For example. A star is 8B light years away, we’re told that that the light has taken 8B years to get to us.
BUT the universe is constantly expanding therefor was the star not much much closer to us therefor making it’s light younger?
In: 10
From whose perspective?
From a particular (pun intended) point of view yes and no. The photon could have been emitted billions of years ago and traveled across space to reach your eyes–from your perspective. From the photon’s perspective (an invalid reference frame) no time has passed since space has zero length in its direction of travel so it traveled instantly from where it was “born” to where it “died”–a photon has no sense of space or time travel.
From a wave perspective the electric and magnetic fields have been oscillating along the path of the wave for billions of year, but there isn’t a real “thing” there since the photon is merely fluctuation in the electromagnetic field. That said “real” particles are just fluctuations in different fields according to QM.
From whose perspective?
From a particular (pun intended) point of view yes and no. The photon could have been emitted billions of years ago and traveled across space to reach your eyes–from your perspective. From the photon’s perspective (an invalid reference frame) no time has passed since space has zero length in its direction of travel so it traveled instantly from where it was “born” to where it “died”–a photon has no sense of space or time travel.
From a wave perspective the electric and magnetic fields have been oscillating along the path of the wave for billions of year, but there isn’t a real “thing” there since the photon is merely fluctuation in the electromagnetic field. That said “real” particles are just fluctuations in different fields according to QM.
Not sure about something, I reckon what we see when we look at the sky is a composition of “photographs” of wayyyyy back in time, when the stars were younger, and also, expansion *oblige*, when those stars were in a different location, the vast majority of them closer. Okay.
But can we really say the photons coming from them are of a certain age? Photons, travelling at the speed of light, massless (at rest, okay), don’t they live in a state of perpetual present, as if for them the entire universe was going to whizz past them while no time has passed for them? (unless they’re caught in an interaction, let’s think of the lucky ones who travel long distances)
Not sure about something, I reckon what we see when we look at the sky is a composition of “photographs” of wayyyyy back in time, when the stars were younger, and also, expansion *oblige*, when those stars were in a different location, the vast majority of them closer. Okay.
But can we really say the photons coming from them are of a certain age? Photons, travelling at the speed of light, massless (at rest, okay), don’t they live in a state of perpetual present, as if for them the entire universe was going to whizz past them while no time has passed for them? (unless they’re caught in an interaction, let’s think of the lucky ones who travel long distances)
Not sure about something, I reckon what we see when we look at the sky is a composition of “photographs” of wayyyyy back in time, when the stars were younger, and also, expansion *oblige*, when those stars were in a different location, the vast majority of them closer. Okay.
But can we really say the photons coming from them are of a certain age? Photons, travelling at the speed of light, massless (at rest, okay), don’t they live in a state of perpetual present, as if for them the entire universe was going to whizz past them while no time has passed for them? (unless they’re caught in an interaction, let’s think of the lucky ones who travel long distances)
There are three distances to discuss in this kind of situation:
1. How far the star is from us now
2. How far the light we are seeing now has traveled
3. How far the star was when it first emitted that light
Those numbers are pretty much never the same thing, though the closer it is to us the closer they probably are.
Another thing to keep in mind about the expansion of the universe, whatever is causing it to occur is VERY weak compared to the other forces. The strong and weak nuclear force, which are responsible for holding molecules and the like together? Dramatically stronger. Thats why the particles in our bodies aren’t being torn apart as the universe expands (yet at least, they might someday in the far distant future). Gravity? Also stronger, much much stronger. The Earth isn’t changing distance relative to the sun as a result of universal expansion for example. Neither is anything in our own galaxy. In fact, neither is anything in the local cluster of galaxies. The Andromeda galaxy, for example, is moving CLOSER to the Milky Way. So much so that in about 4.5 billion years the two will collide and merge. You have to start looking at distant galaxies before you begin to notice the affect of galactic expansion. So any stars closer than say 10 million light years (and probably further, not quite sure how far the expansion starts to kick in) aren’t going to be affected by that. They will be affected by their motion relative to the earth, but not necessarily galactic expansion.
There are three distances to discuss in this kind of situation:
1. How far the star is from us now
2. How far the light we are seeing now has traveled
3. How far the star was when it first emitted that light
Those numbers are pretty much never the same thing, though the closer it is to us the closer they probably are.
Another thing to keep in mind about the expansion of the universe, whatever is causing it to occur is VERY weak compared to the other forces. The strong and weak nuclear force, which are responsible for holding molecules and the like together? Dramatically stronger. Thats why the particles in our bodies aren’t being torn apart as the universe expands (yet at least, they might someday in the far distant future). Gravity? Also stronger, much much stronger. The Earth isn’t changing distance relative to the sun as a result of universal expansion for example. Neither is anything in our own galaxy. In fact, neither is anything in the local cluster of galaxies. The Andromeda galaxy, for example, is moving CLOSER to the Milky Way. So much so that in about 4.5 billion years the two will collide and merge. You have to start looking at distant galaxies before you begin to notice the affect of galactic expansion. So any stars closer than say 10 million light years (and probably further, not quite sure how far the expansion starts to kick in) aren’t going to be affected by that. They will be affected by their motion relative to the earth, but not necessarily galactic expansion.
There are three distances to discuss in this kind of situation:
1. How far the star is from us now
2. How far the light we are seeing now has traveled
3. How far the star was when it first emitted that light
Those numbers are pretty much never the same thing, though the closer it is to us the closer they probably are.
Another thing to keep in mind about the expansion of the universe, whatever is causing it to occur is VERY weak compared to the other forces. The strong and weak nuclear force, which are responsible for holding molecules and the like together? Dramatically stronger. Thats why the particles in our bodies aren’t being torn apart as the universe expands (yet at least, they might someday in the far distant future). Gravity? Also stronger, much much stronger. The Earth isn’t changing distance relative to the sun as a result of universal expansion for example. Neither is anything in our own galaxy. In fact, neither is anything in the local cluster of galaxies. The Andromeda galaxy, for example, is moving CLOSER to the Milky Way. So much so that in about 4.5 billion years the two will collide and merge. You have to start looking at distant galaxies before you begin to notice the affect of galactic expansion. So any stars closer than say 10 million light years (and probably further, not quite sure how far the expansion starts to kick in) aren’t going to be affected by that. They will be affected by their motion relative to the earth, but not necessarily galactic expansion.
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