Do light years mean years in space or on Earth?

[Mordred]

speed of light itself is considered a constant.
however the original definition changed to state its a constant in a total vacuum meaning that it has been shown to slow down or even speed up in one article I posted in another discussion.
their have been experiments where they have slowed light down to a feew meters per second if I remember correct.

However thats besides the point on this discussion which I beleive the poster wanted to know if we defined the speed of light outside the effects of our gravity well or within the effects of our gravity well when all other measuring tools are not constant would this have not affected our perspective on its actual rate. If tyhis is the case then what does that do to all our other measurements of the size of the univers distance to the stars etc. Heres the next quandary what would we see if our point in the expaning universe is FTL? would we see anything at all? or does the photons move with the changes in space time. If not are they somehow outside the effects of space time. After all if everything else is now moving FTL but photons or other massless particles are not what happens then? We already know at some point in the future the universe will expand FTL so this raises some interesting questions.

 

[Mordred]

General relativity predicts that space is expanding or contracting.
origin
solar system

yeah I know general relativity predicts either expanding or contracting which we have since confirmed its expanding.

 

[Mordred]

http://www.orbits.00space.com/vsl.html
http://en.wikipedia.org/wiki/Variable_speed_of_light
http://www.phys.unsw.edu.au/astro/resea ... 03webb.pdf
http://arxiv.org/PS_cache/astro-ph/pdf/ ... 1018v2.pdf
http://www.speed-light.info/speed_of_light_variable.htm


here is several articles that state neiother c or light is constant.
the last one relates most closely to the original posters question

 

[MeteorWayne]

Please don't just post a bunch of links without explaining what each one of them says. We only have so much time, so since you've read them (right) can you give a brief summary of each so we can decide whether we have the time to read them?

Thanx

Meteor Wayne

 

[drwayne]

We also have folks who post here who seem to think that posting a lot of links
lends their arguments a certain gravitas - only thing is - for some of these
posters, the links have nothing to do with their argument, they are just the
first n hits on a Google search.

Trust me, you don't want to look like they do.

 

[Mordred]

The first article discusses the changes in light speed throughout the years describing light speed in the early universe as being much higher than it is duty. They provide some graphs and formulas that support this.
A couple of these articles describe similar findings, one of them went and gathered findings for the past 300 years of from a test of using the lunar eclipse to calculate the speed of light This article states that light speed has changed in the last 300 years essentially its showing that its slowing down.
The second link was merely a wikipedia link that led me to some of these articles and mentioned several of the researchers and mathematics involved in varying speed of light.
The third article is looking more at the fine structure constant variations from the big bang forward. Stating that even the absorbtion of radio isotopes has also changed in time. The found this by studying red shift from quasars and by studying a natural nuclear fission area off Okla. The next article is more supporting mathematics for variations in the fine structure constant.

 

[theridane]

Mordred, isn't that the same thing as what I'm saying? That c is constant and (as a result of time rate changes) the space dilates?

 

[Mordred]

Not really the way I read these is that not even c can really be considered a constant. As over time its values have changed the last article also states that gravity wells also affect our measurements of them from various perspectives.
At one point near the beginning of the Universe the value we would have derived for c would have been a much higher number. Now I could be wrong on some of these but even the fine structure contant has changed over time. For example 20 years ago we could have been taught today that the value for c is x, Today the value would be slightly less.
Least thats how I read them essentially when you think about it its difficult to really state that there is a constant a variable yes but a constant I honestly do not think that this would be accurate.

 

[Mordred]

The last article pertains more to your original question. At least in the importants of a correct answer.
This one describes the speed of light and the effect gravity wells has on it. So when we talk light years are we referring to a value for the speed of light outside the influence of gravity wells? Does this value change over time since we first coined the term a light year?
After all if were showing evidents that the speed of light is changing, is affected by gravity wells, the fine structure constant is changing what then is a light year in actuality?

 

[theridane]

So when we talk light years are we referring to a value for the speed of light outside the influence of gravity wells?

