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## #1 2006-02-12 09:48:16

MathsIsFun

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### Space Travel Near Speed of Light

We might be able to travel to other stars, if the technology can be developed. Maybe.

As an interesting exercise: if you were inside a spaceship, and it was accelerating at 10 m/s² (approx 1g), how much of your time would it take you to reach 99% of the speed of light (with reference to your starting point)?

Because at that speed it won't take you long to reach other stars!

For reference, the time dilation is:

(The only catch is, that if you decide to return, the Earth would have aged a lot.)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #2 2006-02-12 19:08:15

ryos
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### Re: Space Travel Near Speed of Light

The "post Ender's Game" trilogy was pretty much based on this idea.

To me, the more interesting problem is the energy balance. How much energy does it take to accelerate to 99% of c and back down again? I have read that the only known energy source with enough energy density to pull it off is antimatter.

Hmm...I think that would require relativistic physics to figure out, and as a ChemE I don't think I'll ever get there.

El que pega primero pega dos veces.

## #3 2006-02-13 02:00:20

irspow
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### Re: Space Travel Near Speed of Light

What are the prospects of NASA simply sending a probe out using the "new" electron propulsion system to actually test out some of our theories in this area?  I, for one, would think that this would be as good an investment as some of the other programs that we are currently funding.

## #4 2006-02-13 03:39:35

Ricky
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### Re: Space Travel Near Speed of Light

I, for one, would think that this would be as good an investment as some of the other programs that we are currently funding.

Agreed, if not far better.  The International Space Station was not a scientific project, it was a political one.  And the shuttle is currently only being used to supply the international space station.

Meanwhile, Hubble telescope is losing funding.

There are so many great projects that NASA could be funding, but they are choosing all the wrong ones.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #5 2006-02-13 04:37:28

irspow
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### Re: Space Travel Near Speed of Light

I agree.  Let's petition for a solar-powered ion-propulsion probe to see just how fast we can go and put many of our theories to the ultimate test.

Just wondering as I was contemplating approaching the speed of light.  Would a probe moving this fast even be able to transmit data back to us?  I mean if the probe sent the data in the form of light back towards us wouldn't the data be still(?)?

Like this;  If a source ejects something at the speed of light, but is itself moving at the speed of light, wouldn't the ejected something then be stationary?  Maybe we would have to send out successive probes just to retrieve the information left behind by former probes.  It is fun to ponder such circumstances.

In the unlikely(?) event of such a probe actually surpassing the speed of light (assuming that many of our current theories are wrong) would we then be unable to retrieve the data at all?  Or would we have to send out a reciever probe ahead of the one that we want to recieve the data from.  I feel a paradox coming on.  My...mind...is...too...small...ugh.

## #6 2006-02-13 04:47:21

mathsyperson
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### Re: Space Travel Near Speed of Light

#### irspow wrote:

Just wondering as I was contemplating approaching the speed of light.  Would a probe moving this fast even be able to transmit data back to us?  I mean if the probe sent the data in the form of light back towards us wouldn't the data be still(?)?

Yes, but you're assuming that light obeys the same laws of physics as everything else. According to the laws of physics, nothing can reach the speed of light, so light's broken that one already!

We can never be absolutely certain what is right, but at the moment, scientists are saying that the speed of light is constant relative to the observer. That means that despite the fact that the probe is moving, the signal would still travel to us as if the probe were stationary, which means that none of those hypothetical problems will happen.

Why did the vector cross the road?
It wanted to be normal.

## #7 2006-02-13 05:37:48

irspow
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### Re: Space Travel Near Speed of Light

Okay, cool.  I guess that you have made yet another point as to why such a mission should be done.  We would then have confirmation or rejection of these non-problems.

## #8 2006-02-13 11:26:01

Ricky
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### Re: Space Travel Near Speed of Light

According to the laws of physics, nothing can reach the speed of light, so light's broken that one already!

Nothing with mass can reach the speed of light.

We can never be absolutely certain what is right, but at the moment, scientists are saying that the speed of light is constant relative to the observer.

That is a fairly well held fact in physics today.  It has been tested many times.  For example, we wouldn't get the Doppler effect with light if that property wasn't true.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #9 2006-02-13 11:51:41

mathsyperson
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### Re: Space Travel Near Speed of Light

According to the laws of physics, nothing can reach the speed of light, so light's broken that one already!

Nothing with mass can reach the speed of light.

I know, I was joking. But still, what is mass? Light sometimes behaves as a particle and sometimes as a wave, does that mean that it sometimes has mass? Some solar-powered probes are powered by light because the photons hit the probe, get reflected off and because of conservatoin of momentum, the probe gets moved by the photon. But that would mean that the photon has mass, so how is it moving at the speed of light?

