B Revealing Secrets of Distant Supernova DES16C2nm: 10.5 Billion Years Ago

[Viopia]

There is a lot of differemce. It is only locally that it is not distinguishable.

I am still a bit confused. 10.5 billion light years doesn't seem very local. The following statement was made by one of your gold members ''There's no difference between relative motion and "space expanding" between objects. There is a very common misconception that there is a difference, which has even made it into textbooks, but there isn't actually a difference''.

 

[Bandersnatch]

@Viopia I think, for the purposes of this question, it would be sufficient to know that the recessional velocities and local 'real' (or peculiar) velocities can differ in important ways, but can also be compared in some meaningful ways too.
In particular, it's worth reassuring yourself that if you were to put a really long and rigid ruler between yourself and some galaxy receding due to the expansion of space, you would definitely see that galaxy speed past the ruler to ever farther distances. You could also add the local radial velocity of the galaxy (or even a photon) to the recession velocity, just like you would add regular velocity vectors, to obtain a nett velocity that would tell you whether the extra local motion makes the galaxy approach, recede or hover at the same proper distance. I.e. there's an unambiguous physicality here, that you can depend on to visualise what's going on.
At the same time, it's good to keep in mind that there's enough of a difference between the two, that naively extrapolating any further intuitions about velocities should be avoided when thinking about recession.

For example, to bring this into the vicinity of the setup in your question, you can have a rocket moving at such a peculiar (i.e. local) velocity, that the nett approach velocity after adding recession velocity is zero. I.e. the rocket can 'hover' at a constant proper distance from the observer. Just like in your setup - though not for that particular supernova, as it was too far and receding too fast.
For a photon, while locally it always moves at c, it can have any approach velocity whatsoever - including 0 and negative, i.e. being 'stopped' or carried away by expansion despite being sent towards us.

But - here's an example of an important difference - it turns out the redshifts from peculiar velocities and recessional velocities do not add up in the same way. It's not valid to simply add up the two velocities if we want to know the resultant redshift.
A signal, made by the rocket to have the same form at emission as the signal emitted in its rest frame by the supernova it attempts to mimic, and emitted by a rocket with zero approach velocity at the same place and time as the supernova, would not have nett zero redshift, nor would it be redshifted - but would be blueshifted instead.
The details of how this works can be seen here, section III. The figures 5 and 6 can be helpful. (the line marked =(0,0) describe the unaccelerated universe we've assumed here, while =(0.3,0.7) is likely our universe).
So, you would have two photons - one from the supernova, redshifted; another from the rocket, blueshifted. They are therefore distinguishable.

Both photons would arrive on Earth roughly at the same time, as they have the same distance to cover, contrary to your intuition expressed above.
This can be understood by imagining what happens locally, in the immediate vicinity of the supernova. If we arranged for the rocket to emit its photon as it's passing the supernova, we would end up with the two photons traveling together. Since they travel together, in whichever way the expansion affects the intervening space they have to cover, it will do so in the same way for both photons. And since they travel together, and both travel at c, both arrive at the same time, having covered the same distance.

As is mentioned in the article linked earlier, this situation can be observed in nature, with relativistic jets from active galactic nuclei acting out the role of the rocket moving towards Earth, while the faraway galaxies hosting the AGNs are playing the supernova in this scenario.

 

[Viopia]

For a photon, while locally it always moves at c, it can have any approach velocity whatsoever - including 0 and negative, i.e. being 'stopped' or carried away by expansion despite being sent towards us.

Thank you for taking the time to reply in such a detailed way. You have given me so much information that I will need time to digest it all.

 

[Viopia]

For a photon, while locally it always moves at c, it can have any approach velocity whatsoever - including 0 and negative, i.e. being 'stopped' or carried away by expansion despite being sent towards us.

From what you say it appears as though ''space (itself)'' is expanding after all, rather than ''the distance between objects increasing as the objects move apart''. We may be able to do a simple experiment on Earth to prove this. Assume there is a straight train line (track) with distance markers along its length. As we travel along the track (in our track car) at 100mph we pass marker A, and then an hour later we pass marker B, and then an hour later we pass marker C. This means B is 100 miles from A, and C is 200 miles from A. A locomotive passes A at the same time as our car passes B. The locomotive is traveling at 200mph and so when our car has traveled another 100 miles (at C) it is hit from behind by the locomotive. The ''collision'' speed, (ie: the difference in speed between the car and the locomotive) is only 100mph, which is only half of the 200mph that the locomotive is traveling along the track. If we wanted to calculate how fast the locomotive was traveling we could have put two more markers at 1 mile spacings from C towards B. If we check the time the locomotive took to pass these two extra markings we could calculate the speed it was traveling when it crashed into us by measuring how long it took for the locomotive to pass these extra markings (1 mile apart) because we know the distance between them.

