# Do light years mean years in space or on Earth?

## Space or Earth?

• ### Scientists are ignoring gravity's effect on time.

• Total voters
5
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#### rocketmonkey

##### Guest
I have been working on a theory about light years, but I need to find out what scientists are talking about. I always hear about light years, but is that light travel time in space or on Earth? I need to know that before I finish. It is key to the equation.

O

#### origin

##### Guest
Is this some sort of joke?

If it is not a joke, a light year is the distance light travels in 365.25 earth days. Polls are for opinions, a light year is not an opinion.

S

#### SteveCNC

##### Guest
the distance light travels in a vacuum in 1 earth year or 299,792,458 m/s

I know I was wondering the same thing at first , is this a joke ?

the correct answer isn't even one of the options

S

#### Shpaget

##### Guest
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.

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#### ramparts

##### Guest
I took the OP's question to be asking about relativistic effects - time being measured differently in different gravitational fields and whatnot.

To the OP - since the speed of light is constant in all frames. Since the speed doesn't care what kind of gravitational field you're in, whether you're on Earth or in space, etc., you can calculate the distance a light year represents without worrying about differing measures of time.

And I'm very curious what kind of "theory" you have which is so urgently awaiting the results of an SDC poll :lol:

D

#### drwayne

##### Guest
We have had a person or two here (aand at BAUT) that look at the unit of a light year and try
to derive some deep space-time meaning from it.

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#### rocketmonkey

##### Guest
I should rephrase the question. Time is affected by gravity by becoming slower in gravitational fields. Seeing has space has no gravity, light would travel in less time than it would on Earth. I wasn't asking what light years were. I'm asking if scientists talk about light years in space or years on Earth normally. Sorry for the confusion.

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#### ramparts

##### Guest
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.

W

#### Woggles

##### Guest
Isn’t a pound of butter the same as a pound of bacon? So wouldn’t a light year in space be the same as year on earth? A year is a year is a year! Well that how I see it lol!

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#### undidly

##### Guest
ramparts":3sp0jeps said:
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.

T

#### theridane

##### Guest
rocketmonkey, what you're talking about is a well known relativistic phenomenon of length contraction in a gravitational field.

Speed of light is defined as 299,792,458 m/s, one second is defined as 9,192,631,770 periods of the 133 cesium as if it weren't influenced by gravity, so a light-year is also thought of as without any influence of gravity.

Down here in our gravity well lengths contract a little bit, as a result of time contraction due to gravity, so a light-year down here is only 0.999999999305 of the real deal, a difference of about 6500 km.

O

#### origin

##### Guest
undidly":10j5o1bp said:
ramparts":10j5o1bp said:
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.

K

#### Kessy

##### Guest
The fundamental point of Relativity (both special and general) is that light *always* travels at exactly c, to all observers, regardless of frame of reference. Time and length dilation are caused by this, and when applied to light itself cancel each other out by definition.

If I'm on one ship in deep space and you're in another, and we're traveling at 0.5c (or any other speed) relative to each other, we can both observe the exact same photon, and we will both measure it as moving at exactly c in our frames of reference. This is how we derive the formulas for how space and time are distorted in relativistic settings. The same applies to observers near and far from a gravity source.

N

#### nimbus

##### Guest
Rocketmonkey might be confused by extreme case of black hole gravity.

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#### Shpaget

##### Guest
So, if I get this right... the gravity causes time to tick slower (for an outside observer), but it also causes the contraction of the length so the effects cancel themselves out to give us a constant c?

Would that mean that there is more space around objects than there is in empty space (where there are no planets and stars)?

T

#### theridane

##### Guest
The way I understand it is that from an outsider's point of view out clocks tick slower. Because of that, out lenghts apear to be smaller, too, since c is constant and therefore length is effectively a function of c.

From our squeezed point of view photons move at c in the vicinity of our gravity well. However, photons outside of our influence appear to move FTL, because the outside lengths are "longer".

Photons are running c in both frames, but the apparent speed is different because of time dilation.

Right?

K

#### Kessy

##### Guest
Shpaget":11fpwgdh said:
So, if I get this right... the gravity causes time to tick slower (for an outside observer), but it also causes the contraction of the length so the effects cancel themselves out to give us a constant c?

