# The Heisenburg Uncertainty Principle

Created | Updated Feb 21, 2003

The Uncertain Universe

The Uncertain Universe – The Heisenberg Uncertainty Principle

Who is Heisenberg?

Once described as "…the most outstanding theoretical physicist in Germany…" Werner Heisenberg was most definitely one of the leading scientists of the 20th Century.

What did he do?

Like some of the other physicists of his time, Werner was interested in a field of study called Quantum Physics. You can find out more about quantum physics and what a quantum is here. [link to quantum]

He suggested that - regardless of the precision with which they are measured - two so-called "complimentary variables" (related properties describing the physical state of a quantum) could not simultaneously be known with infinite precision . More precisely, he said that the sum of the errors of two complimentary variables must be almost equal to a constant known as "h bar" (the fundamental constant, h, discovered by Max Plank, divided by 2pi [link to pi]).

So What?!

The implications of his suggestion are that if - for example - one was to measure the momentum of a quantum with infinite precision, then we would know absolutely nothing about its position in the universe. Similarly in order to measure the total energy of a quantum with infinite precision, we must do so over an infinite length of time. Generally, the more we know about one variable, the less we know about the complimentary variable as a consequence.

The implications of such a statement are very great. Take, for example, a wave travelling from a musical instrument. We can represent a note being played for some finite length of time by the following diagram:

[link to figure 1]

Since the wave produced must have some real length then we cannot define exactly where the wave exists in space, as it is spread over some distance Dx (as shown). This might then lead us to wonder how accurately do we know the speed of the wave? Using a few fundamental relationships (including the de Broglie relationship, which you can find out more about here [link to de Broglie]), we arrive at the Heisenberg uncertainty principle which tells us that as soon as we know exactly where the wave is we no longer have any information about its speed (and hence momentum).

So, what DO we know about the universe?

Good question! Indeed, who are we to suggest that there exists only one? The so-called "multiverse theorem" addresses this question head-on. Currently championed by a man called David Deutsch, this idea suggests that whenever there is a decision to be made - or, more scientifically, a number of possible outcomes exist for one event - then the universe in which we are conscious will divide into the number of possible outcomes. We are only conscious. Deutsch's ideas on this theorem draw heavily from two other theories called remote superluminal interaction and observer induced collapse of wave-function. I intend to briefly outline those ideas to which I refer and then show how I believe they rely on the two latterly mentioned theories.

"…The key fact is that a real, tangible particle behaves differently according to what paths are open, elsewhere in the apparatus, for something to travel along and eventually intercept the tangible photon. Something does travel along those paths, and to refuse to call it 'real' is merely to play with words."

David Deutsch

The key idea in this quotation is that he believes that the behaviour of a quantum can be influenced by its surroundings; i.e. by the actions of an observer and by other quanta in its vicinity.

This simple idea has strong connections with the afore-mentioned remote superluminal interaction and observer induced collapse of wave-function theories. The much-contested remote superluminal interaction theory suggests that when two quanta must occupy two states simultaneously between them (for example, clockwise and anti-clockwise spin), then as soon as the state of one quantum is observed, then the state of the other quantum must be communicated to it faster than the speed of light. Einstein was famously unconvinced by this, as he had previously shown that the speed of light was an absolute upper limit throughout the universe. Similarly, observer induced collapse of wave-function suggests that in Young's double-slit experiment 4 a single quantum is simultaneously arranged such that it passes through both slits. However, as soon as we observe which slit it does pass through, we collapse the probability projection for each slit. Imagine a single photon approaching the slits; this theory suggests that essentially two photons exist - one passing through each slit. I propose that this then relates back to the multiverse theorem, since it is possible that the photon does in fact pass through both slits, and at this instant the universe divides in two, and we are left to observe the consequences within one universe.

Not much, then?

By way of conclusion, despite the scientific community's endless struggle to define and control uncertainty, including such theories as the Heisenberg uncertainty principle, we cannot say for sure that such efforts hold any relevance. Moreover, we may be basing our ideas and projections of uncertainty on a "pseudo-reality" which we in fact have no control over. Surely, then, uncertainty is forever infinite?