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Knowledge_Seeker
2006-Mar-30, 12:24 AM
I was wondering why it isnt possible to travel at the speed of light. Can someone care to explain?

Because from what i know, there is not resisting force in space except the gravity of large objects such as planets. So from what i know when something leaves the earth's orbit, for example a satelite* leaves at 2,000mph and continues to pluto, since there is no stopping force in space that speed should continue to be 2,000 mph for eternity unless it is in the path of a planet or asteroid or crashes. Correct?

So if this is true, why not send a ship to space, steadily increasing the speed until it is the speed of light, or like 5% of the speed of light? because since no force acts upon the ship, it should never break down, correct?

please feel free to correct me or give comments :D

antoniseb
2006-Mar-30, 12:33 AM
You are thinking in terms of Newtonian Physics, which is an excellent approximation as long as nothing starts getting close to the speed of light, or into gravitational fields strong enough to make a black hole.

As you start approaching the speed of light, it gets harder to accelerate an object. The details of how this works out is called relativity. Why this happens ... we observe that it does.

Knowledge_Seeker
2006-Mar-30, 12:43 AM
You are thinking in terms of Newtonian Physics, which is an excellent approximation as long as nothing starts getting close to the speed of light, or into gravitational fields strong enough to make a black hole.

As you start approaching the speed of light, it gets harder to accelerate an object. The details of how this works out is called relativity. Why this happens ... we observe that it does.

could you explain a little bit more? because i thought that there is no force in space, so your speed shouldnt be a factor.

and are you saying that when traveling as speeds close to the speed of light you kind of create your own gravitational field, eventually becomming a black hole?

Dragon Star
2006-Mar-30, 12:44 AM
It goes like this, moving at faster speeds requires more thrust, which requires more fuel, the added fuel weight requires more thrust to get moving, this effect is exponential, making LS nearly imposable for anything with a rest mass.

Dragon Star
2006-Mar-30, 12:54 AM
Oops, forgot a big part of the problem there (I thought I put it in but I guess not) I typed a huge explanation the first time, pressed some key and the page redirected, loosing all of my work, so I typed some of it again.

This whole concept comes from the theory of special relativity. Einstein has shown that as something accelerates and travels at fast speeds its mass increases, and the larger the mass of the object, the harder it is to give it a push to make it accelerate some more. So when the speed of the object gets really close to the speed of light, its mass becomes almost infinite making it impossible to actually give it the speed of light.

Answer your question?

Knowledge_Seeker
2006-Mar-30, 01:00 AM
Well some of it. i think you are trying to explain how it would be in an earthlike environment.

the way i was thinking about it was like compound speed, the more speed, the more it increases in speed, and then the more increase, the more speed, a cycle (i guess i was thinking about compound interest)

as for your example about the fuel; for arguments sake lets say there was a huge tank the size of the moon or whatever? how about this, take the fuel part out of the equation, would we still be able to travel at the speed of light?

Knowledge_Seeker
2006-Mar-30, 01:01 AM
It goes like this, moving at faster speeds requires more thrust, which requires more fuel, the added fuel weight requires more thrust to get moving,

but would the mass matter in space?

if everything is space in a sense is freefalling would everything have the same "weight"?

umop ap!sdn
2006-Mar-30, 01:13 AM
how about this, take the fuel part out of the equation, would we still be able to travel at the speed of light?
No, because as you approach the speed of light, time slows down, and that last bit of oomph to get your spacecraft to light speed never reaches the propulsion system.

Knowledge_Seeker
2006-Mar-30, 01:19 AM
No, because as you approach the speed of light, time slows down, and that last bit of oomph to get your spacecraft to light speed never reaches the propulsion system.

wait, so you are saying that it is possible to travel maybe 5, 20 or like 80% of the speed of light?

worzel
2006-Mar-30, 01:22 AM
The way you've put it, Knowledge_Seeker, is very insightfull. I was even told by my science teacher at school that she believed that the limit didn't really exist and it was just a matter of technologoy, much like the breaking of the sound barrier, for the same reason you give.

The answer is that from the point of view of the accelerating rocket it can go just as fast as it likes. If you keep accelerating at the same rate you will keep going faster and faster. You could get to that star 10 light years away in hours if you accelerate fast enough, thus breaking the speed of light limit.

But if you start measuring distances while your travelling at this superluminal speed you find a curious thing. All the distances have shrunk, and the faster you go the more they shrink. Using the new distances you'll find that your speed is always less than that of light afterall, although getting ever closer the more you accelerate. You get to the distant star much quicker than you originally thought possible because the distance to it decreases the more you accelerate.

