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What exactly is compressed at very fast speeds?

  1. Sep 18, 2011 #1
    If a spaceship has enough power sometime in the future to achieve an appreciable fraction of the speed of light, what exactly will be compressed along the direction of its travel, relative to our slower frame of reference? Are the atoms (or the particles making up the atoms) which make up the spaceship themselves distorted, or only the space between them, or both, or neither? Or only the macro form of the ship disregarding the atoms? Or what?

    Another way of asking this is what exactly is distorted in a proton going at almost light speed in a supercollider. We know the proton gets more massive, relative to us. Is it actually compressed or shortened along the axis of its line of travel?
    Last edited: Sep 18, 2011
  2. jcsd
  3. Sep 18, 2011 #2


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    Relativiity is not concerned with these issues and before you get overly concerned with them, you should consider that if the spaceship were to do a midcourse maneuver requiring it to point in a direction 90 degrees from its direction of motion, then if it was compressed along its original direction of motion, the spaceship would experience a lot of distortion in the process, but no such distortion is ever evident to a spaceship.
  4. Sep 19, 2011 #3
    I think relativity used to be concerned with these issues, probably still is.

    Lorentz supposed, in writing about electromagnetic phenomena, "...that the electrons, which I take to be spheres of radius R in the state of rest, have their dimensions changed by the effect of a translation, the dimension in the direction of motion becoming RL times and those in perpendicular directions L times smaller", where L is a function of v with L=1 when v=0.

    In writing about the Michelson interference experiment, Lorentz suggested the changing of molecular bond lengths as a mechanism for contraction.

    Einstein, in writing about accelerated electrons calculated a distinction between longitudinal mass and transverse mass, then extended it to include "ponderable material".
  5. Sep 19, 2011 #4
    I think that relativity is mostly concerned with measurements. According to the contraction, when clock is synchronised, the measurement from a slower frame would be contracted conmpare to the frame measures itself. I think nothing is contracted in your sense, since each frame have different contractions, while your contraction is definite.
  6. Sep 19, 2011 #5


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    For 25 years following the Michelson-Morley experiment (1880), Lorentz and others struggled to explain their null result in terms of real physical changes in moving objects. They stuck to the belief that there was an absolute frame of rest, and that changes in the internal electrical forces in moving objects caused them to contract, and clocks to run more slowly.

    Special relativity (1905) brushed those ideas aside, realizing that there is no intrinsic difference between a moving object and a stationary one. Lorentz contraction and time dilation are not physical properties of moving matter, they are effects caused by the structure of spacetime.
  7. Sep 19, 2011 #6
    "Lorentz contraction and time dilation are not physical properties of moving matter, they are effects caused by the structure of spacetime."

    What is the difference between a physical property and an effect? If the effect is real, isn't it physical? Is the structure of spacetime physically real? For example, is the bending of light as it passes in the neighborhood of a star physically real or "an effect?" Or as another example, do the fired protons in supercolliders which get very massive as they approach the speed of light experience a real physical modification or an effect? If only an effect, is not the extreme electromagnetic energy required to make them move at these very fast speeds physically real as opposed to an effect? Please elaborate on this and define your terms more clearly.
  8. Sep 19, 2011 #7


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    I think you are getting a variety of responses because different people are interpreting your question differently. I don't know how to interpret the question. Can you think of a possible experiment that would distinguish between your different options?
  9. Sep 19, 2011 #8
    chaszz, you are asking whether relativity is an "existential" theory (is the contraction real?).

    I think most adept relativists may probably view relativity as a measurement theory, one which is consistent and in agreement with itself, with the math, and with known measurements.

    Quite different from asking, "is it real?"
    As Dalespam hints, relativity likes to entertain questions in the form of an experiment.
  10. Sep 19, 2011 #9
    Is it possible that the advent of quantum mechanics and its failure to agree with most of our common-sense notions based on the macro physical world that we experience directly, has made it customary for many physicists to speak in this way, where there is little direct congruence between mathematical model and reality, and the scientist says this is the best we can hope to do? Yet I humbly point out that Einstein himself did not like this approach and that he was probably an adept relativist. He insisted that reality was real and that physics needed to treat of it. And I think I have seen relativity described as a classical and not a quantum theory. So is it quite enough to say that the model is one thing and reality (whatever it is) quite another and that never (or rarely) the twain shall meet, when we deal with relativity? Will the twin space-traveller really be older than his brother or not?

    I am sure the energy that goes into the magnets in colliders that accelerate particles to nearly the speed of light is both physically real and dollar costly. If a lightning bolt were to hit me and burn off my arm, somehow leaving me still alive and conscious, I somehow don't think I would be musing on it as a mere mathematical model of reality. Yes? No?
  11. Sep 19, 2011 #10


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    The definitive answer to your question is:

    yes, the atoms themselves and the space between them are indeed observed to be flattened along the relativistic direction of travel. If you were to try to take a picture of an atom, it would appear squashed - both the electron cloud and the nucleus.

    Subatomic particles are a different story. They are point particles, not spheres, so trying to observe their shape is a meaningless exercise. However, as mentioned, they do have fields and these fields would also be squashed.
    Last edited: Sep 19, 2011
  12. Sep 19, 2011 #11


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    I think that this is an unreasonable characterization. In science the final arbiter of reality is experiment. So SR accurately models the result of experiments, that is reality. What it does not do is provide nice bedtime stories about how the world works, that is fantasy or philosophy, not reality.

    This is why I asked you to think a little about your own question. Are you asking a purely philosophical question (in which case the question itself is unscientific and not amenable to experimental investigation) or are you asking a truly scientific question (in which case there must be an experiment which could, in principle, distinguish the various alternatives).
  13. Sep 19, 2011 #12
    "...based on the macro physical world that we experience directly..."

    There is no real direct experience of the physical world. Everything is ultimately a measurement, including those made by your nervous system, through which much processing and integration ultimately leads to your inferences about conscious experience, and everything else...

    Science, even in its most ancient early stages, wisely knew primacy of measurement must be its first grounding step. Think about that... it is still the first step.
  14. Sep 19, 2011 #13
    Straight to the point. Good job, DaveC426913. Reports from Brookhaven RHIC experiments refer to observations of flattened formation of gold nuclei (approaching collision point at near-light speeds).

    Another slant on this would be to consider that the object moving at relativistic speed is actually not changing at all. The object in this case is viewed as a 4-dimensional object that has no change in properties at all in 4-D space. Rather, the relativistic phenomena are dependent on the observers: Different lengths are due to observers having different cross-section views of the same unchanging object. The object does not physically change.

    The effect is depicted in the space-time diagram below. We have blue and red observers (blue and red coordinates) moving at the same relativistic speed in opposite directions with respect to the black rest coordinates. The gold represents an object moving at relativistic speed with the blue observer (speed relative to black rest frame and red frame). Note the differences (between blue and red) in cross-section lengths of the gold object.
    Last edited: Sep 19, 2011
  15. Sep 20, 2011 #14
  16. Sep 20, 2011 #15
    Thanks for this also.
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