CONCLUSION: Can we do the same with the photons emitted from the supernova when they hit the Earth? If we liken emitted photons from the supernova to the locomotive, and the Earth traveling through space to the car (because space itself is expanding) we could calculate how fast the emitted photons are traveling from the supernova (in relation to the Earth) by calculating the time it takes for them to pass each of two added markers spaced at (say) 40,000 metres separation distance from the Earth. The one marker could be suspended 40,000 metres from a balloon, and the other marker could be on, or close to, the Earth's surface. We should suddenly block the emitted photons passing the balloon and measure how long it takes for the last emitted photon to reach the Earth's surface rather than measuring the redshift (we already know what the redshift is). It may be worth checking the speed of the photons emitted from the supernova in this way because if the photon speed is less than the speed of light (less than 299,792 Km per second) space itself is indeed expanding and the distance the supernova emitted photons had traveled when they were received on Earth is 10.5 billion light years. However, if the photon speed is the speed of light itself (actually is 299,792Km per second) space itself is not expanding and the distance the supernova emitted photons had traveled when they were received on Earth is only 5.5 billion light years because the distance between objects is increasing, rather than space itself expanding. I suspect you will you will probably say that the speed of the photons will always be at 299,792 Km per second because they are local to the Earth when they arrive, but I believe this makes little sense. This is a simple experiment to do just to rule this possibility out.

 

[Ibix]

You seem to be proposing a local measurement of the one-way speed of light. The answer depends on your clock synchronisation procedure. The obvious one, which assumes that the speed of light is the same in both directions, will have light passing you at $c$ whatever its source. And a two way measure will be $c$ without assumptions.

There is a difference between local speed measures, which will always return $c$ for a round trip speed, and long-distance speed measures where neither distance traveled nor time taken is particularly well defined. It's that lack of clear definition that makes long distance (Mpc and up) speed measures ill defined, and you cannot replicate it locally.

 

[Ibix]

I suspect you will you will probably say that the speed of the photons will always be at 299,792 Km per second because they are local to the Earth when they arrive, but I believe this makes little sense.

Unfortunately, you are attempting to apply "common sense" which is derived from experience with slow moving, small scale systems, to extremely rapid things moving over long distances. The answer "makes little sense" to you because you are attempting to apply an intuitive physical model that is reasonably accurate on some scales to wildly different scales where it is not remotely accurate.

This is a simple experiment to do just to rule this possibility out.

See the "cosmological sources" section of the experimental basis of relativity FAQ, linked from the sticky thread in the relativity forum:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#cosmological

 

[Ibix]

And, finally, there's the point that "has zero mass" and "always passes you at $c$" (give or take my earlier comment about clock synchronisation) are synonymous in relativity. For it to be possible for light to travel at any speed other than $c$ in vacuum, photons would need to have a mass. Cosmological observations are one of the ways people put constraints on any non-zero mass a photon might have, and there's no evidence of any non-zero value so far.

 

[Viopia]

"makes little sense" to you because you are attempting to apply an intuitive physical model

It makes little sense because it's hard to believe that the ''space (itself) expanding'' would suddenly slow down and freeze to satisfy most scientists notion of locality.

 

[Ibix]

It makes little sense because it's hard to believe that the ''space (itself) expanding'' would suddenly slow down and freeze to satisfy most scientists notion of locality.

Why would you want to believe that in the first place? It's nothing like the actual physics.

 

[Viopia]

Why would you want to believe that in the first place? It's nothing like the actual physics.

Space (itself) expanding would permeate the whole of the Universe. If the incoming photons from the supernova were reduced in speed when they hit the Earth because of 'catch up' the only way the photons could actually hit the Earth at the speed of light is if the space (itself) expanding was frozen when the photon - Earth collision occured.

 

[Ibix]

Space (itself) expanding would permeate the whole of the Universe. If the incoming photons from the supernova were reduced in speed when they hit the Earth because of 'catch up' the only way the photons could actually hit the Earth at the speed of light is if the space (itself) expanding was frozen when the photon - Earth collision occured.

This is what I mean about attempting to use an intuitive physical model to understand something that's way outside your direct experience. It has nothing to do with the real physical model - it's entirely your own speculation.

There are at least two meanings of the word "speed" in relativity - the locally measured speed and the coordinate speed. Looking first at the locally measured speed, at every point between the supernova and here an observer will always find the light (whether it comes from the supernova or a lamp in their hand) passing them at $c$. This is a consequence of light following null paths, and the speed of anything on a null path always being measured locally as $c$. The Earth is no different in this respect from any other place in the universe.

