HPS 0410 | Einstein for Everyone |

Back to main course page

Special Theory of Relativity: Adding Velocities

John D. Norton

Department of History and Philosophy of Science

University of Pittsburgh

The speed of light clearly has a special place in this theory. If something is traveling at the speed of light c, then all observers will find it to be traveling at exactly same speed.

A similar thing happens to things
traveling at less than the speed of light. If one observer finds an
object to be traveling at less than light, say, then so must every other. There
is no way that observers can change their states of motion so as to find the
object traveling at faster than the speed of light. And there is a similar
result for objects traveling at faster than the speed of light--if such things
exist. If one observer finds them traveling at faster than the speed of light,
then so must all.

All observers agree on which are: Things that travel slower than light. |
Things that travel at the speed of light. |
All observers agree on which are: Things that travel faster than light. |

One of light's most important roles as a limiting velocity
follows from this: no matter how hard we try, it is impossible to accelerate something through the speed of
light. More generally, the speeds of things are divided into three
groups:

--things that travel slower than light,

--things that travel at exactly the speed of light,

--and things that travel faster than the speed of light.

We cannot slow down or speed up anything so that it crosses the barrier of the
speed of light.

Yet it looks like it would be pretty
easy to violate the limiting character of the speed light by
accelerating something through the speed of light. We might have a gun that can
fire particles at, say, 2,000 miles per second. That is well below the speed of
light. We put the gun on a spaceship that we accelerate up to 185,000 miles per
second--a mere 1,000 miles per second short of the speed of light. If we fire
the gun in the direction of motion, would it not accelerate the particle
through the speed of light?

Is not 185,000 + 2,000 = 187,000?

The limiting character of the speed of light is sufficiently
striking for it to be worth seeing how it follows
from the principle of relativity.

To see it, let us set up the challenge quite solidly. Imagine that a machine that can fire particles at 100,000 miles per second, which is more than half the speed of light, 186,000 miles per second. |

Now we will try to push things past the speed of light. Imagine that the machine is placed on a spaceship that also moves at 100,000 miles per second in the direction that the machine fires the particles; that is, it moves at this speed with respect to a second observer on the earth. |

So, let us ask the obvious question. What will the
earth bound observer find for the speed of the particle? The calculation seems irresistible. The spaceship moves at 100,000 miles per second with respect to the earthbound observer; and the particle moves at 100,000 miles per second with respect to the spaceship. So... |

100,000 + 100,000 = 200,000 ??

But that would be faster than the speed
of light, 186,000 miles per second.

To see that the principle of relativity prohibits this faster than light outcome, imagine that a light signal passes the particle emitting machine at the moment that the particle is emitted. The observer moving with the machine would (obviously) judge that the light signal overtakes the particle. |

Now imagine this same process viewed by the the Earthbound
observer. That observer must also see the light signal overtake the particle. It is just the one experiment, so both observers must judge the same outcome. |

What else would you expect? Might it be that the light signal would overtake a particle emitted by the machine, when the machine is on earth. But when the machine emits on a rapidly moving spaceship, then the particle overtakes the light?

That is exactly what the principle of relativity prohibits! For then we have an experiment that can detect absolute motion. The resting machine emits particles that don't overtake light; the rapidly moving machine emits particle that do overtake light. The principle of relativity demands that the experiment must proceed in the same way when carried out on earth or a rapidly moving spaceship. | (For experts) Those who have read ahead might worry that each observer might find a different outcome, perhaps as an artefact of the relativity of simultaneity (below). That won't happen. Whether light overtakes the particle or not can be reduced to local facts independent of judgments of simultaneity. Imagine that the light signal and the particle are to traverse the same interval in space AB. Both depart A at the same moment--judged locally. If light outstrips the particle, it will arrive at B before the particle. That earlier arrival is once again a local fact that obtains just at point B. |

What this shows is that the principle of relativity prohibits us adding velocities in the usual way. We cannot add velocities by the ordinary rule 100,000 + 100,00 = 200,000. More generally, the classical rule for the composition of velocities fails:

Velocity of A with respect to C |
= | Velocity of A with respect to B |
+ | Velocity of B with respect to C |

In its place we need a new rule for the composition of velocities. It ought to look like the ordinary rule as long as velocities are small--we do know that the ordinary rule works for slow moving things like cars on freeways and trains. But it must look very different at high speeds. If we use it to add two velocities close to light, we must get a resultant that is still less than the velocity of light. Einstein found that the principle of relativity forces a particular rule. For the case of velocities oriented in the same direction in space, the relativistic rule for composition of velocities is:

Velocity of A with respect to C |
= | Velocity of A with respect to B |
+ | Velocity of B with respect to C |

__________________________________ | ||||

reduction factor |

All the work is done in this new rule by the reduction factor. When the velocities are small, this factor is close to 1. So it is as if it isn't really there and Einstein's rule just behaves like the classical rule. But when the velocities get to be close to that of light, the factor starts to get larger and larger and in just the right way to prevent any composition of velocities less than light exceeding that of light. | (For experts only) Click here to see the complete formula. |

If we use the rule to add 100 mph to 100 mph, the reduction factor is almost exactly one, so the ordinary rule works: 100 + 100 = 200.

