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I Can fast objects get cooked by Cosmic Microwave background?

  1. Apr 14, 2016 #1


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    Awhile ago, I was considering what sort of dangers a spacecraft moving at relativistic speeds would face in interstellar space. Aside from the obvious pieces of space dust being relativistic bullets in the ship's frame of reference, the cosmic microwave background (CMB) would become a big problem.

    In particular the CMB in front of the ship would be blue-shifted into the infrared, visible, ultraviolet, and beyond.
    Let's say we put a gamma ray detector on this ship; something whose chemical composition changes by exposure to gamma radiation, but not with lower energy photons. Then, in the ship's frame of reference, it will start detecting gamma rays from the CMB. However, in say, a planet's frame of reference, the CMB is just microwaves, while the ship is moving very fast.

    My question is this
    : How can a gamma ray detector record gamma rays that exist in one frame of reference, but not in others? Surely the detector must register detections in both reference frames, even if where and when those events take place is frame-dependent. However, in the planet's frame of reference, there's no gamma radiation for the detector to detect (it all being microwaves instead).
    It seems a conundrum...
  2. jcsd
  3. Apr 14, 2016 #2


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    Perhaps I'm missing something but it seems to me that your argument is exactly the same as saying that since a swimmer can move about freely on the surface of a lake, it is bizarre that a person hitting it at 500 mph would feel it like at brick wall.
  4. Apr 14, 2016 #3


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    It's a bit different because the speed of the photons in the "cosmic lake" is a constant in both reference frames. In neither frame would the ship be smacking into a wall of photons because light doesn't pile up in front of a spaceship the way sound might (or maybe it does?).
    What's different between the frames (among other things) is due to the relativistic Doppler effect.
  5. Apr 14, 2016 #4
    I don't really understand how gamma rays are detected (Compton ray scattering) so I'll use visible light instead. The extrapolation should still be valid though.

    If you're viewing a spaceship going close to the speed of light with respect to your reference frame on a planet then you will observe the spaceship's visible light detector (the Captain's eyeball) going close to the speed of light as well. In order for the cones and rays in the captain's eyeballs to register a photon, it must encounter a photon with just the right amount of energy to kick an electron in a retinal molecule into a higher energy state.

    When the Captain is encountering those cool microwaves in front of the ship, because the Captain is going so fast those photons only need a tiny amount of energy to kick the retinal electron into a higher energy state. Most of the energy is provided by the Captain, while only a tiny amount is provided by the photon.

    Keep in mind that everything I just described is from the frame of reference of the planetside observer.

    From the Captain's perspective, he is absolutely still. The photon distribution in front has been piled up into a higher energy band (visible light), while the photon distribution behind him has been lowered such that it becomes difficult, if not impossible to measure.
    Those visible light photons give just enough energy to kick an electron into the higher energy state, which transforms the retinal molecule into a different, more stable shape, which triggers a protein conformation that triggers an electrical pulse to the brain.

    I believe you need the Lorentz transformations to figure out mathematically exactly what's going on.
  6. Apr 15, 2016 #5
    Both would agree that the gamma ray detector should register detections. The person on the planet, knowing how to calculate events in the spacecraft's frame of reference, would determine that gamma rays would be encountered there even though they are not in his frame of reference.
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