Coordinate speed, on the other hand, can be more or less anything you want since it depends on how you choose to define your coordinates. Depending on how you choose to define "space" you can cause the coordinate speed of light to vary. If you do pick a coordinate system that has a varying coordinate speed of light, it applies equally to all light passing through a region whether it was emitted locally or remotely. So if you set up some scheme to measure the coordinate speed then it will show the same value for light coming from a lamp or a supernova.

The "expansion of space" does not slow light down in any physically meaningful way. It is not a current in a river dragging things along with it. It just means that light has further to travel than a naive statement of the distance from its source to reception at its time of emission would imply.

 

[Viopia]

The "expansion of space" does not slow light down in any physically meaningful way. It is not a current in a river dragging things along with it. It just means that light has further to travel than a naive statement of the distance from its source to reception at its time of emission would imply.

I would like to remind you of what Bandersnach said earlier in this thread, ie: ''For a photon, while locally it always moves at c, it can have any approach velocity whatsoever - including 0 and negative, i.e. being 'stopped' or carried away by expansion despite being sent towards us''. This ''approach'' velocity of light does seem to be ''meaningful''. You can't have it both ways.

 

[Ibix]

This ''approach'' velocity of light does seem to be ''meningful''.

That's a coordinate speed - as I said, it can be anything you like. In this case, @Bandersnatch is defining the "speed" as the rate of change in remaining distance to travel, which just means that the total distance is growing, not that light has slowed. As for meaningfulness - how would you measure this quantity, given that the light in question can never reach us, and you cannot communicate with that region of spacetime for exactly that reason?

There is certainly a horizon beyond which light will never reach us. Light just this side of it will reach us very far in the future, doing $c$ (as I noted, this has been tested experimentally). Light the other side of it will never reach us, but would always be doing $c$ to any local observer.

 

[Bandersnatch]

Just chiming into say I subscribe to everything Ibix has said, in case there's some perceived disagreement.

 

[Viopia]

defining the "speed" as the rate of change in remaining distance to travel

Most people would define speed as the distance traveled divided by the time it takes to travel that distance eg: d/t. I don't know anyone who would define speed as the rate of change of the distance not yet covered. I think this strange reasoning, and the associated complicated mathematics it seems to generate, are examples of (what I call) the ''Bachelier'' effect. ie: the mathematics (for valuing stock options) became very complicated (because no one understood the problem) until Louis Bachelier's thesis was discovered. After his thesis was discovered the mathematics became much simpler, and even more beautiful, because mathematicians eventually understood the problem.

 

[Viopia]

Light just this side of it will reach us very far in the future, doing c (as I noted, this has been tested experimentally).

I would be interested to now how it has been tested. Thanks.

 

[Ibix]

Most people would define speed as the distance traveled divided by the time it takes to travel that distance eg: d/t.

And they would be able to get almost any answer they wanted in curved spacetime because neither the distance traveled nor time taken are uniquely defined.

I don't know anyone who would define speed as the rate of change of the distance not yet covered.

Do you know many people who try to explain the physics of curved spacetimes without maths? Why would the definitions people use in everyday situations have any bearing on sensible definitions in different circumstances? As I said, "speed" can mean different things in general relativity, and "distance traveled over time taken" has real problems when there isn't an assumption-free way to define either.

the mathematics (for valuing stock options) became very complicated (because no one understood the problem) until Louis Bachelier's thesis was discovered.

General relativity is almost absurdly simple at its core. The maths rapidly grows complex, but that's non-linear differential equations for you. The real problem for you is that the concepts aren't the same as the ones you use every day, and you keep trying to force them to fit in your everyday framework. It's the same type of error moon hoaxers make when they look at a flag waving on the moon - their mental model of a flag is that it stops waving quickly unless there is a breeze and they refuse to accept that a flag in a vacuum does not work that way. So they force that waving flag into their mental model and conclude that everyone else is lying or a fool.

Don't be like the moon hoaxers. Either learn the maths and learn the more general concepts, or accept that there are concepts that don't fit into a Galilean model of the universe.

I would be interested to now how it has been tested. Thanks.

I already linked to the FAQ, which has references.

 

[Viopia]

Thanks for your forebearance. I know that professional cosmologists and physicists, who spend all their time studying these subjects, are bound to know far more about these things than me. The only way I can try to understand these concepts is by challenging you to explain them in a way which is logical to me.

 

[weirdoguy]

by challenging you to explain them in a way which is logical to me.

And all your posts show that what is logical for you is not necessarily logical for physicists and physics. If you wan't to learn physics you have to conform to physics, not the other way around.

 

[Orodruin]

The only way I can try to understand these concepts is by challenging you to explain them in a way which is logical to me.

