The discussion centers on the relationship between time dilation and dark matter, specifically questioning whether time is slower in our galaxy compared to intergalactic space due to gravitational effects. Participants clarify that while gravitational time dilation occurs near black holes, its impact on measurements of distant galaxies is negligible, with calculations showing minimal differences in time perception. The consensus is that time dilation does not significantly contribute to the understanding of dark matter or galactic rotation rates, which are primarily explained by Newtonian gravity and the distribution of visible matter.
PREREQUISITESAstronomers, astrophysicists, and students of physics interested in the interplay between time, gravity, and dark matter in galactic structures.
Dark matter is postulated to explain the rate of galaxy rotation, which is higher than expected if we consider only visible matter. See here, for example:AndrewDes said:If time slows down near a black hole then doesn't it stand to reason that time is slower in our galaxy than inbetween galaxies. If that's the case, wouldn't our measurements of distant galaxies be over estimated do to time being faster intergalacticly. Would this time dilation help explain part of dark matter?
Not to any significant extent. Even super-massive black holes have negligible time dilation effects at this distance.AndrewDes said:If time slows down near a black hole then doesn't it stand to reason that time is slower in our galaxy than inbetween galaxies. If that's the case, wouldn't our measurements of distant galaxies be over estimated do to time being faster intergalacticly.
No. Galactic rotation rates vary with distance from the core because they aren't rigid discs, and the difference between their observed rotation profile and the prediction based on visible matter only isn't a constant factor, which it would need to be if it were to be explained by a mis-calibration of our clocks.AndrewDes said:Would this time dilation help explain part of dark matter?
Just to put a number on this - if you replaced the Sun with a million-solar-mass black hole, Earth clocks would tick 1% slow compared to a clock in intergalactic space (i.e., in the time it took an Earth clock to tick an hour, a clock in intergalactic space would tick an extra 3.6s). If, instead, you replaced Alpha Centauri with that same black hole then Earth clocks would tick something like 0.000005% slow (if I counted zeros right that means the intergalactic clock would get less than an extra millisecond per Earthly hour). The galactic core is a long way further away than that.Ibix said:Not to any significant extent. Even super-massive black holes have
Im suggesting that our preception of time is being slowed because of the combined energy well of our entire galaxy, not just the black hole. Even if it's a small difference between our time and intergalactic Time, it would have a compounded effect on something we are perceiving as being billions of light years away.Ibix said:Just to put a number on this - if you replaced the Sun with a million-solar-mass black hole, Earth clocks would tick 1% slow compared to a clock in intergalactic space (i.e., in the time it took an Earth clock to tick an hour, a clock in intergalactic space would tick an extra 3.6s). If, instead, you replaced Alpha Centauri with that same black hole then Earth clocks would tick something like 0.000005% slow (if I counted zeros right that means the intergalactic clock would get less than an extra millisecond per Earthly hour). The galactic core is a long way further away than that.
No it wouldn't. And, if you say it does, then let's see your calculations.AndrewDes said:Im suggesting that our preception of time is being slowed because of the combined energy well of our entire galaxy, not just the black hole. Even if it's a small difference between our time and intergalactic Time, it would have a compounded effect on something we are perceiving as being billions of light years away.
I don't have the math skills to do such a calculation but if a million solar mass object replacing our sun would slow time down 1%, then looking at the 1.5 trillion solar mass of the milky way as 1 object would slow time down significantly more as compared to space unaffected by gravity. Also speed affects Time, that's why the ISS time travels. So wouldn't the speed at witch our planet is moving+the speed the sun is moving+the speed the galaxy is moving all add up to some effect? Combine our relative speed with being in that 1PeroK said:No it wouldn't. And, if you say it does, then let's see your calculations.
AndrewDes said:I don't have the math skills to do such a calculation
One million solar masses at one A.U. distance versus 1.5 trillion solar masses at some 25,000 light year average radius?AndrewDes said:I don't have the math skills to do such a calculation but if a million solar mass object replacing our sun would slow time down 1%, then looking at the 1.5 trillion solar mass of the milky way as 1 object would slow time down significantly more as compared to space unaffected by gravity.
No. Unless you can do the math, chattering on about small things adding up to big things does not cut it. Sometimes they do. Sometimes they don't. Math is what can tell us which it is.Also speed affects Time, that's why the ISS time travels. So wouldn't the speed at witch our planet is moving+the speed the sun is moving+the speed the galaxy is moving all add up to some effect? Combine our relative speed with being in that 1
5 trillion SM gravity well and we must be getting into some sort of significant difference compared to being stationary in intergalactic space.
No - a (relatively) tiny volume around a black hole is the only place in the galaxy where time dilation is at all significant. All of the rest of the dynamics is well explained by Newtonian gravity, meaning that time dilation is completely negligible.AndrewDes said:Im suggesting that our preception of time is being slowed because of the combined energy well of our entire galaxy, not just the black hole.
I don't know why you'd think that. Typical gravitational time dilation calculations (like the one I did) compare a clock at some finite distance from a massive body to a clock at infinity. There's no further compounding to be done.AndrewDes said:Even if it's a small difference between our time and intergalactic Time, it would have a compounded effect on something we are perceiving a
A 1.5 trillion solar mass object compressed into a few trillion kilometers diameter would cause significant time dilation for objects orbiting close to it, yes. The galaxy is much, much, much more spread out than that, and consequently its gravity is much less intense. Nowhere is its gravitational potential so high that you get significant time dilation.AndrewDes said:if a million solar mass object replacing our sun would slow time down 1%, then looking at the 1.5 trillion solar mass of the milky way as 1 object would slow time down significantly more as compared to space unaffected by gravity.
That's a serious oversimplification, but for orbital motion it's close enough.AndrewDes said:Also speed affects Time,
Nothing significant. Stellar speeds are nowhere close to the speed of light, so the effect is negligible again since kinematic time dilation is proportional to ##1/\sqrt{1-v^2/c^2}##.AndrewDes said:So wouldn't the speed at witch our planet is moving+the speed the sun is moving+the speed the galaxy is moving all add up to some effect?
Most of the mass of the Milky Way is too far away to entail gravitational time dilation on Earth. The Earth is not in a deep gravitational well. In this respect, the calculations are everything.AndrewDes said:I don't have the math skills to do such a calculation but if a million solar mass object replacing our sun would slow time down 1%, then looking at the 1.5 trillion solar mass of the milky way as 1 object would slow time down significantly more as compared to space unaffected by gravity. Also speed affects Time, that's why the ISS time travels. So wouldn't the speed at witch our planet is moving+the speed the sun is moving+the speed the galaxy is moving all add up to some effect? Combine our relative speed with being in that 1
5 trillion SM gravity well and we must be getting into some sort of significant difference compared to being stationary in intergalactic space.
AndrewDes said:dosnt it stand to reason that time is slower in our galaxy than inbetween galaxies.