Is the Sun invisible at relativistic speeds?

[tionis]

By the way, I think that posing vaguely and ambiguously worded questions to random "experts", without clearly explaining the background and intent of the question, is not a very efficient way of seeking enlightenment. I would guess that all of the "experts" that have been cited would quickly agree on the answer, if only the question was posed to them in a clear way - for example, distinguishing between the actual Sun versus an ideal black body. Again, if we're talking about the actual Sun, which has a lower limit to emitted frequencies, the answer is self-evident.

What is vague and ambiguous about my question? This is what I've been sending the experts:

We are having a discussion on our forum whether the Sun disappears from
the visible part of the spectrum the closer we get to c. Let's assume we are on a spaceship a
few light-years away traveling toward the Sun at relativistic speed. For simplicity purpose, the universe is devoid of any other light sources (galaxies, stars, the CMB, etc).

If I were to look out the window of my ship, would I be able to see the
Sun slowly become invisible as the spectrum tilted toward the blue? In other words, is it possible for the Sun to become totally invisible as I approach c?

Thank you,

I think a child can understand it. You guys are the one making it more complicated with ''ideal blackbodies'' and whatnot lol.

 

[Samshorn]

What is vague and ambiguous about my question?

As mentioned previously, the actual Sun has absorption bands that would cause it to alternately fade away and get brighter as our speed increases. It isn't clear if your question is asking about these absorption bands, or only about the ultimate asymptotic behavior, but your wording suggests the latter (because you talk about "approaching c"). If so, the answer clearly depends on the asymptotic spectrum of the Sun. This means it isn't a relativity question at all, it is an astrophysical question, i.e., is there a positive lower limit to the frequencies of EM radiation emitted by the Sun? If there is, then the answer to your question is self-evident (regardless of whether you apply the classical or relativistic Doppler).

But this very self-evidentness may cause some people to suspect that this can't be what you had in mind, because if it was, the answer is just too obvious. So they might try to give you the benefit of the doubt, and assume you had in mind the possibility that the Sun may emit at all frequencies (like a black body, for example, which actually is a fairly accurate representation of the Sun's spectrum over a wide range of frequencies), in which case the question becomes: What asymptotic spectrum should we assume (in place of the actual astrophysical spectrum of the Sun)? This leads on to the issues related to the blackbody spectrum.

Admittedly, it's entirely possible that none of this is of interest to you, but a random respondent may not be sure what your interest is. Do you just want the answer to the braindead obvious question (which is really an astrophysics question, not a relativity question), or are you getting at something more subtle that involves relativity?

Also I note that in your follow-up to Carroll you quoted some comments about black body radiation, so if you were not intending him to provide an answer for that, it was somewhat misleading to introduce those words into your correspondence with him. But Gott seems to have also had black body spectrum in mind... even though you apparently aren't interested in the answer to that question. And in response, Carroll begins to justify his frequency cutoff for the actual Sun based on quantum considerations, which may or may not be a red herring. (Even classically, could a cavity radiator emit wavelengths longer than the size of the cavity?)

Having said all that, I agree that many of the responses in this thread have been off-point, and your question wasn't so ambiguous as to warrant so much confusion. But that's what you get in an open forum, or a survey of "experts". You always get the right answer to your question - along with all possible wrong answers. lol

 

[tionis]

As mentioned previously, the actual Sun has absorption bands that would cause it to alternately fade away and get brighter as our speed increases. It isn't clear if your question is asking about these absorption bands, or only about the ultimate asymptotic behavior, but your wording suggests the latter (because you talk about "approaching c"). If so, the answer clearly depends on the asymptotic spectrum of the Sun. This means it isn't a relativity question at all, it is an astrophysical question, i.e., is there a positive lower limit to the frequencies of EM radiation emitted by the Sun? If there is, then the answer to your question is self-evident (regardless of whether you apply the classical or relativistic Doppler).

But this very self-evidentness may cause some people to suspect that this can't be what you had in mind, because if it was, the answer is just too obvious. So they might try to give you the benefit of the doubt, and assume you had in mind the possibility that the Sun may emit at all frequencies (like a black body, for example, which actually is a fairly accurate representation of the Sun's spectrum over a wide range of frequencies), in which case the question becomes: What asymptotic spectrum should we assume (in place of the actual astrophysical spectrum of the Sun)? This leads on to the issues related to the blackbody spectrum.

Admittedly, it's entirely possible that none of this is of interest to you, but a random respondent may not be sure what your interest is. Do you just want the answer to the braindead obvious question (which is really an astrophysics question, not a relativity question), or are you getting at something more subtle that involves relativity?

Also I note that in your follow-up to Carroll you quoted some comments about black body radiation, so if you were not intending him to provide an answer for that, it was somewhat misleading to introduce those words into your correspondence with him. But Gott seems to have also had black body spectrum in mind... even though you apparently aren't interested in the answer to that question. And in response, Carroll begins to justify his frequency cutoff for the actual Sun based on quantum considerations, which may or may not be a red herring. (Even classically, could a cavity radiator emit wavelengths longer than the size of the cavity?)

Having said all that, I agree that many of the responses in this thread have been off-point, and your question wasn't so ambiguous as to warrant so much confusion. But that's what you get in an open forum, or a survey of "experts". You always get the right answer to your question - along with all possible wrong answers. lol

Samshorn, no worries. I'm sending your questions to Prof. Carroll. I also don't mind you guys going off on tangents. I just want to get the best possible accurate answer to my question. And no, I'm not entertaining any 'subtle' agenda or crackpot theory, if that's what you're asking.

 

[Dale]

Hi Everyone,

After some discussion amongst the mentors we have decided to update the rules to prohibit the posting of personal communications with 3rd parties. This thread contained an extreme amount of such personal communications. The specific posts containing the personal communications have been removed, but as a result the thread has become very disjointed and confused.

Therefore, we are closing this thread. The topic itself is not closed and may be discussed in a new thread which should hopefully be less confusing for all involved.

As a personal recommendation, I would recommend that a follow-up discussion explicitly consider an ideal black-body radiator, rather than the messy complications of absorption bands that are found in the actual sun.