r/askscience • u/brenan85 • Jun 03 '13
Astronomy If we look billions of light years into the distance, we are actually peering into the past? If so, does this mean we have no idea what distant galaxies actually look like right now?
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Jun 03 '13 edited Jun 03 '13
Yep, exactly that. We will only find out what distant galaxies look like now, millions of years in the future (or rather, our descendants, if they exist, will find that out). This is even true on a lesser scale of nearer objects such as the sun. If it suddenly ceased to exist, we would know nothing of that until about 8 minutes had passed.
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u/patefacio Jun 03 '13
So let's say the sun just suddenly disappeared. I understand why we would continue to see it as usual in the sky for 8 minutes or so, but the part of that I'm having trouble grasping is why we would continue orbiting. Does gravity travel at the speed of light along with the sun's rays? Obviously Earth would be flung out into space once the sun disappears, so for those 8 minutes, would we be orbiting nothing?
My question is, does gravity operate on a constant delay like light, if that makes sense?
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Jun 03 '13
I believe gravity travels at the speed of light too but the explanation is beyond me. Basically, gravity is a distortion of spacetime and spacetime can only change at the speed of light.
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u/su5 Jun 03 '13
It is useful, to some, to think of the speed of light as the speed information can propagate. If "gravity" propagated faster than this we could develop some sort of information sharing device.
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u/brainflakes Jun 03 '13
Gravity propagates at the speed of light, which causes gravitational waves. We haven't observed gravitational waves directly yet, but we have seen indirect evidence for them.
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u/Metaphex Jun 03 '13
Here's an interesting video on that very topic-
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u/patefacio Jun 03 '13
Thank you! Great video. I wasn't aware that gravity traveled in waves like light.
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u/alexanderkensington Jun 03 '13
Yes. A good way to view that in action is watching a slinky being dropped in slow motion, you'll notice that the bottom of the slinky doesn't start falling until the top reaches it. It's an information transfer, sort of like a ball on a sheet, if the ball is removed it takes a small amount of time for the sheet to regain it's original shape. The same goes for gravity, it takes time for the bent space to straighten back out, thus earth would continue to orbit for about the same amount of time it'd continue to see the sun.
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u/v1nny Jun 03 '13
I'm assuming you're using the slinky as an analogy and not an example... The slinky effect you describe is due to the spring pulling up on the slinky at the same rate that it is accelerating down from gravity. As far as I know, the effects of relatively across the spring are unobservable with modern equipment.
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u/rlbond86 Jun 03 '13
Gravity travels at the speed of light because it is a form of information. So we would be orbiting "nothing", but from our reference frame the sun hasn't disappeared yet.
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u/berlinbaer Jun 03 '13
the pillars of creation may already have been destroyed by a supernova, but since they are 7000 light years away it will be a while until we see it.
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u/Kalean Jun 03 '13
Thank you for sharing that :) Do we know why they think the pillars might have been destroyed? The article doesn't really explain.
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u/schneidmaster Jun 03 '13
According to the link inside the article,
Red represents hotter dust thought to have been warmed by the explosion of a massive star about 8,000 to 9,000 years ago. Since light from the Eagle nebula takes 7,000 years to reach us, this "supernova" explosion would have appeared as an oddly bright star in our skies about 1,000 to 2,000 years ago.
According to astronomers' estimations, the explosion's blast wave would have spread outward and toppled the three pillars about 6,000 years ago (which means we wouldn't witness the destruction for another 1,000 years or so). The blast wave would have crumbled the mighty towers, exposing newborn stars that were buried inside, and triggering the birth of new ones.
So, from my understanding, the red light indicates the possible supernova event, but the supernova's slower-than-light blast wave took about 1,000 years to actually reach/disrupt the pillars, meaning that the light from the destroyed pillars has not yet reached us.
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Jun 03 '13
I can't remember the exact details but if i remember it correctly, near the pillars of creation you can see a supernova (not sure if it's visible in that photo) or a shockwave that would had reached them already and most likely destroyed them.
