Question about photons

@sonhouse said
@Metal-Brain
Cao says galaxies have been seen receding faster than c. Totally false. If he is talking about the expansion of the universe, it is probably true galaxies are receding faster than c since the further away they are the faster they seem to be receding and therefore at a certain distance they will be receding faster than c BUT in that case we will NEVER see them, ...[text shortened]... we see with telescopes ANYTHING going faster than c. But he uses that to say that kills relativity.

@metal-brain said
Expansion how? That is dependent on the big bang theory. That is merely a theory as well. Did the universe start from one tiny point or not? Someone on this forum claimed the big bang theory did not start at any one tiny point and expanded, no actual big bang. If that did not happen, why the expansion of the universe? Nobody seems to know if there was an actual bang, so why is it called the big bang?

@deepthought said
We know that the light from distant galaxies is red shifted and that the red shift increases with distance. So we have evidence that the universe is expanding, unless there's some other explanation for the phenomenon. Extrapolating backwards eventually everything in the universe would be compressed to the Planck density. The cosmic microwave background provides support ...[text shortened]... nty.

Fred Hoyle named the theory, he didn't believe it and was trying to mock it, the name stuck.

@metal-brain said
Did the universe start from one tiny point or not?

Are you implying that the universe is expanding at half or more of the speed of light?

@Metal-Brain
The rate of expansion goes up with distance, the number comes out at about 70 km/second per 3 million light years so at 3 billion ly, the recession is 70,000 Km per second give or take. So at 9 billion ly, 210,000 km/sec. Light goes about 300,000 km per second so even at 9 billion ly out it is expanding at 2/3 c.
At 13 billion ly, it is very close to c so the universe is thought to be bigger than that which we see in telescopes and therefore receding faster than c and therefore forever out of our view because light from those galaxies would never reach Earth to be seen in ANY telescope no matter how big.

@humy said
Why?
Just look up the facts for yourself.

Are you implying that the universe is expanding at half or more of the speed of light?

Why? What has the speed of expansion got to do with anything?

@deepthought said
We know that the light from distant galaxies is red shifted and that the red shift increases with distance. So we have evidence that the universe is expanding, unless there's some other explanation for the phenomenon. Extrapolating backwards eventually everything in the universe would be compressed to the Planck density. The cosmic microwave background provides support ...[text shortened]... nty.

Fred Hoyle named the theory, he didn't believe it and was trying to mock it, the name stuck.

@metal-brain said
sonhouse is claiming the light has not reached us yet from the other side of the universe. The only way that is possible is if the expansion is at least half the speed of light.

@metal-brain said
If c is constant why is there a red shift?

@humy said
Why wouldn't there be a red shift if c is a constant? With relativistic effects there is no contradiction between the two;

https://en.wikipedia.org/wiki/Relativistic_Doppler_effect

@metal-brain said
I created a thread called Relativistic Doppler effect. Remember? Nobody explained it. Assertions are not explanations. Explain it to me.

@humy said
Explain what to you? Exactly which part of that do you not understand?

@metal-brain said
If c is constant why is there a Doppler effect? A Doppler effect implies c is not constant.

@Metal-Brain
Doppler shift works with audio as well and the speed of travel of sound in atmosphere at ground level is more or less constant. You should know the sound of a train whistle when it comes at you is a different pitch than when it is receding. THAT is doppler shift, a change in frequency of a wave from the same source not moving V a moving source, if the waves are coming at you the wavelength decreases so the frequency increases and the opposite for receding waves.
The same is true of light. Even if c was variable by say 10% there would still be doppler effect, but the change in frequency is small for small velocities, but when it gets to say half the speed of light, the wavelength of light coming at you is compressed 2 to 1, so an IR wave could be seen by humans, maybe at the red end of the spectrum whereas before humans cannot see IR waves, say at 1000 nm. So if the source is receding, a 500 nm source which humans can see, would be doppler shifted at 1/2c to 1000nm and would therefore be out of the visible range and so not able to be seen by humans unaided.
You can think of the constant speed of light kind of like this, a poor analogy but it illustrates the deal:
So think of light as residing on a giant long conveyor belt, say we compare the light to a series of bits of sand and they get deposited on the conveyor belt. The belt is moving at a constant speed so no matter where you are the bits of sand (light particles) are stuck moving at one speed. Now if a space ship shoots by the conveyor belt and drops stuff on the belt, the belt has a glue that stops the motion of the sand (light) so no matter how slow or how fast the ship is moving, the conveyor belt doesn't give a rats ass about that, the conveyor belt says how fast the light waves travel.

@humy said
No, the Doppler effect does NOT "implies c is not constant". The reason; relativity.

The explanation, if you are really curious to know which you obviously are not, is right here;

https://en.wikipedia.org/wiki/Relativistic_Doppler_effect

Read it and tell us which part of that you don't understand.
If you refuse to read it then that just confirms you don't want to know and are just trolling.