Message 47 of 86 (829560) 03-09-2018 12:12 PM

Reply to: Message 46 by nwr 03-08-2018 11:27 PM

Let's go back to your first post in this thread, where you said:

"Talk of "the expanding universe" implicitly assumes that there is a yardstick (for measuring distance) which can be used throughout the cosmos and for all time. I cannot see any reason to believe that. We use a local yardstick defined in terms of local features. And it may well be that every location can have a local yardstick."

So what is "local"? Is it the Earth, the solar system, our arm of the Milky Way, the Milky Way itself, or is it the Virgo supercluster that our galaxy belongs to?

Since the Earth, our solar system, our galaxy, and our galaxy supercluster are all moving we are never in the same place from one moment to the next. I can't see how you can say anything about something being "local" in that sense. Experiments measuring the speed of light over long time periods have not seen any statistical deviation in the speed of light as the Earth, solar system, and Milky Way hurtle through space into new regions of space.

On top of that, the constancy of the speed of light can be indirectly measured by looking at how stars and other cosmological bodies behave, and no significant deviation from the expected values is seen.

So why do you find these measurements to be insufficient? What do you mean by "local"? Do we live in a bubble of spacetime that is different from the rest of the universe?

Message 48 of 86 (829564) 03-09-2018 1:00 PM

Reply to: Message 47 by Taq 03-09-2018 12:12 PM

I applied "local" to our yardstick. Whether or not Virgo counts as local is not relevant to that.

Look around your town. You can describe it quite well in "flat earth" terms. All you need is a suitable coordinate grid.

You can then extend that "flat earth" grid to include the next town, even the whole county, and perhaps the whole state.

What you cannot do, is extend it to the entire earth. And the reason is that the topology of the sphere does not permit such an extension.

I was making a point about an analogous problem.

Message 49 of 86 (829568) 03-09-2018 3:30 PM

Reply to: Message 48 by nwr 03-09-2018 1:00 PM

Yes, it does matter. If you are saying that the laws of physics are different outside of a local area then your definition of local matters.

So what is the cosmological equivalent to a town?

Also, you don't have to move very far to find empirical devations from a flat Earth. With sensitive enough instruments you could probably measure the curvature of the Earth over a distance of less than a mile.

Message 50 of 86 (829570) 03-09-2018 4:14 PM

Reply to: Message 49 by Taq 03-09-2018 3:30 PM

I said nothing about "empirical deviations from a flat earth". I said nothing about "the curvature of the Earth".

Message 51 of 86 (829578) 03-09-2018 5:55 PM

Reply to: Message 50 by nwr 03-09-2018 4:14 PM

Well, your other personality did say that very thing in other posts.

Then you put a lot of effort into writing a post that said just the opposite. If you didn't mean that you can model a town as if it were a flat Earth, then what in the world did you mean?

If we could compare the curvature of the Earth to the expansion of the universe then it may make sense. Over short distances the expansion isn't that noticeable, but over larger distances it is noticeable. Is this what you meant?

Message 53 of 86 (829703) 03-12-2018 12:22 PM

Reply to: Message 52 by nwr 03-12-2018 12:01 PM

Then what did you mean?

Message 54 of 86 (829706) 03-12-2018 1:09 PM

Reply to: Message 53 by Taq 03-12-2018 12:22 PM

There's no point in trying again.

Message 55 of 86 (829714) 03-12-2018 1:50 PM

Reply to: Message 54 by nwr 03-12-2018 1:09 PM

Pushing through the difficulties involved in communicating fine distinctions may be rewarding in a couple of ways. You may eventually find success, and it could lead to unexpected insights, either for you or others.

--Percy

Message 56 of 86 (829778) 03-13-2018 4:38 PM

Reply to: Message 54 by nwr 03-12-2018 1:09 PM

I enjoy talking about theoretical physics and cosmology, so I will give it another try.

In your first post in the thread you talked about not having a yardstick that can be used through the entire cosmos. Cosmologists do think they have a common yardstick, or more precisely a standard candle.

The concept is this. If you stand 1 foot away from a candle it will have a certain measured brightness or luminosity. If you move 10 feet away its luminosity drops, and it keeps dropping at a specific rate over distance. If you use the same candle or a standard candle, you can always measure distance by measuring the luminosity.

The standard candles in cosmology are type Ia supernovae and Cepheid variables.

