Tuesday, July 25, 2006


This podcast will be discussing the nature of time and will talk a bit about time travel. The Creationist claim for the week is in regards to the Law of Biogenesis. I also make mention of a mistake I made in the previous podcast.



Blogger isefra said...

It’s really interesting how the Multiple Worlds Hypothesis is used in this context of space travel. I myself have only heard of it (or similar ideas as this) in a religious context such as in Mormonism where they also seem to believe in multiple dimensions in planetary habitation.

12:52 PM  
Blogger LushJ said...

Thanks for the latest couple of podcasts- I definitely appreciate the info about time and its relativity. I STILL don't 'get' it intuitively, unlike the rest of the science I've learned, but c'est la vie.

This whole discussion of evolution came up in a music forum I am a part of, and someone quoted from answersingenesis.org (not because they believed it, but because it raised questions that they were curious about). There's a number of scientists/grad students who've been answering, it's been pretty interesting.

I suggested your blog & podcast as a decent resource for more information.

6:40 PM  
Blogger Peter Gaffney said...

I have never received a satisfactory answer to a question about the nature of time which I've had for some time: if time and space comprise a unified "fabric" of spacetime (so that, for instance, forces like gravity and acceleration exert a comparable affect on both space and time), was the accelerating expansion of the universe following the Big Bang mirrored or paralleled by anything in the temporal dimension? (Actually, the concept of "acceleration" would seem to imply that time is treated as a constant; otherwise I would think there'd be no way to separate an acceleration of spatial expansion from a deceleration in the "speed" of time.)

Also, isn't it fairly accurate to say that the expansion of the universe is only detectable because gravity and the nuclear forces prevent galaxies, planets, atoms and our own bodies from expanding at the same rate as the universe -- although that could change if expansion dramatically accelerates, causing what has been called the "Big Rip"? (Hey, it just occurred to me that I'm familiar with the Big Bang and the Big Rip... but what about the Big Whoop, or is that spelled Big Whup? I'd imagine that would be some astronomical phenomenon which is extremely anticlimactic.) So even if there were a temporal complement to spatial expansion, if it were uniform at all points and at all scales, how could it even be detected?

Finally three more somewhat involved but perhaps more easily answered questions about time:

(1) Can you explain "frame-dragging" or just the bending of spacetime by gravity in general so that the effect on time is as clear as the effect on space? The presence of a large mass stretches space, so that the minimum distance between two points increases and no longer appears to be defined by a straight line; how would you describe what this stretching does or appears to do along the temporal axis? (I do understand that an object crossing the event horizon of a black hole will appear to a distant observer to be permanently frozen in the moment... although I seem to recall that the event horizon is always hidden; does that mean if the event horizon were visible you could see frozen there EVERYTHING that's ever fallen into the black hole... or at least everything that's fallen in while the event horizon has had its present dimensions?)

(2) I've also read that as you approach a black hole the spatial dimension oriented toward the hole becomes increasingly "time-like," in that as the black hole's gravity captures you, your future begins to lie inevitably in that direction. Is this really any different, except in degree, from saying that on Earth the up-down dimension is very slightly timelike, in that the pull of gravity has the effect that the future of an object TENDS to lie at a lower point, closer to the surface and center of the Earth -- obvious examples being a cannonball flying through the air, a bottle of beer accidentally knocked off a table and a molecule of liquid water; the Earth's gravity being relatively weak, however, other, stronger forces also act upon objects, often counteracting this downward tendency, at least in the short-term. But we still say "What goes up must come down," and indeed for most objects it's highly unlikely that their future movements will ever take them very far in an upward direction, almost certainly not more than a few miles.

(3) You mentioned that the Planck unit for time would probably be the time light requires to travel the length of the Planck spatial unit. Since light zooms along at 186,000 miles per second, that suggests that even though the Planck distance is incredibly miniscule, the Planck duration must be even more astonishingly brief -- after all, on our own scale, a second is considered as about as short a time period as people in ordinary life usually need to be conscious of, whereas 186,000 miles is a huge distance that's hardly even contemplated outside the realms of astronomy, car odometers and frequent flyer accounts. Except on the quantum level and of course as specifically related to the speed of light, however, does this provide the basis for establishing a meaningful, universal correspondence between spatial distance and temporal duration -- with what significance or for what purpose I have no idea?

(I'd be grateful to get insight into any one of these topics; I wouldn't expect anyone to tackle the whole rambling mess.)

8:48 AM  
Blogger Lucretius said...


Sorry for taking so long to get to your questions:

The accelerated expansion of the universe does seem to imply that space and time undergo some sort of change. With space we know that the universe is growing larger at an ever-increasing rate. So what, you ask, happens to time?

Well, according to relativity, time simply appears to pass slower for 1) objects travelling at high, constant velocities and 2) for objects within a gravitational field. It doesn't seem as if either of these are changed by the accelerated expansion, and so it seems as if nothing is really happening to our perception of time.

One could possibly argue that since the gravitational exertion of other bodies on one another is getting weaker as time progresses, a weaker gravitational pull corresponding with a faster expanding universe β€” yet no one would really notice this effect. Local gravitational forces are what really matters in terms of time dilation. So, to sum it up, nothing really happens to time.

The expansion of the universe is detectable using stellar objects known as standard candles, like the Type 1A Supernova. With these objects, we can compare their known brightness to the measured brightness, and calculate the rate they're moving away. I don't think you're statement is quite as accurate as you think though β€” we can observe the universe's expansion because we see further objects away from us moving away at a faster rate, and those closer moving away at a slower rate. Galaxies are like raisins in an expanding loaf of bread which is the universe. The universe is expanding and we are hanging along with the ride, essentially travelling with space. However, if you meant people expanding as well β€” well, we couldn't even say the universe was expanding: everything would be staying the same size, because our tools of measurement would also increase in size, along with us, thus leaving everything the same size in the end. I'll tackle your other questions later.

2:55 PM  

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