# Light seconds, light years, light centuries: How to measure extreme distances – Yuan-Sen Ting

Light is the fastest thing we know. It’s so fast that we measure
enormous distances by how long it takes
for light to travel them. In one year, light travels
about 6,000,000,000,000 miles, a distance we call one light year. To give you an idea of just
how far this is, the Moon, which took the Apollo astronauts
four days to reach, is only one light-second from Earth. Meanwhile, the nearest star beyond
our own Sun is Proxima Centauri, 4.24 light years away. Our Milky Way is on the order of
100,000 light years across. The nearest galaxy to our own, Andromeda, is about 2.5 million light years away Space is mind-blowingly vast. But wait, how do we know how
far away stars and galaxies are? After all, when we look at the sky,
we have a flat, two-dimensional view. If you point you finger to one star,
you can’t tell how far the star is, so how do astrophysicists figure that out? For objects that are very close by, we can use a concept called
trigonometric parallax. The idea is pretty simple. Let’s do an experiment. Stick out your thumb and
close your right eye. It will look like your thumb has moved, while more distant background objects
have remained in place. The same concept applies when
we look at the stars, but distant stars are much, much
farther away than the length of your arm, and the Earth isn’t very large, so even if you had different telescopes
across the equator, you’d not see much of a shift in position. Instead, we look at the change in the
star’s apparent location over six months, the halfway point of the Earth’s
yearlong orbit around the Sun. When we measure the relative positions
of the stars in summer, and then again in winter,
it’s like looking with your other eye. Nearby stars seem to have moved
against the background of the more distant stars and galaxies. But this method only works for objects no
more than a few thousand light years away. Beyond our own galaxy,
the distances are so great that the parallax is too small to detect
with even our most sensitive instruments. So at this point we have to rely
on a different method using indicators we call standard candles. Standard candles are objects whose
intrinsic brightness, or luminosity, we know really well. For example, if you know how bright
the light bulb and walk away from you, you know that the amount of light
you receive from your friend will decrease by the distance squared. So by comparing the amount
of light you receive to the intrinsic brightness
of the light bulb, you can then tell how far away
your friend is. In astronomy, our light bulb turns out to
be a special type of star called a cepheid variable. These stars are internally unstable, like a constantly inflating
and deflating balloon. And because the expansion and contraction
causes their brightness to vary, we can calculate their luminosity
by measuring the period of this cycle, with more luminous stars
changing more slowly. By comparing the light
we observe from these stars to the intrinsic brightness we’ve
calculated this way, we can tell how far away they are. Unfortunately, this is still not
the end of the story. We can only observe individual stars
up to about 40,000,000 light years away, after which they become
too blurry to resolve. But luckily we have another type
of standard candle: the famous type 1a supernova. Supernovae, giant stellar explosions
are one of the ways that stars die. These explosions are so bright, that they outshine the galaxies
where they occur. So even when we can’t see
individual stars in a galaxy, we can still see supernovae
when they happen. And type 1a supernovae turn out
to be usable as standard candles because intrinsically bright ones
fade slower than fainter ones. Through our understanding
of this relationship between brightness and decline rate, we can use these supernovae
to probe distances up to several billions of light years away. But why is it important to see
such distant objects anyway? Well, remember how fast light travels. For example, the light emitted by the Sun
will take eight minutes to reach us, which means that the light we see now
is a picture of the Sun eight minutes ago. When you look at the Big Dipper, you’re seeing what it looked like
80 years ago. And those smudgy galaxies? They’re millions of light years away. It has taken millions of years for
that light to reach us. So the universe itself is in some sense
an inbuilt time machine. The further we can look back,
the younger the universe we are probing. Astrophysicists try to read the history
of the universe, and understand how
and where we come from. The universe is constantly sending us
information in the form of light. All that remains if for us to decode it.

• billy chandler says:

This makes me think now that some of the stars am seeing right now are no longer existing

• เฮียง ปิ่น ซุย says:

รอภาคไทย ขิเกียจอ่าน

• Rajesh Gowda.Y.R. says:

Aaaaa, oops, I'm simply amazed and blown away by the wonders of the Universe, I wonder how majestic and awesome must be the Creator/Yeshua/Jesus Christ who planned, designed and brought the Universe into existence.!! ♉🇮🇳🕎🇮🇱♎.

