Over-complication of science

[hairykrishna]

Mr Whippy said:

If you remove the sun wouldn't our path suddenly be a straight one relative to where the sun once was?

I thought just as a geostationary satellite is perpetually falling towards earth (and so accelerating as it is changing direction all the time), the same is said of earth falling towards the sun?

Isn't that the definition of acceleration? Changing velocity, and the velocity vector is changing all the time?

Yes, yes and yes.

[Mr Whippy]

I'm confused then, surely orbiting anything is generating lots of time dilation in itself, so a supernova orbiting a galaxy is going to be dilating all over the shop relative to our frame of reference depending if it's on the leading or receding edge of a spiral arm or such?

Hrmmmm...

I guess all the maths stacks up, but I guess some assumptions start to get made about acceleration rates? Or do we just assume that on average as many supernovae are moving away as they are towards by similar rates, so we take an overall average?!

Dave

[hairykrishna]

Mr Whippy said:

I'm confused then, surely orbiting anything is generating lots of time dilation in itself, so a supernova orbiting a galaxy is going to be dilating all over the shop relative to our frame of reference depending if it's on the leading or receding edge of a spiral arm or such?

Hrmmmm...

I guess all the maths stacks up, but I guess some assumptions start to get made about acceleration rates? Or do we just assume that on average as many supernovae are moving away as they are towards by similar rates, so we take an overall average?!

Dave

Explaining it terms of the redshift (because it's easier), some stuff has Doppler blue shift superimposed on the universe expansion redshift. In the case of some stuff that's near to us, that means it's actually blue shifted and in the case of very far away stuff it just means it's very slightly less redshifted (does that make sense?). Same applies to time dilation, more or less.

What this boils down to is essentially what you say - we measure an average. Hubbles law (i.e. stuff further away is more redshifted, expanding away from us faster) is an average, a trend. All galaxies (and smaller stuff) have what astronomers call 'peculiar velocity' - some component of their velocity which is not explained by Hubbles law. The further away they are, the smaller this component is relative to their total redshift.

[Mr Whippy]

Ah ok, that is interesting.

So we see blue shifting, until a certain point where the expansion of the galaxies apart from each other offsets the speed of the outer masses of stars (supernova) within galaxies that are moving towards us.

So from that I suppose we can say that at ~ X distance where we stop seeing blue shifts, the speed of expansion is ~ the speed of the orbit of stars around the galaxy.


So from that can we guesstimate the actual speed of expansion rather than simply saying it is expanding at increasing speed?

Then from that shouldn't we be able to get a rough idea of the increase in angle between two objects in the sky (ie, two distant galaxies) with time to confirm the values by another method?

Ie, two unresolvable galaxies may become resolvable in the next 15 years on instrument X?


Hmmm, if all these things fall into place then I'm generally happy, but these things are not often covered when people on the TV explain things, they just assume that you'll be happy without some explanation and proof!


Thank you

Dave

[hairykrishna]

Mr Whippy said:

Ah ok, that is interesting.

So we see blue shifting, until a certain point where the expansion of the galaxies apart from each other offsets the speed of the outer masses of stars (supernova) within galaxies that are moving towards us.

So from that I suppose we can say that at ~ X distance where we stop seeing blue shifts, the speed of expansion is ~ the speed of the orbit of stars around the galaxy.


So from that can we guesstimate the actual speed of expansion rather than simply saying it is expanding at increasing speed?

Then from that shouldn't we be able to get a rough idea of the increase in angle between two objects in the sky (ie, two distant galaxies) with time to confirm the values by another method?

Ie, two unresolvable galaxies may become resolvable in the next 15 years on instrument X?


Hmmm, if all these things fall into place then I'm generally happy, but these things are not often covered when people on the TV explain things, they just assume that you'll be happy without some explanation and proof!


Thank you

Dave

Andromeda's blue shifted. Not sure which others are close enough and moving in the right direction.
We know the speed of expansion well - that's what Hubble's constant is, effectively. The expansion gives them a 'velocity' of ~74km/s for every megaparsec they are away. The distant galaxies are so far away that the increase in angle will be tiny over any reasonable time frame (I think).

TV's not an ideal medium to learn science from. You need to be able to ask questions and ask for explanations to understand at any depth. It's best as an intro i.e. 'that sounds interesting, I'll learn more about it'

[Mr Whippy]

I'd always had books when younger, and did a GCSE Astronomy in my lunch times at school. So I've always had a keen interest, just it's obviously given way to a past time that gets little time, so TV programs are all it's had for a while.

The TV programs are indeed dumbed down far too much which is why I want to throw the book at the TV half the time. A shame there isn't anything truly worthwhile to watch.


I may have to buy a recent book that just covers everything in a more wordy thoughtful kinda way, rather than just heavy maths. Can you recommend anything that might be down my street?


I bought "Just six numbers" by Martin Rees a few years back which was a thoroughly nice read!

While the book "How to build a time machine" by Paul Davies I bought a year ago was really on the edge of tolerance again. Lots of 'this will happen' but very little provision of proof or background to why that was so. Ie, a book that just generated more questions than provided ANY answers.


Many thanks

Dave

[hairykrishna]

Sorry, I don't really have any recommendations. I find that sooner or later you reach the point where explaining without maths gets pretty tricky. If I was pushed to suggest an intro to relativity I'd go for 'Six not so easy pieces' by Feynman. It's an extract from his undergrad lectures though, so he's not shy about launching into calculation.

[Mr Whippy]

I'll give it a try and report back at some point!

Thanks for the time though, you've covered off a few gaps with sensible logical answers that make it now make much more sense.

Cheers

Dave

[hairykrishna]

No problem, glad it made some sense.