Science Questions

Are some galaxies moving so fast as to be invisible?

Tue, 24th Apr 2012

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Claus Schertel asked:

Hi guys,


Thanks for a great show, I just came across the show a couple of months ago and listen every time since.


Since the expansion started in one point with the big bang there must be galaxies that fly away from us in the exact opposite direction of our own galaxy. If these galaxies and our own accelerate there should be cases where the speed relative to each other in the opposite directions is so large that the light can´t reach us anymore (e.g. if both fly away from the big bang center with half the speed of light in opposite direction). If we catch a galaxy that just accelerates and crosses the threshold it should go dark. Are there any cases were such an effect was observed, or are they still too slow, to cause such an effect?


Thanks and keep up the good work.




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Nice question but so hard to answer. It's the inflation that's supposed to make galaxies move away from each other. That the 'space' between them is 'stretching out', getting 'bigger'. So it isn't the 'relative motion' between two galaxies that is expected to raise, if you think of it in form of a 'momentum' gained through that relative motion. There should be no more 'momentum' than without that 'expansion' of space. Eh, and as 'space' actually accelerates, expanding between galaxies, we also might assume more 'gravity' as a result if that was a 'real' acceleration (as an acceleration is the equivalence to a gravity) but I never heard of any such observations?

As for two galaxies moving in opposite directions, each one at half the speed of light? In a relativistic calculation I think that will come out as around 0.8 'c'? As any of those galaxies, for example A looks at B, assuming himself to be 'at rest' without any 'uniform motion'.

As for the question, that would be something huh :)
A whole galaxy going 'dark', but then we would just see the tiniest weakest pinprick of light 'go out' and how should we know why, or what it was we saw? It should be constantly more redshifted too before btw.

Maybe there is some way? yor_on, Thu, 15th Mar 2012

Einstein's laws of relativity state that light moves at a set speed relative to the observer and independant of the speed of the object that emits it. Consider 2 spaceships both travelling at c/2 away from eachother. One shines a torch at the other from the rear window - would it be seen? Since the light from that torch moves at the speed of light as soon as the photon is emitted, even though the torch is moving away at c/2. The light, travelling at c will catch up with the other spaceship moving the opposite direction. Sprool, Fri, 16th Mar 2012

Yep, and the reason is that 'c' is a constant in relativity. Nothing can 'move faster' than 'c' if you want relativity to survive. Tachyons isn't about relativity, they are descriptions of something 'outside' of SpaceTime. So I don't really get people thinking that you can have 'real speeds' inside SpaceTime 'faster' that 'c'. It has to do with the conceptual 'box' we exist in, the 'closed SpaceTime' described in symmetries and constants, as well as the conservation laws. yor_on, Fri, 16th Mar 2012

Try this for size, from an applied physics major at Cornell University:

"A central revelation of electrodynamics which few appreciated until Einstein's time was this: Everyone sees light move at the same speed. To give you a background of history, we knew that there were two quantities, μ and ε, which dealt with magnetism and electricity respectively. (Those are mu and epsilon, the Greek letters for m and e. We physicists were feeling *very* unimaginative at the time. Sorry. I promise that next time we'll use Cyrillic or something.) So, we have these two numbers, μ and ε, and this dude named James Clerk Maxwell goes out and proves that the speed speed v of a light wave in vacuum is given by the relation με = 1/v².

So, v can change, right? We just need either μ or ε to vary, and then the left side changes, and then the right side changes too. Right?

Houston, we have a problem. Actually, two problems.

See, μ is fixed by definition. It can't vary. (If you have trouble imagining how we could do this, think about how an American would respond if we shrunk him to half his height. He would redefine the notion of "foot" and say "dammit, I'm still six feet tall! See my feet right here? That's how long a foot is! And six of them make me up! So I'm still six feet tall, not some pesky three-foot midget!"

You think I'm joking, but what would happen if we shrunk the *whole planet* down with the poor American? ... if we shrunk the whole planet at once, he would probably never realize that he'd been shrunk! For that matter, those who use the metric system wouldn't be able to tell whether they'd been shrunk or the speed of light had doubled; but that's a different story.)

Anyway, so μ is like the "foot" unit -- it's fixed by definition. So, the only thing left is ε, which you measure at lab benches and such. There's a problem with that too: we've never seen ε vary, no matter where we were and what we were doing. And so, if that's all to be believed, then v is fixed -- everyone sees light move at the same speed c. (We have since done *very* careful experiments to confirm this to the highest accuracy possible, and to confirm it much more directly.)

What happens when everyone agrees on the speed of an object?

Well, if we disagree on the distance it travels, then we must also disagree about the time it has traveled. How do we disagree on distance? The classic example is a ball on a train. If I toss the ball up and it falls back into my hand, well, then I think it has moved a short distance, maybe two meters total, if it went one meter into the air. But it will take 1 second to do that. But if I'm on a train that's going, say, at 80 kph or so, someone on the ground just sees the ball shoot mostly-forward, and says that it goes something more like 20 meters, where I think it just went 2.

