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Friction in space as approaching light speed

  1. Feb 21, 2009 #1
    I was talking with a guy at work about relativity. I have VERY limited knowledge in the area of physics ( one college level class on radiation physics, and read a few books and essays on my own) but he has absolutely none. He watched a show on Nova or Discovery that blew his mind about light speed and time travel etc.

    I was explaining to him a little bit about some things I had read when this OTHER guy comes up a starts pontificating a little bit. He's kind of a know-it-all jerk who likes to show off. Granted he has more education than the rest of us (degree in engineering of some sort), but he always tries to lord himself over everyone else. No one was arguing with him, he just started volunteering a bunch of information to everyone. He said one thing that caught my interest and I want to know if it is correct. It sounds like it probably is but I just want to make sure.

    I am not looking to prove him wrong, just verify what he said.

    He said that there is still friction in space due to small particles floating around (dust?) and also miniscule amounts of gas like hydrogen and helium. He said that those elements however are so very little like 1 atom per cubit meter, that they don't really have an effect on objects (spacecraft). But he said that as you approach the speed of light, they DO start to have an effect because now said object is traveling through MUCH more area than it was a slower speeds, so the amount of hydrogen, helium, and dust affecting the object is now a factor. A direct relationship between speed and the amount of friction being created.

    Is that true? This is my first post so I'm sorry if I went too long or whatever. I have an interest in this stuff, but am only starting to realize it at an adult age. Too bad I didn't care enough when I was in high school, perhaps my whole life would be different.
  2. jcsd
  3. Feb 21, 2009 #2


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    There was a thread similar to this not too long ago where I derived a general equation for the temperature of a spaceship as a function of time.:


    You can play with it if you want and see how hot it will get as .5c, .9c, etc, but at such speeds it probably doesn't work too well because I didn't take into account relativity.
  4. Feb 21, 2009 #3
    O.k. What I did was post an incomplete question. I don't know enough about physics to start using your formula my friend.

    The guy was saying that there is friction in space, and that friction in space becomes a factor as something approaches the speed of light.

    So I guess if I had to come up with a hypothetical situation I'd ask:

    If I were traveling at a much slower speed, like the space shuttle, could I turn off the engines and keep traveling infinitely if I ignore gravity or slamming into another object.

    BUT, if I were now traveling at the speed of light to begin with, would I be able to continue at THAT speed, or would friction become such a huge factor that I would not be able to maintain and start slowing down to a speed in which friction is no longer a factor.

    Remember, it was that dude that said that hydrogen, helium, and dust particles are what causes faster moving things to slow down and that at slower speeds those particles do not become a factor.

    I hope I am coming across clear.
  5. Feb 21, 2009 #4


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    Friction is ALWAYS a factor. There is no magical speed at which friction no longer slows your craft -- the effect just becomes minimal with respect to small velocities. A space shuttle would eventually come to a halt, just as a craft traveling at .9999c would.

    As you can see from my example, kinetic energy loss is on the order of .001J/s. So even at .1c the change in speed from ISM collisions is really minimal.
  6. Feb 22, 2009 #5
    Awesome man, you rule. I'm am no where close to being able to understand all that math that you posted in your other threads. I'm very new to all this. I don't understand the formulas, but I'm finding out that I like the theories and real life applications for all this physics stuff.

    So, to press my luck...

    He (the guy that I work with) also said that it is impossible for any object to approach light speed because of this whole friction thing. He said that as you go faster, you go through more area, and therefore more of that space particulae will affect you and serve to slow you down.

    Now, I remember hearing something about approaching light speed is impossible because as you do so something happens to the mass of the object and the amount of energy it takes to keep up light speed approaches infinity. So you could never get going that fast. Is that correct? If it is, leave that aside for another discussion.

    My new question is, does the friction thing also become such a factor as you approach light speed that is serves to prevent you from getting there? Meaning, if I had the fuel and the thrust to approach light speed, could I do it or would friction prevent me?

    Look at me talking like a Star Trek nerd, HA! I'm only asking all of this to satisfy my curiosity and to figure out if I should be giving this guy I work with more credit.
  7. Feb 22, 2009 #6
    It is theoretically possible to approach the speed of light (usually denoted as c), but not to reach it. Because the mass goes to infinity as the speed of your spacecraft goes to c. So when you approach c, the amount of energy put in to get an extra velocity dv gets bigger and bigger. Therefore it is impossible to actually reach c with a finite amount of energy regardless of the friction.
  8. Feb 22, 2009 #7


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    Don't you just hate it when people start pontificating and showing off, interupting YOUR pontificating and showing off!

  9. Feb 22, 2009 #8
    Ha! I hope it didn't really come off that way. I wasn't pontificating so to speak. Just commiserating a little bit with my fellow worker about how little both of us really knew about the subject. I had just happened to be able to clarify a certain thing he was questioning. As I said before my knowledge is quite limited. I didn't even know that the speed of light is represented as c. So thank you engineer for your post.

