Suppose we have 2 spacecraft and one of them (B) is following the other (A) towards a region of greater spacetime curvature.

In A ,which has its own artificial gravity an elctrician(E) climbs a ladder to a fuse box loses his or her balance and falls to the ground .

A backup electician (F) repeats the procedure and again falls to the floor.

B observes that F takes longer to follow the same trajectory than E .

Is this because B actually observes this via a light(or other) signal or because B calculates it from a GR model?

Is this a distinction with no difference or does it have any significance?

(Hope my setup had no holes in it)

Also are the time measured to fall down the ladder and the spatial distance involved the two factors in the spacetime interval in these cases?

47 minutes ago, geordief said:

Is this because B actually observes this via a light(or other) signal or because B calculates it from a GR model?

Both ...
Which makes GR a very accurate model for applications of this type.

56 minutes ago, MigL said:

Both ...
Which makes GR a very accurate model for applications of this type.

Yes ,that answer had also seemed likely to me.

Are there any circumstances where direct observation is not possible ?(aside from the seemingly obvious before T+10'^-43secs)

Philosophically (I hope) is all knowledge related somehow to observation ?(if only of one's own spinning wheels) and you can't have an observation that doesn't imply (fleeting) knowledge?

ps was I right about the spacetime interval applying to the up and down electricians?

On 7/2/2025 at 6:13 PM, geordief said:

Are there any circumstances where direct observation is not possible ?(aside from the seemingly obvious before T+10'^-43secs)

Astronomical observations are limited by the speed of light and the expansion of the universe.

Even travelling at the speed of light, some objects are now travelling much faster than that relative to us so the light they are producing now will never reach us.

For galaxies that is currently about 280 million years after the BB but let's see what the JWST can do in the next couple of years. Gravitational lensing may give it a leg up and push it back further.

The CMBR is from about 380,000 years after the BB and there are also BAO.

That doesn't make sense.
The CMBR represents the limit to which we can see into the past, as it is when atoms were able to combine and make the universe transparent from its previous plasma state.
If that is at 380 million years after the BB event, there is no way to see further to 280 million years after BB.

The observable universe is usually given as a radius or diameter, beyond which there is no causal connection to us on earth, so we can never know any information about it. Due to various rates of expansion throughout its history , the universe has had various horizons, where distant galaxies, now visible, will red-shift and become invisible at times in the future due to accelerating expansion.

If we assume that prior to inflation the universe was in causal contact, to guarantee homogeneity and isotropy, such that its size was approximately equal to the speed of light times its age, then inflation, as per A Guth, would have started at 10^-37 sec after the BB event and inflated the universe such that, at present, the entire universe has a size of at least 1.5x10³⁴ light years, which is 3x10²³ times the radius of the observable universe.
These assumptions, and A Guth's theory, place a lower limit on the size of the entire universe, for the finite but unbounded case, although it could also be infinite.

56 minutes ago, MigL said:

The CMBR represents the limit to which we can see into the past, as it is when atoms were able to combine and make the universe transparent from its previous plasma state.
If that is at 380 million years after the BB event, there is no way to see further to 280 million years after BB.

That's not quite true. The CMBR represents the time the universe became transparent to electromagnetic radiation. However, it is proposed that before then, there was a time when the universe became transparent to neutrinos, and therefore the cosmic neutrino background may be something before the CMBR that could in principle be studied.

59 minutes ago, MigL said:

If that is at 380 million years after the BB event

Thousand.

The CMBR event was all over the entire universe in all directions.

23 minutes ago, KJW said:

That's not quite true. The CMBR represents the time the universe became transparent to electromagnetic radiation. However, it is proposed that before then, there was a time when the universe became transparent to neutrinos, and therefore the cosmic neutrino background may be something before the CMBR that could in principle be studied.

Yes, I think some ambiguity between CMBR and the surface of last scattering is slipping in here, probably. With gravitational waves we can even see further out, because everything is transparent to those.

So one thing is the background of whatever species are out there, and a different thing is the farthest out we can se those things...

OOPS !
Sorry reading comprehension issues.

8 hours ago, pinball1970 said:

For galaxies that is currently about 280 million years after the BB

I had thought you posted we could see galaxies farther back in time than the (origin of ) CMBR, and you had written both in millions of years ( you hadn't ).

But observing further back, whether using neutrino emissions or gravitational wave detection, will not be possible for a while, and most certainly will nor observe galaxies.

6 hours ago, MigL said:

at present, the entire universe has a size of at least 1.5x10³⁴ light years, which is 3x10²³ times the radius of the observable universe

That would fairly put us in our place!

16 hours ago, geordief said:

That would fairly put us in our place!

Even more so if the Universe is infinite.

18 hours ago, MigL said:

Sorry reading comprehension issues.

No problem, to a non physicist (me) this is all pretty confusing. I thought I had a handle on the CMBR but having read several wiki articles this morning it is clear that I don't.

Probably more of an idea now at least

22 hours ago, joigus said:

Yes, I think some ambiguity between CMBR and the surface of last scattering is slipping in here,

I thought the photons from last scattering was the CMBR we see/detect today?

1 hour ago, pinball1970 said:

I thought the photons from last scattering was the CMBR we see/detect today?

Both concepts are related. But the CMBR is a bulk, while the surface of last scattering corresponds to the last photons of that CMBR we can see (and therefore the oldest and hottest at the cosmic time we see them) and are about to get lost forever to our sighting.

6 hours ago, joigus said:

Both concepts are related. But the CMBR is a bulk, while the surface of last scattering corresponds to the last photons of that CMBR we can see (and therefore the oldest and hottest at the cosmic time we see them) and are about to get lost forever to our sighting.

Thanks. +1. Hand on heart I have a low level on this. I will keep reading and maybe start a thread on the parts I do not understand. (Long thread!)