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A few puzzling questions

  1. Apr 23, 2010 #1
    The following ideas have been on my mind for quite some time and are puzzling me to no end.

    1. It is known from the special theory of relativity that a photon of light travels at the speed of light no matter whether it’s coming at the observer or moving away from it. In fact even if the observer were to speed at 99% of the speed of light along the photon, that photon would still be seen as going at the speed of light. I’ve seen the time dilation equations, but those equations tell me nothing about why this paradoxical effect takes place. Can someone explain to me the *physical reason* for this?

    2. Does space have a shape? What is its constitution? According to General relativity, gravitation is explained by a curve in space-time. This suggests that space has a shape. But, a shape made from what? This isn’t clear since time is nonphysical. Can someone explain to me the *physical reality* of space-time?

    3. From point (2), it follows that if space has a shape, then it is constituted of something. In other words, space is made of energy. This is the energy *of* space. Then there is matter and energy *in* space. Some might say that space isn’t made of anything. If so, then matter and energy travel through nothing. If so, then space is an illusion. That doesn’t seem plausible. So space’s fabric is of energy. Can someone explain to me what space is made of?

    4. Related to point (3), the moment after the Big Bang, the universe expanded with energy. Since there was nothing physical before the Big Bang, then that energy expanded into nothing. Is that possible physically? For instance, how could the universe expand from point A to point B, if point B didn’t exist yet?

    Looking forward to your reply.

  2. jcsd
  3. Apr 23, 2010 #2


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    These are some great questions! Unfortunately, you may not appreciate the answers. Theories do not have physical reasons. They are assumptions required to make sense of the world. So no one knows the physical reason that light moves at c OTHER THAN Special Relativity says it is so. You are free to come up with a better theory, but you will quickly see it makes no meaningful predictions that add to our understanding.

    Ditto with General Relativity. General Relativity predicted many new effects which have been subsequently witnessed.

    The properties of Spacetime defy explanations such as "made of something" or "made of energy". Again, current theory describes the world quite accurately although we may prefer more.
  4. Apr 23, 2010 #3
    This is not quantum physics but still..

    1) insert v = 1.0000001*c into the "gamma factor" and you will see what happens :)

    2) explaining the physical reality of space time is like a never ending quest, you will not get an answer here. Also time is highly physical -> se answer on question (3) below

    3) space consists of nothing, it does not follow that space time must consist of anything to be bend. The energy-momentum tensor in Einsteins field equations must have a certain metric - i.e structure.

    You can compare with the field lines from let's say a magnet or an electric dipole. If you sprinkle tiny pieces of metal dust to see how these fields looks like. If you sprinkle mass around a massive object - you will see how the gravitational field looks like. The difference is that the field lines around the magnet are lines IN space-time whereas the field lines around the massive object ARE space-time. The field lines does not consist of anything.

    (however there are some theories of quantum gravity that assigns quanta to space time just in the same way as Quantum field theory assigns photons as the quanta of electro-magnetism)

    So the answer depends, since you seemed to beg for the answer within Einstein Gravity - the answer is that what I wrote above, that the field lines does not consist of anything. But since there is research ongoing on the quantum nature of gravity - there might be quanta - particles - that build up these field lines of space-times also.

    4) this is also related to contemporary research, how did the universe evolve from an "object" with infinite size to the universe of today, and DID the universe evolve from an infinitesimal object at all?

    I hope that you want to take these questions seriously and pursue a career in modern physics towards quantum gravity :)

    best regards
  5. Apr 23, 2010 #4
    Also this is a recent thread about these topics:


    In the future, you might want to give your threads better names than "just a question" or "I have a few questions".. ALL threads are about questions you know ;)
  6. Apr 23, 2010 #5
    Taking an online course in the subject helps give perspective. Books as well. It is best to use the forums to answer more specific questions you did not understand while taking a course.
  7. Apr 23, 2010 #6
    about the invariance of light speed you must first taking account the fact that light is not a particle but a wave with an intrinsic wave lenght, said equal to L. You must also introduced the heisenberg indetermination equation which implies that the impulsion is at least egal to h/L and energy is at least egal to h x wave frequency.
    Since E2 = (mc2)2 + (pc)2 it implies for a pure wave with m=0 that
    c= L x wave frequency.
    If The wave celerity were not c, heisenberg relations would not be respected...
    Considered also that within two referential in relative move light property does not change (from inside the space way appeers shorter and from outside the time inside the second referential look elapses more slowly but c is invariant since there is none physical cause to an not equivalence between the 2 referentials
  8. Apr 24, 2010 #7
    two errors

    1) light is also a particle, the photon

    2) you are using (as many others) Heisenbergs uncertainty relation applied to a single event even though it applies to statists... i.e quantum STANDARD DEVIATION

    so the logical error is then that Heisenberg does not need to be fulfilled for single events
  9. Apr 24, 2010 #8

    Thanks for replying but there is none error
    first what is puzzling in quanta physics is that the fundamental reason of indetermination is not a statistic effect but a intrinsic indetermination what ever the number of event. Look back to Young experiment event when « particles » are sent one to one the interference is maintained. So Heisenberg indetermination inequality can be applied to a single event.
    You should always considered separately the propagation and the detection. Particle is an emergent effect of interaction. The propagation is always described as a wave (complex since phasis is included) thru schrodinger no-relativistic equation or Dirac equation with relativistic effect taken into account.
    Photon is the term used to characterize the interaction for instance for photoelectric effect but there is no particle speed.
    So the only way to justify invariance of c is the use both heisenberg relationships and telativist pythagorian equation linking impulsion and energy

    A precision c is not truly a celerity; it is a factor of dimension conversion like h and G. So it cannot be associated with the other observables like impulsion, time,energy,mass. There is the true reason why c is invariant in front of a referential change.
  10. Apr 24, 2010 #9
    1) you are mixing quantum mechanics with SR, SR does not assume QM to prove that c is max velocity

    2) derive HUP and you'll see that it does not apply to single events.
  11. Apr 24, 2010 #10
    I appreciate your frank and honest answer. Yet I'm stunned when you say that "Theories do not have physical reasons." Well, shouldn't it be the purpose of physicists to come up with theories with solid physical reasons? For instance I feel that General Relativity is lacking in that respect even though experiments have backed up its predictions. But -- as far as I'm concerned -- nobody has yet clearly explained what the space-time curvature is other than it is a geometric structure of space. But then I'm told that there isn't energy *of* space. So then space is a void. How can a void allow for a geometric structure? Maybe I'm just ignorant.

  12. Apr 24, 2010 #11
    Let's see: gammafactor=1/sqrt[1-(v/c)^2], so when v is greater than c then the time dilation is imaginary. What are you trying to explain?

    Your analogy is very good. But I think that the field lines do consist of something. It's just that physicists might not have the answer yet. There's a reason (probably deeply hidden in quantum physics) to those lines being there.

    Deep question indeed. So, I guess that physicists do not have an answer yet.

    I do, and if I had the financial means, I would go back at the university and pursue this further.

  13. Apr 24, 2010 #12


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    You're not ignorant. However, not everyone understands what theory development is about and how theories are constructed and compared. And why.

    Look at my tagline: the map is not the territory. Theories are maps. Good theories are useful maps. But do not confuse the map with what the map represents. They are completely different things. After a while you will realize that the universe is in fact the simplest and complete representation of itself. All models, on the other hand, make assumptions.

    So asking why a map does not do X or Y is not a very important question per se. Maps have purposes. Consider any flat map of the Earth and you will see this in practice: some accurately represent the relative area of the continents, but do not reflect the spherical shape of the planet well - and vice versa. So which is true?
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