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Author Topic:   Astronomers See Evidence of Something Unexpected in the Universe
Taq
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Posts: 8488
Joined: 03-06-2009
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(2)
Message 11 of 86 (829017)
02-28-2018 4:14 PM
Reply to: Message 7 by nwr
02-26-2018 8:47 PM


nwr writes:

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.

The yardstick is the speed of light, spectra, and type Ia supernovae. To use another example, we measure the distance to the Moon using the time it takes for a laser to bounce off of the Apollo reflector on the Moon and return to Earth.

Type Ia supernovae have very similar brightness because they explode with the same mass (E=mcc), so the distance can be determined by luminosity. Next, you have the amount of redshift as determined by the shift in spectral lines which is ultimately determined by the speed of light. When you plot luminosity vs. redshift you get this wonderful correlation:

Another factor to consider is that if redshift is due to relative differences in velocity due to expansion then high redshift type Ia supernovae should brighten and dim at slower rates due to relativistic effects. That is also seen. A cosmological redshift unrelated to expansion should not do this, and yet it is there. Also, there is no other known mechanism that would redshift all wavelengths by the same amount. Other mechanism are wavelength dependent and will shift some wavelengths more than others.


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Replies to this message:
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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 12 of 86 (829018)
02-28-2018 4:16 PM
Reply to: Message 9 by nwr
02-26-2018 10:26 PM


nwr writes:

Our main mistake is in our understanding of "objective". People take it for granted that there is an objective human-independent world. But everything that we know about is a projection from our subjective experience. What we call "objective" is really those aspects of subjective experience that humans can share with one another. And we project that to what we take to be a human independent cosmos. But perhaps it is not nearly as human independent as we like to think that it is.

Two scientists can independently measure redshift and type Ia supernovae luminosity without ever communicating with one another and still produce data sets that agree with one another. I would call that objective.


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Taq
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Posts: 8488
Joined: 03-06-2009
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Message 14 of 86 (829026)
02-28-2018 6:17 PM
Reply to: Message 13 by Percy
02-28-2018 4:44 PM


Percy writes:

The best example I can think of is that we still haven't defined mass in terms of fundamental constants - there's still a standard kilogram kept in a vault outside Paris.

You might be interested in the new proposed standard that is defined in terms of fundamental constants. It is a polished sphere of crystalline silicon-28. They can very precisely measure the volume of the sphere, and from there they can accurately calculate the number of atoms in the sphere due to the consistent nature of crystals formed from silicon-28. Once you have the number of atoms you have the kilogram. This gets around the issues of the old kg standard changing in weight over time due to accumulation of oxidation and other such issues. Using this standard you could travel to an entirely new planet with no standards from Earth and still be able to very accurately produce a kg standard that agrees with the one on Earth.

https://www.nist.gov/...s-and-international-avogadro-project


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Taq
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Posts: 8488
Joined: 03-06-2009
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(1)
Message 18 of 86 (829078)
03-02-2018 4:19 PM
Reply to: Message 15 by nwr
02-28-2018 6:37 PM


nwr writes:

To use the speed of light as a yardstick, you need a fixed clock. But we don't have one.

Due to the fact that E=mc^2 we would see major changes in stellar activity, color, spectra, and isotope production if the speed of light were different elsewhere in the cosmos. The balance between gravity pushing inwards and the energy of fusion pushing outwards is the basis for a star's major characteristics, and if that energy changed then we would see big differences. The speed of light is also a part of many other fundamental properties of matter that would also look different elsewhere in the cosmos if the speed of light were different in those regions. The very fact that type Ia supernovae increase and decrease in brightness at the same rate is yet another piece of evidence for the constancy of the speed of light.


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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 25 of 86 (829288)
03-05-2018 12:47 PM
Reply to: Message 19 by nwr
03-03-2018 2:25 PM


nwr writes:

Rather, I am questioning the meaningfulness of even talking about whether the speed of light is different in other parts of the cosmos.

It is rather important to our whole understanding of how the universe works, so there's that.

Everything that we say about the speed of light depends on earth-based standards. I am questioning whether those earth-based standards are extensible throughout the entire cosmos.

And I am telling you that they can be extended to the entire cosmos, and can be used to ask and answer very important questions about the fundamental aspects of how physics works, both here and elsewhere in the cosmos.


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Taq
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Posts: 8488
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Message 29 of 86 (829346)
03-06-2018 11:20 AM
Reply to: Message 27 by Son Goku
03-06-2018 4:56 AM


Son Goku writes:

Unless I am missing the point General Relativity actually assumes that standard clocks and rulers don't exist and then makes predictions for what we should see here based on that.

GR does posit that the speed of light is the same for all observers in all reference frames.