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u/Yeahjustme Jun 03 '13 edited Jun 03 '13
When you're looking at ANYTHING you're looking into the past. Even the tip of your nose isn't what it looks like. ...Paranoid yet? :D
To answer your question: Yes, we are looking into the past. But we actually do have SOME idea of what distant galaxies look like now. Idea - not fact.
If you take two pictures while a ball is being thrown past your camera, and you know the time between the pictures, you can give a rough estimate of where the ball is going to be at any given time. This prediction is inaccurate, as two pictures would only give you a straight line, and balls do not fly in straight lines, so you take more pictures... And as the evidence increases, it enables you to come up with quite good models for predicting what is going to happen based on previous observations.
If you look at an object, say 10 million lightyears away, what you see is what happened 10 million years ago. But by observing the events of that time, you may be able to predict some of the events that will occur 10 million years later - i.e. now.
Now, the timeframe we've been able to look at stars with any scientific agenda is very very small in the greater scope of things, so, as Ygritte would put it: We know nothing. But we do have ideas.
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u/helicalhell Jun 03 '13
So would the scientists be able to predict an object's behavior based on the observed behavior of other objects that are known to be closer to us or lesser "in the past" than the former object? Does that happen practically?
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u/Yeahjustme Jun 03 '13
In short: Absolutely.
But as always, the answer is a bit more complex than that.
If I think of a car going towards a wall, I would draw the logic conclusion that the car will hit the wall.
This is a scientific theory. (Newtons, actually...)
So, I want to test this theory. I watch x amount of cars on the road, yet none of them seem to be hitting the wall at the end of the road. Puzzling. Why is this? It appears that something DOES prevent the cars from hitting the wall. The drivers.
So I revise my theory: If a car speeds towards a wall and the driver does nothing, the car will hit the wall. Seems better.
So I test it again. I watch x amount of cars, but this time, one of them actually hit the wall, the driver climbs out of the wreckage and say "the damn steering wheel didn't work, nor did the brakes!"
So, I revise my theory again: If a car speeds towards a wall, it will hit it unless the something stops or alters the path of the car. (Yeah Newton)
I test this again - and it seems quite accurate. Until the day a helicopter lifts the wall up JUST before a car slams into it.
Revision: A car headed for a wall will hit the wall if nothing interferes with either the wall or the car.
This is the basics of how a theory uses observations to evolve. It's rarely 100% accurate, but it becomes better and better and better with time.
And yes, this happens practically.
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u/helicalhell Jun 04 '13
It's always fascinating how we draw conclusions from observations through logic.
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Jun 03 '13
I think you lost me with this sentence right here:
If you take two pictures while a ball is being thrown past your camera, and you know the time between the pictures, you cant predict where the ball is going to be at any given time.
Is this supposed to say that you can predict where the ball is going to be?
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u/Yeahjustme Jun 03 '13
Whoops! Yes, that's a typo.
I've corrected it now - thanks for pointing it out.
(Actually: You CAN make prediction based on two images, but since we know it is not a straight line, that prediction would of course be grossly inaccurate... If you had no information up what was up and down.)
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u/DrTrunks Jun 04 '13
To add to this: if I'm standing 1 meter from you, you see me as I was ~0.00000000334 'seconds' ago (1 meter/c) - plus the time it takes your brain to process that information, but that might differ per person.
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u/MisterEggs Jun 03 '13
On a related note; If we had a massive telescope that could watch activity on a planet millions of light years away, and then Earth (for some reason) started hurling towards it..would we see all the activity sped up, like it was being fast forwarded?
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u/idrink211 Jun 03 '13
Yes, it would be in much the same way that the music from a passing ice cream truck is slightly faster as it approaches you and slightly slowly as it moves away. Assuming you're traveling at speeds near the speed of light, your view of this planet would be blue-shifted and thus a higher energy / frequency. Again, this is similar to why the tone of the music is higher as the ice cream truck approaches but lower as it leaves.
