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efanton
2015-Dec-30, 07:00 PM
Earlier today I was reading about how Adam Reiss got a Nobel prize for determining that the universe is expanding at 74.2 kilometres per second per mega parsec.

If that's true, and I have not reason to doubt it, then that means some parts of our universe are indeed moving faster than the speed of light.
With the speed of light at 3x10^8 m/s, and radius of the universe of about 46 billion light years (14103.6 mega parsecs) the outer edge of the universe would be expanding at 14103.6 x 74.2 km = 1,046,487 km/s which would be roughly 3 times the speed of light

If something was travleling faster than the speed of light what would you see?

To put this in context imagine we are looking at, or trying to look at, a galaxy near the outer edge of the universe that has a galaxy behind it.
Would we see either galaxy at all, would we see absolutely nothing or would we see what was in that position before the galaxy moved to it?

grant hutchison
2015-Dec-30, 07:14 PM
In the situation you're discussing, the expansion of the Universe, you see a red-shifted galaxy, and it's not even a particularly extreme redshift - around 1.5. We have observed numerous galaxies with that redshift and higher.
Photons emitted by objects receding faster than light are initially swept away from us by the expanding Universe, but they can (at least in the present epoch), eventually cross a threshold that lets them propagate towards us.

Davis and Lineweaver (http://arxiv.org/pdf/astro-ph/0310808.pdf) (530kB pdf) wrote a very nice review explaining this in detail.

Grant Hutchison

mkline55
2015-Dec-30, 07:57 PM
I depends. Leaving out the redshift/blueshift stuff for a moment and treating light like solid particles, if something is moving directly toward you the light it emits closer to you would arrive before the light emitted farther away. As a result, the object might appear to be receding instead of approaching. If you were to treat light as a wave, though, I believe the trailing edge of each wave would arrive before the leading edge, if that even makes sense or if it makes any difference. Still, as before, succeeding waves would arrive ahead of light waves produced earlier. I think time becomes a major issue here. For something moving away, I don't believe a complete wave could ever be received. As for something moving at a tangent? For some part of its travel, it may not be moving faster than light relative to the observer, however, I believe the wave would have some strange skewing angle.

efanton
2015-Dec-30, 08:16 PM
Thanks Grant.

reading it now, but at the moment trying to wrap my head around this

However, since the radius of the Hubble sphere increases with time, some photons that were initially in a superluminally receding region later find themselves in a subluminally receding region. They can therefore approach us and eventually reach us. The objects that emitted the photons however, have moved to larger distances and so are still receding superluminally. Thus we can observe objects that are receding faster than the speed of light

How can a photon in a superluminal region suddenly find itself in a subluminally receding region. Surely as space expands the photon although moving towards us is also receding with the expansion of the universe? Wouldnt it be a case of one step forwards two steps back, as the receding speed exceeds lights travel speed?

DaveC426913
2015-Dec-30, 08:22 PM
if something is moving directly toward you the light it emits closer to you would arrive before the light emitted farther away.
No, it would not.

The title question amounts to: If the laws of the universe were different from what they are now, what would we observe?
If one wishes to make up new physics, then one might as well make up new observations.

I'm not discounting superluminal motion altogether - the current discussion about recession velocities is a valid example - just that it is nonsensical to discuss local* forms of superluminal motion.

*less than cosmological distances

grant hutchison
2015-Dec-30, 08:54 PM
How can a photon in a superluminal region suddenly find itself in a subluminally receding region. Surely as space expands the photon although moving towards us is also receding with the expansion of the universe? Wouldnt it be a case of one step forwards two steps back, as the receding speed exceeds lights travel speed?Yes, the photon is at first, and for a long time, carried away from us by the expansion of space. But it makes progress into regions of space that are expanding away from us more slowly than where it started from. So the rate at which it is being swept away from us steadily decreases. Eventually, it enters a region of space that is moving away from us more slowly than the speed of light (although it is now much farther from us than when it started!) and thereafter it makes progress towards us. The photons we see from these very distant superluminal galaxies have actually been travelling away from us for much of the lifetime of the Universe.
Put another way, the surface at which objects recede from us at the speed of light (the Hubble Sphere) expands outwards fast enough to "recruit" distant photons that were previously being swept away from us.

Grant Hutchison

grant hutchison
2015-Dec-30, 08:57 PM
The title question amounts to: If the laws of the universe were different from what they are now, what would we observe?
If one wishes to make up new physics, then one might as well make up new observations.Read the full text of the question. It's a question about the real Universe, as we observe it. No new physics requested or required.