This message is a reply to:
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Replies to this message:
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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 37 of 86 (829406)
03-06-2018 5:16 PM
Reply to: Message 32 by nwr
03-06-2018 12:50 PM


nwr writes:

Thus it could be that local clocks and rulers show that the cosmos appears to be expanding locally, yet there is no meaning of "expanding" that applies to the cosmos as a whole.

If the expansion was local then all galaxies outside of the local area would be redshifted by the same amount, but they aren't. More distant galaxies are redshifted more strongly than nearby galaxies, and there isn't a point in the cosmos where redshift caps out. The only explanation that makes sense is that the entire length of space between us and all galaxies is expanding at the same rate per unit of length.


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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 38 of 86 (829408)
03-06-2018 5:20 PM
Reply to: Message 36 by Son Goku
03-06-2018 4:14 PM


Son Goku writes:

Special Relativity posits that all inertial observers agree on the speed of light.
General Relativity, lacking global inertial observers (i.e in general there are no inertial frames) doesn't even posit that. The speed of light will be observer dependent.

Could you describe a situation where the speed of light is observer dependent within GR?


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Replies to this message:
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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 39 of 86 (829409)
03-06-2018 5:28 PM
Reply to: Message 34 by ICANT
03-06-2018 2:43 PM


Re: Time
ICANT writes:

Just one of my foolish thoughts but I thought it was a given that time in space does not behave the same as it does on earth.

In one of the twin's taking a journey into space and returning that twin is supposed to have aged a lot less than the twin on earth.

So wouldn't that mean that the speed something traveled in space would change time compared to here on earth.

Clocks do tick at different rates with respect to gravity and relative velocity. The famous Hafele-Keating experiment used atomic clocks on jetliners to determine that clocks tick faster further away from the Earth's center of gravity. In modern times we observe that the clocks on GPS satellites tick at a higher rate than those same clocks on the surface of the Earth. However, the differences are in the nanosecond range, so not that significant.

Cosmologically, you don't see significant time dilation until you get to the highest redshifted galaxies. In these cases, scientists have seen time dilation in the form of type Ia supernovae taking longer to brighten and dim compared to the same supernovae in nearer and less redshifted galaxies. Within our own galaxy, movements are dominated by gravity so any redshift we see is due to movement through space instead of expansion of space itself.


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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 42 of 86 (829442)
03-07-2018 1:14 PM
Reply to: Message 41 by nwr
03-06-2018 8:32 PM


nwr writes:

This is still a complete misunderstanding of what I have been saying.

You can hardly blame us if you then refuse to clear up any misunderstandings.


This message is a reply to:
 Message 41 by nwr, posted 03-06-2018 8:32 PM nwr has responded

Replies to this message:
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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


(1)
Message 43 of 86 (829449)
03-07-2018 3:37 PM
Reply to: Message 40 by Son Goku
03-06-2018 8:09 PM


Son Goku writes:

I don't know how much detail you want (actual calculations?), but if you shine light on a mirror above the event horizon of a black hole and time the return of the beam and the distance, you won't get the usual speed of light.

I would assume that this would also be true for a laser beam shot from space and bouncing off of a mirror 500 miles below on the face of the Earth (but obviously with less of a difference than with a black hole)?


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Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 47 of 86 (829560)
03-09-2018 12:12 PM
Reply to: Message 46 by nwr
03-08-2018 11:27 PM


nwr writes:

I'm not blaming anyone. I've tried to clear up misunderstandings, but it hasn't worked.

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?


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 Message 46 by nwr, posted 03-08-2018 11:27 PM nwr has responded

Replies to this message:
 Message 48 by nwr, posted 03-09-2018 1:00 PM Taq has responded

  
Taq
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Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 49 of 86 (829568)
03-09-2018 3:30 PM
Reply to: Message 48 by nwr
03-09-2018 1:00 PM


nwr writes:

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

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.

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.

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.


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Taq
Member
Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 51 of 86 (829578)
03-09-2018 5:55 PM
Reply to: Message 50 by nwr
03-09-2018 4:14 PM


nwr writes:

But that is NOT what I have been saying.

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

I keep wondering why it is so important to you to deliberately misconstrue what I am saying.
I said nothing about "empirical deviations from a flat earth". I said nothing about "the curvature of the Earth".

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?


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 Message 50 by nwr, posted 03-09-2018 4:14 PM nwr has responded

Replies to this message:
 Message 52 by nwr, posted 03-12-2018 12:01 PM Taq has responded
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Taq
Member
Posts: 8488
Joined: 03-06-2009
Member Rating: 7.4


Message 53 of 86 (829703)
03-12-2018 12:22 PM
Reply to: Message 52 by nwr
03-12-2018 12:01 PM


nwr writes:

No.

Then what did you mean?


This message is a reply to:
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Replies to this message:
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