Is it true that the telescope can only watch how the universe and celestial bodies were like in the past?

You cannot use it to watch the present universe since the light needs time to travel

Adam D

That is true. It's even true for the eyes in your head, the light that reaches them from across the room took time to travel to you (though not very much).


It's true. But, that's sorta true with *everything*... when you look in the mirror - that's how you looked an instant ago. When you look at the moon - it took the light you're seeing now 1.3 seconds to reach your eye. You see the sun *as it was* about 8 minutes and 20 seconds ago. And, the further away objects are - the further in the past you're seeing. The nearest stars are 4.3 light years away, so we're seeing them as they looked 4.3 years ago. We see the Andromeda galaxy as it was about 2.3 million years ago. So yes - looking through a telescope means you're really looking into the past.


It follows then that, strictly speaking, we only "see" in the past. The delay is just really short in our daily life. You can't see something until it happens, and then the light from that event has to move from where the event happened to reach your eyes. It is so fast when we are close that it beats all heck out of how fast our brain works, so it doesn't matter. But when you get far away, like even the moon, there is a measurable, a noticeable delay (a couple seconds or something). Heck, even tv transmissions where they interview someone on the other side of the planet have a visible delay. So yes, it is true. That is the nature of our reality. So, a star whose light takes years to get here, will only show us what was happening when that light left the star, years ago.



Ronald 7

Yes That much is true For Stars at the outer Edge of our Galaxy their light would have began in the Jurrasic Period The Most distant objects in our Universe are mere echoes of the Big Bang To see proper we would need a Telescope with an aperture the size of our Solar System


Everything you see is from the past. It is because of the time it takes light to go from the source to your eyes. Even the computer screen used to read these words: you see it as it was a billionth of a second "in the past". Of course, pretending that you see the computer screen "in the present" will not cause any problems, so we don't care about the billionth of a second. In fact, there is a much longer delay between the photons entering your eyes, and the image being formed in your brain (around 1/20 of a second). When you look at the Moon (with or without a telescope) you see it as it was a second-and-a-half ago. The Sun, 8m20s (it changes by a few seconds over the year because our orbit is not a perfect circle). There are two groups of scientists who study the universe: astronomers and cosmologists. To avoid confusion, astronomers measure the distance to an object by the time it took light to get here. If light takes 2.5 million years to get here, then the object is at 2.5 million light-years from us (Andromeda galaxy, for example). And they normally time things depending on the date on which we see it here, on Earth. If we see a star explode in the Andromeda galaxy, we normally say "that supernova happened in 2019" even though we all know it blew up two-and-a-half million years ago. That's because all the observations are recorded, based on the date of observation (no one keeps images labelled "1 February 2,497,981 BC", it would be useless). However, if an astrophysicist is studying the immediate effects of that star on its surroundings, she may write: "When that star exploded, 2.5 million years ago, it created a shock wave that took five years to reach the nearest cloud of gas and dust -- the added pressure in the cloud led to a burst of new stars..." But even that is not that useful, as astronomers on Earth would not see any evidence before the year 2024 (for example). Cosmologists, on the other hand, use mathematics (and other sciences) to study space itself. For them, they prefer to work into the structure of the universe. They take the astronomers' observations and consider them to be "observations in the past". Because space itself is expanding, they would apply this correction to the distance. An object located 10 billion light-years away is seen as it was 10 billion years ago. However, during the 10 billion years it took the light to get here, space has expanded (in this case, at a rate close to 200 km/s -- every second, there is 200 km worth of new space added between us and that galaxy). There are a lot of seconds in 10 billion years. A cosmologist would calculate a "comoving distance" representing the distance NOW, to that galaxy. If we could see that galaxy NOW (instead of where it was ten billion years ago), we would measure a much longer distance. Using these corrections, cosmologist can rebuild (mathematically) the "present universe". Of course, we cannot WATCH this present universe, since the light sent now will take even longer to reach us. Astronomy cannot see the present universe, but cosmology can understand some of it.


That is true. There is even a time adjustment necessary for satellites in low earth orbit.

Tom S

Correct, telescopes have nothing directly to do with it, when you look at the Moon (naked eye) you see it as it was about 1.3 seconds ago, the Sun about 8 minutes.


you can NEVER EVER see something at it is at the EXACT moment you are looking at it light travels at 186000 miles/second = 1TRILLION ft/second or 1 ft in 1 nanosecond )=1/1000000000 of a second) So if you are looking at a computer screen say 2 1/2 ft away you are seeing that screen as it was 2.5 nanoseconds ago hold you hand in front of your face - say 1 ft away you are seeing your hand as it was 1 nanosecond ago- cos it takes light 1 nanosecond to get from your hand to your eyes


All light takes some time to reach your eyes and get processed by the brain, so everything you see is in the past.


If you look at our moon with a telescope , it takes the light of the moon 1.3 seconds to reach your eye. If you equip the same telescope with a solar filter, it takes the light of our sun 8.3 minutes to reach your eye. If you look at Saturn it with a telescope, it takes the light of Saturn 1.3 hours to reach your eye. If you look at Alpha Centauri (largest of a triple system) the closest (large) star to earth; it takes the light of that distant (giant yellow) sun 4.39 years to reach your eye. If you look at Betelgeuse the red giant in the constellation of Orion's right shoulder; the light from the red star takes 640 years to reach your eye. If you look at the closest major Galaxy, Andromeda, an unaided eye object under dark skies. The light of that very large galaxy takes 2,500,000 years to reach your eye.


Correct. And so can your eyes, because light has a definite speed so you see even just the other side of the room as it was a tiny fraction of a second ago. But of course the speed of light is very high so this only really matters for things that are very far off. If something is X light-years away, you see it as it was X years ago, because the light took that long to get here. Which usually doesn't matter as things change so slowly it would still look the same now. The only time it would really matter is if you saw a supernova. THEN you would know the star exploded X years ago and it took that time for this interesting information to get here.


Since light has a finite speed, the answer is yes. But even a microscope does it, but then only a fraction of a microsecond. Even your partner to whom you are talking is slightly in the past. Actually, EVERYBODY in relation to you is IN the past. Simply because the speed of light is finite. A better analogy is a thunderstorm. The bolt of lightning occurs, you see the lightning, but you hear the thunder a lot later, because sound is moving only at Mach 1 through our atmosphere. (Mach 1 is the speed of ~1,234km/h).

Jeffrey K

Yes, that is correct.


That is true. There is a 2 to 3 second delay for the Moon. This true for human eye vision too. That train crossing the road is really a little farther ahead than where YOU see it The speed of light IS NOT CONSTANT in different mediums. Some minerals have 3 speeds of light.


true but for most objects the time of travel does not introduce seriously noticeable differences, even as far off as M31