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Speed of light

  1. Nov 23, 2007 #1
    Hello all -

    First, I know nothing about physics. I simply may need some quick advice on certain subjects and this forum seems to be loaded with knowledgeable, friendly people. I trust you won't mind if I occasionally pick your collective brains. I am a freelance writer and at times will need to source my claims and/or refutations. My current need is of a personal nature, however.

    My question: If an object is traveling through space (very close to Earth) at the speed of light, how would it appear to someone standing on the ground just glancing up at the sky at 10:00pm. (Not that the time is too significant...just night w/ a clear sky.) That is...what exactly would that individual see in the nanosecond (?) it took for the object to pass?

    I'll be happy to explain why I would like to know and why I need to have this answer before offering the explanation, after I receive an answer.

    Thanks in advance.
  2. jcsd
  3. Nov 23, 2007 #2
    Well...pushing aside several nit picky physics things, just because of the sheer velocity, unless the thing was emitting large amounts of light or whatever, you probably wouldn't see anything. in the latter case, just a flash of light. The distance from horizon to horizon is nothing for something traveling at c

    however if it IS at c then it can't have mass which makes reflecting (afaik) light a bit of a problem and generating light even harder. There is the case that a massless particle can interact with virtual photons (i beleive) but they would probably not reach the surface of tha earth.

    This is probably wrong so wait for someone else to post =)
  4. Nov 23, 2007 #3
    Thanks, FD.

    That "flash of light" hits home. I'm trying to determine if an incident I recall from circa 1970 in New Cumberland, Pa. was simply a dream that I remembered well enough the following morning and perhaps pondered for a while, or something that actually happened. I was 12 at the time and walking into my cousin's house. As I climbed the stairs, slowly, just star gazing as I went in, a cigar shaped object went by and lit up the night as brightly as if it were high noon. It lasted a fraction of a second. I can't even begin to describe how quickly it occured. But I do remember it being broad daylight for that oh-so-short period of time. My father actually came out of the house and asked, "Are you guys playing with matches out here?" (It was a reasonabale enough question at the time. I did that a few times when I was much younger.) I was alone, however, and when I asked recently if he recalled the incident, my father said no. That might be quite understandable too, as this occured during a summer visit when we had just returned from the Orient, and my folks and aunts and uncles were known for drinking a bit.

    Ultimately, I suppose if I were able to determine that such an object traveling at that speed would, in fact, create that "broad daylight" phenomenon as it flew by, I could reasonably infer that I was not dreaming. It just seems highly unlikely that my uneducated mind could conjure up anything scientifically accurate. Right?

    Anyway...it's a subject that interests me greatly and I wish I knew more.

    Thanks again for the input. More from others would be welcome if anyone has the time and/or inclination.

  5. Nov 23, 2007 #4


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    The object wouldn't have to be travelling that fast.
    A meteor doing mach25 at low altitiude can cover a lot of ground and is pretty bright when it burns up.
  6. Nov 23, 2007 #5


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    A couple of points to ponder:

    1] The culprit
    Obviously, the most likely culprit is a meteor, or perhaps due to the brightness, a fireball (just a big meteor).

    2] The brightness
    To light up the night sky bright as high noon is a lot of brightness - but it may not have been as much as that. Your eyes will be accustomed to the dark, and it wouldn't take much light to "overload" them. It would seem impossibly bright when in fact, wasn't at all compared to daytime. I'm pretty sure that actual daytime is tens-of-thousands-fold brighter than nighttime.

    3] The duration
    I wonder if the fireball didn't so much pass from horizon to horizon as burn up and wink out. They do that. It would explain the rapidity with which it disappeared.
  7. Nov 23, 2007 #6
    Thanks, folks.

    I'm confident that it was milky white in color and cigar-shaped...and about the appearance of mid-day. No way I could have followed it from one horizon to the other. It was just too fast. If the color, speed and shape (and there was a very distinct shape to it) all fall within the realm of the meteor explanation, then so be it.

    Look, I've never been much of one to give much credence to the UFO (as in, intelligent alien beings visiting us) phenom, although I do believe it is very likely there is some kind of life out there...somewhere. How could there not be? But this has been nagging me for a while and the natural explanations don't seem to gel with what I think I witnessed.

    Anyway...I appreciate all your time and effort. Thanks!
  8. Nov 23, 2007 #7


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    There is no way you could actually see anything but black-body radiation (white light) if if it was giving off that much light. And there is no way that if it covered a large fraction of the sky in a fraction of a second that you could discern a real shape. Due to the blurring from not being able to follow it well, it would at best look like an elongated object of pure light. That fits very well with a nearby meteor.
  9. Nov 23, 2007 #8
    "...an elongated object of pure light. That fits very well with a nearby meteor."

