Communicating with galaxies that are moving away from each other

SuprSi

I've read a number of times recently about how galaxies are moving away from each other at the speed of light, or faster, depending on what you read.

While I know the initial aim is to make contact with any advanced lifeform within our own galaxy, if that is ever achieved, how could we ever hope to make contact with anything outside our galaxy if the other galaxies are moving away at such a speed?

ThunderCat

SuprSi wrote: »

I've read a number of times recently about how galaxies are moving away from each other at the speed of light, or faster, depending on what you read.

While I know the initial aim is to make contact with any advanced lifeform within our own galaxy, if that is ever achieved, how could we ever hope to make contact with anything outside our galaxy if the other galaxies are moving away at such a speed?

Fun Fact - Our galactic neighbour, The Andromeda Galaxy, is actually moving towards us and will "collide" with the Milky Way in approx. 4 billion years time. It won't really collide however as the stars from each galaxy are sufficiently far apart that stellar collisions are improbable. Stars being ejected from each galaxy is more likely however. Both galaxies will over time merge together to create one giant elliptical galaxy.


As for the other galaxies that are moving away from us, it is my understanding that it depends on where they are relative to our perspective as to how quickly they are perceived to be moving away from us. For example, a galaxy a few million light years away from us will appear to be moving away from us at a slower rate than one that is hundreds of millions of light years away. The galaxies themselves are probably only moving at a speed of a few hundred miles per second locally but as I said it all depends on where they are in the universe relative to our perspective. The photons of light emitted from galaxies will still reach us of course despite the galaxy moving away from us, until eventually they wont due to the expansion of the universe itself being greater than the speed of light.

SuprSi

Ah yes, I remember seeing a program that showed what the sky will potentially look like when that happens, it might have been Brian Cox. Thanks for the explanation, it makes a lot of sense.

david75

Just reading about a radio signal received from a galaxy 3 billion light years away.

Does anyone know how that works?
How fast do radio signals travel? Or do they? I'm flummoxed

Mickeroo

david75 wrote: »

Just reading about a radio signal received from a galaxy 3 billion light years away.

Does anyone know how that works?
How fast do radio signals travel? Or do they? I'm flummoxed

Radio waves are electromagnetic radiation so they move at the speed of light through space.

david75

Mickeroo wrote: »

Radio waves are electromagnetic radiation so they move at the speed of light through space.

Cool thanks.
But doesn't light and time and gravity all behave differently around different planets/ galaxies?

Maximus Alexander

david75 wrote: »

Cool thanks.
But doesn't light and time and gravity all behave differently around different planets/ galaxies?

Well light can be bent by gravity and obviously won't go through planets... did you mean something more than that? The light from a distance galaxy is travelling through vast empty space to get here, it's not running into any planets.

ps200306

SuprSi wrote: »

I've read a number of times recently about how galaxies are moving away from each other at the speed of light, or faster, depending on what you read.

While I know the initial aim is to make contact with any advanced lifeform within our own galaxy, if that is ever achieved, how could we ever hope to make contact with anything outside our galaxy if the other galaxies are moving away at such a speed?

As Thundercat said, the speed at which the galaxies are receding depends on how far away they are. Think of it like an elastic band on which you draw three dots, A, B, C at one metre intervals. Now start stretching the elastic band and the dots all start moving away from each other. Imagine that in one second you have pulled the elastic band to twice its original length. Dots A and B that were initially 1 metre apart are now two metres apart. Dots A and C that were initially 2 m apart are now 4 m apart. So A will see B receding at 1 m/sec, while it sees C receding at 2 m/sec.

The expansion of space is like that. It is carrying the galaxies away from each other with velocities that depend on the distance between them. The more space between them to begin with, the faster the recession. With our elastic band example we could say that there is 1 m/sec of velocity for every 1 metre of distance. It's the same for space but on a different scale, so we use different units. The recession rate is 70 kilometres per second for each megaparsec of space. A megaparsec is about 3.26 million light years, so we can also convert the recession rate to 21,500 km/sec per billion light years. The speed of light is 14 times that, so a galaxy that is 14 billion light years away would be receding at the speed of light. (We're going to be saying "billion light years" a lot, so we'll abbreviate it Gly; similarly "billion years" will be Gy and "billion years ago" will be Gya).

So what does this actually mean? Distances in Big Bang cosmology are tricky. Some people might remember that the universe is less than 14 billion years old, so how could light have travelled 14 Gly in the available time even if the light from that distant galaxy left it right at the moment of the Big Bang? In this case we are talking about the proper distance -- the distance you would measure if you could lay out a huge measuring tape from here to the galaxy in an instant right now.

