[Chris Caley]

Chris Caley  asked the Naked Scientists:
   
Hi Dr Chris,  

I'm in Sydney, Australia and have been a fan of newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] coming up to 4 years now, and I've been meaning to ask this question for a while.

When I read Stephen Hawking's "A Brief History of Time", I wondered if the concept of time dilation and the twins paradox could be applied at a more macro scale?

If one twin can zoom off, at near the speed of light, watch an episode of the Simpsons and come back to Earth, only to meet up with his brother,
who had aged 20 years, does that idea then have consequences for the relative age(s) of the universe?

If the universe seems 14 billion years old to us, and is expanding
at or near the speed of light, then could there be parts of the universe, let's say galaxies for example, that are moving at or near the speed of light relative to us and are only actually 5 billion years old?   Or maybe galaxies that are actually 50 billion years old?

So could there be parts of the universe that have basically been around a lot longer, and could theoretically have had a lot longer to evolve or develop than our own little corner, tucked away here in the Milky Way?

Anyway, I thought it was an interesting idea, even if the universe all started at the same 'time', given time doesn't seem like much of a constant, then perhaps bits of it are older than other bits at any given instant!  

I'm sure you get a lot of questions but I'll keep an ear out!

Thanks, and all the best!

Chris Caley

What do you think?

[Vern]

This is a good question. We know how time is related to relative movement. We would sense that the time experience of fast moving objects at the edge of the viewable universe is much slower than our own experience of time. We attempt to use this time dilation as a test to reinforce our suspicion that the universe is expanding. So far, observations of 1A supernova and other phenomena seem to verify this.

But we are not yet 100% certain that the expansion of the universe is real; it is simply our best working hypothesis at present. For example, the distance between hydrogen clouds and the distance between galaxies in the early universe seem to be the same as the distance we see now. If the universe has been expanding for the last 14 billion years or so, we would expect to see them closer together in the past than they are today.



This study is called the Lyman alpha forest.

In astronomical spectroscopy, the Lyman alpha forest is the sum of absorption lines arising from the Lyman alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars.

These absorption lines result from intergalactic gas through which the galaxy or quasar's light has travelled. Since the absorption and emission of light follow the laws of quantum mechanics, only photons with specific energies can be absorbed. This causes each individual absorption line. The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that depends on the distance between us and the source of light.

[LeeE]

There is no frame of absolute reference within our universe; everything that exists does so within it's own frame of reference, which can only be relative to other frames of reference.  Because of this it would seem that every measurement of time must be relative.

However, a theoretical frame of absolute reference can be described and is actually used in a lot of theoretical physics, the most common being the 'distant' observer often referred to in relativity.  The ideal distant observer is one who is stationary and infinitely far away, and most importantly, outside any gravitational influence.  For this stationary and infinitely distant observer, who would have never experienced any degree of time-dilation, time would always appear to have run at it's maximum rate (because the rate of time can only be slowed down but not sped up) so it's frame of reference could be considered to be absolute.

Everything within our universe is under the influence of gravity though, so everything that exists within our universe experiences some degree of time-dilation and therefore only relative measurements can be made.

Even so though, this does not stop us from deriving the theoretical measurement that would be seen by the distant observer and comparing that with what we actually measure.

For example, if an observer experiencing a high degree of time dilation watches a clock in a distant frame of reference and sees it running fast they'll know that they must be running slow in comparison because they know that the distant clock cannot really be running faster than the maximum experienced by the theoretical infinitely distant observer, and that in practice every clock in the universe must be running slower than that, even though it may not seem to be doing so.

The stars and galaxies of the universe have the equivalent of clocks running in them.  There are things that happen within stars or galaxies that do so at a known rate, so if we see one of these things occurring faster or slower than it should be, and it seems to be due to time dilation, we can work out the relative degrees of time dilation and then relate that to what the theoretical observer, running in their effective absolute frame of reference would see, giving us an absolute answer, albeit one we could never actually see ourselves.

[Vern]

However, a theoretical frame of absolute reference can be described and is actually used in a lot of theoretical physics, the most common being the 'distant' observer often referred to in relativity.  The ideal distant observer is one who is stationary and infinitely far away, and most importantly, outside any gravitational influence.

I like this; I like the Lorentz treatment of relativity phenomena; so I can easily visualize a special frame of reference that is at rest in space. I know it makes things a bit more complicated and introduces an extra computation to relativity transformations; and is really not necessary, since the answer is the same by either method.

But there is a profound fundamental difference in relativity according to Einstein and relativity according to Lorentz. For Einstein, space-time is distorted by motion of material objects. For Lorentz material objects are distorted by motion in space and time.

I think it will take us about another hundred years for us to realize it, but Lorentz had it nailed. Einstein led us on a goose chase. The frame of reference of the Cosmic Background Radiation is that special frame that is at rest in the universe.

[yor_on]

Chris Caley  asked the Naked Scientists:
   
Hi Dr Chris, 

I'm in Sydney, Australia and have been a fan of the show coming up to 4 years now, and I've been meaning to ask this question for a while.

When I read Stephen Hawking's "A Brief History of Time", I wondered if the concept of time dilation and the twins paradox could be applied at a more macro scale?

