Sam Murphy

Project Storage


Twice the speed of light

The speed of light is as fast as things go, right?

A simple thought experiment indicates otherwise. The maximum speed in the universe maybe twice the speed of light

I am sharing this here because I have not heard a clear explanation as to why it is wrong. Maybe you can help?

2c or not 2c?

Imagine you are at a train station and a train whips by at 30 m/s. At the same time another train travels in the opposite direction at the same speed. After one second either train is 30 m from you but 60 m from each other.

trains passing by at 30 metres per second

Trains moving in opposite directions with you as the point of referene

Relative to you they both have a speed of 30 m/s but relative to each other it is 60 m/s. Let's make one of the trains the point of reference

train point of reference

If a train is the point of reference then the other train is moving twice as fast

More generally, if two trains have the same speed, x m/s, but are travelling in opposite directions then they have a relative speed of 2x m/s. This is because speed is relative. The speed of an object therefore depends on the point of reference.

Now imagine you are holding a lightbulb. Two photons are emitted at the same time in opposite directions. The photons have a speed of c relative to the lightbulb.

two photons travelling away from a lightbulb

Two photons travelling away from a lightbulb

If one of the photons is the point of reference then the other photon is travelling at 2c. In other words, at twice the speed of light.

photon point of reference

A photon can travel at 2c relative to another photon


Two photons travelling in opposite directions can have a relative speed of 2c. All speed is relative. Therefore the maximum speed in the universe is 2c.

Special Limit

Someone once suggested that whilst this might be true for trains it is not true for photons. If this is the case, is there a speed limit at which this logic no longer applies?

Noon & Midnight

Premise: Typically people use 12AM for midnight and 12PM for midday, and this is incorrect (😕)

There are 24 hours in day (obviously) but just 12 hours on a clock face. Usually it is clear which of the 12 hour cycles we are in. If someone says 4 o'clock we know it is in the afternoon or very early in the morning. We can make it explicity by using AM or PM. That is, AM is the 12 hours before midday and PM is the 12 hours after midday.

Here is an example, if we count from 1PM to 2PM then 3PM and so on up to 11PM then the next value should be 12PM (i.e. midnight)

A similar example, if we could count from 1AM to 11AM then the next value should be 12AM (i.e. midday)

I think what confuses people is they may not realize that we are allowed to have 0AM and 0PM

When we count backwards from 2AM to 1AM it is then 0AM (not 12AM!)

In other words, 0AM is 12PM, and in a similar way 0PM is 12AM