The flip side is that us back on earth watching you never see you go faster than light, and never see the distance to that star 10 light years away decrease. But what we do see is your clock running slow. From that we can conclude that you would think you're going much faster than you actually are.

If you returned home the whole journey would have taken a lot less than you oringally expected. But for us it would have been as expected. You would find that you aged less than we did because the journey turned out to be a lot shorter than you originally thought. We would disagree and say that you were just aging much slower while travelling. But we'd all agree that you had aged less by exactly the same amount.

EDIT: I should add that the answers given so far regarding increasing mass and thus less effect from yet more acceleration are all from the earth's point of view. From the accelerating person's point of view their mass doesn't change, and they can always just decide that they are stationay (for an instant) and continue to accelerate as normal.

Dragon Star
2006-Mar-30, 01:24 AM
No, because as you approach the speed of light, time slows down, and that last bit of oomph to get your spacecraft to light speed never reaches the propulsion system.

Umop, sorry, but thats incorrect. For the observer time slows down, not for the subject that is traveling. For the passengers time goes at a normal rate from their point of view.

Space has nothing to do with the relativistic mass changes that occur with fast travel.


From Universe Today(Tue, 13 Apr 2004):


Consider a 100 metre sized spacecraft, for example. To collapse into a black hole, it would need a mass 35 times larger than Jupiter. How much energy would you need to apply to the spacecraft to accelerate it enough to acquire this mass? This is easily computed using E=mc2 - ie. multiply the mass (in kilograms) by the speed of light (in metres per second) squared. The answer is 6 times 10 to the power 45 Joules, which is roughly the combined energy output of all the hundred billion stars in the Milky Way since the Big Bang.

Is this making sense to you?

Dragon Star
2006-Mar-30, 01:27 AM
The way you've put it, Knowledge_Seeker, is very insightfull. I was even told by my science teacher at school that she believed that the limit didn't really exist and it was just a matter of technologoy, much like the breaking of the sound barrier, for the same reason you give.

No, read above, the limit is expressed with E=mc^2.

five_distinct
2006-Mar-30, 02:08 AM
Umop, sorry, but thats incorrect. For the observer time slows down, not for the subject that is traveling. For the passengers time goes at a normal rate from their point of view.

Space has nothing to do with the relativistic mass changes that occur with fast travel.


From Universe Today(Tue, 13 Apr 2004):



Is this making sense to you?


So that's how much energy you'd need to accellerate it to light speed huh? Hypothetically speaking, what would happen if that energy were to be available to accellerate it to that? I understand that to the observer you end up slowing to infinity, as you would above an event horizon, but would you see distances shrink to almost 0 and severe blueshift, and ultimately death?

And what would be left of the ship, would we just have this little black hole flying around the universe very, very quickly?

Enzp
2006-Mar-30, 02:23 AM
KS, mass and weight are not the same thing. Weight is the result of local gravity reacting to your mass. Floating in freefall, you have no weight, and neither does the multiton spaceship you are riding. But you and teh ship still have mass. And that means inertia. If you float up next to the side of the ship and push against it, you won't send it tumbling away, you will just be pushing yourself away. The ship will move a tiny bit and you do most of the moving - the ratio will be as your relative masses.

SO if you magically were already moving at 2000 miles an hour, or 2000 miles a second, or 80% the speed of light, in interstellar space, you would continue to move at that speed until acted upon by something else, yes.

But to accelerate means moving your mass. As your speed increases, so does your mass, so the closer to the speed of light you get, the more and more energy it takes to continue to accelerate yourself. It takes no energy to maintain speed, but it does take energy to increase it - to accelerate.

umop ap!sdn
2006-Mar-30, 02:36 AM
Umop, sorry, but thats incorrect. For the observer time slows down, not for the subject that is traveling. For the passengers time goes at a normal rate from their point of view.
That's just how it was explained to me - that the last drop of fuel never burns.


Is this making sense to you?
Only that it would cause the ship to become a black hole. Hadn't thought of it doing that. :) I think worzel's explanation involving distances makes a lot of sense though - clarifies the why of time dilation.

Dragon Star
2006-Mar-30, 03:14 AM
So that's how much energy you'd need to accellerate it to light speed huh? Hypothetically speaking, what would happen if that energy were to be available to accellerate it to that? I understand that to the observer you end up slowing to infinity, as you would above an event horizon, but would you see distances shrink to almost 0 and severe blueshift, and ultimately death?

And what would be left of the ship, would we just have this little black hole flying around the universe very, very quickly?


