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If you were in a spaceship which was travelling at the same speed as the object you were looking at and it is emitting light in the visible spectrum, yes you will see it.
Galaxies will emit in the visible spectrum, but also in other wavelengths beyond the visible spectrum. It's the same with all celestial bodies - if you are looking at it and you're travelling at the same speed as it, you will see it as long as it emits in the visible spectrum.
What I was talking about before when I said "true colour" is this light it is emitting. The reason why as an observer a long way away probably won't see the true colour is because the expansion of the universe causes the objects to red-shift. Visible light then moves towards infrared and our eyes can't see that. So the "true colour" is correcting for this red-shift. If you're in a spaceship up close and moving at the same speed, you don't get the red-shift.
Sometimes astronomers will represent objects using false colour representation to demonstrate certain phenomena in a way that the layman can understand. If you would like an example of this, I would recommend googling "cartwheel galaxy xray vs visible". As your eye is unable to see x-rays, you would never be able to see the x-ray emissions from the cartwheel galaxy. To make it visible, you observe it with an x-ray telescope and then represent the x-rays as a visible colour.
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If something is red-shifted, it's moving away. If it's blue-shifted, it's coming closer. I always think that if someone's angry, they tend to be red and you want them to go away, if they're blue, they're chill and they want to come in close for a hug.
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I'm not sure I really understand what you're getting at, but you seem to be asking me if the human eye is capable of observing light in the visible spectrum? If you are asking me that question... then yes.
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I am afraid you are getting confused with astrophotography and spectroscopy, but even with that in mind this is a bit of a silly question. I referred to hydrogen absorption spectra, and the absorption lines fall very comfortably within the visible spectrum.
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@mashton unfortunately I'm not in London anymore, I left the UK a couple of years ago because I was particularly upset/terrified with the 2019 General Election outcome.
@moocher there are a number of ways, one of which @frankenbike has pointed out, but it is generally more common to look at absorption lines in the spectrum. Elements absorb certain frequencies of light - if you look at the spectra of stars, you can determine the elemental composition of a star by looking at the gaps in the spectra. Absorption lines appear as black lines, or gaps, in a stars spectrum. Hydrogen is one of the most common ones for stars (and galaxies), and these will always be in the same place. When you look at a red, or blue, shifted star, you will see that these absorption lines are not where you expect them to be as they have been moved with the doppler effect. If you move the whole spectrum back to align with where these gaps should be, you will find the true colour of the star / galaxy.
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It's difficult to explain using words, that's why physics is handled mostly in mathematical notation - but I'll try.
You're using the reference frame of the photons as your measure for time which is the issue - ignore the photons and the fact that they don't experience time. You don't care about that, because you care about the reference frame of the observer. The observer is not travelling at the speed of light, and so it does experience time.
Think of it as a two people in a field at night and it's completely dark. One of the people in the field is on a horse, the other is on foot. The person on foot is trying to catch the person on the horse, but the horse keeps moving. The person on the horse shouts whilst the horse is moving, at that point in time at which the person on the horse shouts, the horse does a poo (bear with me). The person on foot walks, with a trundle wheel and a stopwatch, towards where he heard the shout and then steps in the poo. He knows the horse was there at the point in time when the person shouted, because it was a loud shout and it caused the horse to do a poo. The person on foot now has three very important bits of information. The time elapsed from the observers frame of reference since the shout occurred (person on foot is the observer), the distance to the physical location at which the shout occurred and that the horse existed at the point in time when the sound was emitted (see poo). Why is the poo important? Because of the fact that it proves that the horse existed at the point at which the sound was emitted - from the perspective of the observer, he now has the knowledge that it took ten seconds for the sound to reach him from the horse, so therefore the minimum amount of time (from the observer perspective) that the horse has existed is ten seconds.
The important thing here, is that it doesn't matter how fast the sound travelled, all that matters is that a period of time has elapsed for the observer.
