giving you the chance to observe so many little wonders the winter chill may have left us for spring-like temperatures but don't forget to pack your gear so you don't end up freezing to death On the menu: telescope and naked-eye sky observation evenings stories and legends about the constellations and other surprises for the whole family Find out more about the program in Paris and Île-de-France, on the AFA website Refer your establishment, click herePromote your event, click here The problem with saying autism is underdiagnosed in girls is that Issue no. 3541 published 3 May 2025 Issue no. 3537 published 5 April 2025 say our readers - not only would it require huge amounts of energy but it would have a disastrous effect on marine life but there must be lots of cold water in the depths Couldn’t we just stick a giant egg whisk down there and stir things up a bit to solve the warming problem The cold water at depth is denser than the warm water higher up and therefore it requires energy to mix the layers it will then fall right back down to the depths unless it is mixed with warm water Gravitational waves arise when very heavy objects spiral into each other Do I create one when I crash into the sofa after a day of work you create gravitational waves – there is no lower limit your gravitational influence on other objects changes slightly That change travels outwards from you at the speed of light Most of that influence dies out drastically with distance but a tiny amount doesn’t and continues to propagate as waves You create a gravitational wave by collapsing onto the sofa gravity is by far the weakest of the four fundamental forces of nature Masses create indentations on the fabric of space-time If any of these masses accelerate –acceleration includes slowing down and changing direction as well as speeding up – they generate gravitational waves on the fabric Gravitational waves are the gravitational equivalent of electromagnetic waves The first experimental confirmation that these waves exist came in 2015 when the Laser Interferometer Gravitational-wave Observatory (LIGO) detected some that were generated when two black holes 1.3 billion light years away spiralled into each other Each was about 30 times the mass of the sun and merged to form an even bigger black hole you create gravitational waves that travel at the speed of light Gravitational waves cause the fabric of space-time to stretch in one direction and contract in the perpendicular direction The LIGO detectors were only just sensitive enough to detect this disturbance – a change in length of one part in 1021 If the merger of the two black holes had happened as close to us as the sun then the diameter of Earth would have changed by just 1 metre and the height of a typical person by about 150 nanometres too small a change to be picked up by our senses To answer this question – or ask a new one – email lastword@newscientist.com Questions should be scientific enquiries about everyday phenomena and both questions and answers should be concise We reserve the right to edit items for clarity and style daytime telephone number and email address New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format You can also submit answers by post to: The Last Word Terms and conditions apply More reasons why light passes through some substances but reflects from others Why does light reflect in a mirror but go straight through glass The previous answer was wrong to claim materials are transparent or reflective depending on whether they have a crystalline structure and this has nothing to do with crystalline structure Amorphous materials aren’t necessarily transparent – take obsidian Crystalline materials are often transparent Why do dogs like to shake themselves dry besides a person rather than doing this as soon as they get out of the water describing the physics of the motion that dogs use to shake water off their wet fur referred to the “centripetal” force caused by the movement “Centripetal” means seeking the centre whereas “centrifugal” means fleeing the centre it is the centrifugal force that causes the water to go away from the dog Centripetal force is what holds the water on the dog From a thimbleful of water to a massive tank and an entire sea – would these still be influenced by the pull of gravity from the moon and sun What is the smallest body of water in which tides can be detected This sea doesn’t have a natural oscillation period related to any lunar or solar period I worked in the ship division of the UK’s National Physical Laboratory The facilities included a massive indoor towing tank (sadly long since demolished to make way for a superstore) almost 8 metres deep and contained more than 45 million litres of water Scale models of ships and other marine vehicles were towed down the tank by a motorised carriage at precise speeds so that resistance and propulsive efficiency could be measured in order to help refine hull and propeller designs There was great interest in any disturbance to the supposedly static water because this might introduce errors to the measurements so water level fluctuations and any currents were carefully measured The main components of these disturbances were residual currents which might persist for days following the towing of a large model my recollection is that a semi-diurnal slosh due to the tides could also be clearly seen in the data from the tank It depends on the sensitivity of the detector and the absence of confounding factors like wind blowing the water will respond to gravity and will therefore be influenced by the moon Detection then depends on how much you are prepared to spend on fancy equipment Readers provide more insight into the puzzle of the zigzag path of larger rising bubbles – a phenomenon that has vexxed people since the time of Leonardo da Vinci In carbonated water, larger bubbles oscillate as they float upwards. What causes this, and why is it only big bubbles? (continued) Larger bubbles have lower pressure within them than smaller ones making them less stiff and so more vulnerable to distortion as they flow through the liquid The distortion causes the upward motion of the bubbles to deviate from a straight line This effect is readily seen in soap bubbles blown in air but large ones can assume non-spherical shapes and tend to waver which is related to the speed of an object and the viscosity and density of the fluid it is in vortices are shed by a bubble alternately to one side then the other The fluid simply moves symmetrically around the sphere Larger bubbles have larger Reynolds numbers as they have both a larger diameter and a higher speed Issue no. 3415 published 3 December 2022