    I had no idea meteors could travel that quickly. I accept all of your explanations. They are preferable to little green men, for sure.

    Thanks, Russ (and co.)
  10. Dec 3, 2007 #9
    firstoff if your trying to talk about general relativity, then it shall see as though (although im not sure this is what your asking) timeis going slower for the object traveling near the speed of light. However, the object will "think" that for the non-moving object time is going slower. Again, if this wasn't what you were asking,im sorry, but whatever.
  11. Dec 3, 2007 #10
    PJC01 if you are still out there I have to add my 2 cents. I think that if a hypothetical solid object were to pass earth at, say 90% of the speed of light, and you were observing on the night side, you probably would not see anything. I have just assumed that the object missed touching the earth's atmosphere on the way by. It would have to be absolutely huge and at a very high altitude to reflect enough sunlight to light up the dark side of the earth, and if it did, it would resemble an ultrafast strobe flash. If it were, say, a kilometer in diameter, and passed earth at an altitude of 500 kilometers, it would be in the earth's shadow as it passed over you and you just wouldn't see it. But if it touched the atmosphere at that speed, it would explode and the shock wave would destroy anything or anyone underneath. The 1908 Tunguska event over Siberia would pale by comparison.
  12. Dec 3, 2007 #11


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    As far as the speed of light goes, an object travelling that fast would traverse the diameter of the Earth 24 times in one second, and would pass the Moon in less than two seconds. It's fast.
  13. Dec 3, 2007 #12


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    I think the point is that distance and velocity are utterly impossible to calculate for objects with no external references or visual clues to their size. So my post allows only for the maximum speed a human might observe in nature - a meteor traveling at 30,000 mph (iirc, that's as fast as they get if they come at us head on).
  14. Dec 9, 2007 #13
    [If an object is traveling through space (very close to Earth) at the speed of light, how would it appear to someone standing on the ground just glancing up at the sky at 10:00pm.]

    You're trying to put a classical conceptualization on a non-classical picture. If by object you mean something that has mass or size, it can't move at the speed of light. Only light or other bosons can do that. If you mean the generic notion of "object" the only "object" that you can see and travels at c is light. Profound. Further, light passing the earth can't be distinguished from light that hits it. Thus, your picture is all wrong. Getting away from your picture you can see the answer: The "object" would appear as objects in the night sky, filled with stars and all those other astrological bodies, especially the ones over Hollywood, since the light from such bodies passes everyone's eyes and everyone's earth!

    The max observable speed is the speed of light. We like to believe that because we want to preserve unitarity and Lorentz Invariance(whatever they are). It is humorous that the speed of light is actually unobservable. We only see the light. We don't see it's speed since seeing it would assign light or photons an inertial frame of reference. That can't be done. Light doesn't travel inside the Light Cone.

    [That is...what exactly would that individual see in the nanosecond (?) it took for the object to pass?]

    To make things worse one can't locate the photon that's presumably "passing by". The location of the photon is perfectly indeterminate along its path. By perfect I mean if one asserts that photon is localizable, so as to be amenable to determination, then one must accept that the photon is in contact with every other point in the universe! Hard even to make rigorous the notion of "path" without bringing in a lot of machinery that's dependent on the definition of photon. If this sounds like chicken and egg stuff, how to get the definiens out of the definiendum, you're right.
  15. Dec 10, 2007 #14
    although zillions of people have already clarified this, most of them more recognized than myself, I believe that more information can't hurt at all. Here goes:
    Even though i doubt that the human would be able to process the thing moving so fast, but if he could, he would just see this reeally quick objected zipping past. Now, if this person were wondering how time had an effect on this object, it would be simple: to the object, time of the human would seem to go slower, and vice versa. But also, this brings up the twin theory (even though it is not a direct comparison) the object traveling near the speed of light would "age" less than the human observer thanks to genreal relativity. Hope i was of some assitance, and good luck,
  16. Dec 10, 2007 #15
    i still cannot grasp if im travelling at 186,ooo miles a second light is still 186,000 a miles a second faster
  17. Dec 10, 2007 #16
    Well, for one YOU could never travel at the speed of light because you are not made of light. :wink: I don't think anyone really knows how photons "see" other photons.
  18. Dec 10, 2007 #17


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    Yes we do. Photons do not see other photons. It is not a valid frame of reference. Also, photons do not experience time, so even if you were to propose a thought experiment, waiving the laws of physics to travel that fast, it would still be meaningless to talk about what a photon would experience.
  19. Dec 31, 2007 #18
    You could see the speed of light by using a green 5mW laser pointer at night. It appears virtually instantaneous in how it reaches out.
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