Expert: James Gort - 8/5/2005

Question
here is my last question. light travels at the speed of light so is that affected by time also what propells light over such a large distance.

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Followup To
Question -
if a large object was traveling through space at the speed of light or more, will it hae gravity? gavity travels at the speed of light so wondering if it would. also if einstine theory of relativity is right at what speed will time be affected. if going at the speed of lite makes time completl;y stop at what speed does it slow down?
Answer -
Hello Paul,

That's an interesting thought question, but it might not be possible to test, because Einstein said that the speed of light is an upper limit.  Nothing can attain that speed, since it would take an infinite amount of energy to push it to that speed.  Another way of looking at it, is that the body would attain an infinite mass (as measured from an observer).  As more energy goes into accelerating the object, more and more of the energy goes into increasing its mass instead of increasing its velocity.  

But I'll try and answer your thought question as best as I can, based on our current understanding of relativity (that is, Einstein's predictions).

As an object travels through space, its mass increases (relative to a distant observer).  So its gravity increases.  This can be measured in Mercury's orbit (which is highly elliptical).  When it's closer to the sun, it goes faster, so its mass is slightly more.  When it's farther from the sun, it goes slower in its orbit, and its mass decreases.  This affects the orbit itself, and that can be measured.  And it works out exactly as Einstein predicted.  But if you were living on Mercury, your weight wouldn't change.  To you, the mass of Mercury would be constant!

So if you were travelling on an object going at (or more than) the speed of light, its mass (as measured by a distant observer) would seem to increase to a very large amount (i.e., infinity), but then the object would "disappear" as it passes the speed of light (if it COULD - again, Einstein says this is impossible).  But the gravity waves (or particles called gravitons) or light particles (photons) would never reach you.

Continuing the thought process further, time (as measured by a distant observer) would seem to slow down for the fast-moving traveller, and then stop completely when the traveller reaches the speed of light (we can't actually see the traveller then, because the photons don't reach us).  But the watch on the traveller's hand would seem to run correctly (not run slower).  This has also been tested by using atomic clocks on board spacecraft.  When the spacecraft is launched, a clock on the ground is synchronized to a clock on the spacecraft.  After it makes many orbits, the spacecraft returns to earth, and the spacecraft clock is actually slower than the earth-bound clock.

If the traveller COULD go faster than light, he could return  to earth before he left!  That is, he could go backward in time.

That's why things get a bit confusing when we talk about travelling faster than light - many strange things could happen.

Hope that helps, but it's a very strange and confusing subject.  Until now, all of Einstein's predictions have been correct.

Prof. James Gort


Answer
Hi Paul,

You can think of light as the "carrier of time".  As you go faster, time slows for you (as measured by someone standing still - you wouldn't notice time is going slower).  At the speed of light, time stops for you.  In other words, if you could travel to the nearest star at the speed of light (4.2 light-years away), it would take you 4.2 years to get there (measured by the clocks on earth), but you wouldn't age even one day!

Light consists of small packets of energy called photons.  When a photon is emitted by a hot object (that object actually reduces its energy slightly, and that energy is carried away by the photon), it travels forever UNTIL it interacts with some matter.  If it interacts with some gas in space, it can be absorbed, there would be a delay, and another photon could be re-emitted (usually at a different frequency or color).  Sometimes the photon doesn't interact with anything until it reaches the rods and cones in your eyeball.  When that happens, you see the object!  So nothing has to propel light (or photons), they just travel until they interact with something.

Those are good questions.  It's good to think about such things.  Nature is wonderful and strange at the same time.

Prof. James Gort