How fast/how far must an object move in order for a detectable redshift to occur?

Question

what factors does it depend on?

Answer 1

They're right, any motion will get you doppler effects.

For example, you're sitting at a redlight and in the on-comming lane an ambulance with it's sirens wailing flys past. As it approches, it's pretty close before you hear it and as it passes the sound becomes higher pitch, after it passes the sound becomes more elongated, but you hear it longer than when it was approching.

This is the same phenomena as red shift, but dealing with sound instead of light. The principle is still the same. As a source of wave energy approches, the waves crunch up slightly, they have a shorter wavelength than if it were standing still relative to you. As it receeds from you, the wavelength stretches out.

Back at the red light, you experince this as a percived change of pitch.

Answer 2

Look at the minute-hand on a wall clock. You can barely see it move, yet the redshift at this speed is detectable (about 2 feet per hour).

Detecting a redshift is detecting a change in wavelength (or a change in frequency). To actually see yellow change to orange requires about 3 percent of the speed of light, but high resolution spectrometers can detect tiny shifts like 1/10,000,000 the speed of light (see Peter K's answer).

Shorter wavelengths can give much greater sensitivity. Visible light from atomic transitions may look sharp, but it's really fat compared to gamma ray frequencies from nuclear transitions. I won't try to explain the Mossbauer effect, but it allows the detection of extremely small gamma ray Doppler shifts like one trillionth the speed of light.

Think about a radio station transmitting an extremely sharp frequency, and you have a really good radio in your car tuned to that exact signal. You would lose reception if you drove toward or away from the station. The Mossbauer effect is measured using a gamma ray source and detector, watching the signal come and go as they are moved relative to each other.

Answer 3

The factor is sqrt((1+beta)/(1-beta)) where beta is the velocity of the object expressed as a fraction of the speed of light (between 0 and 1)

Answer 4

for you to see it with the naked eye (which you wont be able to do because the object will be moving away from you so fast that it will be extremely small) id say you need to move at .7x the speed of light. thats kind of the cutoff between where we use newtons laws and einsteins relativity laws. its when time kind of starts to distort and create a noticable effect on you.

Answer 5

Some of the most sensitive experiments have been done on the solar photosphere and have detected red shifts at 0.05km/s. My motorbike could do that speed and although it feels terrifyingly fast at 111MPH compared to any star its like standing still.

Answer 6

Any motion will create a redshift. It just comes down to how sensitive your measurements are, which I don't know.

Answer 7

Are you talking time dilation?

http://www.fourmilab.ch/cship/timedial.html

I would think your starting mass would have a huge impact and could cause maximum time dilation with out reaching the speed of light. Yet, it would require the same amount of energy.

Answer 8

It all depends on how sensative your instrument is.