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View Full Version : Episode 43: Questions, Questions #5



Fraser
2007-Jul-02, 08:37 PM
It's time to answer the questions again. And this time we've got some doozies. Is the Universe rotating? Is space something, or is it nothing? Is dark energy evenly distributed? What would happen if an astronaut went out the airlock, without a spacesuit. Want to know the answers? Well, you've got to listen.

Read the full blog entry (http://www.astronomycast.com/questions/episode-43-questions-questions-5/)

EvilEye
2007-Jul-03, 12:54 AM
I wonder how long it will be before Fraser figures out how to post a working link to the podcast. 789 (789 means kidding)

Episode 43 (http://media.libsyn.com/media/astronomycast/AstroCast-070702.mp3)

Galaxy
2007-Jul-03, 01:28 AM
The link goes to the correct mp3 file, it's only the text that was wrong. It's been fixed on the main page.

-Rebecca
Astronomy Cast
Web Assistant

EvilEye
2007-Jul-03, 02:27 AM
I was just teasing. He uses HTML every week, and this site only allows VB code.

Galaxy
2007-Jul-03, 02:41 AM
I was just teasing. He uses HTML every week, and this site only allows VB code.

It's true - and I knew you were teasing.

We've actually got it set up such that when a post is made on the Astronomy Cast homepage (such as a new show), the excerpt shown on the main page is posted to this forum as a new topic by Fraser's login (regardless of which one of us writes the post in WordPress). WordPress uses HTML, and I guess it's more important that it show up properly on the main site than in the discussion forum.

Nevertheless - given time, this might get changed. It's all a matter of time (and money, but they're linked)...

And this week it was me that posted the show - and forgot to change the link text. Oops! I was wanting to get it posted quickly, since it went up later than usual this week... ah well. Life goes on, right? ;)

-Rebecca
Astronomy Cast
Web Assistant

Carl Cohen
2007-Jul-03, 03:45 AM
Both my Itunes and Zencast does not download today's episode. I was able to get a copy of it by manually downloading the MP3. Is there a problem?:confused:

Carl Cohen
2007-Jul-03, 12:51 PM
It's the next morning and it's working now. Not sure what changed but it downloaded! Thanks!:)

Sticks
2007-Jul-03, 01:25 PM
I keep getting an error message trying to download the MP3 :(

gnosys
2007-Jul-03, 01:45 PM
Scott from Columbus asked exactly the question I've always wanted to ask -- about whether or not time "expands" along with space. But, as Scott notes, if spacetime is all of a piece -- if space and time are not two different things -- doesn't it stand to reason that if space expands, SOMETHING analogous is going to happen to time? It's not clear to me, however, that the passage OF time is synonymous with movement THROUGH time -- isn't it conceivable that the passage of time is a subjective rather than objective process, that all of time exists "simultaneously" and NOW (like HERE) simply marks the point we're passing through? In which case the rate time passes might be a purely subjective/local measurement (like the speed of a train), without any relationship to the "SIZE" of time (or the "expansion" of time, if there is such a thing).

To what extent is the expansion of the universe from the Big Bang analogous to the reverse of the curvature of spacetime approaching a black hole? If we can describe what happens to time as we approach a black hole, could we describe what happens to time after the Big Bang as simply the reverse process? That being said, CAN we describe what happens to time as we approach a black hole?

Galaxy
2007-Jul-03, 02:51 PM
It's the next morning and it's working now. Not sure what changed but it downloaded! Thanks!:)

I'm glad it's working for you now, Carl.

-Rebecca
Astronomy Cast
Web Assistant

Galaxy
2007-Jul-03, 02:53 PM
I keep getting an error message trying to download the MP3 :(

That's odd, Sticks. Are you attempting to download it manually, or with an agregator like iTunes?

-Rebecca
Astronomy Cast
Web Assistant

Sticks
2007-Jul-03, 03:26 PM
That's odd, Sticks. Are you attempting to download it manually, or with an agregator like iTunes?

-Rebecca
Astronomy Cast
Web Assistant

Manually

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Sticks
2007-Jul-03, 03:39 PM
It seems to have worked now from my home PC instead of my works PC :confused:

KimB
2007-Jul-03, 06:33 PM
I'm having difficulty playing the show. It stops just after the first question. I play the mp3 file through a internet enabled netgem iPlayer connected to my tv. All the other shows play ok. Has anything changed? bit rate? server?