Grant Hutchison

Ken G
2015-Dec-30, 09:16 PM
In case it helps, it may be useful to notice that the same thing happens in more mundane situations. If you imagine a picnic sheet made of easily stretchable rubber, with ants crawling on it, and you grab one end of the sheet and start pulling at twice the speed an ant can crawl, ants can still cross that sheet in either direction.

efanton
2015-Dec-30, 09:32 PM
So the rate at which it is being swept away from us steadily decreases.

Got it. Now it makes sense. Thanks Grant.

So if I understand this paper correctly at a certain distance this make redshift measurements untrustworthy if using SR or the approximation v=cz. At this point the need for a standard candle (eg type 1a SN) becomes more important as the redshift can be adjusted using GR. It follows then that at this perceived redshift we dont know whether that particular galaxy is truly at the perceived distance (moving subluminally) or moving superluminally without the aid of a standard candle. Have I got it right?

And DaveC426913 im not looking to change the laws of physics, its hard enough to under the universe we have without inventing a whole new one. I thought my post was very clear, Im talking about this universe, and I posted the question to help me understand current physics theory related only to this universe.

Ken G
2015-Dec-30, 10:48 PM
So if I understand this paper correctly at a certain distance this make redshift measurements untrustworthy if using SR or the approximation v=cz.Redshift measurements are never untrustworthy, but they do only give you redshifts. To turn redshifts into a dynamical story, you need distances to go with the redshifts, and those are the hard thing to get. I think what you are saying is that redshifts don't tell you separation velocities, and that is certainly true-- an observed redshift tells you exactly one thing (if the redshift can be regarded as purely cosmological): 1+z is the factor by which the universe has expanded during the propagation of that light, and that by itself is not a velocity of anything.

efanton
2015-Dec-30, 11:17 PM
Yes, I wasnt explaining myself very well there, but I do understand what you are saying.

thank you Ken and Grant, I was having a hard time trying to get to grips with the topic from reading alone, and some of what I read was downright misleading.
Another mystery solved for me, thanks to a little help from you

DaveC426913
2015-Dec-31, 05:57 AM
Read the full text of the question. It's a question about the real Universe, as we observe it. No new physics requested or required.
I did. Of course.

In both the subject and the body of the OP he talks about " traveling faster than the speed of light". This cannot be.

Cosmological expansion is not travelling. It is the expansion space between objects, even while those objects might be stationary.

In my post I excepted cosmological expansion, but it is nonsensical to talk about objects travelling at > c.

It's fine if he wants to talk about cosmological expansion, but let's be sure not muddy the waters by including impossibilities.

(It may seem like nitpicking, but it's a common error that's best to set straight up front.)

And DaveC426913 im not looking to change the laws of physics, its hard enough to under the universe we have without inventing a whole new one. I thought my post was very clear, Im talking about this universe, and I posted the question to help me understand current physics theory related only to this universe.
Agreed. And in this universe, nothing can travel faster than the speed of light. Just so we're clear.

Ken G
2015-Dec-31, 06:27 AM
Perhaps the best approach is to avoid the term "travelling" altogether, it is more of a common term, like getting on a train or something. Better is to talk about a relative velocity as applying to objects passing each other in the same place and time, and a rate of change of distance as applying to widely separated objects. If we frame our language like that, then it is clear the first cannot exceed c, and the second can and does. Even in special relativity with no gravity and no cosmology, there is a big difference between a relative velocity (attributed to a single object as reckoned from a given reference frame at a place and time the object is passing) and a rate of change of distance between two objects, so cosmology begins with those already very different concepts, and adds the notion of a changing universal scale factor.

What's odd is that in special relativity, relative velocity seems like the objective observation, while rate of change of distance between two objects seems like an artifact of arbitrary coordinates, but in the natural coordinates of cosmology, that situation is exactly reversed. I think there is something going on in that reversal that has never really been appreciated, I feel we are missing something there.

efanton
2015-Dec-31, 01:55 PM
In both the subject and the body of the OP he talks about " traveling faster than the speed of light". This cannot be.

Cosmological expansion is not travelling. It is the expansion space between objects, even while those objects might be stationary.

In my post I excepted cosmological expansion, but it is nonsensical to talk about objects travelling at > c.

