Relativity Question

[sjastro]

Originally Posted by CraigS

So, photons "blink out", and also diminish from our abilities to detect 'em because of extreme redshifts, eh ?

So, which happens first .. and;
Does this phenomenon define the difference between a merely expanding universe .. and an acceleratingly expanding one ?

Cheers

Photons "blink out" because they no longer reach the observer. Due to time dilation they no longer have the time reach us.

In an accelerating Universe, objects that are now at or near the horizon will eventually disappear.
In a merely expanding or inertial expanding Universe there is a gradual slowdown (but still infinite) expansion. The opposite can occur. Objects that are just beyond the event horizon may become visible later on.

Regards

Steven

[CraigS]

Ok .. just drumming it into my brain:

There's light emitting things (galaxies, quasars, S/Ns, etc) that:

i) we can presently see;
ii) we can't presently see, but we will be able to see in the future (and we'll see them in their infancy);
iii) we can't presently see .. and never will see (causally disconnected).

However, for type (ii) above, it is conceivable that the light we eventually receive from these things will be redshifted to the point of being physically undetectable anyway, so we are causally disconnected from it anyway.

We should improve our technologies for detecting sub-mircrowave, otherwise we'll miss all the action !

Who is to say that we're not already missing stuff which happened > 13.7 billion years ago, just because we haven't built a detector which can receive it ?

Cheers

[renormalised (Carl)]

You mean further than 47.5Gly comoving distance. Anything older in 13.7Ga light travel time and we'd be seeing.............................

If they're over the event horizon now, we will never see them because they will always be traveling faster than c. The curvature of spacetime for these objects will always be too great for the light to traverse to us. Especially since the Universe appears to be accelerating in its expansion.

[Robh (Rob)]

Originally Posted by CraigS

Ok .. just drumming it into my brain:

There's light emitting things (galaxies, quasars, S/Ns, etc) that:

i) we can presently see;
ii) we can't presently see, but we will be able to see in the future (and we'll see them in their infancy);
iii) we can't presently see .. and never will see (causally disconnected).

However, for type (ii) above, it is conceivable that the light we eventually receive from these things will be redshifted to the point of being physically undetectable anyway, so we are causally disconnected from it anyway.

We should improve our technologies for detecting sub-mircrowave, otherwise we'll miss all the action !

Who is to say that we're not already missing stuff which happened > 13.7 billion years ago, just because we haven't built a detector which can receive it ?

Cheers

Craig,

I often forget about the scenario of your point (ii).
One of the purposes of the James Webb Space Telescope was to partially bridge this gap i.e. pick up the infra-red spectrum. The Hubble has had great success in the visible spectrum.

A pity that the James Webb is in danger of being canned because of lack of funding (today's news).

Regards, Rob

[renormalised (Carl)]

Originally Posted by Robh

Craig,

I often forget about the scenario of your point (ii).
One of the purposes of the James Webb Space Telescope was to partially bridge this gap i.e. pick up the infra-red spectrum. The Hubble has had great success in the visible spectrum.

A pity that the James Webb is in danger of being canned because of lack of funding (today's news).

Regards, Rob

The US is going backwards...all because of the idiots running the place. Don't be a bit surprised if NASA becomes in danger of folding completely if they keep going along their present course.

It would be a great shame if they can the JWST. And a huge waste of money.

[Robh (Rob)]

A bit off the track but interesting blog on the JWST, funding and purpose.
Love the enthusiasm displayed in the video.
http://blogs.discovermagazine.com/co...ace-telescope/

Rob

[CraigS]

Originally Posted by renormalised

You mean further than 47.5Gly comoving distance. Anything older in 13.7Ga light travel time and we'd be seeing.............................

If they're over the event horizon now, we will never see them because they will always be traveling faster than c. The curvature of spacetime for these objects will always be too great for the light to traverse to us. Especially since the Universe appears to be accelerating in its expansion.

Err … yep.

The limits of the equations in the Wiki article I mention in my previous post are interesting when viewed from accelerating and non-accelerating universe perspectives … might do some more thinking about these ..
.. Interesting ..

Cheers

[CraigS]

Originally Posted by Robh

Craig,

I often forget about the scenario of your point (ii).
One of the purposes of the James Webb Space Telescope was to partially bridge this gap i.e. pick up the infra-red spectrum. The Hubble has had great success in the visible spectrum.

A pity that the James Webb is in danger of being canned because of lack of funding (today's news).