    Anyway, if you knew the other guy, the pontificator (not a word I think), you'd understand. He's very socially awkward and he's the type of guy that thinks breaking the ice with someone is insulting them and then talking about all of his wonderful experience as an engineer. He pretty much dominates every conversation that he's a part of, doesn't listen or accept things that other people say. He's always got an experience that was bigger, better, faster, or in some other way superior to yours. You've met the type I'm sure.

    It sounds like his statements were correct though, which I suspected, so I will have to keep my mouth shut on this one and accept the fact that he really does know somethings I don't (which isn't hard because I don't know much about physics as I have said). I hope I've made it quite clear that I'm not looking to argue with anyone. In fact I never disputed what he was saying as he said it. I merely sought council elsewhere to verify the info he gave.
  10. Feb 22, 2009 #9


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    This is the kind of question from which one can learn a great deal. Truth is, it's two very different questions and the difference between them is absolutely crucial.

    So, short answer; yes, friction could become a factor at very high speeds, even in space. And even that would depend on the region of space through which one travels. Just as jets can achieve higher speeds by travelling through the thinner air at high altitudes, a spacecraft travelling through intergalactic space would experience a lot less drag than one moving between stars within a galaxy.

    But this effect is merely friction, a somewhat "ordinary" force (one with which we all deal on a regular basis). The effects of relativity are much different. The "something" that happens to the mass of the object is a property of relativity. It can be said that the mass of the object approaches infinity as the speed approaches c, so that the last push to achieve c takes an infinite amount of force. But it could also be rephrased to say that time slows down for the object approaching c, so that the last bit of acceleration from "almost c" to "c" would take all of eternity to happen (IOW, never happens).

    Now, there is a similarity between the two, in that they both increase as speed increases. But the big difference is that the friction could be overcome by brute force. If it were possible to travel at c at all (if there were no relativistic effects), then it would be possible, with a big enough thrust, to achieve c in spite of friction. Even if no such source of thrust could be developed, it could at least be imagined. It could even be calculated, simply by determining the density of "stuff" through which the vehicle will travel, the amount of drag that much stuff would cause, and the amount of thrust needed to overcome that much drag.

    But overcoming relativistic effects to achieve c is not even possible in theory. When the amount of thrust required is calculated, the answer is infinite. When the amount of time it would take to get up to that speed is calculated, it is also infinite. So one could achieve c despite friction, if there were no relativistic effects; but one could never achieve c in spite of relativistic effects, even if there were no friction.

    You are among friends; that is the exact reason why (almost) every single one of us is here at PF!
  11. Feb 22, 2009 #10
    WOW! Exactly what I was looking for. Thank you for explaining it so clearly. What I need to work on now for myself is understanding these "relativistic effects". I don't have enough background to fully understand what those effects are or what they mean, but I can accept the idea that it is impossible to reach light speed because of "them". I need to find an article or book that discusses this calculation that infinite energy is required to get an object up to light speed.

    Through this Q&A you guys have so kindly participated in, I have realized or at least suspect that light photons can travel at c because they have no mass. Is that correct? If so, are there other mass-less particles in our universe? I'm sure these are things that I'd learn in an average physics class, but work and family life are preventing my return to college at this time.

    As a bit of background for myself (if you care) I am a Radiologic Technologist. Fancy way of saying X-ray and CT tech. The only physics I ever learned in school were things like the photoelectric effect, compton interactions, differences between alpha and beta particles. Nothing much about motion and certainly nothing about astrophysics or thermodynamics.

    I respect all of the knowledge you folks have, and am quite envious. Someday I suppose...
  12. Feb 22, 2009 #11


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    The derivation of this formula requires a decent knowledge of relativistic mechanics, but if you just want the formula you can easily see why it is impossible to accelerate to light speed.
    The kinetic energy of an object is given by:
    Where [tex]\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/tex]

    Where c is the speed of light. As you can see, as v approaches c, the factor of gamma becomes arbitrarily large as the denominator approaches zero. At v=c there is a 1/0 situation, which is corresponding to an infinite amount of energy.

    Your hunch is correct! A particle with zero rest mass is required to travel at the speed of light!
  13. Feb 22, 2009 #12
    Ha! That whole thing is greek to me man, but I truly appreciate all the effort you guys have put into helping me understand this little situation. I never knew this stuff could be so interesting. Kinda makes me wonder why EVERYONE isn't impressed by it. What a magnificent universe we live in, all of its mysteries...
  14. Feb 22, 2009 #13


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    If you do a search on "relativistic effects" or "tmie dilatin" here in the Forums, you will find many interesting discussions on the topic.
  15. Feb 23, 2009 #14
    Some more on your original question about the "dust" in space, known as the interstellar medium (ISM). There is also an intergalactic medium, which would be the dust between galaxies, and much less dense. Doing a search for both terms should give you a lot more info on them.

    As for a book, Stephen Hawkins' "A Brief History of Time" is a widely available book written for a beginner. I'm sure if you start a new topic looking for book suggestions people will be able to suggest plenty.