Also, the closer something gets to the speed of light the slower time passes in that object's frame of reference. So to us on earth traveling so incredibly fast, everything on the outside would appear to be going faster including that planet we're moving towards. To an outside observer, activity on earth would appear to be slower.
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u/MisterEggs Jun 03 '13
Ok, so, taking it one step further then...
If we could "drive" the Earth around space at near light speeds, both toward and away from the planet we're observing, would we then see a pause in that activity? And would i be right to assume that we could never actually rewind the activity we're observing, as we would need to exceed the speed of light away from the planet in order to do so..?
Thank you for answering, btw!
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u/Gunrun Jun 04 '13
Observed events outside would pause only at light speed, which is impossible to reach, as far as we know. Also as you got closer you got to the speed of light you would notice blue shifting, ie the light being received would get bluer, similarly to the way sound gets higher pitched if the source is approaching at a high speed. There's a really cool program that demonstrates how close to light speed trace would look that I shall edit in here in a moment
Edit : http://gamelab.mit.edu/games/a-slower-speed-of-light/
I am unsure about what would happen if you we're to somehow exceed the speed of light however, sorry.
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u/Hoobleh Jun 03 '13
I would like to see this question answered.
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u/WeenisWrinkle Jun 03 '13
Me too. I would imagine the light would be blue-shifted because of Earth's relative velocity, but hopefully an expert on the topic can answer whether you actually would view events occurring faster than real-time.
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u/zdunn Jun 03 '13
Everyone else is answering the question of "peering into the past," but to answer your second question, we can make a very good educated guess, since our understanding of cosmic structures is pretty damn good. Given what we can see of distant galaxies (and clusters of galaxies), we can very accurately predict what will happen to these structures in the future, since most of it is dependent on dark energy expansion (which we don't understand a lot about, but enough to predict it), gravity, and radiation.
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Jun 04 '13
we are actually peering into the past?
No matter what you look at, you're peering into the past.
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Jun 03 '13
So what happens if a galaxy millions of light years away slowly makes its way towards us? The light would take less time to get here as it got closer... so we'd see it becoming newer over time...? My head hurts...
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u/Panigg Jun 03 '13
Not really. If the galaxy travels at close to the speed of light all you would see is the galaxy being elongated in the direction of travel and maybe some phenomenon such as time dilation, but it wouldn't become younger as it got closer.
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Jun 03 '13 edited Jun 03 '13
Yeah it doesn't get "newer" over time but yes it does have less of a distance between us and some previous time so it would get here at a smaller time than an object moving away from us. Might I add that it would be doppler shifted. Galaxies moving towards us will be blue-shifted and galaxies moving away from us will be red-shifted. The fact that most galaxies are red-shifted means that most galaxies is move away from us.
Edit: Every galaxy is now most, thanks to /u/nomilieu.
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Jun 03 '13
every galaxy is red-shifted
That's not true.
The Andromeda Galaxy, for example, is blueshifted. Our galaxy is due to merge with it in the relatively distant future.
Most, though, are indeed redshifted.
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u/Panigg Jun 03 '13
Well, technically they aren't "moving" away. They mostly remain stationary, it's just the space between the galaxies that gets bigger.
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u/tewas Jun 03 '13
Actually Andromeda is moving towards us, in about 4 billion years we will have head on crash. If we start sending pulses of light every second to Earth from Andromeda, on the receiving end we would get pulses just slighting more often than a second. The approach speed of the galaxy ir about 300 km/s so your light pulses are shorter by .0001 second and you will see as andromeda signaling you every .9999 seconds rather than every second.
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u/Sansasaslut Jun 04 '13
WTF galaxies can move? Can someone explain please?
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u/asr Jun 04 '13 edited Jun 04 '13
Everything moves! It's not possible to stand still.
Galaxies have HUGE gravitational fields, and they attract each other. So they'll orbit each other just like a planet around a sun.