If you were on a train, would you not be 'travelling' at the trains speed even though you yourself were sat down not moving? If you are in a car are you not travelling at the cars speed? It was in this context I was using the word travelling, and although not the best word to use, it was pretty obvious I wasnt implying anything outside the laws of physics

Ken is right of course, I was using a common term to describe a situation and it would have been better to use relative velocity. But had I started using technical terms for a situation I did not fully understand it is quite likely I might have used them wrongly for the situation I was trying to explain, and yet again pedants would jump on it. Sometimes being pedantic is not helpful, better to try grasp what was intended and answer rather than attack the use of words or phrases.
Ken and Grant understood what I was trying to convey and were very helpful.

grant hutchison
2015-Dec-31, 03:59 PM
If you were on a train, would you not be 'travelling' at the trains speed even though you yourself were sat down not moving? If you are in a car are you not travelling at the cars speed? It was in this context I was using the word travelling, and although not the best word to use, it was pretty obvious I wasnt implying anything outside the laws of physicsI think the problem some people have with "travelling" in the context of the expansion of space, is that "travelling" implies that you're going to get somewhere - the train or the car is moving from one place to another place. In the case of the expanding Universe, nothing is going anywhere - it's all just getting farther apart.

However, it's difficult to talk about something that is at rest relative the coordinates of the expanding Universe but nevertheless getting farther away from us. We're conceptually tugged back and forth between the "at rest" bit and the "getting farther away" bit. People use all sorts of phrases that don't stand up to rigorous analysis; the main thing is to avoid the choice of phrase creating the wrong mental picture, or obscuring the underlying meaning. Beyond that, I tend to be fairly relaxed about nomenclature.

Grant Hutchison

DaveC426913
2015-Dec-31, 04:13 PM
If you were on a train, would you not be 'travelling' at the trains speed even though you yourself were sat down not moving? If you are in a car are you not travelling at the cars speed? It was in this context I was using the word travelling, and although not the best word to use, it was pretty obvious I wasnt implying anything outside the laws of physics

I'm not blaming you. It's a distinction that needed to be made for the thread to proceed meaningfully.

... pedants would jump on it. Sometimes being pedantic is not helpful, better to try grasp what was intended and answer rather than attack the use of words or phrases.
Ken and Grant understood what I was trying to convey and were very helpful.
Which is good. I acknowledge your question was sincere. I'm sorry my correction was expressed as an attack.

Ken G
2015-Dec-31, 05:46 PM
What's more, I'm not sure that physics itself has quite come to peace with the whole issue of cosmological "travelling." It is certainly common to say that "space itself is expanding", but this is hardly a scientifically correct statement, because we can't do experiments "on space" to check that it is "doing something", and we also need to choose standard cosmological coordinates to even distinguish space from time. Relativity, which is the very theory that is telling us space is dynamical, tells us that coordinatizations are arbitrary! Finally, and this may be the worst of it, we are often told that the way general relativity deviates from the basic ideas of quantum mechanics means that someday gravity will be returned to the fold of quantum forces, and then what will we say space is doing? I'd have to say the whole business is rather a complete mess in nomenclature at the present time-- a fact rarely mentioned in our haste to correct misconceptions. It is perhaps true to say that we know more about what isn't the right way to talk about it, than we know about what is the right way to talk about it!

2016-Jan-02, 06:06 PM
this thread made me think of something: what would the skies look like in the Star Trek universe? there are ships going everywhere in the sky at high warp, surely they have to leave something that looks like a vapor trail from a modern jet in their wake as they speed along at thousands of times the speed of light. sometimes they are coming right at you, sometimes away from you, sometimes across your field of vision... if you had a telescope, you could look in pretty much any direction and see historic space battles that happened decades ago unfold in real time...

Cougar
2016-Jan-03, 02:12 AM
Earlier today I was reading about how Adam Reiss got a Nobel prize for determining that the universe is expanding at 74.2 kilometres per second per mega parsec.

Going back to this, yes, Adam Riess (https://en.wikipedia.org/wiki/Adam_Riess) did indeed share the Nobel with Saul Perlmutter and Brian Schmidt, but not for determining the expansion rate, but rather developing evidence (from Sne Ia) that the expansion rate was slower in the past (and so it's likely to be greater in the future). In other words, it's been accelerating. The expansion rate is a rather complicated function of cosmic time....

LaurieAG
2016-Jan-03, 10:36 AM
If something was travleling faster than the speed of light what would you see?

Surely, any observer at any specific location and time, could only observe the photons that would have arrived at that place and time, regardless of whether an observer was present or not.