Now wait just a second, that is not the amount of power it would take to get something to go the speed of light, that is the power it takes to get something to go critical mass, pushing it to a black hole. Making something go the speed of light would be infinitely more powerful then that.

Critical Mass would occur quite a while before you reached the speed of light I would think, my guesstimate is around 85-90%, depending on the size of the object.

Umop, it would look like this, if someone were watching this happen, it would take a huge amount of time for them to reach near LS, but for the people traveling, it seems perfectly normal to them, and everything happens around them super fast, so fast even that it would be out of control, colliding with a peanut in space at LS would explode anything solid in the universe except probably Neutron Stars to oblivion.

Five: Nothing would be left of your ship, it would reach critical mass, turn into a black hole and collapse forever making a singularity....your ship disappears but keeps it's mass, making a black hole.

Dragon Star
2006-Mar-30, 03:22 AM
I understand that to the observer you end up slowing to infinity, as you would above an event horizon, but would you see distances shrink to almost 0 and severe blueshift, and ultimately death?

Death would come WAY before you ever reach your event horizon due to the tidal forces that the gravity plays on space time, riping and contorting anything in it's path.

As for seeing distances shrink to 0, I don't know what you mean, blue shift comes from objects coming closer to you, this wouldn't happen, you would just notice that time seems to go slower for you compared to everything else outside of the black hole. Like you standing in the street and the entire day flashes before you eyes in a minute, people and cars, dogs whizzing by you and incredible speed.

five_distinct
2006-Mar-30, 04:22 AM
Yeah you're right the distances shrinking is a terrible analogy..... But because of the fact that time has slowed down for you to such a degree, wouldn't you have a huge amount of radiation bathing you compared to what would normally experience (similar to what would occur near a black hole?)


Thanks for the explaination.

worzel
2006-Mar-30, 08:27 AM
The way you've put it, Knowledge_Seeker, is very insightfull. I was even told by my science teacher at school that she believed that the limit didn't really exist and it was just a matter of technologoy, much like the breaking of the sound barrier, for the same reason you give.No, read above, the limit is expressed with E=mc^2.Huh? I never said she was right.


Five: Nothing would be left of your ship, it would reach critical mass, turn into a black hole and collapse forever making a singularity.It wouldn't collapse. Remember that the ship is equally justified to say that it is stationary and the earth and nearby stars are all whizzing along at near c.


Death would come WAY before you ever reach your event horizon due to the tidal forces that the gravity plays on space time, riping and contorting anything in it's path.That depends on the size of the black hole. In the case of a pseudo gravity field due to acceleration all you ever feel is the accelaration due to your engine, nothing more. There is an event horizon way off behind you in this pseudo grav field (although it is flat, unlike a real gravity well), so clocks in front of you do run fast while clocks behind run slow (this is just a result of integrating the lorentz transformation over successive instantaneous inertial frames). As soon as you stop accelerating, though, all the clocks in front and behind will be running slow because they're now moving very fast relative to you.

worzel
2006-Mar-30, 09:04 AM
Another way of answering this, Knowledge_Seeker, is this:

One of the two postulates of special relativity is that the speed of light is constant for everyone not accelerating. That is, if a photon flys by me I will measure its velocity as c. And if that same photon then carries on and flys by you, then you too will measure its velocity as c, even if you are flying away from me at half c. In fact, everyone who is not accelerating will measure that same photon as moving at c relative to themselves however they are moving relative to each other.

That is very strange indeed. The only reason it was postulated was because all attempts to measure its speed in different setups to see what it was moving against failed, they always came up with the same answer: c. That, and the Maxwell equations also implied it (and were therefore incompatible with Newton's theory).

All this length contraction, time dilation, and relativity of simultaneous events are just consequences of this postulate and the other one that says that if we're both floating by each other then there is no way physically for either one of us of to tell whether it is us or them that is actually moving.

The reason you can't go faster than c is because however fast you go, light still goes at c in all directions relative to you.

astromark
2006-Mar-30, 09:55 AM
This raises a interesting question ; Once in the vacuum of space and away from significant gravity force, could you exceed the speed of light. But what do you use as a gouge. What reference point do you have ? If a object was to come near you and just happened to be doing the same thing you are but, in the opposing direction. Then yes your relative speeds could exceed that of light speed. Its a matter of conception.. Oops Perception.. point of reference.
The fact remains that we can not produce the energy required to get anywhere near light speed. As your mass increases as the speed goes up. the energy requirement would be massive. Of stellar proportions.

worzel
2006-Mar-30, 11:20 AM
This raises a interesting question ; Once in the vacuum of space and away from significant gravity force, could you exceed the speed of light. But what do you use as a gouge. What reference point do you have ? If a object was to come near you and just happened to be doing the same thing you are but, in the opposing direction. Then yes your relative speeds could exceed that of light speed. Its a matter of conception.. Oops Perception.. point of reference.
If you mean that two people could pass by me in opposite directions at over 0.5c relative to me then their relative velocity w.r.t each other, according to me, is >1c, then yes. But from their points of view they are still only separating from each other at less than 1c.