And then you have groups of galaxies (called the Local Group), and each group orbits all the other groups in the local supercluster, and each supercluster orbit all the other superclusters in the universe. Everything is moving constantly. And not slowly either - they move very very fast (some galaxies are moving at 1% of the speed of light toward some other galaxy). But since space is so large it takes a long time to get anywhere.
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u/asr Jun 04 '13
The light would take less time to get here as it got closer... so we'd see it becoming newer over time...?
You would see the light as bluer then it really is. The color (frequency) of light is the speed of light divided by the wavelength. If you are moving toward the light as it moves toward you the wavelength seems shorter (i.e. you keep coming across the next peak in the wave earlier than you expect).
Since you can't change the speed of light, instead the wavelength changes, and this makes the frequency change.
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u/supandi Jun 03 '13
I have a question, could be wrong, could be completely dumb but i've been thinking about this for a while. Can someone help me understand this.
We know that the light we see from a star is what it emitted in the past. Also, if we look into billions of light years into the distance, we will see how a celestial object looked in the past. Now, from my present, neglecting all theories, if I have a vehicle that can travel faster than the speed light(way faster), given sufficient time, is it possible for me to see what happened in the history of this planet. i.e the light should still be traveling, although attenuated over the course of time, but billions of light years away. What happened during continental drift or how did the dinosaurs die. At the end of the day its just light travel in infinite space.
Neglect if I didn't make any sense.
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u/chilehead Jun 03 '13
Since the light gets fainter and fainter the further it travels, and making out details gets harder and harder the further you get from something, you wouldn't be able to see anything that would make any sense to you. Yes, the light you are seeing did come from that time, but seeing something that far away would be like trying to watch your neighbor's television set while you're visiting the moon.
At a distance of 10 billion light years we can't even resolve individual stars in other galaxies, much less anything as small as gas giant planets.
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u/hddoutsider Jun 03 '13
i got a question. lets say the aliens have spotted us using their telescope, are able to see dinosaurs moving in real time, then if they where to traveling at our direction at near the speed of light, all while still viewing us using the telescope, then are they able to see everything happen in fast forward?
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u/derekkered37 Jun 03 '13
Branching onto that idea; if we somehow invent a telescope that can see superly duperly incredibly far, does that mean we will eventually be looking into blackness, before the universe was even created?
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u/zgott300 Jun 03 '13
Yes, exactly! What's even more mind blowing is that many of the stars we see in the sky don't even exist anymore. They've long since gone supernova. There are also new stars that we don't see yet.
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u/CollegeBroski Jun 04 '13
My question:
So after reading all of these responses, I'm guessing that the idea of extraterrestrial beings visiting our planet are very very slim? I mean as mentioned nothing can travel faster than the speed of light, so how could anyone light years away reach us if they can't travel at speeds reasonable enough to reach planet Earth in a short amount of time?
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u/coloumb Jun 04 '13
Probably so. I think even if they did have capabilities to reach our planet, we [as a human race] haven't done anything significant for them to take notice of us...
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u/neon_overload Jun 03 '13
does this mean we have no idea what distant galaxies actually look like right now?
Your use of the word "now" implies that there is a different "now" to the one we observe from our frame of reference, and that there is one universal "now" that is experienced everywhere at the "same time".
Another way of thinking of it is that "now" is a relative concept, that is different for every observer in the universe. At our "now", the state that the universe appears to be in for us is exactly what the universe is like "now" according to our frame of reference.
From a different frame of reference (say, from within another galaxy), their "now" at that point where we are observing them would involve us being at a stage we were 7 million years ago.
It's impossible to link their "now" with our "now", because there is no possible way for anything - light, gravity, any message or information - to travel between these two points at greater than the speed of light.
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u/yunomakerealaccount Jun 03 '13
Doesn't there still have to be a universal "now", though? Say we're observing a supernova seven million lightyears away, for example. It has progressed (invisibly to us) seven million years along its course. Even though the image we perceive is seven million years old, the event occurring in that location right now as we sit here is seven million years in that image's future, though we cannot see it due to distance. We'll see what's happening at this moment in another seven million years.