The distance between our two galaxies wasn't 14 Gly when the light set out. In fact, it was less than 5 Gly and it set out at just over 9 Gya. The universe has been continually expanding while the light was travelling, so the light covered a bigger fraction of the distance between us in its first year of travel than its last year. You have to add up all the little different-sized fractions along the way and we can use integral calculus to do that. We do indeed find that a distance that was 5 Gly at 9 Gya has become 14 Gly of proper distance at the present moment, and that it took light 9 Gy to travel it.

Now, there have been galaxies for more than 9 Gy. So light that has been travelling even longer than that is also reaching us from galaxies that are now even further away. Their current recession speed is even faster than the speed of light. This may be a surprise to people who have heard that the speed of light is some sort of absolute limit. And so it is -- the speed limit at which information can travel between two points -- but there is nothing stopping the expansion of space from carrying galaxies away from us at greater than light speed.

So now back to the original question: how can we possibly communicate with galaxies that are travelling away from us at the speed of light or greater. The answer is we can't. There are closer galaxies with which we could communicate (at least in principle, though it might take millions of years to exchange a light signal). But a galaxy that is receding at light speed would never receive a signal from us. Ironically, we are able to receive ancient light from them because it set off when we were close enough to communicate. But the window for sending a reply has closed forever. (This involves some assumptions about the future expansion rate, but we've probably complicated things enough already :pac: ).

david75 wrote: »

Just reading about a radio signal received from a galaxy 3 billion light years away.

Does anyone know how that works?
How fast do radio signals travel? Or do they? I'm flummoxed

Yep. Light travels at the speed of light, i.e. one light year per year. However, see the previous discussion about how this length changes over time. Mainstream media tends to say "3 billion light years" without mentioning if it's proper distance or something else. But we typically assume proper distance, i.e. the distance right now, this minute. At that distance the light has been travelling about 2.7 Gy, and the distance when it set out was 2.45 Gly.

david75

ps200306 wrote: »

As Thundercat said, the speed at which the galaxies are receding depends on how far away they are. Think of it like an elastic band on which you draw three dots, A, B, C at one metre intervals. Now start stretching the elastic band and the dots all start moving away from each other. Imagine that in one second you have pulled the elastic band to twice its original length. Dots A and B that were initially 1 metre apart are now two metres apart. Dots A and C that were initially 2 m apart are now 4 m apart. So A will see B receding at 1 m/sec, while it sees C receding at 2 m/sec.

The expansion of space is like that. It is carrying the galaxies away from each other with velocities that depend on the distance between them. The more space between them to begin with, the faster the recession. With our elastic band example we could say that there is 1 m/sec of velocity for every 1 metre of distance. It's the same for space but on a different scale, so we use different units. The recession rate is 70 kilometres per second for each megaparsec of space. A megaparsec is about 3.26 million light years, so we can also convert the recession rate to 21,500 km/sec per billion light years. The speed of light is 14 times that, so a galaxy that is 14 billion light years away would be receding at the speed of light. (We're going to be saying "billion light years" a lot, so we'll abbreviate it Gly; similarly "billion years" will be Gy and "billion years ago" will be Gya).

So what does this actually mean? Distances in Big Bang cosmology are tricky. Some people might remember that the universe is less than 14 billion years old, so how could light have travelled 14 Gly in the available time even if the light from that distant galaxy left it right at the moment of the Big Bang? In this case we are talking about the proper distance -- the distance you would measure if you could lay out a huge measuring tape from here to the galaxy in an instant right now.

The distance between our two galaxies wasn't 14 Gly when the light set out. In fact, it was less than 5 Gly and it set out at just over 9 Gya. The universe has been continually expanding while the light was travelling, so the light covered a bigger fraction of the distance between us in its first year of travel than its last year. You have to add up all the little different-sized fractions along the way and we can use integral calculus to do that. We do indeed find that a distance that was 5 Gly at 9 Gya has become 14 Gly of proper distance at the present moment, and that it took light 9 Gy to travel it.

Now, there have been galaxies for more than 9 Gy. So light that has been travelling even longer than that is also reaching us from galaxies that are now even further away. Their current recession speed is even faster than the speed of light. This may be a surprise to people who have heard that the speed of light is some sort of absolute limit. And so it is -- the speed limit at which information can travel between two points -- but there is nothing stopping the expansion of space from carrying galaxies away from us at greater than light speed.