Lord Jubjub
2006-Mar-30, 11:53 PM
For a photon (or anything else travelling at c), there is no time or distance. The universe is a singularity.

Dragon Star
2006-Mar-31, 12:00 AM
For a photon (or anything else travelling at c), there is no time or distance. The universe is a singularity.

What? Wait on a second, the universe is NOT a singularity, a singularity is an infinitely small, infinitely dense "thing".

umop ap!sdn
2006-Mar-31, 04:43 AM
Because a photon travels at c, it "experiences" no time or space. If it could observe, it would perceive its entire journey as being instantaneous and covering no distance.

blueshift
2006-Mar-31, 04:07 PM
We are not going to even succeed at closing the gap between us and light speed via our own acceleration by one mile per second. A microwave taken along on the galactic flight will still take 3 minutes to make hot tea just as it does on Earth.

When we speak of speed c we are speaking of the speed of energy, not the speed of some race car or galactic ship. Speed c acts more like the voltage level in your car battery. Equalizing the energy of the two poles of your car's battery (which equalizes the speeds their of motions jostling around) will slow your car to a dead stop.

If we couldn't maintain a difference of potential between us the speed of energy, then a jogger will not be able to tap into any energy reservoir to speed up or slow down or change directions. Likewise, the result would be that people who exercise should be in worse shape than a couch potato.

There are pictures of each of us in our homes, taken many years ago. The photons that radiated from those scenes still exist. The clocks that may have been in the respective backgrounds all still read the same time that the pics were taken although the scenes have spread out by 1/d^2 over the years. Reaching speed c requires that we too will have to spread out and rip our bodies apart unless we rearrange our molecular structure to function like a laser. I don't think we can do that.

Knowledge_Seeker
2006-Mar-31, 08:38 PM
okay, now i seem to get it a little, but can you tell me why your weight increases when approaching the speed of light?

(p.s i already knew the difference between weight and mass, but thanx for the explanation anyways :D)


Likewise, the result would be that people who exercise should be in worse shape than a couch potato.



Could You explain this?

Grey
2006-Mar-31, 09:43 PM
...a singularity is an infinitely small, infinitely dense "thing".I'll nitpick this. A singularity is not an infinitely small and dense thing. A singularity is a situation under which our physical descriptions using our current theories breaks down. We therefore are unable to describe what happens accurately under such circumstances, and we know it. It is true that at least one such circumstance is when the energy density is extremely high, such as we find in a black hole or the big bang, but it's important to remember that "singularity" doesn't refer to a thing, it refers to the fact that our math doesn't work.

Disinfo Agent
2006-Mar-31, 10:41 PM
okay, now i seem to get it a little, but can you tell me why your weight increases when approaching the speed of light?Does mass change with velocity? (http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html)

The gist: it depends on what you mean by 'mass'. The modern answer is 'no'.


I was wondering why it isnt possible to travel at the speed of light. Can someone care to explain?Read The relativistic rocket (http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html).

Knowledge_Seeker
2006-Apr-01, 02:15 AM
The gist: it depends on what you mean by 'mass'. The modern answer is 'no'.



I meant weight, because obviously the mass would remain the same, because it was pointed out to me earlier that your weight increases thus you need more energy to move that weight.

Ken G
2006-Apr-01, 02:43 AM
Knowledge Seeker, my advice is to read over worzel's comments very carefully, your answers are there. A lot of the other stuff is highly inaccurate and might just confuse you. I would add that even the wording in the OP, "travel at the speed of light", is actually not precise enough to really be answerable. Most in the thread have interpreted this as meaning we cannot achieve a speed that our friends at home will interpret is equal to the speed of light, such that we would arrive at our destination at the same moment as a beam of light emitted alongside us. But as worzel pointed out, you could also be saying "I want to get 1 light year away and I want it to take 1 year of my life to do this", and that is quite possible. Technologically challenging yes, but does not face any theoretical limits at all. In fact, you could do it in one month of your life, in principle. The biggest problem is surviving the g forces associated with such a stupendous acceleration, and yes, the energy requirements are nasty also. But they are not infinite, due to worzel's arguments. The only theoretical limit is that two objects can never whizz past each other with a relative speed exceeding c, so if that's what you meant by "travel at c", then it is impossible yes.