• Legendary Gamer says:

So when you're wishing on a falling star then that star might passed days, weeks, months, or years ago? How incredible. Sorry for my bad english.

Wait wait wait wait wait : I thought earth was FLAT ?

• katt berckley says:

1 light year is 6 trillion miles… wow milk way 100 thousand light years across… that means 6trillion x 100thousand "miles"…woweee

• Odin Isgod says:

Why this guy sounded like he had throat cancer..

hindi sub-title needed

• Terrance Washington says:

No matter what they say, It's all educated guesses.

• skele tom says:

humans are so smart

• James Lee says:

연주시차
케페이드 변광성

• Fukc offdood says:

There has never ever been any Paralax detected

• Iron wolf 2005 says:

• Ness lopez says:

hahahah silly me i thought i could understand this without cute faces drawn on the stars and galaxies xD

• Travis-2313 says:

WOW! How anyone believes this nonsense is beyond me! Talk about unsound science. You can base an “unknown” on an “unknown” that you got from another “unknown” and then suddenly derive a “known.” Give me a break…

• 27kdon says:

Could anyone please explain the part about trigonometric parallax? I don't fully understand how the parallax thing allows us to figure out the actual distance of objects.

I thought that maybe we could use the parallax from some reference object, like the moon, and then, for other objects, simply refer to them as a multiple of the distance to the moon? But then, how do we know the distance to the Moon exactly using this method alone anyway?

• Sandeep Khetawat says:

I will come back here once i understand this…👍

• Boss Life TV says:

Wait so kanye isn't the center of the universe?

• Coin Roll Hunting and Basketball says:

Wait then what is in the middle of the galaxies

• Evo _ says:

So much bs , how can you base something on a brightness that can not be proven. After the first method the rest is hypothetical. Sorry sheeple

• Melchor Pascua says:

• Orane Appleby says:

"We can only observe stars up to about 40 million light years away" I still dont think this is factual

• border boy says:

Who lit the candles

• Rafinha Latvezzi says:

We never went to the moon

• The Almighty says:

Imagine. Civilizations millions of light years away would still be seeing dinosaurs on earth.

• Key Board says:

Nice

• Matthew Gauthier says:

You don't

• Jay Vasco says:

fun fact: if you were to teleport 75 million light years away then zoom to earth you will still see the world war 2 raging

This research and study is very educating in much simpler way. Thanks #TedEd

• Sameer Gh. says:

My friend says that some star we see today they are not live?
Is these make sense?

• HIPERHOGAR ARGENTINA says:

Just perfect! Short, very easy to understand, graphic, well told without complicated and technical terminology. Thanks for helping spread astronomy concepts.

• Laraphil1990 Lara says:

I don't get it.. 🤣🤣🤣🤣

• Azhar Hussein Salih says:

It doesn't make sense to me, you telling me that the image of a star which I see is from past and took some light years to reach me? I thought that the stars are so big and bright and that's the reason we see them, not like they been sending us lights and took million of years to reach us, just like the sun, as the earth moves and we see it in real time, it goes behind the horizon our world becomes darker all in realtime, so can someone make it simpler and explain to me?

• c.j Johns says:

In summary we guess or hypothesize.

• battinisai says:

This is what I keep telling my friends ✌🏼 finally there is a video on it.

• Larry O'Dea says:

So can nothing travel faster than the speed of light? The riddle me this.What about at the beginning, the Big Bang when the singularity expanded to the size of our solar system in a nanosecond, at a speed much greater than the speed of light. It may have only happened once but faster than the speed of light. Just a thought! 😊✌️

• Imuzic Tune says:

So Some lights are still traveling for us to see.. But when it reach us its already part of the past…

• Denz Rocs says:

Many people here thought that the earth emits light like a star. 😂

• Trevor Graham Welch says:

Smoke and Mirrors ???