So now back to the original question: how can we possibly communicate with galaxies that are travelling away from us at the speed of light or greater. The answer is we can't. There are closer galaxies with which we could communicate (at least in principle, though it might take millions of years to exchange a light signal). But a galaxy that is receding at light speed would never receive a signal from us. Ironically, we are able to receive ancient light from them because it set off when we were close enough to communicate. But the window for sending a reply has closed forever. (This involves some assumptions about the future expansion rate, but we've probably complicated things enough already :pac: ).



Yep. Light travels at the speed of light, i.e. one light year per year. However, see the previous discussion about how this length changes over time. Mainstream media tends to say "3 billion light years" without mentioning if it's proper distance or something else. But we typically assume proper distance, i.e. the distance right now, this minute. At that distance the light has been travelling about 2.7 Gy, and the distance when it set out was 2.45 Gly.

That was Brillo thank you.

Can I ask how do radio waves /busrts work in that same understanding?
radio signals do osmosis or what?
They're everywhere at once?

ps200306

david75 wrote: »

That was Brillo thank you.

Can I ask how do radio waves /busrts work in that same understanding?
radio signals do osmosis or what?
They're everywhere at once?

No, as Mickeroo said above, radio waves are electromagnetic waves and like all other electromagnetic radiation -- X-rays, visible light, infrared, microwaves etc. -- they all travel at the same speed of light. The elastic band analogy is limited because the light between the galaxies travels in a straight line along the elastic band, whereas real space is three dimensional and light travels in all directions as a spherical wave front. But it is by no means "everywhere at once" ... you might want to explain a bit more what you mean by that.

Maximus Alexander

david75 wrote: »

That was Brillo thank you.

Can I ask how do radio waves /busrts work in that same understanding?
radio signals do osmosis or what?
They're everywhere at once?

Basically, radio waves are light. Or rather, both radio waves and visible light are the same thing; electromagnetic radiation. The difference is just the frequency.

So a visible light wave would look like this:



While a radio wave would look like this:



(Actually, the difference is much greater but it would be infeasible to show that here)

david75

Got it thanks lads.

c montgomery

Anybody know of any good books that explain electromagnetism without the use of too much maths.

Got a book recommended to me in the physics forum but it's too math based for me.

A broad understanding is what I'm after.

mickmackey1

ps200306 wrote: »

Their current recession speed is even faster than the speed of light. This may be a surprise to people who have heard that the speed of light is some sort of absolute limit. And so it is -- the speed limit at which information can travel between two points -- but there is nothing stopping the expansion of space from carrying galaxies away from us at greater than light speed.

So is there any speed limit on recession? Could galaxies be travelling away from us 'infinitely' fast?

ps200306

mickmackey1 wrote: »

So is there any speed limit on recession? Could galaxies be travelling away from us 'infinitely' fast?

In general no, there is a limit set by cosmological parameters. The recessional velocity is given by:



The 'p' subscripts just refer to the proper velocity and distance, which I mentioned earlier are just the values you'd get if you could instantaneously lay out a giant tape measure. H(t) is the Hubble parameter. It's sometimes erroneously called the Hubble constant, but it is not constant over time. Its current value is the ~70 km/sec per Mpc that I mentioned earlier, but for any time t, H(t) is given by:



Here, R is the so-called scale factor of the universe. You can think of it as a fancy way of referring to the size of the universe at any point in time. However, in order to not have to worry about how it has changed over time, or whether our bit of the universe is embedded in some larger -- possible infinitely large -- structure, we give it a relative value. Think of our elastic band example -- we ignore how much stretching has gone on in the past, or where the ends of the elastic band are right now, by sneakily assigning a value of one to R for the present moment. So right now , by definition. It was smaller in the past and will be bigger in the future.

The infinitesimal fraction just refers to the rate at which the scale factor is changing. However, a lot of complexity is hidden in there as it depends on different solutions to Einstein's equations of General Relativity, and assumptions about the relative importance of matter, energy and dark energy at different epochs. Here are a few possibilities:



You can see that there are some universes that expand and then collapse again, some where the expansion slows down over time but never quite ceases, others where the expansion accelerates over time (perhaps having initially slowed down). These are called Friedmann-Lemaître-Robertson-Walker (FLRW) models, all based on solutions to General Relativity involving a homogeneous isotropic universe (i.e. roughly the same everywhere and in all directions), but with different densities of matter, energy and dark energy denoted by the subscripted (omega) parameters.

