Revealing Secrets of Distant Supernova DES16C2nm: 10.5 Billion Years Ago

[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.

 

[Orodruin]

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.

This is not the point he was making.

The point is that the coordinate speeds of a stationary object in a rotating coordinate system are directly proportional to the distance and if the distance is far enough then the coordinate speeds easily exceed the speed of light.

How fast the spot of light moves on the retina is also not bounded by the speed of light. The light at subsequent moments is not the same light pulse. There is no light moving along the retina.

 

[Ibix]

As a you ''turn around'' the Sun does not circle you,

Sure it does. That motion is no more or less real than any other.

 

[Viopia]

Sure it does. That motion is no more or less real than any other.

If what you say is correct, the Earth traveling around the Sun could be thought of as the Sun rotating while the Earth is stationary. If the the Earth had no orbital velocity it would crash into the Sun. This means there is a difference.

 

[Viopia]

There is no light moving along the retina.

No there isn't any light moving on the retina, just lke the Sun is not moving faster than the speed of light.

 

[weirdoguy]

the Earth traveling around the Sun could be thought of as the Sun rotating while the Earth is stationary.

Well, yes. What's the problem with that? Motion is relative. That is the very basic thing.

 

[Ibix]

Actually, working in a rotating frame where the Earth is stationary is one of the easiest ways to work out orbital dynamics. You get a centrifugal potential that makes it different from the non-rotating case, but it is not any more or less real.

 

[Ibix]

No there isn't any light moving on the retina, just lke the Sun is not moving faster than the speed of light.

Why do you ask questions and then, when we answer or explain, just say "no"?

 

[Orodruin]

No there isn't any light moving on the retina, just lke the Sun is not moving faster than the speed of light.

That’s the point. However, it is not ”just like”. In the former case it is a question of taking a speed of something that is not an actual object and in the latter a coordinate speed that is the coordinate speed of the Sun - an actual object.

Also, please be more careful with your quotes. You just attributed one of your answers to me making it seem I said something I did not.

 

[Viopia]

That’s the point.

Also, please be more careful with your quotes. You just attributed your answer to me making it seem I said something I did not.

I am sorry the quote was not from you. I don't know how this couild have happened. I was trying to answer two questions close together and used the prompt after highlighting the part I wanted to answer.

 

[Viopia]

Why do you ask questions and then, when we answer or explain, just say "no"?

I sometimes don't understand the answers. When this happens I try to say what does not make sense to me.