Stupid question about light years

[Astro Noodles]

Now I'm going to make myself look pretty stupid. Please go easy on me.

Is the speed of light a universal constant?

If I could see something 10 billion light years ago/away, and that thing was travelling away from me at .5 of the speed of light. Would it be that the light was emitted from that object 10 billion years ago, or was the object 6.6 or 7.5 billion light years away or something when the light was emitted?

[vlaiv]

11 minutes ago, Astro Noodles said:

Is the speed of light a universal constant?

One of postulates of Special Relativity is that speed of light in vacuum is constant in any frame of reference - so yes, in that sense it is universal constant - however, light does not always travel at that speed - namely in different medium like in air, or glass or other - it will travel slower than in vacuum.

12 minutes ago, Astro Noodles said:

If I could see something 10 billion light years ago/away, and that thing was travelling away from me at .5 of the speed of light. Would it be that the light was emitted from that object 10 billion years ago, or was the object 6.6 or 7.5 billion light years away or something when the light was emitted?

If something is traveling at very large speed and shines light in your direction - several things will happen:

1. Person that shins light in your direction will measure that light travel at light speed

2. You will also measure that light passing you by at light speed

3. Light will be red shifted due to relativistic Doppler effect

4. That light will reach you in future at time it takes for light to travel the distance from point in your reference system at which origin shone the light all the way to you.

Also note that following things will happen:

1. Person shining the light will not see the time as you do - it can't know what is your local time at which light was sent

2. Person shining the light will see distance between them and you  - differently then you'll see distance between you and them

3. Person shining the light will appear to have their time slowed down in comparison to your clock

etc ...

In the end, I'd like to point out that we did not include effects of expanding universe which is very much relevant at such large distances (but above will be true even in case of light days). Expansion of universe adds another dimension to all of that and although you are not effectively moving against the source in regular terms - space between you is expanding and you are getting further apart - and that affects how long it takes for light to reach you and it also causes light to stretch with expanding space so it ends up further red shifted due to that (longer wavelengths as if it has been stretched).

[JamesF]

I think vlaiv's answer more than demonstrates why asking this question does not make anyone look pretty stupid

James

[Astro Noodles]

Thanks Vlaiv, but I'm not sure I'm any the wiser. Are you saying it is not possible to tell whether the second part of my question can be answered because of the variables involved? I think you are saying that when someone on Youtube says that something is 10 billion light years away, they can't possibly know that?

Sorry if I seem like a numpty but my academic background is in the arts, so a lot of this is difficult for me to conceptualise.

...........Ah, I think I get it. So the light has travelled 10 billion light years from the object, and it doesn't matter how fast it is receding from the viewer. Then you need to add in the expansion of the Universe to know actually how far away it was when the light was emitted?

 

Edited June 25, 2021 by Astro Noodles
light bulb moment

[vlaiv]

2 minutes ago, Astro Noodles said:

Thank Vlaiv, but I'm not sure I'm any the wiser. Are you saying it is not possible to tell whether the second part of my question can be answered because of the variables involved? I think you are saying that when someone on Youtube says that something is 10 billion light years away, they can't possibly know that?

Sorry if I seem like a numpty but my academic background is in the arts, so a lot of this is difficult for me to conceptualise.

Can you be more specific about what you are asking?

36 minutes ago, Astro Noodles said:

If I could see something 10 billion light years ago/away, and that thing was travelling away from me at .5 of the speed of light. Would it be that the light was emitted from that object 10 billion years ago, or was the object 6.6 or 7.5 billion light years away or something when the light was emitted?

If you see the light from object that you perceive as being 10 billion light years away - then that light started a journey 10 billion years ago and it traveled thru space - the distance of 10 billion light years.

We say that object that we are looking at is 10 billion light years away - but that is not actual distance and meaning of "actual" distance to that object is rather ambiguous. For sake of simplicity - let's assume that object we are talking about is at rest with respect to us in terms of local speed. Any speed that we discuss here will be due to expansion of universe.

1. You can ask - what was the distance between us and that object at the time the light was emitted. It was less than 10 Bly.

2. You can ask - what was the distance traveled by light since it departed object until it reached us - that is exactly 10Bly

3. You can ask - what is the distance between us and that object now - it is more than 10Bly

This is nothing strange really - analogy to this can be made by the two of us using simple ball. Imagine that we are 10 meters apart and we are running away from each other. I throw the the ball at one point towards you. At that time we are say 10 meters apart.

Before ball reaches you - you'll move a bit further away from the point you were at when the ball was thrown - so ball will travel a bit more than 10 meters. At the time you catch the ball - I'll also move a bit in opposite direction so I'll be a bit further still.

So we again have three distances in ascending order. Only difference between this case and above case is that here the two of us are running away - but in case of distant galaxy - it is space that is expanding in between - and that makes things as if we are moving away from that galaxy - it looks like galaxy has some speed with respect to us.

As far as distances involved - well we can calculate all of those by just knowing one - 10Bly that took the light to reach us. We can calculate what was the distance between us and that object - before 10By and we can calculate what is the distance now.

In order to do that - and in fact to know that something is 10Bly away - we must rely on our cosmological model and solve it for observed data. In fact - physicist did that - and we are actually in a bit of a pickle there. We have same model calculated from two different sets of data and results don't quite agree. This can mean that either one or both sets of data contain systematic error - or our model is flawed.

When I say model - I mean LambdaCDM model - currently accepted cosmological model. It connects things like age of universe, Hubble's constant, ratio of matter to radiation to dark energy in universe and so on. When someone says that object is 10Bly away - they actually mean that there is certain red shift in spectrum of light coming from that source and we can use that red shift to determine how much space expanded from the moment light was emitted up until now. Since we know how space expanded over time - we can tell how long ago light was emitted and so on and so on.

Problem is that we currently have two different values for present day Hubble constant - 67km/s/mpc vs 73km/s/mpc. On one hand - that is extraordinary match since these two values are calculated from "data separated by 13.7Bly" - one is from cosmic microwave background radiation - light from about 377700 years from big bang, while other is from current measurement of how fast are galaxies around us receding from us as we speak vs their distance.

On the other hand - it is problem that these two values don't match better - as that points to an error somewhere - to quote wiki:

Quote

Multiple methods have been used to determine the Hubble constant. "Late universe" measurements using calibrated distance ladder techniques have converged on a value of approximately 73 km/s/Mpc. Since 2000, "early universe" techniques based on measurements of the cosmic microwave background have become available, and these agree on a value near 67.7 km/s/Mpc. (This is accounting for the change in the expansion rate since the early universe, so is comparable to the first number.) As techniques have improved, the estimated measurement uncertainties have shrunk, but the range of measured values has not, to the point that the disagreement is now statistically significant. This discrepancy is called the Hubble tension.^[68][69][70]

As of 2020, the cause of the discrepancy is not understood. In April 2019, astronomers reported further substantial discrepancies across different measurement methods in Hubble constant values, possibly suggesting the existence of a new realm of physics not currently well understood.^{[60][71][72][73][74]} By November 2019, this tension had grown so far that some physicists like Joseph Silk had come to refer to it as a "possible crisis for cosmology", as the observed properties of the universe appear to be mutually inconsistent.^[75] In February 2020, the Megamaser Cosmology Project published independent results that confirmed the distance ladder results and differed from the early-universe results at a statistical significance level of 95%.^[76] In July 2020, measurements of the cosmic background radiation by the Atacama Cosmology Telescope predict that the Universe should be expanding more slowly than is currently observed.^[77]

Depending on which set of data you take - you'll get slightly different answer to actual distance in light years - that is why astronomers often use red shift value rather than actual distance.

 

[saac]

In general terms yes the distance is known with reasonable confidence.  Two methods are generally used to determine the distance of stellar objects; standard candle method (comparative method) and parallax.   Standard Candles (comparative stars with similar luminosity called Cepheid variables have known distance ) are used to measure distance to distant stars while the parallax method is used to measure the distance to nearby stars.  Remember that the term light year is a distance and not a measure of time.  So when the stellar object is quoted as being 10 by light years away that means exactly that  - the light we see when it was emitted - would place the object 10 by light years distant to "now".  What Vlaiv has made reference to relates to time dilation and length contraction which affects how measurements of time and length are observed by observers moving in relativistic frames of reference (non accelerating) .  In effect moving clocks will not agree with each other. Another consequence and puzzling effect of special relativity leads to the conclusion that a simultaneous experience or definition of "now"  does not really have any meaning - asking where is the star now is fraught with difficulty. 

https://www.scientificamerican.com/video/how-do-we-measure-the-distance-to-s2013-08-06/

 

Jim 

Edited June 25, 2021 by saac

[Astro Noodles]

12 minutes ago, vlaiv said:

Can you be more specific about what you are asking?

