# Is light really billions of years old when we look deep into space?

79 views
0

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

We can calculate the expansion of the universe, so we can say how far away something was when it released the light, as well as how far away it is now, and yes, both of those can be billions of light years.

The universe isn’t expanding *that* fast too, so it doesn’t actually change most distances measured.

There are a few different things going on here. There is the “lookback time” which is how long the light has been traveling to reach us. There is the comoving distance, which is the distance that the object we are seeing is from us *right now* due to expansion. These numbers don’t match, because like you said, for light to take 8 billion years to get here it would have to be emitted from closer than 8 billion lightyears, because of the expansion of the universe. But by the time the light gets here the object is now more than 8 billion lightyears away from us. So the light from a star with a comoving distance of 8 billion lightyears would have a lookback time less than 8 billion light. And light with a lookback time of 8 billion years would come from an object with a comoving distance greater than 8 billion lightyears.

The actual calculations of the lookback time and comoving distance (which is almost always the number reported as the “distance” in cosmology) require a complex model of how the entire universe evolves over time and knowledge of how redshifted the light is when it gets to us.

Light is really that old.

There is may distance to an object in space, let’s look at [https://en.wikipedia.org/wiki/GN-z11](https://en.wikipedia.org/wiki/GN-z11) that in 2016 was one of the farthest away when it was discovered. I use it because I know of what is listed.

The present proper distance is 32 billion light years. That is how far it is from us right now.

The light-travel distance is 13.5 billion light years. That is how far the light has traveled to reach is and if you remove the light from it you get how long time ago it was emitted 13.5 billion years.

The last number is how far away it was then the light that was emitted. That is usually not listening, it is in the notes and why I pick this object. That distance is 2.66 billion light-years.

The expansion of the universe does not make the light younger. It was emitted 13.5 billion years ago and during its travel to us the distance in between expanded so the initial distance of 2.66 billion light-years got longer and the result was the travel distance was 13.5 billion light-years.

compare it to if you walk on a conveyor belt that we tend to call a moving walkway or something similar for human usage. Start 10 meters from the start and try to walk back against its motion. If you walk faster it moves you can reach the star but you will have walked longer than 10 meters. A buddy that did says still on it will have been moving farther away by it without walking.

So you have the distance to your body that is like the resent proper distance.

The distance you walks is the light-travel distance

The distance at the start is the distance at the time the light was emitted.

It is not a perfect analog for the expansion of the universe. The space between objects grows faster if they are farther apart it is not independent of distance like the conveyor belt. The distance to the buddy will be equal to your walking distance. You could modify it with multiple moving walkways when each will move a bit faster than the previous to get a distance difference.

But it does illustrate the different measurements with something a bit like the expansion that is simple to grasp.

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)