• Arman says:

Absolutely amazing ❤️

• Byron Montesflores says:

This is almost scary when u think about the size of our universe. How our lives is so small small to the breath things out there

• Bhavesh Kochra says:

Can you make a brief video about why don't we age in space?

• esa khan says:

The sky I sees tonight is not the one which it is now …it's what it was ….so technically I live in the past ….

• Alex Sage says:

4:54 "The further we move outward within the universe the further we reach towards the beginning of the times or go back in times"… this is incorrect! The more correct understanding is the further we go outward the further we move out within the big bang and NOT go back in time.

• rickyay26 says:

So by this videos logic of distance…we're not seeing life in the universe because what we are observing now is the past of the universe or its very earliest stage? There could be life but because distance is so large, we are unable to see distant life in space?

• Plane Reality says:

Biggest load of pressupositions I've seen.
What is the proof to know how bright a given star is.
There's no experiment to prove it.

• Allan Jason Mburu says:

amazing

• Walter Peter Carreon says:

Amazing universe.

• Pezelj X says:

Use metric..

• SanKha says:

time to go quantum

• denelson83 says:

0:11 – Wrong units!

• Daniel Lopez says:

everybody talking about light this and that but all u people are blind as fuhh

• Ahmed Morsy says:

The earth shouldn’t be a sphere or even rotate, and no human being can specify its full shape.

• Usman Shahid says:

I’ve been searching for this my entire life 🙏

• struggler 87 says:

If this guy would stop smiling when he talks I'd really appreciate it

• dean hepple says:

😏😉😅😪

• James Bond 007 says:

They say nothing can Travel faster then speed of light that is not true Nothing can Accelerate to the speed of light but It is possible to travel from point A ( Earth 🌍) to point B ( Alfa centaury 4.5 LY )faster than 4.5 years is All About the Gravity force they key for interstellar travel is understanding and creating artificial Gravity

• Chris Yunge says:

So you guys have set up this whole hypothesis based on what you do not know, I quote, the whole universe is sending us info and all we have to do is uncode it, even tho the light itself is bringing us images right now. In the present it is millions of light years old, its bringing images that have long ago vaporized, turned to dust, useless tho pretty images, of lost time, heres a thot, travel towards those far reaches, you'll never arrive, 1st the object has pass into oblivion or has dramatically move, 2nd, if the object has passed, you will run out photons, and lost you are with no target reference. You cant just turn around and follow the photon back to earth because it has already disappeared, at the speed of light.

• Rey fer says:

Very good video, congrats

• Tom O'Hart says:

A really long tape measure

• babe kookie says:

Everything we see at night is the past 😴

• Appu Thoonunkal says:

The fastest ting in universe is our mind,we can,believe travel through aganist light speed

• Sachin R says:

Mind = blown

• Shin Lim says:

Light of speed is too slow when compared with universe's space
Human need to invent spaceship that can travel faster than light of speed in billion times to explore universe
I guess that would need billion years too, lol

• M F Islam says:

Understanding space is beyond a three pound human brain. Period

• איציק יצחקוב says:

Big like,einstein!!! Not that I understand anything 🥴

• Sheldon Harris says:

God's creation is amazing.

• krushnarajsinh chudasama says:

So may be aliens dont even see our earth as living planet.. In their present we r just lava and rocks..

• Martin S says:

So, what was going on a trillion years ago?

• Berseker Beast says:

• Gage says:

I haven't watched the video yet but it's a VERY good question. If you are in a space craft and out in the void, there is no ground going underneath you to measure speed with, such as in an aircraft traveling across the face of the Earth. So how do you keep track of your speed…..and out in the void, what exactly IS speed…..relative to…WHAT ? So what if you leave the kindly influence of the Good Earth and our solar system and accelerate to whatever speed and turn off your engines….how would you know if you were slowed down by some external force…or sped up by an external force. What if a distant star started dragging you faster and faster toward it. How would you know?

• Gage says:

I don't buy the concept that the light we see from distant stars is old and that some of those stars might not exist any more. I'm not buying it. All that matters to us is that we DO see them. From our point of view they exist…and that's ALL that matters…because we aren't way out there, we are HERE. That's all that matters. OUR surroundings are all that matters to our experience with the Universe. If we jump in a space ship and on the way to that distant star it winks out….only THEN is it gone in our minds. It doesn't matter if it winked out light years away from us at a sooner date because that is not relative to our reality.

