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

Speed Increases Mass?

  1. Jul 2, 2004 #1
    When particles in cyclotrons increase speed and near the speed of EMR they require much more energy for a further increase in speed. Einstein says this is because the object increases mass with speed.

    Where does this extra mass come from?

    Matter/particles can be created in intense energy fields so is the energy that is added to the particle by accelerating it to EMR somehow converted to matter that adds to the particles mass making it harder to reach light speed?
     
  2. jcsd
  3. Jul 2, 2004 #2

    DW

    User Avatar

    No, mass is invariant. Read section 1 at the link.

    http://www.geocities.com/zcphysicsms/chap3.htm
     
  4. Jul 2, 2004 #3
    Doesn't that seem to violate E=MC^2 then? Why do particles in cyclotrons seem to gain mass as they approach light speed then?
     
  5. Jul 2, 2004 #4

    DW

    User Avatar

    They don't seem to. You need a better text. And [tex]E_{R}[/tex] doesn't equal [tex]mc^2[/tex]. It equals [tex]\gamma mc^2[/tex] as is all thoroughly explained in section 1 at the link I provided if you would please just read it.
     
    Last edited: Jul 14, 2004
  6. Jul 2, 2004 #5
    Not everyone can read Greek.
     
  7. Jul 2, 2004 #6

    turin

    User Avatar
    Homework Helper

    The extra factor that DW is including is a relativistic factor (and a Greek letter "gamma"). It depends in an ever-so-slightly complicated way on speed. To first order approximation, this factor is what gives you the familiar KE = (1/2)mv2. The factor depends on speed in such a way that, as a particle approaches the speed of light, the energy of the particle diverges.
    Note: both speed and energy are frame dependent quantities.
     
  8. Jul 3, 2004 #7

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Think rest mass. Mass does not increase, just the force necessary to accelerate it as it approaches 'c'. This creates much confusion. The problem originates in confusing inertial resistance with rest mass. The mass being moved always 'weighs' the same [according to its reference frame] no matter what velocity it appears to possess relative to any other observer.
     
  9. Jul 3, 2004 #8
    Follow the link which DW gave and meditate on eq. 3.1.8 . Understanding this equation and the text relavant to it will greatly clearify many things.
    :wink:
     
  10. Jul 3, 2004 #9
    For me the math just makes it more confusing.

    If the particle does not increase mass then there must be something preventing it from further accelerating to the speed of EMR. Whatever it is it seems to be somehow connected to the speed of EMR since this effect, this resistance, comes on only as the particle exceeds .5 c. It's virtually unnoticed below that speed. Is this correct?
     
  11. Jul 3, 2004 #10

    turin

    User Avatar
    Homework Helper

    No. It can be noticed at any speed. At 0.5c, the effect cannot be ignored. At 0.05c, for instance, the effect can be ignored for certain approximations, but cannot be ignored for more exact considerations.
     
  12. Jul 5, 2004 #11
    Correct!!
    If the mass is constant (as Einstein believed) , then the "something" is connected to the acceleration. When the particle is at rest in the observer's reference frame, it "sees" spacetime the same way as the observer does. However, as it accelerates, it begins to "see" spacetime differently. It undergoes the famous dilations and contractions. How it "sees" spacetime may be calculated using the Lorentz transforms. The faster it goes, the more differently it "sees" spacetime.
    Remember that the force is being applied to the particle itself, not to the observers reference frame, so it is the particles system of measuring the universe that counts. So we must use it's PROPER velocity, not its reference frame velocity.This is why, it will get closer and closer to c, but never quite gets there.
    DW's great site gives the correct formulas for calculating these things.
     
  13. Jul 5, 2004 #12

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    It's inertial mass increases as it approaches c, you can see this simply follows on from Chronos's last post. As in special relativity problems are usually approached from the pouint of viuew of spacetime, it's considered better to have a defitnion of mass that is Lorentz invaraint (i.e. invaraint with relative speed).
     
  14. Jul 5, 2004 #13

    DW

    User Avatar

    Most people define inertial mass as the resistence a particle has to acceleration. As an equation that is expressed as the m in
    F = mA. In relativity the m in four force is m time four acceleration IS the Lorentz invariant mass. The factors of [tex]\gamma[/tex] one observes in the relation between ordinary force and coordinate acceleration have nothing to do with the mass. They have everything to do with time dilation as pertaining to the time derivatives.
     
  15. Jul 5, 2004 #14

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    Poin taken, but in a certain frame it is as if the inertial mass increases with relative speed.

    That is to say from a purely 3-force and 3-accelartion point of view inertial mass increases with relative speed as I don't see a way, if only using 3-vectors, of having a frame dependnet defintion of inertial mass.
     
    Last edited: Jul 5, 2004
  16. Jul 5, 2004 #15

    DW

    User Avatar

    Its not as if mass increased with speed. Its as if time dilation increased with speed. There is no function of velocity at all for which ordinary force is that function of velocity times coordinate acceleration for arbitrary forces. In other words, in general the equation
    [tex]f^{i} = [M(v)]a^{i}[/tex] is wrong, but it is the mass in that position in such an equation that one calls inertial mass. Again, in relativity that equation of motion is wrong. The closest thing to it for ordinary force and coordinate acceleration is equation 3.2.10 at
    http://www.geocities.com/zcphysicsms/chap3.htm#BM29
    but you can see from it that in the relativistic expression that force isn't even always in the same direction as the acceleration. This means that if you wanted to define a relativistic inertial mass that was velocity dependent and related ordinary force and coordinate acceleration as above that it would have to be a matrix. see last section-
    http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html
    You certainly aren't proposing that are you?
    No, correct way to do the physics is to recognise that the mass as in the four vector general relativistic law of motion F = mA is invariant and that the factors of [tex]\gamma[/tex] in the special relativistic expression 3.2.10 did not come from the mass, but from time dilation related to the time derivatives in the steps that led to that equation. It is not mass dilation that one is seeing in accelerators. It is time dilation.
     
