# Telescopes and Time: Please Explain

I'm a psychotherapist by training so go easy on me here. I would like to know, in simple terms if possible, the basic mechanics of how Hubble can see back in time.

I pretty much understand, in this case, that light has to travel extremely long distances to be captured by a lens. I think the key point I'm missing is how long did, for instance, the light from the deep field shot, take to reach Hubble's lens? Really appreciate any help. And please keep the answer on beginner level please?

• Related (if not dupes of): physics.stackexchange.com/q/150994/25301, physics.stackexchange.com/q/18555/25301, physics.stackexchange.com/q/105980/25301, etc – Kyle Kanos Dec 20 '17 at 12:39
• "I'm missing is how long did, for instance, the light from the deep field shot, take to reach Hubble's lens?" - are you asking how long for the most distant object(s) in the image? The reason I ask is that the light from the more distant galaxies in the image is 'older' than the light from the less distant galaxies. – Alfred Centauri Dec 20 '17 at 15:16
• One possible misunderstanding is that you can't arbitrarily chose to look at any point in time in the past for a given location. For an object 50,000 light years away, you are stuck looking at that object as it was 50,000 years prior to your current time. If you want to look at that object, from Earth, as it was 25,000 years ago, you will need to wait another 25,000 years for the light emitted then to reach Earth. – Michael Richardson Dec 20 '17 at 19:18
• Something you'll probably ask yourself, given the answers below, is how we know the distances to celestial objects. Look up the Cosmic Distance Ladder; the inferences are basically the outcomes of different techniques for different distance ranges, but the techniques overlap in their applicability and agree well where they do, which is why we have confidence in them. – WetSavannaAnimal Dec 20 '17 at 21:25
• One year per light-year. – immibis Dec 21 '17 at 6:58

You seem to already know the answer. You "see back in time" exactly the same way you can "hear back in time" during a thunderstorm...

You know how they tell you to start counting seconds when you see the lightning, stop counting when you hear the thunder, then divide your count by five, and that's how many miles away the storm is?

So the lightning arrives almost instantaneoulsy, while the thunder travels much more slowly. So when you finally hear that thunder, you're hearing what happened in the past. Indeed, five seconds in the past for every mile away the storm is. And that's simply because it takes thunder (i.e., sound) five seconds to travel one mile.

Exactly the same thing for light. One year in the past for every six trillion miles away the star (or whatever you're looking at) is. And that's simply because it takes light one year to travel six trillion miles. (And, by the way, they colorfully name six trillion miles "one light-year").

• And, critically, you don't have to know that lightning struck 5 seconds ago to hear it. If you're shutting your eyes, you'll still hear the thunder and know that there was lightning. In the same way, you can see (for example) a supernova that happened a million years ago just by looking in the right place at the right time. You don't need to know that it happened in order to see it. – bendl Dec 20 '17 at 15:53
• @bendl If it's far enough away, however, you do have to point your massive parabolic sound-mirror in the right direction to pick up the sound. – wizzwizz4 Dec 22 '17 at 10:17

Other answers have detailed exactly what the mechanism is, or have given analogies that demonstrate similar principles, but I wanted to find a better visual representation of what's going on. Unfortunately I wasn't able to find exactly what I'm envisioning, but I got pretty close:

So what are we looking at here? Well, picture the Hubble (or technically any telescope) being on the left, taking a peek at a section of the sky. The whole image that you see through it would be all three of these "planes" combined, it would look like one flat picture. But we know intuitively that not all of them are the same distance from us, so hook that into knowing that light takes time to travel any distance, just like everything else, and you might make the connection that you're looking for.

The light from the closer objects that you're seeing (the left plane in the image) is "newer", in that it took less time to reach you. The light from the farthest objects (the right plane) is "older", having taken a longer time to travel the distance from the source to you. So with that in mind, take another look at the diagram and try to visualize a snapshot of space the same way. Any random view of space works. The light is all arriving at your eye at the same time, but some of it was emitted recently, while some was emitted way further in the past and took a longer time to reach you. Thus you are never seeing things as they are "now" but how they were X number of years ago (X correlating with their distance from you). This is where the term "light-year" comes from. If an object is 1 light-year away, it took the light from it 1 year to reach you. In the diagram that translates to the objects in the left frame being about 0 to 5 billion light-years away, the right frame showing objects that are more than 9 billion light-years away, and the middle being somewhere in between.

That's why telescopes can "see back in time". That phrase is somewhat misleading, because it sort-of implies that they can see things in the present too, which they technically can't. But what it really means is that the farther you can see, the further back in time you are looking. That's part of the reason we're obsessed with building longer-range telescopes, to get a better idea of what the universe looked like further in the past.

Edit: I just remembered an episode of Cosmos (the new version) that deals with this topic, though it delves into the space-time side of things a little deeper. It has a decent explanation of why a telescope can "see back in time" and then goes into explaining the relationship between light and time, and the history of how we discovered it. If you have Netflix you can watch it now, or you might be able to find it through other services. The episode is called "A Sky Full of Ghosts".

