Relativity Question

[avandonk]

The bush I like to beat about is totally obvious!
Bert

[CraigS]

[renormalised (Carl)]

Originally Posted by avandonk

The bush I like to beat about is totally obvious!
Bert

George "Dubya"???

With a baseball bat??

It's all "relative" anyway

[avandonk]

I have enjoyed my whole life by facing the bush face or head on. It cannot be ignored. It needs nurturing as we come from the bush. It is not for nothing all Aussies head for the bush on long weekends.


Bert

[Archy (George)]

Originally Posted by sjastro

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.

What an arrogant reply!

You say:
"time dilation is a property of an expanding Universe cosmology, not a relativity issue."

Yet Wikipedia says:
"Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes. Albert Einstein originally predicted this effect in his theory of relativity and it has since been confirmed by tests of general relativity."

[CraigS]

Originally Posted by Archy

What an arrogant reply!

Accusing others of 'beating around the bush' is guaranteed to solicit a response in kind.
This wording implies that you know the intentions of the respondent before he has stated them.

How do you know this ?

I for one, understood Steven's considered response and I thank him for it.

Cheers

[renormalised (Carl)]

Originally Posted by Archy

What an arrogant reply!

You say:
"time dilation is a property of an expanding Universe cosmology, not a relativity issue."

Yet Wikipedia says:
"Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes. Albert Einstein originally predicted this effect in his theory of relativity and it has since been confirmed by tests of general relativity."

Excuse me, but you need to reread and understand in what context you're trying to place time dilation. What you have contextualised in time dilation is in response to the expansion of spacetime. In which case, there is no issues regarding relativity, only as a property of expanding spacetime cosmology. So, it is you that has misunderstood your own question.

This is precisely why most people get their knickers in a twist over science when they ask questions and expect certain answers. You don't understand the science in the first place and you only paraphrase what you've read. Quoting websites isn't going to give you the answers to your questions, nor is it going to contradict the answers of others unless those others have got the answers wrong themselves. Steven knows exactly where he is coming from...his degrees are in applied maths and physics.

If you want to know about science, fine, read up about it. But if you want to understand it, learn off someone who already knows about it as well. Understanding comes through asking questions about those subjects you read. You won't get that through rote learning.

[sjastro]

Originally Posted by Archy

What an arrogant reply!

You say:
"time dilation is a property of an expanding Universe cosmology, not a relativity issue."

Yet Wikipedia says:
"Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes. Albert Einstein originally predicted this effect in his theory of relativity and it has since been confirmed by tests of general relativity."

You obviously have a problem with basic comprehension skills.

Here is my original quote.

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.

The discussion centres around time dilation effects due to cosmological redshift not gravitational redshift.

[Archy (George)]

Originally Posted by sjastro

You obviously have a problem with basic comprehension skills.

Here is my original quote.



The discussion centres around time dilation effects due to cosmological redshift not gravitational redshift.

You skirt around my original question and you answer with insults: is this the best you can do?

I repeat:

I can understand a redshifted spectrum: how can the "redshift" of a single photon be measured:

[Archy (George)]

Originally Posted by renormalised

Excuse me, but you need to reread and understand in what context you're trying to place time dilation. What you have contextualised in time dilation is in response to the expansion of spacetime. In which case, there is no issues regarding relativity, only as a property of expanding spacetime cosmology. So, it is you that has misunderstood your own question.

This is precisely why most people get their knickers in a twist over science when they ask questions and expect certain answers. You don't understand the science in the first place and you only paraphrase what you've read. Quoting websites isn't going to give you the answers to your questions, nor is it going to contradict the answers of others unless those others have got the answers wrong themselves. Steven knows exactly where he is coming from...his degrees are in applied maths and physics.

If you want to know about science, fine, read up about it. But if you want to understand it, learn off someone who already knows about it as well. Understanding comes through asking questions about those subjects you read. You won't get that through rote learning.

You skirt around my original question and you answer with insults: is this the best you can do?

I repeat:

I can understand a redshifted spectrum: how can the "redshift" of a single photon be measured:

[Archy (George)]

Originally Posted by CraigS

Accusing others of 'beating around the bush' is guaranteed to solicit a response in kind.
This wording implies that you know the intentions of the respondent before he has stated them.

How do you know this ?

I for one, understood Steven's considered response and I thank him for it.

Cheers

Still beating around the bush
You skirt around my original question: is this the best you can do?

