Space, speed of light, and time

LightningTeg

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LightningTeg
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So I've been reading about relativity and the whole time slows down, space contracts thing blows my mind.

If space contracts for an object traveling at close to or at the speed of light, doesn't that mean that light travel is instantaneous? How can that be so if we know it takes 8 minutes for light to get to the earth from the sun? Does this only apply to things with mass?

For example, for a spacecraft traveling at 0.99C to get to the nearest system, alpha centuari (4.7 LY) it would only take 7.4 months.

:confused:

http://www.thebigview.com/spacetime/timedilation.html
 
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LightningTeg said:
If space contracts for an object traveling at close to or at the speed of light, doesn't that mean that light travel is instantaneous?
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A spaceship traveling at nearly the speed of light will get shorter in the direction of travel. How does that make light travel instantaneous? It's not going to contract to zero.

Steve S
 
The speed of light in a vacuum is invariant; that is, it never changes and is always equal to c = 3x108 m/sec no matter who the observer is or from what frame of reference they are making the observation. It is this fact - and yes it is a fact confirmed by experiment - that leads to strange phenomena such as time dilation & length contraction.

Of course, I assume in this discussion we are referring to special relativity - relative motion at a constant velocity.
 
Though from the viewpoint of something travelling at the speed of light, such as a photon, the entirety of its journey, however many billions of lightyears long it may be, is instantaneous.
 
Mister Earl said:
Though from the viewpoint of something travelling at the speed of light, such as a photon, the entirety of its journey, however many billions of lightyears long it may be, is instantaneous.
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The way I understand it, time - as we know it - would cease have meaning if one were to travel at lightspeed. That's basically because our mathematics cannot make sense of things if we assume that a massive object, such as a human through some trick of magic, could travel along with a photon.

Of course, it is a moot point anyway, because any massive object cannot travel at lightspeed.
 
Yes. That's why I referenced photons. Objects without mass can travel at lightspeed.
 
Mister Earl said:
Yes. That's why I referenced photons. Objects without mass can travel at lightspeed.
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But asking "what would it look like to an observer riding along the photon?" is a meaningless question for precisely the reason that I outlined before. At least, that's the way I read your question - if I am in error, I apologize and request clarification.
 
Mister Earl said:
Though from the viewpoint of something travelling at the speed of light, such as a photon, the entirety of its journey, however many billions of lightyears long it may be, is instantaneous.
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'cause for the photon, the universe (which appears to us to be billions of lightyears across) appears to be infinitely thin, hence requiring literally no time to cross.

ETA: Matt, we appear to be in some mild level of disagreement here. ETA^2: Or perhaps not.
 
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shemp said:
So all we have to do is build a spaceship out of photons! Let's get started!
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No, we need a Star Trek style transporter to convert our bodies into photons and reassemble them at the destination.

Note that such a journey to any part of the universe might appear instantaneous to a traveller who uses such a transporter, but many years may pass on earth (assuming that is where the traveller started from) before the traveller gets reassembled and the same number of years again before someone on earth can detect the traveller at his destination.

This is true for "short" distances within our galaxy. Intergalactic distances are measured in billions of light years (and that is the amount of time it would take a person using a Star Trek transporter to travel those distances).
 
So what we really need is a way to traverse from one point to another without traveling through the points in between.
 
LightningTeg said:
If space contracts for an object traveling at close to or at the speed of light, doesn't that mean that light travel is instantaneous?
Click to expand...

You could say that, yes.

LightningTeg said:
How can that be so if we know it takes 8 minutes for light to get to the earth from the sun?
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Time is relative.

For us, it takes light 8 minutes to get here from the sun. For the light, it takes no time at all.

LightningTeg said:
For example, for a spacecraft traveling at 0.99C to get to the nearest system, alpha centuari (4.7 LY) it would only take 7.4 months.
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(Slight correction: Alpha Centauri is 4.4 light years away.)

For the astronauts on the spacecraft, it would take the spacecraft 7.4 months to get there. For us on Earth, it would take the spacecraft 4.4 years to get there.
 
From the point of view of the photon (if such a thing can be said to have a point of view, which it really can't), travel is instantaneous. As mass approaches zero and velocity approaches c, time approaches nothing. At zero mass, there is no time or rather all time is compressed into the same 'moment' and that is all that the photon experiences.

Or something like that, anyway.
 
The forces whose carrier particles are massive have a finite range, or rather a subexponential dropoff in strength because the particles are likely to decay before they get too far. But the forces with massless carrier particles travel at lightspeed have an infinite range.

So travel being "instantaneous" from the "point of view of the photon" is both correct and is a bit more interesting than just a crazy thought experiment.
 
Mister Earl said:
Though from the viewpoint of something travelling at the speed of light, such as a photon, the entirety of its journey, however many billions of lightyears long it may be, is instantaneous.
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A photon doesn't really have a viewpoint though,they are notoriously wishy-washy,can't make up their minds about anything.
 
Is light made up entirely of photons? I thought I heard somewhere that light actually has a bit of mass and that about 4 pounds of light hit the earth every day.
 
This looks like the right place to post yesterday's family relativity puzzle:

I'm familiar with the idea that you can put one of a pair of atomic clocks onto an airliner, fly it around for a while and on its return it will appear to have run slightly slow compared to the clock which stayed at home.

I also understand that a clock on a train will run at normal speed for a passenger on the train, but appear to be running (infinitessimally) slowly to a person on the station platform as it passes by.

But motion is relative, not absolute, so that means that while the train clock looks slow from the platform, surely the station clock also looks slow from the train. That must mean that when the train comes back its clock will not be slow compared to the station clock, mustn't it?

So these two experiments (real and thought) are considering two different relativistic effects, right?
 
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SumDood said:
Is light made up entirely of photons? I thought I heard somewhere that light actually has a bit of mass and that about 4 pounds of light hit the earth every day.
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Light is made of photons. It has zero mass. But it has some energy. When it hits the earth, it warms it up a bit. This increases the earth's mass. My calculation gives about 4.2 pounds a day, assuming all the light is absorbed (actually only about 70% is). So you sort of heard right.
 
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