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View Full Version : Testing relativity with comet ISON

jfribrg
2013-Oct-14, 06:39 PM
I saw on this web page (http://www.cometison2013.co.uk/) that when it reaches perehelion on November 28, comet ISON (C2012/S1) will be traveling at a speed of 845,000 mph. Ths is about 0.001c. This is a rather small number, but could it be large enough to (yet again) test the predictions of SR ? The effect would be tiny, and due to the proximity of the Sun, would have to incorporate GR in the calculations, but I'm wondering if there is anything that could be observed with enough precision to detect the relativistic effects?

Hornblower
2013-Oct-14, 08:45 PM
The difference between predicted trajectories in relativistic and Newtonian models should be straightforward to calculate. The challenge would be to disentangle it from nongravitational effects such as strong outgassing of volatiles from a body of unknown composition and shape, provided any solid body even survives the perihelion passage.

Jens
2013-Oct-14, 11:16 PM
I saw on [url=http://www.cometison2013.co.uk/]calculations, but I'm wondering if there is anything that could be observed with enough precision to detect the relativistic effects?

Which effects? I don't think time dilation would be possible, because there's no clock on it. Length contraction and mass increase would seem difficult because we don't really know it's length and mass very precisely. What were you thinking of measuring?

schlaugh
2013-Oct-15, 12:42 AM
Measuring length contraction would require some amazingly precise instruments. From the Wikipedia article on length contraction:

At a speed of 13,400,000 m/s (30 million mph, 0.0447c), the contracted length is 99.9% of the length at rest; at a speed of 42,300,000 m/s (95 million mph, 0.141c), the length is still 99%.

Relativity of simultaneity would also be almost impossible to measure, again due to lack of adequate observational equipment, both in terms of precision and placement.

jfribrg
2013-Oct-15, 02:10 AM
Which effects?
If I knew which effects, I wouldn't have had to post the question :) I was thinking that maybe the frequency of reflected light could serve as a clock and measure the (admittedly tiny) blue or red shifted spectra, or maybe the small space-time distortions would accumulate and cause the comet to reach perihelion a few seconds off from the time predicted by Newtownian dynamics. Not being an expert on Relativity, I was wondering if there is some other measurement that could be used as a clock or maybe GR could be tested in a similar way as the famous eclipse experiment, perhaps by using the parallax between Earth-based observations and observations from an existing space probe or satellite.

Hornblower
2013-Oct-16, 02:27 AM
For a moving light source the redshift or blueshift in the relativity model will be different from that in the Newtonian model, but I don't think the difference would be apparent with a comet at .001c. Length contraction, even if observable on a changing fuzzy object like this, would be inconclusive because we have no idea what it would look like if stationary. As I pointed out before, the difference in the trajectory would be a confirmation if it were not masked by jet action from outgassing volatiles.

I cannot imagine any observational tests that have not already been done better with objects that are more reliable than a comet.

blueshift
2013-Oct-20, 02:20 PM
Even at .990 c you won't have much effect. Gamma must be at least .995 before SR effects begin to be noticeable.

What does occur when looking at comets is to give us clues as to which elements are the best temperature sensors. When I put my spectroscope on the tail of ISON last winter two stars were conveniently located so that they were aligned with my spectroscope, the comet tail and both stars, whose light was exciting the elements in the tail. If I were able to maintain that alignment and follow the comet all the way to the sun and back out, I would find cyanogen gases changing their quantum states on the approach toward perihelion, going from rattling electrons in relation to each other, then from each other to the nucleus at hotter temperatures and then rattling the nucleus itself into higher energy states at perihelion. I would then be observing things going in the opposite order on the way back toward aphelion.

So the only relativistic effects occur here. The spaces between all fermions and one another along with the space between them and the hadrons and the space between the hadrons will differ from one observer to another but the rate of change of quantum states will not differ, which is what matters.

Anyway, that is my two cents. But don't take my word for it - investigate!

lpetrich
2013-Oct-20, 03:02 PM
It's easy to estimate the relative size of relativistic time-dilation effects. Use (1/2)*(v/c)2. For ISON, that's about 8*10-7. It does have some clocks: the materials that it's composed of. If an atom or molecule jumps down from an excited state, it can release a photon which we can then observe. The comet's time dilation will produce a redshift that's approximately what motion away by 240 m/s will produce. That's close to the velocities of the molecules in its released gases, so it's going to be awfully hard to observe that effect.

blueshift
2013-Oct-20, 03:39 PM
It's easy to estimate the relative size of relativistic time-dilation effects. Use (1/2)*(v/c)2. For ISON, that's about 8*10-7. It does have some clocks: the materials that it's composed of. If an atom or molecule jumps down from an excited state, it can release a photon which we can then observe. The comet's time dilation will produce a redshift that's approximately what motion away by 240 m/s will produce. That's close to the velocities of the molecules in its released gases, so it's going to be awfully hard to observe that effect. Blueshift or redshift has nothing to do with special relativity. Doppler differs from relativity. Anything redshifted has no relationship to special relativity at all or we would have to falsify speed c as being the cosmic speedometer. If you try and hang on to your view you are ignoring the effects of spatial expansion. Many galaxies are receding at speeds above c but that does not violate relativity because the objects are not passing one another. Even with transverse motions one can measure the waves emitted from a distant light bulb separating from one another at double c but that doesn't violate special relativity either or have anything to do with SR.