• Nice answer but that technicality of telescopes not being able to see the present is pretty cheeky though :). I can use my telescope to see a bird only a few yards away from me. While technically I'm really looking at the bird a few attoseconds(?) in the past, it's basically the "present" astronomically speaking. In the technicality you describe, one is never really observing the present, always the past, except for maybe even eye gunk on the cornea, but even then it's still just a bit away from the retina. – coblr Dec 21 '17 at 18:57
• @coblr in fact your eye is seeing where the bird was around 10ns before that moment. BUT your brain is getting that info some tens of milliseconds later, so yes, everything you can see is "in the past" ;) – frarugi87 Dec 22 '17 at 8:10
• @coblr - light travels ~30 cm per ns. At 10 meters, you're already looking 33 ns into the past... – Floris Dec 22 '17 at 14:22
• Haha you're all correct of course! Time isn't as simple a concept as most people think. But that's not exactly the question here ;) – thanby Dec 22 '17 at 18:19

Simple. It sees back in time in the same way that literally all of us see back in time, all the time.

One of the most important observations in all of physics is the finiteness of the speed of light. (One of the first substantial pieces of evidence for this fact was actually demonstrated in 1676 by a Danish astronomer named Ole Rømer, well before Einstein incorporated the finiteness of the speed of light into his theory of relativity). However, the so called "speed of light" actually has very little to do with light; instead, the speed of light determines the maximum speed at which any fundamental interaction of nature -- and therefore transfer of information between two bodies -- can take place. In his Course Of Theoretical Physics Volume 2: The Classical Theory Of Fields, Landau appropriately introduces the speed of light as the maximum velocity of propagation of interaction, and an excerpt from the first chapter of the book in which he talks about the implications of this maximum velocity can be found in this answer.

The main implication important in the situation you bring up is that since light propagates at a finite speed, the light you see coming from any object you look at was emitted in the past, firmly located on what's known as your past light cone. For example, since the sun is approximately 500 light-seconds away from Earth, whenever you look at the sun, you are seeing the sun as it was 500 seconds ago when it emitted the light, and you will not see what the sun looks like now until 500 seconds have passed and the light emitted now has reached you.

Of course, this is not the only consequence of the finite speed of light. All of the unusual affects associated with relativity result from it as well, and I'd highly recommend that you read more about this very interesting subject.

• Between your first sentence and my take on Thanby's answer, I'm a little blown away about thinking that nothing we perceive is in the present, and only always in the past to varying degrees. Never really thought of it like that for the stuff "nearby". – coblr Dec 21 '17 at 19:14
• @coblr For extra mind-blowing, don't forget to add in the cognitive processing delay from your visual cortex. It takes a non-trivial amount of time (up to 0.15s) for your brain to figure out what all those sparkles on your retina are, putting everything you perceive just a little bit more in the "past." – user22a6db72d7249 Dec 21 '17 at 20:56

The Hubble has spotted a galaxy that is 13.4 billion light years away. While we see the moon as it was 1.5 seconds ago, or we watch Mars rovers' "7 minutes of terror" live, as it happened 10 minutes ago [I always wonder if prayer works outside the light cone?]--the significance of the Hubble seeing back in time is that the Universe is only 13.8 billion years old--so it's looking back over 97% of all time that has ever existed in the Universe--and that is amazing.

• "Prayer outside the light cone" - there's one for the physicists and theologians to argue about! :-P – Brad Dec 21 '17 at 17:59

If object emitting a photon (= one ray of light) is so far that it takes a year to reach the observer then what you see now by looking towards the object is actually what it looked like a year ago.

A telescope looking at space will see multiple stars that are each in different distances from the telescope. So the photons from them, while being equally fast, have traveled for different durations.

• Right, I get that part....but the Hubble didn't have to wait 10 million years to gather the deep field protons.....how long did the Hubble have to wait to detect photons from the deep field view? Please be patient with me :) – Tracer Dec 20 '17 at 12:29
• Hmm. The photons were already traveling to the exact location of Hubble at the time when a photo was taken before the Hubble was even made. – Communisty Dec 20 '17 at 12:31
• Analogy: if someone throws a ball to the other side of a field, the receiver doesn't have to be at the right spot when the ball is thrown, but when it finally reaches the spot. – Communisty Dec 20 '17 at 12:33
• Fortunately there are other people answering the question too. – Communisty Dec 20 '17 at 12:47
• @Tracer There is a steady stream of light (photons) coming at us from all over the galaxy - each of them left their place of origin a vastly different time ago from each other. A minute from now, a week, a day, year, or century from now there will still be a stream of photons coming at us from all over the galaxy. If you're looking at light from a star that's a million light years away today you will see what it was doing a million years ago. You can still look at that star a century from now and you will see what it did 999,900 years before today. – J... Dec 20 '17 at 20:20

Light travels with a speed of $300\;000 \mathrm{km/s}$ through space. If you google any space object, it should be possible to find its distance to Earth. Wikipedia is always quick and helpful.

