- #36
tom.stoer
Science Advisor
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Too hasty, I was still editing my post; now it's completed
seriously, we all agree - after some discussions - that the sun looks brighter; "lol" is not appropriate; if you do believe in Kip Thorne only, then there is no reason to start this thread and let us discuss about ittionis said:Thanks, Tom. I will let him know lol.
tom.stoer said:seriously, we all agree - after some discussions - that the sun looks brighter; "lol" is not appropriate; if you do believe in Kip Thorne only, then there is no reason to start this thread and let us discuss about it
about taking this discussion and the time we invest seriouslytionis said:lol What are you on about?
tom.stoer said:about taking this discussion and the time we invest seriously
Nobody did that; we were all talking about a perfect black bodySamshorn said:I think there may be some confusion here, because ... other people are answering it for the actual Sun, which does not emit a perfect black body spectrum.
tom.stoer said:But you never mentioned red-shift fine-tuning shifting absorption bands exactly to the visible spectrum
tom.stoer said:So let's talk about the perfect black body spectrum
tom.stoer said:Nobody did that; we were all talking about a perfect black body
tionis said:Wait a minute, I'm talking about the real Sun here.
tom.stoer said:But you never mentioned red-shift fine-tuning shifting absorption bands exactly to the visible spectrum
tionis said:How am I suppose to know all that? You guys are the physicists here. I merely posted a question based on some online article I read...
Samshorn said:This is the confusion I was talking about. Also, I suspect Kip Thorne hadn't clearly thought about the power spectrum - he didn't mention a black body spectrum - so it's unclear whether he was thinking of some actual cutoff limit for the real Sun's wavelengths, or if he assumed a blackbody spectrum (tacitly) and just overlooked the intensity amplification effect of approaching speed (as Tom did originally), which he also didn't mention.
Samshorn said:What article? That might help clear up the confusion.
tom.stoer said:I think we should try to be more exact.
There is a Doppler shift ω → ω' = βω;
There is a trf. of Ω → Ω' = β-2Ω;
pervect said:Hmmm- well, doing a series expansion, I'm currently disagreeing with Kip :-(. A series expansion indicates that at the limit as ##\nu## goes to zero, the blackbody radiation goes up quadratically, and the factor that fights it, 1/z, is only linear.
tom.stoer said:I think this is not what the formula says. There is not simply a shift like ω → ω' = ω+Δω, but the star looks hotter with T'(v) > T(v=0) when approaching the star, and therefore it looks brighter for every single frequency.
Where's the difference? the point-like source? the sun instead of an idealized black body?pervect said:Unfortunately the problem in the literature isn't quite the one that the OP is stating - I'm not sure if it makes a difference yet.
For referenmces please have a look at post #4; the standard derivation is for CMB, but b/c dΩ is there and the trf. is known, this applies to other sources as well.pervect said:I'd rather like to see "Distribution of Blackbody Cavity Radiation in a Moving Frame of Reference" , which appears to have a more detailed calculation, but I don't have access.
I do not talk about CMB, only abolut black body radiation. It should be irrelevant where the bb radiation comes from. If a different approach yields different results then something must be fundamentally wrong (the isotropic bb radiation should be at least reasonable for a very large and nearby star, omitting geometry effects due to point like emitter, small discs etc.). Using bb radiation with increasing intensity per frequency results trivially in increasing integrated intensity. The only difference could be due to dΩ for pointlike emitters.pervect said:The problem of the sun's appearance is a different (and much simpler) problem than the CMB background.
tom.stoer said:@samshorn: no, this is is not what the formula tells us... The Doppler shift of every single frequency ω can be "absorbed" in a new, velocity-dependent temperature T'(v). So it looks as if the star gets hotter. But for an hotter black body the number of photons is increased for every single frequency ω, therefore the star looks brighter for every single frequency (not only in total).
The reference in #4 , whose formula you ere using, WAS talking about the CMB.tom.stoer said:I do not talk about CMB, only abolut black body radiation.
It should be irrelevant where the bb radiation comes from.
If a different approach yields different results then something must be fundamentally wrong (the isotropic bb radiation should be at least reasonable for a very large and nearby star, omitting geometry effects due to point like emitter, small discs etc.). Using bb radiation with increasing intensity per frequency results trivially in increasing integrated intensity. The only difference could be due to dΩ for pointlike emitters.
I think that is in agreement with all our analyses.So the sun would be bright blue. It's angular size
would become small as you approached c. So bright blue and small is how
it would look.
No, the Sun is not actually invisible at relativistic speeds. It may appear to be invisible due to the effects of time dilation and length contraction, but it is still emitting light and energy.
The speed at which the Sun would become invisible depends on the observer's frame of reference. However, for an observer on Earth, the Sun would appear to be invisible at speeds close to the speed of light, which is approximately 299,792,458 meters per second.
The Sun appears invisible at relativistic speeds due to the effects of time dilation and length contraction. Time dilation causes time to slow down for objects moving at high speeds, which means that the light emitted by the Sun would appear to be moving slower and thus, the Sun would appear dimmer. Length contraction also causes objects to appear shorter in the direction of motion, which could make the Sun appear smaller and less visible.
Yes, the Sun would still provide heat and energy at relativistic speeds. The Sun's energy is not dependent on its appearance, but rather on its nuclear fusion processes. Even if it appears to be invisible, the Sun would still be emitting energy and providing heat to its surrounding environment.
It is theoretically possible for humans to travel at relativistic speeds, but it would require a tremendous amount of energy and advanced technology. Additionally, the effects of time dilation and length contraction would make it difficult to accurately perceive the Sun's appearance at such high speeds. Therefore, it is unlikely that we will ever be able to witness the Sun becoming invisible due to our own travel.