Does it need light speed or not

[DennisN]

What are the exotic matter you mentioned? Are they dark matter?

Hi Huamin! The term "exotic matter" is a general term referring to matter which doesn't behave/can be described as normal matter as we know it, or "normal" matter which simply isn't discovered/described yet.

Examples (hypothetical/theoretical):
* Matter with negative mass; e.g. m = -1 kg. Needed for wormholes.
* Matter with imaginary mass; e.g. m = 1i kg. E.g tachyons (would always travel faster than the speed of light)

There are more examples here. I suppose dark matter could be called "exotic" in the sense that we do not know what it is. But the mainstream scientific view is AFAIK that dark matter is some kind of "normal" matter, we just don't know exactly what it is (WIMPs/Supersymmetrical particles, etc.?)

(If you ask me, I don't believe there's matter with either negative or imaginary mass; I think it's just a purely mathematical feature with no physical relevance, but that's my opinion.)

 

[HuaMin]

Thanks Dennis. I know that the whole universe is having more than 50/60 of its matters in Dark matters. So I don't think they are just some mathematical feature but some matter with some weight (even in negative), aren't they?

 

[DennisN]

I know that the whole universe is having more than 50/60 of its matters in Dark matters. So I don't think they are just some mathematical feature but some matter with some weight (even in negative), aren't they?

Hi again. It seems you misunderstood me or misunderstood the term dark matter, or both. First, I want to repeat that "exotic matter" is a general term which can refer to anything we don't know about, anything which might or might not exist. It is an extremely imprecise term.

Regarding the mass-energy composition of the Universe, these are the approximate numbers;

(From these piecharts here (NASA));

• 5% known matter; atoms etc.
• 23% dark matter; that is, unknown matter (e.g. WIMPs/Superpartners? etc.)
• 72% dark energy; a cosmological constant/vacuum energy?

Regarding dark matter; the mainstream scientific view is AFAIK that dark matter is "normal" matter, we just don't know what it is. What I mean by "normal" in this sense is that the mass of a "dark" particle is a positive, real number. Mathematically speaking, m ≥ 0 and m is part of ℝ.

Now, I don't question that dark matter exists, since there's much evidence pointing in that direction. I however strongly question that any matter can have negative or imaginary mass, but that's entirely another question, and it is not related to dark matter (if I'm wrong about this, I'm happy if a forum member corrects me ).

 

[PlanckShift]

Hi,
If there is really one way to go back the past, must the way be through light speed or not? If the answer is yes, I really do not believe that the human beings or any animals are able to endure light speed technically!

Best Regards

Anti-matter can be thought of as regular matter that's traveling backwards in time.

 

[tiny-tim]

Anti-matter can be thought of as regular matter that's traveling backwards in time.

let's be careful … there's no anti-matter as such …

there's only particles that are the anti-particles of other particles

for example, the positron is the anti-particle of the electron, and vice versa,

but which of them you call matter and which anti-matter is just a matter of preference (or convention)​

if you take a film of particles (including electrons and/or positrons) interacting, and then run it backwards, the electrons will look like positrons, and the positrons will look like electrons

btw, this is why, in the maths of quantum field theory, the creation "field" of an electron includes annihilation operators of positrons of every momentum equally with creation operators of electrons of every momentum

it is in that sense that we can visualise electrons (or positrons) as positrons (or electrons) travelling backwards in time! ​

 

[krd]

Near light speed velocities may not kill you. But an impact with a grain of space dust might. You would need an immense shield to stop debris from obliterating your vessel at those speeds. That seems as big, if not bigger, than the technical hurdles of accelerating to fantastic velocities.

Forget the grain of dust.

An even bigger problem struck me recently.

Outer space is very cold - normally - about 3.2K

But as you speed up, the dim starlight approaching you is Doppler shifted upwards - Those photons would increase in energy. This would cause the background radiation to get hotter.

By my rough calculations (to tell you the truth I'm not doing these by hand as I'm too tired - I'm using dodgy internet calculators), At one thousandth of the speed of light, the temperature at the front of your spaceship would be 2,600,000,000 k (I think that figure is wrong, but I'm too tired to make a full calculation of what it should be ) But you get the idea. Long before you got anywhere near the speed of light, your spaceship would melted by starlight.

If you consider the momentum of the photons as you get anywhere near the speed of light, driving head long into a brick wall would be an understatement.

 

[Drakkith]

By my rough calculations (to tell you the truth I'm not doing these by hand as I'm too tired - I'm using dodgy internet calculators), At one thousandth of the speed of light, the temperature at the front of your spaceship would be 2,600,000,000 k (I think that figure is wrong, but I'm too tired to make a full calculation of what it should be ) But you get the idea. Long before you got anywhere near the speed of light, your spaceship would melted by starlight.

If you consider the momentum of the photons as you get anywhere near the speed of light, driving head long into a brick wall would be an understatement.

I believe that at 86% the speed of light the blueshift would only equal out to be about double the photon energy.

 

[krd]

I believe that at 86% the speed of light the blueshift would only equal out to be about double the photon energy.

How are you doing that calculation?

I used an online calculator - just by a very rough guestimate of E = hf it's nothing like double the photon energy.

 

[Drakkith]

I used a combination of a relativity calculator online and the transverse relativistic equation found here: http://en.wikipedia.org/wiki/Redshift#Redshift_formulae

 

[krd]

I used a combination of a relativity calculator online and the transverse relativistic equation found here: http://en.wikipedia.org/wiki/Redshift#Redshift_formulae

I'm too tired to look at it now. But, isn't the transverse at a 90 degree angle to the motion? You're not calculating the head on shift at 0 degrees. I'm tired so I may have you wrong.

I'm expecting the wavelength to keep shrinking with increased velocity, causing the frequency to rise, and giving the photons more energy. Halving the wavelength should double the energy - or am I missing something (I probably am).

I didn't do a single calculation by hand, just plugged the numbers into various on-line calculators.