# Faster velocity than the speed of light?

by sydfremmer
Tags: faster than c, faster than light
P: 5,632
 If this is the case, does it not mean that some galaxies are moving away from us faster than c?
It is difficult to understand. Our everyday notion of distance doesn’t work on cosmological scales, so everyday intuition is useless.
Yes, that is possible in curved spacetime. But it is a different concept than in your original post which is in flat space-time. At the Hubble radius, the recession velocity is c; beyond the Hubble radius galaxies move away from us at greater than c. [We also move away from such observers at greater than c.]

The 'distance' measure used in FRW cosmology model of our universe is most commonly the proper distance, is based on the FRW metric with a rate of change as measured by co-moving observers; this velocity exceeds c for sufficiently large distances.

All this ‘superluminal’ velocity at great distances tells us is how one of many different possible definitions of distance changes; Other metrics that use different co-ordinates may not contain any apparent superluminal recession.

For more, checkout the Balloon Analogy by pHinds from these forums: http://www.phinds.com/balloonanalogy/
PF Gold
P: 6,324
 Quote by sydfremmer A related topic that I have a difficulty understand is the expansion of the universy. I read that the universe is expanding such that some galaxies move away from each other faster than the speed of light. If this is the case, does it not mean that some galaxies are moving away from us faster than c?
Not in the sense you mean. They are RECEEDING from us at FTL but that is not the same as MOVING. Seems weird, I know. Google "metric expansion". Nothing is moving, it's just that the distance is increasing because space is expanding.
Emeritus
PF Gold
P: 9,389
 Quote by sydfremmer A related topic that I have a difficulty understand is the expansion of the universy. I read that the universe is expanding such that some galaxies move away from each other faster than the speed of light. If this is the case, does it not mean that some galaxies are moving away from us faster than c?
It does. Apparently most of the galaxies that can be seen from Earth are "moving" away from us faster than the speed of light (in the sense that the derivative of the distance with respect to time is >c). But this is a result of expansion of space, and doesn't have a lot to do with motion through space. It's still impossible for a massive particle near such a galaxy to move faster than c relative to that galaxy.

Edit: I agree that "moving" is a misleading word, so I added the quotes in the first sentence, and a clarification.
 P: 5,632 Here is a great paper, but rather long and detailed: Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe Tamara M. Davis, Charles H. Lineweaver http://arxiv.org/pdf/astro-ph/0310808.pdf There is an abbreviated version that used to be available in Scientific American, but I haven't seen it lately. [edit: I found my link to UCLA but it not longer works.] An alternative: Ned Wright.... http://www.astro.ucla.edu/~wright/co...y_faq.html#FTL
 P: 11 Thanks again :) Now I can sleep
Emeritus
PF Gold
P: 9,389
 Quote by Naty1 Here is a great paper, but rather long and detailed: Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe Tamara M. Davis, Charles H. Lineweaver http://arxiv.org/pdf/astro-ph/0310808.pdf There is an abbreviated version that used to be available in Scientific American, but I haven't seen it lately. [edit: I found my link to UCLA but it not longer works.]
Marcus has a working link in his signature. http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf
P: 5,632
 Marcus has a working link in his signature. http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf
Alas, that link no longer seems to work either....
I double checked Marcus signature and that link IS the one he uses....

The abstract is here:

http://www.scientificamerican.com/ar...ut-the-2005-03

at Scientific American the article now costs \$7.95 unless one has a subscription.
 Emeritus Sci Advisor PF Gold P: 9,389 That link is working fine for me. I tried it before I posted it, and I just checked it again.
 P: 5,632 Curious...I keep getting an error message #404?? via a google chrome browser.....at least six attempts now.... ah, the mystery of 'the cloud'.... "That link is working fine for me. " now that's just plain 'showing off' [LOL].....
 Emeritus Sci Advisor PF Gold P: 9,389 What the... NOW I'm getting a 404 error as well. Maybe they reorganized some time after I posted the link last night, and my browser just got the article from the cache when I tried today. Ah, now the web site of that entire university appears to be down. They must be making some major changes right now. Maybe the link will work again when they're done, and if not, maybe we can figure out its new URL.
PF Gold
P: 6,324
 Quote by Didymus Either object can go 0.6c for one year and be just fine. But, it it looks at something else traveling in a way that would break the speed limit, God shrinks time and space so you don't cheat.
This is a physics forum. Let's leave the supernatural out of it. Physical laws do just fine.
Emeritus
PF Gold
P: 9,389
 Quote by Naty1 Curious...I keep getting an error message #404?? via a google chrome browser.....at least six attempts now.... ah, the mystery of 'the cloud'.... "That link is working fine for me. " now that's just plain 'showing off' [LOL].....
Uhm...did you copy and paste the abbreviated text for my link instead of just clicking on it? The link in my post is working fine, but if I click the one in your post, I get the 404 error.

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf