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Time of flight in an expanding universe


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I was just wondering....

If the universe is expanding, when light left a distant object the path to Earth was shorter than it is by the time it arrived here.

Does that mean, for example, that a photon arriving from Andromeda 2 million light years away actually had a journey time of less than 2 million years, as at the start of the journey it would have covered a greater percentage of the current distance in each year of its journey than it covers towards the end? (ignoring relative motions of the galaxies)

A bit non-linear in terms of percentage of journey plotted against time?

Maybe this is nonsense and somebody can put me out of my misery wondering about it?

 

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I think it is the other way around.

Photon actually did travel 2 million years and covered 2 million light years of distance. So integral of space distance for duration of flight of photon adds up to 2 million light years. But when we detect photon, at that point in time actual space distance is greater - universe expanded. At the time photon left for earth, actual space distance was less than both, traveled distance and "current" one.

 

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I was an astrophysicist wanna be - still am, but ended up going into engineering.  Maybe someone can enlighten me.

I can hardly keep up with all the variables involved - seriously.  What does it mean that Andromeda is 2 million ly away?  Does it mean that Andromeda is the same distance away as a specific type of supernova that took place in the galaxy that we use to "calibrate" distances.  Assuming the "calibration" is accurate that means that the photons now arriving from Andromeda are arriving at roughly the same time (think epochs of time) as the photons from the supernova.  The relative intensity of the supernova's photons allow us to ratio up from the same type of supernovas the distances of which we've estimated using other methods (perhaps by comparing to known intensities of variable stars the distances of which we have calibrated by parallax perhaps).

Of course, Andromeda is not only further away now but the space "between" us and Andromeda has been expanding while the photons have been enroute.  This is where I genuinely feel ignorant.  Given that over intergalactic distances and times that even "simultaneity" is relative, is there even an absolute meaning to the term distance.  Would it be more accurate to say that when the photons left in Andromeda that they were a certain "space time" units away?  Would that give us a more objective way of comparing distances?

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Well, first of all, although space is expanding between Milky way and Andromeda, that might not be the best case to discuss effects. Local relative movement of Andromeda is many orders of magnitude larger then space expansion, and hence Andromeda is closing in rather then receding (we will merge in the future).

I think that back in 1923 Hubble actually discovered that Andromeda is another galaxy by measuring variable star distance rather than Type Ia supernova as a standard candle.

By the way, I'm also far from expert on the matter, but perhaps I can shed some light (if I'm correct with all of this).

We know about expanding universe from red shift of distant galaxies (and its relation to galaxy distance). Now there is something to be understood here about red shift. It can be thought of in two different ways. First is just regular Doppler shift - thing that emitted light was moving relative to us at the moment it emitted light - hence light that we see is red shifted. But you might say - well no distant galaxy does not really move but rather space is expanding in between. For that particular case it turns out that following explanation applies: Light emitted at certain point had particular wavelength and on each "leg" of it's journey space that it was traveling thru expanded just a bit, like really small amount, and since light is electromagnetic wave in space - it wavelength became just a tad longer, now we are talking like really small difference one moment to the next, but over wast time spans (and distances :D ) all those contributions add to measurable wavelength change - longer photon travels (greater the distance) - more of this "stretch" will accumulate - bigger the red shift.

Now, which of these two views is correct one? Well since there is no aether in space, and it is relative, both effects are "correct" or are the same thing - evaluate to same values.

There is set coordinates that can be used to represent "current" distances (or at "point in time") - co moving coordinates. Wiki has short introductory article that might be useful : https://en.wikipedia.org/wiki/Comoving_distance

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Here is another wannabe astrophithingy that went into engineering(electronics) instead; but I got a consolation prize of helping spacecraft get to Halley and another polar orbiting the Sun, yea!that was fun, all the way via Jupiter :) :

another bamboozler is that as far as the photon is concerned it took no time at all to get from Andromeda to here !

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I used to be an English teacher so don't listen to me! But...

A light year is a unit of distance. The distance covered by light in a year. The light travel time from an object, in a static universe, would be the same in years but years are not units of distance but of time. So in a static universe an object whose light had been in flight ten million years would be ten million light years away. How easy that would be! But during the 10 million years of light travel time the universe has been expanding so the object is no longer ten million LY away but far more than that.

Consider uncle Sid, the pigeon fancier. He is travelling from Edinburgh to London with his pigeons. In Birmingham at two pm he releases a pigeon with the message, I am in Birmingham. It's two pm. His wife receives this message 6 pigeon hours later, but is he still in Birmingham?

Olly

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1 hour ago, SilverAstro said:

Here is another wannabe astrophithingy that went into engineering(electronics) instead; but I got a consolation prize of helping spacecraft get to Halley and another polar orbiting the Sun, yea!that was fun, all the way via Jupiter :) :

another bamboozler is that as far as the photon is concerned it took no time at all to get from Andromeda to here !

Indeed.

For a photon, time does not pass. Everything happens at once and it takes no time at all to cross the entire universe.

I think that sounds quite boring. Glad I'm not an electron!

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For most mortal (no astrophysicists or others toiling with the fate of the universe) I believe we can simply ignore the expansion effect on distance. The light year as Olly has pointed out is reduced simply to the distance travelled by light in one year - approximates to 9.5 x 10^12 km.  

Jim

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I'm starting to wonder if time stands still if moving at the speed of light.

I was thinking about that fact, and how it can be used to better understand time. I have problem with time :D - can't really understand it quite. So one of my gut feelings is that rate of "passing" of time has something to do with rate at which energy momentum exchange takes place compared to something else, or maybe quantity of it. Imagine simple universe with space and two balls in it. They are not moving relative to each other, nothing happens. Could we tell that time is flowing? Now imagine again such universe and two balls are orbiting common center of "gravity". Does the time flow in such a universe and how would we tell? How many balls do we need in simple universe and doing what so that we can say - time flows.