Nicias
2007-Jul-03, 07:02 PM
Scott from Columbus asked exactly the question I've always wanted to ask -- about whether or not time "expands" along with space. But, as Scott notes, if spacetime is all of a piece -- if space and time are not two different things -- doesn't it stand to reason that if space expands, SOMETHING analogous is going to happen to time? It's not clear to me, however, that the passage OF time is synonymous with movement THROUGH time -- isn't it conceivable that the passage of time is a subjective rather than objective process, that all of time exists "simultaneously" and NOW (like HERE) simply marks the point we're passing through? In which case the rate time passes might be a purely subjective/local measurement (like the speed of a train), without any relationship to the "SIZE" of time (or the "expansion" of time, if there is such a thing).

To what extent is the expansion of the universe from the Big Bang analogous to the reverse of the curvature of spacetime approaching a black hole? If we can describe what happens to time as we approach a black hole, could we describe what happens to time after the Big Bang as simply the reverse process? That being said, CAN we describe what happens to time as we approach a black hole?

Ok, first an introduction. I am graduate student finishing up my doctorate in mathematical general relativity. In particular I study gravitational lensing, so I am quite familiar with the effect of gravity on light.

Second, I completely agree with what Pamela said, so I won't paraphase her here. Thanks for all your hard work Pamela and Frasier, we always can use more outreach in the sciences!

Ok, on to your post, gnosys. Yes, it is mathematically valid to view all of time always existing, just like you say. What this says about free will, I will leave to the philosophers. Your black hole analogy is correct. The traveler falling into the black hole sees nothing wrong with his time. He sees his clocks running normally, until he is destroyed. The observer far away, however, sees the traveler's clock running slower and slower. This is gravitational time dilation. To complete your analogy we would be the traveler. This fits, we don't notice our clocks doing anything funny. (As Pamela pointed out, you *can't* notice your clock doing anything funny, what are you going to compare it to? Your other clock?) The place of the far away observer in the analogy would have to be someone outside the universe, which as Pamela covered, is nonsense. Everything that is, is in the universe. So there your analogy breaks down.

That being said, we can notice relative changes in the speed of time. Deeper in a gravity well, time passes more slowly, so for example, time passes more slowly at the bottom of a building then at the top. (The difference is minuscule, but measurable) This produces a redshift for objects in a gravitational potential. So for example. if we had two equal mass stars, at an equal distance with equal relative motion to us. One of them is a white dwarf, the other a main sequence star. Disregarding convection or rotation of the two stars (which are larger effects) the spectral lines from the red dwarf would be redshifted compared to the main sequence star. This is a known effect and is relevant when studying the spectra of compact objects.

This brings me to the original question. The listener asked if light passing though slow or fast patches would somehow be altered to throw off our measurements. Pamela didn't answer this part. The answer is no. If you do the math, all the matters the gravitational potential where the light was emitted and where it was observed. So if on its way to us light passed close to a white dwarf, and was hence blueshifted (going from high potential to low) it had to climb back out to get to us and so was redshifted by exactly the same amount (going from low potential to high.) The only effect is the relative potentials of the source and observer.

Hope that helps.

-Nicias

crafter
2007-Jul-03, 08:22 PM
Firstly, a most enjoyable episode! Thank you, Fraser and Pamela.

The question about "what is space" included wanting to know what it means for space to be curved. As I understand it, the "curvature" is more of an effect on stuff in space, than on space itself. That is, the effect of gravity is to shape the path taken by particles with mass, as well as massless particles such as photons. However, the point is that it's really just gravity affecting things, rather than a distortion of an underlying "fabric" or anything like that.

Another question, about the expansion of the universe, made the common mistake of assuming that the Big Bang originated at a point (what the questioner called its epicenter), and talked about forces being applied from the direction of expansion. Pamela didn't specifically address this, though it was implied in her answer: space is expanding uniformly, at the same rate, everywhere. That's been happening since the Big Bang. The universe started as a singularity, and has been expanding from there. There's no center of the universe that everything is expanding away from; all space is expanding, everywhere.

five_distinct
2007-Jul-05, 01:31 PM
This second question (Tom's question on "what is space") didn't really seem to be answered. They kind of went off on a tangent about nothing, when I think nothing was more like a hypothetical point to illustrate that space is not nothing. It seemed to me that what he was asking was what space is. He (Tom) seemed to know what it isn't (nothing), but you didn't really expand on what it is.

Pamela (I think that's her name) mentioned a few times that you couldn't have nothing because you have these fields permeating space, whereas the question he seemed to be asking was more about the nature of the "space" that these fields are permeating (and I'm sure he would have used a better word if it were available).

damian1727
2007-Jul-05, 09:14 PM
i like lee smolins view...

there is no space just things and relations between things,,,,:sick:

EvilEye
2007-Jul-06, 03:33 PM
Space is never nothing.

If you put nothing between two objects, they touch each other.