# Why does redshift happen?

If an object B does NOT move away from us, doesn't matter whether it's 10 km or 5,000,000 km away from us, I think redshift wouldn't happen.

Now, if the object is moving away from us, it's said that redshift happens. As we know, redshift is the increase in wavelength. I wonder if this is happening due to the speed, let me bring down my logic.

If object B is 100 km away from us, the light coming to us would take 100/$$x$$. Imagine it starts to move now by 2 km/sec. When it reaches 102 km, another stream of light would come from it, but it would take 102/$$x$$ which is a bit longer the first stream of light (102/$$x$$ > 100/$$x$$). difference like this between these 2 streams mean wavelength must have increased comparing to the first stream and that's what we call "redshift." $$x$$ is the speed of light, but I didn't put it in the equation as it plays no role for the redshift.

Would you agree with me that this is the reason and correct logic why redshift happens?

• The quote says it hasn't anything to do with the speed a signal has from source to observer, ie amount of redshift doesn't matter what the signal speed is but the speed of emitter relative to observer. Apr 22, 2023 at 18:46
• So my logic is correct ? what I'm sayin about 100km distance and so on ?
– Matt
Apr 22, 2023 at 18:54
• you could actually write x instead of 300,000 and you will get 100/x for the first stream of light and 102/x for next stream of light, hence because 102/x > 100/x thats why redshift happens. is this correct ?
– Matt
Apr 22, 2023 at 18:56

Would you agree with me that this is the reason and correct logic why redshift happens?

Yes.

It's just that redshift means the change factor, not the change.

One nano-meters change is a large change factor for a 2 nano-meters long wave. For a 1 meter long wave it's a small change factor. (redshift = change factor)

If speed of light becomes smaller, then waves become shorter, and as the changes of the waves stay the same, as you described, then change factors of those waves become larger.

Wikipedia says:

Redshift (and blueshift) may be characterized by the relative difference between the observed and emitted wavelengths (or frequency) of an object.

Absolute difference = change

Relative difference = change factor