Things happen in their own pockets of relative time, but they're all happening at the same time, if you use the entire universe as a frame of reference. Am I incorrect in assuming that the entirety of the universe is the same age? Is there a sort of "objective clock", progressing from the Big Bang to the present moment, which would read the same time for each constituent part of the universe, even though their relative perception of each other's positions in space would vary?
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u/fakename64 Jun 03 '13 edited Jun 04 '13
A few more mind-blowing facts:
During the "initial" big-bang, the entire universe exploded from a singularity to the size of a galaxy in less than 1 billionth of 1 billionth of 1 billionth of a second. And it's been expanding ever since.
Than means that during that initial very short period of time, literally everything in the universe was moving faster than the speed of light. Physicists explain this by pointing out that time itself is a physical property of the universe -- so you could either say that light moved faster, or time moved slower, or whatever.
The stars didn't start forming until later. It takes a while for all the matter to form up (by gravitational attraction) and then spontaneously start fusion. So the oldest stars we see are still younger than the actual universe itself.
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Jun 03 '13
Not only are we seeing in the past, we're not getting a clear picture of those objects due to gravity distorting the light as it travels through the universe to our eyes. Ever seen gravitational lensing? And that's just light being affected by the gravity of galaxies. Now imagine what the forces of dark energy and dark matter does to light as it travels through space.
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u/CrzyJek Jun 03 '13
Yes we are looking into the past because light took a very long time to reach us. We have no clue what the present is like.
TLDR: Yes.
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u/HiG33k Jun 03 '13
"Look like" can be vague. No, we can't post a visual picture of what our human eyes could see if we could see it. However, there are such things as "neutrinos" that give us a more contemporary "look" at galactic activity. Neutrinos are a type of radiation that doesn't diffuse/scatter like light and therefore can reach us more quickly and are strong signals.
I know the above pertains at least to supernovae; can someone confirm their accuracy at depicting galaxies too?
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u/DevilGuy Jun 04 '13
technically we have little idea what our own galaxy actually looks like, we're on the edge and the milky way is 100-120 thousand lightyears across. that means that what we see, even of our own galaxy is mostly tens of thousands of years out of date.
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u/999999999989 Jun 04 '13
As you say, you have no idea how the galaxy look like right now. But what now? For me.. my now and their now from my location is the same. Knowing how could it be in my now... would be like knowing the future.. and I cannot know my future either. There is no time alone. There is space-time. We can see a galaxy that is many light years away in space as it was. Reality may be different.. but that is at their space location. It is a problem thinking of a universal time equal for all space locations. If the galaxy could do something faster than speed of light that could affect us in present, then I could understand that I am seeing something from the past but something is affecting us that I still cannot see... But nothing the galaxy can do to affect us from the current present time. Only what happened in the past. So for all purposes.. for me.. that galaxy is at I see it now.. and I don't care how can be in their future that could be my present. It's so strange.
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u/Myzon Jun 03 '13
It is pretty interesting to think that every star in the sky that you see could or could not really be there "right now". Something a little easier to grasp would be the speed of sound. If you see someone shooting a gun from a mile or two away, there would be a delay between seeing them shoot and hearing it. The stars are like this. When you look up at the sky the light you are seeing(depending on the star) could be millions of years old and that star could be long dead and gone.
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u/mchugho Jun 03 '13
The concept of "right now" doesn't actually exist as time is relative to the observer.
Edit: an example of this is this experiment with an atomic clock. An atomic clock was placed on Earth and another few clocks were put on planes and flew around the world. When they all returned to each other they were out of sync as the clocks that had been travelling faster had experienced time at a slower rate. Mind blowing stuff.
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u/atticusw Jun 03 '13
Can I ask a follow up question here?
I've always wondered this:
We now know something we see that is distant in space appears as it was when the light left it during that moment in time.
