1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Speed of light times 2

Tags:
  1. May 23, 2018 #1
    If a car drives on the highway at a speed of 100 ft per seconds and a gun in the car shoots a bullet forward at the speed of 1000 ft per second the total speed of the bullet will be 1000 + 100 = 1100 ft per second.

    If a car travels at the speed of light when you turn on the head lights will light come out of the head lights?
     
  2. jcsd
  3. May 23, 2018 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    yes

    and NOT at twice the speed of light

    This style of question has been asked and answered many times on PF
    you should do some searching

    There are those that can answer the reasons better than I am able to

    @ZapperZ @Dale @phinds
     
    Last edited: May 23, 2018
  4. May 23, 2018 #3

    Dale

    Staff: Mentor

    First, a car has mass so it cannot travel at the speed of light.

    However, you could modify the question to be “If a car travels at 0.9999 c ...”. Then the answer is “yes, and the light will travel at c in both frames”
     
  5. May 23, 2018 #4

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    What you did here is what is known as Galilean transformation. We now know that this is not accurate when something is moving close to the speed of light, and that there is a more general description on how velocity adds. This is known as the Lorentz transformation. You do not get the speed of light being greater than c, regardless of the speed of the source.

    I've addressed this particular example in an Insight article. In that example, substitute v'=c in Eq. 2, i.e. the observer in S' sees light moving at speed c. You may also want to continue reading to see under what condition do we get back the velocity addition that you already know.

    Zz.
     
  6. May 23, 2018 #5
    You are making this problem harder than it needs to be. Let say we have created a light bulb that CAN and WILL travel exactly the speed of light. With the light bulb ON traveling at the speed of light your saying, light can not travel faster than light bulb is already traveling. SO light must be piling up in front of the light bulb like dirt piles up in front of a bulldozer blade.
     
    Last edited: May 23, 2018
  7. May 23, 2018 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    no it doesn't, reread Dales response

    Speed of the object emitting the light is isn't relevant .... the speed of light is the same in all reference frames and to all observers

    so if you were able to sit on your light bulb ( travelling at the speed of light), you would see the light emitted from the bulb going out ahead of you at the speed of light relative to you and the globe
     
  8. May 23, 2018 #7

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Here's the problem. The SR equations and postulates do NOT describe a reference frame that is moving at the speed of "c", which is what you are trying to do, and which many other members here BEFORE you had tried to do. This is because one explicit postulate of SR is that light travels at c in ALL inertial reference frame. It means that transforming to the reference frame of the source that is moving at c, and then trying to ask what you are seeing, makes no sense in terms of SR's starting point.

    This, btw, is NOT what you were asking. You were asking what a person in S reference frame (I'm referring to the Insight article) is seeing if someone in S' frame sees light at c. Whether you understood the answer to this or not is unknown, because you've changed your question without acknowledging if the original question has been answered or not. Now, suddenly, you are transforming to the S' frame, and boosting it to c.

    Zz.
     
  9. May 23, 2018 #8

    jbriggs444

    User Avatar
    Science Advisor

    That is, of course, a vacuous if. The resulting assertion is meaningless.
     
  10. May 23, 2018 #9

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award


    ???, your response was not helpful

    I may be not quite correct. but from what I understand from the likes of Dale and others in other threads
    I'm somewhere along the right track :smile:
    hence why I called in the others for comment earlier

    correct where necessary, if you can


    Dave
     
  11. May 23, 2018 #10
    This is the answer I was looking for. If a light bulb is traveling at the speed of light then the light that comes out of the light bulb travels at the speed of light too. That does not mean the light from the light bulb is 2 times the speed of light. This makes total sense.
     
  12. May 23, 2018 #11

    Dale

    Staff: Mentor

    This is simply not possible, even theoretically. You must get rid of this premise in order to get a physical answer.

    What you get is an intense Doppler shift (for an emitter moving barely less than c).
     
  13. May 23, 2018 #12
    We all learn something new every day. You are right it is not possible but it is just a brain storm idea. You do not need to think technical to brain storm. This has helped me to see the gun that shoots a bullet a 1000 ft per second will always shoot a bullet at 1000 ft per second no matter how fast the gun is traveling on a, car, airplane, or rocket. Same thing with the light bulb, light will always travel the speed of light no matter how fast the light bulb travels in any direction.
     
  14. May 23, 2018 #13

    Dale

    Staff: Mentor

    Well, don’t you want us to think technically? Isn’t that the reason you posted the question here, instead of an art appreciation forum?

    Surely you wanted people with a deep scientific background to answer your question in a physically correct manner, so that you could gain some real knowledge.