Only in general relativity does redshift matter and only in GR can refraction match up with time dilation. We measure light approaching us at speed c from the sun but an observer at the Moon measures that same light approaching our eyes as being refracted and slowed down. However, from our view we get faked out because our gravitational field slows time by just the right amount and so we measure the light at c.

Hornblower
2013-Oct-21, 01:05 AM
Even at .990 c you won't have much effect. Gamma must be at least .995 before SR effects begin to be noticeable.

What does occur when looking at comets is to give us clues as to which elements are the best temperature sensors. When I put my spectroscope on the tail of ISON last winter two stars were conveniently located so that they were aligned with my spectroscope, the comet tail and both stars, whose light was exciting the elements in the tail. If I were able to maintain that alignment and follow the comet all the way to the sun and back out, I would find cyanogen gases changing their quantum states on the approach toward perihelion, going from rattling electrons in relation to each other, then from each other to the nucleus at hotter temperatures and then rattling the nucleus itself into higher energy states at perihelion. I would then be observing things going in the opposite order on the way back toward aphelion.

So the only relativistic effects occur here. The spaces between all fermions and one another along with the space between them and the hadrons and the space between the hadrons will differ from one observer to another but the rate of change of quantum states will not differ, which is what matters.

Anyway, that is my two cents. But don't take my word for it - investigate!

Blueshift or redshift has nothing to do with special relativity. Doppler differs from relativity. Anything redshifted has no relationship to special relativity at all or we would have to falsify speed c as being the cosmic speedometer. If you try and hang on to your view you are ignoring the effects of spatial expansion. Many galaxies are receding at speeds above c but that does not violate relativity because the objects are not passing one another. Even with transverse motions one can measure the waves emitted from a distant light bulb separating from one another at double c but that doesn't violate special relativity either or have anything to do with SR.

Only in general relativity does redshift matter and only in GR can refraction match up with time dilation. We measure light approaching us at speed c from the sun but an observer at the Moon measures that same light approaching our eyes as being refracted and slowed down. However, from our view we get faked out because our gravitational field slows time by just the right amount and so we measure the light at c.
I am sorry, but you appear to be mistaken. An emitter in purely transverse motion across your line of sight, with no Doppler effect at all, has a relativistic redshift given by:

1 + z = 1/sqrt(1 – v^2/c^2)

If I understand correctly, this is an artifact of time dilation. In an inertial frame of reference with negligible gravitational effects this is a simple SR calculation. For a hypothetical v = 0.99c, I get a very strong z = 6.09, which I think would be noticeable. For hydrogen emission it would shift the Lyman lines into the visible range and the Balmer lines far into the infrared.

If the object is very close to the Sun, then yes indeed we would need to do the full blown GR calculation to get really accurate redshift and to account for the relativistic advance of perihelion.

I trust the Wiki articles on this because they appear to be in good agreement with my college physics course work.

http://en.wikipedia.org/wiki/Redshift

blueshift
2013-Oct-21, 11:12 AM
I am sorry, but you appear to be mistaken. An emitter in purely transverse motion across your line of sight, with no Doppler effect at all, has a relativistic redshift given by:

1 + z = 1/sqrt(1 – v^2/c^2)

If I understand correctly, this is an artifact of time dilation. In an inertial frame of reference with negligible gravitational effects this is a simple SR calculation. For a hypothetical v = 0.99c, I get a very strong z = 6.09, which I think would be noticeable. For hydrogen emission it would shift the Lyman lines into the visible range and the Balmer lines far into the infrared.

If the object is very close to the Sun, then yes indeed we would need to do the full blown GR calculation to get really accurate redshift and to account for the relativistic advance of perihelion.

I trust the Wiki articles on this because they appear to be in good agreement with my college physics course work.

http://en.wikipedia.org/wiki/RedshiftAh, thanks for putting that up. The transverse motion I was referring to involves a nearby object.

The distance of the objects in the Wikipedia example are of great distances and ISON is not. In the case you brought up from the link the object is not in our solar system and relativistic Doppler does apply and your equation fits that condition.