which I am considerably more confidant about.
On the time thing, this question comes up at least once a fortnight here on the board; the short answer is that the Big Bang is thought to be the origin of time and the expression “before the big bang” is therefore a meaningless one, like (as is often said) “North of the North Pole”
Mangetout:Sure there are negative distances; London (UK) is approximately -70 miles SSW of Southampton (or +70 miles NNE, by normal reckoning).
Didn’t aesth cover this with there are no negative distances, only distances with direction which could arbitrarily be negative or maybe I’m reading that statement wrong?
And I’ve set t=0 in lots of situations where it didn’t start at the big bang. For example, if a car started accelerating and I wanted to find out how far it travelled, I’d just set the point where it started at 0 and anything before that would be negative.
Mangetout: In your system, all negative distances can be `normalized’ by choosing a different frame of reference. From all I understand about distance, that is always the case. (In other words, there is no reason to ever have a negative distance.)
As for t=-1, it doesn’t make sense. The Big Bang is seen as the origin of all dimensions, not just the spatial ones. It originated the timelike dimension, as well, so t=-1 makes about as much sense as saying something is -5 ft from me.*
*There are probably more than three spatial dimensions, but I’ve never heard a serious proposal for more than one timelike dimension, nor any dimension whatsoever that has an existence outside of our current Universe (at least, such that it extends past the Big Bang).
Well, the debate went something along the lines of…
A friend mentioned he didn’t believe in the beginning of time, and thought that the universe always existed in some form.
I said that I found it funny that he didn’t accept the answers that either religion (meaning christianity) or physics offered.
He claimed that the point (meaning the universe before the big bang) still existed in a time dimension, to which I responded that the time dimension, like any ordinary dimension, was also only a point, and that until the time dimension expanded during the big bang, there was no such thing as time. Therefore, t=0 was the start of the big bang, and nothing preceded that.
He believed that the time dimension was special in some way, and wasn’t subject to the same laws as the other three regarding points.
At this point another friend chimed in, told me that even if there was nothing special about it, that there were also negative distances, and therefore could be negative time. He then told me that I shouldn’t make arguments like the ones I did without a math background, like he has. We are all college students, by the way, and I’m at least three years behind both of my friends. This last part was meant to be an insult - don’t talk about things you don’t understand. So I figured I’d ask smarter people than myself.
Speaker: He was using t as a reference to the Universe’s time, not to the time of some local system. I think we all understand that, and I think that the OP understands that t has no special significance in physics.
Saying time didn’t exist before the Big Bang is just as wrong as saying it did. The real answer is that we don’t know, and most probably will never know, since it’s currently assumed that it’s impossible to know about anything that occured before the big bang.
My personal hunch, however, is that there in fact was time before the big bang. And space too. I personally like the theory that the big bang was a result of a quantum fluctuation, possibly within a universe that resulted from another big bang quantum fluctuation, ad infinitum. Time and space are the products of the laws of physics, and i see no reason why the laws of physics had to spring into being at some point or that they are not in some way completely universal and eternal (kinda like god, huh?). I would not be able to grapple with the concept that time and space at some point simply did not exist, or that in their inexistance they suddenly appeared.
But again, all of this is nothing more than speculation and metaphysics and the real answer cannot be known (at least as far as we know :-P).
alex_d: When you define time and distance as being dimensions along the axes of this Universe, the only reasonable way to define them, then they did not exist prior to the Big Bang.
Speculation about objects outside this Universe is more a subject for science fiction than physics.
As for negative distances: No, distances cannot be negative unless the negative sign is defined to mean a distance in the opposite direction (which it usually is). In fact, the universe can be expressed solely in positive integers, which is quite reassuring since a universe with negative quantities would also necessitate imaginary (or rather complex) quantities as well (which would be quite disturbing since almost noone understands what an imaginary number really is).
That’s for pragmatic physics purposes only. The act of defining the axes does in no way bring enlightenment to the question of what came before.