    This does not follow from the above. Only c is invariant, other speeds are not. The velocity addition rule for other speeds is given here: https://en.m.wikipedia.org/wiki/Velocity-addition_formula

    Note how the invariance of c falls naturally out of the velocity addition equation.
     
  15. May 23, 2018 #14

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    This is completely wrong!

    Only light has an invariant speed in all inertial frame. If you try to use your bullet example in a physics class, you will fail miserably. That is like saying that a person walking in a moving walkway isn't walking any faster with respect to the ground than if he/she is walking on the ground. This is clearly wrong!

    Zz.
     
  16. May 23, 2018 #15

    phinds

    User Avatar
    Gold Member

    You are arguing against incredibly well established and incontrovertible evidence that objects with mass (such as your light bulb) cannot travel at c. Dale tried to salvage your question by backing off to less than full c and your objection to that is a non-starter. If your light bulb is traveling at 99.999% of c then the light leaving it travels at c relative to the bulb. Rather than arguing against this, which is futile, try to understand why you are wrong.
     
    Last edited: Jun 7, 2018
  17. May 23, 2018 #16
    I work in engineering sometimes a customer comes to us with an idea an wants to know if we can make his idea work. All the engineers get in a room, lock the door and brain storm ideas for a while. Rules of brain storming is, no matter how stupid it might seem it is still an idea. Ideas from 1 person generate ideas in other people that may not have been generated any other way. After brain storming ideas are tossed around often several ideas combined into 1 good idea to solve the customers problem. Make is simple & make it work, never over engineer the problem. It is funny I work with a guy that has deep physics background that over thinks everything we keep telling him, you don't need to factor in the gravitational pull of the moon on Jupiter to solve this problem. LOL. You need a very good sense of humor to work with us. I often think that guy screws with us as much as we screw with him. LOL :) It make the work day FUN.
     
  18. May 23, 2018 #17

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    And you don't seem to understand that there is a difference between overcomplicating something versus coming up with an IMPOSSIBLE scenario. Do you think if you have a client that wants a real pink unicorn to show up on Thanksgiving, that it is just a matter of "brain storming" to accomplish that? Get real!

    I'm a physicist AND an experimentalist. I am also pragmatic and had to solve many engineering issues (one has to do that when one tries to build a particle accelerator). But we know when Mother Nature tells us what is possible and what isn't. It isn't just a matter of "engineering" when the physical world tells you that certain things can't be done!

    Zz.
     
  19. May 23, 2018 #18

    Dale

    Staff: Mentor

    So then why did you not ask them? If all you wanted was this sort of brain storming, and if this group (to which you have direct personal access) provides it, then why come here? You must have correctly thought that we could provide something different, such as a direct knowledgeable technical answer.

    You might have been brainstorming, but this is an area where we have considerable expertise and don’t need to toss around ideas to come up with an answer. The physics is very clear. I think you realize that is what you needed, even if consciously you like your usual brainstorming process.

    Out of curiosity, what would your group do if the customer’s idea is a standard perpetual motion machine? Wouldn’t you be honest with him and say it is impossible? How much brainstorming time would you spend getting there?
     
    Last edited: May 24, 2018
  20. May 24, 2018 #19

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You're 115 years too late to be brainstorming this! There was something of a collective brainstorming back then which resulted in Einstein realising that - apparently absurdly - time was relative and not absolute.
     
  21. May 29, 2018 #20
    As Dale stated above, something is piling up... It's the waves piling up which makes the frequency much higher. In this case it probably goes out of the visible range. Those headlights in the optical range become death rays to observers in the original rest frame if the car.
     
  22. May 29, 2018 #21

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The problem with this type of argument, is that if we assume that we have a light bulb that can travel at c we are assuming something that is forbidden by the rules that make up our present understanding of the universe. This means we have to abandon those rules. But if we do this, we have nothing to base our answer on, as we would need a whole new set of rules, and we have no way of knowing what those rules are.
     
  23. May 29, 2018 #22
    So, we will attach a timer to a flashlight and begin accelerating it.
    After enormous effort, we get it to 0.999999999c. But that's not good enough.
    So we get it to ##(1-0.1^{-30})c##. But that's still not good enough.
    So we get it to ##(1-0.1^{-80})c##. Perhaps that's close enough. But now it is so massive, it's tidal forces are tearing the rest of the universe apart. So we wait for the timer to go off. But it's so time dilated, we die waiting.
    Long after our galaxy has expired, the timer finally activates. The flashlight sends out its signal (the batteries are still fresh). And that light will be traveling the speed of light relative to anything that is left - including the flashlight.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted