Order of Kilopi
How does velocity effect extreme redshift?
If I travelled at near the speed of light towards a redshifted distant galaxy would it un-redshift?
If I travel at near the speed of light towards a black hole would the redshift of light outside of the EH un-redshift?
Order of Kilopi
Yes with caveats.
If you travelled towards a distant at .99c you would actually see any objects with a z value of less than ~13.11, and you are travelling directly towards, actually become blue shifted.
If you where travelling towards a black hole then only light pointed almost directly radially out, and above the EH, would be visible to you and your speed would counter act some or all of the red shifting caused by gravitational red shift.
Established Member
When there are separate sources of red(blue) shifts, the total effect is the product of the values of (1+z) due to the individual effects. This will hold for wavelengths; in an expanding Universe, you do not recover the intensity that would be seen with no source of redshift when one of the (1+z)s is cosmological.
Order of Kilopi
travelling towards? are you sure you dont mean travelling away? Why the limitation of which light I will see? How is that different than if I was hovering?
Originally Posted by WayneFrancis
Order of Kilopi
Why?
Order of Kilopi
Yes, I meant just what I said.
If you travel away from a black hole any light climbing out of the gravity well will be even more red shifted if you are travelling away.
because if you can't observe the light then it won't be red shifted for you. IE redshift/blueshift of light is a coordinate effect. If you are not in the world line of the photon then the photon is not red or blue shifted for you.
If you are hovering and not in the photons world line then you again won't see the photon and thus it can't be shifted for you.
If you are hovering and in the world line of the photon then you'll see it red shifted from the source if the source is further in the gravity well then you. If the light was from a source further out of the gravity well then you and you are in the world line of the photon then the photon will actually be blue shifted from the source.
Established Member
Because redshift due to cosmological expansion represents a real energy loss -- energy conservation is a local law.
Order of Kilopi
Can you go into a bit more detail on this? How can we distiguish between the many forms of redshift?
Order of Kilopi
The most recent issue of Scientific American contains a good article by Tamara Davis on this very question - given your interest, and assuming that you've found the Lineweaver and Davis material previously referenced too hard to follow (not to mention Ned Wright and John Baez) - you might find getting a copy a very good investment of your time (and, possibly, money).
Given a spectrum alone, the cause of the observed redshift is impossible to tell (though the spectrum of a real object - a galaxy say - will clearly have essentially zero gravitational redshift).
However, with the meta-data about the spectrum of a particular object, putting robust bounds on the contribution from the expansion of the universe vs local 'peculiar' motion is relatively easy and straight forward.
Established Member
Thanks for the pointer to the SciAm article, Nereid. After a quick read-through, though, I'm a bit dubious about the implicit equivalence principle that Davis invokes to argue that, like Doppler, cosmological redshift involves no violation of energy conservation. At least her argument that a collection of locally flat regions of spacetime is equivalent to one large, flat region of spacetime is shaky at best. Perhaps I didn't read the article carefully enough, but my initial impression is that the argument is sloppy, and perhaps wrong.
ETA: The question of whether cosmological redshift implies non-conservation of energy probably deserves a separate thread (and not only because it is OT for this thread). The issue is sufficiently subtle that the experts seem to disagree even on how properly to pose the question. That, in turn, lies at the core of why one sees different (seeming) answers.
Last edited by Geo Kaplan; 2010-Jun-26 at 09:10 PM.
Order of Kilopi
By looking at the photons? We can't. By looking at the system as a whole we can make some educated assumptions.
For example locally we can tell the distance to different object by not only their red shift but but other factors such as cepheid variables and luminosities.
The fact that red shifts so tightly correlate to distance measurements from other measuring mechanisms suggest that red shift can be used to make measurements of other objects.
Going further with this we see locally no object has a peculiar above a certain level. As we look out further and further the red shifts keep going up but there is no reason to think that those objects would have peculiar motions drastically different then what local objects do. So we can then ask if the red shift is then due to gravity but this question fails because our observations doesn't match any known gravity well we have ever seen, ie it is linear and not curved with an inverse square law. That cuts out the 2 types of red shift we are used to. This means there is something else. Some have proposed "tired" light but this again fails when looked at closely. The best fit is that space has a property that causes it to expand and take any objects in it along for the ride, which is actually to have the objects stay still.
It is a processes of logical deduction. Sure there could be other explanations but every other explanation so far are more complicated and don't fit nicely with our observations.
Order of Kilopi
And besides, that should be "affect."
"Effect" is usually a noun. It is less commonly used as a verb, with the sense of "make happen."
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
Yes but now we are travelling at near the speed of light ... I would assume that everything would be blueshifted not redshifted in the direction of travel.
Order of Kilopi
Just curious how you conducted your experiment with the person travelling at near speed of light?
Order of Kilopi
When you put a question to Q&A, it is natural to assume you are looking for the answer as described in our most applicable theories. If you are instead asking for experimental evidence, that is a much harder task and I suggest you embark on it yourself. Theories are compact ways to express the outcomes of many experiments, woven into a complex tapestry of consistencies and simplifying assumptions. The idea is to end up with something simple enough to express in a paragraph or two on a forum. If you wanted something different, you are asking more than anyone here has any good reason to provide for you.
Order of Kilopi
What are you on about tommac? What experiment are you talking about? Wait... I see there is post #13 ...let me go read that one first.
Order of Kilopi
But if you are travelling near the speed of light but still not fast enough to counteract the red shifting from either objects receding from us do to cosmic expansion or GR then the light from the object you are travelling towards can STILL be red shifted.
