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Would anything going the speed of light kill you and would fermions?

  1. Aug 28, 2013 #1
    I had to cut the title short but would anything going the speed of light kill you even though it is impossible? I know that light is going the speed won't kill you but would it count? Because since fermions are transferring forces like photons transfer the electromagnetic force between two objects would it really count as a thing since it transfers a force?
  2. jcsd
  3. Aug 28, 2013 #2
    Well I can't say I'm totally confident that I'm understanding your post correctly but hopefully this helps.

    According to special relativity, if a massive particle approaches the speed of light, then its 3-momentum and kinetic energy approach infinity. You can see this in the formula p=mv(1-(v/c)2)-1/2, where m is the rest mass, v is the velocity, and c is the speed of light. This means that if a massive particle moving arbitrarily close to the speed of light collides with something, it can make an arbitrarily hard impact.

    On the other hand, massless particles like photons do not obey that equation, and even though they move at light speed, they carry a finite amount of momentum. For a photon, the relation is p=hf/c, where f is the frequency of the photon and h is Planck's constant.

    You seem to be a little confused about the names and roles of different particles. Photons are bosons and are massless, and as a result they always travel at c. They are the "mediator" of the electromagnetic force in that they carry the energy/momentum that electromagnetically coupled particles exchange. The archetypal fermion is the electron, which is massive and hence moves slower than c. It interacts via the electromagnetic force by emitting and absorbing photons, but it does not mediate any of the four fundamental forces on its own.

    Now as to whether something would "kill you." Take for example the neutrino, which is a very light fermion which recent evidence suggests has a nonzero mass despite the fact that it typically travels at speeds very close to c. Millions of neutrinos pass through you every second and they do not kill you, mostly because they do not couple electromagnetically (only via another force called the weak force) and hence they pass right through you. Even if one were to hit you it would still not have a high enough momentum to kill you.

    Let's say we put you in front of a particle accelerator that shoots a high energy fermion at you. Even if it couples electromagnetically, since the wave length is so short, there is a good probability it will pass right through you without interacting with anything in your body. [This is similar to how an x-ray machine works--short wavelength/high energy particles can pass right through flesh but only interact with bone. Even bone has a "plasma frequency" such that frequencies higher than the plasma frequency pass right through.] So you would live. But in principle if it collided with something in your body, a huge particle shower could be generated and could kill you.
    Last edited: Aug 28, 2013
  4. Aug 28, 2013 #3


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    Be really careful about what you say there !!

    X-rays are not doing you any immediate harm because the dose it very low/brief.
    But even hospitals etc that do X-rays of patients, are well aware of the damage to tissue from the cumulative effect of multiple exposures.

    I have seen pics in my geology x-ray spectrometer lab, when I was at university, of VERY nasty burns to flesh from X-rays ... ends of fingers burnt off etc.
    I would suggest that gamma rays ... just up the spectrum from X-rays will also do considerable damage to human tissue in appropriate doses.

  5. Aug 29, 2013 #4
    Yes, of course x-rays can be highly toxic. I worked with an x-ray diffractometer that had an x-ray tube which would cause your finger to get necrotic and fall off even if you quickly passed your finger through the beam. X-rays are really harmful, so please don't try this at home without professional supervision.

    But the reason x-rays kill flesh is because there are tons of individual x-ray particles, and even the tiny fraction which interacts with the flesh of your finger is enough to kill it. However, for an x-ray machine to be able to image the inside of your body, the x-rays must have a small probability of being absorbed in the surface layers of flesh. Otherwise you couldn't get a picture of what's inside. So a single x-ray particle indeed has a small probability of interacting with your flesh, and a somewhat higher probability of interacting with bone. [In fact, in order to derive Bragg's law, or to use the Born approximation, one must assume that the probability of the x-ray interacting is small.] When I gave the medical x-ray example and the "standing in front of a particle accelerator" example, I was trying to explain the single particle case, as per the OP, since he seems to be asking about what happens when a single particle like a photon hits you.
    Last edited: Aug 29, 2013
  6. Sep 3, 2013 #5
    Also what would happen if it wasn't x-rays it was the wavelength of visible light would it still have the same reaction?
  7. Sep 3, 2013 #6


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    Visible light with a high intensity can be dangerous, too. It gets absorbed by the skin, and can burn this (and deeper layers afterwards). Intense lasers do exactly that.

    High-energetic massive particles are dangerous as well. With sufficient intensity, they simply cook a human body from the inside. With lower intensity, they break many chemical bonds and destroy cells, which leads to radiation sickness. With an even lower intensity, they can still lead to cancer.
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