I repeat:

I can understand a redshifted spectrum: how can the "redshift" of a single photon be measured:

[CraigS]

I like to skirt around questions .. don't you ?

[Archy (George)]

Originally Posted by CraigS

I like to skirt around questions .. don't you ?

This is the best you can do!

[CraigS]

Originally Posted by Archy

This is the best you can do!

Is that the best you can come up with ?

[renormalised (Carl)]

Originally Posted by Archy

You skirt around my original question and you answer with insults: is this the best you can do?

I repeat:

I can understand a redshifted spectrum: how can the "redshift" of a single photon be measured:

Just what is exactly your game here???. Are you just going to keep repeating yourself and throwing our answers back in our faces??

Or, are you just trying to be funny or think yourself smart by being facetious.

If you can't understand what I (or the others) wrote then you really need to back away from this subject because all you'll do is just make things harder on yourself.

Put it this way, if you can understand a redshifted spectrum, then you should understand how a redshifted singular photon is measured. If you can't, then why should I (or anyone else) even bother trying to explain it to you. It would be a waste of time. However, for the record, you also wanted to know about the immunity of a photon from time dilation. You were told why yet it's very apparent you either didn't believe the explanation and/or didn't understand it. Most likely both. That's your prerogative, but don't come out and arrogantly accuse anyone here of "beating around the bush" when it's supremely evident from your puerile responses that you're all too eminently good at it yourself.

[sjastro]

Archy,

Is it really worth being argumentative at the risk making yourself look more and more foolish in the process?

This thread like your posting history in the Science forum in general, clearly indicates a willingness to pick fights with individuals instead of making a positive contribution.

I am not going to waste my time in engaging in fights.

[Archy (George)]

Originally Posted by renormalised

Just what is exactly your game here???. Are you just going to keep repeating yourself and throwing our answers back in our faces??

Or, are you just trying to be funny or think yourself smart by being facetious.

If you can't understand what I (or the others) wrote then you really need to back away from this subject because all you'll do is just make things harder on yourself.

Put it this way, if you can understand a redshifted spectrum, then you should understand how a redshifted singular photon is measured. If you can't, then why should I (or anyone else) even bother trying to explain it to you. It would be a waste of time. However, for the record, you also wanted to know about the immunity of a photon from time dilation. You were told why yet it's very apparent you either didn't believe the explanation and/or didn't understand it. Most likely both. That's your prerogative, but don't come out and arrogantly accuse anyone here of "beating around the bush" when it's supremely evident from your puerile responses that you're all too eminently good at it yourself.

A long reply, but no answer to the question

[renormalised (Carl)]

Originally Posted by Archy

A long reply, but no answer to the question

It wasn't an answer....that, I and others gave to you beforehand. If you don't want to listen, fine. Find your answers elsewhere.

[bojan]

how can the "redshift" of a single photon be measured

I think that the question above deserves a more precise technical answer (because it is a precise technical question... and myself being an engineer, I am reducing theoretical problems to "how to do something", and not "why something happens").

When we measure the spectrum redshift, what is really measured are the wavelengths of the absorption or emission lines of atoms or ions of specific elements, identified as such - so we know the difference between the original wavelengths (or, we are assuming we know) and the received ones.

If I received one single photon with my test equipment, I must admit I wouldn't know what to do with it - apart from recording it's energy (which then directly translates into wavelength of course... and photomultiplier tube will not do this job, BTW). And that's about it. I wouldn't have a clue about process that generated that particular photon, so I can't possibly determine it's redshift.

I believe THIS was the core of the question in question.

[renormalised (Carl)]

Originally Posted by bojan

I think that the question above deserves a more precise technical answer (because it is a precise technical question... and myself being an engineer, I am reducing theoretical problems to "how to do something", and not "why something happens").

When we measure the spectrum redshift, what is really measured are the wavelengths of the absorption or emission lines of atoms or ions of specific elements, identified as such - so we know the difference between the original wavelengths (or, we are assuming we know) and the received ones.

If I received one single photon with my test equipment, I must admit I wouldn't know what to do with it - apart from recording it's energy (which then directly translates into wavelength of course... and photomultiplier tube will not do this job, BTW). And that's about it. I wouldn't have a clue about process that generated that particular photon, so I can't possibly determine it's redshift.

I believe THIS was the core of the question in question.

That's what he was told before, except that the photon can have its redshift measured. Remember, he's wanting to know the answer for a photon coming from a source that is itself being redshift by cosmological expansion. It's a simple exercise in working backwards to find the wavelength of the photon at its point of origin.