Astronomers observe some galaxy

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Discussion Overview

The discussion revolves around the implications of observing distant galaxies and how these observations relate to the concept of time in the context of relativity. Participants explore the effects of light travel time on scientific conclusions about the universe's past and present, as well as the challenges of integrating ancient astronomical data with contemporary scientific theories.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Kamataat questions whether conclusions drawn from observations of distant galaxies apply to those objects as they were in the past, suggesting that the universe may have changed since then.
  • Garth agrees that distant galaxies are observed as they were in the past and emphasizes the need to consider what "now" means in this context, referencing relativity theory.
  • Another participant notes that the scientific value of observing distant galaxies lies in comparing their earlier appearances to more recent ones, likening this to studying fossils for insights into evolution.
  • Garth further elaborates on the concept of "now" in relation to relativity, discussing how observers experience events based on their own world line and the nature of light travel.
  • There is a reiteration of the idea that while photons experience no time during their journey, observers not moving at the speed of light perceive significant time intervals.

Areas of Agreement / Disagreement

Participants generally agree that observations of distant galaxies reflect their past states, but there is no consensus on the implications of this for understanding the present universe or how to reconcile ancient data with current theories. The discussion remains unresolved regarding the interpretation of "now" in the context of relativity.

Contextual Notes

Participants express various assumptions about the nature of time and observation in relativity, and there are unresolved questions about how to integrate observations from different epochs of the universe.

Kamataat
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Hi!

Let's say that astronomers observe some galaxy or something several million lightyears away and then come to conclusions about that object. But since the light from there has been on the way for millions or even billions of years, then is it correct, that they are actually seeing the object as it was back then? So any conclusions they may draw from these observations apply to that object as it was a long time ago and not today?

So my questions is: what effect, if any, does this have on science? For example when they try to fit together data from current lab experiments and observations of ancient evets, i.e. they sky must have changed since then and so any theories we apply to those events only describe those ancient things and not the current universe (in the cosmological sense)?

- Kamataat
 
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Yes indeed we are seeing distant galaxies etc. 'back then', and this has to be taken into account when observing the very furthest galaxies, say on the Hubble Ultra Deep field plate.

However if we ask, "What are they like 'now'?" we have to ask what do we mean by 'now'? Relativity theory measures the interval across space and time between the object itself and the observation of that object to be zero - as far as the photon is concerned it has made that vast journey instantaneously. So the 'back then' that we see is really the ' here and now'!

Garth
 
Dang Garth, you could cause some innocent soul to blow out a brain fuse talking that way :biggrin:. Because of inflation in the early universe and subsequent expansion, we see distant galaxies as they appeared when the universe was younger. The scientific value of this is in the ability to compare the appearance of the universe in earlier times to its appearance in more recent times. This gives us clues how objects and structures emerged and evolved in the universe. Think of it as the cosmic equivalent of fossils. By studying ancient fossils we obtain clues how life emerged and evolved on earth.
 
Garth said:
Yes indeed we are seeing distant galaxies etc. 'back then', and this has to be taken into account when observing the very furthest galaxies, say on the Hubble Ultra Deep field plate.

However if we ask, "What are they like 'now'?" we have to ask what do we mean by 'now'? Relativity theory measures the interval across space and time between the object itself and the observation of that object to be zero - as far as the photon is concerned it has made that vast journey instantaneously. So the 'back then' that we see is really the ' here and now'!

Garth

Only to the photon. The rest of us, not moving at c relative to the galaxies, see it as billions of years. And yes we are not moving at c relative to the galaxies, even the ones for which the distance is increasing faster than c. Space expansion is not speed.
 
Chronos - SelfAdjoint - Of course!

Garth
 
ok, thanks

- Kamataat
 
Garth said:
However if we ask, "What are they like 'now'?" we have to ask what do we mean by 'now'? Relativity theory measures the interval across space and time between the object itself and the observation of that object to be zero - as far as the photon is concerned it has made that vast journey instantaneously. So the 'back then' that we see is really the ' here and now'!
Let me expand a bit.

Use our own world line as a frame of reference, to foliate space-time into a series of time-like slices, then:

If we ask what is here? (With no specification of when) The answer is to look in the immediate vicinity, at all those events whose space coordinates are 'displaced' from ours by only a 'small' amount. But they could be ages in the past or in the distant future, so we do not generally experience them.
Small is arbitrarily small, tending to zero
If we ask what is happening now? (With no specification of where) The answer is to look at all those events whose time coordinate is 'displaced' from ours by a small amount. This generates a surface of simultaneity, as defined by our own fame of reference, but it is actually unobservable and so we cannot in general experience these events.

What we are experiencing is the here and now, which includes those events that are displaced across space and time by only a 'small' amount. However we also observe the rest of the universe along our back light cone, all the way out and back to the surface of last scattering of the CMB. All these observed events are separated from us by zero 'displacement' across space-time and could be said to be 'here and now' for us!

(Using the SR metric)
ds2 = dx2 + dy2 + dz2 - c2dt2 so along the light cone ds2 = 0

We do experience them by seeing them!
Just a thought
Garth
 
Last edited:

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