Regards, Rob

They (the US House of Representatives Appropriations Subcommittee on Commerce, Justice, and Science), voted against further funding in the next year, eh ?
NASA's backed off and said it'll try and launch it in the next decade (2018 at the earliest ?)

Not good.

Cheers

[CraigS]

Thanks for the help … everybody.

Everytime I revisit this topic, I come away with a different perspective on it all.
(Will that ever change ?)

Cheers

[CraigS]

Just for a laugh ..

I should say I neglected to mention that photon Y was originally emitted by an AGN containing a quasar ... and therefore was 'immune' from the effects of time dilation.

Cheers

[Archy (George)]

Originally Posted by renormalised

Photon....not proton. Very easily, with a photomultiplier tube and in exactly the same way as you measure a redshifted spectrum. You look at (or determine via theory) what the original wavelength/frequency of the photon was when it left the source and compare that with what you measured at the photomultiplier.

Sorry, I meant photon
Yes, you can measure the wavelength/frequency of the photon when it arrives, but how do you determine what the original wavelength/frequency of the photon was when it left the source?

[Archy (George)]

Originally Posted by CraigS

Just for a laugh ..

I should say I neglected to mention that photon Y was originally emitted by an AGN containing a quasar ... and therefore was 'immune' from the effects of time dilation.

Cheers

If relativity applies, how does a photon become immune

[sjastro]

Originally Posted by Archy

If relativity applies, how does a photon become immune

Relativity also "applies" to non expanding cosmological models where time dilation doesn't exist.
Metric expansion (cosmological redshift) is a property of the cosmological model not GR.

The only form of redshift which is derived by GR/SR is gravitational redshift.

[Archy (George)]

Originally Posted by sjastro

Relativity also "applies" to non expanding cosmological models where time dilation doesn't exist.
Metric expansion (cosmological redshift) is a property of the cosmological model not GR.

The only form of redshift which is derived by GR/SR is gravitational redshift.

Don't beat around the bush: how does a photon become immune

[renormalised (Carl)]

Originally Posted by Archy

Don't beat around the bush: how does a photon become immune

He's not, he's trying to answer your question directly.

How does a photon become immune....no one really knows. But if you look at the timing of light pulses in the variability of the quasar's central regions, you find that no matter how far away or close the quasars are, the variability occurs at roughly the same time intervals. Meaning that the light coming from these objects is not subject to time dilation due to cosmological expansion. There could be several explanations for this, the first being there is a population of very massive objects scattered randomly across the universe between us and the quasars...namely small black holes. But observations so far discount this possibility. Another is that the quasars are not at the distances their redshifts say they are, which means the BBT is wrong and we need to figure out why (look at Steven's answer). Or, there is something wrong with SR/GR w.r.t. quasars and we need to find out why, also. There maybe other reasons, but it's late and I'm off to zzzzland

That's the long and short of it.

[sjastro]

Originally Posted by Archy

Don't beat around the bush: how does a photon become immune

Evidently my response went over your head, so let me try to make as simple as possible for you.

The mathematical equations behind GR cannot be solved directly. Solutions have been provided by a process similar to reverse engineering, a solution is constructed based on certain physical assumptions. The solution is then plugged into the field equations. If they satisfy the field equations you have a model you can build on.
Cosmological models are built on this foundation. The important point is that GR provides the gravitational theory for these models and that the physical assumptions of the model do not violate the field equations.

A typical physical assumption for a cosmological model is whether the Universe expands, contracts, oscillates, rotates or remains static.
There are various cosmological models reflecting these scenarios all of which conform to the field equations. GR doesn't determine these assumptions.

As Carl has pointed out time dilation is a property of an expanding Universe cosmology, not a relativity issue.

[CraigS]

Originally Posted by Archy

Don't beat around the bush: how does a photon become immune

I like beating around the bush … why don't you ?

[avandonk]

Actually Craig the two photons are everywhere and everywhen in all the Universes. They have followed all possible paths before they set off. You have no way of proving any alternative until you measure them. Then you are stuck with a result you cannot understand.

It is far more satisfying to beat around the bush! Completely pointless but satisfying.

Bert

[CraigS]

Bert;
Are you tellin' me that we can't gain any understandable knowledge of the physical world from the Science of Astronomy ?

Bear Grylls beats around bushes … and he's a living legend !

Cheers

[renormalised (Carl)]

Originally Posted by CraigS

Bear Grylls beats around bushes … and he's a living legend !

Cheers

And he uses a large stick