So coming back to the original question, could the recession speed of a galaxy be infinite (now or at any other time), if we calculate it from our rearranged formula? :



Well, is infinite if , but that denotes the singularity at the moment of the Big Bang where physics breaks down (apart from which there weren't any galaxies back then). The rate of expansion, , is infinite only in the infinitely far future of the accelerating models. The proper distance, , must be finite in the present or past. So generally speaking the recessional velocity is finite, but the precise value -- especially at large distances -- is model-dependent. For currently preferred models, the furthest galaxies actually observed would have recessional velocities of several times the speed of light, where "several" is some single-digit number. :pac:

ps200306

c montgomery wrote: »

Anybody know of any good books that explain electromagnetism without the use of too much maths.

Got a book recommended to me in the physics forum but it's too math based for me.

A broad understanding is what I'm after.

Maxwell's equations are enumerated in non-mathematical form in a section on History of the Theory of Electromagnetism on Wikipedia. Unfortunately it's incredibly difficult to get from a bald statement of Maxwell's equations to the amazing variety of electromagnetic phenomena without delving into the maths. Here's a site that claims to present the equations in a simple and intuitive form, but it doesn't look to me like it succeeds: all it does is present the equations and the underlying maths in a mixed up, confusing way.

In short, you've got to learn the maths. It's not hugely complicated, about higher level Leaving Cert standard (disclaimer: I didn't do higher level LC maths, I came on it later in life motivated by the same quest for knowledge as you, so I'm not sure). Here's a page that enumerates the mathematical topics involved.

yoppy

Wait a sec, I know the answer to this one, don't you just have to hit different symbols on the DHD thingy?

mickmackey1

ps200306 wrote: »

...

Excellent, thanks for that. I find that the laws governing my capacity for comprehension break down somewhere between the singularity and infinity

starlit

I'm not big into Science but I did not think this would happen yes planets can move and align differently sometimes? Galaxies I didn't think would move? Its like the bright 'star' near the moon I thought was up in the sky lately on a clear night turns out to be a the brightest planet 'Venus' and slightly see 'Mars' at a distance. Where as the other Planets couldn't be seen except during the night, going from night to dawn and then again from dusk/twilight to nigh-time. Think Neptune, Mercury and Saturn are only visible early in the morning. Am I right in saying that?

ps200306

doovdela wrote: »

Galaxies I didn't think would move?

Neither did Einstein, so you're in good company. The problem is that all matter is subject to the attractive force of gravity, so unless you posit some repulsive force to stop things from moving (like Einstein did for a while), and somehow arrange it so that all the clumpy galaxies are in perfect mutual balance like a bunch of pencils balancing on their tips, then things have to move. A static universe is nigh on impossible.

me_right_one

c montgomery wrote: »

Anybody know of any good books that explain electromagnetism without the use of too much maths.

Got a book recommended to me in the physics forum but it's too math based for me.

A broad understanding is what I'm after.

A good start is the "fish in the sea" analogy. PM me if you want to know more:

In order to understand how antenna radiation fields work, we must first be able to grasp how
the electromagnetic (EM) domain works in the physical world. A good analogy is the “fish in the
sea” model; fish are immersed in water, which has waves passing through it constantly. The fish
can’t see them, they can’t hear them, they can’t smell them - but yet these waves are there,
perpetually radiating and reflecting away from their sources, in all directions, at all sorts of
frequencies. The only way the fish would know the waves are there is if they surfaced to the top of
the water, and actually saw waves rolling past. We humans are in a similar situation when it comes
to EM fields; we are immersed in a global sized EM field. During the day, we can “see” EM waves
in the form of light from the sun, but we cannot ever see non-visible Radio Frequency (RF)
waves. And yet we are surrounded by them. Some RF waves radiate to us naturally from objects in
space, and some are manmade being caused by installations such as electrical power lines and
mobile phone base stations.

mickmackey1

doovdela wrote: »

I'm not big into Science but I did not think this would happen yes planets can move and align differently sometimes? Galaxies I didn't think would move? Its like the bright 'star' near the moon I thought was up in the sky lately on a clear night turns out to be a the brightest planet 'Venus' and slightly see 'Mars' at a distance. Where as the other Planets couldn't be seen except during the night, going from night to dawn and then again from dusk/twilight to nigh-time. Think Neptune, Mercury and Saturn are only visible early in the morning. Am I right in saying that?

They both move but planets are waaaay closer so only their movements are easily detectable. The planets can be seen at any time of night depending on where they and the Earth happen to be at a given time. Neptune passed excedingly close to Mars on New Year's Day e.g. but you'd need binoculars to have seen it.