Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black holes and time/velocity

  1. Nov 28, 2011 #1
    Hello! I just want to start off by saying that I am no physicist or astronomer, so my apologies if my questions are really basic lol (although I am an engineer so I do have some technical background). I am very interested in it though, and I had some questions that I was hoping someone could clear up! Thanks in advance for any information provided!

    1. I have read that if an object falls into a black hole, then to an outside observer it will appear to take an infinite amount of time to reach the event horizon and always appear to be outside of the black hole. My question is, if this is true, then why do we not see a huge amount of matter surrounding black holes? (I assume we don't since I have never seen any mention of it) Is it that our telescopes are not powerful enough to observe it directly, or perhaps some other reason?

    2. I'm a bit confused about time and velocities when all of that time dilation and length contraction stuff is involved. Lets say that we are sitting here on earth and some rock that starts on earth begins moving at like .9999*c away from us. In a year's time on earth, will the rock have moved 1 light year away from us from our perspective? Or would that be from the rock's perspective? I hope that this makes sense, to phrase it another way: if we were on that rock rather than earth, would it take us 1 year to go 1 light-year away from earth, or would it take a shorter amount of time?

    3. I have heard that galaxies at the far end of the universe are moving away from us at speeds greater than the speed of light, and I was just curious if there is a basic explanation as to why this is possible without violating any theories?
  2. jcsd
  3. Nov 29, 2011 #2
    Hello Stargazer. I have no educational background on this subject and I, like you, find this subject of astronomy quite interesting. I will try to answer your question though I can probably only answer your last question with a little confidence.

    1) I have never heard of that hypothesis. I would hypothesise that the person who is in the vicinity of a black hole would duo to the gravitation, be pulled down into the black hole and be a victim of the phenomena spaghettification. I do not believe that the observer would see the object or person fall into the hole for an infinite amount of time. I will not deny it though because space and time are distorted around and in a black hole.

    2) I do not understand your question but I will try to answer it anyway. The relationship between velocity and time is not one of my strongest fields but as far as I know, the higher the velocity the slower time goes by. It's an hypothesis of time travelling and our understanding of that have been around since 1905 with Einstein's special relativity theory. There is a famous thought experiment. Lets say you have a twin brother and you're both 10 years old. If I were to send you on a spaceship which has an velocity nearing the speed of light and you were in orbit for lets say 20 years. Your twin brother on Earth would have aged 20 years and would have been 30 years and you would only, depending on the exact speed, have aged 10 years to the age of 20. So time and velocity indeed have a relationship. But again I am very amateur and this is not my strongest field.

    3) Yes we do move away from each other. It was a phenomena first suggested by Edwin Hubble. It have a relevance to the Big Bang theory. Now, gravitation wants to pull everything together but of some unknown reason things are moving away from each other, and the longer away galaxies are the quicker they are moving away from us. This phenomena has so far been called "dark energy", and we do not have anything to address it. Frankly I have not heard of any galaxies moving at greater rate than c, and I doubt it.

    I hope someone else can give more precise answers than this, but I hope that mine has provided you with a little bit better understanding.

  4. Nov 29, 2011 #3


    User Avatar
    Science Advisor

    Yes, this is true. But as the object gets closer and closer to the event horizon, it gets redshifted more and more strongly. So in practice the image would actually disappear from our detectors pretty darned quickly and just fade into the background of the black hole.

    But to answer your question, we haven't ever actually taken a picture of the event horizon of a black hole. But the theoretical prediction that this will occur is on an extremely firm foundation.

    In one year from our perspective, we will observe the rock travel a tiny bit less than a light year. If we have placed a clock on that rock, then that clock will have only gone through a little over five days.

    From the rock's perspective, what has happened is that it has taken a little over five days to travel a little over five light-days from the Earth. The difference, however, is that it sees objects as being flattened along the line of motion: it sees the Earth as basically being a pancake, and it sees the distance that we measure from the Earth as one light year as being only five light-days.

    The fundamental point to be made here is that in General Relativity, there simply is no way to uniquely define the velocity of a far-away object. You can write down equations so that far-away galaxies are traveling faster than light, or are traveling slower than light. It all depends upon how you define your terms.

    In the end, the speed of light limitation in General Relativity just cannot talk about far-away velocities. Instead, it only deals with velocities at a single point: the speed of light limitation in General Relativity is a statement that no object can ever outrun a light beam. The motions of far-away light beams are inconsequential. It is only the motion of light in an object's immediate vicinity that matters. And nothing in the universe is ever able to outrun light rays, nor do any of our observations suggest that they might be able to.

    So are far-away galaxies moving faster than light? It depends upon how you write down your equations. By the obvious way of writing down our equations (where speed equals distance times expansion rate), yes, many absolutely are receding at faster than the speed of light. But that's just an artifact of the way we've written our equations.
  5. Nov 29, 2011 #4
    Awesome, thanks a bunch for the information Chalnoth! Very detailed and easy to understand answers!
  6. Nov 29, 2011 #5


    User Avatar
    Gold Member

    Stargazer, you should also be aware that from the point of view (frame of reference) of the person falling into the black hole, the delay does not happen at all and they are completely oblivious to the event horizon and sphagettification may happen before or after they pass the EH, depending on the size of the BH.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook