Skip to main content

Gravitational redshift

Before I talk about gravitational redshift, let me just briefly talk about doppler shift.

Doppler shift is a high school concept that most of us have learnt at some point or the other in our lives. If you haven’t learnt it, safe to say you’ve surely experienced it. Say you’re on a platform and a train is approaching you. The train is sounding a horn, to catch the attention of passengers on the platform. The increase in the pitch of the horn as the train is approaching you and the decrease in pitch as the train is moving away is the phenomenon of doppler shift.

The same can happen to light. Say a source of light is moving towards you, the frequency you perceive the light to be at is not the same as the frequency of light emitted by the source. This is because of the fact that a clock in the frame of the source ticks slower than your clock, which is at rest. This is the phenomenon of time dilation.

Venturing further now, the Schwarzschild metric describes the spherically symmetric curved space time around a mass. It is the most general vaccum solution. The metric is as follows

Euclidean space in cartesian coordinate system has a metric of the form diag. A matric describes space or spacetime of a manifold, in a specific coordinate system.

Now, let’s say that there is a source at who is emitting radiation and there is an observer at who is receiving the radiation. i.e they are separated radially. Now let’s say that the source emits two pulses of light at and which the observer perceives at and

Given the metric, we can write




Similarly for the second pulse




meaning that for an observer at r = , the time difference between the two events at the source and at the observer are the same.

Now, let’s talk about how clocks at and tick i.e



we can convert the time difference to frequency and therefore energy, therefore changing the above equation to

So, for the case , it can be seen that i.e for a photon emitted at , by the time it reaches , it would have lost a certain amount of energy.

I would like to remind you again that this is curved spacetime and not flat spacetime. In qualitative terms, in a central gravitational potential, a photon will lose energy as it climbs out of the potential.

An interesting nugget of information is the physical implication of clocks in curved spacetime. The spacetime around earth is curved and clocks in space i.e on satellites run a bit slower than a clock on earth. And this difference is crucial to calibrate GPS signals, without which the accuracy with which one can pin point their location becomes abysmal.

This article was written using StackEdit1.


  1. StackEdit is a full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.

Popular posts from this blog

Animation using GNUPlot

Animation using GNUPlotI've been trying to create an animation depicting a quasar spectrum moving across the 5 SDSS pass bands with respect to redshift. It is important to visualise what emission lines are moving in and out of bands to be able to understand the color-redshift plots and the changes in it.
I've tried doing this using the animate function in matplotlib, python but i wasn't able to make it work - meaning i worked on it for a couple of days and then i gave up, not having found solutions for my problems on the internet.
And then i came across this site, where the gunn-peterson trough and the lyman alpha forest have been depicted - in a beautiful manner. And this got me interested in using js and d3 to do the animations and make it dynamic - using sliders etc.
In the meanwhile, i thought i'd look up and see if there was a way to create animations in gnuplot and whoopdedoo, what do i find but nirvana!

In the image, you see 5 static curves and one dynam…

on MOOCs.

For those of you who don't know, MOOC stands for Massively Open Online Course.

The internet is an awesome thing. It's making education free for all. Well, mostly free. But it's surprising at the width and depth of courses being offered online. And it looks like they are also having an impact on students, especially those from universities that are not top ranked. Students in all parts of the world can now get a first class education experience, thanks to courses offered by Stanford, MIT, Caltech, etc.

I'm talking about MOOCs because one of my new year resolutions is to take online courses, atleast 2 per semester (6 months). And I've chosen the following two courses on edX - Analyzing Big Data with Microsoft R Server and Data Science Essentials for now. I looked at courses on Coursera but I couldn't find any which was worthy and free. There are a lot more MOOC providers out there but let's start here. And I feel like the two courses are relevant to where I …

On programmers.

I just watched this brilliant keynote today. It's a commentary on Programmers and the software development industry/ecosystem as a whole.



I am not going to give you a tl;dr version of the talk because it is a talk that I believe everyone should watch, that everyone should learn from. Instead, I am going to give my own parallel-ish views on programmers and programming.
As pointed out in the talk, there are mythical creatures in the software development industry who are revered as gods. Guido Van Rossum, the creator of Python, was given the title Benevolent Dictator For Life (BDFL). People flock around the creators of popular languages or libraries. They are god-like to most programmers and are treated like gods. By which, I mean to say, we assume they don't have flaws. That they are infallible. That they are perfect.
And alongside this belief in the infallibility of these Gods, we believe that they were born programmers. That programming is something that people are born wit…