Saying that it’s reserved only for science fiction is a bit harsh, but that’s exactly my point. Physics cannot give any conclusions on whether or not there was time or space or anything before the big bang. Such things are thought to be unknowable. However, physics, and not just science fiction, still has the right to speculate about such things. It is the point of human curiousity to probe what it does not know. If you stop doing that, physics (and life) becomes very dull.
Enlighten me as to what is so odd about imaginary numbers. I learned them as the square roots of negative numbers, and as being (along with the reals) a subset of the complex numbers.
Having an imaginary distance is meaningless, but it’s no more meaningless than a negative distance.
(It seems like we’re having quite the discussion, eh? Maybe we’ll both learn something here.)
Technically speaking, classical physics can’t tell us anything about the Big Bang, or even that there was one. You need General Relativity to talk about the universe’s structure and growth. Even then, GR will still break down — in fact everything we have breaks down — when you get within a fraction of a second of the magic moment.
Serious astronomers have advocated the following ideas over the years, among others:
[ol]
[li]There was no time before the Big Bang. The Big Bang defines the beginning of our universe, and the beginning of time.[/li]
[li]The universe oscillates over the aeons, going through a Big Bang, expansion, contraction, and a Big Crunch, over and over again, forever.[/li]
[li]Our universe is just an infintesimal part of a larger “multi-verse” in which, essentially, all possible universes get their 15 quintillion minutes of fame.[/li]
[li]We don’t know and perhaps can never know what came before t = 0, or even whether it makes sense to ask the question. (My own favorite, even if it sounds wimpy and indecisive.)[/li][/ol]
Man, your friend is being such a dweeb. (Do you young folk still say that?)
There are many possible mathematical models of our universe that could be correct in principle, or at least could work so well that we wouldn’t know the difference. But to have an idea which model is correct, or best, you of course have to practice physics. Mathematicians aren’t necessarily any help here.
Before the 20th Century — before QM and Relativity and Superstrings — most astronomers believed in a straightforward steady-state universe embedded in a nice 3-D Euclidean space, with time linear and infinite. It was perhaps so “obvious” a model that they wouldn’t have even bothered to describe it that way. How could things be otherwise?
Well! People just had no idea what strange phenomena were waiting out there for us to discover, and what new models would be needed to account for them.
There are many good books on cosmology of course, as well as lots of awful and fluffy-headed ones. Hawking’s A Brief History of Time is a decent place to start though.
I avoid the “negative distance” example and use the “below absolute zero” example. Lots of physical measures can’t go negative. Degrees on the Kelvin scale is one of them. So why should you even expect time to have ever been negative?
(As long as they don’t know about certain quantum fluctuations, base energy, and such, they’ll be satisfied for a short time.)
The way it’s normally used in physics, there’s no such thing as a negative distance. Usually displacement is a vector and distance is the magnitude of the displacement, and magnitudes are always nonnegative. Similarly, speed is the magnitude of velocity, and you can’t have a negative speed.
From a mathematical standpoint, any distance function which is anywhere negative does not satisfy both the first and third metric properties.
Derleth:
As for what’s special about imaginary quantities versus negative quantities. Excepting the physical quantities which are defined to be non-negative you can have negative quantities that have physical meaning, meanwhile imaginary numbers never have a discernable physical meaning. ie you can have a negative velocity, but what does an imaginary velocity mean? Also true for energy, electric charge, momentum, force, accelleration, etc.
You’re right that the imaginary number i is defined to be the square root of -1. Imaginary numbers are a useful mathematical construction for solving mathematical equations. But when you start trying to describe physical quantities with imaginary numbers you find they have no physical meaning.
Derleth a. I know that “Degrees Kelvin” is redundant and so I didn’t say that. I (and many others) use degrees in this context when people who never heard of Kelvins might be looking in. b. “Never has been” is quite definitely wrong.