• rugo kelvin says:

Stop decoding the stars and making sense. It’s hurting my head 🙃😂

• Sabrina Yahisreal says:

Stop lying!

• prajapati ritik arya says:

Im preparing some theories about light and energy

• Mark Burch says:

So, if creationists are right we're seeing light from stars that never existed. If creationists are right and the Universe is only 6,000 years old, and if we see light from a super nova that's 100,000 light years away, The Star that emitted the light never existed. The star exploded 94,000 years before it existed. So God must have just created the light that traveled here, but not The Star. Pretty tricky.

• Mike Strako says:

Awesome!

• Jan van der Kruyk says:

In other words, beyond 1000 light years it's nothing but a guess how far things are. Because you have to assume that this far away star has the same brightness as the one close by you parallaxed the distance off. You can classify stars all you want but to go from max. 1000 ly to a millions of even billions of ly pure by brightness or exploded stars is pure guessing imo.

• TheManChise says:

we'll soon be able to look back in time..see some Dinos..

and some people still do not believe in Allah

• just Dave says:

If you can’t dazzle them with brilliance, baffle them with..
Science.

• Twitch Zaid says:

Hi pravit

• CarmineFragione says:

The furthest stars are actually the closest to the observer , when you consider all possible pathways upon a global pathway, since the Universe is a bubble membrane balloon, and you travel globally around the cosmos, in any direction. So , as it seems absurd, the furthest objective from any observer is the alternative direction from which ever way they aim, or travel in any line. So standing at 12 O'Clock on a circular global pathway, all the apparent old stars are at six o'clock, such that an ancient ring appears all around the horizon and this area of ancient stars are equally distant in any direction you may choose to traverse the cosmos to arrive there. But to get from the right side of yourself to the left side of yourself, without choosing the closest path but electing to choose the longest possible route to move around the world and come to your alternative position, left or right, you find you must traverse the longest possible distance around the Universe, on a global route and arrive at the same apparent place you are already standing, as the observer.
So the furthest stars are equidistant from you , but you are either coinciding yourself, or if you choose to go out and move all the way around the world, choosing not to take the easy path or shortest path , but elect to get to where you are, the longest possible way, you find you are yourself the furthest possible objective away from yourself. Thus the far stars are closer to you overall, considering the average length of all possible routes to there place, but if you want to catch up to yourself, you can stand still and be there in one straight line concept, but globally , any other way on all points of a compass , you move out away from your place and circuit the whole world, to get to the other side of yourself. That creates a conundrum that suggest why a quantum entanglement might exist. The nature of quanta particles escapes the logical global routing of movements around the Universe, and so, particles on the opposite sides of the Universe may be closer than you think.

• Alex West says:

the best way to measure super long distances is with my schlong

• Gaming Prabh says:

Amazing. Today I learn something new.

• koslim says:

So this is how we have been measuring distances all this time? By candles? Lol.. Yea right

• Dylan Warner says:

this is way more confusing than it has to be

the universe is big and light years are long

its that simple

• Orville Bernardo says:

Wow… I can't believe it…

I learned in a video instead of my teacher explaining it to me 200 times.

• Kory Kent says:

I came to hear ludicrous numbers. I'm disappointed

• kabir prakash says:

I really feel grateful to see such videos published by Ted , Vox , Spacerip etc . Just wanted to say THANKS , hope you stay eternal like the universe itself .

• Raniel Simms says:

Powerful first lines. "Let there be light and there was light"

• Anmolika shree says:

Heart of gold is the fastest

• Sky_stider223 says:

8 minutes from earth to the sun? I though it was seconds lol don’t know why but that’s what I remember
It seems light travels slower then I thought

• Anuj Singh says:

awesome sawsome

• love4lust7301 says:

That was super interesting

• W0LF B3AT5 says:

So if a supernova from say…600 light years away from us exploded, does that mean the explosion happened at around the 14th century and that it’ll take EXACTLY 600 years for us in modern day lives to actually see it?