Example. Say you have an object that appears to be receding with a z value of ~198. You speed up to .99c toward that object...you still will see that object red shifted. The only way you'll see it blue shifted is if you exceed the speed that it was receding from you.
So while everything is blue shifted in the direction of travel it is ONLY blue shifted when compared to what you would observe if you where not in motion. It doesn't meant that all of a sudden something that is red shifted over all becomes blue shifted over all.
I'll put it in simple maths terms
A truck is parked next to you. this represents no blue or red shifting.
You drive away from the truck at 30km/hr. this represents red shifting of the truck
The truck starts driving towards you at 60km/hr (relative to the road or 30km/hr relative to you). this represents blue shifting
The truck passes you still at 60km/hr (relative to the road or 30km/hr relative to you). this represents red shifting.
The truck stops and you continue driving towards the truck. this represents blue shifting again
Now I'd be really surprised if you didn't understand this already but history has shown you ask ambiguous open questions seemly hoping to get someone to stumble on what they are trying to say.
NGC3314 specified the same reasoning I have in post #3
Order of Kilopi
After reading and answering post #13 and rereading my other posts in this thread I have no idea where you get the idea where I claimed to conduct any experiment beyond doing something you seem to hate, actually doing a bit of maths using formulas like
Do you have questions about the maths?
Order of Kilopi
I am looking for evidence in the most applicable theories. Just wasnt sure which experiment that WF was talking about.
Order of Kilopi
This I 100% agree with. But ... assuming that I have a near infinite energy source, I can blue shift just about anything.
To me this seems a bit counter-intuitive to an expanding universe. So I am asking the questions.
It seems also a bit peculiar to me ... so I will ask a question ... that if I travel fast enough towards a black hole that the light from an infaller just outside ( 1 planck legnth ) the EH would look blue shifted ... would it?
Now ... I get the difference between GR and SR ... and what I am trying to understand is the relationship between the two of them.
To clarify, at the risk of making this more complex, my confusion. If something is neither blue shifted or red shifted can we assume that there is no space-time dilation? Does that mean that the spaceship travelling towards the BH and the free faller, and the hoverer at near the EH all have watches that tick in sync? I am also confused how space time is curved between the spaceship and the infaller and the hoverer. I am trying to figure this all out.
Order of Kilopi
Yes my question is how does this show the relation between the SR towards the distant galaxy and the expansion of the universe.
Maybe I am missing something but it seems that you are just presenting standard formulae and you arent really showing any math ... what are you trying to propose?
After reading and answering post #13 and rereading my other posts in this thread I have no idea where you get the idea where I claimed to conduct any experiment beyond doing something you seem to hate, actually doing a bit of maths using formulas like
Do you have questions about the maths?
Order of Kilopi
You stated:
I am just asking ... what observations are you talking about? My questions are regarding a traveller travelling at near the speed of light towards a distant galaxy.It is a processes of logical deduction. Sure there could be other explanations but every other explanation so far are more complicated and don't fit nicely with our observations.
Would you propose that traveller could blueshift the most distant galaxies to z=0 ?
Order of Kilopi
Of course no one has achieved velocities anywhere near c with a spectrograph-equipped spacecraft, and it is unlikely that anyone ever will do so. The amount of energy that would be needed simply is prohibitively large.
What has been done successfully is the testing of the theory of relativity with experiments and observations that are technically possible, such as propelling various subatomic particles to within a gnat's whisker of the speed of light. So far, the theory has held up, so we feel confident in concluding that as v approaches c, z increases without limit. Thus we should be able, in a thought experiment with a propulsion system of our dreams, to overcome the redshift of any observable light.
If you can perform a verifiable experiment that gives reliable, contrary evidence, perhaps you should get on the next plane to Stockholm and receive a Nobel Prize in physics.
Established Member
What he's obviously talking about is that SR is a very well-tested theory. We have tested it on earth with laboratory instruments, and regularly measure exactly what the theory predicts. We have also taken full advantage of the universe as a laboratory. When we study, e.g., fast-moving particles (muons) generated outside of the earth, we discover again that SR describes what happens extremely well.
That's why we have theories, tommac. They give us a framework with which we're able to deduce the likely outcome of experiments not yet run, based on a track record of successful predictions of experiments already run.
For more, Google "tests of special relativity."
Order of Kilopi
Well I am at least 2 years away from a nobel prize but none the less I have a few questions ... if we can blueshift out any amount of light then wouldnt that mean that we are traveling towards a galaxy, even though it is moving away from us at faster than the speed of light? If not then how can we detect how fast the distant galaxy is moving away from us, since it is blueshifted with respect to us?
Order of Kilopi
Yes but this question is not just SR. So I am not sure what point was being made. Yes I agree that SR works and is proven ... great now what ... lets get back to the OQ.
Order of Kilopi
I answered your OP questions in post 23.
Order of Kilopi
I'm quite lost tommac; how does this question relate to the one in the OP?
And I don't understand the question you are asking here; for example, what do you intend to mean by "if we can blueshift out any amount of light"?
Established Member
You're being very inconsistent. On the one hand, you post here that you agree that SR "works," as if that were irrelevant to your OQ. But the exchange in question had very much to do with your questioning the basis for SR ("I am just asking ... what observations are you talking about? My questions are regarding a traveller travelling at near the speed of light towards a distant galaxy"). So, to repeat, since you seem not to have understood either your own question, or the answers to it, your question is equivalent to asking what observations support SR. And to that question, answers have been given.
Order of Kilopi
Presumably whatever experiments caused the theory to be accepted in the first place. Remember, no mainstream theories are ever accepted without experimental support, even though we suspect there will always exist some experiment that detects a crack in the foundation.
Posting Permissions
Reply With Quote