If we use the rule for adding 100,000
miles per second to 100,000 miles per second, we are now dealing with
velocities that are 100,000/186,000 = 0.54 the speed of light. For that sum,
the reduction factor is 1.29, so the composition yields:

(100,000 +
100,000)/1.29 = 200,000/1.29 = 155,000

which is still less than the speed of light.

What is most instructive is to see what happens if we start with a velocity of 100,000 miles; and add 100,000 miles per second to it; and add it again; and again; and again.

To picture physically what we are doing, imagine that we start with our base machine "I" that happens already to be moving at 100,000 miles per second. From it we shoot out a second smaller version of the same machine--call it "II"--at 100,000 miles per second with respect to "I."

Now let's repeat the operation. From the smaller machine "II," we'll shoot out a yet smaller version of the same machine at 100,000 miles per second with respect to "II." Call it "III."

Then machine "III" will shoot out machine "IV"; and so on; and so on. As we pass through the series of machines "I," "II,", "III," "IV," etc., we are boosting each with a speed of 100,000 miles per second with respect to the one before.

The cumulative effect of the repeating boosting by 100,000 miles per second is shown below. The total speed of the last boosted machine increases as we proceed along the sequence "I," "II," etc. But the increases become smaller and smaller.

No matter how often we add 100,000 miles per second, we never get past the speed of light--here set at exactly 186,000 miles per second. We get closer and closer to it. But never past it.

One way to think of it is as an "Einstein tax," that copies the way a very severe progressive taxation might increase the amount of tax paid as we get more income. We keep adding 100,000 miles per second to the speed, but the Einstein tax--implemented through the reduction factor--precludes our total speed ever exceeding that of light. |

Public domain image from http://jet.wikia.com/wiki/File:Sm_stack_coins_gold.png

One way to think of it is as an "Einstein tax," that copies the way a very severe progressive taxation might increase the amount of tax paid as we get more income. We keep adding 100,000 miles per second to the speed, but the Einstein tax--implemented through the reduction factor--precludes our total speed ever exceeding that of light.

That the ordinary addition rule fails follows from the principle of relativity. Why does the ordinary rule fail? Here's a way to get comfortable with the the failure. In the original example, the spaceship observer uses rods and clocks that move with the spaceship to measure the speed of the emitted particle as 100,000 miles per second. The earthbound observer now wants to find the speed of the emitted particle. That observer, however, cannot directly use measurements made with the spaceship rods and clocks, for the earthbound observer thinks that they have shrunk and slowed. The earthbound observer must correct the spaceship observer's measurements for effects such as these. The result of the these corrections is Einstein's formula!

Let's try to put this together more explicitly. If a body is moving at 100,000 miles per second, then it will traverse a 100,000 mile rod in one second.

Now imagine that measurement is made by a spaceship observer who moves at 100,000 miles per second with respect to a planet. We are tempted to picture things as that body moving over *two* 100,000 mile rods laid end to end in the one second:

So would mean that the body covers 100,000+100,000=200,000 miles in one second.

What is wrong with this picture is that the two measuring rods belong to observers in relative motion. One belongs to the spaceship observer and the other to the planet observer. Each would judge the other's rod as *not* 100,000 miles long. Similarly the one second traversal time is measured by clocks belonging to observers in relativity motion. Each would judge the other's clock as running slow.

The upshot is that the attractive sum 100,000+100,000=200,000 requires us to mix quantities from different frames of reference that simply do not belong together.

This special role for the speed of light sometimes arouses special wonder. What is so special about light, we may be drawn to ask, that everything else takes such special note of it? Once one starts along this path, all sorts of confusions may arise. Is it that light is used for communication and finding things out? Does everything somehow respond to how we find things out? Does special relativity still work in the dark?

Well--you can forget all this mystical mumbo-jumbo, if ever it attracted you. There is nothing special about light. It's space and time that is special. They have properties we don't expect. Space and time are such that rapidly moving objects shrink and their processes slow down. For a long time, we didn't notice these effects because we did not have a thorough account of a probe of space and time that moves very fast. That changed in the nineteenth century when we developed good theories of light. It is the probe that moves very fast and, for the first time, begins to reveal to us that space and time are not quite what we thought.

There is one further fact about space and time. It harbors a special velocity, one that is the same for all inertial observers. It is an invariant (="unchanging") velocity. Light is just something that happens to go as fast as it possibly can and thereby ends up going at that speed.

There's nothing special about light. What is special is the speed at which it goes.

- Nothing can be accelerated through the speed of light.
- Adding velocities Einstein's way.

Copyright John D. Norton. January 2001, August 30, 2002, July 20, 2006; January 8 2007, January 3, August 21, 27, 2008, January 13, 201, January11, 201..