What if the Universe does not care what is logical to you? Many people get into problems when they encounter situations that do not conform to their owm mental image of how things work. Certainly a huge problem in quantum physics but also many times in relativity. But physics is about finding out and describing how the Universe actually works, not to make the Universe intuitive.

 

[Viopia]

That's a coordinate speed - as I said, it can be anything you like. In this case

If an astonaut is in orbit very close to a supermassive black hole, so he is not spaghettified, his time will be runnng very slowly form the Earth's frame of reference. When you use the word ''coordinate'' do you mean that the astronaut's frame of reference is one coordinate and the Earth's frame of reference is another coordinate. If so, the astronaut's time will be running normaly to the astronaut, but (depending on how close he is to the black hole) the outside universe will be speeded up and could easilly surpass the speed of light from the astronought's frame of reference if he is close enough to the black hole. If this is what coordinates mean, then it would certainly be possible for objects to exceed the speed of light (as they vanish from view) from an observer's frame of reference if you choose the right coordinate.

 

[Viopia]

And all your posts show that what is logical for you is not necessarily logical for physicists and physics. If you wan't to learn physics you have to conform to physics, not the other way around.

Don't you try to get your physics students to understand what the mathematics is telling them? To understand, your students will have to use their own logic, not your logic. When a subject cannot be logically understood, like in quantum physics, students have been told to ''shut up and calculate''. This only means that the mathematical process is undertood.

 

[weirdoguy]

To understand, your students will have to use their own logic, not your logic.

No, they will have to adjust their logic, if it doesn't match with physics. Universe does not care about your faulty logic. Every physics student, including myself, had to go through this process, so I don't know why you think it wouldn't apply to you.

 

[Viopia]

No, they will have to adjust their logic, if it doesn't match with physics. Universe does not care about your faulty logic. Every physics student, including myself, had to go through this process, so I don't know why you think it wouldn't apply to you.

Of course it applies to me. I havn't been able to adjust my logic yet but I may be abe to do so if I learn more. Have you adjusted your logic to understand quantum physics yet?

 

[weirdoguy]

Have you adjusted your logic to understand quantum physics yet?

Long time ago. I had to, otherwise I wouldn't be able to finish my masters in physics.

 

[Orodruin]

Don't you try to get your physics students to understand what the mathematics is telling them? To understand, your students will have to use their own logic, not your logic. When a subject cannot be logically understood, like in quantum physics, students have been told to ''shut up and calculate''. This only means that the mathematical process is undertood.

You are using the word ”logic” in the wrong way. The theory of GR is logically consistent (although it does raise some questions) and all of the computations follow logic. What you are confusing it with is whether or not it appears to fit into your own mental framework, which the theory has no obligation to do. If that is your use of ”logic” then no, students should not use their own ”logic” because it will mostly be wrong. The point of teaching GR is to provide the students with the appropriate ”logic” and teach them what the theory actually states.

 

[Viopia]

can be more or less anything you want since it depends on how you choose to define your coordinates

I am still interested in any response to my question (*51) regarding the use of the word ''coordinate''.

 

[Orodruin]

When you use the word ''coordinate'' do you mean that the astronaut's frame of reference is one coordinate and the Earth's frame of reference is another coordinate.

Frames of reference are not as unambiguous in GR as they are in SR. Coordinates are (local or global) assignments of four numbers to events in spacetime, typically (but not necessarily) three spatial and one time coordinate. Since the assignment is arbitrary, the coordinate speeds (coordinate distance per coordinate time) are arbitrary.

 

[Ibix]

Just to add to Orodruin's response, it's trivially easy to make coordinate speeds faster than light. No black holes needed. Just turn around 360°. In a coordinate frame attached to you the Sun just circled around you (a distance of about 50 light minutes) in a second or so.

But at no point did any massive object overtake a light pulse. That's the distinction between coordinate speeds (which can be anything) and physically meaningful speeds.

 

[Viopia]

For a photon, while locally it always moves at c, it can have any approach velocity whatsoever - including 0 and negative, i.e. being 'stopped' or carried away by expansion despite being sent towards us.

Just to add to Orodruin's response, it's trivially easy to make coordinate speeds faster than light. No black holes needed. Just turn around 360°. In a coordinate frame attached to you the Sun just circled around you (a distance of about 50 light minutes) in a second or so.

But at no point did any massive object overtake a light pulse. That's the distinction between coordinate speeds (which can be anything) and physically meaningful speeds.

It is no wonder I needed time to digest what Bandersnatch said. I thought we were talking about real velocities. Ibix's comment about the Sun can be described in the following way:- As a you ''turn around'' the Sun does not circle you, you merely turn around so that the light hitting your retina (after passing the crystalline lens in your eye) scribes an ark on your retina. The light hitting your retina only tavells around an inch per second which is far less than the speed of light.