We can never be absolutely certain what is right, but at the moment, scientists are saying that the speed of light is constant relative to the observer.

That is a fairly well held fact in physics today.  It has been tested many times.  For example, we wouldn't get the Doppler effect with light if that property wasn't true.

Why not? Sound's speed varies depending on who is observing it, and the Doppler effect still works with that. In fact, I'd have thought that the Doppler effect was dependent on the speed being variable. Obviously that's not true, because we can see the red-shift of all the stars that are moving away from us, but it's counter-intuitive.

Why did the vector cross the road?
It wanted to be normal.

## #10 2006-02-13 12:10:28

Ricky
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### Re: Space Travel Near Speed of Light

Light sometimes behaves as a particle and sometimes as a wave, does that mean that it sometimes has mass?

Light always behaves partially like a particle, and partially like a wave.  A photon is a particle of light.  All particles, including electrons and protons, behave as waves (although I'm not sure if the proton has to be free of the nucleus or not).  But a photon is a type of particle which is always massless.  But a photon does have momentum.

Some solar-powered probes are powered by light because the photons hit the probe, get reflected off and because of conservatoin of momentum, the probe gets moved by the photon.  But that would mean that the photon has mass, so how is it moving at the speed of light?

Yes and no.  The equation is E² = m²c^4 + p²c².  If m=0 (0 mass), E² = p²c², where p is momentum.  So even particles with no mass can have momentum.  Weird, huh?

But this is still up to much debate and is still being explored.

Sound's speed varies depending on who is observing it, and the Doppler effect still works with that.

The speed of sound is a constant in a constant medium.  It does not depend on the movement of the one who produces it, unlike, say, throwing a ball.

In fact, I'd have thought that the Doppler effect was dependent on the speed being variable.

Yes, speed is the variable in the Doppler effect.  But the Doppler effect does not have to do with a change in the velocity of a sound.  Instead, it is a change in pitch (frequency).

Obviously that's not true, because we can see the red-shift of all the stars that are moving away from us, but it's counter-intuitive.

You lost me here.

Last edited by Ricky (2006-02-13 12:27:36)

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #11 2006-02-13 12:40:04

mathsyperson
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### Re: Space Travel Near Speed of Light

Huh. I didn't know about the photon not having momentum but not having mass. It doesn't seem like it should work, because usually momentum is mass x velocity, but then, it doesn't seem like anything to do with light should work really.

I meant that the speed of sound is variable depending on who is observing it. If you are travelling away from the sound source, then the sound will be slower for you than for someone who is stationary.

And red-shift is just the Doppler effect affecting how you see stars. A star might be yellow, but as it is moving away from you then the Doppler effect causes it to appear redder. Red-shift.
It's a standard scientific definition thingy. Sorry if I lost you somewhere else.

Why did the vector cross the road?
It wanted to be normal.

## #12 2006-02-13 13:03:42

Ricky
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### Re: Space Travel Near Speed of Light

It doesn't seem like it should work, because usually momentum is mass x velocity, but then, it doesn't seem like anything to do with light should work really.

Well said.  What we are looking at is a world we can't see, so it's only intuitive that everything is counter-intuitive.

I meant that the speed of sound is variable depending on who is observing it. If you are travelling away from the sound source, then the sound will be slower for you than for someone who is stationary.

Ah, ok.  I thought you meant it was the source of the sound that was moving.

But the main point of the Doppler effect has nothing to do with velocity, but frequency instead.  When you move away from the source, you will hear the sound at a lower pitch or see the light at a lower frequency.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #13 2006-02-13 16:33:14

MathsIsFun

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### Re: Space Travel Near Speed of Light

Doppler: If my hypothetical near-light-speed spacecraft sent an audio message back to earth, the signal would travel back to earth at the speed of light, but Earth would (eventually) hear my voice sounding v-e-r-y  s-l-o-w, right?

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #14 2006-02-13 17:10:04

Tigeree
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### Re: Space Travel Near Speed of Light

omg i'm have'n a de'ja vue

People don't notice whether it's winter or summer when they're happy.
~ Anton Chekhov
Cheer up, emo kid.

## #15 2006-02-13 17:35:23

ryos
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### Re: Space Travel Near Speed of Light

Back to the ionic drive probe: the advantage of ionic drive is that it produces very high exit velocities (around 70,000 mi/sec as opposed to chemical rocketry's 20,000 (IIRC)), so it's a very efficient conversion of electricity to propulsion. The trouble is twofold: one, the stream is very rarified, so that actual acceleration produced is slow. This means that we can't expect to power the thing with the sun, because the sun's rays will fade long before the probe is done accelerating.

The other problem is that even with the high exhaust velocities, energy density is still an issue. You have to carry enough propellent and energy (nuclear is currently the best candidate; most long-range probes have been nuclear powered) to reach those high velocities.

This is why the energy balance is more interesting to me than the time dilation; it's what's holding us back in space.

El que pega primero pega dos veces.

## #16 2006-07-30 06:14:15

Devantè
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### Re: Space Travel Near Speed of Light

A theory I read in a book somewhere: Travelling just over the speed of light in space will lead to time slowing down in the shuttle itself. That would probably look and feel amazing - And probably scary...?

## #17 2006-07-30 07:25:28

Ricky
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### Re: Space Travel Near Speed of Light

A theory I read in a book somewhere: Travelling just over the speed of light in space will lead to time slowing down in the shuttle itself. That would probably look and feel amazing - And probably scary...?

I think you mean just under.  And this effect happens no matter what speed you are traveling.  It just happens to a greater extent when you are traveling faster.

When you walk to your kitchen to get a sandwhich that is sitting on a table, you are moving in time slower than that sandwhich is.  It isn't noticable, but it's happening.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #18 2006-07-30 17:40:26

luca-deltodesco
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### Re: Space Travel Near Speed of Light

#### MathsIsFun wrote:

Doppler: If my hypothetical near-light-speed spacecraft sent an audio message back to earth, the signal would travel back to earth at the speed of light, but Earth would (eventually) hear my voice sounding v-e-r-y  s-l-o-w, right?

well, im not so sure about the (eventually) part, but the being very elongated seems logical

but ofcourse, you could easily get around that, by just being stationary when sending data back or needing to receive it

Last edited by luca-deltodesco (2006-07-30 17:41:01)

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The End Of All Things To Come.

## #19 2006-07-31 01:16:28

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### Re: Space Travel Near Speed of Light

#### mathsyperson wrote:

Yes, but you're assuming that light obeys the same laws of physics as everything else. According to the laws of physics, nothing can reach the speed of light, so light's broken that one already!

Someone correct me if I'm wrong, but I believe relativity implies that the speed of light is a barrier that cannot be crossed. Photons either travel at the speed of light, or their energy absorbed into matter. They have not broken the barrier.

Also, there is nothing to suggest that there are no particles that travel faster than the speed of light (tachyons). Relativity says if those particles exist, they cannot accelerate (slow down) to the speed of light.

You can shear a sheep many times but skin him only once.

## #20 2006-07-31 02:22:29

Ricky
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### Re: Space Travel Near Speed of Light

Also, there is nothing to suggest that there are no particles that travel faster than the speed of light (tachyons). Relativity says if those particles exist, they cannot accelerate (slow down) to the speed of light.

Such particles must have negative mass to be compatible with relativistic equations.  Not saying that it isn't possible, just that it makes no sense.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #21 2006-07-31 02:25:53

luca-deltodesco
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### Re: Space Travel Near Speed of Light

#### Ricky wrote:

Also, there is nothing to suggest that there are no particles that travel faster than the speed of light (tachyons). Relativity says if those particles exist, they cannot accelerate (slow down) to the speed of light.

Such particles must have negative mass to be compatible with relativistic equations.  Not saying that it isn't possible, just that it makes no sense.

doesnt anti-matter, have negative mass?

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The End Of All Things To Come.

## #22 2006-07-31 03:32:34

numen
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### Re: Space Travel Near Speed of Light

No, antimatter has opposite electric charge. As far as I know, negative mass has not been confirmed anywhere in physics.

Bang postponed. Not big enough. Reboot.

## #23 2006-07-31 03:47:28

luca-deltodesco
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### Re: Space Travel Near Speed of Light

#### numen wrote:

No, antimatter has opposite electric charge. As far as I know, negative mass has not been confirmed anywhere in physics.

ah yeh, i was thinking of dark matter :p which as you say, hasnt been confirmed to exist

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The End Of All Things To Come.

## #24 2006-07-31 03:54:13

Ricky
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### Re: Space Travel Near Speed of Light

Darkmatter, again, has a positive mass.

And darkmatter, for all intensive purposes, exists.  It solves problems that we see with galaxy movements as well as problems within the Big Bang theory.  If it didn't exist, we would have to throw all of Einstiens equations away.  It has been indirectly detected in a number of ways.

However, in what form it exists, is still up to much debate.

To put it briefly, we have no knowledge, theoretically or otherwise, of the existance of particles with negative mass.  Just a few physicists who got high and said, "You know, what if matter had negative mass?!?"

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #25 2006-07-31 03:59:28

luca-deltodesco
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### Re: Space Travel Near Speed of Light

i thought that darkmatter was supposed to explain why everything is accelerating away from eachover? i.e. negative force of gravity -> can only occur with a negative mass?

although, thats supposing that its due to a force of gravity at all...

Last edited by luca-deltodesco (2006-07-31 03:59:52)

The Beginning Of All Things To End.
The End Of All Things To Come.