Stupid question about light years

[Astro Noodles]

31 minutes ago, vlaiv said:

Gravitational waves have already been used to measure Hubble constant and so have many different methods - have a look at the list on wiki page:

There are certainly plenty of methods which have been used. It seems that at the moment, Hubble's constant is dependent on the measurement technique used. I notice that some techniques have returned quite a large potential spread of values as well, including the gravitational wave experiments.

It is also interesting that the early universe techniques have returned lower values than those which have used distance ladder measurements.

 

 

[andrew s]

Measuring the Hubble "constant" is not a simple matter of taking a measurement.  To get from the measured values to H requires much theory and in many cases poorly known values for example in nuclear reaction rates and hence stellar evolution.

It is in some ways amazing that it is as close as it is given these constraints.  Just going from the CMB era H to H today requires a model where H changes by many orders of magnitude. 

Regards Andrew 

Edited June 27, 2021 by andrew s

[vlaiv]

@Astro Noodles

Maybe have a look at this youtube channel:

https://www.youtube.com/c/pbsspacetime/playlists

There are bunch of short videos in nice format that deal with host of questions about cosmology (and more). Not overly complex and gives nice introductory overview of what physics says about it all.

For example, this video is on Hubble tension / Crisis in cosmology:

https://www.youtube.com/watch?v=dsCjRjA4O7Y

 

[Astro Noodles]

1 hour ago, vlaiv said:

My friend noted very interesting thing that I'll paraphrase like this: "It's now been almost 100 years that we have two of the most successful physics theories - GR and QM, yet 99.999% of people know nothing about them and have serious trouble comprehending concepts needed to understand those theories".

That is indeed very interesting observation and I can only assume it is so because of the level of mathematics needed to formulate those theories. By contrast - people don't seem to have any problem with Newtonian mechanics and gravity.

Your friend makes a salient point. 

GR and QM ask people to fundamentally re-evaluate their perception of reality. It is all backed up by extremely complicated mathematics which is far beyond the ability of most people to understand which is why people have the problems with comprehension.  We non-Theoretical Physicists have to rely on the words of those teachers who have spent many years of study to achieve the level of understanding and skills necessary to formulate theories and test them.  However, (if they take any interest at all) many people tend to react against anything that has experts or academics telling them what they should think or do. Especially if they are told that they wouldn't understand it anyway even if if it was explained to them. This is certainly not meant to be a pop at science or scientists in any way. I do feel like academia is being undermined, and there is a increasingly large body of people who want to define their own reality/history based on how they feel rather than on evidence or research.

I would dispute that most people have no problem understanding Newtonian mechanics and gravity. I would contend that 'most people' never think about it at all.  People drive cars without understanding how an internal combustion engine works, ride motorcycles without understanding gyroscopic precession, use mobile phones, microwave ovens, the internet etc with no conception of what it is that makes those things work. 

[Astro Noodles]

28 minutes ago, vlaiv said:

@Astro Noodles

Maybe have a look at this youtube channel:

https://www.youtube.com/c/pbsspacetime/playlists

There are bunch of short videos in nice format that deal with host of questions about cosmology (and more). Not overly complex and gives nice introductory overview of what physics says about it all.

For example, this video is on Hubble tension / Crisis in cosmology:

https://www.youtube.com/watch?v=dsCjRjA4O7Y

 

Thanks Vlaiv

I'm currently working my way through everything that Anton Petrov has posted. Working from home, I need something on in the background. I know it's not the ideal learning technique but I can always watch them again in the hope that some of it will sink in. 🙂

[saac]

4 hours ago, Astro Noodles said:

I am just postulating that we might require different sensory equipment to broaden our understanding of the universe.

I agree , I think that is a given Noodles and already reflects our approach to cosmology.   Take for example the many different sensors with which we have equipped our various probes over the years  - each becoming available to us as our technology advances.  We now have at our disposal instruments which will detect across the full frequency range of the em spectrum.   In addition. beyond our mastery of the em spectrum we have already broadened our "eyes and ears"  -  we have used our ability to detect subatomic particles such as neutrinos to conduct observation of the sun.  I first came across this photograph of the sun a few years ago at a lecture at University of Glasgow and found its method of capture  breathtaking - it is a photograph of the sun , taken through the Earth, using neutrinos !  And of course laterally we have started to play with and build expertise in the use of gravitational waves to observe the universe beyond that which the em spectrum would disclose.  I do believe we are made to observe and understand the universe and for sure as we discover and master other phenomena from particle physics or elsewhere we will for sure employ those in our ever expanding "tool box" of observational capabilities. One of the great strengths of science is its incrementing nature - no matter how deep or far we must go we will, step by step, building on what went before.  Perhaps a time will come when we will be using one of the many other exotic particles, perhaps yet to be discovered,  to measure and observe beyond the event horizon of black holes or dark matter .  If it can be done I am sure we will do it . 

