I would explain to him that time is Limted to the work of energy expanding space time, but I haven't got the time.

You have it backwards, technically, we don't experience time, we experience a moment. Time, the past, the future, is simply a container we stitch together. Time is not a thing or event, there is nothing there to grasp or understand.

No one can grasp 'length'; Not even a scientist
 

If there is an infinite amount of length to our left, then it would have taken infinite length for length to reach to our present location. We would never get to the present location. So, we can't postulate that there is an infinite amount of length to the left of our present location. But it's also hard to grasp that there is a fixed amount of length to our left and 'nothing beyond that'. How can we have an idea of what that means? We can't. And it's more complex even than that, because we have to ask whether there is also an infinite amount of length below us and behind us; in fact, it is triply more complex than time. Conclusion: not even a scientist can grasp 'length' and 'infinity'. Our minds can't grasp it. No math or science can discover this part of reality.
That's one of the mysteries of reality we can only meet with 'awe'.

And yet, I'm sitting on a chair that doesn't rock, because it was possible to measure the lengths of all four legs and make them the same.

Maybe maths and science are a bit better at discovering reality than vague, rambling musings about the nature of infinity.

Dave

I think the bong may have come before in this case...

What's the difference between understanding something and understanding it fundamentally?

The "singularity" is just where the current theory breaks down. We don't actually know what the state of the universe was at t=0 because relativity can't deal with that singularity. Perhaps when we get a theory of quantum gravity we'll have something that can describe the state of the universe at t=0. Maybe it will turn out that there was a time before that, or maybe it won't. We don't actually know yet.

But while there may be some things we don't know yet, there's a lot that we do know. For instance, the state of the universe a nanosecond after t=0.

As for QM, while there's still work being done in quantum foundations, the Everett interpretation is pretty clear and mostly understood at the fundamental level, and it's consistent with everything we know. There's some question about where the probabilities arise in a deterministic theory (which Everett is, if you look at it from the entire wave function), but those questions seem to be mostly answered.

Here's a for instance that is I think what he was getting at:

Under the Copenhagen interpretation it's possible to do experimental physics and get clear answers to the questions you want to ask ("What is the outcome theory predicts for this experiment?") say. Because generally the apparatus doing the measurement are very large systems of trillions of particles and the things being measured are very small systems of a few particles. But there's some intermediate size where you might say "When these two systems interact, does the wave function collapse or not?" And whatever the answer is, make the system either larger or smaller and repeat the question. The Copenhagen interpretation won't tell you where the line is drawn between "macroscopic" and "microscopic", or really exactly when wave functions collapse (or what the mechanism is). It's just vague on that. Generally, that's okay because again the difference between the systems being studied and the systems doing the measurements are so large that there's no real need for that kind of specificity. But from a fundamental perspective there must be some mechanism, and collapse clearly doesn't occur when only a few particles are interacting (otherwise entanglement wouldn't exist).

The Everett interpretation makes this all clear: there's no such thing as collapse, there's just decoherence.

There are dynamical collapse models in which collapse does happen, which are specific about when it happens. So those also fulfill the "understood at a fundamental level" requirement that he seems to be asking for.

I don't think it's unreasonable to want to look for an understanding that makes sense at a fundamental level, though I do think we're much further along that he seems to think.

To my understanding, we actually know the state of the universe 10^-43 second after t=0, which is a much smaller length of time than a nanosecond (10^-9).

Yeah, thanks :). Though I think we aren't really confident that far down, since there are still some ambiguities, for instance about the asymmetry between matter and antimatter. But yeah, definitely less a nanosecond. I was intentionally rounding up so I didn't have to look anything up :p

do we really 'know' any of this, aren't we just walking Einstein's equations back in time . . . this alone is not a 'knowing'

R is quite big, yet I can still count to 1.

First, it's as close as we know how to get. And second, we're way past just Einstein here.

No of course we don't "know" this. None of us was around when it happened.

This is what the best models show to be the case. That's all we got. But time and time again that has been more than adequate for our purposes. Science don't really need to "know". All science needs is sufficiently predictive models that given real world inputs spits out what we can expect to observe.

But sure, if you want the literal interpretation then no, we don't "know" any of this. We can't. Does it matter?

For some of it, yes. When we can use the state of the universe to predict the abundances of various elements, for instance, and then look out the universe and see that those predictions are accurate, then I'd say that we do actually know these things.

What we know about the state of the early universe has lead to many falsifiable predictions, and they turned out to be correct.

But there's also still a lot that we don't know. Looking back at it, I think Arth's actually wrong about 10^-43. We really have no idea about times scales close to the plank time, and won't until we get a theory of quantum gravity.

Matters to LarryS because he would love nothing more that to drag you, or anyone, into yet another "philosophy" debate.

Yep as Dara O'Briain said
And to expand on Roborama: it's not just what science knows, it's the degree of certainty it can know things with.

Well, if this were a science thread we could all accept what we know about the Big Bang and have fun with that; but this is a philosophy thread, and I think its acceptable to speak of degrees of certainty, sources of evidence, the models, etc.

The Unknown (we can/can't know?)--------------------------------------? is >
Things others can't know------------------------^ is >
Things others don't know---------------------^ is >
Things I don't know, but others do----^ is >
Things I know--^
Me

(lines are not to scale. The things I know should be a mere fraction of the things I don't, but others do. It should probably increase exponentially from there! but I don't know... ;) )

It may be that the OP is right that there are things no one will never fully understand... but even a broken clock is right twice a day!

I checked the exact number on Wikipedia, but it depends on how much you consider the time before quarks were formed to be "known".

I think Roborama is probably referring to the heading "The very early universe" near the top of your link.

Indeed, but it divides "the very early universe" (<10^-12) into distinct phases, going back to the Planck time (<10^-43), before which we're not even sure that the laws of physics as we know them apply. Although it is not currently possible to experimentally model these phases, we can "know" via other means.

So again, it depends on the precise definition of "know" and how exact the models need to be before you classify them as "known".

Regardless, the number is very, very, very small so quibbling further about details probably isn't necessary.

And I still don't have an answer to my question.

Honestly, quibbling about the details that happened at about a picosecond is probably the interesting part of the discussion. The Planck era is definitely unknown at this point IMO. But that picosecond point has the exact amount of doubt and uncertainty that Magikthise was demanding.

I think my attempt was pretty decent: we can understand something in a way that works very well empirically, even to the point that no observation or experiment that we're currently capable of can contradict it, yet that understanding can be unclear or even contradictory at a fundamental level.

An example is gravity. GR works great on it's own, and it's also completely consistent with experiment. But we know the universe is quantum mechanical in nature, yet when we try to apply the principles of quantum mechanics to gravity, we get results that don't work (infinities that can't be got rid of through the normal methods of renormalization, for instance).

So we understand gravity very well, but on a fundamental level there's something we're missing.

Or to take it a step back, Newton also understood gravity pretty well, but not on a fundamental level. Einstein came much closer to a fundamental level, and if my above paragraphs were wrong we could say that Einstein understood it fundamentally, but that doesn't mean that Newton didn't understand it at all.

Anyway, that seems like a meaningful distinction to me.

But maybe you were complaining not that no one answered your question but that the person you asked didn't answer it... :boxedin:

:D Yes, your answer was decent but I have the feeling MaartenVergu might have had something else in mind.

Fair enough and good point :)