What is the equivalence principle of General Relativity and what does it mean?

Einstein introduced the equivalence principle by saying "We ... assume the complete physical equivalence of gravitational field and a corresponding acceleration of the reference system".

There are many ways of phrasing this but essentially what it is saying is that you can't tell the difference (or there is no difference) being in a gravitational field or 'being accelerated'. If you feel the force you feel normally standing on the Earth, you could be on the Earth ... or you could be in a rocket in deep space accelerating at 1g!

This can then be argued to imply that if you are in a freely falling reference frame, the laws of special relativity hold true for you in that frame (i.e if you are freely falling you don't feel any forces on you, 'a skydiver doesn't feel their weight').

The equivalence principle has important implications, first of all that gravity can bend light! This is the case because if you imagine a freely falling lift with a laser emitter and detector at either end of the lift, in the lift's frame of reference the light travels in a straight line from one side to the next (like in special relativity)! BUT if you change frame of reference and look from the outside into the lift, you see that the light must have travelled in an arc to leave the emitter and land at the detector. Gravity has bent the path of light!

Andrew M. A Level Physics tutor, GCSE Physics tutor, A Level Maths tu...

10 months ago

Answered by Andrew, who tutored A Level Physics with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£20 /hr

Eleanor S.

Degree: Physics (Masters) - Durham University

Subjects offered: Physics, Maths+ 2 more

Further Mathematics

“About Me: Hi, I'm Elle! Science, maths and languages have been my favourite subjects from a young age and studying them in further detail only made me love them more. I am soon to go into my second year of a physics degree at Durham ...”

£22 /hr

Ruby W.

Degree: Engineering Mathematics (Bachelors) - Bristol University

Subjects offered: Physics, Maths+ 3 more

Further Mathematics

“About Me: Hi, my name is Ruby and I am an engineering student at the University of Bristol. I love everything maths and science (especially maths), and teaching has been a passion of mine for many years. As a tutor, I am patient, tho...”

£20 /hr

Ayusha A.

Degree: BEng electrical and electronics engineering (Bachelors) - Newcastle University

Subjects offered: Physics, Maths+ 1 more

Further Mathematics

“About me: I am a final year Electrical and Electronic Engineering student at Newcastle University. I took Mathematics, Further Mathematics, Chemistry and Physics as my A-level subjects. I did peer mentoring in university and also have...”

About the author

Andrew M.

Currently unavailable: for regular students

Degree: PhD Physics (Doctorate) - Imperial College London University

Subjects offered: Physics, Maths


“Hi I'm Andy! I'm a PhD Physics student at Imperial College London, I've also got a Masters degree (Distinction) from Imperial and MSci (First Class Hons) in Theoretical Physics from the University of Glasgow.Basically, I love physics, ...”

You may also like...

Posts by Andrew

What is the equivalence principle of General Relativity and what does it mean?

Why does increasing the temperature of a gas increase the pressure (when the volume is fixed)?

Other A Level Physics questions

How can a car be changing velocity yet not changing speed?

How do capacitors work and what are its units?

Find the magnitude of the force on an electron that is travelling with velocity 2 x 10^4 ms^(-1) in the x direction through a uniform magnetic field of strength 2T in the y direction.

A sample of pure gold has a density of 19300 kgm^-3. If the density of a gold nucleus is 1.47x10^17Kgm^-3, discuss what this implies about the structure of the gold atom. [4 marks]

View A Level Physics tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss