When an object gets pulled into a black hole it seems to slow and stop, but could it be possibly be because the speed of light that hit the object and came back was slowing down as the object got closer and not that time was messed up?

  • $\begingroup$ Time is that which the clock shows. You think a clock gets messed up in a black hole? :-) $\endgroup$
    – CuriousOne
    Commented Apr 9, 2016 at 3:24
  • $\begingroup$ Isaac In other words, on the boundary of a black hole not only the speed of the light is zero, but the speed of the attracted matter is it too? $\endgroup$ Commented Apr 9, 2016 at 4:31
  • $\begingroup$ Say we saw a clock fall into a blackhole, it would seem to speed up as it got closer, but to the clock it is normal, however it might be possible the light hitting the clock and bouncing off then returning to us has slowed down, Making it seem time is going faster for the clock when really the light particles have slowed down. $\endgroup$ Commented Apr 9, 2016 at 11:03

1 Answer 1


"Does a black hole really slow down time?"

No. Gravitational time dilation is an absurd concept. General relativity predicts that gravitational time dilation occurs even in a HOMOGENEOUS gravitational field ("the homogeneous gravitational field is the gravitational field which, in every point, has the same gradient of the potential. Such a field is produced by an infinite material plane with the constant surface density of mass"). That is, two clocks at different heights are in EXACTLY THE SAME immediate environment (experience EXACTLY THE SAME gravitational field) and yet one of them ticks faster than the other. Absurd isn't it? Effect (difference in the ticking rate) without any physical cause.

Photons accelerate in a gravitational field, just as ordinary falling objects do, and this variation of the speed of light causes the gravitational redshift (or blueshift):

http://courses.physics.illinois.edu/phys419/sp2013/Lectures/l13.pdf University of Illinois at Urbana-Champaign: "Consider a falling object. ITS SPEED INCREASES AS IT IS FALLING. Hence, if we were to associate a frequency with that object the frequency should increase accordingly as it falls to earth. Because of the equivalence between gravitational and inertial mass, WE SHOULD OBSERVE THE SAME EFFECT FOR LIGHT. So lets shine a light beam from the top of a very tall building. If we can measure the frequency shift as the light beam descends the building, we should be able to discern how gravity affects a falling light beam. This was done by Pound and Rebka in 1960. They shone a light from the top of the Jefferson tower at Harvard and measured the frequency shift. The frequency shift was tiny but in agreement with the theoretical prediction."

http://www.einstein-online.info/spotlights/redshift_white_dwarfs Albert Einstein Institute: "One of the three classical tests for general relativity is the gravitational redshift of light or other forms of electromagnetic radiation. However, in contrast to the other two tests - the gravitational deflection of light and the relativistic perihelion shift -, you do not need general relativity to derive the correct prediction for the gravitational redshift. A combination of Newtonian gravity, a particle theory of light, and the weak equivalence principle (gravitating mass equals inertial mass) suffices. (...) The gravitational redshift was first measured on earth in 1960-65 by Pound, Rebka, and Snider at Harvard University..."

  • $\begingroup$ If gravitational time dilation is absurd, and light speeds up as it falls, we should measure a variable speed of light and not a blue shift, which is exactly backwards of the observations. Explain that one. $\endgroup$
    – Asher
    Commented Apr 9, 2016 at 16:43
  • $\begingroup$ "we should measure a variable speed of light and not a blue shift". The blue shift is a measure of the increase in the speed of light - see the two quotations. $\endgroup$ Commented Apr 9, 2016 at 16:51
  • $\begingroup$ It's a measure of the change in frequency, which effects the color. But to measure the frequency we must know the speed and wavelength: so which does gravity change - speed, frequency, or wavelength? And if it changes more than one at a time, how? $\endgroup$
    – Asher
    Commented Apr 9, 2016 at 16:55
  • $\begingroup$ "But to measure the frequency we must know the speed and wavelength". No the frequency shift is measured without knowing speed or wavelength, but then we have two hypotheses: 1. The frequency shift is due to a speed change; 2. The frequency shift is due to a wavelength change. The former hypothesis is the correct one (in my view). $\endgroup$ Commented Apr 9, 2016 at 17:07
  • $\begingroup$ Let me rephrase: the frequency has no meaning unless we know the speed or wavelength. Nevertheless, "your view" that the speed changes is not experimentally verified. The frequency change is due to a gain of energy of the photon, just like the changing speed of a falling object is due to a changing energy. Falling massive objects gain velocity, falling light has a constant velocity but "gains" frequency. This is what experiment in the real world shows. $\endgroup$
    – Asher
    Commented Apr 9, 2016 at 20:30

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