If you see the light from object that you perceive as being 10 billion light years away - then that light started a journey 10 billion years ago and it traveled thru space - the distance of 10 billion light years.

We say that object that we are looking at is 10 billion light years away - but that is not actual distance and meaning of "actual" distance to that object is rather ambiguous. For sake of simplicity - let's assume that object we are talking about is at rest with respect to us in terms of local speed. Any speed that we discuss here will be due to expansion of universe.

1. You can ask - what was the distance between us and that object at the time the light was emitted. It was less than 10 Bly.

2. You can ask - what was the distance traveled by light since it departed object until it reached us - that is exactly 10Bly

3. You can ask - what is the distance between us and that object now - it is more than 10Bly

This is nothing strange really - analogy to this can be made by the two of us using simple ball. Imagine that we are 10 meters apart and we are running away from each other. I throw the the ball at one point towards you. At that time we are say 10 meters apart.

Before ball reaches you - you'll move a bit further away from the point you were at when the ball was thrown - so ball will travel a bit more than 10 meters. At the time you catch the ball - I'll also move a bit in opposite direction so I'll be a bit further still.

So we again have three distances in ascending order. Only difference between this case and above case is that here the two of us are running away - but in case of distant galaxy - it is space that is expanding in between - and that makes things as if we are moving away from that galaxy - it looks like galaxy has some speed with respect to us.

As far as distances involved - well we can calculate all of those by just knowing one - 10Bly that took the light to reach us. We can calculate what was the distance between us and that object - before 10By and we can calculate what is the distance now.

In order to do that - and in fact to know that something is 10Bly away - we must rely on our cosmological model and solve it for observed data. In fact - physicist did that - and we are actually in a bit of a pickle there. We have same model calculated from two different sets of data and results don't quite agree. This can mean that either one or both sets of data contain systematic error - or our model is flawed.

When I say model - I mean LambdaCDM model - currently accepted cosmological model. It connects things like age of universe, Hubble's constant, ratio of matter to radiation to dark energy in universe and so on. When someone says that object is 10Bly away - they actually mean that there is certain red shift in spectrum of light coming from that source and we can use that red shift to determine how much space expanded from the moment light was emitted up until now. Since we know how space expanded over time - we can tell how long ago light was emitted and so on and so on.

Problem is that we currently have two different values for present day Hubble constant - 67km/s/mpc vs 73km/s/mpc. On one hand - that is extraordinary match since these two values are calculated from "data separated by 13.7Bly" - one is from cosmic microwave background radiation - light from about 377700 years from big bang, while other is from current measurement of how fast are galaxies around us receding from us as we speak vs their distance.

On the other hand - it is problem that these two values don't match better - as that points to an error somewhere - to quote wiki:

Depending on which set of data you take - you'll get slightly different answer to actual distance in light years - that is why astronomers often use red shift value rather than actual distance.

 

Thanks Vlaiv. I'm going to go away and think about that for a few days.

[Astro Noodles]

Another question. And thanks in advance for your patience,

Does the expansion of the universe have a effect on the length of time that it takes light to travel from point a to point b ?

So, in a static universe an object 10 billion light years away emitted its light 10 billion years ago, but would expansion alter that length of time?

[Soligor Rob]

So not a stupid question Noodles at all I have learnt something or at least started to gain some knowledge, Thanks to Vliav's explanation.

[vlaiv]

8 hours ago, Astro Noodles said:

Another question. And thanks in advance for your patience,

Does the expansion of the universe have a effect on the length of time that it takes light to travel from point a to point b ?

So, in a static universe an object 10 billion light years away emitted its light 10 billion years ago, but would expansion alter that length of time?

Yes it does.

It stretches both space "in front" of traveling light and "behind" it. Only stretched space in front of traveling light affects the time it takes for light to complete the journey, but stretching space affects total distance between two objects at the moment light reaches second one.

[oymd]

Sorry to budge in, but if we take into consideration that the universe is ever expanding at about 73km/s, why do we give stellar objects a particular fixed distance from our viewing point?

