91色情片

I am at the rear wall of my spaceship travelling at the speed of light. I switch on my flashlight. Will the light reach the front wall?

Herman D鈥橦ondt
Sydney, Australia

The quick answer to this question is simple: the scenario is impossible. Only massless objects can travel at the speed of light (c). A spacecraft obviously has mass, and hence can never travel at c. Let鈥檚 assume that you aren鈥檛 going quite that fast, say only 0.99999 c. Albert Einstein鈥檚 theory of relativity assures us that light always travels at c, no matter how we measure it. Hence, the photons will reach the front wall, having traversed the distance at speed c.

Mark Dirnhuber
Bristol, UK

This question is central to Einstein鈥檚 special theory of relativity and the answer is shocking. Strictly, physics determines that no material object can move at the full speed of light (c) relative to an observer, but it can move as near to c as we like without actually reaching it. So the following arguments only hold below that limit.

Even if the spaceship is moving at a mere fraction less than c, not only will the light from the flashlight move at the full c, as measured by you, the observer, inside the spaceship, but also light coming from a star from which the ship is receding will appear to rush past you at that same full speed (albeit with a longer wavelength in your view: this is the famous 鈥渞edshift鈥�).

Imagine you are in your spacecraft, travelling at constant speed in a straight line, and that you are unable to look outside. How can you tell you are moving? No experiment could possibly answer this.

The speed of light is constant relative to any observer, irrespective of how fast they are moving relative to its source. This is the result of the Michelson-Morley experiment in 1887. There can be no absolute stationary frame of reference: all movement at constant velocity is only relative to some other (equally arbitrary) frame. As a corollary, there can also be no absolute time.

These ideas defy common sense, yet the evidence for their veracity is overwhelming. We just have to accept that this is shocking, that common sense fails in this case, and move on in our spaceship.

Steve Hoskins
Reading, Berkshire, UK

If you do get a spaceship made of matter to travel at the speed of light, could you let us know what happens, as relativity can鈥檛 tell us. Please also let us know how you managed it, as relativity says it requires an infinite amount of energy to do so.

@fizzix2020
via Twitter

Your premise is physically impossible. Unless you and your spaceship are massless, travelling at the speed of light violates the laws of physics.

@FullcntctFghter
via Twitter

If you are travelling at that speed, you are a photon. No lamps, no walls. Sorry.

@MarkAlexander5
via Twitter

Time would no longer exist for you to flick the switch.

Ralph Winrich
Dunkirk, Wisconsin, US

The question of turning on a flashlight at one end of a spacecraft while travelling at the speed of light is intriguing on several levels. If you are moving at the speed of light, I wonder what you and your ship would be made of, as, at that speed, your mass would be equal to that of the rest of the universe perhaps.

At any rate, the beam of light emitted by your flashlight would appear to reach the far wall instantly, as the speed of light is independent of your speed and would always be constant at about 300,000 kilometres per second in a vacuum.

Steve Hyams
Manchester, UK

In my Marvel comics, Superman travelled at the speed of light against the rotation of Earth and time went backwards. So, don鈥檛 spoil my childhood please. Or maybe I could use this to go back to my childhood. I鈥檒l leave next Tuesday, so if someone can lend me a torch, I鈥檒l try the experiment. If I get back, which itself is no small question, I鈥檒l let New Scientist know the answer.

To answer this question 鈥� or ask a new one 鈥� 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. Please include a postal address, daytime telephone number and email address.

91色情片 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.

Terms and conditions apply.

听