And then you can calculate the time it has taken the light to reach Earth:

$$time=\frac {distance} {speed}$$

As an example, it takes light from the Sun approx. 8 min to reach Earth. When you look up and see sunlight, you actually see 8 min old sunlight. What the Sun looks like when you look at it is in fact how it looked 8 min ago.

You know when you hear your own voice's echo? That's hearing yourself, say, 2 seconds ago.
1 second of that is for your emitted sound to hit a wall, and 1 second is for it to travel back.

That means the wall is also hearing you back in time 1 second.

Telescopes are just like the wall here; like the wall hearing the past, they see the past.

The only difference is that instead of a 1-second delay it's often more like 1 billion years for stars.

Strictly for the non-scientists:

Light moves amazingly fast by human standards - about 300,000,000 meters per second, which for Americans translates to over 670,000,000 miles per hour. Most things we see in everyday life are seen "now" for practical purposes. If you could guide a beam of light around Earth using prisms and lenses, it would go full circle in about 1/7th of a second.

But when it comes to astronomical objects such as the Moon, the Sun and the planets, the distances are astonishingly large. For the Moon at about 1/4 million miles away, light (or radio) takes around one and a half seconds to span the distance. The Sun takes eight minutes - we say it's eight light minutes away. Using time to describe distance is like when someone talks about someplace to drive to - "it's twenty minutes away."

I worked with the Cassini spacecraft - it was over one hour between when the spacecraft started sending image data, and when we'd start receiving it. Saturn is over one light hour away. It's like slow postal delivery - you get news of what happened, not what is happening. What happens at Saturn remains unknown to us on Earth until light (or radio, or x-rays, or whatever) has had the time to travel the distance. So when we look at Saturn right now through a telescope (or with bare eyes) we see how it was as of one hour ago.

Stars are much much farther away then anything in our Solar System. Light takes years to span the distances between even the nearest stars. Alpha Centuary is about four light years away from us. If it were to suddenly flare up, we wouldn't know until four years later. What we see today looking at Alpha Centuari is how it was four years ago. We can't know any better, since nothing (we know of) travels from the star to us any faster than light.

Most galaxies was can see in typical affordable telescopes are a few million light years away.

Hubble's Deep Field image was taken when aimed at a part of the sky with no stars of our own galaxy in the way, and none of the "nearby" galaxies only millions of light years away. The galaxies visible are several billions of light years away. So, we only know how they looked as of several billion years ago. Reporters who like to say intriguing things then say "Hubble sees back in time" as if it were some magical metaphysical device. No, it's just "slow" messengers of light.

You DO NOT see back in time. I think this type of language borders on PYSCHOBABBLE.

What you are doing is WITNESSING an EVENT that OCCURRED in the PAST.

IMAGINE a far-away planet, where a race of people live in TOTAL DARKNESS because they ran out of GASOLINE. This has caused SEVERE PSYCHOLOGICAL PROBLEMS with many of the ORGANISMS in the POPULATION, so YOUR SERVICES ARE URGENTLY NEEDED.

Thanks to the wonders of APPLES's iPHONE 14 (!!!), it has become possible to INSTANTLY TRANSPORT a VIDEO CAMERA connected to a GIANT LED screen to them with a SWIPE LEFT gesture.

After instantly getting the video camera, the organisms take a LIVE MOVIE of themselves and display it on the GIANT LED display. This occurs in REAL-TIME, immediately after you swiped to the left.

Here is where you have to do some MATH. After they display the picture on their new GIANT LED screen, the iPHOTONS emitted by the GIANT LED travel towards EARTH at 186,282 MILES every SECOND.

Lucky, the planet is only 16,094,764,800 miles away, so it takes exactly 24 hours to reach Earth. You have to WAIT. After a nice meal and some rest, exactly 24 hours later, the iPHOTONS emitted by the GIANT LED when you swiped left have finally reaches your RETINA.

This causes a CHEMICAL REACTION in your EYE in REAL-TIME that your BRAIN interprets as a TINY MOVIE. After about 5 minutes, the organisms being to play LIVE FOOTBALL. In the 37th minute, a TOUCHDOWN is scored.

You witness the touchdown in real-time; HOWEVER, the movie is a TIME-DELAY of the touchdown that occurred 24 hours ago, in REAL-TIME on their planet. You had to wait for the photons to get to your brain.

Now you have a 24 TIME-DELAYED MOVIE of another race. The lights are on so NO FURTHER ACTION IS REQUIRED, the SEVERE PSYCHOLOGICAL PROBLEMS are already GOING AWAY thanks to APPLE INCORPORATED! Another touchdown!!!

There is also a SIMILAR RACE that lives 1,000,000,000,000,000,000,000,000,000,000,000,000,000 miles away but you'll NEVER KNOW if they started playing FOOTBALL too. GOD HELP US if they decide to play SOCCER instead!

SAMSUNG OF KOREA is working on a FLUX CAPACITOR that will send a FANCIER GIANT LED to them MILLIONS OF YEARS AGO. I doubt it's possible, but it's KOREA, who knows?

Disruptions of the electromagnetic field permeating the vacuum of space? Simply SWIPE LEFT!!!

## protected by Qmechanic♦Dec 20 '17 at 12:40

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