Jim

http://strangepaths.com/the-sun-seen-through-the-earth-in-neutrino-light/2007/01/06/en/

https://neutrinos.fnal.gov/sources/solar-neutrinos/

 

 

 

 

Edited June 27, 2021 by saac

[Astro Noodles]

That's an amazing image

[saac]

1 minute ago, Astro Noodles said:

That's an amazing image

It is isn't it .  And just like the recent image of the Black Hole taken by the Event Horizon Telescope the measure of "amazing" extends far beyond the aesthetics of the image itself.  I find that reading into how these images are made is like taking a trip through the looking glass.   I still find that image of the sun breathtaking, beyond any of the high resolution images taken of our star by more conventional means.  And of course the true worth is held in the examination and understanding of the data the image contains.  

Jim

[Astro Noodles]

So, back to the original question for a recap of what I think I know now. 😁

So, an object can be said to be 10 billion light years away if the light travel time between that object and my instrument is 10 billion years.

The light travel time is affected by Hubbles law which uses the red-shifting of light to determine the rate at which an object is accelerating away. Also, the expansion of the universe.

So it would be possible to say that an object was perhaps 10 billion years of light travel time away as observed. But it would also be possible to say, for example that it is 20 billion light years away in distance due to Hubble's law and expansion. Furthermore it can be stated that the two results are not in conflict with each other because they are measuring different things. And then it is also true that 20 billion light years away is not a paradox which makes the object older than the universe, it is just an artifice caused by the standard of measurement selected which is not really appropriate in this case.

Am I on the right track with this?

I can see that the way a question is asked can be just as problematical as providing an answer. Terms of reference and definition are key to this. As I now understand it, asking a question like how far away is the edge of the universe is like asking how far away is forever?

[vlaiv]

53 minutes ago, Astro Noodles said:

And then it is also true that 20 billion light years away is not a paradox which makes the object older than the universe, it is just an artifice caused by the standard of measurement selected which is not really appropriate in this case.

It's not artifact. At current moment in time - object can indeed be 20 billion light years away and no, that does not mean that it is some sort of paradox in terms of age.

Don't confuse distance with time. Only way they are related is in moment of observation.

If we are seeing now - at this moment object that is 10Bly away (calculated by red shift and varying Hubble constant across the time) - it only means that distance traveled by light from emission moment up to now is 10Bly and that object is at least 10By old. If object is now at 20Bly away from us (but we don't see it there now) - well it does not mean that it is 20 By old, but it will be at least 20By old at the time in the future when observer on the Earth sees it as 20Bly away (we don't see it now but we will see it in the future once light emitted now reaches us).

We can't see objects that are older than 13.7By - not ones that are further than 13.7By - first is due to age of universe and later is because light did not have enough time to reach us. That does not mean that there are no objects that are further than that at this moment, nor does it mean that there are no objects that were farther than that at the beginning just 1By after big bang for example.

In the end - there are object that we will never see - since there is accelerated expansion of universe, light that left those objects will never reach us as universe will expand more than the distance light can travel in given amount of time and there will always be "more to travel" before it reaches us.

 

[vlaiv]

1 hour ago, Astro Noodles said:

Am I on the right track with this?

Other than above remark - pretty much yes.

[Astro Noodles]

Thank you all for sharing your knowledge, especially @vlaiv for the detailed explanations.

As you will know, I now have many more questions than I started with. 😁

[Tiny Clanger]

5 minutes ago, Astro Noodles said:

As you will know, I now have many more questions than I started with. 😁

That's science !

[Astro Noodles]

6 minutes ago, Tiny Clanger said:

That's science !

It's certainly the way I know I'm learning something.

I'm certainly enjoying these personal tutorials.

Edited June 28, 2021 by Astro Noodles

[wimvb]

At about this point in the discussion, I'd recommend you get a copy of "The Evolution of Physics", by Leopold Infeld and Albert Einstein. A highly readable book. Paper copies are still available, but it's also available online.

https://archive.org/details/evolutionofphysi033254mbp/mode/2up