Would that mean that there is more space around objects than there is in empty space (where there are no planets and stars)?

The first part is essentially correct. As for "more" space... The geometry of space is different around a massive object then in deep space - it's curved instead of flat. I'm not quite sure if you would consider that "more" space or not... I will say that's not how it's usually described.

theridane":11fpwgdh said:
The way I understand it is that from an outsider's point of view out clocks tick slower. Because of that, out lenghts apear to be smaller, too, since c is constant and therefore length is effectively a function of c.

From our squeezed point of view photons move at c in the vicinity of our gravity well. However, photons outside of our influence appear to move FTL, because the outside lengths are "longer".

Photons are running c in both frames, but the apparent speed is different because of time dilation.

Right?

Only half right, theridane. If you're looking at a particular photon, it will always appear to be moving at c to any observer in their own frame of reference, no matter where that observer is in the universe. It doesn't matter if the observer is in free space or deep in a gravity well, or if the photon is in a gravity well or not.

T

#### theridane

##### Guest
How about photons in distant parts of the universe? Those are expanding away at an apparently FTL speed, and yet they are less than FTL in their own reference frames (locality of velocity in general relativity).

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#### undidly

##### Guest
theridane":rfew2tj4 said:
How about photons in distant parts of the universe? Those are expanding away at an apparently FTL speed, and yet they are less than FTL in their own reference frames (locality of velocity in general relativity).

""Those are expanding away at an apparently FTL ""

How can we know?.
How can we see them?.

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#### ramparts

##### Guest
theridane":1a080yw5 said:
How about photons in distant parts of the universe? Those are expanding away at an apparently FTL speed, and yet they are less than FTL in their own reference frames (locality of velocity in general relativity).

Well, we don't see photons themselves expanding away at speeds faster than light - we see objects moving at speeds faster than light, and those emit photons (which of course travel at c). Of course that's just an illusion, as the velocity comes from space expanding rather than actual movement of the objects.

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#### undidly

##### Guest
ramparts":ymmsjing said:
theridane":ymmsjing said:
How about photons in distant parts of the universe? Those are expanding away at an apparently FTL speed, and yet they are less than FTL in their own reference frames (locality of velocity in general relativity).

Well, we don't see photons themselves expanding away at speeds faster than light - we see objects moving at speeds faster than light, and those emit photons (which of course travel at c). Of course that's just an illusion, as the velocity comes from space expanding rather than actual movement of the objects.

How do we see the objects at all?.
How can we know that what we see is an illusion?.

M

#### Mordred

##### Guest
Now I have a question here does this expansion affect the laws of general relaitivity?
for example if we have an expansion rate of x at one point . Does the same mathematics take into consideration a different expansion rate of say 5x ? If I recall the calculations are vectoral based time being the 4th vector.

O

#### origin

##### Guest
Mordred":vjkme38f said:
Now I have a question here does this expansion affect the laws of general relaitivity?
for example if we have an expansion rate of x at one point . Does the same mathematics take into consideration a different expansion rate of say 5x ? If I recall the calculations are vectoral based time being the 4th vector.

General relativity predicts that space is expanding or contracting.

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#### unclefred

##### Guest
The original question asked if a light year was the same for all observers or unique for an Earth observer. If there are two observers traveling at different speeds, there will be time dilation and length contraction when comparing the two. Yes the two observers will each measure what they believe to the speed of light. But with each using different clock rates and different length standards, will the two measurements of the speed of light be the same or just appear to be the same to the observers? But the original question is the length of a light year, not the speed of light. Assuming that the answer to the above question is that the speed of light is the same in both frames, will the distance traveled in one year be the same? Remember that time dilation says a year is different, so traveling at the same speed for a different length of time should cover a different length. Thus a light years are different lengths depending on your reference frame? I believe that is the original question.

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#### theridane

##### Guest
Speed of light is a constant, rate of time is not, then by definition distance traveled by light in a given amount of time in a given gravitational field has to be different. There's no other solution to that problem. So a light year on Earth has to be shorter than a light year outside of our gravity well. And you know what? It is. It's trivial to derive length dilation from time dilation under gravity by assuming constant c.

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