    But mainly I wanted to comment on the formulas. I know they can look quite intimidating, but really there is nothing there other than basic arithmetic. You just take your numbers put them in the right places and solve the math, and you get your answer. I've found that a lot (not all) of the formulas in physics are surprisingly easy, given what they are calculating. If you can figure out how to work out the formulas yourself it becomes much more exciting. Breaking down the formula Nabeshin gave:

    Where [tex]\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/tex]

    [tex]K_{e}[/tex] is kinetic energy, this is what we are solving for, to know how much energy an object has at a certain speed.

    m is mass, as an example use 1000kg
    c is the speed of light, 299,792,458 meter per second
    [tex]\gamma[/tex] is gamma, you will find this number by first solving the second formula
    v is velocity, or speed of the object, we'll use 299,000,000 as a speed right below the speed of light.

    Now we just plug in the numbers and do the math:


    Solve the exponents, and divide:

    Subtract from 1, then find the square root:

    Divide that by 1 to find our answer:

    Now we have gamma, we put that in the first formula:
    [tex]K_{e}=1000 * 299,792,458^2(13.7624-1)[/tex]

    Subtract 1, do the exponent, and multiply by 1000:

    Now we just multiply these two to get our answer:

    So an object that weighs 1000kg, and is going 299,000,000m/s has an energy of 1,147,027,309,311,076,144,873.6 Joules.
    For reference a nuclear bomb releases 5,020,800,000,000,000 Joules. Our spaceship has the energy of 228,455 nuclear bombs.

    By looking closer at just the gamma formula you can see why traveling at the speed of light would be impossible. As v get closer to c the division of v2 by c2 will get closer to 1, which then gets you closer to 0 for a square root. That leaves you with 1 divided by something closer and closer to 0, which will get larger and larger. Until you actually reach 0 then you have 1/0, which is impossible.
  16. Feb 23, 2009 #15
    Wouldn't it been much easier to to have those hugh numbers like this 5 PJ or 5,020,800 GJ? Those are big numbers and yes that is a lot of energy that a spacecraft has even more if it accidentally crashes into something then all of that energy will be released into 228,455 nuclear bombs going off altogether. But only way to be sure that these particles do act as friction is actually having a spacecraft that goes that fast which of right now is impossible to reach those speeds so we have no way of seeing that these hunches are correct if they cannot be put to the test.
  17. Feb 23, 2009 #16


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    Hunches? These are not "hunches". They follow from scientific principles. If the scientific principles are sound (and I damn well believe in conservation of momentum), then what follows from them must also be sound.

    Simple science.
  18. Feb 23, 2009 #17
    Ok, but do we have a spacecraft that can go at the speed of light to actually test if the matter in space does cause friction. Still, we don't have the technology that will make a spacecraft go at the speed of light; and the "hunches" that I was refering was on the theories of how friction will affect a spacecraft traveling at the speed of light.
  19. Feb 23, 2009 #18


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    We are not discussing spacecraft traveling at c, so I don't know where you got that idea. We were discussing spacecraft traveling at relativistic speeds, that is, close to c. The fact that the interstellar or interplanetary medium would cause friction is simply an extension of the principle that when objects collide things tend to heat up. Really, the problem differs very little from that of the shuttle heating when it re-enters the earth's atmosphere. These two are the same but for two factors, velocity and particle density. That's all.
  20. Feb 23, 2009 #19
    One of the most important uses of science is to predict what will happen based on what we know. What good would it be if the only way we knew anything was based on what we had already done? How would we be able to try anything new without being confidant of what effects to compensate for?

    As for this specifically I can't imagine what problem you have with it. As Nabeshin just said we aren't talking about friction at c, rather at speeds near it. We know friction is a factor at low speeds, why would it stop being a factor at high speeds? It's not very controversial at all. I don't know why you have a problem accepting that matter would cause friction. What specific parts do you doubt? That there is actually interstellar matter? That it is some how different from all other matter? That near c speeds would have some special effect on friction?
  21. Feb 23, 2009 #20
    Endeavor, are you confused as to what the original topic is? I am confused by your statements, perhaps because I don't have as much knowledge as you do in this area. It seems as though you have deviated from the original question which was basically "is friction a factor in space?"

    I originally didn't understand about interstellar or intergalactic matter, but these kind folks have educated me in those matters and I now understand how that matter is ALWAYS a factor, no matter the speed of an object.

    It seems you have added an element to the discussion beyond my current level of understanding when you mentioned the space shuttle smashing into objects and number abbreviations.

    My limited knowledge of physics (pretty much none) however does not stop me from believing Dale Swanson's calculations, even though I'm not good enough to perform the function myself. I think his numbers and his example are solid. He simply created a hypothetical situation with arbitrary numbers and plugged them into the formula. Presto...he arrives at the answer for his hypothetical question. This seems to follow a reasonably scientific process of using info you have to predict other events when given similar info.

    Perhaps you are speaking on another matter entirely however. As I said, I am a bit confused by the extra information you brought up.
    Last edited: Feb 23, 2009
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