# Seeing into the past?



## paranoid marvin (Jun 29, 2020)

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?


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## Elckerlyc (Jun 29, 2020)

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.


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## Brian G Turner (Jun 29, 2020)

paranoid marvin said:


> 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.


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## Ori Vandewalle (Jun 29, 2020)

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


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## paranoid marvin (Jun 29, 2020)

Elckerlyc said:


> 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.


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## .matthew. (Jun 29, 2020)

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?


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## Boneman (Jun 29, 2020)

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.


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## Ori Vandewalle (Jun 29, 2020)

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.


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## Venusian Broon (Jun 29, 2020)

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  
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* back of fag packet calculation, bound to be wrong, yadda yadda yadda etc.


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## -K2- (Jun 29, 2020)

.matthew. said:


> 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


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## Ray Zdybrow (Jul 9, 2020)

paranoid marvin said:


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


Technological advances like... the FTL drive?


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## JohnM (Jul 29, 2020)

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.


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## Danny McG (Jul 29, 2020)

paranoid marvin said:


> 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!


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## Astro Pen (Jul 30, 2020)

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?


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## Ori Vandewalle (Jul 30, 2020)

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.


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## Ori Vandewalle (Jul 30, 2020)

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.


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## Astro Pen (Jul 30, 2020)

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.


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## Ori Vandewalle (Jul 30, 2020)

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


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## Joshua Jones (Jul 30, 2020)

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. 





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						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


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## Ori Vandewalle (Aug 3, 2020)

@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.



Joshua Jones said:


> 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.


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## Joshua Jones (Aug 3, 2020)

Ori Vandewalle said:


> @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.
> 
> 
> 
> ...


Thanks for this! I'm rather out of tea at the moment, so I cannot guarantee that my thoughts here will be either coherent or addressing the nuance of the issue, but I'm going to give it a shot either way...

That makes sense regarding the epistemological ordering of events, but the epistemology and ontology are two distinct matters. More precisely, suppose two stars, 2 light years apart, have supernova reactions 1 year apart from one another. An observer on the side of one which detonated first will observe the first one detonate, then 3 years later, will observe the other. An observer directly and equidistantly opposite will observe the closer one detonate first, followed by the further one a year later. This seems to be the point you are making in paragraph 1. However, neither observation alters the actual ordering of the phenomenon behind the observations.

What I fail to see is how velocity changes this equation apart from altering perception. It seems to me, so long as matter objectively exists, and objectively interacts with other bits of matter, sequencing must exist apart from the order of observation, even when discussing two non-interacting bits of matter. And, if this is true, I fail to see how FTL would permit information to travel backward in time.

Perhaps an example will clarify my point. A 2x FTL spaceship, say, the _Gladius_, departs from Spaceport _Libertas _toward an uninhabited supergiant star 10 light years away. As they hit 5 ly distant, they observe that the star has exploded, rapidly spin and return to the _Libertas _(my argument should hold if they use a 2x FTL communication instead here). The _Gladius _(or data from the _Gladius_) would return 2.5 years before the light from the supernova explosion to inform them of the explosion. It would not, however, be 2.5 years before the explosion itself; it would be 7.5 years subsequent. And, should the Gladius depart again toward the remains of the star, it will re-encounter the observation of the explosion at 1.75 years. However, should it continue on toward the star, it will encounter the remains of the star 10 (plus whatever time is required to report the information, refuel, etc.) years subsequent. So, yes, FTL would enable the communication of information not presently observable, but it would not permit the communication of information before the event occurs which created the information.

I'd go further and suggest that, from the perspective of the the _Gladius_, the observations aft of the direction of travel would appear to be going backward, while the observations fore of the direction of travel would appear to be rapidly advancing. This is due to the _Gladius_ leaving behind the light behind it, and catching up to light in front of it. So, hypothetically, the _Gladius_ could stop and watch it's own departure, but should it return, it would see it's trip in fast forward up to the point it passes itself in the light cone.

It should also be noted that time would likely be experienced much more slowly progressing by the crew of the _Gladius_, but this would only increase the rapidity of the fast forward/reverse phenomena described above. 