What if we had an object emitting light, but moving at us faster than the speed of light (via the object). It's kind of a race condition -- which would reach us first? The light or the object?
Is it like the conveyor belt effect -- a belt moving at 5 units of speed with another object rolling ontop of it at another 5 units of speed would equal a total of 10 units of speed.. The light will already moving at the speed of light (via the object), and it's emitting at the speed of light, so it's now moving 2x speed of light? Ahh mind is starting to blow up a little..
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u/Mattlink92 Jun 03 '13
The answers you seek come with understanding the difference between Lorentz Transformations (what actually happens at high velocities) and Galilean Transformations (your conveyor belt example). Right off the bat, note that your conveyor belt idea only represents slow-travelling things, not things traveling at relativistic speeds i.e. light.
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u/Mudface68 Jun 04 '13 edited Jun 04 '13
I think that they call it red shift. It's the frequency changes like the Doppler effect if I'm remembering correctly
**i see below that I got the color wrong . "blue shift".
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u/esotericsean Jun 03 '13
Followup question, can we create a timescale based on the big bang? Sort of like star date in Star Trek? We might not be able to see the stars as they actually are currently, but we could follow their path and figure out where they are "now", right?
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u/mchugho Jun 03 '13
You can make educated guesses as to where the stars will be but there are a lot of variables to be accounted for and nothing is for certain. Also the concept of now doesn't really exist as time is relative to the observer.
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Jun 03 '13
This is not really scientific but more as a "explain it to me like i am 5" way of explaining this. This is what I used to use to get my little sisters to wrap their heads around this peculiar topic. Please delete this if inappropriate for askscience.
I pointed my telescope at a star 60 light years away (i've forgotten which one it was) and said to them "imagine that on this star an alien is sitting with a super powerful telescope. So powerful that if it would point it at earth it would be able to see houses and people walking around. It decides to point its telescope at earth, and at our house. but the thing is, our house was built 50 years ago so the alien would just see an empty hill where the house stands now. And it would not see us because we aren't born from its perspective. That alien won't see our house until after 10 years or so"
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Jun 03 '13
we sure are and ehh we have a pretty good idea what those galaxies look like today from our understanding of things like gravity and stellar evolution as well as computer simulation
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u/Gravityflexo Jun 03 '13
Even if you look at the person next to you, you are now seeing them as they are "right now". You are seeing them as they were before the light bounces off them and is registered by your brain.
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u/lukehashj Jun 03 '13
However, one could contend that what you are seeing right now is as the planets are - right now - relative to you.
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u/TheTT Jun 03 '13
That's why travelling faster than light is such an interesting thing. If you travel faster than light for a while, you'll be able to see the light that got going before you did. You can catch a glimpse of the past.
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u/SoCo_cpp Jun 03 '13
This means if we are ever successful at traveling faster than the speed of light, as NASA Admits they are working on, then, many challenges permitting, we should be able to send a probe out far enough to observe the creation of the universe. I always wonder if there was ever a universe before the current one.
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u/OriginalityIsDead Jun 03 '13
Well, we have no real idea as to what they actually look like, but from the data we've gathered about their expansion patterns, rotation and direction we can make an educated guess about what they will end up like and simulate an image from that. It wouldn't be spot on or account for the endless number of variables that would be in play that we couldn't hope to predict, but we can make a fair guess about it with reasonable results.
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u/TheZombieHolocaust Jun 03 '13
shouldnt there be a way then to find the Earth and Sun in the night sky (well their light emissions) just by following the known trajectory far enough back in time?
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u/zgott300 Jun 03 '13
Another related idea. If there were a distant mirror, somewhere in space, that we could see the Earth in, we could see it as it was in the past. Theoretically, though not realistically, we could watch Dinosaurs roaming the earth.
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u/whozurdaddy Jun 04 '13
This is actually quite provable, albeit in a different manner. Radio waves also travel at the speed of light. So if we send a signal to a probe that echos the signal back to us, we are receiving a signal from the past. Much like pinging a server on the internet.