Shouldn't the distance be ever increasing by the second?

ie: M31 is described as 2.5 lightyears away? Shouldn't that number be increasing by the second? a year has about 31,536,000 seconds. So, over a year, our distance to M31 should have increased by 2,302,128,000kms, and that is just one year? (equates to about 2 light hours a year)

[Astro Noodles]

5 minutes ago, oymd said:

Sorry to budge in, but if we take into consideration that the universe is ever expanding at about 73km/s, why do we give stellar objects a particular fixed distance from our viewing point?

Shouldn't the distance be ever increasing by the second?

ie: M31 is described as 2.5 lightyears away? Shouldn't that number be increasing by the second? a year has about 31,536,000 seconds. So, over a year, our distance to M31 should have increased by 2,302,128,000kms, and that is just one year? (equates to about 2 light hours a year)

My understanding is that objects are travelling away from earth at rates based on their distance, so the further away an object is, the faster it is travelling away. But I'm sue that Vlaiv  or saac will set us on the right track.

[vlaiv]

2 minutes ago, oymd said:

Sorry to budge in, but if we take into consideration that the universe is ever expanding at about 73km/s, why do we give stellar objects a particular fixed distance from our viewing point?

Shouldn't the distance be ever increasing by the second?

ie: M31 is described as 2.5 lightyears away? Shouldn't that number be increasing by the second? a year has about 31,536,000 seconds. So, over a year, our distance to M31 should have increased by 2,302,128,000kms, and that is just one year? (equates to about 2 light hours a year)

Actual speed is about 73 (or 67) km/s/mpc.

Speed of recession varies with distance. That last bit is mpc - or MegaParsec which is equivalent to about 3261560 Ly

So at a distance of 3.26Mly average recession speed of objects is 73km/s - at twice that distance or 6.52Mly, recession speed is 146Km/s and so on...

M31 that is 2.5Mly should have slightly lower recession speed that 73km/s - but it is in fact not moving away, it is approaching MW due to gravity interaction in local group. At small distances gravity is still dominant force and galaxies bound in local groups and small clusters - act like regular bodies in gravity bound system.

M31 is actually moving at a speed of 110km/s towards us. Which means it will get here in about 4.5By.

[Astro Noodles]

Hi Vlaiv

Why is everything travelling away from us, and not from some other point in the Universe. Or is it that everything is relative, - everything is travelling away from everything else regardless of the position of the observer?

Edited June 26, 2021 by Astro Noodles

[andrew s]

Two points. The speed of light measured locally is a constant. There is a lot of different coordinate speeds of light depending on the coordinate system chosen including ones related to the expansion of the Universe.

Cosmological distance is dependant on the model chosen and on the exact distance requested. I.e. the distance when the light was emitted or when received.

 

8 minutes ago, Astro Noodles said:

Hi Vlaiv

Why is everything travelling away from us, and not from some other point in the Universe. Or is it that everything is relative, - everything is travelling away from everything else regardless of the position of the observer?

The Universe is to a good approximation on a large enough scale homogeneous  and isotropic so all points are equivalent so no special point to expand from. Also the expansion is not any kind of explosion from a point. It is a change in metrical scale everywhere. 

Regards Andrew 

Edited June 26, 2021 by andrew s

[vlaiv]

1 minute ago, Astro Noodles said:

Hi Vlaiv

Why is everything travelling away from us, and not from some other point in the Universe. Or is it that everything is relative, eg everything is travelling away from everything else regardless of the position of the observer?

Everything is moving away from everything else - regardless of position.

That is rather easy to see why on simple example - but for some reason harder to extrapolate to whole universe

Say you have following arrangement:

A---B---C

Three people (Alice, Bob and Charlize) that are in a straight line.

Bob is standing still, Alice is moving to the left at steady pace and Charlize is doing the same to the right. In first instance they are separated by three bars of distance and in some future time they are separated by 5 bars of distance like this:

A-----B-----C

(A and C are moving away from B with steady speed of two bars per some time interval).

Bob sees everyone moving away from him in every direction. So does Alice - but notice something important with Alice - it sees Bob moving with speed of two bars per interval - but what about Charlize? She is further away and moves with 10 - 6 = 4 bars of distance per same time interval. Alice sees Charlize being twice as far away as Bob and receding at twice the speed in comparison to Bob - Hubble's law in action in simple example.

Everyone can see them selves as standing and everything else is moving away from them (choice of reference frame) and further someone is from them - faster it will move with respect to them - but in reality everyone will see the same speed at same distance - Hubble constant.

