Seeing into the past?

[paranoid marvin]

I've been thinking about this. We see stars in the sky from how they were thousands/millions of years ago, because that's how long it takes for light to travel that far.

So if I was on a planet a sufficient distance away, I would be seeing Earth how it USED to look. And if I had a powerful enough telescope I would be able to see the surface of the Earth and what was going on?

So does that mean theoretically if I found a way to travel faster than light, or if I found a wormhole/blackhole that transported me to a different part of the galaxy/solar system. I could go to a planet and view Earth from the past (because light hadn't had the chance to travel that far yet)? And with a really REALLY powerful telescope I might not be able to hear what people were saying, but I could read their lips?

 

[Elckerlyc]

The problem would the really, REALLY powerful telescope that you would need to bring along.
It would furthermore depend on the position of the sun; a position more or less - but not exactly! - between the Earth and your point of view. Unless your telescope has also tremendous infrared capabilities.

 

[Brian G Turner]

So does that mean theoretically if I found a way to travel faster than light, or if I found a wormhole/blackhole that transported me to a different part of the galaxy/solar system. I could go to a planet and view Earth from the past (because light hadn't had the chance to travel that far yet)? And with a really REALLY powerful telescope I might not be able to hear what people were saying, but I could read their lips?

Kind of, yes, which is part of the paradox - because if you could do that, you'd be able to exchange information before you were supposed to know it.

 

[Ori Vandewalle]

You could, but that would be the long way around. If you can travel faster than light, you already have a time machine.

 

[paranoid marvin]

The problem would the really, REALLY powerful telescope that you would need to bring along.
It would furthermore depend on the position of the sun; a position more or less - but not exactly! - between the Earth and your point of view. Unless your telescope has also tremendous infrared capabilities.

But then it simply becomes a case of waiting for the right technological advances. So sooner or later it becomes a probability rather than a possibility. And considering it can take millions of years for light to travel to distant star systems, we have plenty of time to be able to look back into the Earth's past.

 

[.matthew.]

Do you think that's what Musk is doing with all those satellites? Going to spell out a message future people can see from reeaaaallllly far away?

 

[Boneman]

It always amazes me to think that some of the stars we see may not actually exist, it's the light from their past we're seeing, taking light-years to reach us.

 

[Ori Vandewalle]

For stars you can see with the naked eye, it's almost certainly the case they're all still alive. We can't see anything more than about 10,000 light years away, and stars live for millions to billions of years.

 

[Venusian Broon]

Ignoring the likelihood that the light will very likely be absorbed and readily scattered by material between you and the Earth (Given the shear insignificant amount of light the Earth reflects or produces, I'm afraid most of the past light will have been absorbed or scattered into randomness by the surrounding universe - i.e. starlight, dust and gas clouds etc. - by now)

But ignoring that. By a very rough calculation to get the required resolution of say 1mm at only one light year distance, so one year back, which hopefully is enough to see lips move well enough so that you could read them, I think you'd need a 'telescope/interferometer' with a base line aperture of at least 4.7 billion km*. Trying to "go back further in time" requires your telescope to become even more stupendously huge as you go deeper into space.

That may be possible for a spacefaring race...but in order to see tiny details you'd probably need to collect virtually all the light produced. Which makes the construction of this machine even more daunting and even more impossible.

However even Very Long Baseline Interferometers are limited in the resolution that they can achieve. This is nothing that to do with technological advances but hard physics, I'm afraid. Purely by diffraction as the light travels, the miniscule amount of light that deliniates the tiny details in an image such as lips moving will easily be 'smeared out' into a blur quite quickly - at least on large to interstellar scales.

And yes as Ori states, faster than light travel is, as far as our understanding of the current physics of the universe. equivalent to going back in time. If you could do that...then our current view of the universe is wrong. You could probably also poop unicorns and have a disco with a gaggle of angels on the back of pin if that was the case, as anything could be possible
--------------------------------------------------------------------
* back of fag packet calculation, bound to be wrong, yadda yadda yadda etc.

 

[-K2-]

Do you think that's what Musk is doing with all those satellites? Going to spell out a message future people can see from reeaaaallllly far away?

He's no Adrian Veidt on Europa...

K2

 

[Ray Zdybrow]

But then it simply becomes a case of waiting for the right technological advances.

Technological advances like... the FTL drive?

 

[JohnM]

The FTL Drive is required to get man to other planets. Old radio and television broadcasts leave earth at the speed of light. With an FTL Drive you could catch the signals and watch them again.

 

[Danny McG]

I've been thinking about this. We see stars in the sky from how they were thousands/millions of years ago, because that's how long it takes for light to travel that far.