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u/Shiftwire Jun 04 '13
Yes and yes (sort of), I'm mean we can't directly see what they look like right now but we could probably make some halfway accurate assumptions based on distance (and therefore how old our current view is) and the state of said galaxy.
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u/form_an_opinion Jun 04 '13
If we can look that many billions of light years off, is it possible for us to see ourselves in a primordial state?
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Jun 04 '13
I prefer the answer that when we directly perceive the light from an object however distant, we are seeing it as it is right now. And that is the definition of "right now".
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u/Gunrun Jun 04 '13
For a good look at what near light speed travel would look like MIT guys made a cool little game called A Slower Speed of Light, where you collect orbs that slow down light, which for the purposes here would amount to the same thing as moving faster. Demonstrated are effects such as red and blue shifting, time dilation, and the spotlight effect.
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Jun 04 '13
Yes to both.
Betelgeuse, one of the bigger supergiants in our neighbourhood, is expected to go supernova in the next little bit. Thing is, as it's 640 light years away, it could have gone kablooie during the American Revolution and we wouldn't even know about it for another 400 or so years.
The real mindfuck, though? That sentence I just wrote is completely meaningless. It makes no sense to say something we haven't witnessed yet happened in the past, as time travels at the speed of causality. There's no such thing as two causally separated events happening simultaneously. From our frame of reference, Betelgeuse will have gone supernova 640 years before we witness it. But the day before we witness it, it will be completely meaningless to say it had already happened.
Time travels at the speed of causality, which travels at the speed of light. There is no logically coherent way of describing "simultaneous" events if there does not exist an "observer" (needs not be sentient) that witnesses them happen.
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u/EvOllj Jun 04 '13
Theres a maximum speed for information that is the speed of light in a vacuum.
The faster something moves (that can measure time) relative to relative movement directions, the slower the time progresses for that object, making it possible to have a universal speed limit while objects can also move in differenrt directions at speeds up to the maximum speed of light.
From your point of view the light from distant galaxies took a few million years untill it reches you.
But if you travel with nearly the sopeed of light, the speed of time slows down so much, it nearly stands still for point of view of the fast moving item.
From the lights point of view barely any time passed from the moment when it got send from a distant galaxy to the moment when it reached you, if it moved with speeds close to the speed of light.
(the actual speed of light is slower than light speed in a vacuum, if its not in a perfect vacuum, wich it barely ever is).
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u/ukotes Jun 04 '13
i always wonder why there isn't more talk of facing a telescope into regions of our own galaxy. if we want to find 'activity', that would be a good place to look. Not too close to the center but on one of the bands, there are more stars, more planets, higher probability of finding life.
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u/kanzenryu Jun 04 '13
Here's an interesting article about how to view the universe without any time at all http://lesswrong.com/lw/qp/timeless_physics/
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u/megablast Jun 04 '13
This is wrong. We have an idea, because we can see how they look now (then) and extrapolate. But obviously we don't know exactly what they look like.
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u/adamsolomon Theoretical Cosmology | General Relativity Jun 03 '13
It means exactly that.
Moreover - and sit tight, because I'm gonna blow your mind here - there isn't even really any such thing as right now. Because time is relative, so is simultaneity. If an alien came over and we asked her what her planet was up to in the year 2013 (Earth time), how would she even know what that means? There's no way of objective, indisputable time-keeping in the Universe that she could use as a basis of comparison.
There are some very simple clocks you could use that would be visible in far-flung corners of the Universe. The temperature of the cosmic microwave background would be an especially savvy choice. But while would be great for telling you your year (post-Big Bang) to the billions place, for the ones or even the thousands place it wouldn't be much use at all, unless you had some really great instruments.
By the way, these two facts are related very closely. There's no such thing as right now for exactly the same reason that we can only receive information (such as light) from things (such as light) which move at a finite speed. If instantaneous communication were possible, relativity would be wrong, and there might very well be such a thing as a "right now."