Yes, the light year is defined using SI seconds, and those are defined with corrections for time dilation due to gravity.

 

[Mordred]

Well, the question does make some sense...
Quotes from wiki:

As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.

In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of 86,400 SI seconds each, totalling 31,557,600 seconds.

Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

... and since the period described in the last quote changes if we consider the influence of gravity on the passage of time observed by the outside observer, the second that is ticked off on Earth is different than the second that is measured by a clock far away from any mass. Therefore, the distances traveled by light in those different periods will differ.

The definition of second does not include the relativistic dilatations or contractions of time, so I guess the question is valid.

However, wiki also says:

Under the International System of Units, the second is currently defined as
The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.[1]

This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero), and with appropriate corrections for gravitational time dilation. The ground state is defined at zero electric and magnetic fields. The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements. (See History below.) The international standard symbol for a second is s (see ISO 31-1).

but I didn't find the gravity's influence part in the actual document.

Would not the article describing the Isotopes affect this value at some point down the road? Or is the variations part of the relativistic effects in regards to the ceasium atom ?

Next question I learned one value for the distance a light year means say 30 years ago. Now today what would the value for a light year be today or lets go further 2000 years from now? If even radiation decay changes over time, the speed of light changes over time. the fine structure constant changes over time what can we use to actually set a value that does not change year to year? After even the smallest inaccuracy can lead to a huge inaccuracy of our measurements of the distance to far away objects?

forgot to mention that was described in article 3 though they did not mention ceasium itself it does appear to describe variations that could equally affect ceasium.

more specifically this line
coworkers
at the Observatoire de Paris and the Ecole
Normale Supérieure (ENS) compared the rates of
cesium and rubidium fountain clocks over a fiveyear
period. If α is changing, the rate of change,
1/α(dα/dt), must be less than –0.4±16×10–16
per year

so even that measure is changing

 

[MeteorWayne]

IOW, it must be LESS THAN the quoted value, which could be the zero that relativity projects.

 

[Mordred]

I think you just lost me there can you expand on it plz

 

[MeteorWayne]

No, it was pretty clear. The value quoted is a MAXIMUM value. The actual value can be anywhere below that, including the zero that relativity predicts.

 

[Floridian]

As I said, it doesn't matter. Light travels at the same speed in any gravitational field - 299,792,458 m/s or so - and so a light year is independent of that. Time passes at different rates in different gravitational fields, but what a year is, or a second is, remains the same, so there's no difference in the definition of a light year. It's 299,792,458 m/s*1 year = 9.5*10^15 meters. Simple as that. There's no other way to define it.

""Light travels at the same speed in any gravitational field""

Not to an outside observer.

Yes, it does. Light is always measured at c, in a vacuum, by any observer.

Uhhh, speed is relative. Clearly he wasn't talking about lights speed in a vacuum, as in all normal practicality, that never happens.

There may need to be some re-workings of the term speed. As, right now, nobody has been able to manipulate space or term to enhance their distance traveled, it isn't an issue, but, when we use the term speed, I think you can say that light moves much faster through areas where there is little gravitational field. That might be a technically inaccurate statement but logically its sound. In actuality, it would be that the space-time around the light was less warped.

 

[origin]

I think you can say that light moves much faster through areas where there is little gravitational field. That might be a technically inaccurate statement but logically its sound.

What the heck does that mean? Technically wrong, but logically sound? Light does not move at different speeds through different gravitational fields.

 

[ramparts]

Indeed - and speed is not relative - at least, the speed of light isn't! That's the point of special relativity; all speeds are relative, except for the speed of light, which is the same in all reference frames (including gravitational frames).

 

[nimbus]

I've managed to confuse myself thinking about this one, when I thought I had it down pat at first.

If light can't escape a black hole, is it because space past the event horizon is infinitely curved, or that time is slowed infinitely, or that light is perpetually bent around within the EH, or ... ?

 

[ramparts]

Because the escape velocity inside the event horizon is larger than the speed of light, so any light ray sent away from the black hole won't be fast enough to escape the gravitational pull, and will be sucked back in.