There is an interesting discussion on twitter why “nothing can move faster than light”. I could not resist myself from a simple explanation

I also think a similar one might have been given by Einstein himself, long time ago, so forgotten by most when they try to explain it.

How does something go faster !! by burning mass.

In case of a car to move faster and faster one has to burn the fuel, more mass in less time.

In case of light (photons) all of its energy goes into producing its kinetic energy which is therefore it’s speed. SO all it’s mass has been converted into energy that is its speed-energy.

(those who would misinterpret this line it equivalently reads as: all the energy available to the photon is in terms of it’s kinetic energy since it’s mass has to be zero)

To move at that fastest speed of Universe “c” photon has to be massless. Any more mass a photon has to carry, the mass will slow down the photon’s speed from “c” to a smaller and smaller value. That is why anything that is massive is by an “equation-proportionality” slower compared to “c” at the same energy available to a massless particle.

SO nothing can move faster than light because light particles are massless and any other kind of particle has some mass. If somehow these other things can attain the speed of light it just means they have lost all their mass into energy. They have to anyway be massless at the speed of light, “c”.

To move faster than that speed “c” where does this hypothetical particle get more mass which will be produced into energy to be used up towards speed? This particle while moving at speed “c” is already massless. The particle has to go negative in it’s mass to move faster than “c” because the energy available will then go towards speed and mass is negative so this would cause a speed larger than “c”. A negative mass means somebody else has to pay for this particle’s mass.

We don’t have such mass-banks in the world neither do we have negative mass and to add to that if this particle acquires the mass at the speed of light which will provide it more speed than the speed of light it still has to carry that mass, which will slow it down.

Therefore nothing can move faster than light.

## 4 thoughts

1. bert0001 says:

So, then what if a neutrino has a tiny mass say 0.1eV, then it does not go as fast as the light from the sun, and we could measure the difference in arrival time if we managed to couple one foton and one neutrino to one event. (which is of course impossible w<ith the sun as a source – but a supernova might be an interesting candidate for such an experiment. Or am i just dreaming?

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2. Mohan, mdashf says:

You are right. No matter how light unless something is completely massless in its restframe, even though it has a mass as tiny even as 0.1 eV it will move slower than the photon would. So if it were to come from sun and photons were to come from sun we would know this difference irrespective of they constitute one event or not. This is where the laws of Physics and the methods of experiment come in handy. They need not come from one event because they have to satisfy laws and methods. But easier if they are coming from Supernovae b/cos by that time the two would have traveled for so long that their travel-distance would be much larger even if they move slightly differently in terms of their speed.

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3. Mohan, mdashf says:

How to calculate the speed of anything when their speed becomes closer to the speed-of-light.

(to correct the comment I made ” unless something is completely mass-less in its rest-frame ” )

A fact I have realized lately (or rather trapped myself to commit an inconsistent remark) and better late than never; WHEN something is mass-less, it will never have a rest-frame, b/cos by Einstein’s transformation rules known as Theory of Relativity to be consistent this (mass-less) particle will always move at the speed of light no matter which frame you are looking at it from. This then leads to the velocity addition formula of Einstein.

According to Newton/Galileo known by a name Galilean Relativity: if C moves at speed speed_cb wrt B, B moves at speed speed_ba wrt A then C must move at speed speed_ca given by speed_ca = speed_cb + speed_ba. The speeds got added up. A bullet moved at speed 200 km/hr in a train and the train moves at 250 km/hr wrt Jason who is standing still on ground then ground is A, train is B and bullet is C. So Jason will see that the bullet will move at speed 200+250 km/hr.

Galilean Relativity: speed_ca = speed_cb + speed_ba.

But Einstein modified this to make it consistent with how it will work out for light or ant relay of information at that speed. To make light always move at speed c = 2.99 x 10 ^8 meter/sec the simple addition became “warped”. Its still known as addition of velocities or speed and given by speed_ca = [ speed_cb + speed_ba ] / [1 + speed_cb * speed_ba] where you will get the results in terms of speed of light being 1 and speed of anything else as a fraction given by speed_ca. You then simply multiply c = 2.99 x 10 ^8 meter/sec to that fraction to get the exact speed in terms of meter/second or km/hr etc. Not only you have to multiply this value to any result you obtain from this formula, you also have to divide this value 2.99 x 10 ^8 meter/sec before carrying out any calculation in order to use the formula given in this form. (B/cos literature will make it slightly more complicated but less attractive for actual Physicists to get consistent results in meter/sec or km/hr)

So Einstein’s Relativity: speed_ca = [ speed_cb + speed_ba ] / [1 + speed_cb * speed_ba] .

When this fraction [1 + speed_cb * speed_ba] is divided its no more a simple addition but a warped addition and known as Einstein Velocity Addition Formula.

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