If one twin can zoom off, at near the speed of light, watch an episode of the Simpsons and come back to Earth, only to meet up with his brother,
who had aged 20 years, does that idea then have consequences for the relative age(s) of the universe?

If the universe seems 14 billion years old to us, and is expanding
at or near the speed of light, then could there be parts of the universe, let's say galaxies for example, that are moving at or near the speed of light relative to us and are only actually 5 billion years old?   Or maybe galaxies that are actually 50 billion years old?

So could there be parts of the universe that have basically been around a lot longer, and could theoretically have had a lot longer to evolve or develop than our own little corner, tucked away here in the Milky Way?

Anyway, I thought it was an interesting idea, even if the universe all started at the same 'time', given time doesn't seem like much of a constant, then perhaps bits of it are older than other bits at any given instant! 

I'm sure you get a lot of questions but I'll keep an ear out!

Thanks, and all the best!

Chris Caley

What do you think?

I think it falls back to frames of reference
One can imagine a galaxy near the speed of light relative us.
That galaxy if traveling in a so called 'uniform motion' aka non-accelerating, will from its own perspective be years younger than us. That it is traveling at a uniform speed guarantees that it will have no 'gravity well' creating noticeable phenomena that would tip its inhabitants off to any 'speeding'.

But if you imagine yourself having a super telescope pointed at that galaxy from that first creation of the universe, and never losing track of it, then you will have witnessed it for around 13.7 billions years, as seen from your frame of reference.

That is, the light from that galaxy have cometh to hit your retina for as long as the universe is supposed to have existed. That is one of the things that gives me a headache. Which btw, would make you rather old of course

[amrit]

universe do not run in time
universe is timeless
time is run of clocks in atemporal space
easy

yours amrit

[Vern]

But if you imagine yourself having a super telescope pointed at that galaxy from that first creation of the universe, and never losing track of it, then you will have witnessed it for around 13.7 billions years, as seen from your frame of reference.

There might be a measuring stick that will be common to both galaxies. The frame of reference of the CMB might be universal everywhere in the universe. If this is so, entities in any galaxy should be able to determine their speed relative to the CMB. Since we can observe both their galaxy and the CMB we should be able to agree with them about their speed relative to the CMB.

[yor_on]

That is a very interesting idea Vern, using the CMB ad a 'measuring stick'. I know that it is 'isotropic' meaning Invariant with respect to direction, but we measure it as a radiation coming at us. Then the question would be if it is our relative motion in a still sea of radiation that causes that 'motion' or if it is the CMB choosing that direction? There is no way I can think of checking it though, as a uniformly moving system doesn't say a thing about any 'isolated' speed. It's only when comparing it to something else we get a 'relative' velocity or spreed, right?

[LeeE]

I don't think the CMBR would make a good measuring stick.  While it is largely isotropic, there are variations in it so it can only offer limited resolution when used as a yardstick for measuring purposes.

The only thing that seems to be 100% invariant is the measured speed of light.

[yor_on]

And as the speed of light always will be 'c' not caring for your relative velocity too or from any other frame of reference then that 'yard stick' also  won't work. I kind of liked Vern's idea, if, it is possible to measure anything against it, nota bene . We would then have something 'approximately' defining all motion from, even though the isotropy may differ somewhat at different places.

[yor_on]

'Stationary observer' relative what LeeE?

How can there be any 'stationary' distant Observer? There is no way to define 'your' motion an long as it is 'uniform' aka non-accelerating. The only 'way' I know of is the one of 'comparison'. Then you arbitrarily choose another 'frame of reference' to 'define' your motion against. If that is the only way defining it I have grave trouble understanding how to define any distant observer as 'stationary'

What I understand us to be able to do though is to be able to 'count' out the 'true' order of any macroscopic events in spacetime, as long as there are two(?) frames of reference + the events frame itself. No matter in what order those two frames sees those events there will be a 'invariant' cause and effect chain.

Which makes eminent sense to me, as there otherwise would seem to exist either time travel or some very strange sort of universes splitting as we witness different 'event chains' looking at the same events in spacetime (macroscopically that is)

[Vern]

I think that the idea of a special frame of reference for the CMB would be compatible with the Big-Bang scenario. In that case it would simply be the frame of the big bang event. It would also be compatible with the notion of Lorentz relativity phenomena. I think it would be compatible with Einstein GR, although maybe meaningless in that case.
 

[LeeE]

'Stationary observer' relative what LeeE?

How can there be any 'stationary' distant Observer? There is no way to define 'your' motion an long as it is 'uniform' aka non-accelerating. The only 'way' I know of is the one of 'comparison'. Then you arbitrarily choose another 'frame of reference' to 'define' your motion against. If that is the only way defining it I have grave trouble understanding how to define any distant observer as 'stationary'

What I understand us to be able to do though is to be able to 'count' out the 'true' order of any macroscopic events in spacetime, as long as there are two(?) frames of reference + the events frame itself. No matter in what order those two frames sees those events there will be a 'invariant' cause and effect chain.

Which makes eminent sense to me, as there otherwise would seem to exist either time travel or some very strange sort of universes splitting as we witness different 'event chains' looking at the same events in spacetime (macroscopically that is)

This hypothetical stationary distant observer can only exist outside our universe.  This is not only so that it can be regarded as stationary but also so that it is outside the influence of gravity.  Any degree of movement it has is therefore not relative to our universe.