So if I was on a planet a sufficient distance away, I would be seeing Earth how it USED to look. And if I had a powerful enough telescope I would be able to see the surface of the Earth and what was going on?

So does that mean theoretically if I found a way to travel faster than light, or if I found a wormhole/blackhole that transported me to a different part of the galaxy/solar system. I could go to a planet and view Earth from the past (because light hadn't had the chance to travel that far yet)? And with a really REALLY powerful telescope I might not be able to hear what people were saying, but I could read their lips?

This sounds like the start of Pandora's Star!

 

[Astro Pen]

As we go back in time that way we eventually reach the big bang singularity in a spherical shell, the size of the universe, around us.
Could someone explain that apparent paradox?

 

[Ori Vandewalle]

The furthest back we can see is the cosmic microwave background, which is what the universe looked like about 400,000 years after the big bang. At that point, the universe was roughly a thousandth its current size--definitely not a singularity. But also, the standard model of cosmology doesn't really require there to have ever been a singularity per se, and there's no observational evidence there was.

 

[Ori Vandewalle]

Also, the CMB isn't a spherical shell surrounding us; it's just that astronomical observation only ever produces cross-sections of the universe at particular moments in time. When we look at the CMB, we're seeing a particular slice of the universe from 400,000 years ago, but the whole universe was in that state.

 

[Astro Pen]

I think you are talking about the ocean, I'm talking about the coast. They aren't the same thing. The CMB doesn't come into the argument. It's the air, not the balloon.

Nor does the fact that we can't literally see, due to redshift etc', what is over the horizon eliminate it from the model, only from the telescopic view.

We have GN-z11 galaxy at 13.4 billion years ago. Just keep looking back along the radial line beyond that, in the model if not telescopically, you will get to the earliest star then beyond that the hydrogen/helium clumping pre ingition (which will never be visible but we know it happened), and ultimately on to 'the beginning'. The 'zero Hz' image which can only ever be modeled.

 

[Ori Vandewalle]

The big bang happened everywhere, so we see the evidence of it everywhere. I'm not sure what you see as a paradox.

 

[Joshua Jones]

I remember a whole thread about FTL paradoxes and the like from a few years back that covered some related topics (although the main thrust of the discussion there was if you could send an FTL message to warn someone about something which was going to happen), but there is some interesting discussion and an article that I attempted to refute regarding temporal paradoxes involved with FTL, which I still hold to. It also devolved into silliness to try to ease some tension (@Brian G Turner, you may remember Dave, Cathbad, Baylor, and I talking about angels and faeries standing on pinheads during a physics discussion because one guy was taking all this WAY too seriously...good times!), but there is some quite interesting discussion there which may be relevant here.

FTL without paradoxes?

I have a story I'd like to tell which requires both relativistic space travel and some form of FTL travel, but I want to avoid the possibility of someone creating a paradox, and want to minimise the number of arbitrary restrictions I need to employ. Wormholes are the obvious answer, but I'd...

www.sffchronicles.com

 

[Ori Vandewalle]

@Joshua Jones, if you want to continue that discussion (with a new interlocutor), there's a point (or maybe more) I'd like to add to it.

For the sake of discussion, let's say it was a supernova. There is an actual internal sequence happening in the star causing the supernova. Each of the observers may see it differently, but that doesn't change the internal sequence in actuality. Yes, each of the observers have an equally valid (or invalid) perspective, but a failure to identify which perspective is more accurate does not mean that there isn't an actual physical process happening which is the basis of these perceptions.

There's a distinction between what different observers see and what different observers calculate. The order in which you see some series of events is based on when your worldline encounters the light cones from those events. This just comes out of, basically, how close you are to each event and is not a consequence of relativity after all. Like, we will see the light from a nearby supernova before we see the light from a distant supernova just based on light travel times irrespective of the order of events.

But what you calculate comes out of the Lorentz transformation, the mathematical procedure by which you change from one inertial reference frame to another moving at a different speed. This transformation lets you take the spacetime coordinates of some event in one frame (over there, at that time) and translate them to spacetime coordinates in another frame (over here, at this time). When you perform this procedure on two events that are spacelike separated--that is, two events that are outside each other's past light cones and cannot causally influence each other--what you find is that the order of these events is entirely dependent on your velocity (through the Lorentz transformation). So there is no objective fact of the matter about which comes first.

Because this only happens with causally separated events, there is no concern about an "actual" internal sequence leading from one event to the next. This problem doesn't come up for, like, two atoms smashing together and fusing in a red giant, because everything is close together and your past light cones are all overlapping. The trouble with FTL is you can't rely on the light cone to tell you two events are causally separate, but it's still the case that the order is dependent on your frame and the Lorentz transformation. So FTL travel can artificially connect two events causally, but different observers will disagree about which event comes first.