Now, in normal life, we both disagree about the speed of the ball. But with light, we *can't* disagree on speed -- so we are left to disagree on time. So suppose we *had* to believe that your ball was traveling at 2 m/s all the time. (I know, I know, then the ball wouldn't stop when it reaches its peak. So let's have you throw the ball really hard at the ceiling, so that the change in direction doesn't require a slowdown. To do this with light, you'd just use a mirror.)

If it were a fact that we all agreed on the speed of balls as 2 m/s, then I would clearly be saying that your ball was airborne for ~20 seconds total, because I saw it move forward ~20 meters. We disagree on the travel time, because we can't disagree on the speed. But this means even more. It means that when I look at the train, everything happening within there must look to me like it's happening in slow motion -- because the activities that you'd naturally do in 1 second, it takes 10 seconds for me to see them play out.

Even worse, I can't see the train as moving faster than 2 m/s -- otherwise, it would leave the ball behind! In fact, it's a little slower, because the ball moves up and down as well as side to side. So our agreement on this ball speed starts to set up a speed limit that *other* things travel at. It is surprising that this doesn't actually lead to contradictions somewhere down the line -- because it sure looks like it should.

So let's move onto a rocket in a rocket. At this point it's very simple. You're inside a rocket which I would say is moving near the speed of light, and you set off your own rocket, which you would say is moving near the speed of light. What do *I* think of *your* rocket?

Well, remember how I see your world: Everything on your rocket is moving in slow motion, from my perspective. So, when I look at your rocket, it is *also* moving in slow motion. Which means that I wouldn't just add "0.5 c + 0.5 c," now would I? Because when you're moving at 0.5 c, then your world looks to me like a slow-motion world, and what you would say is 0.5 c is, to me, much less than 0.5 c. "

And then read Problem and a paradox. (from Physicsforum) yor_on, Fri, 16th Mar 2012

But when it comes to why galaxies can 'disappear FTL' it have nothing to do with the constant 'c'. That has to do with 'Space' later expanding (as some slower inflation).

When that space expands the wave, that you can describe 'radiation/light' as, is getting 'stretched'. As some rope that you arranged in waves gets stretched by you and your pal drawing it out from both ends. So where the 'observable universe', from our point of view, ends that light/radiation will be quenched, and the rope will be as good as 'straight'. Space is a special property, classically consisting of 'nothing at all'. yor_on, Fri, 16th Mar 2012

Excuse me please but I have a misconception about the big bang and I think I now understand what is meant by the expanding Universe.  I had always assumed that the big bang was like a massive explosion throwing particles out from an infinitely small point, a bit like looking at an explosion in slow motion where you can see the shock wave expanding outwards. 

Would I be correct in stating that in fact the current understanding is that space itself is expanding and that is what causes the Galaxies to be generally flying  apart from each other?  Would it not be both? 

Sounds less like a bang and more like someone blowing a balloon up.  Maybe it should be called the Big Balloon theory? Aaron_Thomas, Fri, 16th Mar 2012

Yeah, been thinking the same AT :)

If you imagine it as an awful lot of balloons maybe? With only their outer skins being a representation of a 2-D description of an expansion. But you can't imagine it as balloons inside balloons inside as every 'point/galaxy' on that skin gets its own expanding 'shell of space' around it.

It's a four dimensional balloon that one, involving all dimensions we have, and it's easy to see why Einstein, long into his late years, in vain, searched for that 'fifth dimension' that would join the four  we see into one 'simple description' of reality. By simple I mean strictly mathematically of course, I doubt it would have made it easier to imagine intuitively though :) yor_on, Sat, 17th Mar 2012

Could the fifth dimension be the source of the inflation?  An analogy would be the source of air for the balloon. 

In relation to the original question about Galaxies moving so fast as to be invisible...... What we can see at the observable edge of our Universe is light so old that we are very out of date as to what is actually there now!  What we can see if 15 Billion light years away.  Right now there must be Galaxies that will always be invisible to us simply because you would have to wait another 15 Billion years to see what is there today! 

Also at a certain point in our Universe according to current theory there will be a place similar to that of an event horizon where the expansion of the space time exceeds the speed of light.  Whatever is beyond that Universal light horizon is in relation to us moving away faster then the speed of light so we will never see it.

Now that we have all this massive amount of data we can search for these red shifted Galaxies and if we are really lucky we might see one go out..... :P

Aaron_Thomas, Sat, 17th Mar 2012

To get back to the original question. 

Claus  You must not forget that all this expansion causes a red shift in the light of the receding galaxy.  After all this is the way that this effect was discovered.  The light from the galaxies does become invisible to our eyes quite quickly but it has just shifted into the infra red region of the spectrum.  This is why all the really big telescopes are working more at infra red than visible light.

This red shift can continue right down to radio frequencies.  The Cosmic microwave background radiation was once when it was emitted visible light very similar to sunlight at around 6000K but now it is microwave radiation at around 2-3K  This red shift stretches and weakens the radiation so as they went over the edge they don't so much a wink out they fade out into the background radiation. It cannot be something that happens suddenly. Soul Surfer, Sat, 17th Mar 2012

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