Now while photon is in flight it is not part of any energy momentum exchange - so time does not flow? Or is it so? According to general relativity even single photon is bending space (any energy contained in volume is related to geometry of space time of that volume, and although photon does not have rest mass it does have energy).

So there is some kind of impact on things around it, but impact is so small so maybe time is running at some incredibly small rate but not quite standing still?

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If a photon can travel to every part of the universe instantly (which it does if travelling at the speed of light) then that surely means that the universe is both tiny and vast at the same time depending who is watching.

Alan

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Hahaa this is a gudun init ! Loadsa fun,

1) Everything is of no concern to the photon ( it doesnt have time,, to consider everything :) )

2) a photon can indeed cross the universe in no time at all, now , this epoch. But only since the time of last scattering ( approx 300,000y give or take after the BigBang ) the time of the CMB

3) a light year is the time we perceive in our frame of reference that it takes the photon to traverse that distance, not for the photon's time of which it has non.

4) where was I , errr, I think I lost some order,,   no electrons dont have a boring time, they have to play tag with the Pauli exclusion principle and exchange photons while doing so :)

5) yes time does stand still for the photon travelling at the speed of itself. Wont for you because you cant.

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2 minutes ago, Alien 13 said:

If a photon can travel to every part of the universe instantly (which it does if travelling at the speed of light) then that surely means that the universe is both tiny and vast at the same time depending who is watching.

would you adam&eve it ,right in the middle ,, my internet electrons stage a revolt :

Yes sort of,

it depends upon who defines tiny and large, you or the photon ?

I blame Einstein !!

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5 minutes ago, SilverAstro said:

Are they gravitationally bound and if not do you admit of Dark Energy ?

It's a simple mind game if nothing is moving - you can't tell that there is time at all. If things are "moving" but not relative to each other - again no time can be detected.

I think we need at least two measurable quantities (comparable to third one), one of which is changing to establish that time is passing, but we have no clue of "rate" of time passing. I have no idea how to tell if time is slowing down, or speeding up unless we say under regular circumstances there is fixed rate of said quantities changing relative to each other and if that rate is to change - rate of passage of time changes.

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Nothing is simple in a gedanken, look where Einstein's mind game got us, there are always assumptions.

The two particles will be moving if there is something else besides eg.  Dark Energy, exclude that then you exclude our present understanding , , then extend two particles to a galaxy of particles. dont forget all pervasive gravity and the Higgs field.

Ah! Here is Andrew he is much better at this sort of thing than I am  :)

 

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14 hours ago, SilverAstro said:

Ah! Here is Andrew he is much better at this sort of thing than I am  :)

Sorry I am late but the sky was clear and I had a new mount to polar align.

So many ideas (and welcome humorous asides) it is difficult where to start or end for that matter.

Firstly for all inertial observers light travel at c (in a vacuum) it never gets anywhere instantly. Light is not in an inertial frame and so you can't apply the equation for proper distance/time to it.

For all inertial observers their clocks always tick at 1 second per second just as their measuring rods measure one meter per meter. The differences apply when you look at what the time and distance measured by an observer in an inertial frame moving with respect to yours. 

The idea that light consists of photon traveling through space is not consistent with modern quantum theory. You can either use classical wave theory (Maxwell) in which light waves travel at c or you can have QED where photon can be created and destroyed with certain probabilities at specific location. But, then you can say nothing about what happens in between. Recall a photon does not have a position operator so you can never even in principle know where "it" is. It is just an excitation in the quantum field and only in very special circumstances is it localised. 

On the two isolated object if they are in a universe where GR applies then time will flow as you can't have a static universe which includes any mass, energy or pressure. The universe will be expanding or contracting.

Regards Andrew

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1 hour ago, andrew s said:

  >Ah! Here is Andrew he is much better at this sort of thing than I am  :)

Sorry I am late but the sky was clear and I had a new mount to polar align.

I have spoken with the judge and he says that is a perfectly acceptable    excuse    reason :blob7:

Yes it did go very quiet, wondered where everyone had got to, then I cottoned-on and checked outside, very murky, could only just make out the 4 ends of Cygnus :(

The you are folks , I told you so, so much better :) . Very succinct, brill in fact, very interesting. May have to come back later on a few things.

Thank you.

 

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Actually, the Milky Way and the Andromeda galaxy are moving closer together. Should we invert our earlier considerations  now? Or can we just handle things getting closer together and moving apart with a bit of blueshift and redshift?

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39 minutes ago, Ruud said:

Actually, the Milky Way and the Andromeda galaxy are moving closer together. Should we invert our earlier considerations  now? Or can we just handle things getting closer together and moving apart with a bit of blueshift and redshift?

If a galaxy is "moving" exactly with the expansion of the universe, the galaxy is said to be comoving. Just as gas molecules have random motions, galaxies have random motions with respect to the expansion of the universe. A galaxy's motion relative to comoving with the expansion is called a peculiar velocity. On small cosmological scales, expansion speeds are small, and peculiar velocities can dominate. On large cosmological scales, expansion speeds are much larger than peculiar velocities, so peculiar velocities are not noticeable.

On a small cosmological scale, when, by chance, a galaxy's peculiar velocity is towards us and larger than the expansion speed, then the galaxy moves towards us, and we its light blueshifted.

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2 minutes ago, George Jones said:

On small cosmological scales, expansion speeds are small, and peculiar velocities can dominate.

For galactic cluster the gravitation attraction between galaxies is 7 order of magnitude gerater than the tension caused by cosmological expansion and so they remain bound. See here https://arxiv.org/abs/astro-ph/9803097v1

Regards Andrew

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