"It is possible to estimate the distance to a Cepheid in a far-off galaxy as follows: firstly, locate the Cepheid variable in the galaxy, then measure the variation in its brightness over a given period of time. From this you can calculate its period of variability. You can then use the luminosity-period graph (below) to estimate the average luminosity. Finally, armed with the average luminosity, the average brightness and using the inverse square law, you can estimate the distance to the star."
http://sci.esa.int/education/35616-stellar-distances/?fbo...

"When the white dwarf reaches 1.4 solar masses, or about 40 percent more massive than our Sun, a nuclear chain reaction occurs, causing the white dwarf to explode. The resulting light is 5 billion times brighter than the Sun.

Because the chain reaction always happens in the same way, and at the same mass, the brightness of these Type Ia supernovae are also always the same. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the astronomer who discovered it.

To find the distance to the galaxy that contains the supernova, scientists just have to compare how bright they know the explosion should be with how bright the explosion appears. Using the inverse square law, they can compute the distance to the supernova and thus to the supernova's home galaxy."
http://hubblesite.org/...rk_energy/de-type_ia_supernovae.php

Do you have any objections to using these standard candles to measure distances to distant galaxies?

Message 58 of 86 (829797) 03-14-2018 12:47 PM

Reply to: Message 57 by nwr 03-14-2018 11:36 AM

Since we have agreement on the yardstick for measuring distances, let's also discuss how redshift is measured and see if the point you are trying to get across has any impact on these measurements.

Over at the SDSS server you can get the actual spectra for galaxies.

http://skyserver.sdss.org/dr14/en/get/SpecById.ashx?id=43...

I'm kind of a nut for liking to look at raw data, but I think it illustrates a point. Those are the adsorption and emission lines for different elements in those galaxies, and they can measure their wavelength. They can then compare those wavelengths to what they are in a stationary frame of reference. As it turns out, these emission and absorption lines are shifted towards lower wavelengths, and that shift is expressed as a z value. Not only that, but the change in wavelength is the same for all wavelengths which is why it is called a wavelength independent redshift. Mechanisms that directly interact with light will do so in a wavelength dependent manner, meaning that different wavelengths will be more strongly redshifted than others. The only known mechanism that produces a wavelength independent redshift is a difference in velocity between two objects.

When you plot distance v. z value you get an extremely strong correlation. Cosmologists interpret this as all galaxies moving away from our galaxy in a distance dependent manner. The only thing that makes sense of these observations is that all space is expanding equally in all directions, everywhere.

Is there something you object to in these measurements of redshift and distance, or the conclusions drawn from them?

Edited by Taq, : No reason given.

Message 59 of 86 (829851) 03-15-2018 10:40 AM

Reply to: Message 51 by Taq 03-09-2018 5:55 PM

I'm not entirely sure either.

But my guess is that your take-away here is slightly off.

The curvature of the Earth analogy wasn't meant to be specifically applied to the expansion of the universe.

The curvature of the Earth analogy was meant to broadly suggest that there are times where we "think things are as we understand them" (early science thinking the world is flat) but they eventually turn out to not be true (science eventually uncovering that the Earth is round).

I think this curvature of the Earth analogy was meant to be applied to the broad sense that our knowledge-of-the-universe is at the level where we "think things are as we understand them" but cannot guarantee that this will not "eventually turn out to not be true."

"Eventually turn out to not be true" does not apply to anything specific.. like expansion of the universe or current-verifications-of-the-speed-of-light.
It only applies to the very general "we don't know as much about the universe as we know about the surface of the Earth" and therefore, there is (significant?) room for a "oh, hey... the earth is round" moment in our understanding of the universe.

It seems unfair that the point is general and vague.
However, the point necessarily must be general and vague. If it had specifics... well, then we would be on the road to beginning to understand if those specifics will lead us to an "oh, hey..." moment.

The entire point is that our knowledge-of-the-universe is at such a level where there is easily enough room to fit in such general, vague and historically-precedent-ed issue-possibilities.
That is, science-history is filled with "oh, hey... " moments where we thought everything was understood this way, until we also uncovered information that shows it's not "this way" but actually "that way."

If that is, indeed, the point.
I would offer that science is well aware of this. It is not a new idea by any means.
And that the only way to make progress into it... is to carry on exactly as we have been:

-making predictions based on things as we "currently know" them to be.
-act as if everything acts as we "currently know" them to be.
-when (if?) anything ever contradicts these assumptions... then study the area for replication/verification and a growth in understanding to update what we "currently know."

Or... maybe I missed the point, too