However, the crux of this argument is the premise that perception does not equal reality, in spite of it being the only means for measuring reality. Hence, my position that if matter objectively exists and objectively interacts (i.e. in the absence of observers or in spite of them), the communication of information into the past is impossible, even with FTL technology.

Is there anything I'm missing here?


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## Ori Vandewalle (Aug 3, 2020)

Joshua Jones said:


> What I fail to see is how velocity changes this equation apart from altering perception. It seems to me, so long as matter objectively exists, and objectively interacts with other bits of matter, sequencing must exist apart from the order of observation, even when discussing two non-interacting bits of matter.



Well, as a rule there is no arrow of time baked into the laws of physics; physical processes are in general reversible. It just so happens that some processes are much more likely than others ("how likely" being a statement about the entropy of a system), so there are plenty of processes we don't end up seeing. But in principle, if a star dies and explodes as a supernova, every interaction in that chain of events could have proceeded the other direction, such that a supernova remnant collapsed into a dying star. It's just, well, even saying "astronomically unlikely" is an understatement there. So there's nothing about a particular sequence of physical events that matters in terms of causality. There's only constant conjunction. 



> A 2x FTL spaceship, say, the _Gladius_, departs from Spaceport _Libertas _toward an uninhabited supergiant star 10 light years away. As they hit 5 ly distant, they observe that the star has exploded...



Ah, but _what_ is the _Gladius_ moving as 2c relative to? That's the key question in relativity. Special relativity isn't well equipped to tell us the direct, relativistic consequences of traveling faster than light (the equations spit out imaginary numbers for observable quantities). Time travel only comes into it when you consider events an FTL thing can be present at from two different (slower than light) inertial reference frames.

As far as how your example leads to time travel, well... like I said, the ordering of events only ever gets out of whack when you consider events that are out of causal contact. Here, you're talking about a supernova, then the light form that supernova interacting with the _Gladius_, and then presumably the _Gladius _wanting to prevent that light from reaching _Libertas_. All of these are events connected by light, which means they are in causal contact. So different observers may disagree about the exact times at which events occur (due to time dilation), but they won't disagree about the order.

Funny business comes in only when you consider spacelike separated events. Because _Gladius _doesn't see the supernova until it's halfway to the star, the event of its leaving the spaceport is causally disconnected from the star exploding. On the other hand, the event of _Gladius_ seeing the supernova is obviously connected to the supernova (by light), but has spacelike separation (because it's moving faster than light) from its own departure from the port.

That means there is some reference frame in which the _Gladius_ is halfway between port and star (witnessing the supernova) before it leaves the port (specifically, some other ship on a parallel trajectory moving close to the speed of light--I can draw very terrible spacetime diagrams to illustrate if that would help). If it gets where it's going before it starts going at all, we can cook up some paradoxical scenarios. I'll note that because we have no idea what FTL travel really means physically, it's possible said scenarios may be prohibited by some as yet unknown physical laws. But a naive depiction of FTL allows for the possibility of paradox.

(Again, this is totally unrelated to when that other ship _sees_ these events; that just depends on where it is in its journey when all this is happening, and it could very well see _Gladius_ leave the port before it reaches the supernova blast. The ship isn't really necessary, only its valid inertial reference frame.)


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## Joshua Jones (Aug 4, 2020)

Ori Vandewalle said:


> Well, as a rule there is no arrow of time baked into the laws of physics; physical processes are in general reversible. It just so happens that some processes are much more likely than others ("how likely" being a statement about the entropy of a system), so there are plenty of processes we don't end up seeing. But in principle, if a star dies and explodes as a supernova, every interaction in that chain of events could have proceeded the other direction, such that a supernova remnant collapsed into a dying star. It's just, well, even saying "astronomically unlikely" is an understatement there. So there's nothing about a particular sequence of physical events that matters in terms of causality. There's only constant conjunction.
> 
> 
> 
> ...


Very interesting. I'll be honest; I spent the day writing budget ordinances and my brain is mostly fried right now, so I'm going to chew on this a bit tomorrow and see if it sticks with me. Right now, I'm at a point where everything is running together... 

Either way, thanks for posting this! I greatly appreciate the time you're taking to engage me in conversation on this topic!


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