In this experiment, we will make our arms feel weightless using 45 seconds of strenuous physical effort.

Equipment
A friend willing to help.

You can also perform the experiment by yourself, if you stand in a place narrow enough that it will prevent your arms from rising up. Watch the video for a demonstration.

Experimental Plan
You can watch the experiment in the following video:

Explanation
Our body is constantly sending our brain information regarding its state by using its senses. The brain processes the incoming information, and this way it knows and understands in what state the body is currently in.

In order to prevent confusion from an excess of information, our brain and entire nervous system know how to disregard the information that does not change over time. For instance, we only sense the shirt we are wearing for a few seconds after wearing it. Within a very short time, that sensation disappears, and we do not notice the touch of the fabric on our skin. The same happens when we visit the hospital – at first we smell a strong scent of disinfectants and drugs, but soon enough we imagine it disappears. The smell does not really disappear, of course – our nervous system (the olfactory sensors in the nose and the processing centers in the brain) simply ignores it as soon as it realizes it does not change over a sufficiently extended period of time – so then we believe the smell has disappeared.  
 

In the current experiment, the arms attempt to rise up, but do not manage to do so despite the great effort our muscles are making, acting as if they were extremely heavy. This is what is happening in the first part of the experiment. If we carry on the experiment long enough, our brain adjusts to the feeling of “heaviness”, and decides it is the normal situation.

The moment our hands are no longer held in place, a sharp transition in the information coming from the senses to the brain occurs – the arms send the regular feeling, that they are no longer heavy. However, since the brain has already adjusted to feeling the heavy weight on the arms, it interprets the situation as if the arms are weightless – compared to the past, of course. 

In this situation, the arms sometimes actually rise up by themselves, since the brain is still sending a message to the muscles in the “heavy” arms to apply a force upwards and sideways in order to stay at their normal distance from the body. After a few seconds, the brain readjusts to the natural weight of the arms, and the normal everyday sensation returns to them – causing the illusion to disappear.

 

Extra Information
Adaptation, habituation and the comparison of sensations are the fundamentals of the nervous system. As in this experiment, they can sometimes give us strange sensations. For example, if you wear a hat on your head for a long time, you will still feel as if it is there a few moments after taking it off.

 

Dr. Avi Saig
Department of Neurobiology and Davidson Institute for Science Education
Weizmann Institute of Science


A note to the readers
If you find the explanations unclear or have further questions, please drop us a line on the forum. We welcome your comments, suggestions and feedback.

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