Now all you need to do is extrapolate that to 3d case. Usual step used is to extrapolate that to 2d case - by use of muffin or balloon. Imagine balloon with spots on it being inflated:

If you choose any two point on that balloon - their distance increases linearly with time (as balloon is being inflated) - but points at distance of "two" will move twice as fast from each other as Alice and Charlize did and so on ...

Next step you have to do in your head as we can't show 3d analogy on 2d screen

 

 

[Clarkey]

18 minutes ago, vlaiv said:

M31 is actually moving at a speed of 110km/s towards us

Oh oh. Don't tell the tabloids🤣

Sorry..... Should be taking spacetime seriously.

[Astro Noodles]

So I started with one question but now I have about 20 more to ask. 🤣

[vlaiv]

33 minutes ago, Astro Noodles said:

So I started with one question but now I have about 20 more to ask. 🤣

Then fire away. Expanding universe is waiting for no one

 

[oymd]

28 minutes ago, vlaiv said:

Then fire away. Expanding universe is waiting for no one

 

What was it that NDT said in his book?

”the universe is under no obligation to make sense to you”

😉

[Astro Noodles]

It seems to me that cosmology as it stands is as much philosophical mathematics as anything that can be observed directly. The models will no doubt evolve as new observations either confirm their validity or not.

Is the problem here something to do with perception? I think that what I'm trying to say is that humans are primarily a visual animal. And, unsurprisingly then our observations have focused on the electro-magnetic spectrum. Gravitational waves, as something entirely different will offer another perspective I imagine. Our other senses seem utterly inadequate for observing the universe beyond our immediate environment. 

The universe may be under no obligation to make sense to us. It may be under no obligation to be sensed at all by us.

 

[saac]

Well we are fortunate that we can sense it directly in the visible spectrum and through our technology via the rest of the em spectrum.  For sure our emerging capability with respect to gravitational waves will help push our understanding further.  It would appear that we were made to observe and understand the universe.  

As for our other senses well they haven't been completely neglected - we have listened  to the atmosphere of the planets and even tasted and smelt space !

https://www.science.org.au/curious/space-time/smells-space

Jim

Jim

Edited June 27, 2021 by saac

[Astro Noodles]

I don't think we were made to observe the universe. I think that we evolved senses to give us a competitive advantage in our own environment. 

Some creatures have very peculiar sensory systems which are different for our own - the ability of many creatures to navigate using the Earth's magnetic field springs to mind.

I am just postulating that we might require different sensory equipment to broaden our understanding of the universe.🤔

[vlaiv]

2 hours ago, Astro Noodles said:

It seems to me that cosmology as it stands is as much philosophical mathematics as anything that can be observed directly

You can really say the same about general relativity or quantum mechanics / quantum field theory.

In fact, cosmology is just the two above applied on observations that we make of the universe around us on many levels.

My friend noted very interesting thing that I'll paraphrase like this: "It's now been almost 100 years that we have two of the most successful physics theories - GR and QM, yet 99.999% of people know nothing about them and have serious trouble comprehending concepts needed to understand those theories".

That is indeed very interesting observation and I can only assume it is so because of the level of mathematics needed to formulate those theories. By contrast - people don't seem to have any problem with Newtonian mechanics and gravity.

Don't assume that cosmology is more "philosophical" than other physics just because of the math involved.

3 hours ago, Astro Noodles said:

Is the problem here something to do with perception? I think that what I'm trying to say is that humans are primarily a visual animal. And, unsurprisingly then our observations have focused on the electro-magnetic spectrum. Gravitational waves, as something entirely different will offer another perspective I imagine. Our other senses seem utterly inadequate for observing the universe beyond our immediate environment. 

I don't think so.

I don't believe that our perception has anything to do with it. We are well equipped mentally to make a measurement as it is comparison of two values.

We know that speed of light in vacuum is a constant regardless of frame of reference - because we measured it and we always get the same result. We know that galaxies are receding from us because again - we measured it, or rather we measured shift in light spectrum that happens due to this. And so on ...

Gravitational waves have already been used to measure Hubble constant and so have many different methods - have a look at the list on wiki page:

https://en.wikipedia.org/wiki/Hubble's_law#Measured_values_of_the_Hubble_constant

 

[Astro Noodles]

13 minutes ago, vlaiv said:

Don't assume that cosmology is more "philosophical" than other physics just because of the math involved.

I'm not making any assumptions. 🙂