  17. Jul 5, 2004 #16

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    hmm, I didn't consider the directional dependence.
     
  18. Jul 5, 2004 #17
    There are two electrodes in the cyclotron which are called "Dee's". When the charge particle enters the region between the Dee's, where there is an electric field, the particle accelerates. The mass of the particle due to an increase in speed comes from the energy gained as the particle is accelerated.
    Not that I know of.
    The increase in mass is a direct result of both time dilation and Lorentz contraction. The proper mass (aka rest mass) of the particle never changes.


    There are a few links to cyclotrons at the bottom of this page

    http://www.geocities.com/physics_world/relativistic_mass.htm

    re - "Think rest mass. Mass does not increase, .." - What that means is "Please don't ask that question." However the weight of a moving particle increases with speed. The weight is given by W = mg where m is a function of the particle's speed.

    The very meaning of inertial mass in relativity comes from the notion of momentum. By this I mean that in special relativity, inertial mass is defined so that momentum is conserved. I.e. mass is the m in p = mv. That's called Weyl's definition of inertial mass. Once you define mass in this way, and you hold the postulates of special relativity to be true, then, for a particle with non-zero proper mass is a function of the particle's speed. The true reason is rooted in time dilation and length contraction and isn't that easy to explain in a few paragraphs with no math.

    The derivation is here - http://www.geocities.com/physics_world/sr/inertial_mass.htm

    re - "... this resistance, comes on only as the particle exceeds .5 c. It's virtually unnoticed below that speed. Is this correct?"

    No. It is not correct. In fact there was an article to this effect published in the American Journal of Physics in 1995.

    Relativistic mass increase at slow speeds, Gerald Gabrielse, Am. J. Phys. 63(6), 568 (1995).

    Its online under Harvard's web site (where the author is from) at -- http://hussle.harvard.edu/~gabrielse/gabrielse/papers/1995/RelativisticMassAJP.pdf

    Its been measured at speeds so low that gamma deviates from 1 by a factor of 10-9

    re - "As in special relativity problems are usually approached from the pouint of viuew of spacetime, it's considered better to have a defitnion of mass that is Lorentz invaraint (i.e. invaraint with relative speed)."

    Its appropriate to have definitions of proper quanties which are Lorentz invariant. Relativistic mass is not such a quantity. Neither is coordinate time and neither is inertial energy, neither of which are Lorentz invariant. Relativistic mass is the time component of the particles 4-momentum where as proper mass is the magnitude of the 4-momentum. (if you wish you can say that inertial energy is the time component and rest energy is the magnitude). Wishiing to have all quantities in relativity be Lorentz invariant would literally mean that we'd no longer be able to speak of time dilation and length contraction. What would we call the components?

    The proper time interval between two events is the magnitude of the spacetime displacement when those events have a spacelike spacetime seperation. The time component of the spacetime displacement is not a Lorentz invariant. That quantity is called the "Time interval between the two events."

    The proper distance between two events is the magnitude of the spacetime displacement when those events have a spacelike spacetime seperation. The space component of the spacetime displacement is not a Lorentz invariant. That quantity is called the "spatial displacement between the events."

    Attempts have been made to rid relativity of the notion or relativistic mass but all such attempts use flawed logic.

    Pete
     
    Last edited: Jul 6, 2004
  19. Jul 5, 2004 #18

    DW

    User Avatar

    No, the mass is invariant. An invariant is the same according to every frame. The mass is the same for the proper frame as for any other coordinate frame. Invariants should not be called by proper or rest because they are the same for all frames, not any different for the proper frame Vs any other frame. There is no mass dilation. You are missassociating factors of gamma that come from time dilation with the mass. That is modern relativity like it or not. Planck's relativity is dead.
     
  20. Jul 6, 2004 #19
    There is no directional dependance on mass. You're incorrectly thinking of longitudinal and transverse mass which is based on the relation m = |F|/|a|.

    For a definition and derivation see
    http://www.geocities.com/physics_world/sr/long_trans_mass.htm

    Its based on an incorrect definition of force as F = ma. It was Euler, not Newton, who tried to define momentum that way. Newton used F = dp/dt as does everyone else today. The original post seemed to speak of relativistic mass, i.e. the mass the most people (e.g. Cern's web site, etc) use when they say that mass increases with speed. It's also the mass that appears in the cyclotron relation. E.g. see

    http://www2.fpm.wisc.edu/safety/Radiation/2000 Manual/chapter12.pdf


    Inertial mass is defined as m = |p|/|v|. This is the momentum and mass that appears in F[/b] = dp/dt.
     
    Last edited: Jul 6, 2004
  21. Jul 6, 2004 #20

    DW

    User Avatar

    He isn't "incorrectly thinking". Inertial mass is the resistence something has to acceleration. As an equation that means the constant m in F = mA. and m is not p/v. The mass is the m in p = mU. You are the one who is wrong.
     
    Last edited: Jul 14, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Speed Increases Mass?
Loading...