Ken G
2006-Apr-01, 02:56 AM
I have to say that this quote from Universe Today, earlier in this thread:


Consider a 100 metre sized spacecraft, for example. To collapse into a black hole, it would need a mass 35 times larger than Jupiter. How much energy would you need to apply to the spacecraft to accelerate it enough to acquire this mass? This is easily computed using E=mc2 - ie. multiply the mass (in kilograms) by the speed of light (in metres per second) squared. The answer is 6 times 10 to the power 45 Joules, which is roughly the combined energy output of all the hundred billion stars in the Milky Way since the Big Bang.
is complete hooey. I have no idea what they are talking about, but neither the physics nor the numbers make any sense. As worzel pointed out, you can't make a black hole by accelerating an object so that its relativistic mass exceeds some limit, that just won't work. But even worse, the total output of the hundred billion stars in the Milky Way is about 10^63 ergs, or 10^56Joules. They're only off by about 12 orders of magnitude...

Dragon Star
2006-Apr-01, 03:02 PM
Well, you did read the date right?:D I don't know why their math is so far off, sorry for posting ** info, but it's from our own damn site so how was I supposed to know? :o

Dragon Star
2006-Apr-01, 03:04 PM
I'll nitpick this. A singularity is not an infinitely small and dense thing. A singularity is a situation under which our physical descriptions using our current theories breaks down. We therefore are unable to describe what happens accurately under such circumstances, and we know it. It is true that at least one such circumstance is when the energy density is extremely high, such as we find in a black hole or the big bang, but it's important to remember that "singularity" doesn't refer to a thing, it refers to the fact that our math doesn't work.

Thank you for clearing that up Grey, great explanation.:)

Ken G
2006-Apr-01, 03:36 PM
Well, you did read the date right?:D I don't know why their math is so far off, sorry for posting ** info, but it's from our own damn site so how was I supposed to know? :o
I don't blame you Dragon Star, I too would normally expect Universe Today to have accurate information. I guess it's "buyer beware" everywhere you go!

blueshift
2006-Apr-02, 04:15 PM
okay, now i seem to get it a little, but can you tell me why your weight increases when approaching the speed of light?

(p.s i already knew the difference between weight and mass, but thanx for the explanation anyways :D)



Could You explain this?First things first. Your measured mass by a distant observer is going to increase. You will not measure a thing changing at all. You are always at rest with respect to yourself. However, a distant observer measures you to have momentum while you do not. You are measuring where the light pulses from your antenna occur and so does a distant observer. The antenna has not moved with respect to you. The antenna has moved as far as the distant observer records.

Momentum is a product of mass x velocity. Your antenna has no momentum with respect to you. It sits right where you mounted it all along. I, the distant observer, measure displacements through space by your antenna.

Take a tennis racket and give an oncoming serve a good enough whack and you can increase the speed of the ball in its return flight. Momentum will drop behind the racket while increasing right in front of it. The tennis ball will increase in measured mass but we will pretend that we don't know that yet.

Now take a mirror and give oncoming sunlight a whack. Momentum drops behind the mirror and increases in front of the mirror just as it did with the tennis racket. But the speed of light cannot change. It remains at speed c. Since momentum = mass x velocity that leaves only mass to be measured as increasing. This is not proper mass but coordinate mass that is measured. Proper mass is what the photons measure their mass to be...unchanged. They are at rest with respect to themselves just as you are always at rest with respect to yourself.

The comment I made about the couch potato applies to a universe where we can decrease the gap between us and the speed of light. The laws of electromagnetism would differ from one reference frame to another with the slower reference frames being able to build up potential energy in a shorter time. Light would be seen accelerating gradually from one reference frame to another, getting faster to the slower moving reference frames. Gamma rays would not be gamma rays to all observers. Neither would visible light. Cell divison and neuron firings would be noticed in the traveler's proper frame of reference as changing. The athlete would catch a chill from running and think slower, losing his ability to react.

Knowledge_Seeker
2006-Apr-03, 05:00 AM
you obviously know what you are talking about, and i seem to get it now.

but now, as a final statement, what is the main reason we cannot travel LS and why. thanx!

worzel
2006-Apr-03, 09:13 AM
what is the main reason we cannot travel LS and why. thanx!
Like I said before, it really boils down to the fact that the speed of light is always the same relative to yourself, so however fast you go, all the photons whizzing by you still overtake you at c in all directions: hence you can never catch them up.

Why that's so is unknown (as far as I know). It was just observed to be the case, so Einstein figured what must give for this to be true.