When we apply this to the galaxy thing, it kind of does its own poo. We know that the universe is expanding at a certain rate, and it's very fast. Fast enough to apply the doppler effect to light and change the frequency at which it is observed. We look at the light emitted by the galaxy and we say, oh - cool, it's not the colour we were expecting it to be, and so it must have been red-shifted. That means that it was travelling at a certain speed at the point of emission. This is the poo. We are the observers - we've seen the poo, and we know how far away the galaxy is, therefore we can conclude that although this light itself has experienced no time, both the emitter and the observer have experienced time because these were not travelling near the speed of light. Therefore we can reasonably conclude that the emitter existed at the point in time it emitted and from the reference perspective of the observer that was X billions years ago - therefore from the perspective of the observer that object is X billion years old.
The light is the messenger - we don't care if it experiences time, but the fact that it does not experience time does not change the fact that the thing that does emit it does experience time. It may not be the same amount of time being experienced by the observer, but everything is measured from the observer frame of reference - so for us, looking at it, that galaxy has been in the state that it's in for x billion of years.
Your thinking isn't flawed - there is no universal concept of now, time bends and changes around things with very large mass and things that travel very fast - but it's just sort of the wrong question to ask. Because now is now, everywhere, it's just it might be a different now.
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You're correct about saying you see everything in the past - it's just a matter of relative distance. You don't want to over complicate it and think about quantum mechanics or general relativity, you can get a good approximation just using Newtonian mechanics. The equation you want is speed = distance / time. We know that light travels at 3*10^8 metres per second, and we know that the distance between you and your phone is about a metre from your eyes. As a result we can calculate that the time elapsed between the light being emitted by your phone and it reaching your eyes is 1/3*10^8 seconds, or 0.00000000336 seconds. This is near instantaneous so you don't notice it.
A scenario where you do notice it is if you're watching a someone hit something from a noticeable distance away (i.e. a cricket game) - where you see the ball get hit and then momentarily later hear the sound of it. It's because sound travels significantly slower than light and so you experience the moment of the ball being hit twice, because it's being reported to you via two different ways (sound and light). Each time, however, you were experiencing the instant at which the ball was hit at that moment in time.
Going back to the example of the phone - the light we see from the sun was emitted around about 8 minutes ago (due to the distance between you and the sun) so if you were to stand the same distance from your phone to the sun and get someone to change what was displayed on the screen - you observing the phone from that distance would not see the phone screen change until 8 minutes later. Therefore, you are not seeing the phone in its current state, but you are always seeing the phone reported to you as it was 8 minutes ago.
When you apply the principle to the galaxies, they are so mind-bogglingly far away that the light hitting your eyes is as it was emitted billions of years ago. So yes, you are looking back in time - in the same way that you are always looking back in time, because your eyes are simply receptors for light which has been emitted by a physical object at a point in time.
You are correct that the concept of a universal "now" is somewhat flawed - but only when you think about time as something constrained by human reporting such as GMT or UTC. Particles experience less time the closer to the speed of light they are travelling, and nothing can travel faster than the speed of light, but in the context of the calculation of speed = distance / time, we know that the speed of light is constant, and as we are measuring the snapshot of when the light was emitted from the object we are observing there is a finite distance between the emitter and the observer - therefore the time being calculated is from the frame of reference of that light which has travelled that distance. That light has been travelling for x number of years therefore it is x number of years old and therefore we are looking x number of years back in time.
I am a physicist, for what it's worth.
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could someone summarise this for me? My time is too valuable to read it for myself
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I went to a private school for free (scholarship, disadvantaged background). The facilities I had access to were amazing and I do not think I would be where I am today were I not exposed to it. I was very strong academically and I wanted to study, it catered to me. I went to school with a lot of very wealthy idiots who did not want to learn and they were also catered for.
Am I happy I went to a private school? Yes. Did I enjoy it? No. Do I openly advertise that I went to a private school? No, but I don't actively take measures to hide it and if it ever comes up in conversation I take pains to explain that I was very lucky to be able to, given my background. Navigating different levels of the British class system on a daily basis (I didn't board, so had to travel back and forth between my school and my decidedly non-posh home) was tricky, but I think it provided me with a resilience which has done me very well in later life.
is the amp still available?